ULTRA THIN DEPOSITED AND SEGREGATED FILMS ULTRA TANKE NANESENE IN SEGREGIRANE PLASTI MONIKA JENKO Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenja Prejem rokopisa - received: 1997-10-01; sprejem za objavo - accepted for publication: 1997-10-21 Dedicated to Prof. Jože Gasperič at the occasion of his 65th birthday Research, development and use of vacuum thin films started at the Institute for Electronics and Vacuum Techniques, IEVT, Ljubljana after its foundation around 1950. With the development of the miniature thin film potentiometers and thin films resistors of high stability at IEVT the first production of thin film electronic components in former Yugoslavia was established. The technology vvas successfully transferred to Slovenian foreign factory in Cormons Italy where Slovenian minority is living. At the same time the high tech research and development of the second and third generation image intensifier tubes was started. Prof. Dr. Jože Gasperič was one of the leading scientists investigating the sputtered cermet thin films in his BS and Ph.D. works. His findings are basic for understanding the mechanism of grovvth of thin sputtered cermet films. In the middle of eighties an experimental ultra high vacuum, UHV, system equipped vvith Auger spectrometer, vvas built at IEVT and the investigation of physical and chemical processes of ultra thin oxide film grovvth on the surface of liquid indium and indium alIoys in situ vvas achieved for the first time. The experimental method based on Auger Electron Spectroscopy for in situ investigation of the initial phase of the ultra thin oxide film grovvth on liquid indium, vvas developed at the institute. The modified experimental method vvas also used for the study of ultra thin segregated Sb, Sn or Se films on the surface of FeSiC alloy, microalloyed by Sb, Sn or Se. The investigation of ultra thin Sn. Sb or Se films on well defined Fe surfaces is the topic of our present research vvhich is close connected vvith the purchase of new AES/XPS instrument vvith the very high spatial resolution. The results of investigations of ultra thin oxide films on the liquid indium and InSn alloy as well as Sb and Sn ultra thin film grovvth on the surface of FeSiC alloy vvere presented. Key vvords: ultra thin films, deposited films, segregated films, surface segregation, oxidation, dissociation Prve raziskave, razvoj in uporaba vakuumskih tankih plasti so se začele na Inštitutu za elektroniko in vakuumsko tehniko v Ljubljani takoj po ustanovitvi, okrog leta 1950. Z razvojem miniaturnega tankoplastnega potenciometra MP in stabilnih tankoplastnih miniaturnih uporov se je pričela na IEVT tudi prva proizvodnja tankoplastnih elektronskih komponent v takratni Jugoslaviji. Tehnologija izdelave MP je bila uspešno prenesena v novo ustanovljeno slovensko zamejsko tovarno v Krminu v Italiji. Obenem pa so se vršile raziskave in razvoj specialnih fotoelektronk. Sb fotokatod druge in tretje generacije za slikovne ojačevalnike z bližinskim prenosom slike, ki je predstavljal "high-tech" v svetovnem merilu. Vakuumisti IEVT so s svojim znanjem sodelovali v svetovno znanih institucijah kot npr.: SAES Geters, Heimann, Leybold Heraeus. Skupne rezultate so že takrat objavili v tuji znanstveni periodiki. Z raziskavami vakuumskih tankih plasti se je na IEVT intenzivno ukvarjal tudi prof. dr. Jože Gasperič, ki je problematiko kermetnih napršenih plasti obdelal v svojem magistrskem in nato še v doktorskem delu. Njegova temeljna spoznanja so pomembeno prispevala k razumevanju mehanizma rasti tankih napršenih plasti. Z izgradnjo eksperimentalnega ultravisoko vakuumskega sistema, opremljenega s spektrometrom Augerjevih elektronov na IEVT v sredini osemdesetih let pa je bila dana možnost raziskav fizikalno kemijskih procesov pri nastanku ultra tankih oksidnih plasti na površini tekočih kovin in zlitin "in situ". Tovrstne raziskave so bile izvedene prvič. Načrtovana in osvojena je bila nova eksperimentalna metoda, ki je omogočila študij začetnih stopenj rasti oksidnih plasti na tekočem indiju, ki smo ga tudi naparili in situ v spektrometru Augerjevih elektronov. Metoda je bila uporabljena tudi za študij ultra tankih segregiranih plasti Sb, Sn in Se na površini zlitin Fe-Si-C, mikrolegiranih z Sb, Sn ali Se. Študij ultra tankih plasti Sb, Sn, Se na dobro definiranih površinah Fe in V pa je predmet naših sedanjih raziskav, ki so vezane na nabavo novega AES/XPS instrumenta