Effect of Fiber Orientation and Sample Geometry on the Effective Thermal Conductivity of a Uniaxial Carbon Fiber-Reinforced Glass Matrix Composite

Abstract

From solutions for the thermal conductivity of anisotropic solids, it is pre dicted that the effect of fiber angle on the thermal conductivity of uniaxial fiber-reinforced composites is a function of sample geometry. Experimental data for the effect of fiber an gle on the thermal conductivity of a uniaxial carbon fiber-reinforced aluminoborosilicate glass matrix composite taken from three different sample geometries are presented in sup port of this conclusion. The thermal conductivity of the center portion of samples with a thickness in the direction of heat flow much smaller than their width agreed very well with the corresponding effect predicted for a composite plate of infinite extent. Similar results were obtained for "angled" samples, with a geometry such that the net direction of heat flow was parallel to the fiber direction. In contrast, for samples of rectangular geometry with a thickness of the order of the width, the thermal conductivity data for fiber angles greater than about 45° agreed very well with those predicted for a narrow composite strip with thermally insulated sides. At lower fiber angles, the data for these latter samples were intermediate to those predicted for the infinite composite plate and the insulated strip. The results of this study imply that care needs to be taken in the selection of specimen geom etry when measuring the effect of fiber orientation on thermal conductivity of uniaxial composites, and when using such data in the thermal analysis of composites with different shape and boundary conditions.

Anisotropy in thermal conductivity of graphite flakes–SiCp/matrix composites: Implications in heat sinking design for thermal management applications

“…the ratio of its dimension in the direction of heat flow and its width. These results are in perfect agreement with previous studies carried out on composites with continuous fibre reinforcements [24]. For "thin" samples like those required in the LFT measurements, the width of the specimens is much larger than their thickness in the direction of temperature gradient.…”

“…The measured values for the dependence of the thermal conductivity on flakes orientation for the two series of specimens (G f -SiC p /Al-12Si and G f -SiC p /epoxy) follow a tendency that is similar to that obtained for uniaxial carbon fiber-reinforced glass matrix composites [24]. In all cases the TC decreases steadily with the angle .…”

“…The experimental results were interpreted with the aid of current approaches to describe the angular dependence of the thermal conductivity of anisotropic solids [24][25][26]. For that sake, let us assume that a thermal gradient is applied along the z-direction in the x-z plane (note that in oriented structures like the one of the material herewith discussed, the x and y directions are equivalent).…”

“…In the case of an infinite thin plate with the thermal gradient perpendicular to it, the thermal conductivity of the composite can be closely approximated by K zz () [25]: (2) If, instead, the sample is infinitely long along the heat gradient direction z, being insulated along its length, heat will be forced to flow solely in the z-direction and the conductivity of the composite will be given by [24,26]:…”

“…On the other hand, the direction of heat flow (note that in anisotropic materials the heat flow direction may not coincide with the direction of the temperature gradient, see [24]) forms an angle  with the flakes plane, given by:…”