VSEBINA – CONTENTS IZVIRNI ZNANSTVENI ^LANKI – ORIGINAL SCIENTIFIC ARTICLES The thermal conductivity of Al73Mn27–xFex Taylor phases Toplotna prevodnost Taylorjevih faz Al73Mn27–xFex D. Stani}, P. Pop~evi}, I. Smiljani}, @. Bihar, J. Lukatela, B. Leonti}, A. Bilu{i}, I. Batisti}, A. Smontara . . . . . . . . . . . . . . . . . . . . . . . 3 Hall effect in the crystalline orthorombic o-Al13Co4 approximant to the decagonal quasicrystals Hallov efekt v kristalini~nem ortorombi~nem pribli`ku o-Al13Co4 dekagonalnim kvazikristalom J. Ivkov, D. Stani}, P. Pop~evi}, A. Smontara, J. Dolin{ek, P. Gille . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Experimental plans method to formulate a self-compacting cement paste Eksperimentalno na~rtovana metoda za dolo~itev samozgo{~ujo~e cementne malte A. Mebrouki, N. Belas, N. Bouhamou . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 The corrosion behaviour of austenitic and duplex stainless steels in artificial body fluids Korozijsko vedenje avstenitnega in dupleksnega nerjavnega jekla v simuliranih telesnih teko~inah A. Kocijan, M. Conradi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Chronic stimulation of an autonomous nerve with platinum electrodes Kroni~na stimulacija avtonomnega `ivca s platinastimi elektrodami P. Pe~lin, M. Zdovc, J. Rozman . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Preparation of Si3N4-TiN ceramic composites Priprava kerami~nih kompozitov na osnovi Si3N4-TiN A. Maglica, K. Krnel, T. Kosma~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Analysis of the material and the actuator influence on the characteristics of a pneumatic valve Analiza vpliva materiala in aktuatorjev na lastnosti pnevmati~nega ventila N. Herakovi~, T. Bevk. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 Stress analysis of a unilateral complex partial denture using the finite-element method Napetostna analiza unilateralno kompleksne zobne proteze z uporabo metode kon~nih elementov A. Todorovi}, K. Radovi}, A. Grbovi}, R. Rudolf, I. Maksimovi}, D. Stamenkovi} . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 ISSN 1580-2949 UDK 669+666+678+53 MTAEC9, 44(1)1–47(2010) MATER. TEHNOL. LETNIK VOLUME 44 [TEV. NO. 1 STR. P. 1–47 LJUBLJANA SLOVENIJA JAN.–FEB. 2010 D. STANI^ ET AL.: THE THERMAL CONDUCTIVITY OF Al73Mn27-xFex TAYLOR PHASES THE THERMAL CONDUCTIVITY OF Al73Mn27–xFex TAYLOR PHASES TOPLOTNA PREVODNOST TAYLORJEVIH FAZ Al73Mn27–xFex Denis Stani}1,2, Petar Pop~evi}1, Igor Smiljani}1, @eljko Bihar1, Jagoda Lukatela1, Boran Leonti}1, Ante Bilu{i}1,3, Ivo Batisti}4, Ana Smontara1 1Laboratory for the Study of Transport Problems, Institute of Physics, Bijeni~ka 46, POB 304, HR-10000 Zagreb, Croatia 2Department of Physics, University of Osijek, Gajev trg 6, HR-31000 Osijek, Croatia 3Faculty Science, University of Zagreb, Bijeni~ka c. 32, HR-10000 Zagreb, Croatia 4Department of Physics, Faculty of Natural Sciences of the University of Split, Nikole Tesle 12, HR-21000 Split, Croatia dstanic@fizika.unios.hr Prejem rokopisa – received: 2009-07-21; sprejem za objavo – accepted for publication: 2009-08-24 The thermal conductivity () of Al73Mn27–xFex (x = 0, 2, 4, 6) complex metallic alloys has been measured in the temperature interval from 2 K to 300 K. All the alloys are Taylor (T) phases, except Al73Mn21Fe6, which is a decagonal (d) quasicrystal. The behaviours of  are typical for complex metallic alloys, i.e., a relatively small magnitude, a change of slope at about 50 K and an increase of the conductivity above 100 K. At room temperature the magnitude of  for all the samples is between 2.7 W/mK and 3.3 W/mK, which is comparable to that of thermally insulating amorphous SiO2 and Zr/YO2 ceramics. The reason for such a low thermal conductivity is because both, the electronic and lattice conductivity are low. The electronic contribution to the thermal conductivity is low because of the large electrical resistivity of the samples. The lattice thermal conductivity is greatly reduced because of the enhanced umklapp process of the phonon scattering (caused by the large lattice constant) and by the disorder in the structure. Keywords: complex metallic alloys, thermal conductivity, spectral conductivity Toplotna prevodnost () kompleksnih kovinskih zlitin Al73Mn27-xFex (x = 0, 2, 4, 6) je bila izmerjena v razponu temperatur od 2 K do 300 K. Vse zlitine, z izjemo Al73Mn21Fe6, ki je dekagonalni (d) kvazikristal, so Taylorjeve (T) faze. Vedenje toplotne prevodnosti  je zna~ilno za kompleksne kovinske zlitine: relativno majhna velikost, sprememba naklona pri pribli`no 50 K in pove~ana prevodnost nad 100 K. Pri sobni temperaturi je velikost  za vse preizku{ance med 2,7 in 3,3 W/mK, kar je primerljivo s toplotno izolirno amorfno SiO2- in Zr/YO2-keramiko. Vzrok za tako majhno toplotno prevodnost sta majhni elektronska in mre`na prevodnost. Elektronski prispevek k toplotni prevodnosti je majhen zaradi velike elektri~ne upornosti zlitin. Mre`na toplotna prevodnost je zmanj{ana zaradi pove~anja procesa "umklapp" razpr{itve fononov (zaradi velikega parametra mre`e) in nereda v strukturi. Klju~ne besede: kompleksne kovinske zlitine, toplotna prevodnost, spektralna prevodnost 1 INTRODUCTION The Al-Mn-Fe system contains several complex metallic alloy phases, which have recently attracted increasing interest. Among them is the orthorhombic Taylor (T) phase, the structure of which is built of atomic layers stacked along the [010]-direction. Along this axis, pentagonal columnar clusters are formed 1. For this reason, they are considered to be approximants of the decagonal (d) Al-Mn phases. The unit cell of the T-phase contains 156 atoms, with many of the sites having either a fractional occupation or a mixed Al/Mn occupation, so that a great inherent chemical disorder exists on the lattice2. As part of a systematic investigation of the transport properties of T-Al-Mn-Fe, here we present the results of the thermal conductivity measurements of T-Al73Mn27–xFex (x = 0,2,4) complex metallic alloys and for a comparison of the d-Al73Mn21Fe6 quasicrystals. 2 EXPERIMENTAL The polycrystalline samples were produced from the constituent elements by levitation induction melting in a water-cooled copper crucible in argon atmosphere. Parts of the samples were annealed in argon at 900 °C and 930 °C for up to 698 h and subsequently quenched into water2. All the samples were T (Taylor) phases of the composition Al73Mn27–xFex (x = 0,2,4,6), except for the d-Al73Mn21Fe6, which was a decagonal quasicrystal. The thermal conductivity  of the Al73Mn27-xFex (x = 0, 2, 4, 6) complex metallic alloys was measured in the temperature interval from 2 K to 300 K using an absolute steady-state heat-flow method. The thermal flux through the samples was generated by a 1-k RuO2 chip-resistor, glued to one end of the sample, while the other end was attached to a copper heat sink. The temperature gradient across the sample was monitored by a chromel-gold differential thermocouple (gold with 0.07 % of Fe) 3. The electrical resistivity  (conductivity  =1/) was measured between 300 K and 2 K using the standard four-terminal technique. 3 RESULTS AND DISCUSSION The temperature dependence of the thermal conduc- tivity (T) for all the investigated samples is shown in Figure 1. The thermal conductivity of the T-phases and d-Al73Mn21Fe6 shows characteristic behaviour for the Materiali in tehnologije / Materials and technology 44 (2010) 1, 3–7 3 UDK 669:538.945:536 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 44(1)3(2010) complex metallic alloys (Mg32(Al,Zn)49, Al74Pd22Mn4, -Al3Mg2 and -phases (Al-Pd-transition metal)4, which is a relatively small value, a change of slope at about 50 K and a rise of the conductivity above 100 K. At room temperature (300 K) the magnitude of the thermal conductivity is in the interval 2.7–3.3 W/mK for all the samples. Such a small magnitude of the thermal conduc- tivity is a characteristic of thermal insulators like SiO2 5 and Zr/YO2 6. The small value of the thermal conducti- vity has also been found in the icosahedral quasicrystals i-Al-Pd-Mn 7,8, which is explained by the small value of electronic density of states at EF (the small contribution of the electrons to the thermal conductivity) and no periodicity of the sample lattice (the small contribution of the phonons to the thermal conductivity). Our samples (T,d)-Al73Mn27–xFex (x = 0, 2, 4, 6) exhibit a very high electrical resistivity (Table 1) compared to simple metals (order of magnitude 1 µ cm), so the contribution of the electrons to the thermal conductivity is much smaller than the lattice (phonon) contribution. The low thermal conductivity can be qualitatively explained by the impact of the structure (disorder) of the studied samples on the thermal transport. Table 1: Values of the electrical resistivity  and the thermal conductivity  at room temperature Tabela 1: Elektri~na upornost  in toplotna prevodnost  pri sobni temperaturi Samples  (µ cm)  (W/mK) T-Al73Mn27 5071 2.69 T-Al73Mn25Fe2 2529 3.01 T-Al73Mn23Fe4 2283 2.83 d-Al73Mn21Fe6 720 3.28 The thermal conductivity model appropriate for complex metallic alloys with a large-scale periodicity of the lattice and a small-scale atomic clustering structure has been described in detail in the previous investigation of the i-Al–Pd–Mn system9. The electrons (e) and lattice/phonon (l) both con- tribute to the thermal conductivity (T): (T) = e(T) + l(T) (1) It is common practice to estimate the electronic part using the Wiedemann–Franz law e = L0T/ (2) where L0 = 2.44 × 10–8 W  K–2 is the Lorenz number, and  is the electrical resistivity. Here we applied a more elaborate analysis based on the Kubo-Greenwood response theory 10,11,12. The central quantity of this formalism is the spectral conductivity function that incorporates both the band structure and the transport properties of the system. All the electronic contributions to the transport coefficients, including the resistivity, the thermopower and the thermal conduc- tivity, are related to the spectral conductivity function. We have analyzed both the electrical conductivity and the thermopower to obtain the properties and the shape of the spectral conductivity function in the vicinity of the Fermi level. Our final results on the spectral conductivity function are shown in Figure 2. The details of the analysis are given in reference13. We should mention that this is a modified version of the procedure originally developed by Landauro and Macia 14,15,16 and is adjusted to suite the experimental data in this class of compounds. D. STANI^ ET AL.: THE THERMAL CONDUCTIVITY OF Al73Mn27-xFex TAYLOR PHASES 4 Materiali in tehnologije / Materials and technology 44 (2010) 1, 3–7 Figure 1: Temperature-dependent thermal conductivity (T) between 2 K and 300 K for samples (T,d)-Al73Mn27–xFex (x = 0, 2, 4, 6) Slika 1: Odvisnost med temperaturo in toplotno prevodnostjo (T) med 2 K in 300 K za zlitine (T,d)-Al73Mn27–xFex (x = 0, 2, 4, 6) Figure 2: The spectral conductivity function for all the samples. The singularity around the Fermi energy (the energy scale is shifted so that EF = 0) is clearly pronounced. The sharpness of the pseudogap is directly related to the convex behaviour of the electric conductivity (T) at low temperatures. Slika 2: Funkcija spektralne prevodnosti za vse zlitine. Singularnost pri Fermijevi energiji (lestvica za energijo je premaknjena tako, da je EF = 0) je jasno pokazana. Ostrina psevdore`e je neposredno povezana s konveksnim vedenjem elektri~ne prevodnosti (T) pri nizki tempe- raturi. The most important feature of the spectral function in Figure 2 is the pronounced pseudo gap around the Fermi level. Within the energy range of ±0.1 eV the spectral function loses around 40 % of its spectral weight. More- over, our analysis, based on transport measurements at low temperatures, reveals the fine structure of the pseudo gap, featuring the |E|1/2 singularity at the Fermi level. Once calculated the spectral conductivity function can be used to determine the electronic contribution, e, to the thermal conductivity. The results of the different contributions at room temperature (RT) are presented in Table 2. The lattice part is significantly larger than the electronic part. This is because the electrical resistivity of these samples is very high, so consequently the elec- trons also have a small contribution to the heat transport. Table 2: Thermal conductivity , electronic contribution e calculated by the model of spectral conductivity, electronic contribution e0 calculated by the WF law, difference between the two former electronic contributions in (%) and ratio effective and normal Lorentz number Leff/Lo at T = 300 K Tabela 2: Toplotna prevodnost , elektronski dele` e, izra~unan po modelu spektralne prevodnosti, in elektronski dele` e0 izra~unan po WF-zakonu, razlika med obema v odstotkih in razmerje med efektivnim in normalnim Lorentzovim {tevilom Leff/Lo pri T = 300 K. Sample /(W/mK) e/ (W/mK) e,o/ (W/mK) (e–e,o) / (%) Leff/Lo T-Al73Mn27 2.69 0.18 0.15 1 1.25 T-Al73Mn25Fe2 3.01 0.43 0.29 4 1.40 T-Al73Mn23Fe4 2.83 0.41 0.32 3 1.30 d-Al73Mn21Fe6 3.28 1.30 1.02 9 1.30 Although the WF law is not valid, the difference between the electronic contribution obtained by the spectral conductivity model e and the one obtain using the WF law e,o is only a few percent (except for d-Al73Mn21Fe6, where it reaches up to 10 %). So, we can still use the WF law to predict the electronic contribution to the heat transport as a rough approximation. The lattice contribution l =  – e is analyzed by considering the propagation of long-wavelength phonons within the Debye model and the hopping of the localized vibrations. This picture assumes that large atomic clusters of icosahedral symmetry strongly suppress the propagation of phonons in the lattice of complex metallic alloys. The exceptions are the long-wavelength acoustic phonons, for which this material is an elastic continuum, and fracton-like localized vibrations within the cluster substructure that can participate in the heat transfer via thermally activated hopping. In the simplest model, the hopping of localized vibrations is described by the single activation energy Ea, yielding a contribution to the thermal conductivity9  H H a B = − ⎛ ⎝ ⎜ ⎞ ⎠ ⎟0 exp E k T (3) where H 0 is a constant. The Debye thermal conductivity is written as:    D D D e (e d= −∫C T x x x T x x 3 0 4 21 ( ) ) / (4) where CD = kB 4 /22v3, v is the average velocity of sound, D is the Debye temperature,  is the phonon relaxation time, x = /kBT, and  is the phonon energy. The different phonon-scattering processes are incorporated into the relaxation time (x) and we assume that Matthiessen’s rule is valid,   = ∑ j , where j is a scattering rate related to the j-th scattering channel. In an analogy with the -phases in Al-Pd-Mn9, we consider two dominant scattering processes in the investigated temperature interval (from 2 to 300 K). First, the scattering of phonons on the structural defects of the stacking-fault type with the scattering rate  j a v N= 7 10 2 2 2 s (5) where a is a lattice parameter, is the Grüneisen para- meter and Ns is the linear density of the stacking faults. D. STANI^ ET AL.: THE THERMAL CONDUCTIVITY OF Al73Mn27-xFex TAYLOR PHASES Materiali in tehnologije / Materials and technology 44 (2010) 1, 3–7 5 Figure 3: Temperature dependent lattice thermal conductivity l (filled symbols) together with the two contributions: the Debye contribution D (dashed line) and the hopping contribution H (dash- dot line) of T-Al73Mn27 and d-Al73Mn21Fe6 shown in different scales. Slika 3: Temperaturna odvisnosti mre`ne toplotne prevodnosti l (polni znaki) z dvema dele`ema: Debyejev dele` D (~rtkana ~rta) in dele` "hopping" H (~rtopi~na ~rta) za T-Al73Mn27 in d-Al73Mn21Fe6, prikazani z razli~nima lestvicama The second scattering mechanism is the umklapp processes with the phenomenological form of the scattering rate pertinent to complex metallic alloys 9,  um = −Bx T 4 and for the total scattering rate we get     = +sf um . The Debye temperature of the investi- gated T-phases is not known; therefore, we have used the D value reported for the related icosahedral i-Al–Pd–Mn quasicrystals, were D was commonly found to be close to 500 K9,19. Since our (T) data are available only up to 300 K, it turns out that the fit is insensitive to a slight change of this D value, so a fixed D = 500 K is used. The Debye constant CD was also not taken as a free parameter, but was instead calculated using v = 4000 m s–1, a value determined for the i-Al–Pd–Mn from ultrasonic data. The results of a fitting procedure using the relation l(T) = D(T) + H(T) to the experimental data are shown for T-Al73Mn27 and for d-Al73Mn21Fe6 in Figure 3. The parameters of the fitting procedure for all the investigated samples are shown in Table 3. From Figure 3 it is easy to see that the Debye contribution D(T) has a maximum at about 50 K, although it becomes smaller at higher temperatures. A similar behavior is conventional for periodic structures, where the origin of such behaviour is in the phonon-phonon umklapp scattering pro- cesses21. Table 3: The fit parameters for the lattice thermal conductivity l = D+H Tabela 3: Parametri ujemanja z mre`no toplotno prevodnost l = D + H Sample A (107 s–1 K–2) B (104 s) H0 (W/mK) Ea (meV) T-Al73Mn27 5.6 1.8 4.8 17.7 T-Al73Mn25Fe2 4.8 1.7 5.2 19.6 T-Al73Mn23Fe4 4.4 2.1 4.9 18.5 d-Al73Mn21Fe6 2.3 2.3 3.3 11.3 The parameter A, which describes the structural defects of the stacking-fault type, is close to 107 s–1 K–2 for all samples. It is possible to estimate from A the linear density of stacking faults Ns. If we take typical values for the lattice parameter a 1.4 nm and the Grüneisen parameter 2, we obtain Ns = 10Av2 / 7 2 2kB 2 = 0.8 µm–1. This micrometer-scale Ns value is comparable to those reported for the -Al-Pd-Mn8 i-Al-Pd-Mn22 and the decagonal d-Al-Mn-Pd23. There- fore, the stacking-fault-like structural defects may be considered as the source of the phonon scattering at low temperatures in the (T,d)-Al73Mn27–xFex (x = 0, 2, 4, 6) samples. The parameters B and define the phonon scattering by the umklapp processes in a phenomeno- logical way. From the fitting procedure we get = 1, so the frequency and temperature dependence of the umklapp term is  um −1 ~ T3. For all the samples, the hopping contribution H becomes significant above 100 K. The activation energy Ea for all the samples is between 10 meV and 20 meV and is smaller by a factor of 2 than the Ea of -Al-Pd-Mn9. This smaller Ea value reflects the considerably less steep (T) increase at temperatures above 100 K for our samples compared to -Al-Pd-Mn. On the other hand, the above Ea values correlate with the inelastic neutron24 and x-ray25 scatter- ing experiments on i-Al-Pd-Mn quasicrystals, where dispersionless vibrational states were identified for the energies higher than 12 meV. In quasicrystals such dispersionless states indicate localized vibrations and are considered to be a consequence of the dense distribution of energy gaps in the phonon excitation spectrum. This prevents extended phonons from propagating through the lattice, whereas localized vibrations may still be excited. Therefore, localized vibrations also appear to be present in the giant unit cell of (T,d)-Al73Mn27-xFex samples, where their origin may be attributed to the cluster substructure. 4 CONCLUSION We have investigated the thermal conductivity of (T,d)-Al73Mn27–xFex (x = 0, 2, 4, 6) samples, which show typical behaviour of (T) for complex metallic alloys, i.e., relatively small values (thermal insulators), a change of the slope at about 50 K and an increase of the conductivity above 100 K. We have separated (T) into the electron e(T) and lattice (phonon) l(T) parts, which both have small values. The electron part was determined by the spectral conductivity model, and it is much smaller than the lattice part. The reason is the very high electrical resistivity of all the samples, which reduces the electron thermal conductivity. The lattice’s thermal conductivity is greatly reduced because of the enhanced umklapp processes of the phonon scattering (caused by a large lattice constant and, consequently, a small Brillouin zone) and by disorder in the structure. Acknowledgement This work was carried out within the activities of the 6th Framework EU Network of Excellence "Complex Metallic Alloys" (Contract No. NMP3-CT-2005- 500140), and has been supported in part by the Ministry of Science, Education and Sports of Republic of Croatia through the Research Projects: 035-0352826-2848 "Thermal and charge transport in highly frustrated magnets and related materials"; 035-0352826-2847 "Modelling physical properties of materials with marked frustration or disorder" and 119-1191458-0512 "Low- dimensional strongly correlated conducting systems". 5 REFERENCES 1 Klein, H.; Boudard, M.; Audier, M.; deBoissieu, M.; Vincent, H.; Beraha, L.; Duneau, M.: Philos. Mag. 