Exploring a Novel, Nanoscale Thermal Switch

ANN ARBOR—Thermal switches that can effectively control the flow of heat are key to enabling a number of applications ranging from the thermal management of nanoscale devices, refrigeration, data storage, thermal computing all the way to the thermal management of buildings. However, in comparison to the vast array of devices, e.g. transistors and diodes, available to control the flow of electricity there exists currently very few proposals for controlling the flow of heat, especially at the nanoscale.

To overcome this challenge researchers have been exploring novel nanoscale phenomena that may enable novel functional thermal devices. Towards this goal, researchers at the University of Michigan (U-M), from the groups of Edgar Meyhofer and Pramod Reddy, have shown for the first time how a nanoscale thermal switch can be built by employing nanoscale effects that arise when heat is transferred between a hot and cold nanoscale-thick membrane via thermal radiation.

“In 2018 we published an important paper in Nature which showed that a famous limit to heat flow called the blackbody limit can be overcome when the dimensions of the objects can be reduced to the nanoscale,” said Pramod Reddy, a professor of
mechanical engineering at the University of Michigan.

Thermal switches that can effectively control the flow of heat are key to enabling a number of applications ranging from the thermal management of nanoscale devices, refrigeration, data storage, thermal computing all the way to the thermal management of buildings. However, in comparision to the vast array of devices, e.g. transistors and diodes, available to control the flow of electricity there exists currently very few proposals for controlling the flow of heat, especially at the nanoscale. Often, the discovery of new phenomena leads to fresh opportunities for creating new devices with functionalities that could not be imagined before. After their 2018 discovery, which highlighted how heat is transported in preferential directions from
nanoscale membranes, Dr. Dakotah Thompson, the lead author of the 2018 study, began exploring potential applications of this new discovery.

“After some thought it became apparent to us that we could potentially create a thermal switch by controlling the emission properties of the nanomembranes by bringing a third object into close proximity of the nanomembrane,” said Edgar Meyhofer, a professor of mechanical engineering at the University of Michigan.

In order to test this hypothesis, Dakotah Thompson, then a graduate student at Michigan, developed a scheme where a planar object can be brought into close proximity (microns) of two co-planar membranes that were exchanging heat.

“In order to accomplish this challenging goal, I nanofabricated both suspended calorimetric devices that had unprecedented calorimetric resolution and a planar mesa-shaped object, and controlled the separation between them using a custom developed nanopositioner,” said Thompson.

From these experiments, the authors could show that heat transfer between nanoscale membranes can be turned on and off by simply modifying the separation between the membranes and the planar mesa.

In order to make precise numerical predictions of the experimental observations Dr. Linxiao Zhu, a postdoctoral fellow at Michigan, and Thompson performed detailed calculations that showed how the observations can be quantitatively related to how the propagation of light, which is the carrier of heat, from one membrane to the other is impeded by the planar mesa which can either absorb the light propagating between the membranes or reflect it away from the membranes.

“US Army investments in basic research are leading to the discovery of new effects and proof of concept demonstrations of novel thermal devices that can have a strong impact on thermal management for next-generation computing,” said Dr. Chakrapani Varanasi, a program manager at the Army Research Office whose program supported this work via a multi-disciplinary university research initiative.

This work is described in a paper in the journal Nature Nanotechnology, titled, “Nanoscale Radiative Thermal Switching via Multi-Body Effects.”

This study was funded by the U.S. Army Research Office and the Department of Energy. The devices were made in the Lurie Nanofabrication Facility at U-M. Meyhofer is also a professor of biomedical engineering. Reddy is also a professor of
materials science and engineering. Thompson is an assistant professor of mechanical engineering at the University of Wisconsin, Madison.

Dakotah Thompson: https://graingerinstitute.engr.wisc.edu/staff/thompson-dakotah/
Edgar Meyhofer: https://me.engin.umich.edu/people/faculty/edgar-meyhofer
Pramod Reddy: https://me.engin.umich.edu/people/faculty/pramod-sangi-reddy
Paper abstract: https://www.nature.com/articles/s41565-019-0595-7

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