UC Irvine

The results of prior experiments have suggested faster diffusion of oxygen in polycrystalline alumina when exposed to water vapor. It is hypothesized that the dissociation of water vapor on the oxide surface helps increase the transport of oxygen in the form of the smaller hydroxide ion through grain-boundary diffusion. This study examines the sintering of ceramics and the oxidation of nickel in an 8YSZ/0.5 vol.% nickel cermet. In YSZ oxygen anion diffusion is fastest in grains due to the spaces in the fluorite unit cell and yttrium cations creating oxygen vacancies. Oxygen diffusion along grain boundaries in YSZ is actually slower than in the grains. In the first part of the study, ceramics are sintered at temperatures of 1000-1500°C for 3 hours in dry air or humid air (0.2 atm P_water). Percent theoretical densities of sintered α-alumina, spinel and 8YSZ sintered in the two atmospheres at various sintering temperatures will be plotted and compared. In the second part of the study, polished surfaces of 8YSZ-Ni cermet are exposed to dry air or humid air (0.38 atm P_water) at temperatures of 1000°C for times from 30 minutes to 2 hours. Preliminary data on the parabolic oxidation kinetics found that the oxidation depth for Ni0 to Ni2+ increased in the presence of humid air. The kinetic rate constants of oxygen diffusion calculated from oxidation depth of nickel front under dry and humid conditions at high temperatures will be reported.