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 plates); 2) formation mechanisms (structures of growth formed through nucleation and structures of breakdown resulting from structural transitions), and 3) synthesis temperatures. The tubes grow from the gaseous phase and their crystal morphological forms differ in phase composition (cylindrical tubes consist of the hexagonal BN phase and faceted tubes of rhombohedral one). At T ≥ 1300 °C, rhombohedral-to-hexagonal phase transition and transformation into multilayered polytypes occur in faceted tubes due to significant anisotropy of the thermal expansion coefficient of graphite-like modifications of boron nitride.