Abstract
This paper examines the decrease in the electromotive force (e.m.f.) and attendant microstructural changes in Rh-Pt thermocouples during their long-term exposure to temperatures in the interval 450–1000 °C. The decline in e.m.f. is ascribed chiefly to the formation of rhodium oxides in the surface layers of the thermocouple and consequent non-homogeneity of the rhodium concentration in the matrix in these layers. The activation energy for the e.m.f. reduction indicates that the mechanism governing this process is diffusion of oxygen from the atmosphere at locations where the periodic character of the lattice is disturbed. Reconditioning by annealing raises the e.m.f. of a thermocouple, the probable mechanism behind this process being decomposition of rhodium oxides and replenishment of the rhodium concentration in the solid solution of the surface layer.
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