Sprinkled with power: How impurities enhance a thermoelectric material
at the atomic level

Date:
October 15, 2020
Source:
Tokyo University of Science
Summary:
Magnesium silicide (Mg2Si) is a thermoelectric material that
can convert heat into electricity. Though it is known that
adding antimony impurities enhances the performance of Mg2Si,
the mechanisms underlying this effect are unclear. Now, scientists
shed light on the effects of these impurities at the atomic level,
taking us closer to arriving at a practical way of efficiently
harvesting waste heat from cars and thermal power plants to produce
clean energy.



FULL STORY ==========================================================================
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 using the enormous amount of waste heat that human activity generates, such as that from automobiles and thermal power plants, thereby providing an eco-friendly alternative to satisfy our 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 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 recent joint study published as a featured article in Applied Physics Letters, scientists from Tokyo University of Science (TUS), the Japan Synchrotron Radiation Research Institute (JASRI), and Shimane University, Japan, teamed up to uncover the mysteries behind the improved performance
of Mg2Si doped with antimony (Sb). Dr Masato Kotsugi from TUS, who is corresponding author of the study, explains their motivation: "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. This information is critical to understanding the mechanisms behind thermoelectric performance
and improving the next generation of thermoelectric materials." But how
could they analyze the effects of Sb impurities on Mg2Si at the atomic
level? The answer lies in extended X-ray absorption fine structure (EXAFS) analysis and hard X-ray photoelectron spectroscopy (HAXPES), as Dr Masato Kotsugi and Mr Tomoyuki Kadono, who is first author of the study, explain: "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. 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, however,
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 Dr Akira Yasui and
Dr 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 Dr 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 Si atoms in
the Mg2Si crystal lattice and introduce a slight distortion in the
interatomic distances.

This could promote 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 by electrons
to circulate. As a result, the electrical conductivity of doped Mg2Si increases, and so does its ZT.

This study has greatly deepened our understanding of doping in
thermoelectric materials, and the results should serve as a guide for innovative materials engineering. Dr Tsutomu Iida, lead scientist in
the study, says: "In my vision of the future, waste heat from cars is effectively converted into electricity to power an environment-friendly society." Fortunately, we might just be one step closer to fulfilling
this dream.


========================================================================== Story Source: Materials provided by Tokyo_University_of_Science. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Tomoyuki Kadono, Naomi Hirayama, Tadashi Nishio, Shingo Yamazawa,
Naoto
Oki, Yoshinobu Takahashi, Natsumi Takikawa, Akira Yasui, Kiyofumi
Nitta, Oki Sekizawa, Mako Tokumura, Shoji Takemoto, Tsutomu Iida,
Masato Kotsugi. Investigation of local structures and electronic
states of Sb- doped Mg2Si by fluorescence XAFS and HAXPES. Applied
Physics Letters, 2020; 117 (14): 143901 DOI: 10.1063/5.0018323 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201015111723.htm

--- up 7 weeks, 3 days, 6 hours, 50 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1337:3/111)