Thermal conductivity the list of thermal conductivities. These should be considered approximate due to the uncertainties related to material definitions. High energy generation rates within electronics or turbines require the use of materials with high thermal conductivity such as copper "(see: Copper in heat exchangers)", aluminium, and silver. On the other hand, materials with low thermal conductance, such as polystyrene foam and fire brick, are used in building construction or in furnaces in an effort to slow the flow of heat, i.e. for insulation purposes. The reciprocal of thermal conductivity is "thermal resistivity", usually expressed in kelvin-meters per watt (K⋅m⋅W). For a
Conductivity near the percolation threshold critical exponents we have the "correlation length", formula_10 formula_11 and the "percolation probability", P: formula_12 For the description of the electrical percolation, we identify the occupied bonds of the bond-percolation model with the metallic component having a conductivity formula_13. And the dielectric component with conductivity formula_14 corresponds to non-occupied bonds. We consider the two following well-known cases of a "conductor-insulator mixture" and a "superconductor–conductor mixture". In the case of a conductor-insulator mixture we have formula_15. This case describes the behaviour, if the percolation threshold is approached from above: formula_16 for formula_6 Below the percolation threshold we have no conductivity, because