Preparation of solid aggregates of high density boron nitride crystals

Abstract

Method and apparatus for compaction of powder of high density boron nitride, having an initial particle diameter of at least two microns, into a high density crystal aggregate of between 1 mm up to about 1 cm in diameter, the resulting crystal aggregate having a zincblende form, the wurtzite form or mixtures of the zincblende and wurtzite forms. High density aggregates of boron nitride, preferably having particle sizes with diameters at least 2 .mu.m, are placed in a hollow, substantially cylindrical first container of metal, and the first container is surrounded by a fluid-like material of metal or metal powder having a shock wave velocity V. The first container and fluid-like material are placed in a substantially cylindrical second container that is purged of substantially all air and sealed. The second container has a rigid side wall and two rigid end walls. A high explosive, having a detonation velocity D of at least 6 km/sec and preferably at least 8 km/sec, is positioned at one end wall and along at least a portion of the side wall of the second container. The high explosive is detonated, and an approximately radially directed pressure wave moves through the fluid-like material and the boron nitride powder toward the center of the second container. A second, approximately radially directed pressure wave, separated in time from the first pressure wave, subsequently moves outward through the fluid-like material and the boron nitride powder. These two pressure waves produce boron nitride aggregates of density at least 95 percent of the theoretical maximum density (3.49 gm/cm.sup.3) and of sizes at least 1 mm in diameter. A solid mandrel can be positioned near a center line of the second container to improve the quality of the resulting product. Grain sizes as small as 0.005-0.05 gm are produced. Boron nitride aggregates with Knoop hardness parameters in the range of 4300-5000 Kgm/mm.sup.2 can be produced in this manner.