Synthetic Diamond Grit

There are basically two different methods for growing synthetic diamond, depending on the final size required. To produce diamond grit, with grain sizes up to about 1 mm, graphite and the metal are mixed together and placed in a growth capsule, which has electrically insulating walls. The capsule is placed at the center of a high-pressure press, and an electric current is passed through the graphite/metal mixture to raise the temper- ature above the melting point of the metal (this is referred to as ‘direct heating’). Once nucleation occurs, carbon flux is transported across a thin film of molten metal between the graphite and the growing diamond. After a few tens of minutes, a substantial fraction of the graphite is con- verted to diamond. The diamond grit is recovered by dissolving the metal in acid.

When GE first announced this process, it was an extraordinary technological breakthrough; today, 50 years later, approximately 200 tonnes (1000 million carats) of synthetic diamond grit is produced each year for industrial cutting and grinding applications.

Diamonds produced in the manner described have an attractive yellow colour because of the presence of single nitrogen atoms. The graphite, metal solvent-catalyst and the slightly porous material, from which the growth capsule is fabricated, all absorb nitrogen from the atmosphere, and the synthetic diamonds typically contain a few hundred parts per million of single nitrogen.

Research in the 1960s by GE showed that colourless synthetic diamond grit could be made if the nitrogen in the growth capsule were removed using ‘nitrogen getters’. These are materials like aluminium or titanium which combine with the nitrogen and prevent it from being incorporated in the diamond. They also showed that blue diamonds could be grown if the nitrogen were removed and a small quantity of boron was added.