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STANI^ ET AL.: THE THERMAL CONDUCTIVITY OF Al73Mn27-xFex TAYLOR PHASES Materiali in tehnologije / Materials and technology 44 (2010) 1, 3–7 7 J. IVKOV ET AL.: HALL EFFECT IN THE CRYSTALLINE ORTHOROMBIC HALL EFFECT IN THE CRYSTALLINE ORTHOROMBIC o-Al13Co4 APPROXIMANT TO THE DECAGONAL QUASICRYSTALS HALLOV EFEKT V KRISTALINI^NEM ORTOROMBI^NEM PRIBLI@KU o-Al13Co4 DEKAGONALNIM KVAZIKRISTALOM Jovica Ivkov1, Denis Stani}1,2, Petar Pop~evi}1, Ana Smontara1, Janez Dolin{ek3, Peter Gille4 1Institute of Physics, Laboratory for the Study of Transport Problems, Bijeni~ka 46, POB 304, HR-10000 Zagreb, Croatia 2Department of Physics, University of Osijek, Gajev trg 6, 31000 Osijek, Croatia 3J. Stefan Institute, University of Ljubljana, Jamova 39, SI-1000 Ljubljana, Slovenia 4Ludwig-Maximilians-Universität München, Theresienstrasse 41, D-80333 München, Germany ivkov@ifs.hr, dstanic@fizika.unios.hr Prejem rokopisa – received: 2009-07-24; sprejem za objavo – accepted for publication: 2009-11-27 We have investigated the anisotropic Hall effect of the o-Al13Co4 orthorhombic approximant to the decagonal phase. The crystalline-direction-dependent measurements were performed along the a, b and c directions of the orthorhombic unit cell. The Hall effect has been measured for all the combinations of the electrical current and magnetic field directions. The Hall coefficients RH change with the crystallographic direction from negative electron-like or zero to positive hole-like for different combinations of the current and field directions. The results for the anisotropy of RH is well correlated with the anisotropy of RH in the d-Al-Ni-Co and d-Al-Cu-Co quasicrystals. The Hall coefficients of the o-Al13Co4 phase were compared to the literature data on single crystals of the Al76Co22Ni2 and the Al80Cr15Fe5 approximants to the decagonal quasicrystals, allowing a study of the evolution of the Hall coefficient with an increasing structural complexity and unit-cell size. Keywords: complex intermetallics, quasicrystalline approximants, Hall effect Raziskali smo anizotropni Hallov efekt v ortorombi~nemu pribli`ku o-Al13CO4 dekagonalni fazi. Meritve, odvisne od kristalne orientacije, so bile izvr{ene v smereh a, b in c ortorombi~ne osnovne celice. Hallov efekt je bil izmerjen za vse kombinacije elektri~nega toka in smeri magnetnega polja. Hallov koeficient RH se spremeni s kristalografsko orientacijo od negativnega podobnega elektronom, do pozitivnega, podobnega vrzelim, po razli~nih kombinacijah toka in smeri polja. Rezultati o anizotropiji za RH se dobro korelirajo z anizotropijo RH v kvazikristalih d-Al-Ni-Co in v d-Al-Cu-Co. Hallov koeficient faze o-Al13CO4 je bil primerjan s podatki iz literature o monokristalnih pribli`kih Al76Co22Ni2 in Al80Cr15Fe5 dekagonalnim kvazikristalom, kar omogo~a {tudij evolucije Hallovega koeficienta z nara{~asjo~o strukturno kompleksnostjo in velikostjo osnovne celice. Klju~ne besede: kompleksni intermetaliki, kvazikristalni pribli`ki, Hallov efekt 1 INTRODUCTION Decagonal quasicrystals (d-QCs) can be structurally viewed as a periodic stack of quasiperiodic atomic planes, so that d-QCs are two-dimensional quasicrystals, whereas they are periodic crystals in a direction perpen- dicular to the quasiperiodic planes. A consequence of this anisotropic structure are the anisotropic physical properties,1–9 when measured along the different crystalline directions. The anisotropy of the Hall coeffi- cient RH of d-QCs is especially intriguing, being positive hole-like (RH > 0) for the magnetic field lying in the quasiperiodic plane, whereas it changes sign to negative (RH < 0) for the field along the periodic direction, thus becoming electron-like. This RH anisotropy was reported for the d-AlNiCo, d-AlCuCo and d-AlSiCuCo, and is considered to be a universal feature of d-QCs.5,6 The lack of translational periodicity within the quasiperiodic layers prevents any quantitative theoretical analysis of this phenomenon. The problem can, however, be over- come by considering periodic approximant phases to the d-QCs that are characterized by a large unit cell, which periodically repeats in space, while the structure of the unit cell closely resembles d-QCs. Atomic layers that correspond to the quasiperiodic layers are again stacked periodically and the periodicity lengths along the stacking direction are almost identical to those along the periodic direction of the d-QCs. Approximant phases thus offer a valid comparison with the d-QCs. Here, we report measurements of the anisotropic Hall coefficient of the orthorhombic o-Al13Co4 complex metallic alloy, which belongs to the derivative of the Al13TM4 com- pound, with four atomic layers within one periodic unit of 0.8 nm along the stacking direction and a unit cell comprising 102 atoms. These measurements complement our previous work on the anisotropic Hall coefficient of the Al76Co22Ni2 and Al80Cr15Fe5 approximants to the decagonal quasicrystals. Al76Co22Ni2 has two atomic layers within one periodic unit of 0.4 nm and 32 atoms in a relatively small unit cell,10–12 and the Al80Cr15Fe5 has six atomic layers within one periodic unit of 1.25 nm and 306 atoms in a giant unit cell.13–14 The o-Al13Co4 Materiali in tehnologije / Materials and technology 44 (2010) 1, 9–12 9 UDK 548.4:537.633.2 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 44(1)9(2010) phase with four atomic layers and 102 atoms in the unit cell is thus intermediate to the other two approximant phases in terms of the number of layers in one periodic unit and the size of the unit cell. A comparison of the three phases can give us an insight into the way the anisotropic Hall coefficient of the approximant to the decagonal quasicrystals evolve with the increasing structural complexity and the unit cell size. 2 EXPERIMENTAL The o-Al13Co4 single crystal used in our study was grown using the Czochralski technique and its structure matched well with the orthorhombic unit cell.15 In order to perform crystalline-direction-dependent studies we cut three bar-shaped samples of dimensions 1×1×7 mm3 from the ingot, with their long axes along three ortho- gonal directions. The long axis of the first sample was along the [100] stacking direction (designated as a), which corresponds to the pseudo-tenfold axis of the o-Al13Co4 structure and is equivalent to the periodic (tenfold) direction in the related d-QCs. The (b, c) orthorhombic plane corresponds to the quasiperiodic plane in the d-QCs and the second sample was cut with its long axis along the [010] (b) direction and the third one along the [001] (c) direction. For each sample, the orientation of the other two crystalline directions was also known. The so-prepared samples enabled us to determine the anisotropic Hall coefficients of the o-Al13Co4 approximant to the decagonal quasicrystals along the three principal orthorhombic directions of the unit cell. The Hall-effect measurements were performed using a five-point method with a standard ac technique in magnetic fields up to 1 T.16 The current through the samples was in the range 10–50 mA. The measurements were performed in the temperature interval from 90 K to 390 K. 3 RESULTS The temperature-dependent Hall coefficient RH = Ey/jxBz of the o-Al13Co4 is displayed in Figure 1. In order to determine the anisotropy of the RH, three sets of experiments were performed with the current along the long axis of each sample (thus along a, b and c, respec- tively), whereas the magnetic field was directed along each of the other two orthogonal crystalline directions, making six experiments altogether. For all combinations of directions, the RH values are typical of a metal, in agreement with the electrical resistivity (Figure 2a), of the order 10–10 m3 C–1. RH exhibits pronounced anisotropy with the following regularity: the six RH sets of data form three groups of two practically identical RH curves, where the magnetic field in a given crystalline direction yields the same RH for the current along the other two crystalline directions in the perpendicular plane. The room-temperature values of the Hall coefficient of these J. IVKOV ET AL.: HALL EFFECT IN THE CRYSTALLINE ORTHOROMBIC 10 Materiali in tehnologije / Materials and technology 44 (2010) 1, 9–12 Figure 2: (a) Temperature-dependent electrical resistivity (T) of o-Al13Co4 along three orthogonal crystalline directions a, b and c. (b) Temperature-dependent thermoelectric power S of o-Al13Co4 along three orthogonal crystalline directions a, b and c Slika 2: (a) Tempereturno odvisna elektri~na upornost (T) za o-Al13CO4 v treh ortogonalnih kristalnih smereh a, b, c. (b) Tem- peraturno odvisna termoelekri~na napetost S za o-Al13CO4 v treh ortogonalnih kristalnih smereh a, b, c Figure 1: Anisotropic temperature-dependent Hall coefficient RH = Ey/jxBz of o-Al13Co4 for different combinations of direction a, b, c, of the current jx and magnetic field Bz (given in the legend). The super- script a, b or c on RH denotes the direction of the magnetic field. Slika 1: Anizotropen tempereturno odvisen Hallov koeficient RH = Ey/jxBz za o-Al13CO4 za razli~ne kombinacije smeri a, b, c toka jx in magnetnega polja Bz (v legendi). Ozna~ba a, b, c na RH je smer magnetnega polja. pairs are RaH = Eb/jcBa = Ec/jbBa = –6.5 · 10–10 m3 C–1, RbH = Ea/jcBb = Ec/jaBb = 3.5 · 10–10 m3 C–1 and RcH = Ea/jbBc = Eb/jaBc = –0.6 · 10–10 m3 C–1, where the additional super- script on the Hall coefficient denotes the direction of the magnetic field. RbH and RcH are practically temperature- independent within the investigated temperature range, whereas RaH shows a moderate temperature dependence that tends to disappear at higher temperatures. The observed RH anisotropy reflects the complicated structure of the Fermi surface. The negative RaH < 0 is electron-like for the magnetic field along the stacking a direction, whereas the positive RbH > 0 is hole-like for the field along the in-plane b direction. For the field along the second in-plane direction c, RcH 0 suggests that the electron-like and hole-like contributions are of compa- rable importance. This orientation-dependent mixed electron-like and hole-like behavior of the anisotropic Hall coefficient is analogous to the anisotropy of the thermopower measured on the same specimens, pre- sented in Figure 2b, which also changes sign with crystalline orientation. In both cases there is no simple explanation of this dual behavior, which would require knowledge of the details of the Fermi surface pertinent to the o-Al13Co4 phase. 4 DISSCUSION We have measured the Hall coefficient of the ortho- rhombic o-Al13Co4 complex metallic alloy, with four atomic layers within one periodic unit. Our main objec- tive was to determine the crystalline-direction-dependent anisotropy of the investigated Hall coefficients when measured within the (b,c) atomic planes, corresponding to the quasiperiodic planes in the related d-QCs, and along the stacking a direction perpendicular to the planes corresponding to the periodic direction in d-QCs. The Hall coefficient results point to a complicated Fermi surface that consists of electron-like and hole-like parts. Comparing the Hall coefficient of the three stacked-layer phases – the Al76Co22Ni2 two-layer phase, the o-Al13Co4 four-layer phase and the Al80Cr15Fe5 six-layer phase – some general conclusions can be drawn on the anisotropic Hall coefficient of the approximant to the decagonal quasicrystal with increasing structural complexity and unit-cell size. The anisotropic Hall coefficient shows the following regularity: the applica- tion of the field along the stacking direction always yields the lowest value of the Hall coefficient (for o-Al13Co4 and Al80Cr15Fe5, the corresponding Hall coefficient is negative, whereas for the Al76Co22Ni2, it is practically zero), whereas the application of the field in-plane results in higher RH values and a change of sign to positive for at least one of the in-plane directions. No systematic change of RH with increasing structural complexity can be claimed. Regarding the anisotropic thermopower, no systematic differences between the three compounds can be inferred from the available experimental data. Comparing the Hall coefficient of the above approximants to the decagonal quasicrystals to the true d-QCs, we find that the two kinds of compounds are in complete analogy. A comparison with the currently best-studied d-Al-Ni-Co-type d-QCs with two atomic layers within one periodic unit shows the following similarities: the Hall coefficient is the lowest, negative and electronic-like for the magnetic field along the periodic direction, whereas RH changes sign to positive hole-like for the field along the in-plane directions.5,6 This duality is suggested to be a universal feature of d-QCs. 5 CONCLUSION The investigated approximants to the decagonal quasicrystals of increasing structural complexity exhibit an anisotropic Hall coefficient qualitatively similar to that of the decagonal quasicrystals. Both types of compounds have in common atomic planes that are stacked periodically. The stacked-layer structure appears to be at the origin of the anisotropy of the investigated Hall coefficient. Acknowledgement This work was done within the activities of the 6th Framework EU Network of Excellence "Complex Metallic Alloys" (Contract No. NMP3-CT-2005- 500140), and has been supported in part by the Ministry of Science, Education and Sports of Republic of Croatia through the Research Projects: 035-0352826-2848 "Thermal and charge transport in highly frustrated magnets and related materials". 6 REFERENCES 1 T. Shibuya, T. Hashimoto, S. Takeuchi, J. Phys. Soc. Jpn., 59 (1990), 1917–1920 2 S. Martin, A. F. Hebard, A. R. Kortan, F. A. Thiel, Phys. Rev. Lett., 67 (1991), 719–722 3 Wang Yun-ping, Zhang Dian-lin, Phys. Rev. B, 49 (1994), 13204–13207 4 Lin Shu-yuan, Wang Xue-mei, Lu Li, Zhang Dian-lin, L. X. He, K. X. Kuo, Phys. Rev. B, 41 (1990), 9625–9627 5 Zhang Dian-lin, Lu Li, Wang Xue-mei, Lin Shu-yuan, L. X. He, K. H. Kuo, Phys. Rev. B, 41 (1990), 8557–8559 6 Wang Yun-ping, Zhang Dian-lin, L. F. Chen, Phys. Rev. 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Stani}, E. S. Zijlstra, B. Bauer, P. Gille, Phys. Rev. B, 76 (2007), 174207–13 14 A. Smontara; D. Stani}; I. Smiljani}; J. Dolin{ek; P. Gille, Zeitschrift für Kristallographie, 224 (2009), 56–58 15 J. Grin, U. Burkhardt, M. Ellner, K. Peters, J. Alloys Compd., 206 (1994), 243–247 16 D. Stani}, Thesis, Physical Department, University of Zagreb (2009) J. IVKOV ET AL.: HALL EFFECT IN THE CRYSTALLINE ORTHOROMBIC 12 Materiali in tehnologije / Materials and technology 44 (2010) 1, 9–12 A. MEBROUKI ET AL.: EXPERIMENTAL PLANS METHOD TO FORMULATE ... EXPERIMENTAL PLANS METHOD TO FORMULATE A SELF-COMPACTING CEMENT PASTE EKSPERIMENTALNO NA^RTOVANA METODA ZA DOLO^ITEV SAMOZGO[^UJO^E CEMENTNE MALTE Abdelkader Mebrouki, Nadia Belas, Nasr-eddine Bouhamou Department of Civil Engineering. Mostaganem University, Algeria mebroukiaek@yahoo.fr Prejem rokopisa – received: 2008-11-23; sprejem za objavo – accepted for publication: 2009-07-15 This paper presents a self-compacting cement-paste formulation using Algerian local materials (a binary cement consisting of natural pozzolana and limestone fillers). In this study, simple laboratory instruments were used, i.e., a mini-slump for spreading out diameters and a Marsh cone for flow-times measurements. A wide variation of combinations was used as preliminary tests to select pastes with acceptable properties and the use of the mixture-plans method has shown that it is possible to define an experimental field inside which optimal measurements can be obtained. This field has been put mathematically into equation form, conditioned by implicit constraints, defining zones of minimal shearing threshold and maximum viscosity and was then solved numerically. The optimization criterion was checked in addition to the interactivity between the components utilizing the multiple combinations of proportioning these materials. From the results given by the ternary diagrams and desirability functions, an optimal self-compacting cement-paste mixture was defined. Experimental checking was performed to validate the obtained results. Keywords: mixture plans method; pozzolana; limestone fillers; superplasticizer; implicit constraints; desirability functions; ternary diagrams; Algerian local materials V ~lanku je prikazan razvoj samozgo{~ujo~e cementne malte z uporabo lokalnih al`irskih materialov (binarni cement iz naravnega vulkanskega pepela pozzolane in apnenca kot polnila). V raziskavi so uporabljene enostavne laboratorijske priprave: mini sto`ec in Marshev sto`ec za meritev ~asa iztekanja. [irok razpon variacij je bil uporabljen pri predhodnih preizkusih za izbiro malt s sprejemljivimi lastnostmi. Uporaba metode me{alnih na~rtov je pokazala, da je mogo~e eksperimentalno opredeliti polje, v katerem je mogo~e dobiti optimalne meritve. To polje je bilo formulirano z matemati~nimi ena~bami, odvisnimi od implicitnih omejitev, ki opredeljujejo cone minimalnih stri`nih pragov in maksimalne viskoznosti in je bilo nato re{eno matemati~no. Merilo optimizacije je bilo preverjeno kot dodatek k interaktivnosti komponent z uporabo multiplih kombinacij proporcioniranja materialov. Iz rezultatov v ternarnih diagramih `elelnih funkcij je bila definirana optimalna samozgo{~ujo~a cementna zmes. Dobljeni rezultati so bili tudi eksperimentalno potrjeni. Klju~ne besede: metoda me{anja planov, pozzolana, apnen~evo polnilo, superplastizator, implicitne omejitve, `elelna funkcija, ternarni diagrami, lokalni al`irski materiali 1 INTRODUCTION Concrete is a composite material that consists essentially of a) a fluid phase called the cement paste and b) a solid phase of aggregates with a fixed gravel/sand ratio. The self-compacting properties of the concrete depend necessarily on those of the cement paste, which is why the study carried out on formulations is based primarily on the paste and its different components. The method used on materials from the Building Materials Laboratory (L.M.D.C - INSA – UPS, Toulouse, France) gave very satisfactory results, and it was then applied to Algerian local materials in order to obtain a self-com- pacting cement-paste formulation. The experiments were carried out using simple equipments that can be afforded by laboratories on moderate budgets. Various combinations of paste-mix parameters have been adopted using mini-cone and Marsh-cone measure- ments, which has made it possible to eliminate all the undesired mixtures, representing segregation, or a lack of capacity to flow, or being poorly proportioned. The remainder of the combinations was used as a database to define an experimental field. From measurements of the spreading out diameters and the out-flow times, ternary diagrams can be produced in order to delimit zones of low shear threshold and high viscosity. Inside these zones, volumetric proportions of the paste components were retained and then treated numerically to obtain an optimal paste mix, satisfying the criteria of the self-compacting properties. 2 MATERIALS AND METHODS The choice of materials is based on their abundant availability and their moderate cost. The cement used is a 'CEM II/A 32.5 N', which according to European standard is ENV 197-1, contains less than 20 % of natural pozzolana added during the clinker crushing and fillers consisting of limestone, obtained from a west Algerian quarry. The physical properties of these two powders are shown in Table 1. The cement used contains about 15% natural pozzo- lana. The results of the chemical analysis of this cement are given in Table 2. Materiali in tehnologije / Materials and technology 44 (2010) 1, 13–20 13 UDK 691:666.94:620.1/.2 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 44(1)13(2010) Table 1: Physical characteristics of cement and limestone filler Tabela 1: Fizikalne zna~ilnosti cementa in apnen~evega polnila density specificsurfaces average diamètre Cement 3150 kg/m3 3400 cm2/g 18.5 µm Limestone 2800 kg/m3 2880 cm²/g 21.2µm Table 2: Cement chemical analysis Tabela 2: Kemi~na sestava cementa SiO2 Al2O3 CaO MgO Na2O K2O Fe2O3 SO3 CaO free Fire loss 23 5.7 60.9 0.7 0.3 0.4 3.3 3.4 0.09 2.1 The mineralogical composition of Clinker according to Bogue is given in Table 3. Table 3: Clinker mineralogical analysis Tabela 3: Mineralo{ka analiza klinkerja C3S C2S C3A C4AF 58.7 16.4 8.1 9.2 The aspects of limestone fillers and cement are shown in Figure 1. Table 4 presents the chemical analysis of the lime- stone fillers. Table 4: Chemical analysis of used limestone fillers Tabela 4: Kemi~na sestava uporabljenega apnen~evega polnila SiO2 Al2O3 CaO MgO Na2O K2O Fe2O3 Fireloss 0.7 0.2 56.8 0.5 0.08 0.1 0.9 41.2 Figure 2 shows the filler's mineralogical analysis, obtained with x-ray diffraction (XRD). This analysis showed a composition of about 97% calcite, with traces of dolomite and quartz. The superplasticizer used in this study is Viscocrete 20 HE, provided by SIKA-Algeria. It is a non-chlori- nated product containing acrylic copolymer in liquid form and containing 40 % of dry extract with a density of 1.085 kg/m3 and a pH of 4.5. It is shown in several studies1,2 that it is possible to prepare self-compacting concretes without using a viscosity agent to remain in the context of local materials promotion. So this parameter will not be integrated into the preparation of the cement pastes. For measurements, a mini-cone inspired from the slump test was used, whose dimensions are proportional to it3, 4, 5 Figure 3. The mini-slump cone has a bottom diameter of 38 mm, a top diameter of 19 mm, and a height of 57 mm. A. MEBROUKI ET AL.: EXPERIMENTAL PLANS METHOD TO FORMULATE ... 