| Thermal conductivity, k, is the intensive property of a material that indicates its ability to conduct heat. |
| In the most materials, there is no correlation between thermal conductivity and electrical conductivity, as shown in the table
below, where highly electrically conductive silver is shown to be less thermally conductive than diamond, which is an electrical
semiconductor. The reason for this difference is that phonons, not electrons, are the primary carriers of heat in diamond. In metals, thermal
conductivity approximately tracks electrical conductivity, as the freely moving valence electrons transfer not only electric current but also heat energy.
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| Thermal conductivity is not a simple property, and depends intimately on structure and temperature. For instance, pure crystalline substances also exhibit highly variable thermal conductivities along different crystal axes,
due to differences in phonon coupling along a given crystal dimension. Sapphire is a notable example of variable thermal conductivity
based on orientation and temperature, for which the CRC Handbook reports a thermal conductivity perpendicular to the c-axis of 2.6 W.m-1.K-1 at 373 K, and 6000 W.m-1.K-1 at 35 K for an angle of 36 degrees to the c-axis.
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| Air and other gases are generally good insulators, in the absence of convection. Therefore, many insulating materials function
simply by having a large number of gas-filled pockets which prevent large-scale convection. Examples of these include expanded
and extruded polystyrene (EPS and XPS, both popularly referred to as "styrofoam") and silica aerogel. Natural, biological
insulators such as fur and feathers achieve similar effects by dramatically inhibiting convection of air or water near an
animal's skin.
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| Thermal conductivity is important in building insulation and related fields. However, materials used in such trades are rarely
subjected to chemical purity standards. Several construction materials' k values are listed below. These should be considered
approximate due to the uncertainties related to material definitions.
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