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Synthetic diamond

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Synthetic diamonds of various colors grown by the high-pressure high-temperature technique

Synthetic diamond (also known as HPHT diamond or CVD diamond depending on the production process used) is diamond produced in a technological process, as opposed to natural diamond, which is produced by geological processes.

Numerous individual attempts to grow synthetic diamond were documented between 1879 and 1928, but none of them have been confirmed to have produced diamonds. In the 1940–1950s, systematic research began in the United States, Sweden and the Soviet Union to grow diamond using chemical vapor deposition (CVD) and high-pressure high-temperature (HPHT) processes. The first reproducible synthesis was reported around 1953. Those two processes still dominate production of synthetic diamond. A third method, known as detonation synthesis, has recently entered the diamond market. In this process, nanometer-sized diamond grains are created in an explosion of carbon-containing explosives. A fourth diamond synthesis variety, using high-power ultrasonic treatment of graphite, has been demonstrated in the laboratory, but has no commercial use yet.

The properties of synthetic diamond depend on the details of the manufacturing processes, and can be inferior or superior to those of natural diamond; in particular, the hardness, thermal conductivity and electron mobility are superior in some synthetic diamonds (either HPHT or CVD). Consequently, synthetic diamond is widely used in abrasives, cutting and polishing tools and in heat sinks. Its applications as a material of active electronic devices, including high-power switches at power stations, high-frequency field-effect transistors and light-emitting diodes are being developed. Detectors of ultraviolet (UV) light or high-energy particles, made of synthetic diamond, are already used at high-energy research facilities and are available commercially. Because of its unique combination of thermal and chemical stability, low thermal expansion and high optical transparency in a wide spectral range, synthetic diamond is becoming the most popular material for optical windows in high-power CO2 lasers and gyrotrons.

Both CVD and HPHT diamonds can be cut into gems of various colors: clear white, yellow, brown, blue, green and orange. The appearance of synthetic gems on the market created major concerns in the diamond trading business, as a result of which special spectroscopic devices and techniques have been developed to distinguish synthetic and natural diamonds.

History

Experimental set-up of Willard Hershey

After the 1797 discovery that diamond was pure carbon, many attempts were made to convert various cheap forms of carbon into diamond—generally with little success. The earliest successes reported in the field were by James Ballantyne Hannay in 1879[1] and by Ferdinand Frédéric Henri Moissan in 1893. Their method involved heating charcoal at up to 3500 °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.[2] 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 1890s.[3]

Many other scientists tried to replicate his experiments. Sir William Crookes claimed success in 1909. Otto Ruff claimed in 1917 to have reproduced diamonds up to 7 mm in diameter,[4] but later retracted his claims.[5] In 1926, Dr. Willard Hershey of McPherson College replicated Moissan's and Ruff's experiments, producing a synthetic diamond. That diamond is on display today at the McPherson Museum in Kansas.[6] Despite the claims of Moissan, Ruff, and Hershey, no other experimenter could reproduce their synthesis.[7][8]

The most definitive replication attempts were performed by Sir Charles Algernon Parsons. A prominent scientist and engineer, known for his invention of the steam turbine, he spent 30 years (1882–1922) and a considerable part of his fortune to reproduce the experiments of Moissan and Hannay but also adapted processes of his own. Parsons was known for his painstakingly accurate approach and methodical record keeping; all his resulting samples were preserved for further analysis by an independent party.[9] He wrote a number of articles—some of the earliest on HPHT diamond—in which he claimed to have produced small diamonds.[10] However in 1928 he authorized Dr. C.H. Desch to publish an article[11] in which he stated his belief that no synthetic diamonds (including those of Moissan and others) had been produced up to that date. He suggested that most diamonds that had been produced up to that point were likely synthetic spinel.[7]

