Understanding

Six non-faceted diamond crystals of 2–3 mm (0.079–0.118 in) size; they are yellow, green-yellow, green-blue, light-blue, light-blue and dark blue. — Lab-grown diamonds of various colors grown by the high-pressure-and-temperature technique

A synthetic diamond or laboratory-grown diamond (LGD), also called a lab-grown diamond,^[1] laboratory-created, man-made, artisan-created, artificial, synthetic, or cultured diamond, is a diamond that is produced in a controlled technological process (in contrast to naturally formed diamond, which is created through geological processes and obtained by mining). Unlike diamond simulants (imitations of diamond made of superficially similar non-diamond materials), synthetic diamonds are composed of the same material as naturally formed diamonds—pure carbon crystallized in an isotropic 3D form—and have identical chemical and physical properties. As of 2023^[update] the heaviest synthetic diamond ever made weighs 30.18 ct (6.0 g);^[2] the heaviest natural diamond ever found weighs 3167 ct (633.4 g).

Numerous claims of diamond synthesis were reported between 1879 and 1928 after English chemist Smithson Tennant demonstrated that diamonds are a form of carbon in 1797;^[3] most of these attempts were carefully analyzed but none was confirmed. In the 1940s, systematic research of diamond creation began in the United States, Sweden and the Soviet Union, which culminated in the first reproducible synthesis in 1953. Further research activity yielded the discoveries of the high pressure high temperature (HPHT) and chemical vapor deposition (CVD) methods of diamond production. These two processes still dominate synthetic diamond production. A third method in which nanometer-sized diamond grains are created in a detonation of carbon-containing explosives, known as detonation synthesis, entered the market in the late 1990s. A fourth method, treating graphite with high-power ultrasound, has been demonstrated in the laboratory, but as of 2008 had no commercial application.

Synthetic diamonds, which have a different shade due to the different content of nitrogen impurities. Yellow diamonds are obtained with a higher nitrogen content in the carbon lattice, and colourless diamonds come only from pure carbon. The smallest yellow diamond size is around 0.3 mm

The properties of synthetic diamonds depend on the manufacturing process. Some have properties such as hardness, thermal conductivity and electron mobility that are superior to those of most naturally formed diamonds. Synthetic diamond is widely used in abrasives, in cutting and polishing tools and in heat sinks. Electronic applications of synthetic diamond are being developed, including high-power switches at power stations, high-frequency field-effect transistors and light-emitting diodes. Synthetic diamond detectors of ultraviolet (UV) light and of high-energy particles are used at high-energy research facilities and are available commercially. Due to 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 CO
₂ lasers and gyrotrons. It is estimated that 98% of industrial-grade diamond demand is supplied with synthetic diamonds.^[4]

Both CVD and HPHT diamonds can be cut into gems, and various colors can be produced: clear white, yellow, brown, blue, green and orange. The advent 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 from natural diamonds.

History

In the early stages of diamond synthesis, the founding figure of modern chemistry, Antoine Lavoisier, played a significant role. His groundbreaking discovery that a diamond's crystal lattice is similar to carbon's crystal structure paved the way for initial attempts to produce diamonds.^[5] After it was discovered that diamond was pure carbon in 1797,^[6]^[7] many attempts were made to convert various cheap forms of carbon into diamond.^[8]^[a] The earliest successes were reported by James Ballantyne Hannay in 1879^[13] and by Ferdinand Frédéric Henri Moissan in 1893. Their method involved heating charcoal at up to 3,500 °C (6,330 °F) with iron inside a carbon crucible in a furnace. Whereas Hannay used a flame-heated tube, Moissan applied his newly developed electric arc furnace, in which an electric arc was struck between carbon rods inside blocks of lime.^[14] 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.^[8]^[15]

Many other scientists tried to replicate his experiments. Sir William Crookes claimed success in 1909.^[16] Otto Ruff claimed in 1917 to have produced diamonds up to 7 mm (0.28 in) in diameter,^[17] but later retracted his statement.^[18] In 1926, Dr. J. Willard Hershey of McPherson College replicated Moissan's and Ruff's experiments,^[19]^[20] producing a synthetic diamond.^[21] Despite the claims of Moissan, Ruff, and Hershey, other experimenters were unable to reproduce their synthesis.^[22]^[23]

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 about 40 years (1882–1922) and a considerable part of his fortune trying to reproduce the experiments of Moissan and Hannay, but also adapted processes of his own.^[24] 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.^[25] He wrote a number of articles—some of the earliest on HPHT diamond—in which he claimed to have produced small diamonds.^[26] However, in 1928, he authorized Dr. C. H. Desch to publish an article^[27] 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.^[22]

ASEA

The first known (but initially not reported) diamond synthesis was achieved on February 16, 1953, in Stockholm by ASEA (Allmänna Svenska Elektriska Aktiebolaget), Sweden's major electrical equipment manufacturing company. Starting in 1942, 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.^[28]^[29] Pressure was maintained within the device at an estimated 8.4 GPa (1,220,000 psi) and a temperature of 2,400 °C (4,350 °F) for an hour. A few small diamonds were produced, but not of gem quality or size.

