A moth frying on a bug lamp proves, suicidally, that an electrical current generates heat. But a current can also cool, if it runs through the right stuff. Electrons carry away heat when they rattle through exotic semiconductors known as thermoelectric materials--but good ones are hard to find. Indeed, a new thermoelectric material described in the 11 February Science is the first one in decades to lower the temperature bar in the thermoelectric limbo.
Like air molecules streaming across hot food, electrons flowing in a current absorb and carry away heat. In most materials, this effect produces no cooling, because the heat quickly flows back in the opposite direction, carried along in vibrations of the solid through which the electrons flow. To keep the heat flowing one way, chemists must keep the number of electrons high, ensure that each electron carries a healthy dollop of heat, and soften the material to damp vibrations. Unfortunately, the three properties tend to work against each other, which is why new thermoelectric materials are hard to find.
A team led by chemist Mercouri Kanatzidis of Michigan State University in East Lansing set out to systematically vary the 30-year-old recipe for the best thermoelectric material available. They hit upon an unexpected crystalline structure of cesium, bismuth, and tellurium that cools nearly as efficiently as the best material currently available. And while its decades-old rival conks out at -50°C, the new stuff keeps working to roughly -100°C.
"In some ways, it's a little bit of serendipity," Kanatzidis says. But others credit the researcher's method more than his luck. Galen Stucky, a chemist at the University of California, Santa Barbara, says "Kanatzidis is doing some super work in this field."