Synthetic diamond

Synthetic diamond (also known variously as lab-created, manufactured, lab-grown or cultured diamond) is a term used to describe diamond crystals produced by a technological process, as opposed to natural diamond, which is produced by geological processes.

Synthetic diamond is not the same as diamond-like carbon, DLC, which is amorphous hard carbon, or diamond simulants, which are made of other materials such as cubic zirconia or silicon carbide. The properties of synthetic diamond depend on the manufacturing process used to produce it, and can be inferior, similar or superior to those of natural diamond.^[1]

Because it can be made for less than it costs to mine natural diamond, synthetic diamond is used in many industrial applications. Reduced costs and the ability to engineer its physical and electrical properties give synthetic diamond the potential to become a disruptive technology in many areas such as electronics and medicine.

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History

The idea of making less expensive, gem-quality diamonds synthetically is not a new one. H. G. Wells described the concept in his short story "The Diamond Maker," published in 1911 ^[2]. In Capital Karl Marx commented, "If we could succeed, at a small expenditure of labour, in converting carbon into diamonds, their value might fall below that of bricks".^[3]

Ever since the discovery that diamond was pure carbon in 1797 many attempts were made to alter the cheaper forms of carbon - generally with little success. One of the early successes reported in the field was by Ferdinand Frédéric Henri Moissan in 1893. His method involved heating charcoal at up to 4000 °C with iron in a carbon crucible in an electric furnace, in which an electric arc was struck between carbon rods inside blocks of lime. The molten iron was then rapidly cooled by immersion in water. The contraction generated by the cooling supposedly produced the high pressure required to transform graphite into diamond. Moissan published his work in a series of articles in the 1890's.

Many other scientists tried to replicate his experiments. Sir William Crookes claimed success in 1909. Ruff claimed in 1917 to have reproduced diamonds up to 7 mm in diameter, but later retracted his claims. ^[4] In 1926, Dr. Willard Hershey of McPherson College read journal articles about Moissan's and Ruff's experiments and replicated their work, producing a synthetic diamond. That diamond is on display today in Kansas at the McPherson Museum. ^[5] Despite the claims of Moissan, Ruff, and Hershey, many other experimenters had enormous difficulty in creating the required temperatures and pressure with similar equipment, leading some to contend that the early successes were the result of seeding by good-willed co-workers. ^[6]

The most definitive duplication attempts ^[4] were performed by Sir Charles Algernon Parsons. He devoted 30 years and a considerable part of his fortune to reproduce many of the experiments of Moissan as well as those of Hannay but also adapted processes of his own. He wrote a number of articles -- one of the earliest on HPHT diamonds -- in which he claimed to have produced small diamonds. ^[7]. However in 1928 he authorized C.H Desch to publish an article in which he stated his belief that no synthetic diamonds (including those of Moisan and others) had been produced up to that date. In fact he found that most diamonds produced so far were more likely than not synthetic Spinel. ^[4]

The GE diamond project

The first person who grew a synthetic diamond according to a reproducable, verifiable and witnessed process was Howard Tracy Hall while working for General Electric in 1954. He received a gold medal of the American Chemical Society in 1972 for his work.^[4] In 1941 an agreement was made between General Electric, Norton and Carborundum to further develop diamond synthesis. However this project soon thereafter ended because of the Second World War. They were able to heat Carbon to about 3000 °C (5432 °F) under a pressure of half a million psi, for a few seconds.^[4]

In 1951 the project was resumed at the Schenectady Laboratories of GE and a high pressure diamond group was formed with F.P. Bundy, H.M. Strong, and shortly afterwards joined by H. T. Hall and others. Following on the work done by Percy Bridgman (who received a Nobel prize for his work in 1946) Bridgman's Anvils were further improved first by Bundy and Strong and later by Hall. The GE team used a tungsten carbide "anvil" within a hydraulic press to squeeze the carbonaceous sample held in a catlinite container, the finished grit being squeezed out of the container through a gasket. It was believed that on occasion a diamond was produced, but since experiments could not be reproduced, such claims could not be maintained. ^[4]

