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Forschungszentrum Jülich - Annual Report 2011

A bout 60% of the energy gen- erated by the engine of a car from fuel is lost in the form of heat. Heat is also released in many industrial processes and in com- bined heat and power plants. However, we may be able to utilize this waste heat thanks to an effect discovered by phys- icist Thomas Johann Seebeck back in 1821: an electric voltage can be created by keeping the contact points of two dif- ferent electrical conductors or semicon- ductors at different temperatures. The strength of this voltage increases the higher the temperature difference is. The problem: thermoelectric generators (TEGs), which exploit the Seebeck effect by arranging two thermoelectric sem- iconductors in pairs, are currently on- ly able to convert a small proportion of waste heat into electric current – to be exact, usually less than 10%. In order to improve this level of effi- ciency, we need materials that conduct electric current well and heat badly. However, good electrical conductors tend to be good heat conductors as both properties are influenced by the mobility of electrons. “But thermal con- ductivity is not just determined by elec- trical charge carriers; thermal conduc- tivity can also be influenced by lattice vibrations. By reducing these vibrations, we can create more efficient thermoe- lectric materials,” says Dr. Raphael Her- mann. His team at the Jülich Centre for Neutron Science uses neutron scatter- ing and synchrotron radiation to investi- gate these mechanisms of thermal con- ductivity. The energy quanta of lattice vibra- tions are called phonons – similar to photons, which are the energy quanta of light. Three factors determine how they transport heat: their velocity, free path length and thermal capacity. “It’s like the transport of goods by road: the fast- er a truck can drive, the fewer traffic jams it encounters and the bigger its cargo space, and so the more goods it can transport,” says Hermann. The re- searchers are looking at materials with chemical formulae such as Yb14MnSb11, FeSb3 and Sr8Ga16Ge30, and determining which factors are decisive in limiting the thermal conductivity of the phonons. For her PhD thesis on this topic, Anne Möchel from Hermann’s team received one of two Young Researchers Awards from the German Thermoelectric Socie- ty (DTG) in 2011. According to the press release, her work represents an impor- tant step towards improving our under- standing of thermal transport. And this understanding is the key to finding effi- cient thermoelectric materials. Wasting less energy in our everyday lives: thermoelec- tric materials could be the answer. They transform waste heat into electricity, albeit not yet efficiently enough. Physicists at Jülich are investigating how these materials can be improved. Electricity from Waste Heat The atomic composition of a thermoe- lectric material with the chemical formula Sr8Ga16Ge30. Car manufacturers test thermoelectric generators to convert waste heat into electric current for on-board electronic systems. They hope that this will reduce fuel consumption by up to 5%. Annual Report 2011 | Forschungszentrum Jülich 23