Selective Internal Oxidation of the Noble Metal-Rich Intermetallic Compound, BaAg5

Abstract

Although the internal oxidation of noble metal-base solid solutions (particularly Ag-base alloys) has been examined extensively, no detailed studies have been reported on the exclusive internal oxidation of noble metal-rich intermetallic compounds. In this paper, the internal oxidation of a silver-rich compound, BaAg₅, has been examined at 400°C in pure O₂. Under these conditions, the BaAg₅ was converted into a mixture of submicron barium-oxide particles in a silver matrix. Image analysis of the oxide content, and chemical analysis with an electron microprobe, revealed the absence of preferential barium segregation to the oxidized zone. Silver nodules were detected on the specimen surface, which was consistent with outward silver migration to relieve the stress associated with internal-oxide formation. Although the oxidation zone thickened at a parabolic rate, the measured rate was higher than expected from the Wagner model. The internal oxidation of BaAg₅ is demonstrated to be a relatively simple means of synthesizing an intimate mixture of BaO₂ and Ag for potential use in gas-phase catalysis.