Influence of SiO2 and Fe2O3 on Properties and Microstructure of .BETA."-Alumina.

Abstract

In order to decrease the cost of β-alumina for solid electrolytes of AMTEC (alkali metal thermo-electric converter), it is desirable to use low purity starting materials. In this paper, we investigated the influence of impurities such as SiO2 and Fe2O3, which are usually contained in low purity alumina, for several hundreds ppm, on properties and microstructure of β″-alumina. The content of β″-alumina phase and the density decreased with the addition of SiO2, and the resistivity of β″-alumina increased even with the addition of amounts of SiO2 as small as 500ppm. Additional SiO2 formed as a glassy phase at multiple grain junctions, but no secondary phase was observed at boundaries between two adjacent grains. Si segregated along the grain boundaries. It was suggested that the segregated Si suppressed the sintering and increased the resistivity. The addition of Fe2O3 did not affect the properties such as content of β″-alumina phase, density and resistance, but the addition of Fe2O3 in ≥0.5 mass% resulted in grain growth. Because Fe ions were suggested dissolved in spinel block of β″-alumina, it was considered that Fe2O3 did not influence the properties of β″-alumina.