Finally Tracy Hall managed the first commercially successful synthesis of diamond on December 16, 1954 (announced on February 15, 1955). Hall's breakthrough was using an elegant "belt" press apparatus which raised the achievable pressure from 6 to 18 GPa and the temperature to 5000 °C, using a pyrophyllite container, and having the graphite dissolved within molten nickel, cobalt or iron, a "solvent-catalyst". Hall was able to have co-workers replicate his work and the discovery was published in Nature. The largest diamond produced by Hall was 150 micrometres across, clearly unsuitable for ornamentation but very useful in industrial abrasives.^[4]

Later developments

Another successful diamond synthesis was produced on February 16, 1953 in Stockholm, Sweden by the QUINTUS project of ASEA (Allemanna Svenska Elektriska Aktiebolaget), Sweden's major electrical manufacturing company using a bulky split sphere apparatus designed by Baltzar von Platen and the young engineer Anders Kämpe (1928–1984). Pressure was maintained within the device at an estimated 83,000 atmospheres (8.4 GPa) for an hour. A few small crystals were produced, but not of gem quality or size. The work was not reported until the 1980s.

During the 1980s a new competitor emerged in Korea named Iljin Diamond, followed later by hundreds of Chinese entrants. Iljin Diamond allegedly accomplished this by misappropriating trade secrets from GE via a Korean former GE employee in 1988.^[8]

Synthetic gem-quality diamond crystals were first produced in 1970 (reported in 1971) again by GE. Hall had continued to work for GE, developing the tetrahedral press with four anvils. Large crystals need to grow very slowly under extremely tightly controlled conditions. The first successes used a pyrophyllite tube seeded at each end with thin pieces of diamond and with the graphite feed material placed in the centre, the metal solvent, nickel, was placed between the graphite and the seeds. The container was heated and the pressure raised to around 55,000 atmospheres. The crystals grow as they flow from the centre to the ends of the tube, the longer the process is extended the larger the crystals - initially a week-long growth process produced gem-quality stones of around 5 mm and one carat. The graphite feed was soon replaced by diamond grit, as there was almost no change in material volume so the process was easier to control.

The first gem-quality stones were predominantly cubic and octahedral in form and, due to contamination with nitrogen, always yellow to brown in color. Inclusions were common, especially "plate-like" ones from the nickel. Removing all nitrogen from the process by adding aluminium or titantium produced a colourless 'white' stone, while removing the nitrogen and adding boron produced a blue. However removing nitrogen slows the growth process and impairs the crystals properties, so most stones are still yellow. In terms of physical properties the GE stones were not quite identical to natural stones. The colourless stones were semi-conductors and fluoresed and phosphoresed strongly under SWUV but were inert under LWUV - in nature only blue stones should do this. All the GE stones also showed a strong yellow fluorescence under X-rays. De Beers Diamond Research Laboratory has since grown stones of up to 11 carats, but most stones are around 1 to 1.5 carats for economic reasons, especially with the spread of the Russian BARS apparatus since the 1980s.

Following on from work by John Angus and Boris Spitsyn researchers at the National Institute for Research in Inorganic Materials in Tsukuba produced diamonds at less than one atmosphere of pressure and only 800 °C through Chemical Vapour Deposition (CVD). The Japanese had begun their research in 1974 and reported their success in 1981.

Properties of synthetic diamond

The gem diamond is just one of many different forms that diamond can take. Natural gem diamond is a single crystal diamond with low levels of impurities. This homogeneity is what allows it to be clear, while its material properties and hardness are what make it a popular gemstone. Most natural diamond removed from the earth's crust does not have the high purity or high crystallinity necessary to be a quality gemstone. Following are some important properties by which various types of diamond are described.

Crystallinity

A mass of diamond may be one single, continuous crystal or it may be made of up many smaller crystals ("polycrystalline"). Single crystal diamond is typically used in gemstones, while polycrystalline diamond is commonly used in industrial applications such as mining and cutting tools. Within polycrystalline diamond the diamond is often described by the average size of the crystals that make it up, called the "grain size." Grain sizes range from hundreds of micrometers to nanometers, usually referred to as "microcrystalline" and "nanocrystalline" diamond, respectively.

Hardness

A diamond's hardness can vary depending on its impurities and crystallinity. Nanocrystalline diamond produced through CVD diamond growth, for instance, can have a wide range of hardness from 30% to 75% of single crystal diamond, and the hardness can be controlled to be used in specific applications. Some single crystal diamonds grown through chemical vapor deposition have been shown to be harder than any known natural diamond.