z visoko lateralno ločljivostjo. Prikazani so rezultati raziskav ultra tankih oksidnih plasti na tekočem indiju in zlitini InSn ter ultra tankih segregiranih Sb in Sn plasti na zlitini FeSiC. Ključne besede: ultra tanke plasti, oksidacija, disociacija, In203, ln20, Sb, Sn, površinska segregacija, AES, XPS INTRODUCTION Advanced technologies are strongly dependent on products of electronic industries such as optoelectronics, sensors, high density integrated components, etc. Thin films of thicknesses from ten to several hundreds of nanometers are very important in the production of these products. Ultrathin films-UTF vvhose thickness is up to fevv monoatomic layers are very decesive in the segregation, corrosion, recrystallization and catalytic processes. UTF are influenced by their interaction vvith the substrate and open a completly nevv perspective in the development of nevv advanced materials vvith desired physical and chemical properties. Research, development and use of thin films started at the Institute for Electronics and Vacuum Techniques, IEVT, Ljubljana Slovenia after its foundation in early fif-ties. With the development of the miniature thin film potentiometers and thin films resistors of high stability at IEVT the first production of thin film electronic components in the country vvas established. The technology vvas successfully transferred to Slovenian factory in Cor-mons-Italy vvhere the Slovenian minority is living. Prof. Jože Gasperič vvas one of the leading scientists investigating the sputtered cermet thin films in his BS and Ph.D. vvorks. His findings are basic for understanding the mechanism of grovvth of thin sputtered cermet films. In the middle eighties the "high tech" research and development of the second and third generation image intensifier tubes vvas started at IEVT. An experimental ultra high vacuum, UHV, system equipped vvith Auger spectrometer, vvas built and the author vvas the first to investigate in situ the physical and chemical processes of ultra thin oxide film grovvth on the surface of liquid in-dium and indium alloys. The experimental method based on Auger Electron Spectroscopy for the in situ investigation of the initial phase of ultra thin oxide film grovvth on liquid indium, vvas developed at the IEVT institute. The simplified ex-perimental method vvas used for the investigation of seg-regated Sb, Sn or Se UTF on the surface of FeSiC alloy, microalloyed vvith Sb, Sn or Se at the Institute of Metals and Technology Ljubljana, vvhere the first studies of P segregation started already in 1962. The investigation of Sn, Sb or Se UTF on vvell de-fined Fe surfaces is being the topic of our present research vvork vvhich is close connected vvith the purchase of nevv AES/XPS device vvith the very high spatial reso-lution. The results of investigations of ultra thin oxide films on liquid In and InSn al!oy as vvell as Sb, Sn and Se ultra thin film grovvth on the surface of FeSiC alloy are presented. 1 AES INVESTIGATION OF INITIAL PHASE OF LIQUID In AND InSn ALLOY OXIDATION AND DISSOCIATION OF I112O3 1.1 Fluxless vacuum soldering In the production of the third generation image intensifier tubes the fluxless vacuum soldering is the most important process for hermetic encapsulation. Extremely clean surfaces are indispensable for obtaining good vvetting of liquid solder. In the first model experiments we found that the leakage of fluxless vacuum soldered seals may often be caused by thin oxide film, covering the liq-uid solder. The solder is lovv melting InBi or InSn alloy. For the basic investigation vve used pure indium as a solder and vve realized that only in situ surface characterization may give direct insight into these complex surface phenomena. For this purpose a very sensitive experimental method based on AES vvas developed at IEVT Ljubljana. The initial phase of surface oxidation on high purity in situ deposited indium and the mechanism of cleaning process of oxidised indium surface ln203 vvas the main goal of the investigation. 