GE diamond project

In 1941, an agreement was made between three companies—General Electric (GE), Norton and Carborundum—to further develop diamond synthesis. They were able to heat carbon to about 3000 °C under a pressure of 3.5 gigapascals (GPa) for a few seconds. The Second World War ended this project soon thereafter. It was resumed in 1951 at the Schenectady Laboratories of GE, and a high-pressure diamond group was formed with F.P. Bundy and H.M. Strong. Tracy Hall and others joined it shortly thereafter.[12]

The group improved on the anvils designed by Percy Bridgman, who received a Nobel prize for his work in 1946. Bundy and Strong made the first improvements and more were made later by Hall. The GE team used tungsten carbide anvils 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. The team recorded diamond synthesis on one occasion, but the experiment could not be reproduced because of uncertain synthesis conditions.[12]

A belt press produced in the 1980s by KOBELCO

Hall achieved the first commercially successful synthesis of diamond on December 16, 1954 (announced on February 15, 1955). His breakthrough was using a "belt" press, which was capable of producing pressures above 18 GPa and temperatures above 2400 °C. The "belt" press utilized a pyrophyllite container where graphite was dissolved within molten nickel, cobalt or iron, a "solvent-catalyst".[13] The largest diamond he produced was 0.15 mm across; it was too small and visually imperfect for jewelry, but usable in industrial abrasives. Hall's co-workers were able to replicate his work, and the discovery was published in the major journal Nature. He was the first person to grow a synthetic diamond with a reproducible, verifiable and well-documented process. He left GE in 1955, and three years later developed a new apparatus for the synthesis of diamond—a tetrahedral press with four anvils—to avoid infringing his previous patent, which was still assigned to GE.[9] Hall received a gold medal from the American Chemical Society in 1972 for his work in diamond synthesis.[12][14]

Later developments

Another successful diamond synthesis was achieved on February 16, 1953 in Stockholm, Sweden by the ASEA (Allmänna Svenska Elektriska Aktiebolaget), one of Sweden's major electrical manufacturing companies. Starting in 1949, ASEA employed a team of five scientists and engineers as part of a top-secret diamond-making project code-named QUINTUS. The team used a bulky split-sphere apparatus designed by Baltzar von Platen and Anders Kämpe.[12][15] Pressure was maintained within the device at an estimated 8.4 GPa for an hour. A few small diamonds were produced, but not of gem quality or size. The work was not reported until the 1980s.[9] During the 1980s a new competitor emerged in Korea, a company named Iljin Diamond; it was followed by hundreds of Chinese enterprises. Iljin Diamond allegedly accomplished diamond synthesis in 1988 by misappropriating trade secrets from GE via a Korean former GE employee.[16]

A scalpel with synthetic diamond blade

Synthetic gem-quality diamond crystals were first produced in 1970 (reported in 1971) by GE. The first successes used a pyrophyllite tube seeded at each end with thin pieces of diamond. The graphite feed material was placed in the center and the metal solvent (nickel) between the graphite and the seeds. The container was heated and the pressure was raised to about 5.5 GPa. The crystals grow as they flow from the center to the ends of the tube, and extending the length of the process produces larger crystals. Initially a week-long growth process produced gem-quality stones of around 5 mm (1 carat (200 mg)), and the process conditions had to be as stable as possible. The graphite feed was soon replaced by diamond grit because that allowed much better control of the shape of the final crystal.[9]

The first gem-quality stones were always yellow to brown in color due to contamination with nitrogen. Inclusions were common, especially "plate-like" ones from the nickel. Removing all nitrogen from the process by adding aluminum or titanium produced colorless "white" stones, and removing the nitrogen and adding boron produced blue ones.[17] However, removing nitrogen slowed the growth process and reduced the crystalline quality, so the process was normally run with nitrogen present. Although the GE stones and natural diamonds were chemically identical, their physical properties were not the same. The colorless stones produced strong fluorescence and phosphorescence under short-wavelength ultraviolet light, but were inert under long-wave UV. Among natural diamonds, only the rarer blue gems exhibit these properties. Contrary to natural diamonds, all the GE stones showed strong yellow fluorescence under X-rays.[9] The De Beers Diamond Research Laboratory has grown stones of up to 25 carats (5.0 g) for research purposes. Stable HPHT conditions were kept for 6 weeks to grow high-quality diamonds of this size. For economic reasons, the growth of most synthetic diamonds is terminated when they reach a weight of 1 carat (200 mg) to 1.5 carats (300 mg).[18]

