Journal of the American Ceramic Society-Hasselman et Vol. 79. No. 3 VLS SiC-reinforced silicon nitride composites, the VLS whisk- EE-2a ● VLS SiCw-LAS ers at room temperature have been estimated to have a thermal ▲ VS SiCM-LAS conductivity value of about 100 W/(mK). These estimates, however,are probably low since they were based on the assumption of perfect whisker-matrix thermal contact In view of these uncertainties, analysis of the experimental data took the following approach. The experimentally measured value of K at room temperature was used with a range of values of whisker thermal conductivity (K) and interfacial thermal conductance(he)in the expressions of Benveniste and Miloh to generate a family of curves for the room-temperature thermal conductivity parallel(K33) and perpendicular(Kn)to the axis of perfectly aligned whiskers. Comparison between the experimental data and these predicted curves should tablishing a window of reasonable values for the hermal conductivity and interfacial thermal conductance for each composite at room temperature. All calculations assumed hat both composites contained 30 vol% uniaxially aligned whiskers with a length-to-diameter ratio of 12. The thermal TEMPERATURE(°C) conductivity of the whiskers was assumed to be isotropic. All combined effects which might contribute to an interfacial ther mal barrier were assumed to be represented by a single interfa- glass matrix and for 30 vol% VLS SiC and VS Sic cial thermal conductance value. The diameter of the VLs orced lithium aluminosilicate glass matrix parallel to whiskers was taken to be 6.5 um and the diameter of the vs pressing direction whiskers was taken to be 0.7 These whisker dimension were used to calculate the descriptive geometry (i,e,eccentric- ity, distance between centerline and foci of the ellipse, etc. )of (2) Analysis the ellipsoids used in Benveniste and Miloh's solutions A direct comparison of the experimental data with those calculated K,3 curves at room temperature for the VLs and VS whiskers are shown in Figs. 4(a)and 5(a), respectively predicted from theory is not possible in view of the preferred In general, for both sets of data, the composite thermal conduc- orientation and differences in the nature of the whiskers (i.e the thickness, thermal conductivity, interfacial characteristic ty increases sigmoidally with increasing he, as theoretically etc. ) The thermal conductivity values for the two Sic whisker predicted for composites with an interfacial thermal barrier types are unknown and cannot be estimated theoretically with any degree of certainty, since thermal conductivity is higl falls below Km. This result is also expected, since the interfacial sensitive to impurity and defect level. An estimate for the inter thermal contact at these low he values is so poor that the whisk facial thermal conductance is even more uncertain, although the ers cannot contribute to the heat conduction and effectively act value is probably not infinite since the approximate factor of 5 as a pore phase with zero thermal conductivity. At the highest mismatch in the elastic moduli of the Sic whiskers and the values of he, the K33 values become constant. Note, however LAS matrix promotes phonon scattering. However, the thermal that the magnitude of K3 for any value of K, falls well below conductivity value for the VS whiskers at room temperature has that predicted by the rule of mixtures. This occurs because the length-to-diameter ratio of the whiskers is finite, and the rule of been estimated from experimental data for mullite, alumina, mixtures is valid only as this ratio approaches infinity and silicon nitride matrix composites to range from about to 30 W/(mK). Similarly, from experimental data for 30 vo Figures 4(b) and 5(b) show the calculated curves for Ki characteristics similar to the ones shown in Figs. 4(a)and 5(a) with the exception that the Ku values show only a slight depen dence on the magnitude of the K, values. As easily verified ■ ◆vssc.LAs from theory, for heat flow transverse to the axis of oriented fibers or whiskers, the dependence of the composite thermal onductivity (K ) on K, i.e., d(ke)/d(K1), approaches zero The curves shown in Figs, 4 and 5 assume that the whiskers are uniaxially aligned. Since the whiskers in the composites of s study are not perfectly aligned the calculated data still annot be directly compared to the experimental data. for known values of K, (the global thermal conductivi to the hot-pressing direction) and K,(the global conductivity normal to the hot-pressing direction), the of Chou and Nomura, which assumes an axially fiber orientation transformed by rotation through an ang describe the fiber misalignment, can be used to derive sions that can be used to obtain K33 and K As shown by Chou and Nomura, K, can be expressed as TEMPERATURE(°C) [sin20K33+(1+ cos O)kulf(e)sin 6d8 id for 30 vol VLS SiC and vs SiC wh er f(0)sin 6de reinforced litl luminosilicate glass matrix normal to the hot pressing direction