1.2 Experimental method for the investigation of surface phenomena at fluxless vacuum soldering The experiments were performed in an adapted Scanning Auger Electron Microprobe, additionally equipped vvith addapted sample holder, heater and thermocouple, evaporation source for in situ deposition of high purity indium, quartz microbalance for determination of In thin film thickness, quadrupole mass spectrometer-QMS for residual gas analysis and precise metal valve for oxygen introduction. Especially designed connections enabled the movement of the sample, figure 1. The sample high purity indium thin film vvas deposited in situ on a pure molybdenum substrate. The oxide ln203 film vvas obtained by exposure of the indium surface to pure oxygen at constant pressure 5x10"5 mbar vvith oxygen time exposure up to 100 minutes in temperature range from 25 to 250°C. The resistive heater for investigating In/In203 film vvas a Mo strip vvhich could be heated up to 1000°C. A thermocouple Fe-CuNi vvas vvelded on the rear side of the Mo strip. The indium source for in situ deposition vvas prepared from indium metal of 6N purity. 1.3 AES studies of the initial phase of liquid indium oxi-dation The surface oxidation of indium in bulk and thin film occurs as (1): 4 In (1) + 3 02 (g) ^ 2 ln203 (c) (1) vvhere (c), (1) and (g) mean crystalline, liquid and gas state, respectively. The solubility of oxygen in pure liq-uid indium is extremely lovv, lxl0~6 atomic percent at 550°C and it is negligible in the temperature range of our investigation from 25 to 250°C. The diffusivity of Figure 1: AES spectrometer adapted for liquid indium surface investigations: 1 - sample, 2 - Thermocouple, 3 - sample holder, 4 -flexible connections, 5 - rigid connections, 6 - In source for in situ evaporation, 8 - flange, 9 - CMA, 10 - ion gun, 11 - QMS, 12 - lead through, 13 - quartz microbalance, 14 - metal valve for oxygen introduction Slika 1: Spektrometer Augerjevih elektronov prirejen za raziskave procesov na tekočem indiju: 1 - vzorec, 2 - termočlen, 3 - nosilec vzorca, 4 - gibljivi priključki, 5 fiksni priključki, 6 - In izvir za "in situ" naparevanje, 8 - prirobnica, 9 - CMA, 10 - ionska puška, 11 - QMS, 12 - prevodnice, 13 - kremenova mikrotehtnica, 14 - vpustni ventil za kisik c .—> O cr 0. 0,6 0,4 0,2 _^250°C f / / / / -/ / i/ 150 °C 1 / f 25 °C 1 1 1 1 1 10 15 Time (min) 20 25 30 Figure 2: Surface oxidation rate of crystalline and liquid indium at temperatures 25, 150 in 250°C at a constant oxygen pressure of 5xl0"5 mbar Slika 2: površinska oksidacija trdnega in tekočega indija pri temperaturah 25, 150 in 250 °C in konstantnem tlaku kisika 5x10"5 mbar. oxygen in liquid indium is very low, 3.7xl0~7 cm2 s"1 at 550°C. It was concluded from these facts that the process of pure Iiquid indium oxidation occurs at the surface. The kinetics of thin oxide film growth on crystalline and liquid indium was investigated by AES, following the peak height ratio-PHR of amplitudes between O(KLL) and In (M5N45N45) Auger transition at kinetic electron energies of 512 eV and 402 eV (for In0) and 405 eV (for In3+) respectively. For a defined geometrical sample position in the AES spectrometer, the kinetics of the thin oxide film growth was followed up to the film thick-ness of 3.5 nm, vvhich corresponds to an effective electron depth escape for ln203. Surface indium oxidation vvas investigated at the temperatures of 25, 150, 250 in 550°C, figure 2. Thin oxide film thickness of 3.5 nm on pure indium vvere obtained at conditions listed in Table 1. Table 1: ln203 films, 3.5 nm thick obtained at different temperatures and different oxygen exposures Temperature (°C) Oxygen exposure (L) 25 6x104 250 3xl04 550 1.5x104 At higher temperatures T > 360°C, a volatile oxide was formed by the reaction: ln203 (c) + 4 In (1) <-» 3 ln20 (g) (2) The reaction betvveen the ln203 thin film and the un-derlying liquid indium, corresponding to equation (2) was studied at the temperature of 360, 400, 450 and 550°C in vacuum of lxl0"9 mbar. The results are shovvn in figure 3. OC ZC O. Figure 3: Isothermal dissociation of ln203 ultra thin solid film on liquid indium at the temperatures 360, 400, 450 in 550°C in a vacuum below lxl0"9 mbar Slika 3: Izotermna disociacija ultra tanke plasti ln203 na tekočem indiju pri temperaturah 360, 400, 450 in 550 °C in vakuumu pri tlaku < lxl0~9 mbar. 1.4 Investigation of huOj dissociation in uhv by AES At temperature T > 360°C the evaporation of volatile ln20 corresponding to equation (2) proceeded vvith per-ceivable velocity. At 550°C the process vvas so fast that AES studies of ln203 to In vvere not possible. The use of indium as a solder for vacuum fluxless soldering depends upon the fact that the thin ln203 film spontaneously disappeared - dissociated at T > 360°C by ln20 evaporation follovving the equation (2) ln203 (c) + 4 In (I) o 3 ln20 (g). 