Amorphous TiNiSn thin films for mechanical flexibility in thermoelectric applications

Abstract

Thermoelectric devices convert heat to electricity without greenhouse gas emissions and have the potential to serve as energy sources in wearable devices. Ongoing efforts are focused on designing materials that offer both high conversion efficiency and mechanical flexibility. Half-Heusler materials, such as TiNiSn, exhibit promising chemical stability and thermoelectric efficiency, but their inherent brittleness poses challenges for applications in flexible devices. Here, TiNiSn thin films were deposited by DC magnetron sputtering at room temperature to investigate their response to bending for flexible devices applications. Therefore, different substrates were considered: Si, Kapton, silk, and printer paper, whereas Si was used as a reference. The composition and structure of the deposited thin films were analyzed by employing energy-dispersive X-ray spectroscopy and wide-angle X-ray scattering, respectively. The film morphology was examined via scanning electron microscopy. Additionally, density functional theory (DFT) was employed to explore interfaces between the flexible substrates and amorphous TiNiSn and calculate the Cauchy pressure, a key indicator of ductile/brittle behavior. Amorphous TiNiSn thin film exhibits good adhesion to flexible Kapton, silk, and paper substrates. Mechanical loading, namely bending up to 154°, was applied to assess crack formation, revealing only a few cracks at 78° and 154°, thus indicating a certain level of flexibility. DFT data support these findings, showing intermediate adhesion strength between amorphous TiNiSn and monomers from the flexible substrates. The calculated Cauchy pressure of 30 GPa suggests the ductility of TiNiSn in the amorphous state. Therefore, replacing alternative time-consuming synthesis methods, eliminating the demand for high temperatures, and providing a nontoxic and cost-effective material with good adhesion to various substrates are reasons why amorphous TiNiSn thin film emerges as a good candidate for flexible thermoelectric devices.