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Forschungszentrum Jülich - Research in Jülich 2_2012

2|2012 Research in Jülich 21 cause both properties are influenced by the mobility of electrons. “However, thermal conductivity is due not only to electrical charge carriers, but also to lat- tice vibrations. By reducing their share, it is possible to produce more efficient thermoelectric materials,” says Her- mann, who lectures at the University of Liège in Belgium as a guest professor. As the name of his young investiga- tors group – “Lattice dynamics in emerg- ing functional materials” – suggests, he specializes in experiments on lattice vi- brations and mechanisms of heat trans- port. In fact, his group, which currently includes five PhD students and one post- doc, conduct research not only on ther- moelectric materials, but also phase- change materials, for example, which are important for optical data storage. In the labs of the Jülich Centre for Neutron Science (JCNS) on campus, the scientists measure elastic constants and other macroscopic properties of materi- als related to movements of the lattice. However, in order to take a look at lat- tice vibrations under the microscope, they require access to large-scale facili- ties at international research institu- tions. For example, they regularly use the measuring facilities of JCNS at the FRM-2 research reactor in Garching near Munich and at the Institute Laue- Langevin in Grenoble, France. Further- more they study the materials at institu- tions such as the European Synchrotron Radiation Facility, which is also located in Grenoble, and at the photon source of the Argonne National Laboratory, USA. ROLE OF LATTICE VIBRATIONS Microscopic images of the lattice vi- brations show that their energy is “quan- tized”, meaning that it can only take spe- cific values. Quantized lattice vibrations are known as phonons – comparable to photons, which are energy quantums of light. There are three factors that deter- mine heat transport by phonons: veloci- ty, free path, and thermal capacity. “It can be compared to the transport of goods by road: the faster a truck goes, the less time it spends stuck in traffic jams and the larger its cargo space is, the more it can transport,” explains Her- mann. The researchers determined the key limiting factor for the thermal conductiv- ity of phonons in thermoelectric materi- als described with chemical formulas such as Yb14MnSb11, FeSb3 and Sr8Ga16Ge30. Hermann’s former PhD stu- dent Anne Möchel, now Anne Houben and working at the Max Planck Institute for Plasma Physics, received a young scientists award from the German Thermoelectric Society (DTG) for her PhD thesis in 2011. According to the DFG press release, her work represents an important step towards improving our understanding of thermal transport. Her- mann is confident: “This understanding of heat transport is a key factor in the quest for thermoelectric materials with higher efficiency.” He adds that Jülich provides ideal conditions for him to con- tinue to be successful with his research. He has obviously convinced the German Research Foundation (DFG) and the Fed- eral Ministry of Education and Research (BMBF) as well. The funding they provide now considerably bolsters Hermann’s re- search budget from the Helmholtz pro- gramme. :: The atomic structure of two thermoelectric materials: Sr8Ga16Ge30 (left) und Yb14MnSb11 (right). Car manufacturers test thermoelectric generators in the exhaust gas system in order to convert exhaust heat into electric power for on-board electronics. EARLY-CAREER SCIENTISTS | Thermopower

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