1.5 AES investigation of initial phases oxidation of InSn liquid solder In the second part of our investigation the knovvledge of the initial phase of surface oxidation on pure liquid In to InSn solder (20 at.% In, 80 at% Sn) vvas applied. The samples high purity InSn film, 2.5 jim thick vvere prepared in Balzers Sputron plasma beam apparatus, figure 4. On the oxidised InSn surface a difference betvveen AES spectra of pure metal In (402, 408 eV) and ln203 (399, 405 eV) vvas found. For tin (428, 435 eV) the char-acteristic chemical shift vvas not determined and only changes in the shape and intensity of Auger spectra vvere recognized. Figure 5 shovvs the AES spectra of the liquid InSn alloy exposed to pure oxygen 1.5x104 L, covered by thin oxide film, approximately 3.5 nm thick (figure 6), corresponding to an effective electron depth escape A^f for ln203. Figure 4: AES spectrum of InSn solder (20 at.% In and 80 at% Sn) after ion etching approximately 3 nm under surface Slika 4: AES spekter InSn spajke(20 at.% In and 80 at% Sn) po ionskem jedkanju približno 3nm pod površino The samples were oxidised in situ in Auger Spectrometer by exposure of a clean surface of Iiquid InSn alloy to pure oxygen (104 - 10" L) at 250°C. The results indicate that ali oxide films on the surface of liquid InSn solder vvere enriched in indium in varying amounts, depending on oxygen pressure, time exposure etc. In the first part of the investigation it was found that the surface oxidation of In, in bulk or in the form of a thin film can be formulated by equation (1): 4 In (1) + 3 02 (g) 2 ln203 (c) (1) Figure S: AES spectrum of 3.5 nm thick oxide UTF on the surface of liquid InSn alloy Slika 5: AES spekter 3.5 nm debele oksidne plasti na površini tekoče spajke InSn. where (c), (1) and (g) mean crystalline, liquid and gas state, respectively. The surface oxidation of Sn, in bulk or in thin film, occurs by the reactions: 2 Sn (1) + 02 (g) <-» 2 SnO (c) (3) Sn (1) + 02 (g) <-> SnO, (c) (4) AES chemical shifts of SnO and Sn02 are approxi-mately the same. On tin exposed to oxygen (104 - 106 L) both oxides SnO and Sn02 were found. As the same occurs for SnO and Sn02, chemical shift can not be used to identify the oxidized state of the tin film. It vvas pro-posed that a mixture of oxides SnO, Sn02 and ln203 vvas formed on InSn al!oy at the exposure to oxygen (104 -106 L). The mixture of oxides appeared to be thermody-namically unstable near the alloy-oxide interface; SnO and Sn02 oxides vvere reduced to Sn vvith the tendence to the formation of additional ln203 3SnO(oxidcc) + 2In(alloy]) <-> In203(oxidec) + 3Sn(a]loyl) (5) 3Sn02(oxide c) + 4In(alloyl) 2In203(oxide c) + 3Sn(alloyl) (6) Free energy AG" of reactions (5) and (6) are obtained by using the data for free energy of formation SnO, Sn02 and ln203 AG°(SnO) = = -69670 + 3.06 TlogT - 1.5xl0'3T2 - 0. + 18.39T (7) AG°(SnOj)= -143080 - 7.37 TlogT - - 0.7x10'3T2 + 2.38xl05T'+76.53T (8) AG°(In203)= -220970 + 24.22TlogT - 3xl0"3T2 - 0.3xl05T' + 41.36T (9) Figure 6: AES depth profile of oxide UTF on surface of liquid InSn solder. Oxide UTF was made in situ Slika 6: AES profilni diagram ultra tanke oksidne plasti na tekoči spajki InSn; oksidna plast je bila narejena in situ At the temperature 250°C AG° (5) = -525 kJmol1 and (6) = -285 kJmol"1 and the equilibrium constants: K(S) = (aSn3 x aIn203)/(asn03 * aIn2) = 2.2x1052 (10) K(6) = (as„3 x aIn2032)/(aSn03 x aIn4) = 2.8xl028 (11) where ai s are the activities of the reactants and products. The mixed oxide thin film formed during oxidation vvould be a mixture of pure I^Oi, S nO and Sn02. The driving force for reactions (5) and (6) is therefore AG(5) = AG" + RT ln[(l - NIn)3/N2,J (12) AG(6) = AG° + RT ln[( 1 - NIn)~VN4In] (13) Nin is the indium concentration in the Snln alIoy. At 250°C AG for Ni„ 10'6 (Ippm) is -108 kJmol'1 for reac-tion (5) and -351 kJmok1 for reaction (6). In other words, SnO and Sn02 are thermodynami-cally unstable are in the mixed oxide even when in contact vvith extremely dilute In in Sn. The only stable oxide formed at InSn alloy should be ln203 at 250°C. Since ali oxidation processes are of the nonequilibrium type, the amounts of SnO and Sn02 and the overall SnO, Sn02/In203 ratio depend on 02 pressure, temperature, diffusion coefficient, solubilities and other factors. Con-sequently the mixed oxides SnO and Sn02 in the oxide-alloy interface tend to be converted into Sn and ln203. 