Influence of SiO₂ and Fe₂O₃ on properties and microstructure of β″-alumina

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 500 ppm. 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 Fe ₂O₃ did not affect the properties such as content of β″-alumina phase, density and resistance, but the addition of Fe ₂O₃ 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.