In today’s electronics, a lot of heat is released as waste during use – which is why devices such as laptops and mobile phones heat up during use and require cooling solutions. Over the last decade, the concept of controlling this heat using electricity has been put to the test, leading to the development of electrochemical thermal transistors, devices that can be used to control heat flow using electrical signals.

Currently, liquid-state thermal transistors are used, but they have critical limitations: basically, any leakage leads to device termination. A research group from Hokkaido University led by Professor Hiromichi Ohta of the Electronic Sciences Research Institute has developed the first solid-state electrochemical thermal transistor. Inventions described in the journal Advanced Functional IngredientsIt is as efficient and much more stable than current thermal transistors in liquid form.

“A thermal transistor usually consists of two materials: an active material and a switching material,” Ohta explains. “The active material has a variable thermal conductivity (𝜅), and the switching material is used to control the thermal conductivity of the active material.”

The team built their thermal transistor based on zirconium oxide stabilized by yttrium oxide, which also acts as a switching material, and used strontium cobalt oxide as the active material. Platinum electrodes were used to provide the energy needed to control the transistor.

The thermal conductivity of the active material in the open state was comparable to some liquid thermal transistors. Overall, the thermal conductivity of the active material was four times higher in the “on” state than in the “off” state. Additionally, the transistor was stable over 10 usage cycles, which was better than some existing liquid state thermal transistors. This behavior has been tested on more than 20 separately manufactured thermal transistors, ensuring repeatability of results. The only drawback was the operating temperature of about 300°C.

“Our results show that solid-state electrochemical thermal transistors have the potential to be as efficient as liquid electrochemical thermal transistors, without any limitations,” Ohta said. “The main barrier to the development of practical thermal transistors is the high resistance of the switching material, and therefore the high operating temperature. This will be the focus of our future research.”