1.6 AES investigation of Itn03 dissociation on the surface ofliquid InSn solder in UHV The last part of fluxless vacuum soldering in UHV investigation dealt vvith the "cleaning process" by dissociation of thin ln203 film from the surface of the liquid Figure 1-. Changes in AES peaks of In, Sn and O betvveen heating of oxidized InSn solder at constant temperature of 450°C Slika 7: Spremembe AES vrhov značilnih za In, Sn in O med segrevanjem oksidirane spajke InSn pri konstantni temperaturi 450°C InSn solder. The reaction betvveen the thin ln203 film and the underlying liquid InSn solder correspond to equation 4 In (1) + 3 02 (g) 2 ln203 (c) (1) vvas investigated at 550°C and the results are shovvn in figure 7. It vvas found that at T>250°C in UHV the thin In203 film formed on the surface of liquid InSn alloy spontaneously disappeared by In203 dissociation according to equation (1). The oxides SnO and Sn02 or In203 are unstable in very thin oxide films on the surface of a liquid InSn solder at T>250°C. For fluxless vacuum vacuum soldering vvith liquid indium, previously cleaned at T > 360°C, recontamination is negligible in UHV in the temperature range: Tm < T < 360°C At fluxless vacuum soldering vvith liquid InSn solder, previously cleaned at T > 360°C, recontamination is negligible in UHV in the temperature range Tm < T < 360°C. 2 CHARACTERIZATION OF SEGREGATED Sb AND Sn UTF BY AES 2.1 Introduction Physical properties of metals and alloys depend on the composition and on the surface and interface structure of the material. These properties are affected by seg-regation processes of alloying elements and impurity elements during their manufacturing and use. Some of these elements vvhich segregates on free surfaces (surfaces, grain boundaries, interfaces) and ultra thin segregated films specifically affect adsorption, corrosion, adhesion, surface diffusion, recrystallization, catalytic activity, friction and vvear7 8. The atomic composition of grain boundaries is also very important because it affects physical properties as vvell as corrosion behaviour of metals and alloys. For materials applied at high temperatures, the composition of interfaces may be drastically changed by segregation, by enrichment of dissolved surface active atoms diffusing on the surface or grain boundaries and can cause metal embrittlement. The aim of the investigation vvas to examine the na-ture of segregation of antimony and tin and its effect on recrystallization process, grain grovvth and texture devel-opment of a cold rolled and annealed silicon non ori-ented electrical sheet to be used in generation of electrical energy. It has been experimentally confirmed that a small addition of antimony, tin and selenium into the melt of silicon iron by microalloying affect the magnetic properties of electrical sheets by enrichment on free surfaces, i.e. surfaces and grain boundaries91315"19. Such enrichment affects grain grovvth, producing an increase in the number of ferrite grains vvith soft magnetic lattice orientation vvhich grovv on the account of grains vvith other crystallographic orientations, and in this way improves the magnetic properties. Our investi-gations show a strong correlation between antimony and tin surface segregation and the orientation of surface grains17"19. The kinetics of surface segregation is determined by bulk diffusion of the segregate to the respective interface. Surface segregation kinetics was measured on non ori-ented silicon steel sheet in situ by Auger Electron Spec-troscopy-AES, by annealing and simultaneous analysis of the sputter cleaned sample at higher temperature in UHV. Non oriented silicon steel is a polycrystalline multi-component alloy of Fe, Si, Al, C, P and S. The addition of approximately 0.05 to 0.1% of single elements i.e. an-timony or tin starts a competition for free surface sites7'25. The grain boundary segregation of Sb and Sn was studied initially on polycrystalline non oriented silicon steel. The alloys were annealed in the temperature range betvveen 450 and 650°C to establish the equilibrium grain boundary segregation on samples quenched and mounted into UHV system, fractured in situ after cooling and analysed by AES. The main point of the research work was the determination of the physical nature of the surface segregation and its relationship to the internal grain boundary segregation grains at the sheet surface. The investigation also included the determination and evolution of the texture of grains at the sheet surface. Emphasis vvas placed on the understanding hovv texture affects the electrical en-ergy losses, as vvell as magnetic properties. 