Thermoelectric crystal conductivity reaches a new high

Just as a voltage difference can generate electric current, a temperature difference can generate a current flow in thermoelectric materials governed by its “Peltier conductivity” (P). Now, researchers from Japan demonstrate an unprecedented large P in a single crystal of Ta₂PdSe₆ that is 200 times larger than the maximum P commercially available, opening doors to new research avenues which can potentially revolutionize modern electronics.

We all know that current flows inside a metallic conductor in presence of a voltage difference across its ends. However, this is not the only way to generate current. In fact, a temperature difference could work as well. This phenomenon, called “Seebeck effect,” laid the foundation of the field of thermoelectrics, which deals with materials producing electricity under the application of a temperature difference.

Similar to the concept of an electrical conductivity, thermoelectricity is governed by the Peltier conductivity, P, which relates the thermoelectric current to the temperature gradient. However, unlike its electrical counterpart, P is less explored and understood. For instance, is there a theoretical upper limit to how large P can be? Far from being a mere curiosity, the possibility of a large P could be a game changer for modern-day electronics.

A new study published in the Journal of Physics: Energy has raised hopes on this front, reporting the observation of a P of 100 A cm^-1 K^-1 at a temperature of 10 K in a Ta₂PdSe₆ crystal, the largest ever reported for a bulk solid. “Ta₂PdSe₆ is structurally similar to Ta₂NiSe₅, a material which we found shows unique structural and transport properties. This led us to focus on the properties of Ta₂PdSe₆ and we ended up discovering the giant Peltier conductivity,” explains Assistant Professor Akitoshi Nakano from Nagoya University, Japan, who led the research team conducting this study.

The team prepared high-quality single crystals of Ta₂PdSe₆ and characterized its chemical composition using scanning electron microscopy with energy dispersive x-ray spectroscopy and x-ray diffraction. Next, they measured its electrical conductivity, thermopower, and Hall resistivity and performed calculations based on a two-carrier model considering electrons and holes (the absence of electrons, represented by particles with a positive charge equal to the electronic charge) to explain their findings.

They found that while other thermoelectric materials reached a P of the order of 1 A cm^-1 K^-1 at best before going down as the temperature decreased further, Ta₂PdSe₆ showed a rapid growth in P below 100 K, reaching a value higher by two orders of magnitude! This suggested that the large P stemmed from a low residual resistivity.

The modeling calculations revealed that the surprising P value was caused by the high mobility, low concentration, and heavy mass of the holes inside the crystal.

What practical ramifications could this finding entail? “Ta₂PdSe₆ could be used as a current source for a superconducting solenoid isolated in a cryogenic space. Due to its large P, a cubic sample of 1 cc could supply a current of 100 A to the solenoid for a temperature difference of 1 K. Furthermore, an absence of external current leads would make the system compact and concise, reducing cooling costs,” says Dr. Nakano. “This could be then carried over to the design of a compact, low-cost MRI machine, making medical diagnosis more convenient in the future.”

We certainly hope his visions will be realized soon!

The paper, "Giant Peltier conductivity in an uncompensated semimetal Ta₂PdSe₆," was published in Journal of Physics: Energy on September 15, 2021, at DOI: 10.1088/2515-7655/ac2357.

Authors:

Akitoshi Nakano¹, Ai Yamakage¹, Urara Maruoka¹, Hiroki Taniguchi¹, Yukio Yasui² and Ichiro Terasaki¹

¹ Department of Physics, Nagoya University

² Department of Physics, Meiji University

Media Contact:

Akitoshi Nakano

Graduate School of Science, Nagoya University

Email: nakano.akitoshi@nagoya-u.jp

Funding:

Kakenhi Grant No. 19H05791, 20K20898, and 21K13878 of Japan

About Nagoya University, Japan

Nagoya University has a history of about 150 years, with its roots in a temporary medical school and hospital established in 1871, and was formally instituted as the last Imperial University of Japan in 1939. Although modest in size compared to the largest universities in Japan, Nagoya University has been pursuing excellence since its founding. Six of the 18 Japanese Nobel Prize-winners since 2000 did all or part of their Nobel Prize-winning work at Nagoya University: four in Physics - Toshihide Maskawa and Makoto Kobayashi in 2008, and Isamu Akasaki and Hiroshi Amano in 2014; and two in Chemistry - Ryoji Noyori in 2001 and Osamu Shimomura in 2008. In mathematics, Shigefumi Mori did his Fields Medal-winning work at the University. A number of other important discoveries have also been made at the University, including the Okazaki DNA Fragments by Reiji and Tsuneko Okazaki in the 1960s; and depletion forces by Sho Asakura and Fumio Oosawa in 1954.

Website: https://en.nagoya-u.ac.jp/