Compression Annealing of Pyrolytic Boron Nitride

Abstract

PYROLYTIC boron nitride is usually prepared by the thermal decomposition of boron trichloride and ammonia vapours on graphite substrates at 1,900° C. It has a density of 2.0–2.2 g/cm³ and a crystallite size of 50–100 Å and the hexagonal crystallites are aligned with their c-axes preferentially perpendicular to the substrate^1,2. The preferred orientation of the boron nitride crystallites, ΔφFWHM(002), defined as the full width at half-maximum intensity of the 002 orientation distribution function, is 50°–100° (ref. 3). These properties are similar to those for pyrolytic graphite deposited at 1,900° C. Pure boron nitride sublimes in one atmosphere of nitrogen at temperatures of 2,330° C⁴ to 3,000° C², compared with 3,700° C for graphite. Pyrolytic graphite, usually deposited at 2,100–2,200° C, can be converted into a material resembling single crystal graphite by annealing at temperatures to 3,600° C⁵, by tensile deformation parallel to the predominant direction of the basal planes of the crystallites at temperatures above 2,700° C^6,7, or by compression annealing perpendicular to the basal planes at temperatures above 2,800° C^8,9. Because pyrolytic boron nitride and pyrolytic graphite are similar in structure and in many properties, it was of interest to determine whether annealing or hot working could cause a significant structural transformation in the boron nitride.