When we mention “diamond,” the image that often comes to mind is a sparkling gemstone. However, in the field of materials science, diamond is far more than a precious gem. It is not only one of the hardest substances found in nature but also a critical functional material in electronics, optics, and mechanical engineering. Among its forms, Single-Crystal Diamond (SCD) and Polycrystalline Diamond (PCD), though both composed of pure carbon with sp³ bonding, exhibit remarkably different properties and applications due to their structural differences.

From Atomic to Macroscopic: Structural Origins of Differences

Single-crystal diamond consists of carbon atoms arranged in a perfect cubic lattice, free of grain boundaries and with minimal impurities. Its properties are highly anisotropic, meaning hardness and thermal conductivity can vary slightly along different crystallographic directions.
In contrast, polycrystalline diamond is formed by sintering countless diamond grains at the micro- or nanoscale. These grains are randomly oriented, giving rise to macroscopically isotropic behavior, though residual catalysts or minor non-diamond phases may remain at the grain boundaries.
This structural distinction—“perfect lattice vs. aggregate grains”—underpins the contrasting mechanical, thermal, and electronic performance of SCD and PCD.

Performance Comparison: Each Has Its Strengths

Single-Crystal Diamond (SCD)

Hardness: Theoretically the highest, Mohs 10;
Thermal Conductivity: Exceeds 2000 W/(m·K), the highest known among materials;
Electronic and Optical Properties: High carrier mobility, wide bandgap (5.5 eV), high transparency from UV to IR;
Limitations: Anisotropy can cause brittleness in certain directions, size is limited, and production cost is high.

Polycrystalline Diamond (PCD)

Hardness and Toughness: Slightly lower than SCD, but wear-resistant and less prone to directional fracture;
Thermal Conductivity: Several hundred W/(m·K), sufficient for most engineering applications;
Isotropy: Uniform mechanical properties and superior impact resistance compared to SCD;
Limitations: Grain boundaries reduce electronic performance and optical transparency, and high temperatures may induce graphitization at grain boundaries.

Differences in Fabrication: Determining Size and Cost

Single-crystal diamonds are primarily grown via High Pressure High Temperature (HPHT) or Chemical Vapor Deposition (CVD), resulting in high-purity crystals but limited size, suitable for high-end electronic and optical applications.
Polycrystalline diamonds can be produced by HPHT sintering of diamond powders (often with metal catalysts) or CVD deposition into films or bulk materials. This enables large-area fabrication at relatively lower cost, making PCD more suitable for industrial-scale applications.

Application Scenarios: High-End vs. High-Efficiency

SCD is commonly used in high-power electronic substrates, RF devices, quantum chips, precision optical windows, and sensors—applications requiring peak performance.
PCD excels in cutting tools, mining drill bits, wear-resistant components, and thermal management substrates, where durability and large size are prioritized.

Comparison Summary: Key Parameters

 

Property	Single-Crystal Diamond	Polycrystalline Diamond
Composition	Pure carbon, no grain boundaries	Pure carbon, grain boundaries may contain catalysts or impurities
Structure	Perfect cubic lattice, anisotropic	Randomly oriented grains, isotropic
Hardness	Highest (Mohs 10)	Slightly lower, but uniform wear
Thermal Conductivity	>2000 W/(m·K)	Several hundred W/(m·K)
Electronic Performance	High carrier mobility, wide bandgap	Reduced due to grain boundaries
Optical Performance	High transparency, low scattering	Lower transparency than SCD
Toughness	Brittle along some directions	Excellent impact resistance
Fabrication	HPHT or CVD single-crystal growth, size-limited	HPHT sintering or CVD deposition, large-area possible
Applications	High-power devices, quantum devices, optical windows	Cutting tools, drill bits, wear parts, thermal substrates

Conclusion: Same Carbon, Distinct Roles

Although single-crystal and polycrystalline diamonds both share the sp³ carbon structure, they differ markedly in structure, properties, cost, and applications. SCD represents the performance limit and the frontier of technology, while PCD emphasizes cost-effectiveness and engineering practicality. Looking ahead, as CVD techniques mature and nanoscale fabrication improves, the boundaries between their applications may blur, and hybrid approaches—high-performance PCD or cost-effective SCD—could further expand the industrial landscape for diamond materials.