March 1996 Thermal Conductivity ofVLS and VS Silicon Carbide Whisker-Reinforced LAS Glass-Ceramic Composites :要 民二吗 (a) Fig. 1. Optical micrographs of a 30 vol% VLS SiC whisker-reinforced lithium aluminosilicate glass:(a) parallel and (b)normal to hot-pressing suggests that the VLS whisker composite contained approxi- approximately 3%0, the data for the LAS matrix and SiC-LAS 2%0 porosity. Fig ptical micrographs of composites are identical sections of the VLS whisker composite parallel and Figures 2 and 3 show the therma ctivity data for the to the hot-pressing direction, respectively. During he natrix and the composites with the vls and vs whiskers for ressing the whiskers achieved a preferential orientation, heat flow parallel and normal to the hot-pressing direction aligned normal to the hot-pressing direction within an angle of that is significantly higher than that of the matrix. This increase symmetric). A similar preferred orientation was observed for ared with a corresponding factor of 2 for those with the VS whiskers. It should also be noted that only the thermal (2) Determination of Thermal properties conductivity values for the VLs whisker composite normal the hot-pressing direction show a significant negative tempera The thermal diffusivity was measured by the flash ture dependence. All the other sets of data shown indicate little nique, .26 using a glass-Nd laser as the flash source. The if any, dependence on temperature mens were rectangular plates approximately 5 mm x8 mm with a thickness of 3 mm. The transient temperature was moni mnhp足t Table L. Specific Heat of LAS Matrix and SiC-LAS Composite to assure linearity between the specimen temperature change Specific heat (/(gK)) and detector output. 2 Up to temperatures of approximately LAS matri 200C the thermal diffusivity measurements were made in air. Above this temperature to about 500.C the samples were con tained in a carbon-resistance furnace with a nitrogen atmosphere. The specific heat was measured by differential scanning calo- rimetry. The thermal conductivity was calculated by multi- plying the experimental thermal diffusivity data with the orresponding data for the density and specific heat, Because of 0.73 the low coefficient of thermal expansion for the matrix pha 0.894 nd composites of this study, changes in specimen dimensions with temperature had no significant effect on the thermal con ductivity values obtained in this manner 511 1.140 VS SIC-LAS IlL. Results, Analysis, and Discussion 0.745 0.9 ()Experimental Results 070 Table e weithine nthe swas tice per of the dsc method of 507 1.145