1. SYNTHESIS OF BARIUM CERATE AND STRONTIUM CERATE SOLID ELECTROLYTE BY A REACTION-SINTERING PROCESS Yi-Cheng Liou*, Song-Ling Yang Department of Electronics Engineering, Kun Shan University, Tainan Hsien 71003, Taiwan, R.O.C. *Corresponding author. ycliou@mail.ksu.edu.tw Synthesis of BaCe 0.9 Nd 0.1 O 3-δ and Sr 0.995 Ce 0.95 Y 0.05 O 3-δ ceramics by a reaction-sintering process was investigated in this study. Without any calcination involved, the mixture of raw materials was pressed and sintered directly. Monophasic BaCe 0.9 Nd 0.1 O 3-δ ceramic was obtained at 1450 o C/2 h sintering. A maximum value 5.82 g/cm 3 (91.5% of the theoretical value) was found at 1500 o C/2 h. Some weak reflections of SrCO 3 and CeO 2 were found in sintered Sr 0.995 Ce 0.95 Y 0.05 O 3-δ. Density of Sr 0.995 Ce 0.95 Y 0.05 O 3-δ ceramics reached a maximum value 5.69 g/cm 3 (98.4% of the theoretical value) at 1350 o C/2 h. The reaction-sintering process has proven a simple and effective method in preparing BaCe 0.9 Nd 0.1 O 3-δ and Sr 0.995 Ce 0.95 Y 0.05 O 3-δ ceramics for solid electrolyte applications in solid oxide fuel cells. The XRD patterns of BCNB ceramics produced using the reaction-sintering process are shown in Figure 1. The BaCeO 3 phase formed as a major phase and some weak peaks of unreacted BaCO 3 and CeO 2 were also detected in pellets sintered at 1350 o C/2 h. Monophasic BCN ceramic was obtained at 1450 o C/2 h sintering. Reaction-sintering process is proven effective in producing BaCeO 3 -based ceramics. In BCN studied by Chen and co-workers, they found the decomposition of BaCO 3 and the formation of the perovskite phase take place at about 900 o C and finish completely below 1200 o C from the DTA-TG curves at a heating rate 2 o C/min. Therefore, the unreacted BaCO 3 and CeO 2 in Fig.1 may exist due to the fast heating rate 10 o C/min. The shrinkage results for BCNB ceramics are shown in Fig. 2. It increased with increasing sintering temperature and time and reached 21% at 1500 o C/6 h. 1450 o C is high enough for densification. In Figure 3, the density of BCNB ceramics increased with the sintering temperature at similar trend in Fig. 2 and saturated at 1450 o C. A maximum value 5.82 g/cm 3 (91.5% of the theoretical value) was found at 1500 o C/2 h. In the study of Lu and Jonghe, 6.02 g/cm 3 was obtained after 1100 o C/2 h calcining and 1500 o C/2 h sintering. SEM photos of as-fired BCNB ceramics are shown in Figure 4. Porous pellets were found among the pellets sintered at 1350  C for 2 h, which was in good agreement with the density values in Figure 3. As the sintering temperature was raised to 1400  C, pores drastically decreased. A dense electrolyte is needed to prevent gas mixing. Fig. 5 shows the XRD profiles of SCY and SCYB ceramics sintered at 1350-1500 o C. These reflections match with those of SrCeO 3 in ICDD PDF # 01-081-0026. This proves the perovskite phase SCY could be obtained via the reaction-sintering process. This simple process is effective not only in preparing BaTi 4 O 9, (Ba x Sr 1-x )(Zn 1/3 Nb 2/3 )O 3, Ba 5 Nb 4 O 15, Sr 5 Nb 4 O 15 and Pb-based complex perovskite ceramics but also effective in preparing perovskite BCN and SCY ceramics. Some weak reflections of SrCO 3 and CeO 2 are also found in the profiles. In the study of Liu et al., SrCe 0.95 Yb 0.05 O 3-δ powders prepared by ethylenediaminetetraacetic acid (EDTA) and citric acid methods calcined at 400, 600 and 700 o C were composed of carbonates (SrCO 3 ) and oxides (CeO 2 ). However, the XRD spectra of powders prepared by the glycine method indicate that cubic perovskite SrCe 0.95 Yb 0.05 O 3-δ, CeO 2 and SrCO 3 phases coexist. At sintering temperatures of 800 o C and 900 o C, the XRD patterns of powders prepared by these three methods display the existence of SrCe 0.95 Yb 0.05 O 3-δ, CeO 2 and SrO crystal phases and the disappearance of SrCO 3 phases. At sintering temperature above 950 o C, only the cubic perovskite phase of SrCe 0.95 Yb 0.05 O 3-δ was detected. The shrinkage percentage of SCYB pellet increases from 1300 o C and saturates at temperatures above 1350 o C as shown in Fig. 6. It indicates a full sintering occurred at 1350 o C and is 100 o C lower than BCNB ceramics. In Fig. 7, the density of SCYB ceramics increases with sintering temperature and reaches a maximum value 5.69 g/cm 3 (98.4% of the theoretical value) at 1350 o C/2 h then slightly decreased. In the study of Sammes et al., a density close to 97% of the theoretical value was obtained in SrCe 0.95 Y 0.05 O 3-δ ceramics calcined at 1000 o C/0.5 h and 1300 o C/1 h then sintered at 1500 o C for 11 h. Therefore, reaction-sintering process is effective to produce SCY ceramics with high density even without the calcination. The SEM photos of SCYB ceramics sintered at 1300-1450 o C for 2 h are shown in Fig. 8. Grains began to grow at 1300 o C (2-4 μm) and the size increased significantly with the sintering temperature. As shown in Fig. 9, larger grains are found in pellets sintered for 6 h. 5-10 μm grains could be easily found in pellets sintered at 1450 o C. Fig. 1 XRD patterns of BCNB ceramics sintered at (a) 1350 o C and (b) 1450 o C for 2 h. (BaCeO 3 : ICDD PDF # 01-070-1429; ▲ : BaCO 3 ; ● : CeO 2 ) Fig. 4 SEM photographs of as-fired BCNB ceramics sintered at (a) 1350 o C, (b) 1400 o C, (c) 1450 o C and (d) 1500 o C for 2 h. Fig. 8 SEM photographs of as-fired SCYB ceramics sintered at (a) 1300 o C, (b) 1350 o C, (c) 1400 o C and (d) 1450 o C for 2 h. Fig. 6 Shrinkage percentage of SCYB ceramics sintered at various temperatures and soak time. Fig. 7 Density of SCYB ceramics sintered at various temperatures and soak time. Fig. 2 Shrinkage percentage of BCNB ceramics sintered at various temperatures and soak time. Fig. 3 Density of BCNB ceramics sintered at various temperatures and soak time. Fig. 5 XRD patterns of SCY ceramics sintered at (a) 1500 o C for 6 h and SCYB ceramics sintered at (b) 1350 o C, (c) 1400 o C and (d) 1450 o C for 2 h. (SrCeO 3 : ICDD PDF # 01-081-0026; ▲ : SrCO 3 ; ● : CeO 2 ) Fig. 9 SEM photographs of as-fired SCYB ceramics sintered at (a) 1300 o C, (b) 1350 o C, (c) 1400 o C and (d) 1450 o C for 6 h. Materials and Austceram 2007 July 4 - 6, 2007, Sydney, Australia