14 Materiali in tehnologije / Materials and technology 44 (2010) 1, 13–20 Figure 2: Limestone filler's mineralogical analysis (XRD) Slika 2: Minaralo{ka sestava apnen~evega polnila Figure 1: Aspects of limestone fillers and cement. (a) Limestone Fillers, (b) Fillers in substitution into cement Slika 1: Videz apnen~evega polnila in cementa, (a) polnilo apnenec, (b) polnilo za zamenjavo apnenca Figure 3: Mini cone Slika 3: Mini sto`ec This apparatus will be used primarily for a deter- mination of the spreading out diameters on a horizontal metal plate with respect to the mix parameters variation (water/binder, limestone/binder ratios and superplasti- cizer). These diameters were measured after 1 min of spreading out, and the same procedure was applied for all the other mixtures. Generally, there is a certain correlation of the test with the threshold of shearing or with the apparent viscosity at a low velocity gradient4, and the mini-slump test results correlate in certain cases with the yield stress6. The main advantages of this test are the facility of its implementation, since it requires simple preparation and a small quantity of materials (a volume less than 40 mL). These tests are reproducible and often used in North America for a determination of the superplasticizer's saturation point of a cementing mixture4. It has been shown that the paste rheology model is useful to the SCC mix design and reducing the laboratory work testing time and materials used7. It would be more interesting to investigate the flow and workability of the concrete by studying the cement paste, which is the main component responsible for these properties8. For a consistency determination, a Marsh cone (Figure 4) was used to measure the out-flow times of a reference volume of pastes with different mixtures, and is a measure of the out-flow time of the cement paste flowing out through the cone by gravity to fill up a given reference volume (150 mL in this study). Since the tested volume was small, the test was simple and short in duration5. The rheological properties of the concrete (Steel Fiber Reinforced Self-Compacting Concrete) can be deduced from those of the paste which constitutes it9. It was concluded10 in their study that the Marsh cone with an orifice of 5 mm was not suitable for measuring the out-flow time and bigger orifices of 8 mm or 10 mm may be used. The time required for a paste sample to flow through the cone is proportional to the paste visco- sity. The flow time increases with the increase in viscosity; therefore, it will be considered as an index of fluidity. Table 5 presents various pastes mixtures (mass pro- portions) for which spreading out and out-flow times measurements were carried out. The compositions shown below, as broad as they are, take into account all the possible mix parameters variations that can contribute to elaborate the cement pastes. The quantity of the binder (cement + filler) was maintained as a constant and for superplasticizer, a maximum proportioning of 3 % was recommended by the manufacturer. Table 5: Compositions of the studied pastes Tabela 5: Sestava preiskanih muljev Cement, /% 100 – 95 – 90 – 85 – 80 – 75 – 70 – 60 Limestone substitution in the cement, /% 0 – 5 – 10 – 15 – 20 – 25 – 30 – 40 Water / Binder (E/L), (W)/(B) 0.22 – 0.24 – 0.30 – 0.40 Superplasticizer (Sp), (Sp)/% 0, 0.5, 1, 1.2, 1.3, 1.5, 2, 3 The substitution percentage is calculated in mass terms. The experimental procedure used for the pastes' preparation is shown in Table 6. Table 6: Experimental procedure of preparation of a standard paste mixture Tabela 6: Eksperimentalna procedura priprave standardne zmesi mulja Step Moment Duration Measured parameter Result Materials Preparation and weighing. – 10 min Materialsmasses Components proportion Materials Mixture – 5 min – – Visual aspect with the trowel. – During mixing – Paste aspect Measure with the min-cone. t0 : end of mixing 2 min Flow Spreading out (cm) Visual aspect with glass tube. t0 5 min In parallel with the previous one – Consistency/sedimentation Measure with Marsh cone. t0 + 2 min 1 min Flow time Time (s) 3 RESULTS AND DISCUSSION The selected results are based on the standard deviations calculated for a chosen paste mixture that was repeated three times under the same experimental conditions. The standard deviation will have the same value for all mixtures. 3.1 Consistency: Visual aspect The visual aspect is a preliminary but very important stage, which allows checking the validity of the mixture visually. The paste mixture can be dry or of very firm A. MEBROUKI ET AL.: EXPERIMENTAL PLANS METHOD TO FORMULATE ... Materiali in tehnologije / Materials and technology 44 (2010) 1, 13–20 15 Figure 4: Marsh cone Slika 4: Marsh sto`ec state, Figure 5a, when it is prepared with insufficient water/binder ratio (Water/Binder m(W)/m(B) = 0.22) and with an important filler substitution quantity (F) = 30 %). It is shown in Figure 5b that the paste formed was plastic and not able to flow (F) = 10 %, m(W)/m(B) = 0.23 and (Sp) = 1 %). Contrary to the previous case, it was noticed that a paste can be capable of flowing but presents a white layer ((F) = 15 %, m(W)/m(B) = 0.24 and (Sp) = 1.5 %) Figure 5c, which is synonymous with a segregation between the solid and the liquid phases of the paste. For other cases, the paste was well formed, but segregation was noticed at the time of the measurement of the spreading out diameter: bubbles appeared on the surface of the wafer with a liquid halo around it. Figure 5d shows a well-formed and homo- geneous wafer, which was kept to measure the spreading out diameters and the out-flow time (F) = 15 %, m(W)/m(B) = 0.24 and (Sp) = 1.2 %). The same proce- dure was followed during the experiment, and only mixtures without any abnormality were retained for measurements. 3.2 Experimental plans for the cement pastes 3.2. 1 Experimental field Several mixtures of normal consistencies were useful for the rheological measurements and have contributed to delimit the experimental field inside of which the measurements give the required results. This concerns the spreading out diameter within the interval 14.4–16 mm, in accordance with what was found,11 and in accordance to the flow without rupture of the paste volume, which is a characteristic of good fluidity. The parametric analysis allows the understanding of the influence of each mix parameter on the fluid suspensions and on the prepared paste. However, the important parameter introduced is the solid volumetric concentration (G) defined by the ratio of the volume of solids on the total volume (solid particles coming from the cement, limestone fillers and superplasticizer in dry extract form). The use of the experimental plans method contributes to collecting the maximum amount of information about components, their influences taken separately and about their possible interactions. It makes it possible to reduce considerably the number of experiments, to plan and facilitate the study. The main objective of the study is then to obtain mixtures having optimal responses, or satisfying certain requirements fixed on departure12. To achieve this, a wide variation of combinations between the mixing parameters was used as preliminary tests to select the pastes with acceptable characteristics and the use of the experimental plans method showed that it is possible to delimit an experimental field bound by the volumetric proportions of the materials composing the paste. The field was transformed mathematically into equations conditioned by implicit constraints, defining zones of minimal shearing threshold and maximum viscosity. The required response depends on the volumetric proportions /% of the components used. Thus, for an experimental plan with four factors, (C) (cement), (F) (limestone filler), (W) (water) and (Sp) (superplasticizer), taken in volu- metric proportions, with a total volume equal to unity, implies that there is a dependence and an interaction between each component. The experimental field was constrained by the following expression: (C) + (F) + (W) + (Sp) (1) Considering a complete mixture plan, taking account of the required accuracy of the response and the number of admitted experiments, the choice of a mathematical model converged towards a polynomial of degree 2, relating the response Y (Y1 for the spreading out diame- ter or Y2 for the out-flow time) to the proportions of the components, which can be written in the following form: Y X X Xi i j k j kk = × + × × = < ∑ ∑∑  ij i j i j (2) The polynomial coefficients i and ij have to be determined and are expected to be different for each response. The parameters Xi and Xj correspond to the volumetric proportions of the components. Equation (2) can be expanded to give: Y = 1·(C) + 2·(F) + 3·(W) + 4·(Sp) + + 12·(C)·(F) + 13·(C)·(W) + 23·(F)·(W) + + 14·(C)·(Sp) + 24·(F)·(Sp) + 34·(F)·(Sp) (3) Equation (3) can be rewritten in matrix form as follows: [Y] = [X] · [] + [] (4) A. MEBROUKI ET AL.: EXPERIMENTAL PLANS METHOD TO FORMULATE ... 16 Materiali in tehnologije / Materials and technology 44 (2010) 1, 13–20 Figure 5: Visual aspects of the elaborate pastes Slika 5: Videz pripravljenih zmesi Where [X] is the experiment matrix, [] is the vector of the model coefficients and [] is the vector of the experimental errors. After preliminary tests, a parametric analysis was used to define zones that are checked at the same time for a high viscosity and a minimal shearing threshold, and also to define the experimental field bounded by the lower and higher constraints (in mass proportion), given as follows: 10 %  limestone  20 % 1 %  Superplasticizer  1.5 % (5) 0.24 %  water ratio  0.3 % 0.57  G  0.59 % Where (G) is the volumetric concentration in the solids. The transformation of these constraints to a system of equations (6) allowed modeling of the experimental problem, which can be solved numerically. (C) + (F) + (Sp) = 1 –0.1125 (C) + (F)    (C) – (F)  0 – (C) – 0.0258 (F) + (Sp)  0 (6)  (C) + 0.0387 (F) – (Sp)  0  (C) + 0.43 (F) – 0.17 (Sp) – 0.57 (W)  0 – (C) – 0.41 (F) + 0.19 (Sp) + 0.59 (W)  0 Here, a numerical example is presented, which shows a way of obtaining the system of equations: (F) is the limestone's filler volume rate = V(F). M(F) is the limestone's filler mass. (Limestone's mass proportion)  20 %  M M ( ) ( ) F C  0.2  M M ( ) ( ) F C – 0.2  0 V V ( ) ( ) F C = M M ( ) ( ) F C 2800 3150 = M M ( ) ( ) F C × 3150 2800  M M ( ) ( ) F C = V V ( ) ( ) F C × 2800 3150 = = V V ( ) ( ) F C × 0.889 V V ( ) ( ) F C × 0.889 – 0.2  0  V(F) – 0.225 × V(C)  0  –(F) + 0.225 × V(C)  0 Thus: 0.225 × (C) – (F)  0 3rd equation of system (6). Table 7 gives the solutions computed by the excess and default and are illustrated in the form of the higher and lower constraints, respectively. Table 7: Implicit constraints Tabela 7: Implicitne omejitve Component C F Sp E Implicit higher Constraints 0.5159 0.1150 0.0246 0.4198 Implicit lower Constraints 0.4491 0.0568 0.0160 0.4033 For an experimental mix plan with four factors, the field formed was a space with four dimensions. The model calculation points and the obtained experiment matrix have produced a geometric form of a hyper polyhedral. These points are located at the tops, at the mid-sides, at the middle of the faces and at the gravi- tational centre. An analytical solution for this complex problem is almost impossible; numerous solutions were obtained by using software packages for experimental mixture-plans processing, and the one used in this study among others is called "NemrodW", developed for the design and the analysis of an experimental plan. 3.2.2 Experiment matrix The determination of the experiment matrix was carried out with the analysis of the exchange algorithm generated by the software, which is a procedure applied to N = 10, as a number of variables (number of poly- nomial coefficients), until N = Nmax satisfies the follow- ing optimization criteria: – Criterion D: optimization of the information quality. – Criterion A: optimization of the model coefficients' quality. – Criterion G: optimization of the model prediction quality. Once determined, the basic matrix was used to calcu- late the model's polynomial coefficients, which will be different for each response. Table 8 gives the necessary information about the experiment matrix and the main characteristics of the studied problem generated by the software. Table 8: Characteristics of the problem Tabela 8: Karakteristike problema Aim of the study Mixing study Variables number 4 Experiments number 35 Coefficients number 10 Responses number 2 The characteristics of the experiment matrix for the volumetric proportions of the components to be used in the preparation of the cement pastes are shown in Table 9. These values were generated by the software and taken inside the experimental field. Table 9: Characteristics of the experiment matrix Tabela 9: Zna~ilnosti eksperimentalne matrice Properties Values Determinant (X'X) ** 1/p 3.9588 Determinant (M) 5.5578975E-0017 Determinant (M) ** 1/p 0.082476 Function of maximum variance 0.333333 Trace (X'X)–1 6.0948 Effectiveness G/% 93.75 Index of Khuri (%) 97.85 The volumetric values of the composition parameters provided by the software were used to prepare the pastes and followed by the measurements. In order to define the A. MEBROUKI ET AL.: EXPERIMENTAL PLANS METHOD TO FORMULATE ... Materiali in tehnologije / Materials and technology 44 (2010) 1, 13–20 17 optimal proportion values of the cement, limestone fillers, superplasticizer and water, the measured responses of the spreading out diameters and the out-flow times were again input into the software as new data. Figure 6 shows the ternary diagrams in space and in the plane illustrating the influence of each parameter on the paste mixture. Indeed, by fixing one parameter and while varying the three others, their sum should always be equal to unity. For example, if the "water" parameter is fixed at 0.305, and while varying the volumetric propor- tions of the others components, the parameter "(Sp)" will be dominating i.e., the responses are more sensitive to the variations of this parameter than to those of the "cement" or those of the "filler". There would then be interactions between the "(Sp)" and "cement" according to the site of the influence field, which is closer to the cement than to the filler, Figure 6. It should be noted here, that the same work has been carried out for the out-flow time response. 3.2.3 Experimental responses Figure 7a and 7b show the curves obtained for G = 0.58 at the end of the processing. Figure 7a shows that the substitution of 15 % of limestone fillers decreases the shearing threshold, whereas Figure 7b shows that a proportioning of 1.5 % of superplasticizer increases the viscosity. These two complementary rheological aspects define the required self-compacting cement paste's property. The values of (Sp) and (F)/(C), satisfying these two properties, are surrounded by circles in the graphs of the two responses. As shown13, the two essential properties "high flow- ability and segregation resistance" have been obtained with the use of the superplasticizer and fine particles (limestone fillers) and without use of a viscosity-modi- fying admixture. Table 10 presents the statistical characteristics of these two responses. Table 10: Characteristics of the responses Tabela 10: Zna~ilnosti odgovorov Response Average Type –gap Mini- mum Maxi- mum Center Spreading out (cm) 18.377 2.275 13.600 23.000 18.300 Time flow (s) 12.217 2.444 8.000 18.000 13.000 A. MEBROUKI ET AL.: EXPERIMENTAL PLANS METHOD TO FORMULATE ... 18 Materiali in tehnologije / Materials and technology 44 (2010) 1, 13–20 Figure 7: (a) Spreading out for G = 0.58, (b) Flow Time for G = 0.58 Slika 7: (a) [irjenje za G = 0.58, (b) ~as iztekanja za G = 0.59 (a) Variation of the spreading out response in the plan Cement- Filler-Sp. Fixed component: Water, (W) = 0.305 (b) Variation of the spreading out response in the plan Filler-Sp-Water. Fixed component: Cement, (C) = 0.511 (c) Variation of the spreading out response in the plan Sp-Water- Cement. Fixed component: Filler, (F) = 0.112 Figure 6: Ternary diagrams for the spreading out response Slika 6: Ternarni diagram odgovora pri {irjenju 4 OPTIMAL PASTE Digital processing of the experimental plans has allowed us to optimize simultaneously two responses. It is a purely numerical procedure that consists mathema- tically to find a formulation or a combination of para- meters for which the desired responses are either of optimal values or belonging to an interval of optimal values. This is called the multi-criteria case of an optimization, based on desirability functions. Using these functions to solve the postulated problem for each response, a profile curve of the desirability function was selected, Figure 8. The desirability is null for an unsui- table response and is a maximum when the response given is very satisfactory, and it takes intermediate values to a lesser extent than the satisfactory responses. The total required desirability Dg (paste) for the required optimal paste is a function of the elementary desirabi- lity's 'd(spread)' and 'd(flow)', necessary for the spreading out and out-flow time, respectively, and it is defined by the following relationship: Dg paste d spread d flow( ) ( ) ( )= × (7) The graphs of the choice of desirability functions are shown in Figure 8. It is of the right unilateral type, without tolerance for the Y1 responses (spreading out) and of the bilateral type with tolerance for Y2 responses (out-flow time). On the basis of this choice, Table 11 presents the characteristics of the elementary functions of desirability and the function of total desirability, defined by the relationship (7). Table 11: Characteristics of desirability functions Tabela 11: Zna~ilnosti `elelnih funkcij Response Value di /% di(min)/% di(max) /% Spreading out diameter 17.15 cm 100.00 65.31 100.00 Out-Flow time 15.75 s 100.00 100.00 100.00 Desirability (Dg) 100.00 80.82 100.00 The formulation of the optimal paste mix was obtained by satisfying the desirability criterion relating to the paste's homogeneity and fluidity. The volume and proportions of each component of the optimal paste are given in Table 12. Table 12: Composition of the optimal paste Tabela 12: Sestava optimalne me{anice Component Volume proportions Dosage, g/L Cement 0.538 1694.7 Limestone 0.116 324.8 Sp 0.014 15.19 Water 0.332 3320 Using the proportions obtained, a cement paste was produced in order to check, to compare and then to validate the theoretical results. Visually, the paste aspect was acceptable, without any apparent segregation. Table 13 gives the experimental measurements of the spreading out diameter and the out-flow time. Table 13: Comparison of the theoretical and experimental results Tabela 13: Primerjava teoreti~nih in eksperimentalnih rezultatov Spreading out diameter, cm Out-flow time Target values  14.00 15.00  time  20.00 Theoretical values 17.15 15.75 Experimental values 17.20 15.86 The small difference between the theoretical and experimental values, which is about 0.05 cm for the spreading out diameter and 0.11 s for the out-flow time, means that the proposed model gives satisfactory results and can then be validated. The remaining part of the work consisted of injecting aggregates (sand and gravel for a fixed G/S ratio) at the same time and balancing with water to reach the desired fluidity. At the end of the experimental procedure, a self-consolidating concrete could be achieved, checking its characteristics in its fresh state according to the recommendation of the French Association of Civil Engineering. 