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About two months ago I reported on research being done to make thermoelectric materials using a standard microwave oven and some innovative nanotechnology. The process, developed by RPI, aims to make thermoelectric devices both more affordable and more effective. But new research from the California Institute of Technology provides another possible solution: a new liquid-like compound with a higher conversion efficiency.
Thermoelectrics: A Brief Summary
If you're unfamiliar with thermoelectrics, they are devices that can create electricity from a difference in temperature or create a temperature difference using electricity.. This is accomplished by creating a system in which the movement of charge carriers from one side to another is induced by a temperature gradient between these sides (or vice versa where charge induces temperature changes), called the thermoelectric effect.
Thermoelectrics currently have a niche in refrigeration where reliability and system life are more important than cost or efficiency. They are also used in cars to convert waste heat to electricity. They have the potential to make any process that involves waste heat more efficient, but the low conversion rates and high material prices don't usually provide a positive ROI.
Making Thermoelectrics Better
Effective thermoelectric materials must have high electrical conductivity but low thermal conductivity. Good electrical conduction means electrons can flow easily through the material with little resistance. Poor heat conduction is needed to prevent the material from quickly reaching thermal equilibrium - once the two sides are the same temperature, the electrons stop flowing.
To make thermoelectrics more effective, researchers have been looking to combine properties of amorphous and crystalline structures to develop a material with the most ideal thermoelectric properties. In solids, heat is transferred through vibrations both longitudinally and laterally. In liquids, these vibrations only travel via longitudinal waves. Because of this, a liquid-like material that is also good at conducting electricity should be more efficient than traditional amorphous materials.
Caltech's study was on copper-selenium material, which exhibited these combined characteristics. Although it is physically a solid, the flow of its copper atoms through the crystal lattice gives it liquid-like behavior. The team found that the crystal structure of the selenium provided better electricity conduction while the heat transfer was dampened by free flowing copper atoms. Copper-selenium was actually used in spacecraft design by NASA around 40 years ago, but its unique properties were not understood at that time.
Opening a New Door
While copper-selenium may be more efficient, it's likely to be difficult to produce on a large scale because of its liquid-like nature. The importance of this research is that it provides scientists a new approach to finding other, more viable thermoelectric materials that exhibit similar qualities.
I'm always excited to hear about developments in this technology - there are so many possibilities that stem from thermoelectric development. Thermoelectrics in refrigeration could cut the costs of commercial and industrial cooling systems and replace the types of coolers we use today involving compressors and refrigerants (more moving parts). In addition, any technology/process that generates waste heat, (be it combustion engines or incinerators) could potentially benefit from the integration of thermoelectrics into the system. Nifty little gadgets that harness untapped sources of heat energy, such as human body heat, are another outcome of progress in thermoelectrics. I mean, who wouldn't want a cellphone that could be charged just by holding it?
Sources:
Caltech News
Thermoelectrics - Caltech Materials Science
Wake Forest University - Power Felt Gives a Charge
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