Impurities and inclusions

No crystal is absolutely pure. Any substance other than carbon found in a diamond is an impurity, and may also be called an inclusion, due to the way these impurities fall in the crystal lattice. While inclusions can be unwanted, they can also be introduced on purpose to control the properties of the diamond. For instance, while pure diamond is an electrical insulator, diamond with small amounts of boron added is an electrical conductor, possibly allowing it to be used in new technological applications.

Gem-quality diamonds grown in a lab can be chemically, physically and optically identical to naturally occurring ones although they can be distinguished by spectroscopy in infrared, ultraviolet, or X-ray wavelengths. The DiamondView tester from De Beers uses UV fluorescence to detect trace impurities of nickel or other metals in HPHT diamonds, or hydrogen in some LP CVD diamonds.

Manufacturing technologies

There are two main methods to produce synthetic diamond. The original method is High Pressure High Temperature (HPHT) and is still the most widely used method because of its relative low cost. It uses large presses that can weigh a couple of hundred tons to produce a pressure of 5 GPa at 1,500 degrees Celsius to reproduce the conditions that create natural diamond inside the Earth. The second method, using chemical vapor deposition or CVD, was invented in the 1980s, and is basically a method creating a carbon plasma on top of a substrate onto which the carbon atoms deposit to form diamond.

High pressure, high temperature

The GE method is called HPHT (High Pressure, High Temperature). There are two main press designs used to supply the pressure and temperature necessary to produce synthetic diamond. These basic designs are the belt press and the cubic press. There are a number of other designs, but only belt press and cubic press are used for industrial scale manufacturing.

The original GE invention by H. Tracy Hall, uses the belt press, wherein upper and lower anvils supply the pressure load and heating current to a cylindrical volume. This internal pressure is confined radially by a belt of pre-stressed steel bands. A variation of the belt press uses hydraulic pressure to confine the internal pressure, rather than steel belts. Belt presses are still used today by the major manufacturers at a much larger scale than the original designs.

The second type of press design is the cubic press. A cubic press has six anvils which provide pressure simultaneously onto all faces of a cube-shaped volume. The first multi-anvil press design was actually a tetrahedral press, using only four anvils to converge upon a tetrahedron-shaped volume. The cubic press was created shortly thereafter to increase the pressurized volume. A cubic press is typically smaller than a belt press and can achieve the pressure and temperature necessary to create synthetic diamond faster. However, cubic presses cannot be easily scaled up to larger volumes. To illustrate, one could increase the pressurized volume by either increasing the size of the anvils, thereby increasing by a great factor the amount of force needed on the anvils to achieve a similar pressurization, or by decreasing the surface area to volume ratio of the pressurized volume by using more anvils to converge upon a different platonic solid (such as a dodecahedron), but such a press would be unnecessarily complex and not easily manufacturable.

Chemical vapor deposition

Main article: Chemical vapor deposition of diamond

Chemical vapor deposition of diamond is a method of growing diamond by creating the environment and circumstances necessary for carbon atoms in a gas to settle on a diamond substrate in diamond crystalline form. This method of diamond growth has been the subject of a great deal of research since the early 1980s, especially due to its potential applications in the cutting tool, semiconductor and diamond gem industries.

In their pioneering work in the area, the Japanese passed a mixture of carbon-containing gas (methane in their case) and hydrogen into a quartz tube at a pressure of 0.05 atmospheres. Using microwaves the mixture was heated to 800 °C, disassociating both the methane and hydrogen into elemental forms. The carbon is deposited on a substrate, the majority as graphite but a very small proportion as diamond crystal. the graphite is 'removed' by the hydrogen leaving a thin layer of diamond, initially the layer was around 25μm in thickness.

Applications

Given the extraordinary set of physical properties diamond exhibits, diamond has and could have a wide-ranging impact in many fields.

Machining

Diamonds have long been used in machine tools, especially when machining non-ferrous alloys. While natural diamond is certainly still used for this, the amount of synthetic diamond is far greater. The most common usage of diamond in cutting tools is done by distributing micrometer-sized diamond grains in a metal matrix (usually cobalt), hardening it and then sintering it onto the tool. This is typically referred to in industry as “PCD” diamond. PCD tipped tools are often used in mining and in the automotive aluminium cutting industry.

