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
Thermal conductivity containing "I" terms have to be zero. Now if we assume that where "δ" is the Dirac delta function, "I" terms get the following expressions By substituting these expressions to the equation for "λ", we see that it goes to zero. Therefore, "P"("x") has to be delta function. In an isotropic medium the thermal conductivity is the parameter "k" in the Fourier expression for the heat flux where formula_2 is the heat flux (amount of heat flowing per second and per unit area) and formula_66 the temperature gradient. The sign in the expression is chosen so that always "k" >