The formation process of diamond is closely related to key parameters such as pressure, temperature, and synthesis time.

1. Pressure

    Determines the number and size of crystal grains: Within the diamond phase region, the number and size of diamond crystal grains are primarily determined by pressure. As pressure increases, the number of diamond crystal grains rapidly increases, and their particle size also becomes larger.

    Affects nucleation and conversion rate: When pressure is low or temperature is high, the critical radius for diamond crystal grains is larger, making nucleation difficult and resulting in a low conversion rate. Conversely, when pressure is high or temperature is low, the critical radius is smaller, crystal grains form easily, are numerous and dense, and the conversion rate is high.

    Role in secondary pressurization: Secondary pressurization experiments showed that when the oil pressure increase amplitude is small (e.g., from 67MPa to 69MPa, an amplitude of 3%), the number of crystal grains basically does not increase. However, when the pressure increase amplitude is larger (e.g., from 67MPa to 71MPa, an amplitude of 7%), the number of crystal grains significantly increases. If the pressure increase amplitude is too large (exceeding 12%), even with high heating power, the number of crystal grains remains high, and their color turns black.

    Impact on crystal grain size: Whether in primary or secondary pressurization, crystal grain size changes significantly only within the first few minutes after the pressure stabilizes (about 6 minutes); even if the synthesis time is extended afterwards, the increase in grain size is not obvious.

    Used for controlling crystal grain quality: When the pressure increase amplitude is small, the increase in crystal grains is minimal, thus allowing control over the number and quality of crystal grains, providing a possible way to grow high-quality coarse-grained diamonds.

2. Temperature

    Determines the color of diamond: The color of diamond is largely related to temperature. Yellow diamonds are typically distributed in higher temperature regions. When the second heating power is high, the resulting crystal grains are yellower.

    Affects the number and size of crystal grains: At the same pressure, when the temperature increases from low to high, the number and size of crystal grains will change from zero to a maximum, and then decrease again until no diamond appears.

    Affects nucleation and conversion rate: When temperature is high or pressure is low, nucleation is difficult, and the conversion rate is low.

    Choosing growth temperature: Although at lower temperatures the conversion rate is low and nucleation is sparse, impurities are not easily excluded from the crystal. Therefore, it is more appropriate to choose to grow diamonds at higher temperatures to control the growth rate, which can yield high-quality crystal grains grown in the so-called diamond "optimal crystal region".

    Combined effect with pressurization: When the second heating power (W2') is high, even if the pressure increase amplitude is large, the increase in the number of crystal grains is not significant, and the crystal grains are yellower. If only pressure is increased without increasing power, the resulting crystal grains will be black and mostly clustered.

3. Synthesis Time

    Affects crystal grain size: Crystal grain size changes significantly only within the first few minutes after the pressure stabilizes (about 6 minutes); even if the synthesis time is extended afterwards, the increase in grain size is not obvious.

    Potential impact: The sources indicate that extending synthesis time may contribute to the synthesis of high-quality large-grained diamonds.

In addition to the above main parameters, the research also involves the following factors:

Catalyst: The study used Ni~oMn2sCos catalyst. The catalyst promotes the rapid formation of diamond crystal grains by distorting the graphite lattice through electron attraction. After diamond formation, the crystal grains are surrounded by the catalyst melt, and their continued growth depends on the diffusion and deposition of carbon atoms or atomic groups.

Synthesis Method and Materials: The direct-heating static pressure catalyst method was adopted. Specific experimental methods included sheet-layered direct-heating assembly. Measures such as graphite pretreatment, improved sample assembly, or the use of new catalysts have also been explored to increase diamond grain size.

Pressurization Process: Experiments were conducted using two processes: primary pressurization and secondary pressurization. By selecting pressure and temperature conditions, segmenting pressure and temperature increases, and strictly controlling the amplitude, high-quality coarse-grained diamonds can be obtained in a relatively short time.

Diamond Formation Region: Diamond only forms within specific pressure and temperature ranges. The left boundary of this region is parallel to the catalyst melting curve, and the lower boundary is parallel to the graphite-diamond phase equilibrium curve.

In summary, given specific materials such as graphite, catalyst, and pressure-transmitting medium, pressure and temperature are the decisive factors in the formation and growth process of diamond. A deep understanding of their relationships, roles, and mechanisms will be crucial for guiding research and production practices for growing high-quality large-grained diamonds.