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Thermal diode works in space

Technology News |
By Nick Flaherty


Researchers in the US has developed a thermal management technology for aircraft that can be used in data centres and space designs for removing heat.

Jonathan Boreyko, an associate professor in mechanical engineering, and his team at Virginia Tech developed a bridging-droplet thermal diode as a novel approach to thermal management.

“We are hopeful that the one-way heat transfer of our bridging-droplet diode will enable the smart thermal management of electronics, aircraft, and spacecraft,” said Boreyko.

Thermal diodes allow heat to conduct in only one direction by use of engineered materials. For management of heat, diodes are attractive because they enable the dumping of heat entering one side, while resisting heat on the opposite side. In the case of aircraft (the focus of Boreyko’s funding), heat is absorbed from an overheated plane, but resisted from the outside environment.

Boreyko’s team created a diode using two copper plates in a sealed environment, separated by a microscopic gap. The first plate is engineered with a wick structure to hold water, while the opposite plate is coated with a water-repelling (hydrophobic) layer. The water on the wicking surface receives heat, causing evaporation into steam. As the steam moves across the narrow gap, it cools and condenses into dew droplets on the hydrophobic side. These dew droplets grow large enough to “bridge” the gap and get sucked back into the wick, starting the process again.

If the source of heat were instead applied the hydrophobic side, no steam can be produced because the water remains trapped in the wick. This is why the device can only conduct heat in one direction.

What does this look like in practice? An object producing heat, like a CPU chip, overheats if this heat is not continually removed. Boreyko’s invention is affixed to this heat source.

Used as a heatsink for a high performance CPU in a data centre, the generated heat is transferred through the conducting plate, into the water. Water turns to steam and moves away from the source of the heat. The hydrophobic, nonconducting side prevents heat from entering via the air or other heat sources that may be near, allowing the diode to manage the heat only from its main subject.

Boreyko’s team measured a nearly 100-fold increase in heat conduction when the wicked side was heated, compared to the hydrophobic side. This is a significant improvement to existing thermal diodes. According to Boreyko, current diodes are either not very effective, only conducting a few times more heat in one direction, or require gravity. This new bridging-droplet thermal diode can be used upright, sideways, or even upside-down, and would even work in space where gravity is negligible.

The team has filed a provisional patent and is in search of industry partners to carry on the work.

The research was published in Advanced Functional Materials.

www.vtnews.vt.edu

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