5 CONCLUSION In this study, an extensive experimental program has been carried out, adopting a new paste-mix concept for a self-compacting concrete (SCC), which considers that A. MEBROUKI ET AL.: EXPERIMENTAL PLANS METHOD TO FORMULATE ... Materiali in tehnologije / Materials and technology 44 (2010) 1, 13–20 19 Figure 8: Graphs of the desirability functions Slika 8: Graf `elelnih funkcij fresh concrete self-compacting properties come from those of the cement paste. The experimental mixture method was applied to the cement pastes to get the maximum amount of information about the components, their influences taken separately and their possible interactions. This method has allowed us to define an experimental field in which all the mixtures can show measurable characteristics, reduce considerably the number of experiments, and plan and facilitate the study. Indeed, it has allowed us to build an experiment matrix and to propose a formulation according to fixed target values. The optimal paste mix was produced simply with the measurements of the spreading out diameters and the out-flow times. This was achieved by combining the criteria for a low shearing threshold, a high viscosity and the optimum flow-viscosity ratio of the paste. The NemrodW software used has produced ternary diagrams, which showed interactions between compo- nents taken two by two. The proposed model has produced satisfactory results when compared to the experimental measurements. The proposed numerical model has yielded a total desirability of 100 %, which is proof of a satisfactory formulation of a self-compacting cement paste within the experimental field. 6 REFERENCES 1 Youjun Xie, Baoju Liu, Jian Yin, Shiqiong Zhou: Optimum mix parameters of high strength self-compacting concrete with ultra- pulverized fly ash. Cement and Concrete Research, 32 (2002), 477– 480. Doi: 10.1016/S0008-8846(01)00708–6 2 Naadia, T. Mouret, M. and Kharchi, F. Effect of the aggregate size on the rheological behaviour of the concretes. Application to the SCC. 1st International conference on the technology and concretes durability, CITDUB1, USTHB, Algiers, Algeria (2004) 3 Kantro, Dl. Influence of water-reducing admixtures on properties of cement paste – a miniature slump test. Cement, Concrete and Aggre- gates, 2 (1980) 2, 95–102. Doi: 10.1520/CCA101903 4 Cyr, M. Contribution to the characterization of mineral additives and to the comprehension of their effect on the rheological behavior of the cementitious materials. PHD thesis. INSA- L.M.D.C, Paul Sabatier University, Toulouse III, France (1999) 5 Roussel, N. Stefani, C., Leroy, R. From mini-cone test to Abrams cone test: measurement of cement-based materials yield stress using slump tests. Cement and Concrete Research, 35 (2005), 817–822. Doi: 10.1016/j.cemconres.2004.07.32 6 Ferraris, C F., Obla, K H., Hill, R. The influence of mineral admixtures on the rheology of cement paste and concrete. Cement and Concrete Research, 31 (2001), 245–255. Doi: 10. 1016/S0008- 8846(00)454– 7 Bui Van, K., Akkaya, Y. and Shah, SP. Rheological model for self-consolidating concrete. ACI Materials Journal, 99 (2002), 6 8 Phan, T H., Chaouche, M. Rheology and stability of self-compacting concrete cement pastes. Applied Rheology. 15 (2005) 5, 336–343. http://www.ar.ethz.ch/AR_browse_reports.html 9 Ferrara, L., Yon-Dong, P., Shah SP. A method for mix-design of fibre -reinforced self-compacting concrete. Cement and Concrete Research, 37 (2007) 6, 957–971. Doi: 10.1016/j.cemconres.2007. 03.014 10 Sonebi, M., McKendry, D. Effect of mix proportions on rheological and hardened properties of composite cement pastes. The Open Construction and Building Technology Journal, 2 (2008), 15–23. Doi: 10.2174/1874836800802010015 11 El-Barrak, M. Contribution to the study of the flow property of the self-compacting concretes at the fresh state. PHD thesis. INSA-LMDC, Paul Sabatier University, Toulouse III, France (2005) 12 Mathieu D., Phan-Tan-Luu R., Sergent M.: Design methodology. LPRAI Marseille (2000) 13 Boel, V., Audenaert, K. and De Schutter, G. Pore size distribution of hardened cement paste in self-compacting concrete. ACI special publication, volume 234 (2006), 167–178. http://hdl.handle.net/1854/ LU-372328. ISBN 0-87031-207-3 Used symbols: C: Cement. F: Limestone fillers. Sp: Superplasticizer. W: Water. D1 = Y1: First response: Spreading-out diameter (measured by mini-slump). D2 = Y2: Second response: Flow time (measured by Marsh cone). (C), (Sp), (F) and (W): volumetric proportions of cement, superplasticizer, limestone filler and water. A. MEBROUKI ET AL.: EXPERIMENTAL PLANS METHOD TO FORMULATE ... 20 Materiali in tehnologije / Materials and technology 44 (2010) 1, 13–20 A. KOCIJAN, M. CONRADI: THE CORROSION BEHAVIOUR OF AUSTENITIC AND DUPLEX STAINLESS STEELS ... THE CORROSION BEHAVIOUR OF AUSTENITIC AND DUPLEX STAINLESS STEELS IN ARTIFICIAL BODY FLUIDS KOROZIJSKO VEDENJE AVSTENITNEGA IN DUPLEKSNEGA NERJAVNEGA JEKLA V SIMULIRANIH TELESNIH TEKO^INAH Aleksandra Kocijan, Marjetka Conradi Institute of metals and technology, Lepi pot 11, 1000 Ljubljana, Slovenia aleksandra.kocijan@imt.si Prejem rokopisa – received: 2009-09-29; sprejem za objavo – accepted for publication: 2009-11-11 The evolution of the passive film formed on duplex stainless steel 2205 and AISI 316L stainless steel in artificial saliva and a simulated physiological solution was studied using cyclic voltammetry and potentiodynamic measurements. The extent of the passive range slightly decreased with the increasing chloride concentration from artificial saliva to the simulated physiological solution. The formation of pits during the potentiostatic conditions was studied using atomic force microscopy and the results showed an increasing growth of pits for the AISI 316L compared to duplex stainless steel 2205 and, furthermore, a decreased corrosion resistance of both materials in the simulated physiological solution compared to the artificial saliva. Keywords: duplex stainless steel, AISI 316L, potentiostatic, cyclic voltammetry, artificial saliva, simulated physiological solution S cikli~no voltametrijo in z meritvami potenciodinamske polarizacije smo raziskovali tvorbo pasivne plasti na povr{ini dupleksnega nerjavnega jekla 2205 in avstenitnega nerjavnega jekla AISI 316L v raztopini umetne sline in simulirani fiziolo{ki raztopini. Z nara{~ajo~o koncentracijo kloridnih ionov v simulirani fiziolo{ki raztopini v primerjavi z umetno slino se je pasivno obmo~je pri obeh materialih rahlo zo`ilo. Z uporabo mikroskopije na atomsko silo smo raziskovali pojav jami~aste korozije pod potenciostatskimi pogoji. Rezultati so potrdili pove~ano rast jamic pri AISI 316L v primerjavi z dupleksnim nerjavnim jeklom 2205 in bolj izrazito tvorbo le-teh z nara{~ajo~o koncentracijo kloridnih ionov. Klju~ne besede: dupleksno nerjavno jeklo, AISI 316L, potenciostatske meritve, cikli~na voltametrija, umetna slina, fiziolo{ka raztopina 1 INTRODUCTION Austenitic stainless steel AISI 316L is the most commonly used orthopaedic and orthodontic bracket material. Its mechanical properties, such as ductility and wear resistance, make it attractive for particular appli- cations. The corrosion resistance of stainless steel is relatively good. However, it is challenged by the hostile environment in the human body, as it is susceptible to localised corrosion in any environment containing chloride. Furthermore, it is well known that up to 22 % of the population may exhibit allergic and hypersensi- tivity reactions to nickel 1,2. The 2205 duplex stainless steel is being investigated as a material for orthodontic bracket fabrication. The microstructure of this duplex stainless steel is a mixture of austenitic and delta-ferritic phases. The delta-ferrite is hard and relatively less ductile; the austenite is softer and more ductile. The combination of both phases results in a steel that is harder than the single-phase austenitic and more ductile than the single-phase ferritic stainless steel. The combination of both phases has a beneficial influ- ence on the corrosion characteristics in various aqueous environments. The high Cr content together with high Mo and N contents gives rise to a high pitting-corrosion resistance in chloride solutions. The chromium adds to the overall resistance through a passivation process by forming a complex spinel-type passive film (Fe, Ni)O(Fe, Cr)2O3. Molybdenum increases the stability of the passive film and, therefore, the ability of the stainless steel to resist the localised corrosion, including pitting and crevice corrosion, particularly in environments containing chloride ions 3–8. The evolution of the passive film formed on duplex stainless steel 2205 and AISI 316L stainless steel in artificial saliva and a simulated physiological solution was studied using cyclic voltammetry and potentio- dynamic measurements. The formation of pits during the potentiostatic conditions was studied using atomic force microscopy (AFM, Nanoscope V, Veeco Instruments) in the contact mode. 2 EXPERIMENTAL Duplex 2205 stainless steel and AISI 316 stainless steel were investigated. Their compositions were con- firmed with analytical chemical methods, as shown in Table 1. The experiments were carried out in artificial saliva (0.4 g/L NaCl, 0.4 g/L KCl, 0.795 g/L CaCl2 × 2H2O, 0.780 g/L NaH2PO4 × 9H2O, 0.005 g/L Na2S × 9H2O, Materiali in tehnologije / Materials and technology 44 (2010) 1, 21–24 21 UDK 669.14.018.8:620.193 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 44(1)21(2010) 1 g/L Urea) and in a simulated physiological solution – Hank’s solution (8 g/L NaCl, 0.40 g/L KCl, 0.35 g/L NaHCO3, 0.25 g/L NaH2PO4 × 2H2O, 0.06 g/L Na2HPO4 × 2H2O, 0.19 g/L CaCl2 × 2H2O, 0.41 g/L MgCl2 × 6H2O, 0.06 g/L MgSO4 × 7H2O, 1 g/L glucose). All the chemicals were from Merck, Darmstadt, Germany. The test specimens were cut into discs of 15 mm diameter. The specimens were ground with SiC emery paper down to 1000 grit prior to the electrochemical studies, and then rinsed with distilled water. The specimens were then embedded in a Teflon PAR holder and employed as a working electrode. The reference electrode was a saturated calomel electrode (SCE, 0.242 V vs. SHE) and the counter electrode was a high-purity graphite rod. All the potentials described in the text are stated with respect to a SCE. The cyclic voltammetry and potentiodynamic measurements were recorded using an EG&G PAR PC-controlled potentiostat/galvanostat Model 263 with M252 and Softcorr computer programs. In the case of potentiodynamic measurements the specimens were immersed in the solution 1 h prior to the measurement in order to stabilize the surface at the open-circuit potential. The potentiodynamic curves were recorded starting at 250 mV more negative than the open-circuit potential. The potential was then increased, using a scan rate of 1 mV s–1, until the transpassive region was reached. In cyclic voltammetry a scan rate of 50 mV s–1 was used, unless stated otherwise. The passive layers on the surface of the 2205 DSS and AISI 316L were formed under potentiostatic condi- tions for 30 minutes at potentials of –0.6 V and 1.4 V vs. SCE, respectively. These potentials were selected in the region of transpassive oxidation of the polarization curve (see Figure 1). After the electrochemical preparation, the specimens were rinsed with distilled water and ethanol, dried and transferred to the analyzer chamber. To characterize the surfaces of the 2205 DSS and AISI 316L before and after exposing the samples to different corrosion conditions, such as artificial saliva and Hank’s solution, AFM in the contact mode was used. The surface was probed with a commercial, sharpened, silicon nitride (Si3Ni4) cantilever, with a spring constant of 0.12 N/m. The topology of the surfaces was then analyzed with the Nanoscope V software. 3 RESULTS Figure 1a shows the potentiodynamic curves for 2205 DSS and AISI 316L in artificial saliva and Hank’s solution. After 1 h of stabilization at the open-circuit potential, the corrosion potential (Ecorr) for the 2205 DSS in artificial saliva was approximately –0.44 V. Following the Tafel region, the alloy exhibited passive behaviour. However, the passive range is limited by the breakdown potential (Eb), which corresponds to the oxidation of water and the transpassive oxidation of the metal species. The breakdown potential for the 2205 DSS in artificial saliva was approximately 1.23 V. In the case of AISI 316L, the Ecorr was approximately of –0.47 V and Eb was 0.37 V, respectively. The extent of the passive range slightly decreased with the increasing chloride concen- tration from artificial saliva to the simulated physiolo- gical solution. The corrosion potentials in Hank’s solution were –0.32 V for 2205 DSS and AISI 316L (Figure 1b). The passive region was narrower compared to the results in artificial saliva due to the increased concentration of chloride ions. The breakdown potential shifted to more negative potentials for the 2205 DSS and AISI 316L in Hank’s solution, i.e., 1.05 V and 0.35 V, respectively. The cyclic voltammograms of the 2205 DSS and AISI 316L recorded in artificial saliva and Hank’s solution make it possible to compare the current peaks A. KOCIJAN, M. CONRADI: THE CORROSION BEHAVIOUR OF AUSTENITIC AND DUPLEX STAINLESS STEELS ... 22 Materiali in tehnologije / Materials and technology 44 (2010) 1, 21–24 Figure 1: Polarisation curves recorded for the 2205 duplex stainless steel and the AISI 316L in (a) artificial saliva and (b) the simulated physiological solution Slika 1: Polarizacijske krivulje dupleksnega nerjavnega jekla 2205 in AISI 316L v (a) umetni slini in (b) simulirani fiziolo{ki raztopini Table 1: The composition of the 2205 duplex stainless steel and the AISI 316L (w/%) Tabela 1: Sestava dupleksnega nerjavnega jekla 2205 in AISI 316L (w/%) Cr Ni Mn Si P S C Mo 2205 22.7 5.7 1.37 0.38 0.032 0.001 0.03 2.57 316L 17.0 10.0 1.40 0.38 0.041 <0.005 0.021 2.10 and the corresponding electrochemical processes taking place on the materials investigated (Figure 2). The cyclic voltammograms were recorded for the AISI 316L and 2205 DSS at a scan rate of 50 mV/s, in the potential range from –1 V to 0.5 V and 1.5 V, respectively. As can be seen, the main characteristics of the cyclic voltammo- grams are similar in both solutions. In the presence of artificial saliva four peaks are observed in the cyclic voltammogram for the 2205 DSS (Figure 2a). The first anodic peak A1 at a potential –0.2 V can be ascribed to the electro-formation of Fe(II) hydroxide upon the Cr(III)-containing passivating layer, existing on the electrode at such negative potentials 5. It is followed by the region with a constant current density, up to 0.9 V, where another peak A2 is observed in the transpassive region associated with the oxidation of Cr(III) to Cr(VI) 9. The Ni(II) species formed during the passivation process might have been oxidised to Ni(IV) oxide (NiO2) in this potential range, too. In the reduction cycle in the potential range of peak C2 at 0.2 V Cr(VI) is reduced to Cr(III) and the iron oxide-hydroxide layer is largely reduced in the potential range of peak C1 at a potential of –0.5 V 9. In the case of AISI 316L in artificial saliva, only two peaks were observed in the cyclic voltammo- grams, the first one in the anodic cycle at –0.2 V, which can be ascribed to the electro-formation of Fe(II) hydroxide, and the second one in the cathodic cycle at –0.5 V, which corresponds to the reduction of the iron A. KOCIJAN, M. CONRADI: THE CORROSION BEHAVIOUR OF AUSTENITIC AND DUPLEX STAINLESS STEELS ... Materiali in tehnologije / Materials and technology 44 (2010) 1, 21–24 23 Figure 4: AFM images of the 2205 duplex stainless steel (a) and the AISI 316L (b) after oxidation in the simulated physiological solution at 1.4 V and 0.65 V, respectively Slika 4: AFM posnetek dupleksnega nerjavnega jekla 2205 (a) in AISI 316L (b) po oksidaciji v simulirani fiziolo{ki raztopini pri 1,4 V in 0,65 V Figure 3: AFM images of the 2205 duplex stainless steel (a) and the AISI 316L (b) after oxidation in artificial saliva at 1.4 V and 0.65 V, respectively Slika 3: AFM posnetek dupleksnega nerjavnega jekla 2205 (a) in AISI 316L (b) po oksidaciji v umetni slini pri 1,4 V in 0,65 V Figure 2: Cyclic voltammograms recorded for the 2205 duplex stainless steel and the AISI 316L in (a) artificial saliva and (b) the simulated physiological solution Slika 2: Cikli~ni voltamogrami dupleksnega nerjavnega jekla 2205 in AISI 316L v (a) umetni slini in (b) simulirani fiziolo{ki raztopini oxide-hydroxide layer. In the presence of the simulated physiological solution the results were similar to the results obtained for artificial saliva (Figure 2b). How- ever, due to the increased chloride concentration the corrosion current density enlarged compared to the artificial saliva. Similar results were obtained in different investigated media 10–13. Figures 3 and 4 show AFM 3D height images of the 2205 DSS and AISI 316L surfaces when treated with artificial saliva and the simulated physiological solution, respectively. As a consequence of the corrosion, the formation of pits was observed on both surfaces. The degree of corrosion was reflected in the density of the pits that appeared on the surface, their shape and depth. Following this idea, the 2205 DSS sample indicated more resistance to corrosive solutions compared to the AISI 316L sample. The density of the pits on the 2205 DSS surface was small, the pits were shallow (few hundreds of nm) and not very well shaped, i.e., non- spherical. In contrast, on the surface of the AISI 316L sample an increased density of the pits was observed with depths up to a micrometer and more. In addition, the simulated physiological solution appeared more aggressive than the artificial saliva for both samples due to the increased chloride solution. This effect was more pronounced for the AISI 316 L sample where the formation of corrosion pits was considerably enhanced over a larger area of the sample (Figure 5). 4 CONCLUSIONS An electrochemical study of the passive film gene- rated in artificial saliva and a simulated physiological solution on the DSS 2205 and AISI 316L was performed, and the results were analysed. Cyclic voltammograms obtained in the chloride solution showed characteristic peaks. The anodic peaks at –0.1 V and 0.9 V were identified with the formation of an iron oxide-hydroxide layer and the transpassive oxidation of Cr(III) to Cr(VI) and Ni(II) to Ni(IV), respectively. The reduction peaks at 0.2 V and –0.5 V are attributed to valence transitions, occurring in the solid state, associated with the chromium and iron in the oxide, respectively. None of the current peaks detected in the voltammetric curves can be attributed to the Mo species alone, which indicates that molybdenum mainly enhances the effect of other passivating species, i.e., Cr, more than acting directly in the passivating process in the chloride media. The results of the electrochemical study showed an improved corrosion resistance of the 2205 DSS com- pared to the AISI 316L, which was also confirmed by the AFM imaging. Therefore, the present study indicates a possible exploitation of 2205 DSS in orthopaedic and orthodontic applications. 5 REFERENCES 1 J. A. Platt, A. Guzman, A. Zuccari, D. W. Thornburg, B. F. Rhodes, Y. Ossida, D. W. Thornburg, B. F. Rhodes, Y. Ossida, B. K. Moore, American Journal of Ortodontics and Dentofacial Orthopedics, 112 (1997), 69–79 2 R. D. Willenbruch, C. R. Clayton, M. Oversluizen, D. Kim, Y. Lu, Corros. Sci., 31 (1990), 179–190 3 R. M. Souto, I. C. Mirza Rosca, S. Gonzales, Corrosion, 57 (2001), 300–306 4 F. Bernard, V. S. Rao, H. S. Kwon, J. Electrochem. Soc. 152 (2005) 10, 415–420 5 F. J. Torres, W. Panyayong, W. Rogers, D. Velasquez-Plata, Y. Oshida, B. K. Moore, Bio-Medical Mat. Eng., 8 (1998), 25–36 6 A. Kocijan, ^. Donik, M. Jenko, Corros. Sci., 49 (2007), 2083–2098 7 K. T. Oh, Y. S. Kim, Y. S. Park, K. N. Kim, J. Biomed. Mat. Res., 69B (2004), 183–194 8 G. T. Burstein, C. Liu, Corr. Sci., 49 (2007), 4296–4306 9 N. Ramasubramanian, N. Preocanin, R. D. Davidson, J. Electrochem. Soc., 132 (1985), 793–798 10 ^. Donik, A. Kocijan, M. Jenko, A. Drenik, B. Pihlar, Corros. Sci. 51 (2009) 827–832 11 A. Kocijan, ^. Donik, M. Jenko, Mater. tehnol., 43 (2009), 39–42 12 ^. Donik, A. Kocijan, M. Jenko, I. Paulin, Mater. tehnol., 43 (2009), 137–142 13 A. Kocijan, ^. Donik, M. Jenko, Mater. tehnol., 43 (2009), 195–19 A. KOCIJAN, M. CONRADI: THE CORROSION BEHAVIOUR OF AUSTENITIC AND DUPLEX STAINLESS STEELS ... 