Surface Oxidation of TiNiSn (Half-Heusler) Alloy by Oxygen and Water Vapor

Abstract

TiNiSn-based half-Heusler semiconducting compounds have the highest potential as n-type thermoelectric materials for the use at elevated temperatures. In order to use these compounds in a thermoelectric module, it is crucial to examine their behaviour at a working temperature (approximately 1000 K) under oxygen and a humid atmosphere. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) were utilized to study the surface composition and oxidation of the TiNiSn alloy at elevated temperatures. It was found that during heating in vacuum, Sn segregates to the surface. Exposing the alloy to oxygen at room temperature will cause surface oxidation of Ti to TiO2 and Ti2O3 and some minor oxidation of Sn. Oxidation at 1000 K induces Ti segregation to the surface, creating a titanium oxide layer composed of mainly TiO2 as well as Ti2O3 and TiO. Water vapor was found to be a weaker oxidative gas medium compared to oxygen.

The Initial Stage in Oxidation of ZrNiSn (Half Heusler) Alloy by Oxygen

“…In the MNiSn family, where M denotes Ti, Zr or Hf, these elements and Ni are the cations, transferring charge towards the anionic Sn. This charge transfer tendency is expressed as chemical shifts of the elements in the alloy, compared to the known binding energy (BE) of the pure elements, as previously reported for the TiNiSn [11]. Table 1 depicts the measured BE of the elements from the clean ZrNiSn alloy.…”

“…During our previous work on TiNiSn, it was found that heating the alloy in vacuum causes Sn segregation to the surface, and it was attributed to the considerably lower surface energy of Sn compared to the other components [11]. Since the surface energy of Sn (~0.7 J/m 2 ) is considerably lower than the surface energy of Zr (~2 J/m 2 ) and Ni (~2.4 J/M 2 ) [17], it is expected that Sn will segregate to the surface, in the present case also.…”

“…As part of the motivation to map the surface properties and the oxidation behavior of other MNiSn HH thermoelectric alloys at the expected operation temperature, in the present work, temperature effects on the surface composition and the initial oxidation behavior of ZrNiSn alloy at RT and 1000 K were studied, utilizing X-ray photoelectron spectroscopy (XPS). The results are compared to those recently reported [11] for the TiNiSn alloy.…”

“…In our previous study [11], the surface properties of the TiNiSn alloy and their interaction with oxygen and water vapor at room temperature (RT) and 1000 K were characterized for the first time. It was found that during the heating of the sample in a vacuum, Sn readily segregates to the surface due to the considerably lower surface energy of the Sn compared to the Ni and Ti.…”

High Power Factor vs. High zT—A Review of Thermoelectric Materials for High-Temperature Application

“…The materials exhibit remarkable oxygen stability, but recent research has shown a sensitivity to oxygen at working temperatures for TiNiSn and ZrNiSn, resulting in the formation of oxides at the surface. Thus, the oxygen influence on the thermoelectric properties still needs to be investigated [ 165 , 166 ].…”