Experimental thermal diffusivity data transverse to the fiber direction for composites composed of ;a reaction bonded silicon nitride matrix reinforced with uniaxially aligned carbon-coated silicon carbide fibers indicate the existence of a significant thermal barrier at the matrix-fiber interface. Calculations of the interfacial thermal conductances indicate that at 300°C and 1-atm N2, more than 90% of the heat conduction across the interface occurs by gaseous conduction. The magnitude of the interfacial cainductance is decreased significantly under vacuum or by removal of the carbon surface layer from the fibers by selective oxidation. Good agreement is obtained between thermal conductance values for the oxidized composite at 1 atmi calculated from the thermal conductivity of the N3 gas and those inferred from the data for the effective composite thermal conductivity. [
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.
The role of an interfacial carbon coating in the heat conduction behavior of a uniaxial silicon carbide nitride was investigated. For such a composite without an interfacial carbon coating the values for the thermal conductivity transverse to the fiber direction agreed very well with the values calculated from composite theory using experimental data parallel to the fiber direction, regardless of the ambient atmosphere. However, for a composite made with carbon‐coated fibers the experimental values for the thermal conductivity transverse to the fiber direction under vacuum at room temperature were about a factor of 2 lower than those calculated from composite theory assuming perfect interfacial thermal contact. This discrepancy was attributed to the formation of an interfacial gap, resulting from the thermal expansion mismatch between the fibers and the matrix in combination with the low adhesive strength of the carbon coating. In nitrogen or helium the thermal conductivity was found to be higher because of the contribution of gaseous conduction across the interfacial gap. On switching from vacuum to nitrogen a transient effect in the thermal diffusivity was observed, attributed to the diffusion‐limited entry of the gas phase into the interfacial gap. These effects decreased with increasing temperature, due to gap closure, to be virtually absent at 1000°C.
A new and simpler design of thin-lm heat ux gauge has been developed for use in high-heat-ux environments. Heat ux gauges of the same design were fabricated on three different substrates and tested. The heat ux gauge comprises a thermopile and a thermocouple junction, which measures the surface temperature. The thermopile has 40 pairs of S-type thermocouples and is covered by two thermal resistance layers. Calibration and testing of these gauges were rst carried out in an arc-lamp calibration facility. Sensitivity of the gauge was discussed in terms of the relative conductivity and surface temperature. The heat ux calculated from the gauge output was in good agreement with the precalibrated standard sensor. The steady-state and the transient response characteristics of the heat ux gauge were also investigated using a carbon dioxide pulse laser as a heat source. The dynamic frequency response was evaluated in terms of the nondimensional amplitude ratio with respect to the frequency spectrum of a chopped laser beam. The frequency response of the gauge was determined to be about 3 kHz. The temperature pro les in the thin-lm heat ux gauge were obtained numerically in steady-state conditions using FLUENT and compared with the experimental results.
The transverse thermal conductivity of an aluminoborosilicate glass uniaxially reinforced with carbon fibers was found to be lower under near-vacuum than in nitrogen, whereas no such difference was found for the longitudinal thermal conductivity. This effect was attributed to the existence of an interfacial gap resulting from the thermal expansion mismatch between the matrix and fibers. The presence of this gap permits the gaseous environment access to the fiber-matrix interface and thereby contributes to the interfacial heat transfer. Its presence does not affect the longitudinal thermal conductivity, however, because the gap is aligned parallel to the fibers and, therefore, the direction of heat flow. Analysis of the experimental data indicates that, in nitrogen at atmospheric pressure, the gaseous conductance constitutes about one-third of the total interfacial conductance.
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