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Cheap, Efficient Thermoelectrics

Nanomaterials could be used for lower-emission cars and solar panels.

Thermoelectric materials promise everything from clean power for cars to clean power from the sun, but making these materials widely useful has been a challenge. Now researchers at MIT and Boston College have developed an inexpensive, simple technique for achieving a 40 percent increase in the efficiency of a common thermoelectric material. Thermoelectric materials, which can convert heat into electricity and electricity into heat, hold promise for turning waste heat into power. But thermoelectric materials have not been efficient enough to move beyond niche applications. The new jump in efficiency, achieved with a relatively inexpensive material, may finally make possible such applications as solar panels that turn the sun’s heat into electricity, and car exhaust pipes that use waste heat to power the radio and air conditioner.

Efficient crystals: Researchers increased the efficiency of a commonly used thermoelectric material, bismuth antimony telluride, by grinding it into a fine powder and pressing it back together. This technique creates random crystal lattices (lines in this tunneling-electron microscope image of the material) that interrupt the flow of heat.

The researchers started with bismuth antimony telluride, a thermoelectric material used in niche products such as picnic coolers and cooling car seats. Then Gang Chen, a professor of mechanical engineering at MIT; Institute Professor Mildred Dresselhaus; and Boston College physics professor Zhifeng Ren crushed it into a powder with a grain size averaging about 20 nanometers, and pressed it into discs and bars at high heat. The resulting material has a much finer crystalline lattice structure than the original material, which is made up of millimeter-scale grains. Chen and Ren’s nanocomposite formulation of the material is 40 percent more efficient than the conventional form of the material at 100 °C, and it works at temperatures ranging from room temperature to 250 °C.

“Power-generation applications [for thermoelectrics] are not big now because the materials aren’t good enough,” says Chen. He believes that his group’s more efficient version of the material will finally make such applications commercially viable.

Thermoelectric materials must be able to maintain a heat gradient, which means that they must be good conductors of electrons and good thermal insulators. When one end of a bar of thermoelectric material is heated, electrons move from the hot side to the cold, creating an electrical current. If a material conducts heat well, this current-generating temperature gradient will dissipate. Unfortunately, in most bulk materials, electrical conductivity and thermal conductivity “go hand in hand,” says John Fairbanks, who heads thermoelectrics efforts in the Department of Energy’s Vehicle Technologies Program.

One approach to making better thermoelectric materials has been to build nanostructured materials from the bottom up. Interfaces in these materials reflect the flow of heat without impeding electrical current. Researchers who have grown arrays of silicon nanowires, pressed silicon and germanium nanowires into millimeter-scale bars, and tested single organic molecules have had success on a small scale, but making such materials in bulk is a major hurdle.

The researchers’ nanocomposite technique creates many interfaces in the material that reflect thermal vibrations, says Chen. Peidong Yang, a professor of chemistry at the University of California, Berkeley, says that the work is “a great example of how defect engineering can significantly impact on the [vibration] transfer in solids.”

Ren says that it’s easy to make large amounts of the nanocomposite material: “We’re not talking grams; we’re not talking kilograms. We can make metric tons.” Because bismuth antimony telluride is already used in commercial products, Ren and Chen predict that their technique will be integrated into commercial manufacturing in several months.

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