2.2 Surface segregation of antimony and tin Most elements dissolved in iron tend to enrich at ele-vated temperatures at surfaces, grain boundaries and in-terfaces and distribution equilibrium Ajissoivcd <-> A segregated are established at sufficiently high temperature7-25'26 34. 2.2.1 Antimonv Antimony surface segregation vvas studied using AES method on a 2.0% Si steel, alloyed vvith different mass contents of Sb (0.05 and 0.1%). The dependence of surface segregation on grain orientation and on the presence of other solute atoms of S, P, C, Al and Si vvas investigated in situ under UHV conditions by AES in the temperature range from 450 to 900°C. The antimony enrichment at the surface vvas esti-mated by follovving the peak height ratio - PHR of ara-plitudes betvveen the dominant SKMsN^N^s) and Fe(L Mi,3V) Auger transitions at kinetic energies of 454 and 651 eV, respectively. The mole fraction of Sb 0.05% and 0.1% in investigated steels, is clearly in the range of solubility in a-Fe at ali temperatures investigated but belovv the detection limit of AES method. The enrichment of Sb, caused by equilibrium segregation at the surface, can only be measured at or after annealing at elevated temperatures by AES method. AES measurements shovved a different quantity of segregated antimony on different grains (figure 8). Ali samples vvere metalographically polished, the orientation of single grains vvas determined by the etch pitting method. The temperature dependence of antimony surface segregation a) on grain vvith (001) and b) (111) orientation is shovvn on figure 9. The surface of investigated steel alloyed vvith 0.1% Sb vvas clean sputtered vvith Ar+ ions and in situ annealed in the analyzing chamber of the Auger spectrometer. The temperature vvas increased every 20 minutes for 50°C. The antimony segregation rate vvas perceived at temperature T > 650°C and it increased vvith the increasing temperature, vvhile at T > 850°C the antimony segregation rate declined. If the influence of a possible channelling effect is neglected, it is possible to estimate the Sb surface concentration by comparison vvith the results on Sb surface segregation on single crvstal surfaces of Fe - 4% Sb of defined orientation. For the same primary energy of exciting electrons, the saturation PHR vvere measured for single crystal surfaces of (100), (110) and (111) orientation. For the (100) oriented surface, the saturation coverage is half of a monolayer corresponding to LEED c(2x2) overlay pattern. For other surface orientations, no vvell defined ordered structure of surface coverage vvas observed. The PHR vvas of the same order; for (111) oriented grain 0.6, and for (100) oriented grain 0.4. In the investigated steels other solute elements such as C, S and P are present. Tvvo or more elements can simultaneously segregate to the surface. In such cases a competition for the sites available occurs24-32. The relative amount of seg-regating elements on the surface depends on their free energy of segregation and their concentration in the bulk29'32'347"37. Also the kinetics of antimony surface segregation vvas measured (figure 10). There are tvvo possible explanations for this effect, simultaneous antimony and sulphur segregation and competition for sites available on the surface, and/or desorp-tion from the segregated layer. Figure 8: SAM image of the surface of steel vvith 0.05% Sb. A different quantity of segregated antimony vvas measured on grains vvith different orientation in the plane of the sheet Slika 8: SAM posnetek površine jekla legiranega z 0.05%Sb. Površinska segregacija Sb je odvisna od kristalografske orientacije zrn. Slika 9: Temperaturna odvisnost površinske segregacije na jeklu z 0.1 %Sb a) na zrnu (100), b) na zrnu (111) Figure 10: Sb surface segregation on steel with 0.1% Sb on a) (100) and b) (111) oriented grain at 800 and 850°C Slika 10: Površinska segregacija Sb na jeklu z 0.1% Sb a) (100) zrno in b) (111) zrno pri 800 in 850 °C The competitive surface segregation of antimony and QSb _ sulphur is described by the follovving equations25: i_©s_esb - *Sb exP(-A 750°C. 2.2.2 Tin A scanning Auger image - SAM of non-oriented elec-trical steel annealed 10 minutes