As an important sealing and pressure-transmitting medium in the synthetic diamond system, the internal chemical composition and impurity mineral composition of pyrophyllite will directly affect the pressure transmission, sealing, and high-temperature/high-pressure stability of the medium, thereby significantly affecting the synthesis quality and yield of diamond. The specific impacts are mainly reflected in the following aspects:
1. Impact of chemical composition
Silicon dioxide (SiO₂): Pyrophyllite with a higher SiO₂ content usually has better pressure transmission, but if the content is too high, it will cause the raw material to become hard, thereby reducing its sealing performance.
Aluminum oxide (Al₂O₃): Pyrophyllite with a higher Al₂O₃ content often exhibits better sealing properties.
Iron oxide (Fe₂O₃): Pyrophyllite with a higher iron content has a higher internal friction coefficient.
Moisture (structural water/loss on ignition): Pyrophyllite with a higher loss on ignition has a softer texture and good resilience, but an excessively high water content is harmful to diamond synthesis. Under high-temperature and high-pressure environments, free moisture and crystal water will promote the phase transition of pyrophyllite (generating coesite and kyanite) and form a harmful hard crust layer, which will hinder the effective transmission of pressure and interfere with diamond synthesis.
2. Impact of impurity mineral composition
Layered and non-layered impurity minerals: Pyrophyllite ore is often accompanied by various impurity minerals. Among them, as long as the content of layered structure impurities (such as kaolinite, sericite, chlorite, etc.) does not exceed 10%, it will not affect the pressure transmission performance of pyrophyllite. However, the content of non-layered structure impurities must be strictly controlled within 5%, otherwise, it will destroy the static pressure rheology of the pressed pyrophyllite powder block, resulting in uneven pressure transmission.
Impurity minerals containing iron, titanium, etc.: Although pure pyrophyllite has extremely high pressure transmission, its sealing performance is poor. Experiments show that the presence of small amounts of impurity minerals such as hematite, ilmenite, limonite, and rutile in pyrophyllite can actually effectively improve the sealing performance of the medium.
Quartz: An appropriate amount of quartz is beneficial to the synthesis. Studies have pointed out that when pyrophyllite contains about 20% quartz as a pressure-transmitting medium, it not only helps to reduce the synthesis pressure, but also significantly improves the quality of diamond products.
In summary, to ensure the high quality of synthetic diamonds, the selected pyrophyllite not only needs to have moderate hardness (to balance pressure transmission and sealing) but also needs to have its specific impurity minerals and moisture controlled within an optimal ratio range. For instance, the loss on ignition should be controlled at 5.5%~7.5%, aluminum oxide at 27%~35%, and silicon dioxide at 47%~65%. Excessively high purity or excessive specific impurities will destroy the cavity environment, thereby reducing the synthesis quality of the diamond.