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How are Synthetic Diamonds Made

Synthetic diamonds are a reality now. The processes developed to produce these are a marvel of advanced research in chemistry.
Omkar Phatak Feb 17, 2019
Making synthetic diamonds has been the goal of technologists all over the world, since ages. To create something so unsurpassed in beauty and perfection as a diamond, was a very challenging task.
The reason people thought it was possible, was the fact that a diamond is essentially a huge pure carbon crystal, with some impurities. It is a beautiful array of covalently-bonded carbon atoms, arranged in a particularly unique manner.
In solid state physics jargon, the structure of diamond is a modified face centered cubic (FCC), with a unique stacking of layers of carbon atoms. This arrangement makes a diamond, the toughest known solid, the best thermal conductor, and gives it transparency, as well as optical dispersion properties.
Beauty of a thing essentially lies in its simplicity and elegance. Also, as Leonardo da Vinci has said, simplicity is also the ultimate sophistication. Therefore, achieving this simple arrangement of carbon atoms to create artificial diamonds, requires the ultimate sophistication in technology.
A natural one forms in the fiery chasm, in the belly of the Earth, 190 km to 140 km below the planet's surface, where pressures are of the order of billions of pascals and temperature is in excess of 3000° Celsius.
For novices, 1 Pascal is a pressure of 1 N/m2. Normal atmospheric pressure is around 100 thousand pascals. So, one has to achieve a pressure which is ten thousand times the atmospheric pressure and a temperature that is of the order of 3000° Celsius, to create the simple arrangement of carbon atoms, that is a diamond.
The reason that diamond structure takes so much energy to form, might be that a lot of stable-structured carbon compounds, formed at lower temperatures, need to be broken and overcome before diamond can form.
It takes more energy to create and maintain order, than disorder. It is a process of large and subtle energy changes that must precisely place every atom in its place in the crystal, with their specific bonding.
A crystal grows from a seed. A seed is a small crystal which acts as the template, from which the whole crystal takes form, as a cyclic repetition of the seed structure. It's a slow and gradual process which takes a lot of time. This property is exploited in the forging of synthetic diamonds.
Many scientists around the world, funded by private industrialists, worked round the clock to recreate the high pressure and high temperature environment of natural diamond formation, that could forge artificial or synthetic diamonds.
Finally, in 1954, the first successful and verified production of a synthetic diamond was achieved by Tracy Hall's group at the General Electric company, by using the belt press method. It is classified as a 'High temperature and high pressure' (HTHP) method now. After the initial success, many modifications and other alternative processes were invented.
They are mainly classified into two categories broadly. One is the 'High pressure, High temperature' type, second is the 'Chemical vapor deposition' type, and lastly, a recently developed method of 'Explosive detonation'. Let us take a brief look at these methods of synthesis.

High Temperature, High Pressure Method

This method in principle, is compressing or squeezing a carbon sample with heavy anvils and heating it at the same time. The sample is placed in a soft solid shell of materials like pyrophyllite, that acts as the pressure-transmitting medium.
The sample may be melted graphite, mixed with other metals like iron, nickel, or cobalt which work as catalysts. The three methods in this category are different in the way they squeeze the carbon sample. Different geometrical patterns were experimented with.
The first one used by Hall was a pair of two heavy anvils placed on top of each other, with the sample in between and compressed together by a belt arrangement. The anvils are also electrodes, through which high voltage current is passed to heat the sample to the required temperature. This is called the belt press method.
The other two are variations of the belt process with even more anvils. The first is the cubic press which, as the name signifies is a cubic arrangement of anvils with one anvil pressing down on each of the six faces of the cube. It can deliver more pressure to a greater surface area.
Overall, it achieves more pressure than the belt press. These machines have achieved a pressure in excess of 18 billion Pascals or 18 GPa and a temperature in excess of 2400oCelsius.
The third method, developed by Russians, is the sleekest of the three. It is called 'BARS', which is a short form for the phrase 'Split Sphere'. As the name suggests, the machine consists of a huge metallic sphere split into eight pieces, each working as an anvil pressing down on a square compressible box, containing the carbon raw material to be compressed.
The whole sphere is inserted into another concentric sphere filled with oil and heated to achieve high temperatures. Synthetic diamonds up to 25 karat or 5 gm in weight, have been produced by these methods.

Chemical Vapor Deposition

This technology uses an entirely different principle for creating synthetic diamonds. It operates at very low pressures, compared to other processes. The process consists of heating a 1:99 ratio mixture of hydrocarbons, with hydrogen gas in an enclosure, which also contains the preheated substrate.
A heating device is also placed inside for ionizing the gas mixture. The carbon ions get deposited on the preheated, diamond powder scrubbed target, forming a diamond coating. The hydrogen acts as a scavenging agent, bonding with all the non-diamond carbon ions.
The advantage of this process is the large surface area which can be coated at a time and it's inexpensive in comparison. It cannot be used for commercial production, but serves well in research purpose synthesis. However, this method has a lot of impurity problems.

Detonation Synthesis

In this method, specially manufactured hydrocarbon explosives are detonated inside an enclosure. This creates a phenomenal amount of internal pressure and heat, which creates very small, nanometer-sized diamond crystals, that can be used in many industrial applications.
Synthetic diamonds are mostly used in industrial applications and they are slowly making inroads into the jewelry markets. These mass production techniques have made diamonds commercially available for electronic and machine tool applications. The once alchemist-like dream of creating diamonds is now a reality.