The view inside the Ultra High Vacuum Scanning Thermal Microscope, which was used to measure temperature fluxes at the nanoscale. Credit: Joseph Xu
When heat travels between 2 objects that aren’t touching, it flows differently at the smallest scales – distances on the order of the diameter of DNA, or 1/50,000 of a human hair. While researchers have been aware of this for decades, they haven’t understood the process. Heat flow often needs to be prevented or harnessed and the lack of an accurate way to predict it represents a bottleneck in nanotechnology development.
Now, in a unique ultra-low vibration lab at the University of Michigan, engineers have measured how heat radiates from one surface to another in a vacuum at distances down to 2nm. While the thermal energy still flows from the warmer place to the colder one, the researchers found it does so 10,000X faster than it would at the scale of, say, a bonfire and a pair of chilly hands. “Faster” here refers to the speed at which the temperature of one sample changes the temperature of the other -and not the speed at which the heat itself travels. Heat is a form of electromagnetic radiation, so it moves at the speed of light. What’s different at the nanoscale is the efficiency of the process. “We’ve shown, for the first time, the dramatic enhancements of radiative heat fluxes in the extreme near-field,” said A/Prof Pramod Reddy.
The view inside the Ultra High Vacuum Scanning Thermal Microscope, which was used to measure temperature fluxes at the nanoscale. Image credit: Joseph Xu
APPS: They could advance next-generation information storage such as heat-assisted magnetic recording. They could push forward devices that more directly convert heat into electricity, including heat generated in cars and spacecrafts that is now being wasted. Those are just a few potential uses.
The phenomenon studied is “radiative heat”, electromagnetic radiation, or light, that all matter above 0K emits. It is the emission of the internal energy of matter from movement of particles in matter- movement that only happens above absolute zero.
In the middle of the last century, the Russian radio physicist Sergei Rytov proposed a new theory called “fluctuational electrodynamics” to describe heat transfer at smaller-than-10-micrometer distances. Since then, research hasn’t always resulted in supporting evidence. Because of the sophistication of the U-M lab, the researchers say their findings close the case, and Rytov was right.
METHOD: In the chamber, they used custom-built “scanning thermal microscopy probes” that allowed them to directly study how fast heat flows between 2 surfaces of silica, silicon nitride and gold. These materials arecommonly used in nanotechnology. For each material, they designated one sample that would be heated to 305F, and they coated the tip of the probe with the same material, but kept it at a cooler 98F. They slowly moved the sample and the probe together, beginning at 50 nm until they were touching, and they measured the temperature of the tip at regular intervals.
MOA: The cause of the rapid heat transfer, the researchers discovered, is that in nanoscale gaps there can be an overlap of the 2 sides’ surface and evanescent waves, both of which carry heat. When these waves from two different devices overlap, that’s when they allow tremendous heat flux.”
http://ns.umich.edu/new/releases/23361-heat-radiates-10-000-times-faster-at-the-nanoscale
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