Thermal Conductivity of Carbon Fiber/Liquid Crystal Polymer Composites

Fuel cells require thermally conductive bipolar plates. These are typically composed of a thermoplastic resin filled with a single type of graphite added for conductivity. In this study varying amounts of two types of carbon fibers, Fortafil 243 and Panex 30, were added in volume percentages ranging from 0 to 55% to a thermoplastic matrix, Vectra A950RX Liquid Crystal Polymer. The resulting single filler composite materials were tested for through-plane thermal conductivity with the ASTM F433 guarded heat flow meter method. Furthermore, the in-plane and through-plane thermal conductivity was tested using the transient plane source method.

The two types of composite materials produced similar through-plane thermal conductivities until about 40 vol% fiber. After that, the Panex/Vectra composites had higher through-plane thermal conductivity. Panex/Vectra composites also had higher in-plane thermal conductivity at all filler levels. Due to these results, Panex/Vectra composites have a consistently higher in-plane to through-plane conductivity ratio.

A model was created from these results showing an exponential relationship between the volume fraction of filler and square root of the product of the in-plane and through-plane thermal conductivity. Since through-plane thermal conductivity models currently exist, they can be used with this model to predict in-plane thermal conductivity.