Based on the characteristics of synthetic diamond crystal growth, the main growth methods can be categorized into three types: the Direct Method, the Flux-catalyst Method, and the Epitaxial Method. Here is a detailed explanation of these three methods and their main differences:

1. Direct Method：

Growth Mechanism: This method utilizes transient static ultra-high pressure and high temperature technology, dynamic shock wave technology, or static-dynamic mixed ultra-high pressure and high temperature technology to convert carbonaceous raw materials directly from a solid or molten liquid state into diamond. Under such extreme ultra-high pressures (e.g., above 100 kilobars), the carbon atom groups in graphite undergo compression, shear, and thermal vibration, leading to shortened interlayer spacing and stacking faults, thereby exciting the sp3 hybridization state required for diamond formation.

2.Flux-catalyst Method：

Growth Mechanism: This uses a much lower pressure (e.g., 50-100 kilobars) than the direct method and static high-pressure and high-temperature technology to grow diamonds through the reaction of carbonaceous raw materials with specific metals and their alloys (such as Group VIII metals, chromium, manganese, etc.). These metals and alloys play a composite role of flux and catalyst, which can lower the pressure and temperature required for the phase transition and increase the phase transition rate. The geometric and electronic structures of the metals can prompt carbon atoms to enter interstices or interdiffuse, thereby exciting the transformation of non-diamond crystalline carbon atoms into the sp3 hybridization state.

3. Epitaxial Method：

Growth Mechanism: This method utilizes carbon atoms and their groups precipitated during the pyrolysis (such as the pyrolysis of methane, ethane, etc.) or electrolysis of carbon-containing substances to perform epitaxial growth on diamond seed crystals or certain substances acting as substrates.

Nucleation Characteristic: Also primarily non-spontaneous nucleation. This method requires a high-quality seed crystal surface and usually necessitates purification of the diamond seed crystal before and during the reaction to remove unfavorable impurities, thereby prompting carbon atoms to preferentially grow epitaxially at defects.

Main Differences between the Three Methods:

Thermodynamic Reaction Zones: The Direct Method and Flux-catalyst Method are achieved in the diamond stable zone; whereas the Epitaxial Method is carried out in the diamond metastable zone.

Nucleation Mechanisms: The Direct Method relies mainly on spontaneous nucleation, nucleating independently under extreme pressure and temperature conditions; while the Flux-catalyst Method and Epitaxial Method rely mainly on non-spontaneous nucleation, requiring the assistance of foreign substances (such as metal catalysts or diamond seed substrates) to aid nucleation and growth.

Reaction Conditions and Auxiliary Materials: The direct method requires extremely harsh ultra-high pressure (often above 100 kilobars) and high temperature, without the need to add catalysts. The flux-catalyst method significantly lowers the required pressure (e.g., drops below 100 kilobars) and temperature by adding specific metals/alloys. The epitaxial method must rely on the pyrolysis/electrolysis processes of carbon-containing substances and the guidance of the seed crystal substrate.