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.
Highlights
Thermoelectric materials have a great potential to convert waste heat into electricity and are being widely investigated, mainly for decreasing the dependence on conventional fossil fuels and to reduce harmful emissions [1]
The experiments were performed in ultra-high-vacuum (UHV) systems, pumped by turbo- molecular and titanium sublimation pumps to a base pressure of ~2 × 10−10 Torr, monitored by a Bayard-Alpert type ionization gauge and a quadrupole residual gas analyser (RGA, SRS 100, Stanford Research Systems, Sunnyvale, CA, USA)
No measurable oxygen or carbon contamination could be observed after the sputter cleaning
Summary
Thermoelectric materials have a great potential to convert waste heat into electricity and are being widely investigated, mainly for decreasing the dependence on conventional fossil fuels and to reduce harmful emissions [1]. Galazka et al [17] utilized thermogravimetric analysis (TGA) measurements on Ti0.33 Hf0.33 Zr0.33 NiSn compound, to follow the surface oxidation. They found that oxidation started at ~545 K and a significantly mass change was measured above 673 K. Berche and Juned [18] studied the oxidation behaviour of TiNiSn by ab initio methods. Temperature effects on the surface composition (segregation) were studied with a major focus on the oxidation of the TiNiSn (HH) alloy at room temperature (RT) and 1000 K, utilizing Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS)
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