In the 1950s, research started in the Soviet Union and the US on the growth of diamond by pyrolysis of hydrocarbon gases at the relatively low temperature of 800 °C. This low-pressure process is known as chemical vapor deposition (CVD). William G. Eversole reportedly achieved vapor deposition of diamond over diamond substrate in 1953, but it was not reported until 1962.[19] Diamond film deposition was independently reproduced by Angus and coworkers in 1968[20] and by Deryagin and Fedoseev in 1970.[21] Whereas Eversole and Angus used large, expensive, single-crystal diamonds as substrates, Deryagin and Fedoseev succeeded in making diamond films on non-diamond materials (silicon and metals), which led to massive research on inexpensive diamond coatings in the 1980s.[9]

Manufacturing technologies

There are several methods used to produce synthetic diamond. The original method uses high pressure and high temperature and is still widely used because of its relatively low cost. It uses large presses that can weigh a couple of hundred tons to produce a pressure of 5 GPa at 1500 °C. The second method, using chemical vapor deposition, creates a carbon plasma over a substrate onto which the carbon atoms deposit to form diamond. Other methods are explosive formation (forming detonation nanodiamonds) and sonication of graphite solutions.[22][23][24]

High pressure, high temperature

Schematic of a belt press

In the HPHT method, there are three main press designs used to supply the pressure and temperature necessary to produce synthetic diamond: the belt press, the cubic press and the split-sphere (BARS) press.

The original GE invention by Tracy Hall uses the belt press, wherein the 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.[25] Belt presses are still used today by the major manufacturers at a much larger scale than the original designs.[9]

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.[26] 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. 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 difficult to manufacture.[27]

File:Bars1.jpg
Schematic of a BARS system

BARS is the smallest, most efficient and economical of all the diamond-producing setups. In the center of a BARS device, there is a ceramic cylindrical reaction cell of about 2 cm3 in size. The cell is placed into a cube of pressure-transmitting material, which is pressed by elements made from cemented carbide (VK10 hard alloy).[28] The outer octahedral cavity is pressed by 8 steel sectors. After mounting, the whole assembly is locked in a disc-type barrel with a diameter about 1 meter. The barrel is filled with oil, which pressurizes upon heating, and the oil pressure is transferred to the central cell. The central cell is heated up by a coaxial graphite heater. Temperature is measured with a thermocouple.[29]

Chemical vapor deposition

Main article: Chemical vapor deposition of diamond

Chemical vapor deposition of diamond is a method of growing diamond from a hydrocarbon gas mixture. This method 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.

CVD diamond growth typically occurs under low pressure (1–27kPa) and involves feeding varying amounts of gases into a chamber, energizing them and providing conditions for diamond growth on the substrate. The gases always include a carbon source, and typically include hydrogen as well, though the amounts used vary greatly depending on the type of diamond being grown. Energy sources include hot filament, microwave power, and arc discharges, among others. The energy source is intended to generate plasma in which the gas molecules are broken down to chemically active radicals. The actual chemical process for diamond growth is still under study and is complicated by the very wide variety of growth processes used. The advantages of CVD diamond growth include the ability to grow diamond over large areas, the ability to grow diamond on various substrates, and the fine control over the chemical impurities and thus properties of the diamond produced.[22]

Explosive detonation

Detonation nanodiamond, TEM image
Main article: Detonation nanodiamond

Diamond nanocrystals (5 nm in diameter), called detonation nanodiamond, can be formed by detonation of certain carbon-containing explosives. Detonation nanodiamond is only now beginning to reach the market in bulk quantities, for polishing applications, principally from Russia and China.[23]

Ultrasound cavitation

Diamond nanocrystals can be synthesized from a suspension of graphite in organic liquid at atmospheric pressure and room temperature using ultrasonic cavitation. The yield is about 10%. Cost of nanodiamond produced by this method is estimated to be competitive with that for the HPHT process.[24][30] This technique is relatively simple, but it has been reported by two research groups only, and has no industrial use as of 2009.

