The Initial Oxidation of HfNiSn Half-Heusler Alloy by Oxygen and Water Vapor

Abstract

The MNiSn (M = Ti, Zr, Hf) n-type semiconductor half-Heusler alloys are leading candidates for the use as highly efficient waste heat recovery devices at elevated temperatures. For practical applications, it is crucial to consider also the environmental stability of the alloys at working conditions, and therefore it is required to characterize and understand their oxidation behavior. This work is focused on studying the surface composition and the initial oxidation of HfNiSn alloy by oxygen and water vapor at room temperature and at 1000 K by utilizing X-ray photoelectron spectroscopy. During heating in vacuum, Sn segregated to the surface, creating a sub-nanometer overlayer. Exposing the surface to both oxygen and water vapor resulted mainly in Hf oxidation to HfO2 and only minor oxidation of Sn, in accordance with the oxide formation enthalpy of the components. The alloy was more susceptible to oxidation by water vapor compared to oxygen. Long exposure of HfNiSn and ZrNiSn samples to moderate water vapor pressure and temperature, during system bakeout, resulted also in a formation of a thin SnO2 overlayer. Some comparison to the oxidation of TiNiSn and ZrNiSn, previously reported, is given.

Effects of Stepped Heating on the Initial Growth of Oxide Scales on NiCrAlHf Bond Coat Alloy under Air and Water Vapor Atmospheres

“…A portion of singlet Hf also diffused into the alloy along the grain boundaries and increased the bond strength of the oxide scale. Constant heating caused HfO 2 to form a dense diffusion barrier in the middle of the oxide scale, which hindered the entry of O [28]. In addition, Hf promoted the bidirectional diffusion of Al and improved the density of the oxide scale.…”