Buy Online: The Properties of CVD Diamond
Optical Properties
| Property | Polycrystalline CVD diamond | Single crystal CVD diamond |
| Refractive Index | 2.432 for L=0.5 um | 2.432 for L=0.5um |
| dn/dt | 9.6 x 10-6 K -1 | 9.6 x 10-6 K -1 |
| Absorption Coefficient 8-12 um |
<0.07 | |
| Absorption Coefficient 3-5 um |
Min 0.8 at 3.7 um | |
| Emissivity at 10 um | 0.20 at 573 K 0.03 at 773K |
|
| Integrated forward scatter 8-12 um | 0.1 - 0.7% | <0.6% |
| Integrated for scatter visible | <4% | <0.6% |
| Transmission 8-200 um (1 mm thick) |
71.4% | 71.4% |
Transmission 633 nm |
>64% | >66% |
Note the maximum transmission of 71.2% results from reflection at the two diamond/air interfaces because of the high refractive index of diamond.
Thermal Properties
| PROPERTY | Polycrystalline CVD diamond | Single crystal CVD diamond |
| Thermal Conductivity | 2000* | >2000 |
| @ 20 Deg C (W/m.K) | ||
| Thermal Conductivity | 500-1500 | >1000 |
| @ 200 Deg C (W/m.K) | ||
| Thermal Diffusivity (cm2 /s) | 2.8 - 11.6 | 10 - 12 |
| Thermal Expansion Coefficient | 1.21 | 1.21 |
| 100 Deg C at 250 Deg C (ppm/K) | 3.84 | 3.84 |
| 500 Deg C (ppm/K) | 4.45 | 4.45 |
| 750 Deg C (ppm/K) | ||
| Thermal Shock Figure of merit W/m | >1100 |
*Optical Grade CVD
Mechanical Properties
The high strength of the carbon to carbon bond is the source of the exceptional mechanical properties of diamond. Dislocation of the atoms is difficult and consequently diamond is the hardest known substance.
On any measurement scale, diamond is the hardest known material. Mohs hardness is a scratch hardness test for solids related to indentation hardness. The relationship between the Mohs number, M, and the indentation hardness, H, measured in kg/mm2 is
Log H = 0.2M + 1.5
While there is reasonable "interval equality" between the first 9 integers on the Mohs scale, the interval between 9 (silicon carbide, aluminium oxide, tungsten carbide) and 10 (diamond) represents a much larger difference in indentation hardness than a single unit on this scale would suggest.
Indentation hardness (Knoop scale).
For crystalline solids, the Knoop indenter is considered to be most accurate. Indentation hardness varies according to crystallographic direction. For the {001} diamond surface the value is 570 -10,400 kg mm-2 and with normal loads of 500g, 1 kg and 2 kg. For diamond ({111} surface <110> direction, 500g load), the Knoop hardness is 9000 kg mm-2 ; for cubic boron nitride under the same conditions and orientation the figure is 4500 kg mm-2 which is the next hardest material to diamond.
In addition, the mechanical properties of CVD diamond will vary according to the grade of the material and its application area.
Summary of Mechanical Properties for CVD diamond
| PROPERTY | POLYCRYSTALLINE CVD DIAMOND | SINGLE CRYSTAL CVD DIAMOND |
| HARDNESS (GPa) | 85-100 | 70-100 |
| Fracture Toughness (MPa) | 5.5 | 3.4 |
| Tensile Strength* (MPa) | 400 800 (Growth) (Nucleation) | 2000 - 3000 |
| Compressive strength | 9 | 9 |
| Rain Impact DTV 2 mm drop size | 525 ms-1 | |
| Sand erosion at 80 ms-1 C25/52 sand | 0.18mgkg-1 |
* depends on grade of polycrystalline diamond
Electronic properties
Diamond has immense potential in electronics. Intrinsic diamond has a combination of high mobility, breakdown and thermal conductivity which results in the largest Johnson and Keyes figures of merit obtained for any material. These figures combine a material's electrical and thermal properties to indicated their relative capacity to handle high power or high speed device operation.
| PROPERTY | Polycrystalline CVD diamond | Single crystal CVD diamond |
| Electronic grade | Electronic grade | |
| Hole mobility (cm2 /Vs) | 1,000 | 3,800 |
| Electron mobility (cm2 /Vs) | 1,800 | 4,500 |
| Carrier lifetime (ns) | ~ 1-10 | ~ 2,000 |
| Voltage breakdown (MV/cm) | ~ 0.5 | ~ 4 |
| Charge collection distance um | ~250 at 1 V/um field | CCD is thickness limited* |
| Bandgap (eV) | 5.47 | 5.47 |
| Electron saturation velocity (cm.s-1 )X10 7 | 2 | 2 |
| Johnson's figure of merit | 8,200 | |
| Keyes' figure of merit | 32 | |
| Baligas figure of merit | 17,200 |
*In available sizes, eg 500um thick
Diamond doping
The most common dopants used in CVD diamond, with their associated ionization energies, are:-
| Element | Activiation Energy (eV) | |
| n-type | Nitrogen | 1.7 |
| Phosphorous | 0.60 | |
| p-type | Boron | 0.37 |
Electrochemistry
Boron Doped Single Crystal Diamond
Boron doped diamond is unique as an electrochemical material for a number of reasons:
The material is very chemically inert and is capable of withstanding very aggressive chemical environments.
Related to its chemical inertness, it can operate over a very wide potential window without parasitic surface reactions either breaking down the solvent or causing local surface reactions with the solute. As such, in electrochemical sensing applications it enables the solute to be fully and accurately characterised, and in processing applications it enables efficient electrochemical processing of the solute, without expensive parasitic reactions, such as the breakdown of water in aqueous solutions.
The surface of diamond is not prone to fouling, enabling long reliable operation without maintenance, and in circumstances where fouling does occur it can be easily removed either mechanically or chemically without damaging the electrode.
Properties of Boron Doped Single Crystal Diamond
Wide Potential Window in Aqueous Solution
Oxygen Reduction Significantly Retarded
Low Background Currents
Corrosion Stability in aggressive media and at high temperatures and pressures
Uniform Electrical Conductivity
Note:- The electrochemical performance of boron doped diamond is dependant on the surface finish.
Chemical Properties
Diamond is chemically inert and (due to the high strength of the covalent bonds) is highly resistant to chemical attack by acids or other chemical reagents. The only exceptions are materials that at high temperatures act as oxidising agents - these provide the only effective way to attack diamond at normal pressures and temperatures (below about 1000 Deg C). Salts, such as sodium nitrate, are known to attack diamond when in the molten state at temperatures as low as 450 Deg C. In the presence of oxygen, diamond starts to be oxidised at around 650 Deg C.
The only other possible form of chemical attack is by two groups of metals. The members of the first group are strong carbide formers, and include tungsten, tantalum, titanium and zirconium. At high temperatures, these will react chemically with diamond to form their respective carbides. The second group of metals includes iron, cobalt, manganese, nickel and chromium (and also the platinum group of metals). In the molten state these metals are true solvents for carbon.