SYNOPSISEffective thermal conductivities of powder-filled epoxy composites were experimentally obtained using a transient method. Fillers used were aluminum and cupric oxide. Com
INTRODUCTIONWith increasing demand for plastics or resins in different branches of engineering, the addition of other materials-metals, graphite, oxides, glass, etc.-has been widely applied. The resulting composites combine the advantages of plastics or resins with those of the additives, which could provide improvements in the physical properties. In particular, cupric oxide-filled plastics can be used in microwave heating applications. For such heterogeneous systems, then, more information on the physical properties is required. The determination of effective thermal conductivity, for example, has led to numerous experimental and theoretical studies. In this work, the effective thermal conductivities of epoxy resins containing aluminum or cupric oxide powder with various filler concentrations were obtained by a transient method, based on direct contact condensation of steam on a sphere. The results for aluminumepoxy composites were compared with data from the literature, and both sets of data compared with predictions from several well-known models.
Models in the literatureFor a two-component composite, the simplest alternatives would be with the materials arranged in To whom correspondence should be addressed.Journal of Applied Polymer Science, Vol. 49, 1901-1908 (1993) suming a parabolic distribution of the filler: where ments need to be completed before it can be fully established.From experimental results, Sundstrom and Lee* concluded that the Bruggeman equation gave better prediction up to 10% of filler by volume, whereas the Cheng-Vachon equation was more accurate in the range of 15-25% by volume. Nielsen' indicated that their data could be fitted by the Lewis and Nielsen" equation, in which effects of the shape of particles and type of packing were included:where where A is a constant related to the generalized Einstein coefficient kE, and 4m, the maximum packing fraction of the filler. Some values of A and 4m were given. l1 Oleynikova and Dushchenko l2 showed that several formulae available in the literature were different expressions of the Maxwell-Eucken correlation. They recommended eqs.( 1) and (2) for calculation of thermal conductivities of two-component systems containing less than 12% by volume of filler. Progelhof et al.13 reviewed numerous correlations in the literature and concluded that the Lewis and Nielsen correlation fitted the data best for the range of fillers tested.Based on the generalization of models for parallel and series conduction in composites, Agari and Uno14 developed a new model:Two new parameters were introduced, which could be obtained by regression from experimental data. C, is a measure of the effect of particles on the secondary structure of the polymer and C2 measures the ease of particles to form conductive chains. Later, they15 modified it with an aspect ratio to take the shape of pa...
