Two teams of US scientists have demonstrated silicon-based 'thermoelectric' materials that could convert waste heat back into electricity[1,2] - potentially giving a boost to the efficiency of everything from power stations to refrigerators.

Until now, efficient thermoelectric materials have used pricier metals such as bismuth or tellurium. Switching over to silicon could have a global impact, allowing large-scale thermoelectric devices to be produced more cheaply. 

SEM image of the silicon nanowire array (green, centre) showing all electrical connections. The temperature difference is controlled by two Joule heaters, shown in white and red © Nature

The thermoelectric effect occurs when one end of a material, such as a wire, is heated. Electrons travel to the colder end, producing an electrical current. However, to harness this energy effectively it is critical that the temperature difference is maintained. This requires a material with an unusual combination of properties - excellent electrical conductivity but poor heat conductivity. As silicon conducts heat well, no-one was expecting it to be a good candidate.

But now, two teams have shown that silicon nanowires can act as efficient thermoelectric materials. James Heath of the California Institute of Technology led one of the teams. 'It was a surprise,' he told Chemistry World. 'We found that when you make extremely small silicon nanowires there are some fundamental differences in how these materials behave and transfer heat.'

Heath believes that with further research the efficiency of these materials could be boosted even further. The technology could help to cool and power microchips in the future, he added.

A second team, led by Peidong Yang at the University of California in Berkeley, took a different approach and used much larger nanowires. 'Our trick was to use lots of defects,' said Yang. 'We made silicon nanowires with very rough surfaces and then doped these nanowires with boron. The defects help to block the atomic vibrations that produce heat flow, but still allow electrical current to flow.'

Francis DiSalvo has worked extensively on thermoelectric materials at the Cornell Fuel Cell Institute, Cornell University, New York. 'This research is exciting but there is still some way to go,' he told Chemistry World.

One important role for thermoelectric materials would be capturing heat that is lost during energy production by power stations or car engines, DiSalvo said.   'If these materials can be made cheaply enough, capturing even a fraction of this wasted heat would make them worthwhile,' he added.

Lewis Brindley