High Pressure High Temperature (HPHT) method

HPHT BARS Schematic
The only economical process for growing large (1 + ct) single crystals of synthetic diamond is still the high-pressure/high-temperature flux method. General Electric, Sumitomo Electric, and De Beers researchers all use a reaction vessel in what is referred to as a "belt" type of apparatus mounted in a high-pressure hydraulic press, although the design details of their respective equipment apparently differ. When a metal solvent/catalyst is used as a flux, typical growth conditions are 1400° to 1600°C and 50 to 60 kbar. Such conditions are within the field of temperatures and pressures where diamond is the stable phase of carbon. With this type of equipment and these conditions, diamond crystals are produced by the "temperature gradient" or "reconstitution" technique. Growth periods for a 1-ct diamond crystal are reportedly on the order of five days or more.

At Novosibirsk, synthesis of large, single diamond crystals is accomplished using a different type of high-pressure/high-temperature apparatus, known as the "split sphere". In this type of growth apparatus, pressure is applied to two sets of anvils (figures 3). The outer set of eight anvils forms an octahedron-shaped cavity. Within this space, an inner set of six additional anvils is positioned to form a cube-shaped central cavity that contains the high-pressure cell in which the diamond crystals are actually grown (again, see figure 3). Diamond crystal growth experiments have been conducted at pressures of about 55 to 65 kbar and temperatures in the range of about 1350° to 1700°C. Transition metals (Fe, Ni, Mn, etc.) and their alloys are used as solvents/catalysts. The best-quality cuboctahedral crystals (grown up to 1.5 ct) reportedly have been obtained using growth rates of not more than 5 mg/hr. Thus, the conditions of pressure, temperature, time, and flux composition are similar to those used in the "belt" type of growth apparatus. However, the growth cell in the BARS equipment is smaller than that in the typical belt- type growth apparatus, so there is less space available to grow synthetic diamonds.
Schematic of a BARS system
Experiments have also been carried out at Novosibirsk and elsewhere in Russia to study the effects of additional heating (or "annealing") at high pressure on synthetic diamonds produced using the BARS equipment. These experiments were conducted using the same "split sphere" growth apparatus. These researchers report that heating for four to five hours at up to 2000° to 2200°C and 70 to 80 kbar can both affect the color of the synthetic diamonds and produce changes in their visible and infrared absorption spectra.
High Pressure High Temperature (HPHT)