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Diamonds

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From 07/18/2014 through 5/29/2012

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Diamonds: Applications

Diamonds: Nanocomposites

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Notes

3. “Diamond may be considered to be a very unique material. It has relatively high hardness, high thermal conductivity, high electric resistivity, a low coefficient of friction and is substantially inert to attacks from most chemicals. For tribological applications, diamond may be considered an excellent material to inhibit erosion, abrasion and sliding wear.”

[Diamonds, US Patent 8,496,992 (7/30/2013)]

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1. “Monocrystalline diamonds, as found in nature, can be classified according to color, chemical purity and end use. The majority of monocrystalline diamonds are colored, and contain nitrogen as an impurity, and are thereby used primarily for industrial purposes; these would be classified as type Ia and Ib. The majority of gem diamonds (which are all considered "monocrystalline" diamonds) are colorless or various light colors and contain little or no nitrogen impurities; and would be classified as type IIa. Types Ia, Ib and IIa are electrical insulators. A rare form of monocrystalline diamond (classified as type IIb) contains boron as an impurity, is blue in color and is a semiconductor. In nature these characteristics are uncontrolled and therefore the color, impurity level and electrical characteristics are unpredictable and cannot be utilized to produce large volumes of specialized articles in a predictable manner.

Monocrystalline diamond provides a wide and useful range of extreme properties, including hardness, coefficient of thermal expansion, chemical inertness and wear resistance, low friction, and high thermal conductivity. Generally monocrystalline diamond is also electrically insulating and optically transparent from the ultra-violet (UV) to the far infrared (IR), with the only absorption being carbon-carbon bands from about 2.5 .mu.m to 6 .mu.m. Given these properties, monocrystalline diamonds find use in many diverse applications including, as heat spreaders, abrasives, cutting tools, wire dies, optical windows, and as inserts and/or wear-resistant coatings for cutting tools. The engineering and industrial uses of diamonds have been hampered only by the comparative scarcity of natural monocrystalline diamond. Hence there has been a long running quest for routes to synthesize monocrystalline diamond in the laboratory.

Synthetic monocrystalline diamonds, for industrial use, can be produced by a variety of methods, including those relying on a "high pressure method" and those involving controlled vapor deposition (CVD). Diamond produced by either the "high pressure method" or the CVD method can be produced as monocrystalline diamond or polycrystalline diamond. High pressure diamond is usually formed as micron sized crystals, which can be used as grit or loose abrasive, or set into metal or resin for cutting, grinding or other applications.

Both methods, i.e., "high pressure method" and "CVD method" make it possible to control the properties to a high degree and thereby control the properties of color, impurity level and electrical characteristics on a theoretical level. However, on a practical level, in order to manufacture useful objects by the "high pressure method", there are limitations imposed by the presence or absence of impurities. As an example, it has been suggested that the addition of nitrogen might assist in the growth of large crystals, although the elimination of nitrogen, or the addition of boron, can make it more difficult to grow large crystals. In addition, it appears that it is not possible to make monocrystalline structures having layers of varied composition without having to remove the seed crystal from the reactor after each layer is grown, and then replacing the seed crystal in the reactor in order to grow a subsequent layer having a different composition. Moreover, large seeds cannot be accommodated in the "high pressure method". In the CVD method, most work has been confined to production of polycrystalline diamond, as opposed to the growth and control of single crystals.

It is actually difficult and expensive to produce high quality pure monocrystalline diamond by the high pressure method. It has been shown that the addition of boron to a synthetic monocrystalline or polycrystalline diamond makes it useful for constructing a semiconductor device, a strain gauge or other electrical device although monocrystalline diamond is to be preferred. See U.S. Pat. No. 5,635,258. See also, W. Ebert, et al. "Epitaxial Diamond Schottky Barrier Diode With On/Off Current Ratios in excess of 10.sup.7 at High Temperatures", Proceedings of IEDM, pp. 419-422 (1994), Published by IEEE, and S. Sahli, et al., "Piezoelectric Gauge Factor Measured at Different Fields and Temperatures", pp. 95-98, Applications of Diamond Films and Related Materials, A. Feldman, et al. editors, NIST Special Publications 885.

So called `industrial diamond` has been synthesized commercially for over 30 years using high-pressure high-temperature (HPHT) techniques, in which monocrystalline diamond is crystallized from metal solvated carbon at pressures of about 50 to 100 kbar and temperatures of about 1800 to 2300K. In the high pressure method the crystals grow in a three dimensional manner and the crystal is all of one impurity level, except for possible discontinuities arising from fluctuations in the growth cycle. See, for example, R. C. Burns and G. Davis, "Growth of Synthetic Diamond", pp. 396-422, The Properties of Natural and Synthetic Diamond, J. E. Field, editor, Academic Press (1992), U.S. Pat. Nos. 3,850,591 and 4,034,066.

