New material aids heat recycling.

A new thermoelectric material that is twice as efficient as the best on the market could help cars to recycle waste heat as electricity. With as much as 60% of the energy produced by a car engine thought to be lost as heat, this is potentially a big source of power.

The thermoelectric effect is the direct conversion of heat into electricity as electrons flow from hot to cold. To maintain this effect the material needs to have unusual properties: the ability to conduct electricity well whilst conducting heat badly.

Japanese and US researchers took a well-known thermoelectric material, lead telluride, and added tiny quantities of thallium. Adding thallium enabled the researchers to take advantage of a quantum-mechanical quirk to double the material's thermoelectric efficiency (Science 2008, 321, 554). Author Joseph Heremans, professor of mechanical engineering at Ohio State University, says that the addition of thallium increases the number of available energy states that the electrons can occupy, improving the material's ability to generate a current.

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The thermoelectric material Heremans' group has developed works best in temperature ranges that are useful for car engines: 230500[degrees]C. 'The most interesting use of this material is for propulsion in hybrid vehicles,' Heremans says.

This work is part of a renaissance in thermoelectric materials, according to David Rowe, an honorary research professor at Cardiff University. He says that it may be possible to further improve the material's thermoelectric potential by engineering nanostructures into it.

Rowe says that BMW is already interested in thermoelectric materials to power cars' electrical systems and hopes to have them in its 5 Series range by 2010. However, he sounds one note of caution: 'Tellurium is an expensive material and is becoming quite rare.'

Heremans also comments that the toxic legacy of thermoelectric devices cannot be ignored. With both lead and thallium in this material it would need to be treated in a similar way to old batteries. Happily, thermoelectric materials can be recycled and could continue generating electricity for years. 'Some thermoelectric generators in satellites have been going for 30-40 years,' Heremans says.