at 800°C was taken. The orientation of individual grains vvas determined by the etch pit method. Figure 11 shovvs SEM and SAM im-ages of the surface of the investigated steel. A different surface tin segregation rate on different grains vvas measured. Different grain orientation provided different sites for segregated tin atoms. Figure 12 shovvs the temperature dependence of surface segregation of alloying and tramp elements of non-oriented electrical steel alloyed vvith 0.05% Sn on grain orientations (001) and (111) respectively. Electrical steel is a multicomponent system vvith a very complicated temperature dependence behaviour of surface segregation on binary alloys. The relations of the surface segregation enthalpies and volume diffusivitiy are as follovvs: AH"Si < AH"C < AH"P and Dcv » DSiv > Dpv At lovver temperature, about 300°C, C segregated to the surface due to very high diffusion coefficient in com-parison to Si and P, although the bulk concentration vvas very lovv, only 15 ppm. At higher temperature, C atoms vvere displaced by Si atoms and P vvhile S atoms dis-placed silicon at higher temperatures. Their bulk diffusion coefficient is rather lovv, but their segregation en-thalpy is very high, therefore tin started segregating significantly above 600°C. Kinetics study confirmed the orientation dependence of tin surface segregation and of the thickness of the segregated layer. It vvas ascertained17 that on grains vvith (100) and (111) orientation in the sheet plane, the segregation of tin vvas beyond one monolayer, due to the strong decrease of surface energy. On a surface vvith a (111) orientation FeSn intermetallic compound of one unit celi thickness vvas found. Our measurements shovved that tin surface coverage dependence on tin bulk concentration and 0 value approached one for (100) and (111) orientations. 2.3 Grain boundary segregation 2.3.1 Antimonv Grain boundaries of FeSi steel alloyed vvith 0.05 and 0.1% Sb vvere also analyzed by AES after ageing for 200 and 500 hours at 550°C. The fracture facets vvere almost completely transgranular, only on some areas intergranu-lar decohesion vvas noticed. In the investigated alloys there vvas no indication of antimony grain boundary segregation (figure 13). Only a negligible grain boundary segregation of other solute elements such as carbon, silicon and aluminium vvas established. Figure 12: Temperature dependence of surface segregation on steel with 0.1% Sn on a) (100) and b) (111) oriented grains Slika 12: Temperaturna odvisnost površinske segregacije na jeklu z 0.1 %Sn a) (100) in b) (111) orientirana zrna. Figure 13: AES spectra taken on a) transgranular facets and SEM image of fractured sample and b) an intergranular facet of non oriented silicon steel with 0.1% Sb and SEM image of transgranular facet Slika 13: AES spekter posnet na a) transkristalni ploskvi in SEM posnetek prelomljenega vzorca in b) interkristalni prelom silicijevega jekla za neorientirano pločevino z 0.1 %Sb 111111 'I " 11!11'1!" 1 700 900 Kmetic / eV .................I'" S 00 700 900 Kinetic energy / eV b) SAMPLE B K)00'C 24N.A,r . 550*C.500h H,0 - 170 min a/lef (focluff SAMPLE B K)00"C . 24h .Air • 550»C. SOOh . H,0 -15 min olter fracture Figure 14: XPS spectra taken on intergranular facet and SEM image of fractured non oriented silicon steel vvith 0.1% Sn Slika 14: XPS spekter posnet na interkristalni ploskvi in SEM posnetek prelomljenega vzorca silicijevega jekla za neorientirano elektro pločevino Figure 15: Pole figure of (200) grains obtained by X-ray difractometry shows the share of grains with (001)(100) texture in the steel sheet vvith 0.05% Sb Slika 15: Polove figure zrn (200) narejene z rentgensko difraktometrijo prikazujejo delež zrn (001)(100) v teksturi elektro pločevine z 0.05% Sb. Figure 16: Texture fibres in the middle plane a) and b) on the surface of electrical steel vvith 0.05% Sn and vvithout Sn Slika 16: Tekstura vlaken v sredini vzorca a) in b) na površini elektro pločevine z 0.05% Sn in brez Sn. 2.3.2 Tin The equilibrium segregation of tin was attained after annealing the specimen alloyed vvith 0.1% Sn for 200 hours at 550°C, figure 14. Considering that tin is equally distributed on both fractured sides, a coverage of grain boundary surface of 0.05 ML vvas estimated. The scattering of results vvas rather large due to the strong dependence of tin segregation to grain boundary orientation. Steel alloyed vvith 0.05% Sn had much less intergranular facets. The evalu-ated equilibrium segregation vvas smaller than in steel al-loyed vvith 0.1% Sn. Detailed AES analyses of free sur-faces betvveen inclusion (A1N, A1203) and matrix clearly indicated that a considerable tin segregation occurs also at their interfaces. 