New coating helps phones stay cool by sweating

Mammals sweat to regulate their body temperature, and researchers from Shanghai Jiao Tong University in China are exploring whether our phones could do the same. In a paper in Joule, the researchers present a coating for electronics that releases water vapor to dissipate heat from running devices – a new thermal management method that could prevent electronics from overheating and keep them cooler than is currently possible.

"The development of microelectronics puts great demands on efficient thermal management techniques, because all the components are tightly packed and chips can get really hot," says senior author Ruzhu Wang, who studies refrigeration engineering at Shanghai Jiao Tong University. "For example, without an effective cooling system, our phones could have a system breakdown and burn our hands if we run them for a long time or load a big application."

Larger devices such as computers use fans to regulate temperature. However, fans are bulky, noisy, and energy consuming, and thus unsuitable for smaller devices like mobile phones. Up to now, manufactures have been using phase change materials (PCMs) such as waxes and fatty acids for cooling in phones. These materials absorb the heat produced by devices when they melt, but the total amount of energy exchanged during the solid-liquid transition is relatively low.

In contrast, the liquid-vapor transition of water can exchange 10 times more energy than the PCM solid-liquid transition. So, inspired by mammals' sweating mechanism, Wang and his team studied a group of porous materials that could absorb moisture from the air and then release water vapor when heated. Among them, metal-organic frameworks (MOFs) proved the most promising because they could store a large amount of water and thus take away more heat when heated.

"Previously, researchers have tried to use MOFs to extract water from the desert air," Wang says. "But MOFs are still really expensive, so large-scale application isn't really practical. Our study shows electronics cooling is a good real-life application of MOFs. We used less than 0.3g of material in our experiment, and the cooling effect it produced was significant."

The team selected a type of MOF called MIL-101(Cr) for the experiment because of its good water-absorbing capacity and high sensitivity to temperature changes. They coated three 16cm² aluminum sheets with MIL-101(Cr) of different thicknesses – 198µm, 313µm and 516µm – and heated them on a hot plate.

The team found that the MIL-101(Cr) coating was able to delay the temperature rise of the sheets, and that this effect increased in line with coating thickness. While an uncoated sheet took 5.2 minutes to reach 60°C, the thinnest coating doubled this time, so that it took 11.7 minutes for a sheet to reach the same temperature. For the sheet with the thickest coating, it took 19.35 minutes of heating to reach 60°C.

"In addition to effective cooling, MIL-101(Cr) can quickly recover by absorbing moisture again once the heat source is removed, just like how mammals rehydrate and ready to sweat again," Wang says. "So, this method is really suitable for devices that aren't running all the time, like phones, charging batteries and telecommunications base stations, which can get overloaded sometimes."

To investigate the cooling effect of MIL-101(Cr) on actual devices, Wang and his team tested a coated heat sink on a microcomputing device. Compared to an uncoated heat sink, the coated one reduced the chip temperature by up to 7°C when the device was run at heavy workloads for 15 minutes.

Looking forward, the team plans to improve the material's thermal conductivity. "Once all the water is gone, the dried coating will become a resistance that affects devices' heat dissipation" says first author Chenxi Wang. Incorporating thermal conductive additives such as graphene into the material may help address this problem, he says.

At the moment, the cost of these MOFs will prevent them from being incorporated into phones, but this research could help to resolve that problem. "By finding MOFs a practical application, we hope to increase the market demand for them and encourage more research on MOFs to bring down the costs," Wang says.

This story is adapted from material from Cell Press, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.