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John O | March 2018

Researchers develop method for switching heat on and off in electronics

by josh perry, editor


a team of researchers at the university of illinois at urbana-champaign have developed a new technology that breaks the heat flow between a hotter region and a colder region, in effect allowing engineers to turn heat on and off.


schematic of the thermal switch showing the (a) on-state with the liquid metal droplet bridging the heat source and sink and (b) off-state with liquid metal removed from the channel. (university of illinois at urbana – champaign)


according to a report from the university website, “engineers have long desired a switch for heat flows, especially in electronics systems where controlling heat flows can significantly improve system performance and reliability. there are however significant challenges in creating such a heat switch.”


the new technology is based on the motion of a liquid metal droplet, which can be positioned to connect the path of the heat flow or moved away from the path to limit the heat flow.


“on one side of the switch there was a heat source representing the power electronics component, and on the other side of the switch, there was liquid cooling for heat removal,” the report explained. “when the switch was on, they were able to extract heat at more than 10 w/cm2. when the switch was off, the heat flow dropped by nearly 100x.”


researchers will continue to work on a prototype that integrates the switch with power electronics on a board.


the research was recently published in applied physics letters. the abstract stated:


“devices capable of actively controlling heat flow have been desired by the thermal management community for decades. the need for thermal control has become particularly urgent with power densification resulting in devices with localized heat fluxes as high as 1 kw/cm2.


“thermal switches, capable of modulating between high and low thermal conductances, enable the partitioning and active control of heat flow pathways. this paper reports a millimeter-scale thermal switch with a switching ratio >70, at heat fluxes near 10 w/cm2.


“the device consists of a silicone channel filled with a reducing liquid or vapor and an immersed liquid metal galinstan slug. galinstan has a relatively high thermal conductivity (≈16.5 w/mk at room temperature), and its position can be manipulated within the fluid channel, using either hydrostatic pressure or electric fields.


“when galinstan bridges the hot and cold reservoirs (the “on” state), heat flows across the channel. when the hot and cold reservoirs are instead filled with the encapsulating liquid or vapor (the “off” state), the cross-channel heat flow significantly reduces due to the lower thermal conductivity of the solution (≈0.03–0.6 w/mk).


“we demonstrate switching ratios as high as 15.6 for liquid filled channels and 71.3 for vapor filled channels. this work provides a framework for the development of millimeter-scale thermal switches and diodes capable of spatial and temporal control of heat flows.”

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