Diamonds and diamond industrial applications

A mineral with such extraordinary characteristics needs extremely high temperatures and pressures to appear naturally in the Earth's lithospheric mantle. It forms at depths between 140 and 300 km, at pressures between 45 and 60 kilo bars (up to 60000 times the atmospheric pressure at sea level) and temperatures of 900-1300 degrees Celsius.

That makes it a rather rare material, with only about 26 to 30 tons of diamonds mined annually (mainly in central and southern Africa, Russia, Canada, India, Brazil and Australia). Although diamond is mainly known and valued as being the most precious gemstone, actually less than 20% of the total extracted diamonds are pure enough and have the necessary optical qualities (like clarity and color) to be used in jewelry. The rest is destined for industrial use. And it is far from being enough: annually, about 115 tons of industrial diamonds are synthetically produced for industrial applications (a small part of them being also used as gemstones, because of their fancy colors).

Synthetic diamonds are produced using two main techniques: High-Pressure High-Temperature synthesis (HPHT) and Chemical Vapor Deposition (CVD). The first one uses huge presses (weighing hundreds of tons) to produce pressures and temperatures similar to those needed for naturally occurring diamonds. The second one creates carbon plasma over a substrate, which produces carbon atoms deposits which eventually form diamond.

The former method is used for manufacturing large synthetic diamonds, like those which can be used as gemstones. The latter is used mainly for coatings, but can also produce crystals of 2-3 millimeters in diameter. There are two other techniques which begin to gain attention: one of them is by detonating carbon-containing explosives in a metal chamber, which forms diamond nanocrystals (with a diameter of 5 nanometers) used for polishing applications; the other is using graphite, subjecting it to ultrasonic waves; this technique is for the moment purely experimental and it remains to be seen if it can have any industrial use.

The industrial usage of diamonds is mainly based on their hardness. They are mainly used to polish and cut any other material (including other diamonds). Drill-bits and saws have diamond tips, and diamond powder makes an excellent abrasive material. There are also diamond-core drill bits (which have small diamonds embedded in metal segments on the steel shell ) used mainly for masonry. Special cutting tools and drill-bits are made from polycrystalline diamond.

These are made from a layer of about 0.5 mm of diamond particles on a tungsten carbide support, and are used in automotive and aerospace industry, to drill and cut very abrasive materials, like aluminum alloys and carbon fiber reinforced plastics. Polycrystalline diamond tools are also made using micrometer-sized diamond grains which are dispersed in a cobalt matrix and sintered on the cutting tool for different purposes. Diamond powder is used to polish or cut tile and stone, generating large amounts of heat.

For home use, special scalpels with synthetic diamond blades are manufactured for cutting many materials, like paper and plastic. One thing which is worth mentioning is that diamond is not suitable for cutting and machining ferrous alloys, because carbon is soluble in iron at high temperatures, so diamond tools are wearing very rapidly compared to other materials.

Other diamond applications are: high pressure experiments (like diamond anvil cell, which can create pressures exceeding 3 million atmospheres), high performance bearings and special windows. Lately, high-tech applications started to take advantage of diamond properties.

Diamonds are used as heat-sinks for high-power semiconductor lasers, laser arrays and high-power transistors. They start to replace zinc selenide for manufacturing output windows for high power CO2 lasers, gyrotrons and synchrotrons. Synthetic diamonds are also used as radiation detectors in laboratories and as special electrodes for biomolecular research. Experimental research proved the potential of using diamonds for manufacturing high-performance semiconductors and transistors for micro-chips. Light emitting diodes (LEDs) made from synthetic diamond doped with boron and phosphorus produce UV light at a wavelength of 235 nanometers.