Properties

Traditionally, purity and crystallinity are considered the basic diamond properties. Purity and high crystalline perfection make diamond transparent and clear, whereas its hardness, optical dispersion (luster) and chemical stability (combined with marketing), make it a popular gemstone. High thermal conductivity is also important for technical applications. Whereas high optical dispersion is an intrinsic property of all diamonds, other properties vary, and are expanded below.

Crystallinity

Diamond can be one single, continuous crystal or it can be made up of many smaller crystals (polycrystalline). Large single-crystal diamond is typically used in gemstones, whereas polycrystalline diamond goes to 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 nanometers to hundreds of micrometers, usually referred to as "nanocrystalline" and "microcrystalline" diamond, respectively.[9]

Hardness

A diamond's hardness strongly varies depending on its impurities and crystallinity. Nanocrystalline diamond produced through CVD diamond growth, for instance, can have hardness from 30% to 75% to that of single crystal diamond, and the hardness can be tuned for specific applications. Some single-crystal diamonds grown through HPHT synthesis or CVD, and HPHT nanocrystalline diamonds (see hyperdiamond) are harder than any known natural diamond.[31][32][33]

Impurities and inclusions

Main article: crystallographic defects in diamond

No crystal is absolutely pure. Any atom other than carbon found in a diamond is an impurity, and those atoms can aggregate into macroscopic phases called inclusions. While impurities can be unwanted, they can also be introduced on purpose to control the properties of the diamond. For instance, pure diamond is an electrical insulator, but diamond with boron added is an electrical conductor and even superconductor,[34] allowing it to be used in new technological applications. Nitrogen impurities hinder movement of lattice dislocations (defects within the crystal structure) and put the lattice under compressive stress, increasing hardness and toughness.[35]

Thermal conductivity

Unlike most electrical insulators, diamond is a good conductor of heat because of the strong covalent bonding within the crystal. The thermal conductivity of pure diamond is already the highest among any known solid. However, its value is reduced by small amount (natural abundance 1.1 %) of 13C isotope, which ruins the ideality of the 12C diamond lattice in terms of thermal conduction. Therefore, monocrystalline synthetic diamond enriched in 12C isotope (99.9%) has the highest of all thermal conductivity, 30 W/cm·K at room temperature, five times more than copper.[36]

Applications

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

Machining and cutting tools

Diamonds in an angle grinder blade

Diamond has long been used in machine tools, especially when processing non-ferrous alloys, as carbon dissolves in iron at the high temperatures created by high-speed machining. While natural diamond is still applied for this, the usage of synthetic diamond is far greater. The most common form of diamond in cutting tools is micrometer-sized grains dispersed in a metal matrix (usually cobalt), sintered onto the tool. This is typically referred to in industry as poly-crystalline diamond (PCD). PCD tipped tools can be found in mining and in the automotive aluminum cutting industry.[37]

For the past fifteen years, work has been done to coat metallic tools with CVD diamond, and though the work still shows promise it has not significantly displaced traditional PCD tools.[38]

Heat sinks

Because of superior thermal conductivity and dielectric properties of diamond, it is commercially sold for heat sink applications.[39] Those heat sinks are used in high-power semiconductor lasers;[40] in semiconductor technology, they prevents silicon and other semiconducting materials from overheating.[41]

Diamond's thermal conductivity is made use of by jewelers and gemologists who may employ an electronic thermal probe to separate diamonds from their imitations. These probes consist of a pair of battery-powered thermistors mounted in a fine copper tip. One thermistor functions as a heating device while the other measures the temperature of the copper tip: if the stone being tested is a diamond, it will conduct the tip's thermal energy rapidly enough to produce a measurable temperature drop. This test takes about 2–3 seconds.[8]

