Can your phone sweat to keep cool?
Researchers propose a new strategy for transient thermal management.
By Nick Carne
Chinese engineers are investigating whether our phones can regulate their temperature by sweating – sort of.
Writing in the journal Joule, a team from Shanghai Jiao Tong University describes a coating it has developed that releases water vapour to dissipate heat from devices that are too small for fans, which are used to regulate the temperature of larger ones.
"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 refrigeration engineer and senior author Ruzhu Wang.
Some manufacturers use phase change materials (PCMs), such as waxes and fatty acids, for cooling in phones. These can absorb heat produced by devices when they melt.
However, the researchers say the total amount of energy exchanged during a solid-liquid transition is relatively low. In contrast, the liquid-vapour transition of water can exchange 10 times as much.
Wang and his team studied a group of porous materials that could absorb moisture from the air and release water vapour when heated. Metal-organic frameworks (MOFs) were found to be 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 0.3 grams of material in our experiment, and the cooling effect it produced was significant."
The team selected a type of MOFs called MIL-101(Cr), which has good water-absorbing capacity and high sensitivity to temperature changes, and tested three different thicknesses on aluminium sheets.
While an uncoated sheet reached 60 degrees Celsius after just 5.2 minutes, the thinnest coating extended this to 11.7 minutes and the thickest to 19.35.
"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."
When tested on heat sinks in a microcomputing device, the coating reduced chip temperature by up to seven degrees when the device was run at heavy workloads for 15 minutes.
The next steps, the researchers say, are to further improve the material's thermal conductivity and to bring the costs down so the idea becomes a commercial reality.