A Technique for Locally Increasing Surface Heat Spreading and through-Thickness Thermal Conductivity of Graphite/Epoxy Laminates

Abstract

The polymer matrix composite through-thickness thermal conductivity is particularly important in applications such as composite spaceborne electronics enclosures where the heat dissipation is primarily dependent on thermal conduction to a heat sink. The spreading of heat at the composite surface and subsequent localized conduction in the through-thickness direction down to high thermal conductivity fiber may be the key to designing a lightweight, thermally efficient enclosure. A finite element model was constructed of a composite with heat applied to a central area. The laminate consisted of a hybrid of high thermal conductivity pitch fiber/epoxy on the outside surfaces interlaminated with low thermal conductivity carbon fabric/epoxy. Three configurations were modeled: (A) a heat source in the middle, (B) Cu plating under the central heat source and (C) Cu plating under the heat source with a centrally located hole that was also Cu plated. The model with Cu on the surface under the heat source had a maximum surface temperature 35% lower than the model with no Cu to spread the heat. The model with a central Cu plated hole had a maximum surface temperature 58% lower than that with no Cu plating on the surface. Therefore, the surface Cu plating with Cu plated hole spreads the heat and increases the through-thickness thermal conductivity. Samples were prepared using the aforementioned hybrid pitch fiber/epoxy interlaminated with carbon fabric/epoxy laminates sandwiched between .015 mm thick Cu foil. In the fabricated samples, all Cu (except that in the immediate vicinity of the heat source) would be removed by etching. A drill size of 2.29 mm diameter and feed rate of 152 cm/min were selected to minimize pitch fiber damage and matrix smearing, which results in increased thermal resistivity. Some of the drilled holes were etched with sulfuric acid and Cu plated and some were just cleaned and Cu plated. SEM photomicrographs of the holes and of the hole edges showed no appreciable increase in Cu adherence of the etched over the unetched samples. The unetched samples appeared to have as intimate contact with the Cu as the etched samples. The etched samples had areas with thicker epoxy layers insulating the pitch fibers from the Cu. The co-cured surface Cu foil with and without a Cu plated hole to spread the heat and increase through-thickness thermal conductivity, respectively, are currently being incorporated into heat transfer test samples.