UDK 666.3/.7 ISSN 1580-2949 Original scientific article/Izvirni znanstveni članek MTAEC9, 49(4)573(2015) PREPARATION OF POROUS CERAMIC MATERIALS BASED ON CaZrO3 PRIPRAVA POROZNE KERAMIKE NA OSNOVI CaZrO3 Edyta Sniežek, Jacek Szczerba, Ilona Jastrz^bska, Elžbieta Kleczyk, Zbigniew P^dzich AGH University of Science and Technology, Faculty of Materials Science and Ceramics, al. A. Mickiewicza 30, 30-059 Krakow, Poland esniezek@agh.edu.pl Prejem rokopisa - received: 2014-08-08; sprejem za objavo - accepted for publication: 2014-09-22 doi:10.17222/mit.2014.187 The present study was devoted to an investigation of the synthesis conditions and the influence of Sn ions on the production of a CaZrO3 porous structure. Porous ceramics based on CaZrO3 with a SnO addition were prepared by means of pressureless sintering. The study presents the influence of the type of the starting materials and the firing procedures on the microstructures of the CaZrO3 materials. Two different firing procedures were applied. The samples were obtained from pure chemical reagents CaCO3 or CaO and ZrO2. SnO was added in the mass fraction of 2 %. The prepared materials were investigated in terms of phase composition with the XRD. The microstructure was analyzed using the SEM/EDS and mercury porosimetry methods. It was found that using CaCO3 in a one-step firing process at 1650 °C with a soaking time of 10 h allowed us to obtain porous zirconate ceramics with a porosity of about 44 %. The second synthesis, where CaO was used, allowed us to obtain a porosity of about 36 %. During the firing solid solutions containing Sn ions in CaZrO3 and ZrO2 were formed. No other compounds containing Sn ions were identified. It was found that these ions played a significant role in the formation of a stable porous microstructure. The final materials mainly consisted of CaZrO3 and a small amount of ZrO2. The obtained porous CaZrO3 materials with an excellent oxidation and alkali resistance in a wide temperature range could be potential candidates for the use as membranes and filters. Keywords: calcium zirconate, porous ceramics, solid solution Ta študija je namenjena preiskavi razmer pri sintezi in vplivu ionov Sn na izdelavo porozne strukture CaZrO3. Porozna keramika na osnovi CaZrO3 z dodatkom SnO je bila pripravljena s sintranjem brez tlaka. Študija predstavlja vpliv vrste izhodnega materiala in procesa žganja na mikrostrukturo materiala CaZrO3. Uporabljena sta bila dva načina žganja. Vzorci so bili izdelani iz čistih kemijskih sestavin CaCO3 ali CaO in ZrO2. Masni delež dodanega SnO je bil w = 2 %. Fazna sestava pripravljenega materiala je bila analizirana z rentgensko difrakcijo. Mikrostruktura je bila analizirana s SEM/EDS in s porozimetrijo z živim srebrom. Ugotovljeno je, da uporaba CaCO3 v enostopenjskem postopku žarjenja 10 h na 1650 °C omogoča pridobitev porozne cirkonske keramike s poroznostjo okrog 44 %. Druga sinteza, kjer je bil uporabljen CaO, omogoča doseganje poroznosti okrog 36 %. Med žganjem je nastala trdna raztopina, ki je vsebovala ione Sn v CaZrO3 in v ZrO2. Ni bila ugotovljena nobena druga sestavina, ki bi vsebovala ione Sn. Navedeno je, da ti ioni igrajo pomembno vlogo pri nastanku stabilne porozne mikrostrukture. Končni materiali so vsebovali pretežno CaZrO3 in majhno količino ZrO2. Dobljen porozni keramični CaZrO3-material z odlično odpornostjo proti oksidaciji in alkalijam v širokem temperaturnem intervalu je lahko potencialni kandidat za uporabo v obliki membrane in filtrov. Ključne besede: kalcijev cirkonat, porozne keramike, trdna raztopina 1 INTRODUCTION gallium change the conduct of CaZrO3 and in this state it acts as a proton conductor.1-6 