Polycrystalline Diamond not only has hardness and wear resistance close to that of single-crystal diamond but also exhibits superior anisotropy and impact resistance comparable to that of carbide. Hence, it is widely used in fields such as oil and gas extraction, coal and geological exploration, and mechanical processing.
Since the discovery of natural single-crystal diamond, another diamond called "carbonado" (black diamond) has also been discovered. This diamond is very rare in nature. Studies have found that it is a bulk polycrystal composed of numerous single-crystal diamond particles and trace impurities. The disordered arrangement of diamond micro-particles gives it the characteristic of having no cleavage planes, making its hardness, strength, and wear resistance superior to traditional single-crystal diamond.
Explosive Synthesis
Using high-pressure resistant containers, the high temperature and kinetic energy generated by an explosion are used to rapidly impact metal sheets onto graphite sheets, creating a momentary high-temperature, high-pressure environment that transforms graphite into micro-powder state polycrystalline diamond. This method leaves graphite residue within the polymer and cannot synthesize large, regular polycrystals, thus is only suitable for producing abrasive-grade polycrystalline diamonds where quality is not a major concern.
Low-Pressure Chemical Vapor Deposition (CVD)
The core principle of this method involves introducing carbon-containing gaseous raw materials into a reaction chamber at less than 1 standard atmosphere (1.01×105 Pa). Under these conditions, a series of complex chemical reactions ultimately allow carbon atoms from the gaseous raw materials to deposit on a substrate's surface in the diamond phase, forming a thin film of polycrystalline diamond. This method produces films with high thermal stability and allows doping with other elements to create semiconductor materials, making it especially useful in electronics and optics. However, current issues with this method include long synthesis cycles and low productivity.
Direct Conversion
This method involves directly converting high-purity graphite micro-powder into polycrystalline diamond under ultra-high pressure and temperature (over 2000℃ and 13GPa). This method can synthesize high-purity diamonds with performance indicators close to natural diamonds. However, it demands extremely high synthesis equipment standards, operates under stringent conditions, has low productivity, and high costs, making industrial-scale production currently unfeasible and limiting this method to the experimental stage.
High-Pressure High-Temperature Solvent Method
Under constant high pressure and temperature (5-7GPa, 1300~1700℃), this method converts diamond micro-powder, graphite powder, and other raw materials into polycrystalline diamond composites using transition metals or alloy catalysts. The advantages of this method include short synthesis cycles and relatively low requirements for the performance indicators of the synthesis equipment, indirectly reducing costs and making it highly suitable for industrial production.
Polycrystalline diamond is a polycrystalline material with the common characteristics of polycrystals. Due to the random arrangement of grains within its internal environment, it exhibits long-range disorder but short-range order, making its physical and chemical properties consistent in all directions. Its lack of cleavage planes gives it superior impact resistance compared to large single-crystal diamonds, allowing for fewer limitations and a broader range of applications.
Polycrystalline Diamondhas lower synthesis costs compared to natural and synthetic large single crystals, can be fashioned into different shapes as needed, and features high hardness and strength. It is an ideal material for industrial applications and is now used in various fields including industry, technology, and national defense.
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