24 Materiali in tehnologije / Materials and technology 44 (2010) 1, 21–24 Figure 5: The surface of the AISI 316L treated with the simulated physiological solution. The formation of corrosion pits is significantly enhanced due to increased chloride concentration Slika 5: Povr{ina nerjavnega jekla AISI 316L v simulirani fiziolo{ki raztopini. Tvorba korozijskih jamic je pove~ana zaradi vpliva vi{je koncentracije kloridnih ionov P. PE^LIN ET AL.: CHRONIC STIMULATION OF AN AUTONOMOUS NERVE WITH PLATINUM ELECTRODES CHRONIC STIMULATION OF AN AUTONOMOUS NERVE WITH PLATINUM ELECTRODES KRONI^NA STIMULACIJA AVTONOMNEGA @IVCA S PLATINASTIMI ELEKTRODAMI Polona Pe~lin1, Miro Zdovc2, Janez Rozman1 1ITIS, d. o. o., Ljubljana, Centre for Implantable Technology and Sensors, Lepi pot 11, 1000 Ljubljana, Slovenia 2University of Nova Gorica, Vipavska 13, Ro`na Dolina, 5000 Nova Gorica, Slovenia polonapeclin@gmail.com Prejem rokopisa – received: 2009-02-10; sprejem za objavo – accepted for publication: 2009-03-10 A spiral nerve cuff (cuff), including thirty-nine platinum electrodes, arranged in thirteen spiral sets of three electrodes (triplet), was installed on the mid-cervical left vagus nerve (vagus) of a Beagle dog. The relevant position of the particular compartment and the optimal stimulation intensities were identified by delivering stimulating pulses to each triplet. The largest mean change in the components of both the ECG and hemodynamic changes was elicited with a stimulating intensity ic = 2.5 mA applied to triplet9. Every two weeks the aforementioned compartment was stimulated for 20 min with rectangular, charge-balanced, biphasic, current pulses with the parameters: width 100 µs, intensity ic = 2.0 mA, frequency 20 Hz, and a time delay between the biphasic phases of 100 µs for the time period of 18 months. It was calculated that to induce neural damage a total injected charge of approximately 172.8 mA s per phase at a charge density of 0.1 µA s/mm2 was injected. At the end of the trial, the neural and encapsulation tissue subjacent to the central group of triplet 9 and the neural and encapsulation tissue subjacent to the silicone insulation were dissected free and analyzed. The extent of the pathology of the neural tissue caused by the presence of the cuff and/or by the electrochemical reactions and the surface of the stimulating cathode in triplet 9 were investigated using light microscopy. The results showed that the usage of a cuff was associated with a build up of connective tissue around the cuff as well as within the cuff. The results also showed that some tissue injury occurred beneath the stimulating cathode of triplet 9. Finally, the results showed an absence of any anomaly at the flat geometric surface of 2 mm2 in stimulating the cathode of the triplet 9, which could be attributable to irreversible electrochemical reactions. Keywords: electrical stimulation, platinum electrodes, left vagus nerve, electrochemistry, electrical charge Spiralna objemka, ki je vsebovala devetintrideset platinastih elektrod, razporejenih v trinajst skupin po tri elektrode (troj~ek), je bila kirur{ko vstavljena na osrednji del levega vratnega `ivca vagusa psa pasme Beagle. Polo`aj dolo~enega podro~ja `ivca in optimalna jakost stimulacije sta bila dolo~ena z dovajanjem stimulacijskih impulzov na vsakega od troj~kov. Najve~jo povpre~no spremembo komponent elektrokardiograma in hemodinamskih sprememb je izzvala jakost stimulacije ic = 2.5 mA, dovedena na troj~ek 9. Omenjeni predel je bil v obdobju 18 mesecev vsaka dva tedna stimuliran 20 min s pravokotnimi, nabojsko uravnote`enimi izmeni~nimi tokovnimi impulzi z naslednjimi parametri: {irina 100 µs, jakost ic = 2.0 mA, frekvenca 20 Hz in fazni zamik 100 µs. Izra~unano je bilo, da je bil za povzro~itev po{kodbe `ivca skupni vnesen naboj pribli`no 172.8 mA s. @iv~ni in ovojni~ni tkivi ob osrednji skupini elektrod v troj~ku in tkivi ob silikonski izolaciji so bili na koncu poskusa izrezani in analizirani. Razse`nosti patologije `iv~nega tkiva zaradi elektrokemijskih reakcij in prisotnosti objemke ter povr{ina stimulacijske katode troj~ka 9 sta bili preiskani s svetlobno mikroskopijo. Rezultati so pokazali, da je uporaba objemke povezana z gradnjo veznega tkiva okoli objemke in v njej. Rezultati so tudi pokazali, da je pod stimulacijsko katodo troj~ka 9 nastala po{kodba `ivca. Kon~no so rezultati pokazali, da na ravni geometrijski povr{ini stimulacijske katode troj~ka 9 ni bilo po{kodb, ki bi jih lahko pripisali nepovratnim elektrokemijskim reakcijam. Klju~ne besede: elektri~na stimulacija, platinaste elektrode, levi `ivec vagus, elektrokemija, elektri~ni naboj 1 INTRODUCTION The electrical activation of the nervous system provides a means to exert external control over body systems that are normally under the control of the nervous system.1,2,3 Peripheral nerve stimulation often requires the development of electrode systems that stimulate the selectively a certain group of fibers in a nerve.4 However, the long-term use of such electrical stimulation requires that it is applied selectively and without causing any tissue injury.5,6,7 Tissue injury and the corrosion of the stimulating electrode are both associated with high-charge-density stimulation.8,9 For this reason, the long-term stimulation of the nervous tissue requires the absence of irreversible electroche- mical reactions, such as the electrolysis of water, the evolution of chlorine gas or the formation of metal oxides.10,11 For a given electrode there is a limit to the quantity of charge that can be injected in either the anodic or cathodic direction with reversible surface processes.12 This limit depends upon the parameters of the stimulating waveform,13 the size of the electrode, and its geometry. The charge required for stimulation with miniature electrodes, however, often exceeds the limits for reversible charge injection. Therefore, an important component in the design of stimulating-electrode systems is the stimulating electrode itself; its properties determine the nature and kinetics of the charge transfer between the electrons moving in the external circuit and the ions moving through the electrolytes within the tissue. Nerve cuffs are among the most successful of stimulating electrode systems.3,14,15 Acute studies from Materiali in tehnologije / Materials and technology 44 (2010) 1, 25–29 25 UDK 611.8:621.357 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 44(1)25(2010) our laboratory have demonstrated that cuffs could be used to selectively activate the different superficial regions of a peripheral nerve.15 However, the long-term effectiveness and potentially harmful effects of these types of electrodes on neural tissue in various applica- tions is still not completely defined. The present study addressed the mechanisms involved in the production of neural damage induced by both the chronical physical presence of the implanted cuff and by the prolonged selective stimulation of the mid-cervical left vagus of a dog. The study reported here also seeks to investigate the changes on the surfaces of particular stimulating electrodes that could result the long-term selective stimulation of the vagus of a dog, using the method of light microscopy. 2 METHODS The cuff design was devised to induce as low as possible radial pressure when installed on the vagus so that any mechanically induced vagus damage might be minimized. Accordingly, a cuff was made by bonding two 0.1-mm-thick silicone sheets together. One sheet, stretched and fixed in that position, was covered by a layer of adhesive and by a second unstretched sheet. Afterwards, the composite was compressed to a thick- ness of 0.3 mm. When released, after the curing process was completed, the composite curled into a spiral tube as the stretched sheet contracted to its natural length.4,15 Thirty-nine rectangular electrodes (1.5 × 0.6 mm) were made of cold-rolled and annealed 0.05-mm-thick plati- num ribbon (99.99 % purity), yielding a geometric area of 2 mm2 in each electrode. However, according to Brummer et al. (1977),5,8 one geometric square milimeter of a smooth platinum electrode corresponds to about 1.4 ("real") mm2. Therefore, the real surface of these elec- trodes may have been an area of at least 2.8 mm2. Furthermore, the faradaic processes available on Pt have been classified as reversible or irreversible.12 For some platinum electrodes, reversible charge injection limits for cathodic pulses range from 0.25 × 10–6 A s/mm2 to 35 × 10–6 A s/mm2 for Ir oxide electrodes. The relatively large contact area of the single platinum electrode shown in Fig. 1d, when implanted on the vagus, resulted in low impedances /Z/ (about 1.73 k at 1 kHz, and about 1.36 k at 10 kHz). The impedance of the single electrode was measured "in vivo" 20 d after implantation. The electrodes were mounted on a third silicone sheet with a thickness of 0.1 mm where they were arranged in three parallel groups, each containing 13 electrodes at a distance of 0.5 mm. The distance between the groups was 6 mm. As a result, thirteen triplets in a longitudinal direction were formed (Figure 1b). To avoid as much as possible an occurrence of electrochemical potentials between the parts made of different metallic materials and consequently unwanted irreversible electrochemical reactions that could occur in physiological media, the electrodes were connected to the lead wires using the technology of a simple mecha- nical connection (Pressing). The lead wires were made of fine-stranded and Teflon-insulated stainless-steel wires (Cooner Wire, Chatsworth, USA, Type AS631). The sheet with the arranged electrodes was then bonded on the inner side of the mechanically opened cuff. Finally, the cuff with the inner diameter of 2.5 mm was trimmed to a length of 20 mm, as shown in Figure 1a. To stimulate mostly B-fibers, the model proposed a stimulating pulse train composed of charge-balanced biphasic stimuli with a frequency of 20 Hz containing a combination of current cathodic and anodic phases.13 The cathode and anode current steps of the stimuli used were 2 mA, while the cathode and anode halves were 100 ms in width. The time delay between the biphasic phases was settled at 100 ms. Finally, the maximum charge required for the selective stimulation of B-fibers within the particular region of the vagus was proposed. Q/mm2 (geometric)/phase = = 2·10–3A × 100·10–6s/2 mm2 = 0.1·10–6 A s/mm2 (1) The relationship between the charge density per phase, or Q/mm2 (geometric)/phase (expressed in units of microcoulombs per square milimeter per phase of the charge balanced waveform) expressed in the equation (1), and the total injected charge to tissue injury was investigated by light microscopy after 18 months of selective stimulation. The authors followed the guidelines contained in the Declaration of Helsinki and the U. S. National Institutes of Health Guide for the Care and Use of Laboratory Animals. Under fully sterile conditions a gas sterilized (ethylene oxide) cuff was implanted on the mid-cervical left vagus (Figure 2a), according to the protocol approved by the ethics committee at the Veterinary Administration of the Republic of Slovenia, Ministry of Agriculture, Forestry and Food (VARS), (Telephone: +386 1 300 1300, URL: http://www.vurs.gov.si. P. PE^LIN ET AL.: CHRONIC STIMULATION OF AN AUTONOMOUS NERVE WITH PLATINUM ELECTRODES 26 Materiali in tehnologije / Materials and technology 44 (2010) 1, 25–29 Figure 1: (a) 39-electrode cuff, (b) Simplified perspective illustration of the cuff, (c) Conceptual illustration of a current stimulating pulse pair, (d) Magnified view of one electrode within the triplet. Slika 1: (a) 39-elektrodna objemka, (b) enostavna prostorska risba objemke, (c) shematska risba tokovnega stimulacijskega impulza, (d) pove~ana slika ene elektrode v troj~ku Four weeks after the implantation the first stimu- lating session was performed. To determine in which position a triplet would be in contact with a particular compartment, innervating mainly the atrioventricular and to some extent the sinoatrial node, a stimulating pulse train of the aforementioned stimuli with an intensity of ic = 2.5 mA was delivered for about 10 s to all 13 triplets. The triplet that elicited the largest change in the heart rate and the blood pressure, i.e., triplet 9, was considered as being relevant for the selective stimulation. More precisely, when the stimuli were delivered to triplet 9, the heart rate and the blood pressure began to decrease. Afterwards, in each session the compartment was continuously stimulated for 20 min, using the afore- mentioned stimuli, while the intensity ic was settled at 2.5 mA, as shown in Figure 1c. The session was repeated every two weeks for a time period of 18 months. It was calculated that to induce neural damage a total injected charge of approximately 172.8 mA s per phase was injected. After the stimulation sessions were completed, the animal was euthanized by an overdose of an inhalant anesthetics isoflurane introduced from a vaporizer. The authors followed the American Veterinary Medical Association (AVMA) Guidelines on Euthanasia from June 2007. Finally, for the histo-pathological analysis the vagus together with the cuff represented in Figure 2b, was dissected free and cut at both sides of the cuff, thus providing a 20-mm-long composite segment.6,9,17 After- wards, two cylindrical specimens were cut out from a composite segment: 1) a 6-mm-long specimen,was cut from the composite subjacent to the silicone insulation between one of the outer and the central group of triplet electrodes (stimulating cathode) (Figure 2b+) and 2) a 1.5-mm-long specimen was cut from the composite subjacent to the central group of triplet electrodes (sti- mulating cathodes) (Figure 2b*). For the histo-pathological analysis, the specimens were fixed in 4 % formaldehyde solution phosphate buffered at pH 7.4, processed according to standard histological procedures and embedded in paraffin. Transverse, 4-µm tissue sections obtained by an ultra- microtome were stained with hematoxylin and eosin or by Giemsa stain. For the metallurgical observations, however, a central electrode (stimulating cathode) of triplet 9 in the second 1.5-mm-long specimen was gently, without any mechanical deformation, removed from the silicone, cleaned and rinsed with acetone. In an evaluation of the galvanometric behaviour of this electrode using the electrochemical technique of cyclic voltammetry, an operational potential window between hydrogen and oxygen evolution in a protein-containing solution (Eliott’s buffered solution, pH 7.3) was delineated. In other words, keeping the electrode potential within this window during the stimulation guaranteed that no water-electrolysis reaction occurred. This electrode, a stimulating cathode, was then introduced into the optical microscope. 3 RESULTS Compared with the region of the vagus remote from the cuff, the vagus subjacent to the cuff showed a granuloma formation and a thickening of the connective P. PE^LIN ET AL.: CHRONIC STIMULATION OF AN AUTONOMOUS NERVE WITH PLATINUM ELECTRODES Materiali in tehnologije / Materials and technology 44 (2010) 1, 25–29 27 Figure 3: (a) Low-power view of a transverse section through the specimen (+), cut beneath the silicone insulation, (b) Magnified view of detail in (a), representing a granuloma-like tissue and a branch of vagus tissue, (c) Low-power view of a transverse section through the specimen (*), cut beneath the central group of triplet electrodes (stimulating cathodes), (d) Magnified view of detail in (c), repre- senting a granuloma-like tissue and a branch of vagus tissue, (e) Silicone insulation. Slika 3: (a) Malo pove~ana slika prereza skozi vzorec (+), odrezan ob silikonski izolaciji, (b) pove~an detajl v sliki (a), ki je slika tkiva granuloma in veje tkiva vagusa, (c) malo pove~ana slika prereza skozi vzorec (*), odrezan ob osrednji skupini elektrod v troj~ku (stimulacij- ske katode), (d) pove~an detajl v sliki (c), ki je slika tkiva granuloma in veje tkiva vagusa, (e) silikonska izolacija Figure 2: (a) X-ray of the cuff implanted on the vagus of a dog, (b) Insulated vagus within the cuff and sites where the segments (*) and (+) for histo-pathological analysis were cut out Slika 2: (a) Rentgenska slika objemke, implantirane na pasji vagus, (b) izprepariran vagus znotraj objemke in mesta odvzema segmentov (*) in (+) za histopatolo{ko analizo tissue (the radial pressure measured in the cuff was about 245 Pa). Furthermore, the histological changes seen in the region of the vagus tissue that was stimulated with the adjacent triplet9 were obviously different from the region of the vagus tissue that was surrounded by the cuff, but was not in contact with the triplets. Figure 3 (a) and (b) contains a transverse section at magnifications of 2 and 10 through the middle of the specimen (+), cut from the vagus subjacent to the silicone insulation. Frame (a) is a low-power view (2X) of the section through the vagus specimen and silicone insulation (e). A precise reconstruction of an "in vivo" situation showed that the vagus was well accommodated within the cuff. There was a build up of connective- tissue encapsulation around and within the cuff. Frame (b) shows a magnification of detail 1 at 10X, including granuloma-like tissue on the left, and a branch of neural tissue on the right. This tissue included a mix of fibroblasts, collagen, and mostly foreign body cells. Most of the fibres appeared histologically normal. However, proliferation of subperineural connective tissue and a decrease in the density of axons could be observed. Figure 3 (b) and (c), shows an analogous transverse section at magnifications of 2 and 10 through the middle of a specimen subjacent to the central group of triplet electrodes (stimulating cathodes). Frame (c) is a low- power view (2X) of the section, showing that the vagus was easily accommodated within the cuff. Frame (d) is a magnification of detail 1 at 10X, showing a vagus tissue and a build up of connective tissue. The vagus tissue is seen on the left, and the connective tissue encapsulation is seen on the right. Through most of the vagus tissue, the density of fibres in this stimulated region appeared normal, except at the right side, where a decrease in the density of axons could be observed. The cyclic voltammogram showed that hydrogen evolution began at about –0.8 V, while the evolution of oxygen began at about 1.0 V. As expected, a train of the biphasic, rectangular, current pulses applied during the stimulation resulted in a series of potential steps between –0.8 V and 1.0 V, with respect to the Saturated Calomel Electrode. Finally, Figure 4a shows a low-power microscopic image of a triplet 9 central electrode (stimulating cathode) before implantation, while Figure 4b shows a low-power microscopic image of the aforementioned electrode, pulsed with described stimulating pulses, after 18 months of stimulation. 4 DISCUSSION The majority of fibres comprising the vagus of a dog are small-diameter non-myelinated C-fibers; the rest are intermediate-diameter myelinated B-fibers and large- diameter myelinated A-fibers are organized longitu- dinally along the length of the nerve.16 However, the acute responses of a cardio-vascular system on vagus stimulation generally requires mostly B-fibre stimu- lation, while stimulation of the A- and C-fibres is to be avoided. Since in reality, B-fibres are located close to the stimulating electrode and also at a certain distance from the electrode, the electrode should be able to inject enough charge to depolarize these fibres.1,2 For multi- electrode stimulating systems containing miniature stimulating electrodes working at relatively high charge densities, it is very important that they are electroche- mically stable. The technology of such systems usually includes the use of numerous parts made of different metals. To ensure an appropriate electrochemical stability all the connections and parts except the surface of the stimulating electrode must be covered by a material with as high as possible electrical resistivity. The characteristics of the morphological abnorma- lities observed after 18 months of stimulation were consistent with those observed in previous studies of cuff electrodes.9 In fact, it was recently shown that the mere presence of an electrode invariably resulted in a thickened epineurium and in some cases increased peripheral endoneurial connective tissue beneath the electrodes.18 It could be explained by the radial pressure within the cuff, which could induce an interference with intraneural blood flow. Nevertheless, the stimulus parameters used in the study somewhat exceeded those that would be required in most clinical applications of electrical stimulation. Therefore, an increased thickness of the connective tissue could be expected. In the study, a vagus was exposed to a relatively high rate of movement between the cuff and the nerve trunk. A direct mechanical interaction between the cuff and the vagus was an obvious means by which such damage might been inflected. However, there are other mechanisms that may also contribute: pressure caused by the formation or P. PE^LIN ET AL.: CHRONIC STIMULATION OF AN AUTONOMOUS NERVE WITH PLATINUM ELECTRODES 28 Materiali in tehnologije / Materials and technology 44 (2010) 1, 25–29 Figure 4: (a) Low-power microscopic image of the triplet9 central electrode (stimulating cathode) before implantation, (b) Low-power microscopic image of the triplet9 central electrode (stimulating cathode), pulsed with the described stimulating pulses, after 18 months of stimulation. Slika 4: (a) Malo pove~ana mikroskopska slika osrednje elektrode (stimulacijske katode) troj~ka 9 pred implantacijo, (b) malo pove~ana mikroskopska slika osrednje elektrode (stimulacijske katode) troj~ka 9 po 18-mese~ni stimulaciji. excessive fibrous encapsulation around the cuff, the transmission of forces from adjacent muscles to the cuff and hence to the nerve, and some undue tension in the cuff’s leads, even if they are carefully routed during the implantation. 