Thermoelectric materials, which convert heat into electricity, are valuable tools for capturing waste heat and turning it into usable electricity. These materials are especially useful in industries and vehicles where engines produce a lot of waste heat, improving energy efficiency by converting it into additional power. They also exhibit potential for portable power generation, in remote sensors and satellites where traditional power sources may be impractical.
Traditional thermoelectric devices, known as parallel thermoelectric devices, generate a voltage in the same direction as the heat flow. These devices typically use two types of parallel materials, namely p- and n-type, which generate voltages in opposite directions. By connecting them in series, a stronger voltage can be generated. However, this also leads to a greater number of contact points, which increase electrical resistance and consequently, power loss. In contrast, transverse thermoelectric devices do something unique: they generate electricity perpendicular to the heat flow. This allows them to use fewer contacts and, hence, achieve a more efficient thermoelectric conversion. Materials with "axis-dependent conduction polarity (ADCP)" or goniopolar conductors, that conduct positive charges (p-type) in one direction and negative charge (n-type) in another, are promising candidates for transverse thermoelectric devices. Unfortunately, a direct demonstration of the transverse thermoelectric effect (TTE) has been less studied—until now.
In this view, a research team from Japan, led by Associate Professor Ryuji Okazaki from the Department of Physics and Astronomy at Tokyo University of Science (TUS), including Mr. Shoya Ohsumi from TUS and Dr. Yoshiki J. Sato from Saitama University, achieved TTE in the semimetal tungsten disilicide (WSi2). Although previous studies have shown that WSi2 exhibits ADCP, its origin and the anticipated TTE have not been detected in experiments. "Transverse thermoelectric conversion is a phenomenon that is gaining attention as a new core technology for sensors capable of measuring temperature and heat flow. However, there are only a limited number of such materials, and no design guidelines have been established. This is the first direct demonstration of the transverse thermoelectric conversion in WSi2," explains Prof. Okazaki. Their study was published online in the journal PRX Energy on November 13, 2024.
The researchers analyzed the properties of WSi2 through a combination of physical experiments and computer simulations. They measured the thermopower, electrical resistivity, and thermal conductivity of a WSi2 single crystal along its two crystallographic axes at low temperatures. They found that the ADCP of WSi2 originates from its unique electronic structure, featuring mixed-dimensional Fermi surfaces. This structure reveals that electrons and holes (positive charge carriers) exist in different dimensions. A Fermi surface is a theoretical geometrical surface that separates occupied and unoccupied electronic states of charge carriers inside a solid material. In WSi2, electrons form quasi-one-dimensional Fermi surfaces and holes form quasi-two-dimensional Fermi surfaces. These unique Fermi surfaces create direction-specific conductivity, enabling the TTE effect.
The researchers also observed variations in how these charge carriers conduct electricity from sample to sample, consistent with previous studies. Using simulations based on first principles, the researchers showed that these variations were due to differences in how charge carriers scatter due to imperfections in the crystal lattice structure of WSi2. This insight is key to fine-tuning the material and developing reliable thermoelectric devices. Further, they demonstrated direct TTE generation in WSi2 by applying a temperature difference along a specific angle relative to both crystallographic axes, resulting in a voltage perpendicular to the temperature difference.
"Our results indicate that WSi2 is a promising candidate for TTE-based devices. We hope this research will lead to the development of new sensors and the discovery of new transverse thermoelectric materials," says Prof. Okazaki.
By elucidating the mechanism of TTE generation in WSi2, this study takes a step further toward advanced materials that can convert heat into electricity more efficiently, leading to a greener future.
