Diamond, known as the "king of hardness," holds a pivotal position in the field of materials science. Based on differences in crystal structure, diamonds can be categorized into monocrystalline diamond powders and polycrystalline micron diamonds. Although these two types of materials share many commonalities, the differences in their microstructures result in significant distinctions in performance, preparation methods, and application ranges.
Monocrystalline diamond Powders possess a highly ordered, near-perfect crystal structure. At an atomic scale, carbon atoms are arranged in a tightly ordered three-dimensional periodic pattern, with the lattice being complete and continuous. The entire crystal originates from a single nucleus, growing steadily in a specific crystallographic direction without being interfered by grain boundaries. This results in a uniform and symmetric electron cloud distribution, with consistently strong chemical bond energies, maintaining extreme uniformity in physical properties on a macroscopic level.
In contrast, polycrystalline micron diamonds resemble a "micro-mosaic" artwork composed of innumerable tiny "crystal puzzles." They are aggregated from numerous small nanometer-sized particles. These small particles have a crystal structure similar to monocrystalline diamond powders but are arranged disorderly with inconsistent orientations. The particles are interconnected through unsaturated bonds, with clear grain boundaries and relatively more defects in their crystal structure.
Monocrystalline diamond Powder
It is one of the hardest substances in nature, with a Mohs hardness of 10. It has extremely high wear resistance, with edges that can reach atomic-level flatness and sharpness. When cutting, the perfect state of the edge can be transferred to the processed material, producing a highly polished mirror finish. It is suitable for ultra-thin cutting and ultra-precision machining.
Polycrystalline Micron Diamond
Although it has slightly lower hardness compared to monocrystalline diamond powders, it still possesses excellent wear resistance. Its unique toughness and self-sharpening properties make it effective in grinding and polishing. Coarse particles can break into smaller particles, continually exposing new cutting edges. This ensures the grinding and polishing quality of the sample surface while improving grinding and cutting efficiency, making it more suitable for processing workpieces composed of materials with different hardnesses.
Monocrystalline diamond Powder
At room temperature, its thermal conductivity is above 2000 W/(m·K), mainly derived from the vibration of carbon atoms, i.e., the propagation of phonons. Its highly ordered lattice structure practically eliminates the influence of grain boundary scattering, maintaining stable physical and chemical properties under high-temperature and high-pressure environments. It can be used in high-power semiconductor cooling, laser equipment temperature control, and other fields.
Polycrystalline Micron Diamond
Although its thermal conductivity is somewhat reduced due to grain boundary scattering, within a specific temperature range, the interference of grain boundaries on phonon scattering paths can become advantageous for controlling thermal conductivity. It can be used as a heat sink for semiconductor power devices. Its precipitation technology is relatively easier to achieve, and the preparation cost is more advantageous.
Monocrystalline diamond Powder
It has a high optical refractive index and extremely low absorption coefficient, with minimal loss when light passes through. Once polished, it can be used for infrared optical windows, high-end microscope objectives, etc. High-quality monocrystalline diamond powders prepared by CVD methods can be completely colorless and transparent, with excellent transmittance from ultraviolet to infrared, and even microwave ranges. The theoretical transmittance can be as high as 71.6%, making it an excellent crystal Raman material as well.
Polycrystalline Micron Diamond
Due to grain boundary scattering, its optical uniformity is limited. However, after enhancing crystal grain size and reducing grain boundary impact through special processes, it can be applied in the fields of lighting and display devices where cost sensitivity and moderate optical precision are required.
High Temperature High Pressure (HTHP) Method
Graphite or other carbon sources are subjected to high temperatures (1200-2000℃) and high pressures (5-6 GPa) in the presence of a metal catalyst, causing carbon atoms to rearrange and form monocrystalline diamond powders. This method can yield large gem-quality single crystals but comes with high equipment costs, complex operations, and size and growth rate limitations.
Chemical Vapor Deposition (CVD) Method
In a low-pressure high-temperature chamber, carbon-containing gases are activated using plasma or hot filament, causing carbon atoms to deposit layer by layer on a substrate, growing monocrystalline diamond powders. The crystals produced by this method have high purity, with highly controllable growth parameters, suitable for the mass production of semiconductor-grade single crystals. However, the growth rate is relatively slow.
Direct Conversion Method
Graphite powder mixed with a metal binder is subjected to short-term high temperature and high pressure sintering, causing rapid transformation into polycrystalline micron diamond. This process is simple, low-cost, quick, and capable of producing bulk and shaped polycrystalline products commonly used in industrial cutting tools.
CVD Method
By adjusting deposition pressure, temperature, gas flow, etc., it promotes the generation and growth of numerous crystal nuclei, resulting in polycrystalline micron diamonds. This method allows flexible control over grain size and grain boundary characteristics, used in tool coatings, MEMS fine components, and more.
Semiconductor Field: As an ideal substrate material, its wide bandgap and high carrier mobility help transistors and power modules overcome high-frequency and high-voltage operation bottlenecks, driving innovations in 5G base stations, electric vehicle power system chips, and more.
Optical Instrument Manufacturing: Used to make high-precision optical windows and lenses, such as infrared detectors and space telescopes, enhancing the light capture and resolution capabilities of equipment.
Ultra-precision Machining: For instance, in the manufacturing of cutting tools, achieving high-precision, high-efficiency material processing. When processing non-ferrous metals, the surface roughness can reach Rz0.1-Rz0.05μm, and the shape precision of the processed workpiece can be controlled below 50nm.
Grinding and Polishing: As a precision abrasive, it is widely used in the fine grinding and polishing of hard and brittle materials such as sapphire wafers, silicon carbide wafers, functional ceramics, ensuring machining quality while improving efficiency.
Electronic Packaging: Polycrystalline micron diamond films can be used as heat sink substrates, addressing chip heating issues, supporting the miniaturization and performance enhancement of electronic products.
Semiconductor Cooling: Polycrystalline micron diamond films can serve as heat sinks for semiconductor power devices, improving heat dissipation performance and ensuring stable operation.
Currently, in the research direction of monocrystalline diamond powders, research teams are focusing on overcoming two main challenges: reducing production costs to achieve mass production and exploring growth methods for large-size monocrystalline diamond powders. Simultaneously, there is active exploration of new doping technologies and defect control technologies to uncover more potential properties.
For polycrystalline micron diamonds, research is focused on the intelligent design of grain boundary structures, through precise control, optimizing the performance of polycrystalline micron diamonds in predetermined directions to meet specific industrial needs. As technology continues to advance, single-crystal and polycrystalline micron diamonds are bound to continue playing key roles, aiding the realization of more unknown technological innovations.
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