Suzuki et al.7 investigated porous, In-doped CaZrO3/ Zirconate materials with a perovskite structure are MgO composites with respect to the CH4-sensitivity in interesting for many engineering fields, especially for air. The samples were prepared from a high-purity natu- high-temperature structural applications. Due to their ral dolomite, ZrO2, In2O3 and LiF. To obtain porous characteristics they can be applied in the sensors, mecha- composites the samples were sintered in air at 1300 °C. nical filters or coatings used at high temperatures and in It was found that the porous composite, consisting of corrosive environments. It is interesting to obtain porous CaZrO3, MgO and CaIn2O4 (amount fraction x = 10 % of materials based on calcium zirconate (CaZrO3). In2O3), was characterized by the porosity of 57 %. A The synthesis conditions and properties of CaZrO3 higher porosity (60 %) of the samples was obtained with can be modiefied with an addition of selected ions, such an addition of x = 5 % of In2O3; these samples were as scandium, indium, gallium, yttrium, aluminum, mag- composed only of the CaZrO3 and MgO phases. The nesium, etc. CaZrO3 doped with Al2O3, Y2O3 and MgO is In-doping decreased the CH4-sensitivity in argon, but it an oxygen-ion conductor. Undoped CaZrO3 is a p-type was effective at improving the CH4-sensitivity in air.7 semiconductor used at low temperatures (< 1200 °C). The CaZrO3/MgO composites without In obtained with Moreover, trivalent cations, e.g., indium, scandium, the one-step heat treatment were also characterized by a high porosity of 30-50 % which depended on the sintering temperature.8 The method of preparing the MgO-CaZrO3-)ö--Ca2SiO4 porous materials with an interconnected porosity and a controlled size in the range of micrometers was presented in9. Dolomite-zirconia mixtures were used to obtain porous materials for refractory applications. The samples were fired in the temperature range from 800 °C to 1740 °C. After the final sintering at 1740 °C the porosity was at a significant level due to the decarbonization process associated with the loss of CO2.9 Individual properties of CaZrO3 and SnO2 may lead to an assumption that the Sn-doped CaZrO3 has a stable porous structure. Therefore, it is interesting to study the synthesis process, the influence of the Sn ions on it and the structure of CaZrO3 ceramics. 2 EXPERIMENTAL WORK CaZrO3 porous ceramics were prepared by means of a conventional solid-state reaction method. Calcium carbonate (CaCO3), zirconium dioxide (ZrO2) and tin oxide (SnO) were used as the starting raw materials. The characteristics of the starting raw materials are presented in Table 1. The firing was carried out in two ways: CaCO3 was used with the first firing method (designation: ICZSn) and CaO was used with the second one (designation IICZSn). The compositions of the materials were designed taking into account the CaCO3 or CaO to ZrO2 ratio corresponding to the CaZrO3 stoichiometry. SnO was added in the mass fraction of 2 %. The oxides were mixed together for 2 h. The homogenized mixtures were pressed into pellets (a diameter 20 mm, a thickness 10 mm) at a pressure of 70 MPa. The synthesis of the two series of the samples was carried out with pressureless sintering as shown in Figure 1. The pellets were heated up to 1650 °C with different hating rates, held at this temperature for 10 h and then cooled down in the furnace. Because CaCO3 was used in the first firing, the samples were heated at two different heating rates: 2 °C/min up to 1000 °C and 5 °C/min up to the final temperature. Table 1: Specification of the starting materials Tabela 1: Pregled izhodnih