Cross property connection between the electric and the thermal conductivities of copper graphite composites

The cross property relationship between thermal and electrical conductivity of the copper matrix–graphite composite was characterised in the composition range up to 100 vol.% of graphite. It was observed that the investigated cross property is linear in the whole composition range. This linearity averages out nonlinear dependence of single conductivity on composition. It means also that the behaviour of investigated composite under applied external electric potential and thermal gradient are qualitatively identical. Both conductivities are realised by free electrons in copper and in basal plane of graphite. The only difference between both cases is phonon thermal conductivity in graphite phase. The possible explanation for linear cross property is that all non linear effects are probably ruled by the scattering of valence electrons on copper–graphite interface via converting of some part of kinetic electron energy to the vibrations of phonons in graphite phase. Further Wiedemann–Franz law was used to determine composition threshold above which the phonon conductivity in graphite phase starts to play significant role. This threshold was observed around 25 vol.% of graphite. As linearity of cross property is valid also above the composition threshold it means probably that above this composition electrical conductivity composition dependence start to be nonlinear as well.