Converts Room Temperature Waste Heat Into Electricity!
– Expected to be applied to energy harvesting technology that contributes to IoT society in the near future –

The research team of Osaka Prefecture University (President: Masahiro Tatsumisago) Graduate School of Science comprising Associate Professor Atsuko Kosuga (concurrent post: JST PRESTO Researcher), graduate student Tomohiro Oku, and Professor Yoshiki Kubota; Kindai University Technical College Associate Professor Hiroki Funashima; and Japan Synchrotron Radiation Research Institute Senior Researcher Shogo Kawaguchi succeeded in increasing the thermoelectric power factor (Keyword1) near room temperature up to twice that of existing materials by precisely controlling the electronic structure of germanium telluride. The result of this research is related to the elemental technology of waste heat utilization that reuses room temperature waste heat, which is the largest amount among the waste heat generated in the world, as electricity, and it is expected to contribute to the application of energy harvesting technology (Keyword2) and the realization of an energy-conserving society. The results of this research were published online in Materials Today Physics published by Elsevier on August 12, 2021, Japan time.

Research Points

•  Succeeded in developing a material that exhibits a thermoelectric power factor that is approximately twice as much as that of existing materials at near-room temperature.
•  Materials that were known to exhibit high performance in the high temperature range have now been improved to deliver high performance at near-room temperature due to precise control of the electronic structure.
• Expected to contribute to energy harvesting and energy-saving by using room temperature waste heat, which has the largest amount of waste heat.

Journal information

Journal name: Materials Today Physics
Article title: Superior room-temperature power factor in GeTe systems via multiple valence band convergence to a narrow energy range
Authors: Tomohiro Oku, Hiroki Funashima, Shogo Kawaguchi, Yoshiki Kubota, and Atsuko Kosuga

• Materials Today Physics (Elsevier Website)

Summary of Research

Approximately 70% of the primary energy is wasted as heat, and its use is important from the viewpoint of effective use of unused energy. In particular, waste heat at near-room temperature is often dispersed in a small scale and diluted amount, although it is known that there is a large abundance of room temperature waste heat due to its dependence of the waste heat temperature distribution. Therefore, it is known that such waste heat is difficult to collect by any method other than thermoelectric power generation (Keyword3). However, the development of “room temperature thermoelectric materials” that exhibit excellent thermoelectric properties at near-room temperature, which is necessary for such thermoelectric power generation, has not progressed, very few such materials have been developed beyond the existing materials discovered about half a century ago.

In this study, the research group succeeded in increasing the thermoelectric power factor of the thermoelectric material GeTe (germanium telluride), that exhibited high performance at a temperature range of 250 to 600 ℃, to the range of room temperature to 150 ℃, that is, near-room temperature (Fig. 2, left). It was shown from experiments and calculations that this performance improvement was caused by band convergence (Keyword4) (Fig. 2 right) of the additional valence as well as valence bands that were conventionally known to contribute to thermoelectric performance into a very narrow energy region by converting GeTe into a solid solution (Keyword5) with Sb₂Te₃ (antimony telluride). Normally, in order to achieve thermoelectric performance improvement at room temperature in this way, accurate crystal structure and electronic structure of the prepared material is required. However, this material system has the characteristic that it can have a structure that is difficult to distinguish depending on the sample preparation conditions and temperature. Furthermore, due to the characteristics of the solid solution system, it was difficult to obtain accurate information on the electronic structure. In this study, the research team of Associate Professor Kosuga and others acquired the high-precision powder diffraction data of the prepared sample at the powder crystal structure analysis beamline BL02B2 of the large-scale radiation facility (SPring-8) (Keyword6)and investigated the crystal structure. Using that as input data, the electronic structure of the sample was calculated. It is generally known that it is difficult to calculate electronic structure accurately in a solid solution sample like this one without extensive calculations. However, the calculation code has been improved so that it can be calculated efficiently and accurately for material systems with such characteristics, making it possible to perform precise control of the electronic structure.

The newly developed GeTe solid solution sample achieved a thermoelectric power factor that is approximately twice that of Bi₂Te₃ (bismuth telluride) in the room temperature to 150 ℃ range; which was prepared by the similar simple sample preparation process as that used in this study (Fig. 1). This material, just like Bi₂Te₃, has the potential for further improvement in thermoelectric performance by optimizing the microstructure using nanoparticles (Fig. 1). By applying the principle of high performance applied in this research to other material systems, there is a possibility that new room temperature thermoelectric materials will be discovered from the material group that was previously excluded from the search target of room temperature thermoelectric materials, and it is expected that the development of room temperature thermoelectric materials will accelerate.

Social significance, future plans

Based on the results of this research, room temperature thermoelectric materials have been developed and their control methods have been clarified. By applying the results of this research to device development, we will be one step closer to the realization of a technology that efficiently converts room temperature waste heat, which takes up a large proportion of waste heat generated in the world, into electricity.

In the future, it is expected to contribute to an energy-saving society as an effective energy utilization technology and the realization of the IoT (Internet of Things) as a self-sustaining power source for micro energy harvesting that supports the sensor network society advocated by Society 5.0.

In addition, the material obtained in this study will greatly contribute to resource saving strategies, as germanium is about 30 times more abundant than the rare metal bismuth.

Funding Information

This research is the result of Japan Science and Technology Agency (JST) Strategic Basic Research Program PRESTO (JPMJPR17R4).

• “Scientific Innovation for Energy Harvesting Technology” Area Webpage

Part of this research was supported by the Thermal & Electric Energy Technology Foundation and the Tanigawa Thermal Technology Promotion Fund.

Keywords

Keyword1: Thermoelectric power factor

It is a factor that measures the materials performance of thermoelectric materials. It is expressed as the product of the square of the Seebeck coefficient (thermoelectromotive force per 1 ℃ temperature difference) and the electrical conductivity (a measure of the ease of flow of electricity).

Keyword2: Energy harvesting technology

Technology that uses energy such as heat, light, and vibration that exist around us to generate electricity.

Keyword3: Thermoelectric power generation

Clean power generation technology that directly converts heat (temperature difference) into electricity. It is based on Seebeck effect, which is a physical phenomenon of solids. This technology is suitable for small-scale distributed waste heat recovery. The materials used in this technology are called thermoelectric materials or thermoelectric conversion materials.

Keyword4: Band convergence

Aligning the energy positions at the edges of multiple energy bands within a certain energy range is called band convergence. By increasing the degree of convergence, the thermoelectric power factor can be improved. If you want to improve the thermoelectric power factor in the room temperature range, it is necessary to converge multiple bands in a very narrow energy region compared to the case of the high temperature range.

Keyword5: Solid solution

A solid in which two or more substances are mixed to form a completely uniform solid phase is called a solid solution. There are two types of solid solutions: an atom at the lattice point of one crystal phase replaces another atom completely irregularly, or another atom invades the gap of the lattice.

Keyword6: SPring-8

SPring-8 is owned by Riken and produces the world’s highest performance synchrotron radiation in Harima Science Garden City, Hyogo Prefecture, is supported by JASRI. The name of SPring-8 comes from Super Photon ring-8 GeV. At SPring-8, extensive research is being conducted using synchrotron radiation, for nanotechnology, biotechnology, and industrial use.

Contribution to SDGs

7: Affordable and clean energy
9: Industry, innovation and infrastructure

Related Link

• Osaka Prefecture University, Graduate School of Science, Department of Physical Science, Thermoelectric Group website