|MadSci Network: Engineering|
Heat is conducted in metals, according to current theory, by lattice waves, phonons, and free (or valence) electrons. In a non-metal, heat is conducted only by phonons. (A phonon is equivalent to one quantum of energy in the form of a thermoelastic wave of fixed frequency. In this sense, a phonon is analogous to the concept of a photon in electromagnetic radiation theory.) Because there are three methods of heat transport in metals, compared with only one in non-metals, one would expect that metals would be better conductors of heat than non-metals. The transport of heat through a metal is affected by the structure of the metal. A single pure crystal of metal will conduct heat much better than a large number of very small crystals because of scattering of the phonons at the crystal boundaries. Likewise, if you alloy a metal, the differences in sizes of metal atoms will cause a large decrease in thermal conductivity. Mechanical damage to a metal structure by cold working or radiation will also decrease the conductivity of the metal. Based upon these considerations, pure copper and silver have the highest thermal conductivity of the metals. In 1923, a German named Schott measured the thermal conductivity of a single crystal of copper at 12,200 Watts per meter Kelvin (W/mK) at a temperature of 20.8 K. However, at room temperature, commercially pure copper typically has a conductivity of about 387 W/mK. The material with the highest thermal conductivity is the most pure, single crystal form of carbon, which we call a diamond. When you want to verify if a diamond is real or not, thermal conductivity is an infallible method. The thermal conductivity of diamonds is around 1200 W/mK. As a result, there is interest in learning how to grow diamond films thick enough and cheaply enough to allow them to be used for thermal management. Other materials with high conductivity are certain other forms of carbon, such as high purity graphite fibers and pyrolytic graphite. As you might expect, both have very high chemical purity and large crystalline structures.
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