Thermal air-oxidized coating on Yb14−xRExMnSb11 ceramics

Abstract

A high sublimation rate of Yb14MnSb11 as a promising thermoelectric material at working temperatures is a critical factor for its space power application. In search of a way to suppress the sublimation, an isothermal thermogravimetric procedure under controlled conditions was developed and ability of Yb14MnSb11 doped by trivalent rare-earth elements to form the protective air-oxidized scale was studied. A series of ceramic rods Yb14−xRExMnSb11 with RE = La–Lu (except Ce and Eu) and x = 0.1, 0.3 and 0.4 that is close to the solubility limit was examined. Thermogravimetric curves as a time function of a progressively increasing mass of the rods in dry air at temperatures 773, 873, 973 K were recorded, and the kinetic and energetic characteristics of the oxidation process were calculated for the whole series of the isostructural Yb14−xRExMnSb11 solid solutions. Changes of the oxidation curves were considered in the context of the classic Wagner’s theory, and the general kinetic description of the process was made based on the parabolic model. This thermoanalytical investigation for the row of the isostructural Yb14−xRExMnSb11 compounds provided new vision of the oxidation mechanism and the role of the RE additions in the scale formation. For all these compounds, the oxidation kinetics followed the parabolic rate law and solid-state diffusion was the rate-limiting step for the reaction of a dense scale formation from the mixed Yb–RE oxides.