materialov Pure (%) Median particle size (^m) Reagents CaCO3 (POCH) 41.83 ZrO2 (Acros Organics) 8.5 4.53 SnO (Aldrich Chemistry) 97 23.66 The phase composition of the sintered samples was examined using the powder X-ray diffraction (XRD) technique at room temperature. The measurements were performed with a Panalytical X'Pert-Pro diffractometer using Cu-^a radiation at a 10 angle ranging from 10 ° to 90 °. The obtained data were analyzed using the X'Pert Pro Highscore Plus software. The open porosity of the sintered samples was measured using the water-displacement method based on Archimedes' principle. The pore-size distribution was analyzed with the mercury-intrusion method (Porosimeter PoreMaster 60, Quanta-chrome Instruments). A cylindrical-pore model was used for the calculation. The changes in the microstructure of the products were discussed on the basis of SEM observations (NovaNanoSem 200) accompanied by an EDS chemical analysis of micro-areas. Figure 1: Firing curves: a) ICZSn, b) IICZSn Slika 1: Krivulie žgania: a) ICZSn, b) IICZSn 3 RESULTS AND DISCUSSION Figure 2 shows the XRD analysis of the samples. The X-ray diffraction patterns of ICZSn and IICZSn indicated that CaZrO3 in the amounts of 95 % and 99 %, respectively, was the main phase. Cubic ZrO2 stabilized with calcium oxide (4 %) and monoclinic ZrO2 (1 %) were identified in ICZSn. Furthermore, when CaO was used as the starting material (IICZSn) only cubic ZrO2 (1 %) was determined. No phases containing tin were identified. This may indicate that the SnO2-ZrO2 solid solution was created in accordance with reference10. An effective ionic radius of Sn4+ (0.069 nm) is close to Zr4+ (0.072 nm) and considerably lower than Ca2+ (0.112 nm).11 It is worth mentioning that, in the Sn-O system, tin oxide is present in various forms, such as SnO, SnO2, Sn2O3, Sn3O4 and Sn5O6. Only SnO and SnO2 are stable. Above 270 °C, SnO decomposes into Sn3O4 and metallic tin in accordance with Equation (1): 4SnO ^ Sn3O4 + Sn(l) (1) V • ^ • Figure 2: X-ray diffraction patterns of the samples prepared with a solid-state reaction: a) ICZSn, b) IICZSn Slika 2: Rentgenski difrakcijski posnetek vzorcev, pripravljenih z reakcijo v trdnem: a) ICZSn, b) IICZSn Above 450 °C Sn3O4 melts incongruently into Sn and SnO2.12'13 The SEM micrographs of the samples are presented in Figures 3 to 6. The chemical compositions of the samples were confirmed with the EDS measurements (Tables 2 and 3). These allowed us to identify the most probable phase compositions of individual grains, CaZrO3 (dark grey grains - point 1) and ZrO2 (light grey phase - point 2). Figures 4 and 6 present different forms of ZrO2. In ICZSn, ZrO2 created clearly identifiable I • « i * -Jf % V % ■■■ ' t V W \ A. ^ 'i % 2 z* f.' ° •- Figure 4: SEM micrograph of the ICZSn sample with marked points where the EDS analysis was performed Slika 4: SEM-posnetek mikrostrukture vzorca ICZSn z označenima točkama, kjer je bila izvršena EDS-analiza Figure 5: SEM micrograph of the IICZSn microstructure Slika 5: SEM-posnetek mikrostrukture IICZSn Figure 3: SEM micrograph of the ICZSn microstructure Slika 3: SEM-posnetek mikrostrukture ICZSn Figure 6: SEM micrograph of the IICZSn sample with marked points where the EDS analysis was performed Slika 6: SEM-posnetek mikrostrukture vzorca IICZSn z označenima točkama, kjer je bila izvršena EDS-analiza areas. In IICZSn, ZrO2 occurred as individual inclusions. This can be explained with a higher concentration of ZrO2 in the ICZSn samples (5 %). Furthermore, it can be established that the Sn ions incorporated in CaZrO3 and Figure 7: a) Cumulative pore-volume changes and b) pore-size distribution (pore-frequency curve) of the ICZSn sample Slika 7: a) Kumulativna sprememba volumna por in b) razporeditev velikosti por (krivulja frekvence por) vzorca ICZSn ZrO2 are in different amounts. The SEM/EDS investigations confirmed the results of the XRD analysis. The average diameter of the CaZrOs grains changed from about 5-15 ^m for ICZSn to 50 for IICZSn. The grain growth was associated with the type of the used raw material (CaCOs-ICZSn or CaO-IICZSn). Moreover, when only small amounts of ZrO2 were observed in the samples (IICZSn - 1 %) the Sn ions were incorporated into the CaZrOs structure. Table 2: Average chemical compositions (EDS) of CaZrO3 and ZrO2 grains according to Figure 4 Tabela 2: Povprečna kemijska sestava (EDS) CaZrO3 in ZrO2 zrn, skladno s sliko 4 Figure 8: a) Cumulative pore-volume changes and b) pore-size distribution (pore-frequency curve) of the IICZSn sample Slika 8: a) Kumulativna sprememba volumna por in b) razporeditev velikosti por (krivulja frekvence por) vzorca IICZSn Table 4: Properties of ICZSn and IICZSn materials determined with mercury porosimetry Tabela 4: Lastnosti materialov ICZSn in IICZSn, določene s porozi-metrijo z živim srebrom Cumulative pore volume mm3/g Median pore diameter Mm Bulk density g/cm3 Porosity % ICZSn 173.6 10 2.49 43.2 IICZSn 132.3 9 2.84 37.6 Point Amount fraction, x(ICZSn)/% O Zr Sn Ca 1 53.9 23.8 0.4 21.9 2 57.9 33.6 0.7 7.8 Table 3: Average chemical compositions (EDS) of CaZrO3 and ZrO2 grains according to Figure 6 Tabela 3: Povprečna kemijska sestava (EDS) CaZrO3 in ZrO2 zrn, skladno s sliko 6 Point Amount fraction, x(IICZSn)/% O Zr Sn Ca 1 51.1 24.5 0.9 23.5 2 62.2 29.9 0.2 7.7 The total pore volume and the median pore diameter of the samples analyzed with the mercury-intrusion method are shown in Figures 7 and 8 and summarized in Table 4. The figures show slightly different types of curves but having the same mean pore size. The ICZSn sample (Figure 7) was characterized by one main pore population of about 10 ^m in diameter. Two other populations were also distinctly detectable: of less than 0.1 ^m and of about 100 ^m. In contrast, the IICZSn sample had a very narrow pore-size distribution with the mean size of 9 ^m (Figure 8). This proved that, in this case, the pores were more uniform and monomodal. This difference may have resulted from the CaCO3 decarbo-nization which occurred, as generally known, in the 600-950 °C temperature range. During the heating, CaCOs decomposed into CaO (solid) and CO2 (gas) according to Equation (2). The formed CaO reacted with ZrO2 creating CaZrOs. Moreover, a CaZrOs structure with incorporated Sn ions can reveal unique electrical properties. CaCOs ^ CaO + CO2 f (2) Acknowledgement The porosity measured in accordance with Archimedes' principle varied from 44 % to 35 % for ICZSn and IICZSn, respectively, being in good agreement with the results of the mercury-intrusion analysis. These porous materials, being comprised of CaZrOs and a small amount of ZrO2 have an excellent oxidation and alkali resistance in a wide temperature range. The presented materials can be used as filters, membranes and insulation materials. Moreover, an incorporation of Sn ions in the CaZrOs and ZrO2 structure can lead to obtaining unique electrical properties. 4 CONCLUSIONS The article focuses on porous CaZrOs materials with SnO additions. The presented results describe the influence of the starting raw materials and the firing procedure on the final properties of the CaZrOs ceramics. It is shown that using CaCOs, in comparison with CaO, allowed us to obtain a material with the porosity exceeding 40 %. 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