Correlation between microstructure and electrical resistivity of hexagonal boron nitride ceramics

Hexagonal boron nitride is a material with a unique combination of mechanical, chemical and electrical properties and therefore of considerable technical and commercial interest. Nevertheless, there exists only very limited knowledge concerning the microstructure and electrical properties of such materials. In this work different materials produced by SPS from 'turbostratic' and well-crystallised powders are compared with commercial materials in terms of densification, microstructure and electrical properties. The turbostratic powders could be densified at temperatures as low as 1500-1600 °C, but recrystallisation of the grains took place at much higher temperatures (1800-1900 °C). The electrical resistivity of the investigated materials reached values of up to 1015 Ω cm and strongly depended on the microstructure. The main factor influencing the resistivity was the amount and nature of the grain boundary phase.