In thermoelectrics, vibrations are key

Posted on 30. Aug, 2012 by in Academic Departments, Annual Report, Issues, Materials Science and Engineering, Research

Students in a clean room

Foreground, Materials Science Program PhD student Kyle McElhinny, and materials science postdoc Gokul Gopal work in a college cleanroom. Photo: Renee Meiller.

More and more, researchers are finding that things behave differently when observed on the nanoscale. Materials Science and Engineering Associate Professor Paul Evans has devised an x-ray analysis technique that allows researchers to study vibrations or how electrons travel in nanoscale samples of silicon crystal. Researchers knew the vibrations were occurring, but 99 percent of them were invisible because the tools to look at them were not sensitive enough. “The physics boils down to a pretty solid set of rules,” Evans says. “The issue is that when you change the number of atoms, the total number of atoms, and you put some boundaries in, the frequencies of the vibrations change, just a little bit.”

Understanding how the frequencies change at such a miniscule level ultimately could help engineers manipulate the vibrations of small structures and create more efficient thermoelectric devices.

The thermoelectric effect is the direct conversion of temperature differences to electric voltage, and vice-versa. Thermoelectric materials can be used to capture waste heat and convert it to electricity or can convert electricity to cool lasers or small portable refrigerators. The advantage is that thermoelectrics are small and require no coolants or moving parts.

The disadvantage is that current materials cannot operate at low cost with high power efficiency. Evans’ discovery could lead to improved or even new materials for use in thermoelectrics. “People have looked at materials for use in thermoelectrics that have large amplitude vibrations in the crystal,” Evans says.  “There are crystals that make big vibrations and scatter phonons effectively and don’t conduct heat very well but still conduct electricity. So the things that are in your thermal electric cooler are bulk thermal electrics. But if you can make things small and manipulate their thermal conductivity—make nanomaterials that have excellent thermal electric properties—that’s the hook. If you want to make better thermal electrics then you need to understand the fundamental inputs into the heat cell.”

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