The crystal structure of diamonds is unique. Each carbon atom in a diamond has a strong chemical bond with four others. This renders diamonds the hardest natural material. The other properties of diamonds are high thermal conductivity, resistance to chemicals and exceptionally high transparency.
Natural diamonds were formed at great depths below the earth’s surface, typically below 120 km. These are environments of extreme pressure of the order of 45 thousand atmospheres and intense heat with temperatures of between 900°C and 1300°C. Under these conditions, over millions of years, carbon crystallises to form natural diamonds. They emerge into the surface transported by magma carried by volcanic eruptions. A particular kind of magma cools and forms kimberlite rocks which are the most significant sources of diamonds.
Diamonds can be made in the laboratory following two main processes. The High-Pressure High-Temperature (HPHT) process recreates ome conditions of the natural process. Man-made diamonds have been commercialised by HPHT techniques since the early 1990s. HPHT is a very cumbersome process and it has inherent impurities from catalysts. Producing large-size defect-free diamonds, in an economical time frame is not possible by HPHT. In 1956, General Electric brought out commercial HPHT diamonds for the first time. HPHT simulates conditions of natural diamond formation with special catalyst materials. This process produces billions of carats of diamonds annually, mostly for industrial applications.
In 1954, before the HPHT process was developed, the chemical vapour deposition (CVD) process was patented. However, in the late 1980s, scientists began to understand the process of reproducibly growing diamonds using the CVD process.
The CVD process operates in sub-atmospheric pressure. It starts with diamond fragments or seeds placed on a silicon substrate which are exposed to a plasma of methane gas made by powerful microwave radiation. Methane (CH4 ) dissociates into methyl (CH3) and Hydrogen. The carbon-based radicals attach themselves to the diamond seed to crystallise into layers of diamond on the diamond seed much like the way salt crystals can be grown in a salt solution. The temperature is in the 1000ºC range. The high Hydrogen concentration drives the surface chemical reactions toward the diamond formation. Nascent hydrogen helps in the formation of the diamond phase over the graphite form of carbon by preferential etching of non-diamond deposits.
Homoepitaxial growth of diamond under microwave-assisted plasma CVD with the addition of nitrogen in the methane- hydrogen precursor gas mixture at ppm level could result in single crystal diamonds. High power high-density plasma could produce SCD at a high growth rate even at high methane percentages. In such plasmas, uniform heating of the diamond substrate is realized and the H/CH3 ratio is enhanced, both of which have an impact on the size of the crystal grown.
However, the exact mechanism of CVD diamond growth is still not clear because of the lack of probing techniques to extract in-situ information about the diamond growth environment. Optical emission spectroscopy and mass spectroscopy techniques are used to probe the plasma. Modelling and numerical simulations are employed to understand microwave CVD growth. Researchers at the University of Bristol, LIMHP, France, Naval Research Laboratory, USA etc. have done pioneering work in understanding the diamond growth mechanism.
CVD growth rates were not high enough until recently for making gem-sized diamonds. However, recent developments of high quality and high growth rate microwave plasma SCD technologies have raised immense interest in the jewellery industry. Many lab-grown diamond companies are starting up, like US-based Apollo Diamond (now known as SCIO diamond), Gemesis Corp., Washington Diamonds, Microwave Enterprises, Singapore-based IIa Technologies and many others all over the world.
An alternative to the microwave plasma process is the laser-plasma CVD. In the laser-plasma CVD process, mirrors concentrate a CO2 laser beam into a reaction chamber where it interacts with CH4 and H2 present in an inert gas plasma. Gas mass flow controllers ensure optimal gas composition pressure controllers regulate the chamber pressures. Laser-plasma CVD diamond manufacturing is achievable at ambient pressures whereas microwave plasma CVD happens in sub-atmospheric pressure of the order of 100 torr requiring evacuation of the chanber.
Laser-plasma CVD scores over microwave plasma CVD due to its high diamond deposition rates: 100 μm/hr. This, though lower than theoretically predicted rates, exceeds plasma-enhanced CVD growth rates. The higher deposition rate of the laser process is because the high laser power density in laser-plasma CVD creates more ionization. Operating at higher pressure also helps enhance the plasma density. Achieving the theoretical values of high diamond growth rates for laser-plasma CVD is a research frontier. It is being pursued because of the potential of the process to reap high profitability.
Impurities can affect the growth rate, quality, and colour of the synthetic diamond. A small amount of oxygen inhibits crack formation, thus maintaining single-crystalline growth over polycrystalline types. Trace amounts of Nitrogen in the feed gases can enhance the growth rate by a factor of eight. However, nitrogen incorporated into the CVD synthetic diamond creates a yellowish or light brown colour, necessitating further treatment.
Chemical vapour deposition (CVD) growth technology has made substantial progress. The largest CVD gem diamond the Gemological Institute of America (GIA) has considered is an emerald-cut diamond weighing 34.59 ct, measuring 24.94 × 13.95 × 9.39 mm, produced by Ethereal Green Diamond in India [1].
There are several diamond groups world-over, that are designing and manufacturing reactors for efficient and fast production of diamonds by CVD techniques. They are exploring new recipes and efficient plasma formation for rapid SCD growth. So the research effort was put into making colorless purer SCD with high-pressure high-power density microwave plasma CVD. It was found that using very high methane percentages at high pressure would rapidly grow SCD and high microwave power density ensured the dissociation of precursor radicals effectively.
Man-made diamonds are formed of Carbon atoms arranged in a unique variation of the face-centred cubic crystal structure designated the diamond lattice. They possess the same optical and chemical characteristics of natural diamonds and can be their alternative. Spectroscopic measurements prove that their purity is comparable to or even superior to that of the natural diamond.