内部酸化したAu-Sn合金のメスバウアー効果

Abstract

Mössbauer spectra of 119Sn nuclei have been measured for an Au-2.0 at%Sn alloy annealed in air at temperatures between 673 K and 1073 K. An internally oxidized phase was observed and determined to be SnO2. The internal oxidation process was discussed by the changes in Mössbauer spectra and their parameters. The results are summarized as follows:(1) The oxide phases formed during the internal oxidation process were found to be only SnO2, in which the 119Sn isomer shift ΔSI=0.02±0.04 mm/s relative to BaSnO3, and the magnitude of quadrupole splitting ΔEQ=0.49±0.02 mm/s.(2) By annealing in H2 gas flow for 1.8 ks at 1123 K, the oxide was completely reduced and Sn atoms dissolved into Au matrix.(3) The oxide phase was localized near the specimen surface within the depth of 0.8 μm. No preferential oxidation along grain boundary was observed.(4) During a short-period annealing, the formation of oxide proceeded with the internal oxidation, the rate of which was controlled by the diffusion of oxygen in Au matrix. Dependence of the oxidation process on the annealing temperature was of the Arrhenius type, and its activation energy was obtained to be 0.49±0.07 eV. After annealing at 973 K for 0.6 ks, transition from the internal to the external oxidation occurred, and the oxidation rate was sharply diminished.(5) Using Wagner’s theory for the internal oxidation, the sum of EOS, the heat of solution for oxygen in Au and EOD, the activation energy of diffusion for oxygen in Au, were determined. The obtained value of EOS+EOD, 1.2±0.1 eV, suggests that the mechanism of diffusion of oxygen in Au is the same as in Cu and Ag.