Interest in diamond has been further increased by the much more recent discovery that it is possible to produce polycrystalline diamond films, or coatings, by a wide variety of chemical vapor deposition (CVD) techniques using, as process gases, nothing more exotic than a hydrocarbon gas (typically methane) in an excess of atomic hydrogen. CVD diamond grows two dimensionally, layer by layer and it is therefore possible to build up a bulk crystal (or plate or film) which can be of a single composition or composed of layers of many compositions (called a "structure"). CVD diamond grown in this manner can show mechanical, tribological, and even electronic properties comparable to those of natural diamond. See, for example, Y. Sato and M. Kamo, "Synthesis of Diamond From the Vapor Phase", pp. 423-469, The Properties of Natural and Synthetic Diamond, J. E. Field, editor, Academic Press (1992). See also U.S. Patents for background; U.S. Pat. Nos. 4,940,015; 5,135,730; 5,387,310; 5,314,652; 4,905,227; and 4,767,608.

There is currently much optimism that it will prove possible to scale-up CVD methods to such an extent that they will provide an economically viable alternative to the traditional high pressure methods, e.g., for producing diamond abrasives and heat spreaders. The ability to coat large surface areas with a continuous film of diamond, in turn, will open up new potential applications for the CVD-prepared materials. Today, however, the production of monocrystalline diamond by the CVD process is considerably less mature than high pressure, and the resultant materials tend to have higher defect levels and smaller sizes.

Chemical vapor deposition, as its name implies, involves a gas-phase chemical reaction occurring above a solid surface, which causes deposition onto that surface. All CVD techniques for producing diamond films require a means of activating gas-phase carbon-containing precursor molecules. This generally involves thermal (e.g., hot filament) or plasma (e.g., D.C., R.F., or microwave) activation, or the use of a combustion flame (oxyacetylene or plasma torches). Two of the more popular experimental methods include the use of a hot filament reactor, and the use of a microwave plasma enhanced reactor. While each method differs in detail, they all share features in common. For example, growth of diamond (rather than deposition of other, less well-defined, forms of carbon) normally requires that the substrate be maintained at a temperature in the range of 1000-1400 K, and that the precursor gas be diluted in an excess of hydrogen (typical CH.sub.4 mixing ratio .about.1-2 vol %).

The resulting films are usually polycrystalline (unless a monocrystalline diamond seed is provided) with a morphology that is sensitive to the precise growth conditions. Growth rates for the various deposition processes vary considerably, and it is usually found that higher growth rates can be achieved only at the expense of a corresponding loss of film quality. Quality is generally taken to imply some measure of factors such as the ratio of sp3 (diamond) to sp2-bonded (graphite) carbon in the sample, the composition (e.g. C--C versus C--H bond content) and the crystallinity. In general, combustion methods deposit diamond at high rates (typically 100 .mu.m/hr to 250 .mu.m/hr), but often only over very small, localized areas and with poor process control, thereby leading to poor quality films. In contrast, the hot filament and plasma methods tend to provide have much slower growth rates (0.1-10 .mu.m/hr), but produce high quality films.

One of the great challenges facing researchers in CVD diamond technology is to increase the growth rates to economically viable rates, (to the level of 100+.mu.m/h, or even one or more mm/hr) without compromising film quality. Progress continues to be made in the use of microwave deposition reactors, since the deposition rate has been found to scale approximately linearly with applied microwave power. Currently, the typical power rating for a microwave reactor is .about.5 kW, but the next generation of such reactors have power ratings up to 50-80 kW. This gives a much more realistic deposition rate for the diamond, but for a much greater cost, of course.

Thermodynamically, graphite, not diamond, is the stable form of solid carbon at ambient pressures and temperatures. The fact that diamond films can be formed by CVD techniques is inextricably linked to the presence of hydrogen atoms, which are generated as a result of the gas being `activated`, either thermally or via electron bombardment. These H atoms are believed to play a number of crucial roles in the CVD process: They undergo H abstraction reactions with stable gas-phase hydrocarbon molecules, producing highly reactive carbon-containing radical species. This is important, since stable hydrocarbon molecules do not react to cause diamond growth. The reactive radicals, especially methyl, CH.sub.3, can diffuse to the substrate surface and react, forming the C--C bond necessary to propagate the diamond lattice. H-atoms terminate the `dangling` carbon bonds on the growing diamond surface and prevent them from cross-linking, thereby reconstructing to a graphite-like surface. Atomic hydrogen etches both diamond and graphite but, under typical CVD conditions, the rate of diamond growth exceeds its etch rate, while for other forms of carbon (graphite, for example) the converse is true. This is believed to be the basis for the preferential deposition of diamond rather than graphite.”

[Linares and  Doering, US Patent 8,187,380 (5/29/2012)

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(RDC 6/5/2012)

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Roger D. Corneliussen
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Copyright 2012 by Roger D. Corneliussen.
No part of this transmission is to be duplicated in any manner or forwarded by electronic mail without the express written permission of Roger D. Corneliussen
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* Date of latest addition; date of first entry is 6/5/2012.