On the mechanism of Ag exudation during internal oxidation

Internal oxidation experiments under different oxidation conditions focusing on the characteristics of noble metal exudation are presented together with finite element method (FEM) simulation calculations of the build-up of internal stresses. In a high-temperature bending test with a fully oxidized Ag–SnO2–In2O3 sample stress states that were comparable with those occurring during internal oxidation were generated, but exudation of Ag was not observed. From further targeted internal oxidation experiments in combination with FEM simulation calculations of internal stress distributions it can be concluded that internal stresses play a minor role in the occurrence of noble metal exudation. It is suggested that exudation occurs due to a gradient in the chemical potential of the noble metal. The potential gradient is a consequence of the concentration gradient of inward diffusing oxygen. The influence of the oxygen-induced driving force on the development of microstructure considering internal and external noble metal layers is discussed.