Effective Thermal Conductivity of Uniaxial Composite with Cylindrically Orthotropic Carbon Fibers and Interfacial Thermal Barrier

Abstract

The theories of Hasselman et al. and Benveniste et al. were extended to derive expressions for the longitudinal and transverse effective thermal conductivity of an isotropic matrix uniaxially reinforced with dilute concentrations of cylindrically orthotro pic fibers for finite interfacial thermal conductance values. The transverse thermal conduc tivity of the composite was found to be a function of both the radial and tangential compo nents of the transverse thermal conductivity of the fibers. Comparison with the effective thermal conductivity of an isotropic matrix reinforced with isotropic fibers indicates the effective thermal conductivity of cylindrically orthotropic fibers equals the geometric mean of the radial and tangential transverse thermal conductivity. As for isotropic matrices reinforced with isotropic fibers, a finite value for the interfacial thermal conductance in composites reinforced with cylindrically orthotropic fibers leads to a lowering of the com posite transverse thermal conductivity and also introduces a dependence on fiber diameter. Based on experimental data for the transverse thermal conductivity of a uniaxial carbon fiber-reinforced aluminoborosilicate glass matrix composite in nitrogen at atmospheric pressure and in vacuum, estimates of the interfacial thermal conductances indicate that in nitrogen at atmospheric pressure interfacial gaseous heat transfer contributes about one- third of the total interfacial thermal conductance. The theory should be applicable to other isotropic matrices reinforced with cylindrically orthotropic fibers.