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Stuart H Robertson

Geoffrey John Davies

Lesley Kay Hedges

A method is provided for producing large cubic boron nitride crystals having the size of 0.1 carat or larger. The method involves providing a reaction vessel which contains CBN seed material (32), a CBN source (26) separated from the seed material and sandwiched between two masses (22) (24) of solvent/catalyst for CBN synthesis which is capable of melting under CBN synthetic conditions, and contains sufficient hexagonal boron nitride to saturate the catalyst with boron nitride when molten, the separation of seed material and source material being such that under the operating conditions of CBN synthesis a temperature gradient is created between the seed material and the source material with the seed material being located at a point near the minimum value of temperature of the temperature gradient and the source material being located at a point near the maximum value of temperature of the temperature gradient, placing that reaction vessel in the reaction zone of a high temperature/high pressure apparatus and subjecting the contents of the reaction vessel to conditions of temperature and pressure in the CBN stable region of the boron nitride phase diagram for a period of at least several hours and not more than 24 hours.

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Johannesburg, Transvaal, South Africa

Stuart H Robertson