Thermally Conductive Epoxy Nanocomposites

Abstract

The quest for improvement of thermal conductivity in aerospace structures is gaining momentum. This is even more important as modern day aerospace structures are embedded with electronics which generate considerable amounts of heat energy. This generated heat if not dissipated might potentially affect the structural integrity of the composite structure. The use of polymer based composites in aerospace applications has also increased due to their obvious superior specific properties. But the thermal conductivity of the polymer matrix is very low and not suited for the design demands in aerospace applications. Several research studies have been conducted to improve the thermal conductivity of the polymeric composites. Different fillers have been used to improve the thermal conductivity of the polymeric matrix. Fillers may be in the form of fibers or in the form of particles uniformly distributed in the polymer matrix. The thermophysical properties of fiber filled composites are anisotropic, except for the very short, randomly distributed fibers, while the thermophysical properties of particle filled polymers are isotropic. Numerous studies have also been conducted in recent years where nanoparticles have been dispersed in the polymeric matrix to improve the thermal conductivity. Putman et al. [1] used the 3ω method to study the thermal conductivity of composites of nanoscale alumina particles in polymethylmethacrylate (PMMA) matrices in the temperature range 40 to 280 K. For 10% of 60 nm of alumina particle filler by weight (3.5% by volume) thermal conductivity of the composite slightly decreased at low temperatures. Whereas, above 100 K, thermal conductivity of the nanocomposite increased by 4% at room temperature. Kruger and Alam [2] studied the thermal conductivity of aligned, vapor grown carbon nanoscale fiber reinforced polypropylene composite. They measured thermal conductivity by laser flash instrument in the longitudinal and transverse directions for 9%, 17% and 23% fiber reinforcements by volume. The values of thermal conductivity as reported by them were 2.09, 2.75, 5.38 W/m.K for the longitudinal directions and 2.42, 2.47, 2.49 W/m-K for the transverse direction respectively, while the thermal conductivity of unfilled PP was 0.24 W/m-K. Exfoliated graphite platelets are another filler material of promise for improving the thermo-mechanical properties of the polymeric matrix. Aylsworth [3, 4] developed and proposed expanded graphite as reinforcement of polymers in 1910s. Lincoln and Claude [5] in 1980s proposed the dispersion of intercalated graphite in polymeric resins by conventional composite processing techniques. Since that time, research has been conducted on exfoliated graphite reinforced polymers using graphite particles of various dimensions and a wide range of polymers. Drzal et al. [6] have demonstrated the use of exfoliated graphite platelets to enhance the thermal and mechanical properties of polymeric resins. They concluded that composites made by in situ processing have better mechanical properties compared to composites made by melt-mixing or other ex situ fabrication methods due to better dispersion, prevention of agglomeration and stronger interactions between the reinforcement and the polymer. In the present study we use silver nano-filaments, nickel nano-filaments, alumina and exfoliated graphite platelets to enhance the thermal conductivity of an epoxy thermoset resin. The objective of this research is to identify the right filler to achieve the thermal conductivity as required by aerospace design engineers which is around 10 W/ m-K. An arbitrary filler loading of 8 wt% was chosen to compare the different fillers used in this study.