Scientists demonstrate solid-state thermal transistor

A research team at Hokkaido University led by Professor Hiromichi Ohta at the Research Institute for Electronic Science has developed a solid-state electrochemical thermal transistor. The innovation, described in the journal Advanced Functional Materials, is more stable than and as effective as current liquid-state thermal transistors. Modern electronic devices use electrochemical thermal transistors to manage the heat produced as waste during usage. Electrochemical thermal transistors are used to control heat flow with electrical signals. Currently, liquid-state thermal transistors are in use, but have some limitations, as leakage can cause the device to stop working.

“A thermal transistor consists broadly of two materials, the active material and the switching material. The active material has changeable thermal conductivity (κ), and the switching material is used to control the thermal conductivity of the active material,” Ohta said.

Image caption: Design of a solid-state electrochemical thermal transistor. In the “off” state (left), the active material has reduced oxygen, which reduces its thermal conductivity. In the “on” state (right), the active material is rich in oxygen, greatly increasing its thermal conductivity. Image credit: Qian Yang, et al. Advanced Functional Materials, February 21, 2023.

The researchers constructed the thermal transistor on a yttrium oxide-stabilised zirconium oxide base, which functioned as the switching material, and used strontium cobalt oxide as the active material. Platinum electrodes were used to supply the power required to control the transistor. The thermal conductivity of the active material in the “on” state was comparable to some liquid-state thermal transistors. In general, thermal conductivity of the active material was four times higher in the “on” state compared to the “off” state. The transistor was also stable over 10 use cycles. This behaviour was tested across more than 20 separately fabricated thermal transistors, to determine if the results were reproducible. The only drawback was the operating temperature of around 300°C.

“Our findings show that solid-state electrochemical thermal transistors have the potential to be just as effective as liquid-state electrochemical thermal transistors, with none of their limitations. The main hurdle to developing practical thermal transistors is the high resistance of the switching material, and hence a high operating temperature. This will be the focus of our future research,” Ohta said.

Top image caption: The pioneering solid-state electrochemical thermal transistor developed in this study. Image credit: Hiromichi Ohta.