December, 2019 Continued from previous page


Fluctuational Electrodynamics When heat travels between two separated


objects, it flows differently at the smallest scales — distances on the order of the diameter of DNA, or 1/50,000th of a human hair. For example, heat radiates 10,000 times faster at the nanoscale. Researchers have been aware of this for decades, but have not understood the process. Now at ULVL, researchers have measured how heat radi- ates from one surface to another in a vacuum at distances down to 2 nm. “We’ve shown, for the first time, the dramat-


ic enhancements of radiative heat fluxes in the extreme near-field,” says Reddy. “Our experiments and calculations imply that heat flows several orders of magnitude faster in these ultra-small gaps.” In the middle of the last centu-


ry, Russian radio physicist Sergei Rytov, proposed a new theory called “fluctuational electrodynamics” to describe heat transfer at distances smaller than 10 µm. Since then, research has not always resulted in supporting evidence. “Experiments were undertaken


in the 1990s and early 2000s that tried to test those ideas further. They found large discrepancies between what the theory would predict and what their experiments revealed,” explains Meyhofer. Because of the sophistication of the University of Michigan’s new laboratory, the researchers say their findings close the case — and Rytov was right.


Custom Low-Vibration Chamber The ULVL chamber was cus-


tom-designed for performing


nanoscale experiments, so precise, that they are sensitive to mere foot- steps. The vibration isolation system in the chamber consists of a seismic mass and a sophisticated low-vibra- tion table, which reduces vibrations from outside and inside, such as the mechanical vibrations associated with heating and cooling systems or elevators. The chamber limits acoustic


noise, temperature and humidity variations, as well as radio frequency and magnetic interference. “Our facility represents the true state of the art,” Meyhofer says. “When creat- ing nanoscale gaps, such as those required for our nanoscale heat radi- ation experiments, the slightest per- turbation can ruin an experiment.” In the chamber, the research -


ers use custom-built “scanning ther- mal microscopy probes” that allow them to directly study how fast heat flows between two surfaces of silica, silicon nitride and gold. The researchers chose these materials because they are commonly used in nanotechnology. For each material, they desig-


nated one sample (the planar sub- strate) that would be heated to 305°F (151.7°C). Using a probe, they coated the tip with the same material, but kept it connected at the base to a thermal reservoir that was main- tained at a cooler 98°F (36.7°C). The sample and the probe were slowly moved, together, in small steps, beginning at 50 nm until they were touching. The temperature of the tip was measured at regular distances between 2 nm and 50 nm. The cause of the rapid heat transfer, the researchers discovered, was the result of an overlap of the two sides’ surfaces, and evanescent waves, both of which carry heat. A phenomenon


Customized Minus K Technology negative-


stiffness vibration isolation table installed in the ULVL.


www.us- tech.com


present in nanoscale gaps. “These waves reach on a small distance into


Page 59 University of Michigan’s Ultra-Low Vibration Lab


the gap between materials,” says Bai Song, former graduate student in mechanical engineering. “And, their intensity at the extreme near-field is enormous compared with the electromagnetic waves at larger distances. When these waves from the two different devices overlap, that is when they allow tremendous heat flux.


Cooling Supercomputers with LEDs LEDs with electrodes can reverse-cool adjoin-


ing devices a mere few nanometers away. This approach could lead to new solid-state cooling technology for future microprocessors, which will have so many transistors packed into small spaces that current methods will be incapable of removing heat quickly enough. “We have demonstrated a second method for using photons to cool devices,” says Reddy, who co-


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