2.4 Texture determination 2.4.1 Antimonv The results support the hypothesis that the texture formation results from orientation dependent effects of antimony and tin on the surface energy. The texture of sheets from antimony and antimony free steels vvas de-termined by X-ray difractometry. The pole figure ob- tained (figure 15), shovvs that only a small share of grains vvith (001)(100) texture vvas found in the investigated 0.05% Sb steel. 2.4.2 Tin The results support the hypothesis that the texture formation results from orientation dependent effects of tin on surface energy. The texture vvas measured on the surface and in internal plane after the half of the sheet thickness vvas removed. Taking into account that ap-proximately six crystal grains constitute the 0.5 mm thick cross-section steel sheet and the fact that penetra-tion depths of X-rays vvas less than 0.01 mm one might conclude that also some grains whose grovvth vvas not af-fected by the surface tin segregation vvere analysed. Nev-ertheless, there vvere not more than 10% of such grains. The orientation distribution functions (ODF) f (g) vvere calculated from the (200), (110) and (211) pole figures. The textures vvere presented as a, y and r| fibres. Figure 16 shovvs texture fibres in the internal plane (a) and on the surface (b) of electrical steels alloyed vvith and vvithout tin. The volume fraction of grains vvith the (100) planeš measured on the surface and in the middle plane increased by the order of tvvo vvhen compared the (nO> (ml (m} (00i> x 0 •/. Sn . o 0.05 V.Sn a- fibre 0* 30* 60* 90» y-(ibre steel vvithout tin and the steel vvith 0.05% tin. Softer magnetic orientations vvere found on the surface. Steel vvith 0.05% Sn, vvhich had previously been aged 25 hours at 550°C, shovved an inerease of (100) planeš par-allel to the roiling direetion by three times, compared to the steel vvithout tin.36 During annealing antimony and tin segregated at steel surface at temperatures T > 650°C. A strong correlation betvveen the antimony or tin surface segregation and the orientation of the grains at the sheet surface vvas estab-lished. The maximum equilibrium Sb surface segregation of 0.6 monolayer vvas measured after annealing at 750°C on grains vvith the (111) crystallographic orientation in the sheet plane. The maximum equilibrium Sn segregation on the surface vvas reached also at 750°C and ap-proached in the majority of orientations one monolayer. Grain boundary segregation of antimony and other solute elements, sueh as C, S, P, Al and Si vvere negligi-ble in non oriented silicon steel vvith 0.05 and 0.1% Sb. Tin surface segregation vvas much higher than grain boundary segregation. At equilibrium grain boundary segregation only 7 and 3% of tin atoms vvere found on a grain boundary for steel alloyed vvith 0.1 and 0.05% Sn, respectively. Antimony as vvell as tin surface segregation de-creased the surface energy of grains vvith (100) surface orientation in the sheet plane, and these grains grovv on account of grains vvith other surface crystallographic orientations, sueh as (110) and (111). Only a certain level of surface segregation promoted selective grain grovvth. By excessive surface coverage of segregated atoms, the surface energy of ali orientations is not affected selective^ and no preferential grain grovvth is obtained. Textures represented as sections through three-di-mensional orientation distribution space in fixed direc-tions shovved that the volume fraction of magnetically soft grains inereased for three times in tin steel sheets vvhen compared to steel vvithout tin. Better textures vvere obtained near the surface than in the middle plane of 0.5 mm thick steel sheet. The best results vvere obtained for steel alloyed vvith 0.05% Sn. It is concluded that only a certain level of segregation promoted the desired selective grain grovvth. The results of the present investigation support the hypothesis that the texture formation results from orientation dependent effects of antimony and tin on the surface energy. 3 REFERENCES 11, p. Csorba, Image Tubes, Hovvard W. 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