Roles of alumina in zirconia-based solid electrolyte

Abstract

Up to 5 mol% Al₂O₃ was added to 9 mol% Y₂O₃-stabilized ZrO₂, and the roles of Al₂O₃ were systematically studied by means of the complex impedance approach, the positron annihilation technique, SEM, TEM, and electron probe microanalysis from the following aspects: (1) the existence of forms of Al₂O₃ in ZrO₂, (2) the effects of Al₂O₃ on the microstructure of ZrO₂, (3) the effects of Al₂O₃ on the resistance of ZrO₂, (4) the microstructure and property changes of ZrO₂ with Al₂O₃ addition during ageing at 940‡C. Two types of grain boundary phase, crystal and amorphous, were discovered. The Al₂O₃ segregation at grain boundaries can promote the mobility of the grain boundaries and thus results in a low density, because of entrapped pores. The Al₂O₃ addition decreases the grainboundary resistance in two ways: to scavenge the SiO₂ and CaO located at grain boundaries, and to form crystal grain-boundary phases with very high crystal defect concentrations. Ordered microdomains of Zr₃Y₄O₁₂ were precipitated from ZrO₂ grains during ageing, and aluminium was found to facilitate the precipitation.