Thermal conductivity is defined as the ratio of the flux of thermal energy in a given direction to the temperature gradient in the same direction:

\[\label{eq1} \lambda=\dfrac{\dot{q}}{\partial T/\partial l}\;\;\;\mbox{W/m K} \tag{1}\]

where q̇ is the flux of thermal energy in W/m². This is, of course, also true for a fluid as long as no convective heat transport has to be considered. In a gas, the kinetic theory of gases provides an easy explanation for the thermal conductivity λ, relating it to the heat capacity per unit volume of the gas c_p, the average particle velocity \(v\), and the mean free path l:

\[\label{eq2} \lambda=\frac13c_pvl \tag{2}\]

In solids, as in gases and liquids, the thermal energy is mainly stored in the motional energy of the atoms, but in solids they are fixed to their respective positions within the lattice by elastic forces. Explaining thermal conductivity is therefore explaining the different kinds of interaction by which the lattice vibrations are transmitted throughout a crystalline or amorphous solid when the equilibrium is distorted by a temperature gradient.