Selective internal oxidation of the noble metal-rich intermetallic compound, BaAg5

Although the internal oxidation of noble metal-base solid solutions (partic ularly Ag-base alloys) has been examined extensively, no detailed studies h ave been reported on the exclusive internal oxidation of noble metal-rich i ntermetallic compounds. In this paper, the internal oxidation of a silver-r ich compound, BaAg5, has been examined at 400 degreesC in pure O-2 Under th ese conditions, the BaAg5 was converted into a mixture of submicron barium- oxide particles in a silver matrix. Image analysis of the oxide content, an d chemical analysis with an electron microprobe, revealed the absence of pr eferential barium segregation to the oxidized zone. Silver nodules were det ected on the specimen surface, which was consistent with outward silver mig ration to relieve the str ess associated with internal-oxide formation. Alt hough the oxidation zone thickened at a parabolic rate, the measured rate w as higher than expected fr am the Wagner model. The internal oxidation of B aAg5 is demonstrated to be a relatively simple means of synthesizing apr in timate mixture of BaO2 and Ag for potential use in gas-phase catalysis.