Heat conductivity of non-metallic crystals

Some non-metallic crystals can have heat conductivities comparable with the best metals, while other non-metals are better heat insulators than most metallic alloys, and materials at both extremes have practical uses. The temperatures at which the conductivities of pure crystals are a maximum vary widely, and for any application which requires an electrical insulator with high heat conductivity, the most suitable crystal depends on the temperature range concerned. Imperfections in the lattice can mask the ideal behaviour, and materials having very disordered atomic arrangements or composed of small crystallites are used as heat insulators. The theory of phonon conductivity can explain the observed phenomena quite well, but the magnitude of the intrinsic conductivity Cannot be predicted accurately from a knowledge of other crystal properties. Some analogies can be drawn between heat flow in a crystal and gas flow in a tube, but the analogy does not cover ‘Umklapp’ processes, which are peculiar to a periodic lattice structure.