Electrical properties of boron nitride matrix composites:: III, observations near the percolation threshold in BN-B4C composites

In this third paper of the series, we discuss the electrical resistivity of BN-B4C composites with compositions ranging from 0% to 100% B4C. After establishing the response of samples whose compositions Iie far away from the percolation region, where effective medium models apply, we focus attention on samples with compositions at or near the percolation threshold (similar to 60% BN-40% B4C). The large differences in electrical properties among samples with the same nominal composition can be explained by invoking a connectivity parameter. Since the difference in the electrical resistivity of BN and B4C is about 9 orders of magnitude, the degree of connectivity of the two components at the percolation threshold determines the resultant composite resistivity. Connectivity in these composites was quantified by taking BN peak height ratios in X-ray diffraction patterns of all samples containing 60% BN-40% B4C. The degree of preferred orientation of the BN platelets can be correlated with systematic increases in the electrical resistivity of the composites.