NIMS and Hitachi Ltd., have jointly discovered that the thermoelectric performance of a weakly ferromagnetic alloy dramatically increases in a wide range of temperatures near the ferromagnetic transition temperature (Tc) at which they become non-magnetic. This study demonstrated for the first time that the thermoelectric performance of ferromagnetic materials substantially increases in response to changes in magnetic properties and may serve as a new approach to designing highly efficient thermoelectric materials.

Thermoelectric technology capable of converting heat into electricity has a variety of potential applications, such as efficient utilization of waste heat from industrial plants and supplying power to IoT devices. Global research efforts are underway to develop highly efficient thermoelectric materials. Improving thermoelectric performance has been challenging, however, because many interacting physical properties need to be taken into consideration. Therefore, a new approach to developing thermoelectric materials which goes beyond existing frameworks has attracted great interest. NIMS previously confirmed that adding magnetic elements to non-magnetic thermoelectric materials increases the power factor—a power generation capacity indicator—of such materials. NIMS has since been engaged in research to increase the performance of thermoelectric materials through analysis of the correlation between magnetic and thermoelectric properties.

The NIMS-Hitachi research team expanded the range of its target thermoelectric materials to include magnetic metal materials and discovered that the thermoelectric performance of a weakly ferromagnetic alloy containing Fe, V, Al and Si can be greatly enhanced. When the alloy was subjected to a wide range of temperatures in proximity to the ferromagnetic transition temperature (Tc), its thermoelectric performance increased. An alloy’s energy conversion efficiency increased particularly prominently (approximately a twofold increase) at near-Tc temperatures when the alloy’s Tc was close to room temperature. This phenomenon is presumably attributable to spin fluctuations unique to metallic ferromagnetic materials, which efficiently absorb heat energy and convert it into electronic energy.

These findings indicated that highly efficient thermoelectric materials may be created using materials that become ferromagnetic at approximately room temperature. The effect discovered in this research should be taken into account in electronic state calculations and data-driven materials searches, which may accelerate the development of high-performance thermoelectric materials and practical use of thermoelectric devices in the everyday environment.

This research project was carried out by Naohito Tsujii (Principal Researcher, International Center for Materials Nanoarchitectonics [MANA], Research Center for Functional Materials [RCFM], NIMS), Takao Mori (Group Leader, MANA, RCFM, NIMS), Akinori Nishide (Researcher, Hitachi) and Jun Hayakawa (Senior Researcher, Hitachi). This study was conducted as part of the project entitled “Development of novel magnetic semiconductor thermoelectric materials and power generation devices” under the JST CREST Program entitled “Scientific innovation for energy harvesting technology” (Research Supervisor: Kenji Taniguchi, Deputy Research Supervisor: Hiroyuki Akinaga).

This research was published in Science Advances on February 22, 2019, Eastern Standard Time.