Graphene foam "thermal switch" keeps batteries warm or cool on demand
Electronics, and batteries in particular, are vulnerable to very high or very low temperatures, but a new thermal switch could help deal with both. It's made from (what else) graphene foam that can dynamically trap heat when it’s cold out, or allow heat to escape when it gets too warm.
Lithium-ion batteries are recommended to only be used between 0 and 45 °C (32 and 113 °F), particularly when charging. Anywhere outside that range, and you risk degrading the battery’s performance. Keeping components from overheating is a constant battle that informs design of electronic devices, which give off plenty of heat of their own. But when the external temperature drops, it would be useful to keep that heat trapped in the device.
“As electronic devices get smaller and more powerful, managing heat becomes a more crucial issue,” says Xiulin Ruan, co-corresponding author of the study. “Most devices use passive thermal management, such as conduction and convection, to move excess heat. But this system isn’t tunable or adjustable, and doesn’t help at all in cold conditions.”
Now researchers at Purdue University have developed a way to handle both. The key, as it so often is, is graphene, an overachieving material made up of sheets of carbon just one atom thick. It’s normally an excellent conductor of heat, but graphene foam acts like an insulator due to the air pockets inside. So the team tapped into that dual nature to make a layer that switches back and forth between insulator and conductor.
In the test pictured above, the researchers placed a sample of graphene foam between a heater and a heat sink, and measured the temperature and heat flow when the foam is compressed or relaxed. It starts off 1.2 mm thick, acting like an insulator, but when it’s squeezed down to 0.2 mm it becomes eight times more thermally conductive.
“It functions like a resistor in an electrical circuit,” says Amy Marconnet, co-corresponding author of the study. “Instead of varying the amount of current flow, it varies the amount of heat it allows to pass.”
In another experiment, the team simulated real-world ambient temperatures of between 0 and 30 °C (32 and 86 °F), with the graphene foam placed on top of a heat source that simulated an electronic device. The results were similarly promising, stabilizing the operating temperature of the device across the whole range of ambient temperatures.
The team says this system could be most useful for personal electronics like phones, but perhaps could be scaled up to larger components like electric vehicle batteries, which can be particularly sensitive to temperature.
The research was published in the journal Nature Communications.
Source: Purdue University