Get ready for thermal transistors

A Stanford-led engineering team has developed a way to not only manage heat but help route it away from delicate devices. Writing in Nature Communications, the researchers describe a nanoscale thermal transistor, something that researchers have been trying to develop for years. Previous thermal transistors proved too big, too slow and not sensitive enough for practical use. The challenge has been finding a nanoscale technology that could toggle on and off repeatedly, have a large hot-to-cool switching contrast and no moving parts.

Cross-sectional view of a device under operation. Li ions enter and leave the MoS₂ film through the exposed edges. Below, researchers used an optical technique dubbed TDTR to measure the operating properties of the thermal transistor.

The thermal transistor design starts with a thin layer of molybdenum disulfide, a semiconducting crystal that is made up of layered sheets of atoms. Just 10 nm thick and effective at room temperatures, this material could be integrated into ordinary semiconductors, a critical factor in making the technology practical.

The researchers bathed the material in a liquid rich with lithium ions. Applying small electrical current forces the lithium atoms to infuse into the layers of the crystal, changing its heat-conducting qualities. As the lithium concentration rises, the thermal transistor switches off i.e. becomes less conductive. Working with researchers at the University of California, Davis, the team discovered this happens because the lithium ions push apart the atoms of the crystal, making it harder for heat to get through.

Circuit symbol of the thermal transistor device: The gating voltage V_WE, applied between Li and Al/MoS₂ electrodes, changes the thermal conductance G to heat flow between the Al transducer and SiO₂ substrate, which are at temperatures T_Al and T_SiO2 respectively.

Specifically, the working device is a 10-nm-thick layered MoS₂ crystal, prepared on SiO₂ (90 nm) on p-type Si. An 80-nm Al layer patterned on top of the MoS₂ serves as an electrical contact and as an opto-thermal transducer for measurements. A solid Li pellet acts as the reference and counter electrodes, and 1.0 M LiPF₆ in ethylene carbonate/diethyl carbonate is the liquid electrolyte. Researchers were able to modulate thermal conductance reversibly in the device by a factor of nearly 10×, on time scales of minutes.

The researchers envision that thermal transistors connected to computer chips would switch on and off to help limit the heat damage in sensitive electronic devices.

Besides enabling dynamic heat control, the team’s results provide new insights into what causes lithium ion batteries to fail. As the porous materials in a battery are infused with lithium, they impede the flow of heat and can cause temperatures to shoot up. Thinking about this process is crucial to designing safer batteries.

In a more distant future, the researchers imagine that thermal transistors could be arranged in circuits to compute using heat logic, much as semiconductor transistors compute using electricity.