5 CONCLUSIONS – The use of a cuff was associated with the build up of connective tissue around the cuff as well as within the cuff. – A moderate tissue injury occurred beneath the stimulating electrode of the triplet 9. – No changes were observed on the surfaces of the stimulating triplet9 electrode investigated after 18 months of stimulation using optical microscopy. – It can be concluded that irreversible electrochemical reactions potentially induced by relatively high charge biphasic stimulating pulses were not elicited. – The technical solutions described may find potential applications in neuroprosthetic technology for neurologically impaired patients, especially for the selective control of internal organs and glands, such as the cardiovascular system, and their relation to bodily changes and diseases. – One weakness of the cuff manufacture was the technically demanding and a time consuming process. – The latter technology of the mechanical connection could be the solution for further development of multi-electrode systems for the electrical stimulation of the nerve tissue. – Acknowledgements: This study was financed through Research Programme P3-0171, by the Ministry of Education, Science and Sport, Ljubljana, Republic of Slovenia. 6 REFERENCES 1 J. 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Bullara: Relationship between stimulus amplitude, stimulus frequency and neural damage during electrical stimulation of sciatic nerve of cat. Med. Biol. Eng. Comput., 33 (3 Spec No) (1995), 426–9 18 T. G. H. Yuen, W. F. Agnew, L. A. Bullara, S. Jacques, D. B. McCreery: Histological evaluation of neural damage from electrical stimulation: considerations for the selection of parameters for clinical application. Neurosurgery, 9 (1981) 3, 292–9 P. PE^LIN ET AL.: CHRONIC STIMULATION OF AN AUTONOMOUS NERVE WITH PLATINUM ELECTRODES Materiali in tehnologije / Materials and technology 44 (2010) 1, 25–29 29 A. MAGLICA ET AL.: THE PREPARATION OF Si3N4-TiN CERAMIC COMPOSITES PREPARATION OF Si3N4-TiN CERAMIC COMPOSITES PRIPRAVA KERAMI^NIH KOMPOZITOV NA OSNOVI Si3N4-TiN Aljo{a Maglica, Kristoffer Krnel, Toma` Kosma~ Engineering Ceramics Department, Jo`ef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia aljosa.maglica@ijs.si Prejem rokopisa – received: 2009-07-24; sprejem za objavo – accepted for publication: 2009-09-04 In this work we report on the preparation of particulate ceramic composites based on a Si3N4 or SiAlON matrix phase. The composites were prepared with reaction sintering of Si3N4/TiO2 powder mixtures using Y2O3, Al2O3 and, in case of SiAlON, also AlN as sintering additives. The results of X-ray diffraction investigation confirmed that TiN was formed during sintering in a nitrogen atmospere with chemical reaction of Si3N4 and TiO2. A comparison of the materials sintered with addition of TiO2 in the starting-powder mixture with the matrix-phase ceramics (Si3N4 or SiAlON) showed that materials with addition of TiO2 have higher densities and better flexural strength. The electrical conductivity of the sintered composites with addition of TiO2 in the starting powder mixture were also investigated. Their electrical conductivity was found to be highly dependent on the amount of added titania and on the sintering conditions. Key words: Si3N4, TiN, electrically conductive ceramics, ceramic heater V delu poro~amo o pripravi del~nih kerami~nih kompozitov na osnovi matri~ne faze iz silicijevega nitrida ali SiAlON-a. Kompozite smo pripravili z reakcijskim sintranjem me{anice prahu Si3N4 in TiO2, kot dodatke za sintranje pa smo uporabili Y2O3 in Al2O3, v primeru SiAlON-ov pa {e AlN. Rezultati rentgenske difrakcije so potrdili, da med sintranjem v du{ikovi atmosferi pri reakciji TiO2 s Si3N4 nastane TiN. Ko smo primerjali kompozite, pripravljenje z dodatom TiO2 v za~etni me{anici z materiali matri~ne faze (sintrani Si3N4 ali SiAlON), smo ugotovili, da dose`emo vi{jo gostoto in bolj{o upogibno trdnost pri materialu z dodanim TiO2. Raziskali smo tudi elektri~no prevodnost kompozitov z dodanim TiO2 v za~etni me{anici in ugotovili, da je njihova elektri~na prevodnost odvisna od dodane koli~ine TiO2 in od pogojev sintranja. Klju~ne besede: Si3N4, TiN, elektri~no prevodna keramika, kerami~ni grelec 1 INTRODUCTION Recently, much attention has been devoted to the production of particulate-reinforced silicon nitride and SiAlON materials, not only because of their improved fracture toughness, strength and mechanical reliability, but also because of their potential multi-functionality, especially their electrical conductivity, which can be obtained with incorporation of electrically conductive particles into the matrix phase1. The most commonly used electroconductive particles are WC, MoSi2, TiN, TiC, TiCN, TiB2 and ZrN2,3. Electro-conductive compo- site ceramics with good thermal conductivities are intere- sting for the production of various heating elements, such as ceramic glow plugs, igniters, ceramic heaters, etc4,5. Such materials have also received great attention due to their compatibility with electrical discharge machining6,7. The electrical conductivity of ceramic materials is strongly affected by the distribution of the electrically conductive second phase, and so many studies have been performed to increase the electrical conductivity of ceramic materials8,9. Composites of silicon nitride and titanium nitride have been investigated in order to obtain a combination of high hardness, high strength, good fracture toughness, and low electrical resistivity10,11,12. Some attempts were made to use a silicon nitride matrix with titanium nitride as the conductive phase. There are three main processing routes to fabricate the Si3N4-TiN composites13. The first is conventional liquid-phase sintering of Si3N4 and TiN powder mixtures with a small amount of sintering additives. The second one is the chemical vapour deposition (CVD) method using gas components of SiCl4–TiCl4–NH3–H2. The third method, where Si and Ti are used as the starting materials, is called an "in-situ synthesis" process. By contrast to the conventional powder processing route and the CVD process, the in-situ chemical reaction method enables the production of composites with the following advantages: low cost, desirable microstructures and enhanced sinterability. The aim of the work was to prepare Si3N4/TiN and SiAlON/TiN ceramic composites with sintering in a nitrogen atmosphere by the in-situ chemical reaction of Si3N4 and TiO2. The microstructure, mechanical pro- perties and electrical conductivity of the composites were investigated. The results indicated that the flexoral strength improved and that electrically conductive materials can be prepared if the amount of added TiO2 is sufficient. w()/w(( + ) – Si3N4) = 0.8 2 EXPERIMENTAL The starting powders used were Si3N4 SILZOT HQ (SKW, DE, d50 = 1.7 µm; BET = 3.2 m2/g, w () / w Materiali in tehnologije / Materials and technology 44 (2010) 1, 31–35 31 UDK 666.3/.7:537.311.3 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 44(1)31(2010) (+)-Si3N4) = 0.8, Al2O3 (Alcoa, USA, d50 = 0.5 µm; BET = 3–7 m2/g), Y2O3 grade fine (H. C. Starck, DE, d50 (max) = 0.9 µm; BET = 10.0–16.0 m2/g), AlN grade C (H. C. Starck, DE, d50 = 1.2 µm; BET = 4.1 m2/g) and TiO2 RC8 (Cinkarna Celje, SLO, rutil, d50 = 0.35 µm; BET = 6.5–8.5 m2/g). The nominal compositions for the preparation of Si3N4 and SiAlON with and without the addition of TiO2 are listed in Table 1. The powders were mixed with Si3N4 ball milling in a planetary mill for 2 h in isopropanol. After evaporation of isopropanol using a rotating evaporator, the dry powder mixture was cold pressed at 100 MPa into bars with dimensions of 3 mm × 5 mm × 42 mm and subsequently cold isostatically pressed at 790 MPa. The pressed samples were then sintered at 1800 °C for 2 h in nitrogen atmosphere. The sintered samples were examined using X-ray powder diffraction (D4 Endeavor, Bruker-AXS, Germany) and a scanning electron microscope (SEM, Jeol–5800, Japan). The density of the sintered samples were determined using Archimedes’ method. The flexural strength was measured on an Instron–1362 testing machine (Instron, USA), using the four-point bending method with a lower-span length of 20 mm and an upper-span length of 10 mm, and a crosshead speed of 1 mm/min. The bodies for the strength tests had the following dimensions: 2.3 mm × 3.9 mm × 38.5 mm. The electrical resistivity of the sintered specimens with dimensions of 2.3 mm × 3.9 mm × 19.2 mm was measured on a Multimeter 3457A testing machine (HP, USA) using four-probe measure- ments at room temperature (25 °C) with a direct current. Table 1: Compositions of the starting powder mixtures (mass frac- tions, w/%) Tabela 1: Za~etna sestava me{anic prahov (masni dele`, w/%) Compositions Si3N4 Y2O3 Al2O3 AlN TiO2 SN 92 5 3 0 0 SN/TiO2 82.8 4.5 2.7 0 10 SiAlON 83.9 2.2 5.5 8.4 0 SiAlON/TiO2 75.5 2 4.9 7.6 10 3 RESULTS AND DISCUSSION 3.1 Ceramic materials based on Si3N4 The X-ray diffraction pattern of Si3N4 with and without the addition of TiO2 in the starting mixture (denoted as SN and SN/TiO2) after sintering at 1800 °C for 2 h in flowing N2 is shown in Figure 1. In both samples we could observe the presence of the -Si3N4 and YAG (Yittrium Aluminium Garnet) phases, while in the sample SN/TiO2 we also observed TiN. The peaks of TiO2 could not be detected, and it could be concluded that the transformation of TiO2 into TiN was completed. During the sintering of such composites the following chemical reactions take place14,15,16: 6TiO2 + 4Si3N4  6TiN + 12SiO (g) + 5N2 (g) (1) 6TiO2 + 2Si3N4  6TiN + 6SiO2 +N2 (g) (2) The TiO2 and Si3N4 react in the temperature range from 1150 °C to 1350 °C (equations 1 and 2) in a nitro- gen atmosphere. In both reactions gaseous species are formed, which can influence the density of the final A. MAGLICA ET AL.: THE PREPARATION OF Si3N4-TiN CERAMIC COMPOSITES 32 Materiali in tehnologije / Materials and technology 44 (2010) 1, 31–35 Figure 1: X-ray analysis of SN and SN/TiO2 after sintering at 1800 °C for 2 h in flowing N2 Slika 1: Rentgenska analiza materiala SN in SN/TiO2, sintranega pri 1800 °C 2 h v pretoku N2 Figure 2: Microstructure of sintered silicon nitride ceramic at 1800 °C for 2 h in N2: a) SN and b) SN/TiO2 Slika 2: Mikrostruktura sintrane silicijeve nitridne keramike pri 1800 °C, 2 h v N2: a) SN in b)SN/TiO2 sample, especially in the samples with larger amounts of titania. The microstructures of the SN and SN/TiO2 com- posites are presented in Figure 2. In both SEM images (Figure 2a and 2b) we can see elongated -Si3N4 grains (dark region), a brighter transient liquid phase based on Y2O3 and Al2O3, and some black pores. The material with the addition of TiO2 in the starting–powder mixture (Figure 2 b) also indicates white TiN particles with size around 0.5–1.0 µm. The bright TiN particles were analyzed with EDXS, and the results confirm the presence of Ti and N. Figure 3 shows the fracture surfaces of sintered SN and SN/TiO2 samples. As shown in Figure 3 a, the matrix -Si3N4 grains are surrounded by the secondary bright phase, while in Figure 3 b, the white TiN particles are located in between the -Si3N4 grains. From the SEM micrographs we can conclude that these materials have intergranular fracture, which is characteristic for this kind of ceramic17,18. The materials SN and SN/TiO2 (Table 2) exhibited around 90 % relative density, and suitable flexural strengths. However, due to the cheaper submicron silicon nitride powder and the pressureless sintering process the relative density of materials could not be higher than 97 %. Sample SN/TiO2 had a higher flexural strength due to the presence of TiO2 in the starting–powder mixture, which contributed to the larger amount of transient liquid phase. The electrical conductivity of this composite was relatively low, because it had the mass fraction only 10 % of conductive phase, which is not enough to exceed the percolation threshold for particles of this size. Table 2: Comparison of a relative density, a flexural strength and a electrical conductivity of sintered Si3N4 samples Tabela 2: Primerjava relativne gostote, upogibne trdnosti in elektri~ne prevodnosti sintranih vzorcev Si3N4 Material rel./% fl./MPa el. /( m)–1 SN 89.8 380 ND* SN/TiO2 90.0 410 7.1 · 10−8 * ND … the measured electrical resistivity is higher from the range of measurement / izmerjena elektri~na upornost je ve~ja od obmo~ja merljivosti 3.2 Ceramic materials based on a SiAlON matrix phase For the formation of the SiAlON matrix the Si3N4 powder with the addition of sintering additives such as AlN, yttria and alumina was chosen (Table 1), together with and without the addition of TiO2 in the starting–powder mixture. The phase analysis was conducted on the sintered specimens with and without the addition of the mass fraction of 10 % of TiO2 in the starting–powder mixture (denoted as SiAlON and SiAlON/TiO2) using XRD analysis. The results of the XRD analysis, presented in Figure 4, revealed that in the case of sample without TiO2 we could observe -SiAlON and some signals from Y2O3, whereas in the SiAlON/TiO2 material again -SiAlON was formed together with TiN, implying that complete transfor- A. MAGLICA ET AL.: THE PREPARATION OF Si3N4-TiN CERAMIC COMPOSITES Materiali in tehnologije / Materials and technology 44 (2010) 1, 31–35 33 Figure 4: XRD patterns of sintered SiAlON and SiAlON/TiO2 mate- rials at 1800 °C for 2 h in nitrogen atmosphere Slika 4: Rentgenska difraktograma sintranega materiala SiAlON in SiAlON/TiO2 pri 1800 °C 2 h v du{ikovi atmosferi Figure 3: The SEM micrographs of fracture surface of sintered silicon nitride samples: a) SN and b) SN/TiO2 Slika 3: SEM-posnetki mikrostruktur prelomov sintranih vzorcev silicijevega nitrida: a) SN in b) SN/TiO2 mation of TiO2 occurred during the sintering process, in accordance with the following chemical reactions16,19–21. 2TiO2 + 3AlN + 5Si3N4  3Si5AlON7 + 2TiN + + 12O2 (g) (3) 2TiO2 + 2AlN  2TiN + Al2O3 + 12O2 (g) (4) From equation 3 it is clear that TiO2 reacts together with AlN and Si3N4 to form SiAlON, TiN and O2. The second chemical reaction (equation 4) leads to the formation of TiN, O2 and Al2O3. The formation of Al2O3 contributes to a larger amount of transient liquid phase and therefore could increase the densification of the material. The microstructures of the sintered SiAlON and SiAlON/TiO2 ceramics (Figure 5) contained as bright intergranular phase in a darker -SiAlON matrix. In the SiAlON/TiO2 ceramic, additional submicron (white) TiN particles with size around 0.8–1.2 µm are homogenously distributed around the elongated -SiAlON grains. The presence of TiN was also confirmed by EDXS analysis, where signals of Ti and N were observed. The fracture surfaces of these materials (Figure 6) show dark -SiAlON grains, a brighter transient liquid phase and uniform white TiN particles. The sintered SiAlON with and without the addition of TiO2 in the starting–powder mixture reached a higher relative density, flexural strength and comparable electrical conductivity (Table 3) compared to the SN and SN/TiO2 samples. The SiAlON/TiO2 material had a A. MAGLICA ET AL.: THE PREPARATION OF Si3N4-TiN CERAMIC COMPOSITES 34 Materiali in tehnologije / Materials and technology 44 (2010) 1, 31–35 Figure 6: The SEM micrographs of fracture surface of sintered SiAlON samples: a) SiAlON and b) SiAlON/TiO2. Slika 6: SEM-posnetki mikrostruktur prelomov sintranih vzorcev SiAlONa: a) SiAlON in b) SiAlON/TiO2 Table 3: Comparison of a relative density, a flexural strength and a electrical conductivity of sintered SiAlON samples Tabela 3: Primerjava relativne gostote, upogibne trdnosti in elektri~ne prevodnosti sintranih vzorcev SiAlONa Material rel./% fl./MPa el./( m)–1 SiAlON 95 406 ND* SiAlON/TiO2 96.5 610 1.1 · 10–8 * ND … the measured electrical resistivity is higher from the range of measurement / izmerjena elektri~na upornost je ve~ja od obmo~ja merljivosti Figure 5: SEM microstructure of sintered SiAlON samples: a) SiAlON and b) SiAlON/TiO2 at 1800 °C for 2 h in N2. Slika 5: SEM-posnetki mikrostruktur sintranih vzorcev SiAlONa: a) SiAlON and b) SiAlON/TiO2 pri 1800 °C, 2 h v N2 higher density compared to the SiAlON and conse- quently the flexural strength was higher by about 33 %. The electrical conductivity of this composite is relatively low, and is in the same range as the conductivity of the SN/TiO2 material. This could be explained by the grain growth of the TiN particles during sintering. 4 CONCLUSIONS The results show that by sintering in nitrogen at atmospheric pressure we were able to sinter Si3N4 and SiAlON ceramic materials, with and without the addition of TiO2 in the starting–powder mixture, to a relatively high density and with suitable mechanical properties. The SN and SN/TiO2 materials exhibited 90 % of relative density, while the SiAlON material reached 96 % of relative density and consequently a higher flexural strength. The addition of AlN enhanced the densification of SiAlON under the same sintering conditions compared to the Si3N4. The composites with the addition of TiO2 had a higher flexural strength, due to the larger amount of transient liquid phase. The SN/TiO2 and SiAlON/TiO2 composites were electrically conductive. However, their electrical conductivity is somewhat low and is not high enough to fulfill the requirements for the production of ceramic heaters. The reason for this is the amount of secondary conductive phase and the micro- structure of the samples. 5 REFERENCES 1 S. De Bondt, L. Froyen, A. Deruyttere, Electrical conductivity of composites: a percolation approach, J. Mater. Sci, 27 (1992), 1983–1988 2 A. Bellosi, S. Guicciardi, A. Tampieri, Development and characteri- zation of electroconductive Si3N4-TiN composite, J. Am. Ceram. Soc., 9 (1992), 83–93 3 M. Ade, J. Haußelt, Electroconductive ceramic composites with low-to-zero shrinkage during sintering, J. Eu. Ceram. 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ANALYSIS OF THE MATERIAL AND THE ACTUATOR INFLUENCE ON THE CHARACTERISTICS OF A PNEUMATIC VALVE ANALIZA VPLIVA MATERIALA IN AKTUATORJEV NA LASTNOSTI PNEVMATI^NEGA VENTILA Niko Herakovi~, Toma` Bevk Laboratory for Handling, Assembly and Pneumatics, Faculty of Mechanical Engineering, University of Ljubljana, A{ker~eva 6, SI-1000 Ljubljana, Slovenia niko.herakovic@fs.uni-lj.si Prejem rokopisa – received: 2009-06-26; sprejem za objavo – accepted for publication: 2009-09-04 Development of pneumatic valves dictates improvements in the area of valve miniaturization, lower price, better dynamic characteristics and lower consumption of electric energy which can hardly be achieved with commonly used electromagnetic actuators. This is also the reason why alternative actuators started to appear on the market, among them piezo actuators which offer the best possibilities for practical use in pneumatic valves. The article treats the analysis of dynamic characteristics of an ISO 3 5/2 directional pneumatic valve for different parameters. The influence of three different piston materials on the valve reaction and response time is analyzed, showing the influence of the mass of moving parts on valve dynamics. A special emphasis is given to the analysis of the actuator energy consumption for a pneumatic valve actuated by an electromagnetic actuator in one case and by a piezo actuator in another case during only one switch. The results obtained from the analysis indicate that the material of moving parts (piston) influences significantly the dynamics of the valve. Experimental results prove that the lower the mass, the faster is the displacement of the valve piston, and consequently, the switchover and the response time of a pneumatic valve gets shorter. The analysis has shown that the valve, actuated by a piezoelectric actuator consumes much less electric energy than the valve, actuated by the electro magnets and also achieves better dynamic characteristics. Key-words: pneumatic valve, electromagnetic actuator, piezoelectric actuator, energy consumption, dynamic response Razvoj pnevmati~nih ventilov narekuje izbolj{ave na podro~ju miniaturizacije, ni`jih cen, bolj{ih dinami~nih lastnosti in ni`je porabe elektri~ne energije, kar pa je zelo te`ko dose~i z obi~ajno uporabljenimi elektromagnetnimi aktuatorji. Zaradi tega so se na tr`i{~u za~eli pojavljati alternativni aktuatorji, med katerimi nudi najve~ mo`nosti za prakti~no uporabo v pnevmatiki piezo aktuator. Prispevek obravnava analizo dinami~nih karakteristik pnevmati~nega potnega ventila ISO 3 5/2 za razli~ne parametre. Analiziran je vpliv treh razli~nih materialov bata na odzivnost in odgovor prehoda ventila, ki prikazuje vpliv mase gibalnega elementa na dinamiko ventila. Poseben poudarek je posve~en analizi porabe elektri~ne energije za pnevmati~ni ventil, ki je krmiljen v enem primeru s piezoelektri~nim aktuatorjem in v drugem primeru z elektromagnetnim aktuatorjem za ~as enega preklopa. Rezultati, pridobljeni iz meritev ka`ejo, da material gibalnega elementa (krmilni bat) ob~utno vpliva na dinamiko ventila. Eksperimentalni rezultati so dokazali, da manj{a kot je masa krmilnega bata, kraj{i je ~as pomika ventila in posledi~no je preklopni ~as, ter ~as odgovora prehoda pnevmati~nega ventila kraj{i. Analiza je tudi pokazala, da porabi ventil, krmiljen s piezoelektri~nim aktuatorjem, precej manj elektri~ne energije kot ventil, krmiljen z elektromagnetnim aktuatorjem, poleg tega pa dose`e tudi bolj{e dinami~ne lastnosti. Klju~ne besede: pnevmati~ni ventil, elektromagnetni aktuator, piezoelektri~ni aktuator, poraba energije, dinami~ni odziv 1 INTRODUCTION Design and development of pneumatic valves con- stantly dictates improvements in valve miniaturization, lower price, better dynamic characteristics and lower consumption of electric energy 1,2,3,4. Better dynamics of valves can be achieved with use of light-weight piston materials and alternative actuators for their actuation. The most commonly used actuators in industry are elec- tromagnetic actuators which are robust and reasonably priced, however, they are questionable when shorter switchover and response times are needed, which is desirable in high dynamic systems. Shorter switchover and response times occur due to magnetic induction and eddy current when switching the current on. With the use of alternative actuators, among which piezoelectric actuators offer the best possibilities for practical use 4,5,6,7,8,9, much shorter response times and lower electric energy consumption can be achieved. On the one hand, switchover times of the electromagnetic actuators lie in the range between 10 to 20 ms, whereas on the other hand, piezoelectric actuators achieve shorter switchover times in the range of 500 µs 3. For consumers also the switchover repeatability in the whole life time of a valve is of great importance. Today’s valve pistons are made of steel for hydraulic valves and of aluminium for pneumatic valves and they are known for their relatively high density (especially those, made of steel), which influences negatively the valve dynamics, especially the response time. The lower are the masses of moving parts, like pistons and elec- tromagnetic coils, the shorter is the switchover time and the dynamics is better 10. A piezoelectric actuator works on the principle of the inverse piezo effect. It converts electric energy directly to mechanical energy, while the electromagnetic actuator converts electric energy indirectly to mechanical energy Materiali in tehnologije / Materials and technology 44 (2010) 1, 37–40 37 UDK 621.22:620.1/.2 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 44(1)37(2010) (Figure 1). Because of the indirect energy conversion, longer response times are achieved with electro magnets and consequently, worse dynamic characteristics of a pneumatic valve are obtained 3,5. 