Thermoelectric materials can directly convert heat into electricity, making them promising for recovering waste heat from factories, vehicles, and other sources. They can also transport heat when an electric current is applied through a phenomenon known as the Peltier effect. These properties have attracted considerable interest for energy-saving technologies and thermal management.
However, the performance of a thermoelectric device is not determined by the thermoelectric material alone. Electrodes, lead wires, and the way heat escapes from the device to its surroundings—known as heat leakage—can also strongly influence its behavior. In conventional thermoelectric research, performance has often been evaluated using the dimensionless figure of merit, or zT, after the electrical and thermal conditions have reached a steady state. As a result, less attention has been paid to how the coupled electrical and thermal response develops over time and how the different components of a real device contribute to that response.
Associate Professor Yasuhiro Hasegawa of the Graduate School of Science and Engineering at Saitama University has now developed a theoretical framework that treats the thermoelectric material, electrodes, and heat leakage within a single unified description.
The approach is based on time-domain impedance spectroscopy (TDIS), a method in which a step-like electric current is applied to a thermoelectric material and the resulting change in electrical resistance is monitored over time. Immediately after the current is applied, the system first shows an electrical response. The current then transports heat through the Peltier effect, gradually changing the temperature distribution inside the material. This evolving temperature distribution, in turn, affects the electrical response.
Because these processes occur simultaneously, the measured time-dependent signal reflects the combined influence of the thermoelectric material, electrodes, lead wires, and heat leakage. Until now, separating these contributions and understanding their individual roles has been challenging.
Hasegawa theoretically analyzed the TDIS response and showed that the different contributions can be quantitatively distinguished. This makes it possible to interpret what previously appeared to be a single complex transient signal in terms of the individual elements that make up the thermoelectric system.
Importantly, the analysis also revealed that the time-dependent response follows a common scaling law, even when the thermoelectric materials or measurement conditions differ. This finding provides a unified way to describe how coupled electrical and thermal phenomena evolve over time and establishes a theoretical foundation for understanding thermoelectric devices as complete systems rather than simply as individual materials.
The framework could also change how thermoelectric devices and measurement systems are designed. Electrode structures and measurement conditions have often been determined through experience and trial and error. The new theory could instead allow researchers to predict in advance how these factors will affect the measured response and to choose conditions that make thermoelectric behavior easier and more reliable to evaluate.
More broadly, the work points toward an integrated approach in which the material, electrodes, thermal environment, and measurement system are optimized together as a single thermoelectric system. This could contribute not only to more accurate evaluation of thermoelectric materials but also to the design of higher-performance thermoelectric devices.
Such an approach may be particularly useful for thermoelectric technologies requiring rapid thermal response, including wearable cooling devices and thermal-management systems for electronic equipment. In the longer term, the framework could also support technologies that more effectively convert currently unused waste heat into electricity.
The study was published in the Journal of Applied Physics on July 30, 2026, and was selected as a Featured Article, a designation given by the journal editors to particularly noteworthy papers.