For the past fifteen years work has also been done in the hope of using CVD diamond growth to coat tools with diamond,^[9] and though the work still shows promise it has not significantly displaced traditional PCD tools.

Electronics

CVD diamond also has applications in electronics. Conductive diamond is a useful electrode under many circumstances.^[10] University of Wisconsin-Madison chemistry professor Robert Hamers developed photochemical methods for covalently linking DNA to the surface of polycrystalline diamond films produced through CVD. In addition, the diamonds can detect redox reactions that cannot ordinarily be studied and in some cases degrade redox-reactive organic contaminants in water supplies. Because diamond is almost completely chemically inert it can be used as an electrode under conditions that would destroy traditional materials. For such reasons waste water treatment of organic effluents,^[11] as well as production of strong oxidants, have been published.^[12] A number of companies produce diamond electrodes.

Diamond shows great promise as a potential radiation detection device. Diamond has a similar density to that of soft tissue, is radiation hard and has a wide bandgap. It is employed in applications such as the BABAR detector at Stanford.^[13]

Diamond also has potential uses as a semiconductor.^[14] This is because the diamonds can be "doped" with impurities like boron and phosphorus. Since these elements contain one more or one less valence electron than carbon, they turn the diamonds into p-type or n-type semiconductors. Diamond transistors are functional to temperatures many times that of silicon and are resistant to chemical and radioactive damage. While no diamond transistors have yet been successfully integrated into commercial electronics, they show promise for use in exceptionally high power situations and hostile environments.

CVD diamond growth has also been used in conjunction with lithographic techniques to encase microcircuits inside diamond. Researchers at Lawrence Livermore National Laboratory and the University of Alabama, Birmingham use this process to create designer diamond anvils as a novel probe for measuring electric and magnetic properties of materials at ultra high pressures using a Diamond Anvil Cell.^[15]

HPHT "type IIa" diamonds are, as of 2007, approaching the very high purity and crystallographic structure perfection required to replace silicon in applications like X-ray tomographic imaging at synchrotrons;^[16] they will be able to sustain the increased intensities of next generation light sources.^[17]

Gemstones

Several companies currently produce gems made through HPHT technology. They are grown in split sphere high-pressure, high-temperature (HPHT) crystal growth chambers that resemble washing machines. The device bathes a tiny sliver of natural diamond in molten carbon at 1500 °C and 58,000 atm (5.9 GPa). This produces a rough diamond which can be cut down to a polished size close to half its original carat weight. Gemesis diamonds have an orange tint that is rare in natural diamonds. The yellow tint occurs when approximately five out of each 100,000 carbon atoms in the diamond crystal lattice are replaced with nitrogen atoms. Adia Diamonds produces diamonds in various shades of yellow and orange as well as blue and white (colorless). The blue color comes from doping the diamond with boron, rather than nitrogen, during the growth process. White diamonds must be grown in an environment free of nitrogen and boron, which makes them very difficult to produce. Yellow diamonds are more profitable because they can be made more quickly and cost less to manufacture than blue or colorless diamonds. The largest synthetic diamond crystal grown to date via this method was a 34-carat yellow stone. Apollo Diamond is a company that currently produces gem diamond through chemical vapor deposition and sells clear diamond gemstones.

The mined diamond industry is evaluating marketing and distribution countermeasures to these less expensive alternatives. The three largest distributors have made public statements about selling their diamonds with full disclosure and have implemented measures to laser-inscribe serial numbers on their gemstones.^[18]

LifeGem is a company offering to synthesize diamonds from the carbonized remains of people or pets.

See also

• Diamond simulant
• Moissanite

Notes

Further reading

• Hazen, Robert M (1992). The New Alchemists (hardcover), Random House, New York: Times Books, 286 pages. ISBN 0812922751. 

• Davis, J. (2003). The New Diamond Age Wired Magazine, Issue 11.09 (about CVD diamond growth)
• Yarnell, A. (2004). The Many Facets of Man-Made Diamonds. Chemical and Engineering News 82 (5), 26-31.
• Hall, H. T. (1961). The Synthesis of Diamond Journal of Chemical Education, 38, 484
• Carnegie Institution's Geophysical Laboratory
• Putting the Squeeze on Materials.