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First-ever videos show how heat travels through materials

Press Trust of India  |  Washington 

In an "extremely exciting" discovery, scientists with the aid of an ultrafast electron microscope have recorded the first-ever videos of how heat travels through materials at the speed of sound.

The research by University of Minnesota in US provides unprecedented insight into roles played by individual atomic and nanoscale features that could aid in the design of better, more efficient materials with a wide array of uses, from personal electronics to alternative-energy technologies.

Such work would be greatly aided by actually watching heat move through materials, but capturing images of the basic physical processes at the heart of thermal-energy motion has presented enormous challenges, researchers said.

They used a brief laser pulse to excite electrons and very rapidly heat crystalline semiconducting materials of tungsten diselenide and germanium.

They then captured slow-motion videos of the resulting waves of energy moving through the crystals.

"As soon as we saw the waves, we knew it was an extremely exciting observation. Actually watching this process happen at the nanoscale is a dream come true," said lead researcher David Flannigan, from the University of Minnesota.

Flannigan said the movement of heat through the material looks like ripples on a pond after a pebble is dropped in the water.

The videos show waves of energy moving at about 6 nanometres per picosecond. Mapping the oscillations of energy, called phonons, at the nanoscale is critical to developing a detailed understanding of the fundamentals of thermal-energy motion.

This is because the fundamental length scales are nanometres and the speeds can be many miles per second. Such extreme conditions have made imaging this ubiquitous process extraordinarily challenging.

Materials scientists and engineers have spent decades researching how to control thermal energy at the atomic level in order to recycle and use it to increase efficiencies and ultimately drive down the use of fossil fuels.

To overcome these challenges and image the movement of heat energy, the researchers used an ultrafast electron microscope (UEM) capable of examining the dynamics of materials at the atomic and molecular scale over time spans measured in femtoseconds.

"In many applications, scientists and engineers want to understand thermal-energy motion, control it, collect it, and precisely guide it to do useful work or very quickly move it away from sensitive components," Flannigan said.

"Because the lengths and times are so small and so fast, it has been very difficult to understand in detail how this occurs in materials that have imperfections, as essentially all materials do," he said.

"Literally watching this process happen would go a very long way in building our understanding, and now we can do just that," he added.

The study was published in the journal Nature Communications.

First Published: Sat, April 16 2016. 19:48 IST
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