Due to questions on the patent process and the reasonable belief that no other serious diamond synthesis research occurred globally, the board of ASEA opted against publicity and patent applications. Thus the announcement of the ASEA results occurred shortly after the GE press conference of February 15, 1955.^[30]

GE diamond project

A 3-meter tall press — A belt press produced in the 1980s by KOBELCO

In 1941, an agreement was made between the General Electric (GE), Norton and Carborundum companies to further develop diamond synthesis. They were able to heat carbon to about 3,000 °C (5,430 °F) under a pressure of 3.5 gigapascals (510,000 psi) for a few seconds. Soon thereafter, the Second World War interrupted the project. It was resumed in 1951 at the Schenectady Laboratories of GE, and a high-pressure diamond group was formed with Francis P. Bundy and H. M. Strong. Tracy Hall and others joined the project later.^[28]

The Schenectady group improved on the anvils designed by Percy Bridgman, who received a Nobel Prize in Physics for his work in 1946. Bundy and Strong made the first improvements, then more were made 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 into a gasket. The team recorded diamond synthesis on one occasion, but the experiment could not be reproduced because of uncertain synthesis conditions,^[31] and the diamond was later shown to have been a natural diamond used as a seed.^[32]

Hall achieved the first commercially successful synthesis of diamond on December 16, 1954, and this was announced on February 15, 1955. His breakthrough came when he used a press with a hardened steel toroidal "belt" strained to its elastic limit wrapped around the sample, producing pressures above 10 GPa (1,500,000 psi) and temperatures above 2,000 °C (3,630 °F).^[33] The press used a pyrophyllite container in which graphite was dissolved within molten nickel, cobalt or iron. Those metals acted as a "solvent-catalyst", which both dissolved carbon and accelerated its conversion into diamond. The largest diamond he produced was 0.15 mm (0.0059 in) 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.^[34]^[35] 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 violating a U.S. Department of Commerce secrecy order on the GE patent applications.^[32]^[36]

Further development

Synthetic gem-quality diamond crystals were first produced in 1970 by GE, then reported in 1971. 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 (800,000 psi). 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 (0.20 in) (1 carat or 0.2 g), 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.^[35]^[37]

The first gem-quality stones were always yellow to brown in color because of 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.^[38] Removing nitrogen also 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. Unlike natural diamonds, all the GE stones showed strong yellow fluorescence under X-rays.^[39] 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 six weeks to grow high-quality diamonds of this size. For economic reasons, the growth of most synthetic diamonds is terminated when they reach a mass of 1 carat (200 mg) to 1.5 carats (300 mg).^[40]

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 (1,470 °F). 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.^[41]^[42] Diamond film deposition was independently reproduced by Angus and coworkers in 1968^[43] and by Deryagin and Fedoseev in 1970.^[44]^[45] 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.^[46]

From 2013, reports emerged of a rise in undisclosed synthetic melee diamonds (small round diamonds typically used to frame a central diamond or embellish a band)^[47] being found in set jewelry and within diamond parcels sold in the trade.^[48] Due to the relatively low cost of diamond melee, as well as relative lack of universal knowledge for identifying large quantities of melee efficiently,^[49] not all dealers have made an effort to test diamond melee to correctly identify whether it is of natural or synthetic origin. However, international laboratories are now beginning to tackle the issue head-on, with significant improvements in synthetic melee identification being made.^[50]

Manufacturing technologies

There are several methods used to produce synthetic diamonds. The original method uses high pressure and high temperature (HPHT) and is still widely used because of its relatively low cost. The process involves large presses that can weigh hundreds of tons to produce a pressure of 5 GPa (730,000 psi) at 1,500 °C (2,730 °F). The second method, using chemical vapor deposition (CVD), creates a carbon plasma over a substrate onto which the carbon atoms deposit to form diamond. Other methods include explosive formation (forming detonation nanodiamonds) and sonication of graphite solutions.^[51]^[52]^[53]

