Reaction of Beta-Alumina Solid Electrolyte with SO_x(x=2, 3) Gas

Abstract

The stability of beta-alumina (β- and β″-alumima)solid electrolyte in the SO_x atmosphere at high temperatures was investigated by electron probe X-ray micronalysis and thermodynamic considerations. Samples of β- and β″-alumina were heat-treated in the SO₂ standard gas containing 1054 ppm of SO₂ in air at 973 K for 10.8 and 260 ks. The formation of Na₂SO₄ on the beta-alumina surface was confirmed by the measurements of SK_β spectra and line-scanning profiles of sodium, sulfur and aluminum. The thickness of the Na₂SO₄ films was almost independent of the heat-treatment time longer than 10.8 ks and was a few micrometers. The cause of the rapid reaction of the beta-alumina with SO_x gas was discussed in connection with the structure of the beta-alumina. From the thermodynamic considerations using Na–S–O potential diagrams at 900 and 1100 K, β″-alumina rather than β-alumina was found to be a more suitable solid electrolyte to detect lower SO_x partial pressures at high temperatures. It was proved that the SO_x sensor using beta-alumina solid electrolyte had a good selectivity for SO_x in the SO_x–CO₂–NO_x gas atmosphere.