Electron microscopy of hexagonal boron nitride powder

Abstract

The morphology and crystalline state of h-BN powder after one-stage or two-stage baking process were investigated extensively. The particles are scale-shaped and the flat surfaces have a (001) habit plane of hexagonal close-packed structure. The side shape of particles after one-stage baking is round, while that after two-stage baking is dodecan, a twelve-faced prism, with the side habit of (100), (110) or their variants. Lattice image observation shows that the side surface of a one-stage baked particle is wavy and thin, while that of a two-stage baked particle is straight and thick. Many particles after one-stage baking are composed of overlapped grains contacting with each other at (001) flat surfaces forming a twist boundary. These grains have relative rotation angles ranging from 5 ° to 26 ° around the common [001] axis and have a coincidence lattice relation with respect to (001) flat planes. X-ray photoelectron spectroscopy analysis shows that both C and O segregate onto the surface of one-stage baked particles, while only C segregates onto the surface of two-stage baked particles. Formation of coincidence lattice grain boundary and impurity segregation both restrict the growth of diameter and thickness in scale-shaped particles. It is concluded that two-stage baked particles, having side surface habits, are stable against various environments.