Developed by Jonathan Boreyko, an associate professor in mechanical engineering at Virginia Tech university, the ‘planar bridging-droplet thermal diodes’ are reportedly highly efficient and extremely versatile.
“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.
Diodes use engineered materials to allow heat to conduct in only one direction. They are attractive for thermal management because they enable the dumping of heat entering one side, while resisting heat on the opposite side. On an aircraft, for example, 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. 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 are sucked back into the wick, starting the process again.
If the source of heat is applied to the hydrophobic side, no steam is produced because the water remains trapped in the wick. This is why the device can only conduct heat in one direction.
The new diode can be affixed to heat sources, such as CPU chips. 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 nearby heat sources, 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,” a research announcement said. Boreyko said current diodes only conduct a few times more heat in one direction, or require gravity to assist. The new bridging-droplet thermal diode can be used upright, sideways or upside-down, and could even work in space.
The research was published in Advanced Functional Materials.
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