Running Hot and Cold

Nanotech makes thermoelectric materials viable.

Institute Professor Mildred S. Dresselhaus wants to put a new twist on a 19th-century idea. Scientists have known for nearly 200 years about the thermoelectric effect: certain materials generate an electrical voltage when their temperature is different on each side. And when a voltage is applied to them, they heat up on one side and get colder on the other. Making materials with these properties, however, has always been a challenge: most materials that conduct electricity also conduct heat, so their temperature equalizes quickly. This makes them inefficient at generating electricity, and impractical for most heating or cooling applications.

Credit: Asia Kepka

But Dresselhaus, who researches the physics of nanoscale solids, says thermoelectric devices will become viable when new nanostructured materials that she and others are designing in the laboratory are commercialized. With Gang Chen at MIT, ­Zhifeng Ren at Boston College, and Jean-Pierre ­Fleurial at NASA's Jet Propulsion Laboratory, ­Dresselhaus is manipulating materials' energy transport properties at the nanoscale in order to develop good electrical conductors that are poor heat conductors. By working with composites made of complementary semiconducting materials such as bismuth telluride and silicon germanium, the researchers hope to create thermo­electric materials twice as efficient as their conventional counterparts.

Existing thermoelectric materials already have some applications, such as individually ­temperature-controlled car seats that can be efficiently warmed up when they're cold or chilled when they're sweaty. The system, made by a Michigan-based company, enhances fuel economy as well as comfort: "If you're sitting on a cool seat, you need less air conditioning," ­Dresselhaus points out. But even better would be applications that capture waste heat--say, from a car's tailpipe--and convert it into electricity. "We're all concerned about sustainable energy," she says. "If we could recycle waste heat to generate energy, we could use it for something useful."

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The current challenge is to incorporate nanoparticles into structures big enough to plug into a human-scale system. To this end, Dresselhaus squeezes silicon and germanium nanoparticles into a mold, then rapidly heats and cools them in a vacuum to compact them into millimeter-scale bars. By squeezing together small particles that differ in composition and size, she increases the surface area within the material, creating an ­obstacle course of internal nanoscale substructures that slow heat transfer while letting electrical energy zip through.

The new materials could help researchers build cooling systems into microchips, replace Freon-based HVAC systems in vehicles, make car engines more efficient, and improve photovoltaic efficiency by harnessing solar heat as well as light. "If we had improved materials that could be produced cheaply and in large quantities, certainly the thermoelectrics industry could move forward more quickly," Dresselhaus says. "It won't end with car seats."