Electrical properties of boron nitride matrix composites: I, analysis of McLachlan equation and modeling of the conductivity of boron nitride-boron carbide and boron nitride-silicon carbide composites

The McLachlan equation, which incorporates both effective medium models and percolation, was used to predict the volume fraction-conductivity relationships of insulator-conductor composites, and results were compared with experimental data. Two composite systems were investigated (BN-B4C and BN-SiC), Both systems are anisotropic, because of the orientation of BN platelets perpendicular to the hot-pressing direction. For BN-B4C composites, with increasing B4C content, the ac and de conductivities are relatively constant to similar to 40% B4C (the critical volume fraction). At this composition, the conductivity suddenly increases to a value closer to that of B4C and then resumes a gradual increase. Little difference is seen for measurements made perpendicular or parallel to the hot-pressing direction, i.e., perpendicular or parallel to the BN platelets, Similar results are found for the BN-SiC composites, except that the critical volume fraction is similar to 20% SiC in this case. The experimental curves are in good agreement with those predicted by the McLachlan equation. The parameters s and t of the McLachlan equation relate to the morphology of the phases present in the microstructure. The critical volume fraction relates to the connectivity of the phases in the composites.