hpht - high pressure high temperature

To recreate the conditions of a diamond's birth some 150-200 km below the Earth's surface, several HPHT methods have been developed. The two most common are the BELT system and its Russian counterpart, the BARS system.
The first type of press is the BELT system. It is used by, among others, De Beers and General Electric and is basically a huge hydraulic press with anvils and a ring-shaped structure; hence the name ‘belt'. Belt presses are still used today by several major manufacturers at a much larger scale than the original designs.

This is a schematic example of a Belt type HPHT press.  Diamond seeds are placed at the bottom of the press. 
The internal part of press is heated by a tubular, graphite heater.  This generates temperatures above 1400°C
and melts the solvent metal.  The molten metal dissolves the high purity carbon source, which is then
transported to the diamond seeds and precipitates.  If nitrogen is removed by mixing small quantities
of Ti with the metal, a colourless diamond is synthesized.


The second type of press is the BARS system, which was developed at the Russian Academy of Sciences in Novosibirsk and is made up of eight outer anvils with a spherical outer shape to which pressure is applied, and 6 inner anvils to multiply the pressure to the sample.

The third type of press design is the cubic press. A cubic press has six anvils which provide simultaneous pressure onto all faces of a cube-shaped volume. The first multi-anvil press design was actually a tetrahedral press, using only four anvils to converge upon a tetrahedron-shaped volume. The cubic press was created to fulfill the need to increase the pressurized volume. A cubic press is typically smaller than a belt press and can achieve the pressure and temperature necessary to create synthetic diamond faster.
All methods use a core reaction cell in which the diamonds are grown. This reaction cell is submitted to the extreme temperatures and pressures needed. Inside the reaction cell a carbon source is placed, as well as some seed crystals. One of the most important elements in the process is the presence of a metal solvent or ‘flux', which acts as a solvent and transport material of the carbon source to the seed.

This solvent can be iron (Fe) or nickel (Ni) or mixture of the two. This flux makes it possible to synthesize a diamond at much lower temperatures and pressures than necessary for direct conversion of graphite into diamond.

The exact composition of the flux strongly influences the properties of the synthetic diamond. For example, aluminum will remove nitrogen atoms from the synthetic diamond. This results in the growth of a colorless diamond. Adding boron to the growth capsule will result in a blue diamond.

A slow, well-controlled growth is essential for growing high quality diamonds. Only a few minutes are needed to convert graphite into powder sized diamond, but it takes about three days to crystallize a 1 ct, rough gem-quality stone.

A HPHT press used for HPHT synthesis.