Synthetic diamond

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A colourless synthetic diamond produced via chemical vapour deposition
A colourless synthetic diamond produced via chemical vapour deposition

Synthetic diamond is diamond produced through chemical or physical processes in a factory. Like naturally occurring diamond it is composed of a three-dimensional carbon crystal. Due to its extreme physical properties, synthetic diamond is used in many industrial applications, and has the potential to become a serious disruptive technology in many new application areas such as electronics and medicine. Synthetic diamond is also called manufactured diamond, artificial diamond or cultured diamond. Synthetic diamond is not the same as Diamond-like Carbon, DLC, which is amorphous hard carbon, or diamond imitation, which can be made of other materials such as cubic zirconia or silicon carbide.

Despite being occasionally characterized as 'fake', synthetic diamond is molecularly identical to the carbon allotrope defined as diamond when referring to naturally occurring diamond. As such, it shares the same material properties and is potentially of an even higher quality than its natural counterpart.

Contents

[edit] History

The idea of making cheap, 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 (text from Project Gutenberg). In Capital (Volume 1, 1867), 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".[1]

The first artificial diamonds were synthesized by Henri Moissan in 1893 by heating charcoal at high temperatures 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 iron contracted on rapid cooling, generating the high pressure required to transform graphite into diamond. This experiment was successfully repeated by Ruff in 1917, which resulted in the production of very small diamonds, the largest of which measured 0.7mm. In 1926, Dr. Willard Hersey 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.

Another successful diamond synthesis was produced on February 16, 1953 in Stockholm, Sweden by the QUINTUS project of ASEA, 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. The discovery was kept secret.

The first commercially successful synthesis of diamond was produced on December 16, 1954, by H.Tracy Hall at General Electric, using an elegant "belt" apparatus. Hall was able to have co-workers replicate his work and the discovery was published in Nature.

This gave rise to an industrial diamond industry that was for decades represented by two main players: GE Superabrasives and De Beers Industrial Diamonds. 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 (General Electric v. Sung, 843 F. Supp. 776). In 2003 GE sold GE Superabrasives to a private equity firm called Littlejohn and it was renamed to Diamond Innovations. Littlejohn sold Diamond Innovations to Sandvik in January of 2007. Also, in 2002, De Beers Industrial Diamonds rebranded to Element Six and is operating as an independent company from De Beers. Many more companies have become important players in the industrial diamond market. The main ones are Sumitomo Electric Hardmetal, US Synthetic, Smith Megadiamond and Novatek. Some smaller companies have signaled their intent to enter the market for gems using synthetic diamond. These are Adia Diamonds, Apollo Diamond, Gemesis and Tairus. Today, in the year 2006, the industrial diamond industry is an annual US$1 billion market, producing some 3 billion carats, or 600 metric tons, of synthetic diamond. This should be put in comparison with the 130 million carats (26 metric tons) mined annually for gem purposes.

[edit] 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.

[edit] High Pressure, High Temperature (HPHT)

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 none of them 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.

[edit] Chemical Vapor Deposition (CVD)

Chemical vapor deposition of diamond has received a great deal of attention in the materials sciences because it allows many new applications of diamond that had previously been either too expensive to implement or too difficult to make economical. CVD diamond growth typically occurs under low pressure (1 to 27 Pa) 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 a plasma in which the gases are broken down and more complex chemistries occur. The actual chemical process for diamond growth is still under study and is complicated by the very wide variety of diamond growth processes used.

The advantages to CVD diamond growth include the ability to grow diamond over large areas, the ability to grow diamond on a substrate, and the control over the properties of the diamond produced. In the past, when high pressure high temperature (HPHT) techniques were used to produce diamond, the diamonds were typically very small free standing diamonds of varying sizes. With CVD diamond growth areas of greater than fifteen centimeters (six inches) diameter have been achieved and much larger areas are likely to be successfully coated with diamond in the future. Improving this ability is key to enabling several important applications.

The ability to grow diamond directly on a substrate is important because it allows the addition of many of diamond’s important qualities to other materials. Since diamond has the highest thermal conductivity of any material, layering diamond onto high heat producing electronics (such as optics and transistors) allows the diamond to be used as a heat sink[1],[2]. Diamond films are being grown on valve rings, cutting tools, and other objects that benefit from diamond’s hardness and exceedingly low wear rate. In each case the diamond growth must be carefully done to achieve the necessary adhesion onto the substrate.

