Structural types of boron nitride particles produced by carbothermal synthesis

Abstract

Transmission electron microscopy and microdiffraction are used to examine boron nitride produced by carbothermal synthesis (using saccharose as a carbon-containing component) in the temperature range 1000-1450°C in nitrogen. It is established that onion-like particles form during structural ordering of turbostratic boron nitride in the range 900−1000°C. The structural types of BN particles are classified according to: 1) crystal morphological features (onion-like particles, cylindrical and faceted tubes, and lamellas), 2) formation mechanisms (structures of growth formed through nucleation and structures of breakdown formed through structural transitions), and 3) synthesis temperatures. The tubes grow from the gas phase and their crystal morphological forms differ in phase composition (cylindrical tubes consist of the hexagonal BN phase and faceted tubes of the rhombohedral one). At T ≥ 1350°C, rhombohedral-hexagonal phase transition and transformation into multilayered polytypes occur in faceted tubes due to a highly anisotropic thermal expansion coefficient of graphite-like modifications of boron nitride.