Windows

Diamond is hard, chemically inert, it has a high thermal conductivity and low thermal expansion, in contrast to many window materials used for the infrared and microwave parts of the spectrum. Therefore, diamond is starting to replace zinc selenide and other materials as the output window of high-power CO2 lasers[42] and gyrotrons. Those windows should have shapes of disks having large diameter (about 10 cm for gyrotrons) and small thickness (to reduce absorption). In these shape requirements, CVD diamond outperforms any other diamond varieties.[43][44]

Electronics

Diamond has potential uses as a semiconductor,[45] because it can be doped with impurities like boron and phosphorus. Since these elements contain one more or one less valence electron than carbon, they turn diamond into p-type or n-type semiconductor. Making a p-n junction by sequential doping of diamond with boron and phosphorus produces UV light emitting diodes (LEDs, at 235 nm).[46] Another useful property of diamond for electronics is high carrier mobility, which reaches 4500 cm2/(V·s) in single-crystal CVD diamond.[47] High mobility is favorable for high-frequency field-effect transitors. The wide band gap of diamond (5.5 eV) leads to the excellent dielectric properties. Combined with high mechnical stability of diamond, those properties are being used in prototype high-power switches for power stations.[48]

Conductive CVD diamond is a useful electrode under many circumstances.[49] Photochemical methods have been developed for covalently linking DNA to the surface of polycrystalline diamond films produced through CVD.[50] In addition, 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 mechanically and chemically stable, it can be used as an electrode under conditions that would destroy traditional materials. For such reasons, waste water treatment of organic effluents using diamond,[51] as well as production of strong oxidants, have been reported.[52]

Diamond transistors are functional to much higher temperatures than silicon devices and are resistant to chemical and radiation 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.[53]

Diamond shows great promise as a potential radiation detection device. It is radiation hard and has a wide bandgap of 5.5 eV (at room temperature). Therefore, it is employed in applications such as the BaBar detector at the Stanford Linear Accelerator[54] or BOLD (Blind to the Optical Light Detectors for VUV solar observations).[55][56] Pure single-crystal synthetic diamonds are approaching the very high purity and crystallographic structure perfection required to replace silicon in devices such as synchrotrons used in CT scanning;[47][57] they will be able to sustain the increased intensities of next generation light sources.[58]

CVD diamond growth has been used in conjunction with lithographic techniques to encase microcircuits inside diamond. The CVD process is also used 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.[59]

Gemstones

Colorless gem cut from diamond grown by chemical vapor deposition

Diamonds for use as gemstones are grown by HPHT[18] or CVD[60] methods. They are available in yellow or blue colors, and to a lesser extent colorless (or white). The yellow color comes from nitrogen impurities in the manufacturing process while the blue color comes from boron.[17] Other colors such as pink or green are achievable after synthesis using irradiation.[61] Several companies also offer memorial diamonds grown using cremated remains. Gem-quality diamond is currently produced by the following producers listed in the table below.

Company Method Color
Apollo Diamond CVD colorless
Chatham HPHT yellow, blue
D.NEA yellow, blue, white
Gemesis yellow

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 nitrogen, nickel or other metals in HPHT or CVD diamonds.[9]

The mined diamond industry is evaluating marketing and distribution countermeasures to these less expensive alternatives. The three largest distributors of natural diamonds have made public statements about selling their diamonds with full disclosure and have implemented measures to laser-inscribe serial numbers on their gemstones.[60]

See also

Wikimedia Commons has media related to Synthetic diamond.

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Further reading

  • R. M. Hazen (1992). The New Alchemists. Random House, New York: Times Books. ISBN 0812922751. {{cite book}}: |format= requires |url= (help); Unknown parameter |subtitle= ignored (help)
  • Laboratory-grown Diamonds (Second ed.). Vancouver, Canada: Gemology Headquarters International (GHI). 2007. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |subtitle= ignored (help)
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