Beta" -alumina is a promising material for the solid electrolyte in sodium-sulfur batteries because of its high sodium ion conductivity, chemical inertness and negligible electronic conductivity.
Beta" -alumina is thermally unstable and transformed, at the temperatures above 1500℃, irreversibly into beta-alumina having lower ionic conductivity and mechanical strength than those of beta" -alumina. The properties of the solid electrolyte are linearly dependent on the proportion of the beta" -alumina and beta-alumina phase. For obtaining high operating efficiency as an electrolyte, high proportion of beta" -alumina is required.
The beta" -alumina phase is known to be stabilized by addition of lithium oxide($Li_2O$) and/or magnesium oxide(MgO) at the temperature above 1600℃.
In the present study, magnesium oxide was used as a beta" -alumina phase stabilizer. The effect of concentration of magnesium oxide on the phase formation, microstructures, and mechanical and electrical properties of the beta" -alumina was investigated.
Bending strengths of beta" -alumina specimens containing 0, 1.13, 2.25, 4.5 and 6.75 wt% magnesium oxide were measured. Bending strength increased with addition of magnesium oxide up to 4.5 wt% and decreased with further addition of magnesium oxide. Beta" -alumina containing 4.5 wt% of magnesium oxide had the highest bending strength. It was found to be 170 MPa.
The densities of magnesium-stabilized beta"-alumina specimens were measured by iso-butyl alcohol immersion method. The relative densities in all cases were higher than 96.5% of its theoretical densities.
Ionic resistivity was measured by two-probe a.c. impedance method using Pt electrode. Ionic resistivity decreased with addition of magnesium oxide up to 4.5 wt% and increased with further addition of magnesium oxide. Beta"-alumina containing 4.5 wt% of magnesium oxide had the lowest ionic resistivity. It was found to be11∼18Ω·㎝ at 300℃.
Phase analysis was performed by using X-ray diffraction method. Lattice parameters and proportion of beta"-alumina phase increased with addition of magnesium oxide up to 4.5 wt%. Magnesium-spinel was formed as a second phase in beta"-alumina containing 6.75 wt% of magnesium oxide.
