The synthesis of large single-crystal diamonds using the HPHT Hydraulic Cubic Press (cubic press) primarily employs the Temperature Gradient Method under High-Temperature and High-Pressure (HTHP) conditions.
This technology, originally developed by General Electric in the US in the 1970s, has only achieved large-scale commercial production, particularly on domestic Chinese cubic presses, in the last 5–6 years.
Here is a detailed explanation of how large single-crystal diamonds are synthesized using an HTHP cubic press (such as the domestic SPD6×2800 model with an 850mm cylinder bore):
1. Synthesis Principle: The Temperature Gradient Method
The core function of HTHP technology is the conversion of graphite raw material into diamond. The Temperature Gradient Method utilizes the varying solubility of carbon in a metallic solvent (catalyst) based on temperature:
HTHP Environment: Synthesis generally requires temperatures around  and pressures of 5.5 GPa. Specific experiments detailed in the source were carried out at a pressure of 5.5 GPa and a temperature range of .
Creating the Concentration Gradient: The synthesis cavity is deliberately set up with a temperature gradient, which, in turn, creates a concentration gradient of carbon within the metallic solvent.
Diffusion and Crystallization: Driven by this concentration gradient, the carbon diffuses through the metallic solvent. When the carbon reaches the low-temperature area where a seed crystal is present, the homoepitaxial crystallization of the diamond is achieved.
Growth: The crystal gradually grows as the high pressure and temperature are maintained (e.g., for 168 hours).
Growth Rate Advantage: Compared to Chemical Vapor Deposition (CVD), the HTHP method offers an advantage in growth speed, typically requiring 7–8 days to grow a single diamond crystal of 2–3 carats.
2. Key Components and Materials
Specific materials and components are essential for successful synthesis within the cubic press cavity:
A. Raw Materials and Catalyst
 Carbon Source: Natural flaked graphite is used as the raw material. This graphite requires intensive graphitization purification treatment beforehand, including heating at  for 5 hours under nitrogen gas protection, to remove ash and increase the degree of graphitization.
Metallic Catalyst (Solvent): An iron-cobalt (Fe-Co) alloy is used as the catalyst. The alloy is prepared by mixing elemental iron and cobalt in a 6:4 ratio, vacuum smelting, stirring the liquid metal for 1 hour, and then slowly cooling the alloy rod.
Seed Crystal: To control the directional growth of the large single crystal, the  crystal plane is chosen as the epitaxial growth surface. The seed crystals are typically  square crystals.
B. Nitrogen Scavenger (De-Nitrogenizing Agent)
Goal: To synthesize high-quality Type IIa gem-grade large single-crystal diamonds.
Material: Metallic Titanium (Ti) powder is added to the graphite carbon source, typically ranging from 0.5wt.% to 1.5wt.% of the catalyst weight.
Mechanism: Ti acts as a nitrogen scavenger by reacting within the synthesis cavity to form Titanium Nitride, which reduces the nitrogen concentration in the crystallization environment. Lower nitrogen concentration increases the solubility of carbon in the catalyst, thereby increasing the crystal growth rate.
Efficiency: Quantitative analysis using infrared spectroscopy shows a high de-nitrogenizing efficiency. Nitrogen concentration, which is around 185 ppm without Ti, can be reduced to about 1 ppm when 0.7wt.% Ti is added, reaching the detection limit of infrared spectroscopy. Diamond crystals synthesized under this condition are classified as Type IIa.
C. Temperature Gradient Regulation
Heating Method: Temperature regulation is achieved by controlling the heat generated by the auxiliary heat source.
 Mixed Auxiliary Heat Source: A customized design uses a mixed body of oxide and graphite. Graphite content is varied to establish the required temperature gradient: the upper auxiliary heat source contains 8wt.% graphite, which is lower than the 12wt.% content in the lower component.
Temperature Difference: This composition ensures that, under the same current, a temperature distribution is created where the upper end is hotter than the lower end, with a temperature difference of approximately .
3. Energy Consumption Optimization
The synthesis of large single-crystal diamonds using the 850mm cylinder bore cubic press is challenging due to the high conventional operating power required (8.0∼8.5kW). Energy optimization is critical for sustainability.
 Procedure: The assembled synthesis block is pre-heated (100 °C for 2 hours). After insertion, the press is loaded to 5.5 GPa and heated to 1280∼1300 °C. The actual temperature inside the cavity is monitored using a Type B double platinum-rhodium alloy thermocouple.
Resistance and Power Improvement: By optimizing the synthesis cavity structure and employing the mixed auxiliary heat source, the synthesis resistance was significantly increased (from 1.8∼1.9mΩ to 4.5∼5.0mΩ).
Reduced Power: This structural optimization addresses the issue of lowered synthesis resistance caused by larger cavity size, allowing for a substantial reduction in operating power. Single-crystal diamond growth can be achieved effectively in the range of , achieving a power reduction of about 15% compared to traditional assembly structures.