Owing to its structure, diamond has excellent performance in many fields such as optics, mechanics, thermal behaviour and chemistry. The parameters are shown in the following table.
Table of diamond capabilities
| Physical property | Natural diamond | High quality CVD diamond polycrystalline film |
|---|---|---|
| Hardness (kgf/cm²) | 10000 | 9000 – 10000 |
| Young's modulus (GPa) | 1200 | Approaches natural diamond |
| Coefficient of friction | 0.1 | 0.1 |
| Coefficient of thermal expansion (10⁻⁶/°C) | 0.8 | 0.8 |
| Thermal conductivity (300 K, W/cm·K) | 20 | 10 – 20 |
| Longitudinal wave velocity of sound (m/s) | 18000 | — |
| Density (g/cm³) | 3.6 | 2.8 – 3.5 |
| Refractive index (590 nm) | 2.41 | 2.4 |
| Band gap width (eV) | 5.5 | 5.5 |
| Light transmission | 225 nm to far-IR | Approaches natural diamond |
| Resistivity (Ω·cm) | 10¹⁶ | > 10¹⁰ |
| Dielectric strength (V/cm) | 10¹⁷ | > 10¹⁸ |
| Electron mobility (cm²/V·s) | 2200 | 4500 |
| Hole mobility (cm²/V·s) | 1600 | 2300 |
| Dielectric constant | 5.5 | 5.5 |
| Saturated electron velocity (cm/s) | 2.7 × 10⁷ | — |
Optics
The diamond band gap is 5.5 eV. It has a suitable refractive index (≈ 2.4) and a low absorption coefficient from 225 nm to the far-IR band. For example, the absolute absorption coefficient of optical CVD diamond is 0.07 – 0.029 cm⁻¹ at a wavelength of 10.6 μm and a thickness of 0.5 – 1.5 mm.
Mechanical properties
The hardness of diamond is about 90 GPa and Young's modulus is 1050 GPa. Compressibility is 1.7 × 10⁻⁷ cm²/kg and the longitudinal wave velocity of sound is about 18000 m/s. Young's modulus is the maximum among all currently synthesised materials. Moreover, the coefficient of kinetic friction is only 0.05, as low as that of tetrafluoroethylene.
Thermal properties
Diamond has the highest thermal conductivity, reaching 20 W/cm·K at 300 K. At room temperature the coefficient of thermal expansion is 0.8 × 10⁻⁸/K and the thermal conductivity is 5 times that of copper. The melting point and boiling point are 3500 °C and 4200 °C respectively.
Electrical properties
The Hall hole mobility of natural diamond is 1800 cm²/V·s and the electron mobility is 2000 cm²/V·s. The drift velocity of hole and electron carriers saturates when the electric field intensity reaches 10⁴ V/cm; the saturation velocities of holes and electrons are 10⁷ cm/s and 2.0 × 10⁷ cm/s respectively. The resistivity is 10¹⁵ Ω·cm. The hole mobility of synthesised diamond can reach 1400 cm²/V·s. Synthesised diamond films in particular have negative electron affinity (NEA) and show excellent field emission performance, which is affected by dopant, film surface roughness, measurement scale and the like. According to studies, nano diamond enhances field emission performance.
Chemical properties
Diamond does not react with ordinary acids as temperature rises; it is also stable in hot chromic acid or a mixture of sulfuric and nitric acid. However, at high temperature diamond is easily oxidised and reacts with some metals such as tungsten, titanium and zirconium to form carbides. It also decomposes after reacting with iron, cobalt, nickel, manganese and so on.