Although nobody wants a hot smartphone, there's no way that that things could ever incorporate tiny cooling fans. Some day, though, they may stay frosty through the use of a material that absorbs and releases water.

We have already seen cooling systems that utilize what are known as phase change materials. Often taking the form of a wax, these turn into a liquid when the temperature rises, absorbing ambient heat as they do so – this helps keep their environment cool. Once the temperature drops, the materials revert to a solid form, releasing the stored heat.

It's a clever form of cooling, but according to scientists from China's Shanghai Jiao Tong University, the total amount of energy that's exchanged through the solid-liquid transition isn't very significant.



Instead, the researchers looked to compounds called metal organic frameworks (MOFs). One in particular, by the name of MIL-101(Cr), excels at absorbing water vapor from the surrounding air while temperatures are low, storing it as a liquid, and then releasing it in vapor form at higher temperatures. As the vapor is released, it carries heat from the MOC with it, cooling the material down.

Led by Prof. Ruzhu Wang, the researchers started with three 16-square-centimeter (2.5-sq-in) aluminum sheets, and covered one side of each with a coating of MIL-101(Cr). The thickness of those coatings was different on each sheet, ranging from 198 micrometers on the thinnest up to 516 on the thickest.

When the sheets were subsequently heated on a hot plate, the MOF was found to make a big difference in how quickly they reached a temperature of 60 ºC (140 ºF). For instance, while an uncoated sheet took just 5.2 minutes to hit 60 degrees, the sheet with the thinnest coating stretched that out to 11.7 minutes. The sheet with the thickest coating took 19.35 minutes.

In a more practical test of the technology, the scientists applied a coating of MIL-101(Cr) to a heat sink on a microcomputer. As compared to an uncoated counterpart, that sink stayed a total of 7 ºC degrees cooler while running at heavy workloads for 15 minutes.

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Wang and colleagues are now looking at improving the thermal conductivity of the material, as it can actually hamper heat-dissipation once it thoroughly dries out. They're also hoping to find ways of reducing its cost, as MOFs are currently quite expensive.

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