Optimizing the crystal quality of synthetic diamonds by controlling pressure and temperature mainly centers on precisely adjusting the degree of overpressure and degree of supercooling (supersaturation) during the phase transition process. These thermodynamic and kinetic conditions directly determine the nucleation rate, growth speed, and the crystal's ability to expel impurities. Specific optimization strategies are as follows:
Adopt low overpressure and supercooling to grow high-quality large single crystals To obtain high-quality large single-crystal diamonds, the overpressure and supercooling must be controlled within a small range. Under these temperature and pressure conditions, although the diffusion ability of carbon atoms and their groups is relatively strong, the energy fluctuation required to generate a three-dimensional diamond crystal nucleus is very high, so the number of generated nuclei is extremely small. In an ideal state, only one crystal nucleus is generated in the system, avoiding multiple nuclei competing for carbon sources or squeezing each other, which is a necessary condition for growing high-quality large single-crystal diamonds. Furthermore, under these conditions, adopting the method of adding diamond seed crystals to promote the preferential generation and layer-by-layer spreading of two-dimensional nuclei on the seed crystal surface is an effective means to cultivate high-quality large single crystals.
Adjust appropriate overpressure and supercooling to cultivate polycrystalline or abrasive-grade diamonds If the goal is to grow large-particle polycrystalline diamonds, appropriate (moderate) overpressure and supercooling need to be provided. At this time, a larger number of three-dimensional crystal nuclei are generated, and they will cross-link and combine with each other during the growth process, eventually forming polycrystals. The suitable nucleation conditions for ordinary abrasive-grade diamonds happen to lie at the junction of the single-crystal and polycrystal nucleation zones. What needs to be strictly prevented is the extreme situation where the overpressure and supercooling are too large—at this time, although the energy threshold required for nucleation is lowered, because the diffusion ability of carbon atoms and their groups becomes extremely weak, very few nuclei are generated, and it may even be difficult to successfully complete the phase transition.
Strictly control the cooling rate to reduce crystal impurities Crystal purity is a key indicator of quality, which is closely related to the rate of temperature drop. If the cooling rate is too fast in the late stage of growth or during the shutdown phase, exceeding the diamond crystal's ability to expel impurities such as flux-catalysts out of its body, a large number of impurities will be trapped inside the crystal. These impurities that fail to exsolve under supersaturated conditions will severely degrade the crystal quality, making this kind of diamond extremely prone to carbonization and graphitization cracking when reheated in the metastable zone.
Overall, the control of pressure, temperature, and time is essentially about influencing macroscopic/microscopic compression, shear, and thermal vibration to govern the movement, diffusion, and interaction of carbon atom groups. Only by providing stable and minimal overpressure/supercooling degrees, coupled with a slow cooling and impurity-expelling process, can the integrity of the crystal lattice structure be optimized to the greatest extent, thereby obtaining high-quality synthetic diamonds.