Thermal Conductivity of Nanoceramics
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The heat carriers in ceramic insulators are lattice waves. The thermal conductivity is governed by their attenuation length or mean free path. Scattering of these waves by grain boundaries can reduce the mean free path and thus decrease the thermal conductivity. The lattice waves, which carry heat range over a wide frequency spectrum, and only waves of the lower part of that spectrum are scattered by grain boundaries. This will be discussed with emphasis on thermal barrier coatings, where nanoscale grains and point defects, acting on different parts of the spectrum, each cause an independent thermal conductivity reduction. In coatings protecting against wear one can use nanoscale inclusions to enhance mechanical properties without reducing the thermal conductivity. Finally, there are applications where the thermal conductivity of ceramics is to be enhanced by appropriate high-conductivity inclusions, such as carbon nanotubes.