2 PIEZOELECTRIC ACTUATOR In 1880 it was discovered that electric potential could be generated if pressure is applied to quartz crystals (piezoelectric material). This is called piezo effect. Vice versa, if electric potential is applied to piezoelectric material it changes its shape – inverse piezo effect. Piezo effect exhibited by natural materials as quartz, tourma- line, Rochelle salt etc. is very small so new materials with improved characteristics of piezo effect were deve- loped. These materials are polycrystalline ferroelectric ceramic materials such as barium titanate and lead (plumbum) zirconate titanate (PZT), which is also a most widely used material for actuator applications today 11. The biggest advantages of piezoelectric actuators over electromagnetic actuators are short response time and low energy consumption in the stationary state. Other advantages and also disadvantages are presented in Table 1 5,11,12. Extension of these actuators is propor- tional to the applied voltage. Table 1: Properties of piezoelectric actuators Tabela 1: Lastnosti piezoelektri~nih aktuatorjev Advantages Disadvantages High resolution High force generation Good efficiency High dynamics – fast expansion No wear (no moving parts) High material stiffness Low power consumption (stationary state) Operation at cryogenic temperature No magnetic field Hysteresis Drift (Creep) Small strain (0,1-0,2 %) Problem of depolarization High supply voltage (60–1000 V) Two main types of piezo actuators on the market are stack piezo actuators and bending piezo actuators (Figure 2). Main properties for selecting the right piezo actuator for a particular application are: force generation, deflection, dimensions and response time. Stack actuators (Figure 2a) can consist of monocrystal or of several piezoceramic plates. An advantage of the con- struction out of several plates is that a lower voltage is required for supplying the stack actuator, which is especially important for applications in fluid power technology and in assembly automation. They also generate high forces up to several tons but have small deflection – several 100 µm 11. A bending actuator usually consists of a passive metal substrate which is glued to a piezoceramic strip – bimorph actuator 11. There are also bending actuators with several layers – trimorph or multimorph. A bending piezoelectric actuator has high deflection – greater than 1 mm but generated forces are very low, only several N. For the operation of a pneumatic valve two types of piezoelectric bending actuators can be chosen (Figure 2): • Cantilever piezo element (Figure 2b) • Crossbow piezo element (Figure 2c) The bending cantilever piezo element is fixed only at one end; it has higher deflection and smaller forces than the crossbow piezo element. The crossbow piezo element is fixed on both ends 3,13. 3 TEST RIG A test rig (Figure 3) was set up to carry out the measurements of the valve characteristics. Measurements were performed on a modified directional pneumatic valve ISO 3 5/2 which has a maximum nominal flow of 3539 L/min. Actuation with a piezoelectric actuator was N. HERAKOVI^, T. BEVK: ANALYSIS OF THE MATERIAL AND THE ACTUATOR INFLUENCE ... 38 Materiali in tehnologije / Materials and technology 44 (2010) 1, 37–40 Figure 1: Conversion of electric energy to mechanical energy for electromagnetic and piezo actuator Slika 1: Pretvorba elektri~ne energije v mehansko za elektromagnetni in piezoaktuator Figure 2: Different types of piezoelectric actuators Slika 2: Razli~ni tipi piezoelektri~nih aktuatorjev performed using the nozzle/flapper principle (Figure 4) 3,14. The pneumatic valve was actuated with the electro- magnetic actuator in one case and with the piezoelectric actuator in the other case. In both cases the energy consumption was analyzed. The analysis was made for a single switchover when the actuator should hold the given position for one hour. To represent the influence of the matereial of the moving part, i.e. valve piston, on the valve dynamics, three different piston materials were used: • Aluminium piston (commonly used in pneumatic) • Steel piston (commonly used in hydraulics) • Light-weight material piston Using the LabView program package, the following parameters (signals) from the test rig were measured and analyzed: • Pressure in both chambers • Stroke of the valve piston • Energy consumption of the actuator The pressure in each chamber of the valve was measured with a piezo pressure gauge HBM P4VK, and the voltage signal was then amplified with an HBM KWS 3073 amplifier. The stroke was measured with a CLP13-100 potentiometer, which was supplied from a BNC-2120 shielded block, and the energy consumption was measured with special electronics, designed for powering a piezo-electric and electromagnetic actuator. All the signals were brought to a BNC-2120 shielded connector block which was connected to a 6023E card for acquiring signals in the PC. 4 RESULTS In order to analyze the dynamic characteristics of valves, the amplitude and phase diagram are used – Bode diagram. Valve characteristics are compared at the frequency of – 3 dB and at the phase of – 90°. The amplitude characteristic of the valve shows how fast the N. HERAKOVI^, T. BEVK: ANALYSIS OF THE MATERIAL AND THE ACTUATOR INFLUENCE ... Materiali in tehnologije / Materials and technology 44 (2010) 1, 37–40 39 Figure 4: Nozzle/flapper principle Slika 4: Princip {oba/odbojna plo{~a Figure 3: A block diagram of the test rig Slika 3: Blokovni diagram preizku{evali{~a Figure 5: Influence of valve piston material on the valve dynamics Slika 5: Vpliv materiala bata na dinamiko ventila Table 2: Influence of valve piston material (mass) on valve dynamics Tabela 2: Vpliv materiala (mase) ventilskega bata na dinamiko ventila Material Steel (st) Aluminium(al) Lightweight (lw) Mass 140,9 g 59,4 g 39,6 g Stroke + delay 38 ms 33 ms 29 ms Stroke 21 ms 15 ms 12 ms Delay 17 ms 18 ms 17 ms Frequency 3 dB 74 Hz 110 Hz 151 Hz Phase 90° 26 Hz 28 Hz 30 Hz displacement of the valve piston is and the phase depends on the delay time between the input signal and the displacement (output signal). Figure 5 and Table 2 show the comparison of the dynamic characteristics of the valve with different piston materials. The input signal is a step function – start of supplying the actuator with electric energy. The response signal is a displacement of the valve piston. Aluminium (al), steel (st) and light- weight (lw) material pistons are implemented and their responses analyzed. The time delay of the displacement was for all pistons the same because of almost equal static friction between the valve piston and valve housing guidance. The difference is evident in the piston displacement time. The aluminium piston achieves 40 % shorter response time in comparison to the steel piston and the switchover time with the aluminium piston was thus 15 % shorter than with the steel piston. The light-weight material piston achieved even better results; the response time was 25 % shorter compared to the aluminium piston and switchover time was 14 % shorter. This proves how big the influence of the valve piston material on valve dynamics is. Energy consumption of the piezoelectric and electro- magnetic actuator was analyzed for one switchover, when the actuator stays under power for one hour (Figure 6). The results show that the energy consump- tion of the piezoelectric actuator is 3,03 · 10–6 Wh and is 3 · 106 times smaller in comparison to the electromag- netic actuator which has a consumption of 8,96 Wh (Table 3). The energy consumption of the piezoelectric actuator is so small because it works on the same principle as a capacitor. Electric energy is consumed when the piezoelectric actuator moves to its maximum displacement position. When the piezoelectric actuator expands, there is almost no energy consumption – stationary state. 5 CONCLUSIONS The paper is focused on the experimental analysis of the material and actuator influence on the dynamic and stationary characteristics of a pneumatic valve. In the research three different materials have been used for a valve piston: steel, aluminium and a light-weight material. The results of the experimental analysis show that a pneumatic valve with a light-weight material valve piston enables much faster moves - better short response time and the shortest switchover time which is essential for high dynamic automation systems. It is obvious, that the material (mass) of the valve piston has a big influence on the valve dynamics. The analysis of the energy consumption of the electromagnetic and the piezoelectric actuator shows a big advantage in lower energy consumption of piezoelectric actuator in the stationary state. 6 REFERENCES 1 Herakovi~, N.: Alternativni aktuatorji in mehatronski sistemi – pregled in uporaba v fluidni tehniki, Ventil, (1997) 3–4, 135–141 2 Murrenhoff, H.: Trends in valve Development; 3rd International Fluid Power Conference (Volume 1); Shaker Verlag GmbH, Aachen, 2002 3 Herakovi~, N. in Noe, D.: Analiza delovanja pnevmati~nega ventila s predkrmilnim piezoventilom, Strojni{ki vestnik, (2006) 12. 834–851 4 Bevk T., Eksperimentalna analiza dinami~nih lastnosti pnevmati~ne- ga ventila in optimizacija energetske porabe, diplomska naloga univerzitetnega {tudija, FS – Ljubljana 2008 5 Herakovi~, N.: Die Untersuchung der Nutzung des Piezoeffektes zur Ansteuerung fluidtechnischer Ventile, Verlag Mainz, 1996 6 Topçu E., Yüksel I., Kamýº Z.: Development of electro-pneumatic fast switching valve and investigation of its characteristics, Mecha- tronics, (2006) 6, 365–378 7 Choi, S. B., Yoo, J. K., Cho, M. S., Lee, Y.S.: Position control of a cylinder system using a piezoactuator-driven pump, Mechatronics, 2005, {t. 2, str. 239–249 8 Choi, S. B., Han, S. S., Lee, Y. S.: Fine motion control of a moving stage using a piezoactuator associated with a displacement amplifier, Smart Materials and Structures, (2005) 1, 222–230 9 Miyajima, T., Fujita T., Sakaki K., Kawashima K., Kagawa T.: Development of a digital control system for high-performance pneumatic servo valve, Precision Engineering, (2007) 2, 156–161 10 Fabjan, R.: Dolo~itev dinami~nih karakteristik pnevmati~nih potnih ventilov, diplomska naloga univerzitetnega {tudija, FS – Ljubljana, 2008 11 N.N.: Nanopositioning & Micropositioning Catalog, PI, 2006 12 Novotny M., Ronkanen P., Piezoelectric Actuators, 13 Koch, J.: Piezoxide (PXE) Eigenschaften und Anwendungen, Valvo Unternehmensbereich Bauelemente der Philips GmbH, 1988 14 Slamnik, G.: Razvoj krmiljenja pnevmati~nega ventila s piezoaktu- atorjem, diplomska naloga visoko{olskega {tudija, FS – Ljubljana, 2002 N. HERAKOVI^, T. BEVK: ANALYSIS OF THE MATERIAL AND THE ACTUATOR INFLUENCE ... 40 Materiali in tehnologije / Materials and technology 44 (2010) 1, 37–40 Figure 6: Energy consumption of piezoelectric and electromagnetic actuator at one switchover Slika 6: Poraba elektri~ne energije s piezoelektri~nim in elektromag- netnim aktuatorjem Table 3: The analysis of the actuator electric energy consumption Tabela 3: Analiza porabe elektri~ne energije aktuatorjev Actuator Energy consumption Piezoelectric 3,03 · 10–6 Wh (one switch) Electromagnetic 8,96 Wh A. TODOROVI] ET EL.: STRESS ANALYSIS OF A UNILATERAL, COMPLEX, PARTIAL DENTURE ... STRESS ANALYSIS OF A UNILATERAL COMPLEX PARTIAL DENTURE USING THE FINITE-ELEMENT METHOD NAPETOSTNA ANALIZA UNILATERALNO KOMPLEKSNE ZOBNE PROTEZE Z UPORABO METODE KON^NIH ELEMENTOV Aleksandar Todorovi}1, Katarina Radovi}1, Aleksandar Grbovi}2, Rebeka Rudolf3,4, Ivana Maksimovi}1, Dragoslav Stamenkovi}1 1 School of Dentistry, University of Belgrade, Rankeova 6, 11000 Beograd, Serbia 2 Faculty of Mechanical Engineering, University of Belgrade, Serbia 3 University of Maribor, Faculty of Mechanical Engineering, Smetanova 17, 2000 Maribor, Slovenia 4 Zlatarna Celje, d. d., Kersnikova ulica 19, 3000 Celje, Slovenia a.todorovic@sbb.rs Prejem rokopisa – received: 2009-10-10; sprejem za objavo – accepted for publication: 2009-11-13 Different types of dental restorations are used in the treatment of a unilateral, free-end saddle. A unilateral, complex, partial denture is one of the indications for this case of partial edentulousness. Consequently, the aim of this study was to stress test the unilateral complex partial denture model and its parts, under load, when changing the length of the free-end saddle. The stress distribution in canines and the first premolar, as the retention teeth, was examined under the influence of physiological and excessive occlusal forces by moving the point of attack in a distal direction. CATIA software was used for the creation of the 3D, fixed restoration unit model, in real size, with the appropriate supporting structures (canine and first premolar with present crowns, alveola, periodontal space) that are connected by the SD snap-in-latch attachment to the mobile portion of a partial denture. The mobile portion consists of an acrylate-coated metal base with three teeth (second premolar, first and second molars). The stress analysis, using the finite-element method, was performed under the application of physiological loads of 25 N, 50 N, 75 N and 100 N, and excessive loads of 300 N, 500 N and 700 N in the second premolar region, as well as in the first and second molar region. The results of the analysis showed that the largest amount of load under the application of physiological occlusal forces is positioned on the abutment teeth. Excessive forces are borne by the attachment. The stress analysis, performed on the unilateral complex partial denture model, suggested that the obtained stress values are lower than the limit values at which the plastic deformation in the model occurs. Key words: unilateral complex partial denture, SD snap-in-latch attachment, physiological load, excessive load Razli~ne vrste popravil zob so v uporabi pri obdelavi enostransko prostega sedla. Enostransko kompleksna delna proteza je ena od indikacij za primer delne edentuloze. Zato je bil namen te {tudije napetostni preizkus enostransko kompleksnega modela dela proteze in njenih delov pri napetosti pri spremembi dol`ine sedla s prostim koncem. Porazdelitev napetosti v podo~njakih in v prvem premolarnem kot retencijskem zobu je bila raziskana pri vplivu fiziolo{kih in prevelikih sil ugriza pri premiku to~ke napada v distalni smeri. Softver CATIA je bil uporabljen za pripravo 3D fiksnega modela obnove pri pravi velikosti z ustrezno podporno strukturo (podo~njaki in prvi premolarni zob s krono, dlesnijo in peridontalnim prostorom), ki so povezani z vezjo SD snap-in-latch k mobilnemu delu partialne proteze. Ta del je iz kovinske podlage pokrite z akrilatom s tremi zobmi (drugi premolarni, prvi in drugi molarni). Napetostna analiza je bila pripravljena z obojim, fiziolo{kimi silami 25N, 50N, 75 N in 100 N ter s prevelikimi silami 300 N, 500 N in 700 N v drugem premolarnem ter v prvem in v drugem molarnem delu. Rezultati analize ka`ejo, da je najve~ja obremenitev zaradi fiziolo{kih ugriznih sil na opornem zobu, prevelike sile pa prena{ajo pritrditve. Napetostna analiza je pokazala, da so izra~unane napetosti ni`je od limitnih vrednosti, pri katerih nastane plasti~na deformacija modela. Klju~ne besede: enostransko kompleksna proteza, pritrditev SD snap-in-latch, fiziolo{ka obremenitev, prevelika obremenitev 1 INTRODUCTION Different types of dental restorations are used in the treatment of a unilateral, free-end saddle. The final decision on the selection for indication is made on the basis of the patient’s consent to treatment, the state of the mouth cavity, the degree of oral hygiene, the periodontal status of the remaining teeth and the degree of residual ridge resorption. Patients with poor oral hygiene repre- sent a contraindication to prosthetic treatment.1 Reduced retention, stability and the visibility of the retentive wire extension represent major disadvantages and the cause of dissatisfaction in patients with partial dentures and implant-supported overdentures placed only in the frontal region. The implementation of the osseo- integrated implants in the side region increases stability, partial denture retention and eliminates the need for wire extensions, as Mitrani, Brudvik et al.2 pointed out in their retrospective study. The implementation of the minimum number of implants, in order to increase the support structure of a partial denture, leads to a change in the Kennedy class I or II of partial edentulousness into a Kennedy class III, denture stabilization and a reduction in the rotational movements. Ohkubo et al.3 showed that there is no significant difference in movements, during mastication, between a removable partial denture and an implant-supported removable partial denture. The masti- catory center, with implant-supported partial dentures, is moved in a more distal direction than with the conven- tional dentures. Materiali in tehnologije / Materials and technology 44 (2010) 1, 41–47 41 UDK 616.31:539.3:519.8 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 44(1)41(2010) The implementation of implants in the side region is sometimes an impractical solution due to anatomical and/or financial limitations, or a patient's refusal to undergo the necessary extensive surgical procedure. Kuboki, Okamoto et al.4 compared, in their study, the quality of life in patients with implant-supported den- tures, conventional removable partial dentures, and the patients with no dental treatment in a mandibular, unilateral, free-end saddle. The quality of life in the patients with implant-supported dentures was signifi- cantly higher than in the others and, as for the patients of the remaining two groups, it was approximately the same. Aesthetics, the absence of pain, chewing and speech are the key factors that influence the quality of life. 5–7 The progress in technology, along with the one's aspiration to regain the function and appearance of lost natural teeth and to raise the quality of life, leave open the possibility to make dental restorations that do not affect the patient’s health and do not require additional surgical procedures. A unilateral complex partial denture is a mobile dental prosthesis, localized to one side of the jaw only. Its role is teeth compensation in a unilateral, shortened, dental series, to restore lost function and to rehabilitate the patient aesthetically. Unlike other types of partial dentures, it does not have a prosthetic plate and arch. This type of dental restoration treatment is consi- dered to have high functional, aesthetic and preventive values.8-10 The whole complex consists of milled crowns made of a Co-Cr-Mo alloy covered by ceramics and attached by the mobile part. The mobile part is a denture saddle made of the Co-Cr-Mo alloy covered by acrylic. The attachment consists of the primary part (Co-Cr-Mo alloy) and the secondary part (titanium). As far as the dentures and their parts are made of different materials, we tried to show the behavior of this system under physiological and excessive forces by using the FEM. The aim of this study was to assess the stress distribution in a denture model and its parts, under load, when changing the length of the free-end saddle. The more detailed objectives were: – stress testing in canines and the first premolar as retention teeth, under the influence of physiological occlusal forces by moving the point of attack in a distal direction; – stress testing in the canines and the first premolar as retention teeth, under the influence of excessive occlusal forces by moving the point of attack in a distal direction. 