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

Abstract

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 dc conductivities are relatively constant 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 ∼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.