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Thermal conductivity (TC) is a material-specific constant which indicates the capability of the material to conduct heat. In the SI unit system, thermal conductivity is given in “watt per meter and kelvin” (W/m K).

Calculation

The rate of heat energy transferred (the power, P, given in W) along a body can be calculated according to the equation:

P = k A (Thot – Tcold)/d

where

k = thermal conductivity (W/m K)
A = cross-sectional area of the body (m2)
Thot = temperature of the hot end of the body (K)
Tcold = temperature of the cold end of the body (K)
d = length of the body (m)

Metals have high TC, non-metals low TC

Metals have a high thermal conductivity, due to the availability of freely movable valence electrons, while non-metals (with their electrons bound to the nuclei), normally have low thermal conductivity.

Diamond the exception

A most remarkable exception is diamond, where the extremely strong sp3 bond between the carbon atoms in the diamond crystal greatly enhances the thermal conductivity of the crystal lattice (see the node Diamond, the writeup "Heat conductivity of diamond"). Diamond actually has 5 times higher thermal conductivity than the best-conducting metals.

Below are examples of the thermal conductivity of some materials:
```METALS
Thermal
conductivity
(W/m K)
------------
Silver         420
Copper         390
Brass          109
Aluminium      220
Steel           50

NON-METALS

Glass            0.8
Concrete         0.8
Fiberglass       0.04
Styrofoam        0.01
Wood (approx)    0.1
Diamond       2000
```
Thermal conductivity varies with temperature - the above figures are given for room temperature.

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