Yi-Cheng Liou*, Chung-Che Lan, Song-Ling Yang

1. Materials and Austceram 2007 July 4 - 6, 2007, Sydney , Australia PREPARATION OF ZINC-DOPED LANTHANUM STRONTIUM GALLATE SOLID ELECTROLYTE USING A REACTION-SINTERING PROCESS Yi-Cheng Liou*, Chung-Che Lan, Song-Ling Yang Department of Electronics Engineering, KunShan University, Tainan Hsien 71003, Taiwan, R.O.C. *Corresponding author. ycliou@mail.ksu.edu.tw La0.8Sr0.2Ga0.8Zn0.2O2.8(LSGZ) ceramics prepared using a reaction-sintering process was investigated in this study. Without any calcination involved, the mixture of raw materials was pressed and sintered directly. SrLaGa3O7 is the major phase in 1150C sintered pellets and decreased at higher sintering temperature 1250C. La0.8Sr0.2Ga0.8Zn0.2O2.8 became the major phase in pellets sintered at 1250oC/2 h.Density of La0.8Sr0.2Ga0.8Zn0.2O2.8 ceramics increases with sintering temperature and reaches a maximum 6.71 g/cm3 at 1230oC for 6 h.The reaction-sintering process has proven a simple and effective method in preparing La0.8Sr0.2Ga0.8Zn0.2O2.8 ceramics for solid electrolyte applications in solid oxide fuel cells. The XRD patterns of LSGZ sintered at 1150oC and 1250C for 2 h are shown in Fig. 1. It can be seen reflections of SrLaGa3O7, LSGZ, SrLaGaO4 and La4Ga2O9 were detected. SrLaGa3O7 is the major phase in 1150C sintered pellets and decreased at higher sintering temperature 1250C. LSGZ became the major phase in pellets sintered at 1250oC/2 h. Therefore, reaction-sintering process can efficiently transform the mixture of raw materials into LSGZ phase. La4Ga2O9 phase decreased obviously at 1250oC. Using a Pechini-type process, Tas et al. reported that SrLaGaO4 and SrLaGa3O7 phases appeared in the calcined La0.9Sr0.1GaO2.95 at 1200oC and disappeared at 1340oC/6h calcination. SrLaGa3O7 phase appeared after 6 h calcined at 700oC and about 4-5 wt% of a mixture of the undesired SrLaGaO4 and SrLaGa3O7 phases still remained even after calcined at 1400oC for 6 h. They also found SrLaGa3O7 as a major phase in LSGZ calcined at 1000oC via aqueous chemical precipitation and almost pure LSGZ at 1200oC. SEM photographs of as-fired LSGZ ceramics sintered at 1150-1250oC/2 h are shown in Fig. 2. Porous pellet with fine grains of about 2μm formed at 1150oC and 1170oC. This means 1150-1170oC sintering is not high enough for grain growth and densification in LSGZ ceramics prepared using the reaction-sintering process. It can be easily observed that pores decreased at 1200oC and grain size increased at temperatures 1200-1250oC. The amount of pores decreased in LSGZ ceramics sintered for 4 h as shown in Fig. 3. Dense LSGZ could be obtained at sintering temperatures above 1230oC. Dense structure is needed in electrolyte of SOFC to avoid mixing and reaction between fuel and oxygen gas. Tas et al. reported LSGZ powders consisted of round particles with an average diameter of 0.25μm after calcined at 1000oC for 6 h. The non-compacted LSGZ powder body consisted of dense 20 to 30 μm chunks was observed at 1300oC 6 h calcination. Fig. 4 shows the shrinkage percentage of LSGZ ceramics sintered at various temperatures for 2-6 h. It increases from about 19-22% at 1150oC to 24-25% at 1250oC. In our study of La0.8Sr0.2Ga0.83Mg0.17O2.815 ceramics via reaction–sintering process, shrinkage of 17-20% at 1300oC and 26-28% at 1400oC were found. It implies a lower temperature is needed in Zn-doped than in Mg-doped LaSrGaO3-δ ceramics prepared via reaction-sintering process. In Fig. 5, density of LSGZ ceramics increases with sintering temperature and reaches a maximum 6.71 g/cm3 at 1230oC for 6 h. Fig. 2 SEM photographs of as-firedLSGZ ceramicssintered at (A) 1150oC, (B) 1170oC, (C) 1200oC, (D) 1230oC, and (E) 1250oC for 2 h. Fig. 3 SEM photographs of as-firedLSGZ ceramics sintered at (A) 1150oC, (B) 1170oC, (C) 1200oC, (D) 1230oC, and (E) 1250oC for 4 h. Fig. 4 Shrinkage percentage of LSGZceramicssintered at various temperatures for 2-6 h. Fig. 1 XRD patterns of LSGZ sintered at (A) 1150oC and (B) 1250oC for 2 h. (1: LaGaO3 ICDD PDF # 00-054-0870; 2: SrLaGaO4 ICDD PDF # 01- 080-1806; 3:SrLaGa3O7 ICDD PDF # 00-045-0637; 4: La4Ga2O9 ICDD PDF # 00-037-1433) Fig. 5 Density of LSGZ ceramics sintered at various temperatures for 2-6 h.