The most important attribute of CVD diamond growth is the ability to control the properties of the diamond produced. In the area of diamond growth the word “diamond” is used as a description of any material primarily made up of sp3 bonded carbon, and there are many different types of diamond included in this. By regulating the processing parameters—especially the gases introduced, but also including the pressure the system is operated under, the temperature of the diamond, and the method of generating plasma—many different materials that can be considered diamond can be made. Single crystal diamond can be made containing various dopants[3]. Polycrystalline diamond consisting of grain sizes from several nanometers to several micrometers can be grown[4],[5]. Some polycrystalline diamond grains are surrounded by thin, non-diamond carbon, while others are not. These different factors affect the diamond’s hardness, smoothness, conductivity, optical properties and more.

There are several problems facing CVD diamond growth in the future. First, because research in the area is so heavily application oriented, there are basic questions which have had very little work done on them, and this continues to be a problem for the field. This problem is exacerbated by the fact that small changes in chemistry can require a great deal of research to understand. Another problem is that while CVD diamond growth occurs over large areas compared to other methods of diamond growth, these areas are still too small for some applications, such as large scale transistor manufacturing. There is no better method of producing semiconducting, doped diamond than CVD, but until large scale wafers can be efficiently produced CVD electronics will only have niche applications. CVD diamond growth has had historically low growth rates, usually a few micrometers an hour. While growth rates have been improved dramatically in a few very specific areas, in most applications they are still very slow. The biggest problem with CVD diamond growth is cost; cheaper alternatives are used instead of CVD diamonds whenever possible.

The Carnegie Institute's Geophysical Laboratory can produce 10 carat (2 g) single-crystal diamonds rapidly (28 nm/s) by CVD, as well as colorless single-crystal diamonds. Growth of colorless diamonds up to 60 g (300 carats) is believed achievable using their method.[6]

[edit] Synthetic diamond types

[edit] HPHT diamond grit

Grit are single crystal diamond particles that range from approximately 1 millimeter down to 1 μm. Its suitability for different applications is determined not only by size, but also by shape and metal content. The more crystalline the grit is normally means higher strength, and lower metal content means better thermal properties, both important factors in abrasive applications. Grit is normally yellow in color stemming from nitrogen that is used in the HPHT process. It can also be made blue if boron is used.

[edit] HPHT polycrystalline compact diamond, PCD

PCD are diamond particles sintered together using HPHT technology. The PCD properties can be engineered to a much larger extent than grit and be made into large black compacts that can be manufactured into special shapes or cut into segments from larger pieces for use in abrasive wear applications.

[edit] HPHT large single crystal diamond

These synthetic diamond crystals are much larger than the normal grit and can reach up to 10 mm in length. To produce such large crystals, it is necessary to maintain the HPHT synthesis process for up to a week and sometimes longer, which is technically very difficult.

The applications for this type of synthetic diamond are the same as for PCD, but because of its higher cost and better performance, in the more demanding environments. Some of the larger synthetic diamond crystals are also used in the gem industry as yellow artificial diamond.

[edit] CVD polycrystalline diamond

CVD polycrystalline diamond is grown flat as a wafer up to 5 millimeters thick. The diamond wafer can be up to 30 centimeters in diameter depending on the technology used to manufacture. Recently technologies have been developed to make 3D shapes of CVD polycrystalline diamond. The diamond is usually black but can be made completely transparent.

The applications fields are wide, varying from abrasive to optical to medical to environmental.

[edit] CVD single crystal diamond

The synthesis of single crystal diamond using the CVD process is usually done on a single crystal diamond substrate, but other substrates have been used - for instance, sapphire. Available sizes are typically limited to a few millimeters in length and height but larger gems are also grown.

CVD single crystal diamond is mainly used in abrasive, electronic, sensor and detector applications.

[edit] Applications

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

Diamonds have long been used in machining tools, especially when machining non-ferrous alloys. This is most commonly 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. It is not uncommon to find large PCD diamond drills used in drilling for oil, but the primary use for PCD diamond tools in recent years has been machining aluminum for the automobile industry. The automobile industry uses a number of aluminum alloys that produce extreme wear on tools and diamond is the only cost-efficient way of machining these alloys. For the past fifteen years work has been done on using CVD diamond growth to coat tools with diamond[7], and though the work still shows promise it has not significantly displaced traditional PCD tools.

CVD diamond also has applications in electronics. Conductive diamond has been demonstrated as a useful electrode under many circumstances[8]. For example, University of Wisconsin-Madison chemistry professor Robert Hamers has developed a photochemical methods for covalently linking DNA to the surface of polycrystalline diamond films produced through CVD. Also, the diamonds have been shown to detect redox reactions that can't 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[9] as well as production of strong oxidants have been published[10]. There are already a number of companies producing diamond electrodes.

