Properties and application

Diamond is a solid, the cubic modification of carbon and a precious mineral. Its crystal structure is formed by a three-dimensional framework of tetrahedrons in which each carbon atom is surrounded by four neighbors They are  covalently bonded. All neighbors have the same distance to the central carbon atom (see Fig. 1).
 

Pure diamond is colorless. By impurities and defects in the crystal lattice the diamond may become brown, yellow, green, orange, blue, pink, red and gray to black (see Fig. 1).  Due to its high refractive index and noble gloss, it is used as gem in jewelry in a variety of cut shapes.
Diamond is the hardest material known. It’s the only material that achieves the Mohs hardness number 10, the highest value on this hardness scale. The Greek name "Adamas" means the "unconquerable", since it can not be scratched by any substance but can scratch all other known substances. His abrasion resistance is 140 times larger than that of corundum. Therefore, it is used for making drilling, cutting and grinding tools and as an additive in polishing pastes.  The hardness is dependent on the crystal orientation of the diamond (anisotropy). This makes it possible to grind diamond with diamond. Its high modulus of elasticity, thermal conductivity and electrical resistance are also of importance. Small amounts of boron, phosphorus or nitrogen may transform diamond from an insulating to a conductive material whereby a semiconductor or a superconductor may be formed. Diamond electrodes are used in waste water treatment for oxidation and disinfection processes.  Due to its good thermal conductivity (1000-2500 W / (m * K)) it is used to cool thermally highly loaded electronic devices by dissipating heat. Diamond films made by chemical vapour deposition (CVD) serve primarily for wear protection of tools.

Nano-diamond has a diameter from a several nanometers up to a few hundred nanometers. Preferentially, particles with a diameter of less than 50 nm are used.  Nano-diamonds play a role as plastic filler and in polishing processes for highest demands, particularly in micro-electronics. In chain oil advertisings they are (surprisingly) said to reduce friction. “Ultra-nanocrystalline diamond” is a brand name of Argonne National Laboratory covering a dispersed powder made by detonation techniques and a chemical vapor deposited film composed of individual crystallites in the range of a few nanometers. Nano-diamond is studied intensively as it is expected to enable new industrial technologies and to reduce raw material costs.

Occurrence and production
 

Natural diamond is formed under high pressure (100-150 kbar) and temperatures (1200 ° C-1500 ° C)  in the mantle of the Earth at about 150 km depth. The starting material of diamond of the natural diamond synthesis is still unclear. Usually, it is assumed to be formed from carbon dioxide (CO₂). Diamond crystallizes very slowly. By volcanic eruptions and lava diamond achieves the surface of Earth. If cooled down quickly, the conversion into graphite may be suppressed. The most important mining areas are located in Russia, Africa and South Africa, Australia and Canada. Diamond nanoparticles have already been shown to exist in outer space, so they should haved reached earth, too. But, terrestrial nanodiamond has not been detected hitherto.
Synthetic diamonds were made for the first time by the physicist Erik Lundblad in 1953 via a high-temperature high-pressure process. The conversion of carbon into diamond occurs at 1200-1400 ° C under a pressure of 5-7 GPa, with the help of metal catalysts. Today artificial diamonds are produced for industrial use at large scale.  In the 1960s Soviet researchers first demonstrated that diamond also can be deposited from the gas phase at atmospheric pressure by chemical vapour deposition. Today, several micrometers thick wear layers are produced at industrial scale. Also centimeter-sized crystals, which are used as optical windows in measuring devices, can be produced in this way. Nanoscale diamonds are obtained by means of shock waves or detonation operation where high temperatures and pressures can be reached for a short time.

 

Literature:

1. Kruger, Anke, nanodiamond in: New Carbon Materials, 2007, Springer, 331-388
   
2. RÖMPP Online, version 3.20, diamonds
   
3. Atlas Minerals: Diamond
   
4. Wikipedia: Diamond