Thermal Characterization of Plain and Carbon Nanotube reinforced Syntactic Foams

Composite materials fabricated using hollow microspheres are called syntactic foams. Particulate filler composites such as syntactic foams, consisting of glass microballoons and epoxy resin, are desirable for applications that require high compressive and impact strengths and low thermal conductivities. However, for heat dissipation applications, filler additions are required to increase the thermal conduction capacity of syntactic foams. Carbon nanotubes are structures that have exhibited a unique combination of mechanical, electrical and thermal properties making them excellent candidates for replacing conventional fillers such as carbon black, silicas, clays, aluminum and copper [1]. In this research, the effect of nanotube addition on the thermal properties of syntactic foam is studied. Addition of nanotubes to the syntactic foam composite is performed in two ways. In the first method, the nanotubes are separately mixed with resin and further with microballoons. In the second, nanotubes are grown over the surface of microballoons. These nanotube grown microballoons are mixed with resin to fabricate syntactic foam. Multi-walled carbon nanotubes (MWCNTs) are used in the first method of fabricating syntactic foams. These nanotubes are commercially obtained and added to the Fig 1. Effect of nanotube addition to thermal conductivity of syntactic foam at 50% volume fraction of glass microballoons. epoxy resin using ultrasonication and three-roll milling. Further, the microballoons are added to the mixture of resin and nanotubes. Nanotube reinforced syntactic foams are fabricated with 10, 20 and 50% volume fraction of S22 type glass microballoons. The nanotube volume fraction in the syntactic foam is varied from 0.1 to 0.5%. In order to compare the enhancement of thermal conductivity with an addition of nanotubes, plain syntactic foams of the corresponding volume fractions of microballoons are also fabricated. Composite foams are tested for thermal conductivity, thermal diffusivity and specific heat experimentally using a Flashline 5000 thermal analyzer. Theoretical values of conductivity in syntactic foams are calculated using the rule of mixtures. Theoretical values matched to those of plain syntactic foam, but not for the nanotube reinforced syntactic foams. Even though the inclusion of CNTs forms a percolated network in the matrix of the syntactic foams, the improvement is only modest (Fig. 1). This is mainly due to thermal resistance at the interface of nanotubes and matrix [2,3], along with nanotube agglomeration in resin (Fig. 2). Transmission electron microscopy (TEM) images are obtained to observe the dispersion of the CNTs in the epoxy resin using mixing processes such as ultrasonication and three-roll milling. The influence of filler contents of carbon nanotubes and glass microballoons is discussed. Fig 2. Agglomeration of carbon nanotubes in epoxy resin