Distorting impurities help improve thermoelectric materials

In the search for solutions to ever-worsening environmental problems, such as the depletion of fossil fuels and climate change, many have turned to the potential of thermoelectric materials to generate power. These materials exhibit what is known as the thermoelectric effect, which creates a voltage difference when there is a temperature gradient between the material's sides. This phenomenon can be exploited to produce electricity from the enormous amount of waste heat that human activity generates, such as by automobiles and thermal power plants, thereby providing an eco-friendly alternative to satisfy mankind's energy needs.

Magnesium silicide (Mg₂Si) is a particularly promising thermoelectric material with a high 'figure of merit' (ZT) – a measure of its conversion performance. Though scientists have previously noted that doping Mg₂Si with a small amount of impurities improves its ZT by increasing its electrical conductivity and reducing its thermal conductivity, the underlying mechanisms behind these changes were unknown – until now.

In a novel study, reported in a paper in Applied Physics Letters, scientists from Tokyo University of Science (TUS), the Japan Synchrotron Radiation Research Institute (JASRI) and Shimane University, all in Japan, teamed up to uncover the mechanisms behind the improved performance of Mg₂Si when doped with antimony (Sb).

"Although it has been found that Sb impurities increase the ZT of Mg₂Si, the resulting changes in the local structure and electronic states that cause this effect have not been elucidated experimentally," says Masato Kotsugi from TUS, who is corresponding author of the paper. "This information is critical to understanding the mechanisms behind thermoelectric performance and improving the next generation of thermoelectric materials."

But how could the scientists analyze the effects of Sb impurities on Mg₂Si at the atomic level? The answer lay in two advanced analytical techniques: extended X-ray absorption fine structure (EXAFS) analysis and hard X-ray photoelectron spectroscopy (HAXPES).

"EXAFS allows us to identify the local structure around an excited atom and has strong sensitivity toward dilute elements (impurities) in the material, which can be precisely identified through fluorescence measurements," explains Kotsugi. "On the other hand, HAXPES lets us directly investigate electronic states deep within the bulk of the material without unwanted influence from surface oxidation."

Such powerful techniques are not performed using run-of-the-mill equipment. The experiments were conducted at SPring-8, one of the world's most important large X-ray synchrotron radiation facilities, with the help of Akira Yasui and Kiyofumi Nitta from JASRI.

The scientists complemented these experimental methods with theoretical calculations to shed light on the exact effects of the impurities in Mg₂Si. These theoretical calculations were carried out by Naomi Hirayama of Shimane University. "Combining theoretical calculations with experimentation is what yielded unique results in our study," she says.

The scientists found that Sb atoms take the place of silicon (Si) atoms in the Mg₂Si crystal lattice and introduce a slight distortion in the interatomic distances. This distortion promotes a phenomenon called phonon scattering, which reduces the thermal conductivity of the material and in turn increases its ZT.

Moreover, because Sb atoms contain one more valence electron than Si, they effectively provide additional charge carriers that bridge the gap between the valence and conduction bands. In other words, Sb impurities unlock energy states that ease the energy jump required for electrons to move around. As a result, the electrical conductivity of doped Mg₂Si increases, and so does its ZT.

This study has greatly deepened scientists understanding of doping in thermoelectric materials, and the results should serve as a guide for innovative materials engineering. "In my vision of the future, waste heat from cars is effectively converted into electricity to power an environment-friendly society," says Tsutomu Iida, lead scientist of the study.

This story is adapted from material from Tokyo University of Science, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.