Evaluation of elastic properties of reduced NiO-8YSZ anode-supported bi-layer SOFC structures at elevated temperatures in ambient air and reducing environments

Elastic properties of Ni-8YSZ anode-supported bi-layer SOFC structures were studied at elevated temperatures up to 1,000 A degrees C in both ambient air and H-2 environments. The anode samples with desired porosity and microstructure were fabricated by reducing a NiO-8YSZ anode precursor structure in a gas mixture of 5% H-2-95% Ar at 800 A degrees C for selected time periods up to 8 h. The development of the essential porous microstructure in forming the Ni-8YSZ cermet phase was analyzed with SEM. It was observed that the room temperature elastic moduli and hardness of the anode samples decrease significantly with increasing fraction of reduced NiO. Since the elastic properties of fully dense Ni, NiO, and 8YSZ are comparable to each other, the decrease in the magnitude in elastic moduli and hardness is evidently due to the colossal increase in porosity in the reduced Ni-8YSZ cermet anodes because of the reduction of NiO to Ni. At elevated temperatures, the Ni-8YSZ anodes show a complex profile of Young's modulus as a function of temperature, which is significantly different from the unreduced NiO-8YSZ samples. When studied in ambient air, the Young's modulus of the Ni-8YSZ samples decrease slowly up to 250 A degrees C, then more rapidly from 250 to 550 A degrees C, and finally it increases monotonically with the increase in temperature. However, in reducing environment, the Young's moduli values decrease continuously throughout the temperature range. Two sets of samples of different thicknesses were studied simultaneously to highlight the effects of the sample thickness on the elastic properties of the anodes.