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Nature of bonding determines thermal conductivity
Date:5/3/2011

nce, who is also currently a visiting professor at the University of Lige. "Phase change materials are very well suited because they are poor conductors of heat not only in the unordered but also in the ordered state, in contrast to semiconductors, for example," adds Prof. Matthias Wuttig from RWTH Aachen University. As part of an international research team, Hermann and Wuttig are investigating the reasons for this surprising material behaviour on alloys of germanium, antimony and tellurium. With the aid of sophisticated scattering experiments at the European Synchrotron Radiation Facility (ESRF) in Grenoble, they demonstrated that the bonding conditions between the atoms in the crystalline state as well as deviations from the perfect lattice structure influence the transmission of these vibrations through the material and thus reduce its thermal conductivity.

"The starting point for our investigations was the observation by our Japanese colleagues that the amorphous material is harder than the crystalline," says Hermann. "This contradicted all assumptions, but the measured stronger bonding forces between the atoms in the amorphous state fitted the picture." The Jlich scientists investigated how the atoms in the specimens vibrate both locally in the atomic range and also over longer distances. "In the crystalline material, we found harder vibrations for the long-range order and better conductivity for sound than in amorphous material. This is normal and is related to an increase in the order. However, we were surprised by the results for short-range vibrations in the crystal. They were softer. The short-range order in crystalline material is therefore lower than in amorphous material. This is very unusual."

On the basis of all the experimental results, the Aachen research group headed by Wuttig developed a model to explain the apparent contradictions. "Normally, the propagation of sound waves in material correlates with the thermal conductivit
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Contact: Angela Wenzik
a.wenzik@fz-juelich.de
49-246-161-6048
Helmholtz Association of German Research Centres
Source:Eurekalert

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