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Shanghai researchers want to make your phone 'sweat'

Professional Engineering

An infrared image shows a heat sink coated with MIL-101(Cr) is cooler than one without (Credit: Chenxi Wang)
An infrared image shows a heat sink coated with MIL-101(Cr) is cooler than one without (Credit: Chenxi Wang)

A sweating phone sounds like a cause for concern, but a group of researchers hope that replicating the natural process could help devices stay cool under intense use.

To mimic the process that mammals use to regulate body temperature, researchers from Shanghai Jiao Tong University in China developed a coating for electronics that releases water vapour to dissipate heat. The coating could offer a new thermal management method to stop mobile electronics from overheating and keep them cooler compared to existing strategies.

“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,” said refrigeration engineer Ruzhu Wang. “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 intensive, making them unsuitable for smaller devices like mobile phones. Manufacturers use phase change materials (PCMs) such as waxes and fatty acids for cooling in phones. The materials absorb 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-vapour transition of water can exchange 10 times the energy of PCM solid-liquid transition. Wang and his team studied a group of porous materials that could absorb moisture from the air and release water vapour when heated. Among them, metal organic frameworks (MOFs) were the most promising because they could store a large amount of water and therefore take away more heat.

“Previously, researchers have tried to use MOFs to extract water from the desert air,” said Wang, who was senior author of the piece. “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 aluminium sheets with MIL-101(Cr) of different thicknesses – 198, 313, and 516 micrometres respectively – and heated them on a hot plate.

The team found that MIL-101(Cr) coating was able to delay the temperature rise of the sheets, and the effect increased with coating thickness. While an uncoated sheet reached 60°C after 5.2 minutes, the thinnest coating didn't reach the same temperature until 11.7 minutes. The sheet with the thickest coating reached 60°C after 19.35 minutes of heating.

MIL-101(Cr) can quickly recover by absorbing moisture again once the heat source is removed, making it suitable for devices that aren't running all the time such as phones, charging batteries and telecommunications base stations, which can get overloaded.

The team also coated a heat sink on a microcomputing device. Compared to an uncoated heat sink, it reduced the chip temperature by up to 7°C when the device was run at heavy workloads for 15 minutes.

The researchers hope to improve the material's thermal conductivity, as the dry coating could effect heat dissipation after water has evaporated. Incorporating thermal conductive additives such as graphene into the material could address the problem.

High cost is a ‘major’ issue, but the team said they hope to help bring down the costs of MOFs by creating a practical application for them.

The research was published in Joule.

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Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.


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