Effect of a metallic coating on the thermal conductivity of carbon nanofiber–dielectric matrix composites

A theoretical model is developed to evaluate the thermal conductivity of composites made of a dielectric matrix material containing randomly oriented and aligned carbon nanofibers coated with a metallic layer. The effect of the metallic coating on the phononic thermal conductivity of the matrix material and the electron–phonon coupling inside the metallic coating are both taken into account in this model. It is shown that: (1) the metallic coating has an extraordinary effect on the enhancement of the composite thermal conductivity. For a volume fraction of 30% of fibers with radius of 50 nm and 10 nm-coating of copper, the increase in the thermal conductivity is as high as 27%, which increases significantly with the fiber volume fraction. (2) Although the thermal conductivity of silver is 453% as that of indium, the composite thermal conductivity is only increased slightly by changing an indium coating to a much more expensive silver coating, due to the relatively high thermal conductivity of these metals in comparison with the one of the matrix. (3) The composite thermal conductivity increases with the volume fraction of the fibers when their radius and the radial thermal conductivity are greater than the Kapitza radius at the matrix-coating interface and the effective thermal conductivity of the matrix, respectively. The obtained theoretical results agree fairly well with experimental data reported in the literature for the thermal conductivity along the axis of aligned carbon fibers with copper coating and embedded in an epoxy matrix. This model is expected to be valid for composites in the absence of percolation with the length-to-radius aspect ratio of fibers in the range of 10–100, and it provides theoretical guides for optimizing cost-efficient high thermal conductivity composites.