High Pressure High Temperature (HPHT)
As it applies to man made diamonds
Diamonds are the hardest known naturally occurring substance on Earth. Mined for their brilliance, luster and durability, these single crystals have applications in the jewelry industry and in the world of technology.
Since English chemist H. Davy proved conclusively in 1814 that diamond is a crystalline form of carbon, attempts have been made to duplicate nature. After over a hundred years of attempts, in the early 1950s, General Electrical (GE) created the first officially recognized synthesis of a diamond. The process used to create this first man made diamond was HPHT, high pressure, high temperature.
HPHT is the original method of producing man made diamonds and, due to its relatively low cost, is still widely used. The HPHT process is an attempt to reproduce the conditions that create natural diamonds. Large presses that can weigh a couple of hundred tons are used to produce a pressure of 5 GPa (giga pascals) at 1,500 degrees Celsius. There are two main press designs used to supply the required pressure and temperature: the belt press and the cubic press.
The original GE invention by H. Tracy Hall uses the belt press. Upper and lower tungsten-carbide anvils supply pressures and the heating current to a cylindrical capsule, with the pressures reaching between 50,000 and 70,000 atmospheres. A carbon source is placed at the top of the capsule, a metal slug is placed in the center, and tiny grit-sized synthetic diamond seed is placed at the bottom.
Heat of 1,200-1,500 degrees Celsius is applied to the capsule by passing an electric current through heaters contained within. The top of the capsule is kept at a higher temperature than the bottom to create a temperature difference of 10 to 50 degrees. When the pressure-temperature reaches ideal conditions, carbon from the upper capsule source dissolves in the metal and is driven to the bottom. The carbon precipitates out of the solution onto the seed crystal in the form of a diamond.
The second type of press design is the cubic press. A cubic press has six anvils which provide pressure simultaneously onto all faces of a cube-shaped capsule. 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 shortly thereafter to increase the pressurized volume. Typically smaller than a belt press, a cubic press can achieve the pressure and temperature required to create man made diamond at a faster rate.
Cubic presses, however, can’t be easily scaled up to produce larger volumes of product. For example, increasing the size of the anvils could increased the pressurized volume, but that would also skyrocket the amount of force needed on the anvils to achieve that pressure. Decreasing the surface area to volume ratio of the pressurized capsule could also be achieved by using more anvils to converge upon a different solid, but a press able to achieve that level of complexity would be difficult to manufacture.
The HPHT process takes only a few hours and produces tiny diamond bits and dust. These small diamonds, polycrystalline in structure (diamonds are single crystals) are too small for use as gemstones. For tools, however, these diamonds are extremely useful. Coated on edges of cutting tools, drill bits, and in resin on a flexible backing, these crystals can cut, drill, grind and polish a wide variety of materials. The high pressure, high temperature (HPHT) method of diamond production is a major contributor, and research continues to improve both the creation process and its applications.
For more information, see the High Pressure/High Temperature Data Center
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