2 MATERIALS AND METHODS To obtain an effective and accurate analysis for the stress conditions and movements by using the finite-element method, it is necessary to design a model that more closely resembles the real object. A fixed restoration piece, with appropriate supporting structures (canine and first premolar with present crowns, alveola, periodontal space) that are connected by the SD snap- in-latch attachment to the mobile portion of a partial denture, was created in the CATIA software. The mobile portion consists of an acrylate-coated metal base (Co-Cr-Mo alloy) with three teeth (second premolar, first and second molars). This model was made in actual size (in a 1:1 ratio) to establish the validity of the results obtained. According to the literature11, the crown length (height) of the canine model was 9.5 mm, and the mesiodistal width was 7.5 mm. The root length of the model was 16.62 mm. The crown height of the first premolar was 8.5 mm, the mesiodistal width was 7.5 mm, and the root length was 14.5 mm. The average distance of 2 mm between the cement-enamel junction and alveolar bone crest was used in the model, providing the precise root length within the bone. The length of the free-end saddle was 28.27 mm. As for the second premolar, the crown width was 5.16 mm, and the height was 7.5 mm. The crown height of the first molar was 7.5 mm, and the mesiodistal width was 10.5 mm. The crown height of the second molar was 7 mm and the width was 8.22 mm (Figure 1). For the calculation model of the unilateral complex denture a mesh of the appropriate density was generated. All the materials used in the model were isotropic. In an isotropic material, the properties are the same in all directions, and therefore there are only two independent material constants. In most reported studies12,13 the materials are assumed to be homogeneous, linear and have elastic material behavior characterized by the two material constants of the Young’s modulus and the Poisson’s Ratio. Therefore, both the teeth and the denture were simplistically modeled as linear, homogeneous and isotropic materials because it was very hard to determine the properties of non-homogenous, anisotropic, composite structures of the analyzed objects. For the structural model the type of finite element used was a 3D ten-node tetrahedral (the option of the 20-node A. TODOROVI] ET EL.: STRESS ANALYSIS OF A UNILATERAL, COMPLEX, PARTIAL DENTURE ... 42 Materiali in tehnologije / Materials and technology 44 (2010) 1, 41–47 Figure 1: 3D unilateral complex denture model with Snap-in-latch attachment Slika 1: 3D model enostransko kompleksne proteze s pritrditvijo snap-in-latch brick element).14 The finite-element mesh of the unilate- ral complex partial denture model consisted of 357,829 nodes and 186,859 elements. The visual appearance of the mesh for the unilateral complex partial denture model, with supporting struc- tures, is shown in Figure 2. Applying the finite-element method, the analysis of the stress conditions was performed, under a load of different occlusal forces at the same points along the entire model. The data on the characteristics of the materials used in the stress and movement analysis, using the finite-element method for the tested model, are shown in Table 1. Table 1: Mechanical characteristics of the materials Tabela 1: Mehanske karakteristike materialov Material Young's Modulus of Elasticity, E/MPa Poisson's ratio Author Enamel 4.1 × 104 0.30 Rubin Dentin 1.9 × 104 0.31 Rubin Root cement 1.37 × 104 0.35 Peters Pulp 0.000207 × 104 0.45 Rubin Periodontal Iigament 0.00689 × 10 4 0.45 Reinhard Gingiva 0.00196 × 104 0.30 Reinhard Alveolar bone 0.137 × 104 0.30 Gtingor Gold alloy 7.70 × 104 0.33 Reinhard Co-Cr-Mo 23 × 104 0.33 Stamenkovi} Ceramics 6.9 × 104 0.33 Anusavice Using the finite-element method for the stress and load analyses, the applied force was converted into pressure using the formula P = F/S in order to obtain more realistic images and results. Using the finite-element method for the stress and load analyses, the applied force was converted into pressure in order to obtain more realistic images and results. The unilateral complex denture model was loaded with different reference forces, while the degree of strain in the retention teeth was being observed. According to various literature data,15-17 the intensity of the occlusal force is within the range of 50 N in edentu- lous patients to 1000 N in extreme cases. According to Martinovic11, their value in an intact tooth is around 780 N, and 210 N in edentulous patients. The values in the range 25–300 N are considered physiological for denture wearers. Forces larger than 300N are considered excessive forces, because they could jeopardize the functional properties of partial dentures. During the testing, all three parts of the free-end saddle (the second premolar region and the first and second molar regions) of a unilateral complex partial denture were loaded, with the aim being to observe the behavior of the whole system and some of its parts in different conditions. A total of 25–700 N of force was applied, with the load in the range from the physiological to the excessive one. The behavior of the denture in relation to the length of the saddle, i.e., in relation to the movement of the point of attack in a distal direction, was also observed. A load analysis of the unilateral complex partial denture was performed using the finite-element method18-21. The finite-element method was applied in dentistry, for the first time, in the early 1970s by Farah, Craig and Sikarskie, to optimize the design of dental restorations. The procedure for the analysis of a model using the finite-element method (FEM) included: – A definition of the virtual model; – Load inputs and the calculation; – Analysis of the results. Four types of finite elements were used for modeling: SOLID 187, CONTA 174, TARGE 170 and SURF 154. Stress analysis, under load, of the unilateral complex partial denture model using the FEM, can be performed in a four-step process: – Model geometry for the calculation and finite-ele- ment mesh generation; – Selection of the material and its characteristics; – Definition of a support; – Size, direction and load input determination. 3 RESULTS Using the finite-element method, the calculations were performed with the planned load of the unilateral complex denture model (Kennedy class II). The stress values in canines, the first premolar and the attachment of the unilateral complex partial denture model, under different physiological loads, with the point of attack in the second premolar region, are shown in Table 2. The stress values in canines, the first premolar and the attachment of the unilateral complex partial denture model, under excessive loads, with the point of attack in the second premolar region, are shown in Table 3. With the increased intensity of the applied forces from 25–100 N, the reduction in stress on the retention teeth occurs, while it is increased simultaneously on the attachment. Applying forces larger than 100 N leads to a change in the system behavior and an increase of the stress was A. TODOROVI] ET EL.: STRESS ANALYSIS OF A UNILATERAL, COMPLEX, PARTIAL DENTURE ... Materiali in tehnologije / Materials and technology 44 (2010) 1, 41–47 43 Figure 2: The finite-element mesh of the unilateral complex partial denture model Slika 2: Mre`a kon~nih elementov za enostransko kompleksno protezo observed in both the retention tooth and the attachment. Under the influence of the forces within the range of 100–300 N, the change of the whole system behavior occurs and the stress is transferred from the retention tooth to the attachment. By applying forces larger than 300 N, the maximum load is concentrated on the attachment, leading to plastic deformation and fatigue destruction. The stress values in canines, the first premolar and the attachment of the unilateral complex partial denture model, under different physiological loads, with the point of attack in the first molar region, are shown in Table 4. The stress values in canines, the first premolar and the attachment of the unilateral complex partial denture model, under excessive loads, with the point of attack in the first molar region, are shown in Table 5. The stress values obtained in the attachment were slightly lower after the point of attack was moved to the first molar region and with the same amount of load applied. An increase in the force intensity, from 25 N towards 100 N, leads to a reduction in the stress on both the retention teeth and the attachment. Excessive forces (300 N, 500 N and 700 N) lead to an increase in stress on both the retention teeth and the attachment. The stress values in canines, the first premolar and the attachment of the unilateral complex partial denture model, under different physiological loads, with the point of attack in the second molar region, are shown in Table 6. The stress values in canines, the first premolar and the attachment of the uni- lateral complex partial denture model, under excessive loads, with the point of attack in the second molar region, are shown in Table 7. Stress reduction in the retention teeth, with the increase in load, was also observed in this case. 4 DISCUSSION The planning phase represents the most important initial phase for making dental restoration, due to the very specific environment in which it should be located. The features of the tissues to be restored and their A. TODOROVI] ET EL.: STRESS ANALYSIS OF A UNILATERAL, COMPLEX, PARTIAL DENTURE ... 44 Materiali in tehnologije / Materials and technology 44 (2010) 1, 41–47 Table 4: The results of FEM analysis under the application of physiological loads of 25 N, 50 N, 75 N, 100 N to the unilateral complex denture model, with the point of attack in the first molar region Tabela 4: Rezultati analize pri fiziolo{kih obremenitvah 25N, 50 N, 75 N in 100 enostransko kompleksne proteze z napadom v prvem premolarnem delu Force values (N) Canine (MPa) First premolar (MPa) Attachment (MPa) 25 36.18 38.49 203.40 50 34.50 35.38 202.92 75 33.03 32.37 202.47 100 30.12 29.27 202.04 Table 5: The results of FEM analysis under the application of excessive loads of 300 N, 500 N, 700 N to the unilateral complex denture model, with the point of attack in the first molar region Tabela 5: Rezultati analize pri prevelikih obremenitvah 300 N, 500 N in 700 N enostransko kompleksne proteze z napadom v prvem premolarnem delu Force values (N) Canine (MPa) First premolar (MPa) Attachment (MPa) 300 17.77 9.46 199.70 500 20.12 27.80 199.26 700 27.68 54.46 200.74 Table 6: The results of FEM analysis under the application of physiological loads of 25 N, 50 N, 75 N, 100 N to the unilateral complex denture model, with the point of attack in the second molar region Tabela 6: Rezultati analize pri fiziolo{kih obremenitvah 25N, 50 N, 75 N in 100 enostransko kompleksne proteze z napadom v drugem premolarnem delu. Force values (N) Canine (MPa) First premolar (MPa) Attachment (MPa) 25 37.76 40.05 193.89 50 37.66 38.38 185.43 75 37.43 37.10 178.79 100 37.02 35.78 174.15 Table 7: The results of FEM analysis under the application of excessive loads of 300 N, 500 N, 700 N to the unilateral complex denture model, with the point of attack in the second molar region Tabela 7: Rezultati analize pri prevelikih obremenitvah 300 N, 500 N in 700 N enostransko kompleksne proteze z napadom v prvem premolarnem delu Force values (N) Canine (MPa) First premolar (MPa) Attachment (MPa) 300 36.08 31.42 213.29 500 46.84 31.15 332.98 700 61.05 34.31 475.90 Table 2: The results of the FEM analysis under the application of physiological loads of 25 N, 50 N, 75 N, 100 N to the unilateral complex partial denture model, with the point of attack in the second premolar region Tabela 2: Rezultati analize pri fiziolo{kih obremenitvah 25,N, 50 N, 75 N in 100 enostransko kompleksne proteze z napadom v drugem premolarnem delu Force values (N) Canine (MPa) First premolar (MPa) Attachment (MPa) 25 35.58 37.54 221.24 50 33.18 33.60 240.88 75 30.73 29.77 262.18 100 28.31 25.65 285.16 Table 3: The results of the FEM analysis under the application of excessive loads of 300 N, 500 N, 700 N to the unilateral complex denture model, with the point of attack in the second premolar region Tabela 3: Rezultati analize pri prevelikih obremenitvah 300 N, 500 N in 700 N enostransko kompleksne proteze z napadom v drugem premolarnem delu. Force values (N) Canine (MPa) First premolar (MPa) Attachment (MPa) 300 12.98 11.39 492.97 500 20.19 38.97 717.18 700 29.86 73.59 946.21 different behavior in function and at rest, require a through approach to the problem and a full examination of all the aspects of the situation we are to be involved in. When planning prosthetic restorations in the treatment of the unilateral free-end saddle, we are facing the presence of two different biological tissues and the need for the even distribution of the occlusal and other forces on the paradoncium tissue of the remaining teeth and in the mucoperiosteum on the edentulous alveolar ridge. It is almost impossible to achieve an absolute equal load distribution to the supporting tissues; however, the closest load distribution to these two tissues can be achieved by using the elastic bond between the free-end saddle and the retention tooth, together with the extension of the free-end saddle. An important factor in planning is to disable the movements of the free-end saddle that tend to destabilize the denture. Masticatory forces are found to be variable within the range of minimum and maximum values, they reflect activity and muscle power and are determined by the capacity of the periodontal ligament in patients with natural teeth, or by the sensory capacity of the muco- osseal fundament in patients with removable dental restorations. Literature data by various authors show variable values of the maximum masticatory force. According to Trenouth22 the maximum bite force in the molar region is within the range 500–700N, and 100–200 N in the incisal region. Tumrasvin et al.23 came up with the value of 240 N by measuring the maximum bite force of an intact tooth in the upper jaw, and 300 N in the lower jaw. The values for the bite force in the patients with a lack of three teeth were 150 N. Zeljkovic24 describes the maximum functional force in patients with fixed dental restoration as being similar to patients with preserved dentition, while in the patients with total dentures it is one-third or one-quarter less than in patients with natural dentition. Data obtained by Miyaura et al.25 pointed out the value was 500 N for an intact tooth and 300 N for a removable denture. Pellizzer et al.26 examined, using the finite-element method, the behavior of the implant-supported, partial removable denture regarding a unilateral, free-end saddle A. TODOROVI] ET EL.: STRESS ANALYSIS OF A UNILATERAL, COMPLEX, PARTIAL DENTURE ... Materiali in tehnologije / Materials and technology 44 (2010) 1, 41–47 45 St re ss , /M Pa  Figure 5: A graph of all the applied forces, their effects, and obtained values of stress in the first premolar of the unilateral complex denture model, when moving the point of attack in a distal direction Slika 5: Grafikon z vsemi obremenitvami in napetosti v prvem premolarnem zobu modela enostransko komplekne proteze pri pomiku to~ke napada v distalni smeri (a) (b) (c) Figure 3: Changes in the retention teeth and the attachment after force application of 25 N (a), 100 N (b) and 300 N (c) to the second pre- molar region, using FEM analysis Slika 3: Spremembe v retencijskem zobu in v pritrditvi pri obremenit- vah 25 N (a), 100 N (b) in 300 N (c) v drugem premolarnem prostoru Figure 4: Change in the attachment under the application of excessive load of 500 N with the point of attack in the second premolar region Slika 4: Spremembe v pritrditvi pri prevelikih obremenitvah 500 N s to~ko napada v drugem premolarnem prostoru under the application of 150 N, 210 N and 300 N forces, in the region of the first and second molars, and came to the conclusion that supporting structures showed a satisfactory behavior when the load was applied. The variety of the maximum bite-force values influenced our selection of the excessive forces (300 N, 500 N, 700 N), so that the unilateral complex denture and its constituent elements could be found under the most adverse conditions. During the process of modeling and the calculation of the unilateral complex partial denture model, apart from the geometry and the ways to support them, the mechanical characteristics of the materials used for the restorations, as well as the characteristics of the biological supporting structures must also be taken into account. The Young's modulus of the elasticity and the Poisson's ratio were used for the mentioned materials (Table 1). Each material has its elastic properties, usually expressed by the modulus of elasticity, in the domain of the elastic behavior of materials.27 The results of the FEM analysis showed that the physiological forces (25 N, 50 N, 75 N, 100 N) act primarily on the retention teeth, which accept the major part of the load, while the excessive forces (300 N, 500 N, 700 N) are accepted by the attachment. The appli- cation of the physiological forces leads to a stress reduction in the canine and the first premolar, which is confirmed by the presence of the elastic bond between the fixed and mobile portions of the unilateral complex partial denture. On the basis of the FEM analysis applied, and obser- ving the region of the retention teeth and attachment with the point of attack in the second premolar region under the applied force of 25 N, the intrusion of the first premolar was observed, as the distal edge of the retention tooth. With the increased force intensity up to 100 N, the maximum tooth intrusion and stress reduction in the retention tooth occur. Within the range 100–300 N the increase in stress in the attachment occurs (Figure 3, Table 2). Applying a force larger than 300 N leads to plastic deformation and fatigue of the attachment, which protects the retention teeth (Figure 4). Various intensities of the physiological (25–100 N) and excessive forces (300–700 N) at all points of attack cause load distribution in the retention teeth that is less than the fatigue limit (the tensile strength is far larger). The lower stress values obtained in the retention teeth and the attachment when moving the point of attack in a distal direction in the first molar region, are the results of the mesh imperfection and the differences between the first molar and premolar regions (P = F/S). By applying the 300 N force in the second premolar region, the 500 N force in the first molar region and the 700 N force in the second molar region, a linear increase in the stress of the retention teeth was observed. Physiological forces of 25 N, 50 N, 75 N and 100 N cause stress on the first premolar showing a growth tendency, when moving the point of attack in a distal direction. This indicates that the excessive force applied at the optimal point of the model causes identical behavior to the physiological load (Figure 5). 5 CONCLUSIONS Applying different load forces within the range of the minimum physiological to the maximum excessive on the unilateral complex partial denture model, and observing the stress values obtained, we came to the conclusion that the stress values obtained in the retention teeth are less than the limit ones at which plastic deformation and fatigue would occur. 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