Near Field Heat Transfer: Where Radiation Becomes Conduction

We show in this article that the radiative heat transfer exchanged in the near field can be interpreted as a conduction heat transfer due to the propagation of optical phonons. We consider two situations. In the first one, two heated bodies at different temperatures are separated by a gap and in the second one a temperature gradient is imposed to a bulk material. In both situations, the radiative heat transfer is calculated by means of fluctuational electrodynamics in order to take into account near field effects. Asymptotic expressions of a thermal conductivity are obtained from the radiative heat transfer calculation. We interpet this conductivity as a consequence of the heat transfer by propagation of optical longitudinal phonons.