Fabrication And Properties Of Barium Zirconate-based High Temperature Proton Conductors

Y-doped BaZrO₃ ceramics is the best one among several high temperature proton conductors in theory. With the combining merits of good chemical stability and high bulk conductivity, it is a promising candidate for electrolytes in solid oxide fuel cells. However, the material is difficult to densify. Its total conductivity is low due to the large area and high resistance of the grain boundary. This inhibits its high-drain applications. Methods of lowering the sintering temperature and improving the conductivity of BaZrO₃-based ceramics are investigated in this work.Several sintering aids, such as P₂O₅, CuO and NiO, which can densify BaZr_0.9Y_0.1O_3-δ at a lower temperature, were chosen and researched thoroughly. BaZrO₃-based ceramics were densified by P₂O₅ through liquid sintering mechanism. 94.2% of theoretical density was reached for BaZr_0.9Y_0.1O_3-δ sintered at 1600°C for 4 h by adding 4 mol% P₂O₅. CuO and NiO promoted the densification of BaZr_0.9Y_0.1O_3-δ by solid solution mechanism. 95.4% of theoretical density was reached for the sample sintered at 1600 ?C by adding 2 mol% of CuO, much higher than the 67.9% of theoretical density for the sample without CuO. The proton transport number of BaZr_0.9Y_0.1O_3-δ with 1² mol% of CuO ranged from _0.95 to 0.85 between 600⁸00°C, which could basically fulfill the request for the fuel cells. NiO could promote the densification of BaZr_0.9Y_0.1O_3-δ much more effectively. 1² mol% NiO could make the BaZr_0.9Y_0.1O_3-δ sample to reach >95% of theoretical density when sintered at 1500°C. Therefore, the sintering temperature of BaZr_0.9Y_0.1O_3-δ at >1700°C as previously reported in literature could markedly be lowered by sintering aids. This provides a guarantee for the co-firing process in the preparation of solid oxide fuel cells.Gel-casting process was studied to prepare BaZrO₃-based powders. As the mixing of the constituents at a molecular level during the gel formation is achieved, the powders have a very high degree of chemical homogeneity. Lower calcining temperature and shorter holding time were required to synthesize the uniform BaZr_0.9Y_0.1O_2.95 powders compared with the traditional solid-state reaction method. Pure perovskite phase was obtained at 1200°C for 4 h by gel-casting process, whereas 1400°C for 4 h was required by solid state reaction. Sinterability of the powders was obviously improved by gel-casting method. 92.8% of theoretical density was reached for the ceramics prepared by gel-casting method, much higher than the 67.9% of theoretical density for the samples made by the solid state reaction. Different rare earth elements doped BaZr_0.9M_0.1O_2.95 (M= Yb, Dy, La) powders were prepared by gel-casting methods. The results demonstrated that the gel-casting process is a simple, fast and convenient method for preparing a high-temperature proton conductor BaZr_0.9M_0.1O_2.95 powders.Two times sintering process was applied to prepare the composites of Y-doped BaZrO₃/sulphates and BaZrO₃/carbonates. The original phases of the salts were preserved as high as possible by this process; meanwhile the reaction between BaZrO₃-based ceramics and the salts were reduced to the lowest level. The conductivities of BaZrO₃-based ceramics were increased by one or two orders of magnitude, reaching the order of 10-2 S/cm for BaZr_0.9Y_0.1O_3-δ/salt composites at 600⁸00°C, much higher than that of the monolithic BaZr_0.9Y_0.1O_3-δ. Superprotonic conductivity phenomena were observed in the composites with sulphate (Na2SO4, K2SO4 and Li2SO4:K2SO4=1:1) and carbonate (Na₂CO₃, K₂CO₃ and 2Li₂CO₃+Na₂CO₃).