9 October 2008

Thermoelectric materials aid in energy efficiency

With a new understanding of thermoelectric materials, there may soon be a way to develop motors that are more fuel-efficient and have environmentally friendly cooling methods.

Thermoelectric materials can assemble into units, which can transform the thermal difference to electric energy or vice versa-electrical current to cooling. An effective utilization, however, requires the material to supply a high voltage and has good electrical, but low thermal conductivity, according to new data unveiled by researchers at the University of Århus, Risø-DTU and the University of Copenhagen.

Crystal structure of a nano-cage
The crystal structure of a

The new data shows why some thermoelectric materials can have the desired low thermal conductivity without degrading electrical properties. This can be crucial for the conversion of wasted heat from vehicle exhaust emissions. Leading car manufacturers are now working to develop this possibility, and the first models are close to production. That technology can give cars considerably improved fuel economy, said Bo B. Iversen, professor at iNANO at the University of Århus. That can also contribute to the development of new cooling methods, so one avoids the most common, but very environmentally damaging greenhouse gas, R-134a.

One of the most promising thermoelectric materials in the group of clathrates, which create crystals full of "nano-cages," is now under investigations, researchers said.

By placing a heavy atom in each nano-cage, researchers can reduce the crystals' ability to conduct heat. Until now, they thought it was the heavy atoms random movements in the cages that were the cause of the poor thermal conductivity, but this ended up not being true, said Asger B. Abrahamsen, senior scientist at Risø-DTU.

Researchers used the technique of neutron scattering, which gives them opportunity to look into the material and see the atoms' movements.

"Our data shows that it is rather the atoms' shared pattern of movement that determines the properties of these thermoelectric materials. A discovery that will be significant for the design of new materials that utilize energy even better," said Kim Lefmann, associate professor at the Nano-Science Center, the Niels Bohr Institute at the University of Copenhagen.

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