Diamond has shown 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. These qualities suggest it has potential to be an excellent radiation detection material, and it has already been employed in some applications, such as the BABAR detector at Stanford[11].

Diamond also has potential uses as a semiconductor[12]. 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 n-type or p-type semiconductors. There are also studies being conducted about impregnating boron-doped CVD diamonds with deuterium to produce n-type semiconducting diamonds. 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 incase microcircuits inside diamond. Researchers at Lawrence Livermore National Laboratory and the University of Alabama, Birmingham use this process to create designer diamond anvils[13] as a novel probe for measuring electric and magnetic properties of materials at ultra high pressures using a Diamond Anvil Cell.

[edit] Synthetic gems

Adia Diamonds, Chatham Created Gems, Gemesis and Tairus all 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.

Another company, Boston, Massachusetts-based Apollo Diamond, uses the low-pressure technique of chemical vapor deposition (CVD) to produce larger, more expensive diamonds with greater control over impurities. The diamond produced is a single crystal, as opposed to the polycrystalline patchworks formerly produced by CVD. This greater measure of control allows Apollo Diamond to produce diamonds of various colors, from pink to black. The ability to control the intentional introduction of impurities, doping, is necessary for the creation of diamond semiconductor devices.

The mined diamond industry is evaluating countermeasures to these cheaper alternatives. Gem-quality synthetic diamonds are visually identical to naturally occurring ones, but 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 LP CVD diamonds. Furthermore, all three manufacturers laser-inscribe serial numbers on their gemstones. [14]

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

[edit] See also

  • Mars trilogy, science fiction series featuring a space elevator which uses a cable made of a chain of synthetic diamond crystals, each over 30,000 meters long


[edit] Notes

  1. ^ Costello et al. (1994) Diamond protective coatings for optical components. Diamond and Related Materials 3(8), June, 1137–1141
  2. ^ Woong Sun Lee and Jin Yu. (2005) Comparative study of thermally conductive fillers in underfill for the electronic components. Diamond and Related Materials 14(10), October, 1647–1653.
  3. ^ J. Isberg , J. Hammersberg , D. J. Twitchen and A. J. Whitehead. (2004) Single crystal diamond for electronic applications. Diamond and Related Materials 13(2), February, 320–324.
  4. ^ Stan Vepek. (1998) New development in superhard coatings: the superhard nanocrystalline-amorphous composites. Thin Solid Films 317 (1-2), April 1, 449–454.
  5. ^ Krauss et al. (2001) Ultrananocrystalline diamond thin films for MEMS and moving mechanical assembly devices. Diamond and Related Materials 10(11), November, 1952–1961
  6. ^ Real big diamonds made real fast. Science Blog. Retrieved on September 14, 2005.
  7. ^ Ahmed et al. (2003) Diamond films grown on cemented WC–Co dental burs using an improved CVD method. Diamond and Related Materials 12(8), August, 1300–1306.
  8. ^ M. Panizza and G. Cerisola (2005) Application of diamond electrodes to electrochemical processes. Electrochimica Acta 51(2), October, 191–199.
  9. ^ D. Gandini, E. Mahé, P.A. Michaud, W. Haenni, A. Perret, Ch. Comninellis (2000) Oxidation of carbonylic acids at boron-doped diamond electrodes for wastewater treatment. Journal of Applied Electrochemistry 20;1345.
  10. ^ P.A. Michaud, E. Mahé, W. Haenni, A. Perret, Ch. Comninellis (2000) Preparation of peroxodisulfuric acid using Boron-Doped Diamond thin film electrodes. Electrochemical and Solid-State Letters 3(2), Letters online.
  11. ^ M. Bucciolini (2005) Diamond dosimetry: Outcomes of the CANDIDO and CONRADINFN projects. Nuclear Instruments and Methods in Physics Research A 552, 189–196.
  12. ^ A. Denisenko and E. Kohn (2005) Diamond power devices. Concepts and limits. Diamond and Related Materials 14(3-7), March-July, 491–498.
  13. ^ D.D. Jackson and C. Aracne-Ruddle and V. Malba and S.T. Weir and S.A. Catledge and Y.K. Vohra, Rev. Sci. Instrum., 74, 2467-2471 (2003)
  14. ^ Sources : Wired.com, Chemical and Engineering News: The Many Facets of Man-Made Diamonds).

[edit] External links

[edit] Further reading

  • The New Alchemists: Breaking Through the Barriers of High Pressure, Robert M. Hazen, Times Books, Random House, New York, 1992, hardcover, 286 pages, ISBN 0-8129-2275-1
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