Conventional thermoelectric devices rely on parallel thermoelectric materials, which produce voltage in the same direction as heat flow. While effective, these devices require multiple contact points to connect p-type and n-type materials, resulting in increased electrical resistance and power loss. Transverse thermoelectric devices, by contrast, generate electricity perpendicular to heat flow, reducing contact points and enhancing efficiency. Materials exhibiting axis-dependent conduction polarity (ADCP), such as goniopolar conductors, are ideal for such devices. However, direct demonstrations of the transverse thermoelectric effect (TTE) in such materials have been rare - until recently.
A research team led by Associate Professor Ryuji Okazaki from the Tokyo University of Science, along with collaborators from Saitama University, successfully demonstrated TTE in the semimetal tungsten disilicide (WSi2). "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," said Prof. Okazaki. Their findings were published in the journal PRX Energy on November 13, 2024.
The team conducted extensive physical experiments and computer simulations to analyze the properties of WSi2. By measuring thermopower, electrical resistivity, and thermal conductivity along its crystallographic axes at low temperatures, they discovered that WSi2's ADCP arises from its unique electronic structure. This structure features mixed-dimensional Fermi surfaces, where electrons and holes occupy different dimensions, resulting in direction-specific conductivity that enables the TTE effect.
Further analysis revealed variability in conductivity due to imperfections in the crystal lattice structure, consistent with prior studies. Simulations based on first principles confirmed that these variations stem from differences in how charge carriers scatter. This insight provides a foundation for optimizing WSi2 for practical use in thermoelectric devices. The researchers demonstrated direct TTE generation in WSi2 by applying a temperature difference at a specific angle relative to its crystallographic axes, generating a perpendicular voltage.
"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," Prof. Okazaki explained.
This study's findings pave the way for advanced thermoelectric materials that can efficiently convert heat into electricity, contributing to a more sustainable energy landscape.
Research Report:Transverse thermoelectric conversion in the mixed-dimensional semimetal WSi2
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