High pressure, high temperature

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. Diamond seeds are placed at the bottom of the press. The internal part of the press is heated above 1,400 °C (2,550 °F) and melts the solvent metal. The molten metal dissolves the high purity carbon source, which is then transported to the small diamond seeds and precipitates, forming a large synthetic diamond.^[54]

The original GE invention by Tracy Hall uses the belt press wherein the upper and lower anvils supply the pressure load to a cylindrical inner cell. This internal pressure is confined radially by a belt of pre-stressed steel bands. The anvils also serve as electrodes providing electric current to the compressed cell. A variation of the belt press uses hydraulic pressure, rather than steel belts, to confine the internal pressure.^[54] Belt presses are still used today, but they are built on a much larger scale than those of the original design.^[55]

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.^[56] The first multi-anvil press design was a tetrahedral press, using four anvils to converge upon a tetrahedron-shaped volume.^[57] The cubic press was created shortly thereafter to increase the volume to which pressure could be applied. A cubic press is typically smaller than a belt press and can more rapidly achieve the pressure and temperature necessary to create synthetic diamond. However, cubic presses cannot be easily scaled up to larger volumes: the pressurized volume can be increased by using larger anvils, but this also increases the amount of force needed on the anvils to achieve the same pressure. An alternative is to decrease the surface area to volume ratio of the pressurized volume, by using more anvils to converge upon a higher-order platonic solid, such as a dodecahedron. However, such a press would be complex and difficult to manufacture.^[56]

A schematic drawing of a vertical cross-section through a BARS press: the synthesis capsule is surrounded by four tungsten carbide inner anvils. Those inner anvils are compressed by four outer steel anvils. The outer anvils are held a disk barrel and are immersed in oil. A rubber diaphragm is placed between the disk barrel and the outer anvils to prevent oil from leaking. — Schematic of a BARS system

The BARS apparatus is claimed to be the most compact, efficient, and economical of all the diamond-producing presses. In the center of a BARS device, there is a ceramic cylindrical "synthesis capsule" of about 2 cm³ (0.12 cu in) in size. The cell is placed into a cube of pressure-transmitting material, such as pyrophyllite ceramics, which is pressed by inner anvils made from cemented carbide (e.g., tungsten carbide or VK10 hard alloy).^[58] The outer octahedral cavity is pressed by 8 steel outer anvils. After mounting, the whole assembly is locked in a disc-type barrel with a diameter about 1 m (3 ft 3 in). The barrel is filled with oil, which pressurizes upon heating, and the oil pressure is transferred to the central cell. The synthesis capsule is heated up by a coaxial graphite heater, and the temperature is measured with a thermocouple.^[59]

Chemical vapor deposition

Chemical vapor deposition is a method by which diamond can be grown from a hydrocarbon gas mixture. Since the early 1980s, this method has been the subject of intensive worldwide research. Whereas the mass production of high-quality diamond crystals make the HPHT process the more suitable choice for industrial applications, the flexibility and simplicity of CVD setups explain the popularity of CVD growth in laboratory research. The advantages of CVD diamond growth include the ability to grow diamond over large areas and on various substrates, and the fine control over the chemical impurities and thus properties of the diamond produced. Unlike HPHT, CVD process does not require high pressures, as the growth typically occurs at pressures under 27 kPa (3.9 psi).^[51]^[60]

The CVD growth involves substrate preparation, feeding varying amounts of gases into a chamber and energizing them. The substrate preparation includes choosing an appropriate material and its crystallographic orientation; cleaning it, often with a diamond powder to abrade a non-diamond substrate; and optimizing the substrate temperature (about 800 °C (1,470 °F)) during the growth through a series of test runs. Moreover, optimizing the gas mixture composition and flow rates is paramount to ensure uniform and high-quality diamond growth. The gases always include a carbon source, typically methane, and hydrogen with a typical ratio of 1:99. Hydrogen is essential because it selectively etches off non-diamond carbon. The gases are ionized into chemically active radicals in the growth chamber using microwave power, a hot filament, an arc discharge, a welding torch, a laser, an electron beam, or other means.

During the growth, the chamber materials are etched off by the plasma and can incorporate into the growing diamond. In particular, CVD diamond is often contaminated by silicon originating from the silica windows of the growth chamber or from the silicon substrate.^[61] Therefore, silica windows are either avoided or moved away from the substrate. Boron-containing species in the chamber, even at very low trace levels, also make it unsuitable for the growth of pure diamond.^[51]^[60]^[62]