The thermal expansion of NiO-8 mol % Y203-stabilized Zr02 (YSZ) composites and Ni-YSZ cermets in air and hydrogen have been investigated in the temperature range from 50 to 1000°C. The average linear thermal expansion coefficient (TEC) of NiO-YSZ composites in air increased with NiO content over the entire composition range. While NiO in the composites was changed to Ni in the H2 stream, their expansions were governed by the reduction of NiO. For reduced, Ni-YSZ cermets, the TEC increases significantly with Ni content in the composition range >60 vol % Ni. The TEC increased gradually during repeated thermal cycles between room temperature and 1000°C. When cermets were measured in air, the Ni particles were fully oxidized to NiO above 900°C, and many cracks appeared in the samples.

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A novel Ni-YSZ cermet anode for solid oxide fuel cell application has been developed in order to solve problems with our previous anode, which was prepared from a powder mixture of NiO and precalcined YSZ. In contrast to homogeneously sized YSZ in our previous anode, the new material has a YSZ frame made of coarse (average size of 27.0 pm) and fine (average size of 0.6 tim) powders to sustain the network of Ni and pore. Unlike the previous anode, no significant changes in shrinkage and microstructure of the new anodes were detected during sintering in air and upon transformation of NiO to Ni metal in a reducing atmosphere. The influence of the content of coarse YSZ powder was examined by fixing the Ni volume ratio at 40%. It has been found that with an increasing amount of coarse YSZ powder, the electrical conductivity increased by three orders of magnitude around 20 weight percent (w/o) coarse YSZ powder and also that the anode with an 80 w/o one was found to be the most stable during cell fabrication and cell operation. It is concluded that the consolidated framework made from two distinctly sized particles leads to a more stable YSZ frame and Ni network.

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The sintering behavior and electrical conductivity of highpurity S-rnol% Y,O,-stabilized ZrO, (SYSZ) with AI,O, additions were investigated. The addition of 1 wt% AI,O, to 8YSZ provided dense, sintered samples with 94% relative density at 1400°C without a holding time. Addition of 1 wt% SiO, enhanced the sinterability of 8YSZ. Na,O addition of 0.1 wt% remarkably lowered it. Electrical conductivity at 1000°C in air increased slightly with increased AI,O, content up to 1 wt% and then monotonously decreased. 8YSZ with 1 wt% AI,O, showed the maximum conductivity of 0.16 S/cm at 1000°C.

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