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November 23, 2011

Nanocaviities for better thermoelectric materials

Thermoelectric materials are also currently used in the type of cooler bags that keep things cold without making use of their own cooling elements. These cooler bags are full of the elements Lead and Tellurium. Both of these substances are also toxic.

"We want to replace them with inexpensive and readily available substances. Moreover, there is not enough Tellurium to equip all of the cars in the world," says Ole Martin Løvvik, who is both an associate professor in the Department of Physics at the University of Oslo and a senior scientist at SINTEF.

With the current technology, it is possible to recover scarcely ten per cent of the lost energy. Together with the team of scientists led by Professor Johan Taftø, Løvvik is now searching for pollution-free, inexpensive materials that can recover fifteen per cent of all energy losses. That is an improvement of fully fifty per cent.

"I think we will manage to solve this problem with nanotechnology. The technology is simple and flexible and is almost too good to be true. In the long run, the technology can utilise all heat sources, such as solar energy and geothermal energy. The only limits are in our imagination," states Løvvik.

The new technology will initially be put to use in thermoelectric generators in cars. Several major automobile manufacturers are already interested. Løvvik and his colleagues are currently discussing the situation with General Motors.

Replacing the expensive material should allow the process to eventually create thermoelectrics as cheap as paint. But that goal has not been reached yet




If we create barriers in the material so that some atoms vibrate at different frequencies from their adjacent atoms, the heat will not be so easily dissipated.

"Moreover, the atomic barrier must be created in such a way that it does not prevent the electric current from flowing through it.”

Grinding nano-cavities at minus 196 degrees.

The scientists have found a method of creating these atomic barriers. The barriers are introduced densely in the special semi-conductors.

"We have achieved this by using a completely new "mill". Just as the miller grinds grain, the scientists will grind down semi-conductors to nano-sized grains. They will do that by cooling them down with liquid Nitrogen to minus 196 degrees. That makes the material more brittle, less sticky and easier to crush. It is important to grind down the grains as small as possible. Afterwards the grains are glued back together again, and in this way the barriers are created."

"The small irregularities in the barriers reflect the heat waves," says Løvvik.

The team of scientists uses an electron microscope to examine the micro-structures in the material.

"We have now discovered new nano-cavities in the materials and learned more about how they reflect heat waves.”

The scientists are now searching for the next generation of thermoelectric materials. They have just tested the cobalt arsenide mineral, skutterudite, which may be found at Skutterud at Blåfarveværket in Modum, Norway.

"It was just recently discovered that skutterudite may have atoms located in small nano-cavities. These cavities act as barriers to heat dissipation," concludes Løvvik.

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