Synthetic Polycrystalline Diamond

A polycrystalline synthetic diamond electrode was used, values of A o range between 6-10 and 2.4-10 cm s . 

Natural diamonds used for jewellery and for industrial purposes have been mined for centuries. The principal diamond mining centres are in Zaire, Russia, The Republic of South Africa, and Botswana. Synthetic diamonds are made by dissolving graphite in metals and crystallising diamonds at high pressure (12-15 GPa) and temperatures in the range 1500-2000 K [6] see section 3. More recently, polycrystalline diamond films have been made at low pressures by... 

Figure 4-150 shows the major components and design of the PDC bit. The polycrystalline diamond compacts, shown in Figure 4-151. The polycrystalline diamond compacts (of which General Electric s) consist of a thin layer of synthetic diamonds on a tungsten carbide disk. These compacts are produced as an integral blank by a high-pressure, high-temperature process. The diamond layer consists of many tiny crystals grown together at random orientations for maximum strength and wear resistance.

The overview by Pleskov covers the literature on electrochemical behavior of synthetic diamond films, as well as the use of electrochemical methods in their characterization. The rapid advancement of the field of diamond electrochemistry was triggered by progress in the technology of deposition of polycrystalline diamond thin films on diamond and other substrates. Advances around the world have by now led to formation of a self-consistent, but as yet incomplete, view of electrochemical behavior of diamond. While discrepancies and scatter between data from different research groups still exist, the rapid advance in film quality and in reliable methods of evaluation point to a promising future. 

In the diamond stmcture, carbon atoms are present in sp hybridization, with a tetrahedral stereochemistry and a face-centered cubic stmcture that is shown in Fig. 2.1. Besides natural diamond, synthetic diamond has been produced since General Electric first announced its successful high-pressure synthesis in 1955. Sintered polycrystalline diamond, different types of diamond films, and diamondlike carbon are other types of diamond-related synthetic materials, some of which are noncrystalline [13, 19] these solids have their own terminology [10, 20]. Unhke other carbonaceous solids, diamond has a rather limited and specific relevance to adsorption. Indeed, ever since the publication of a pioneering work... 

Faceted, polycrystalline structure Synthetic diamond Blocky... 

Apart from naturally occurring diamond there is by now a variety of artificial carbon materials that feature diamond structure as well. These include the synthetic diamond generated by high pressure and temperature, but also films, polycrystalline materials resembling the carbonados (Section 1.3.2) and the so-called... 

Similar differences were found between natural and synthetic diamonds on oxidation in air atmospheres. The lower oxidation resistance of synthetic diamonds is probably due to the higher surface area, their polycrystalline character, and the presence of metallic ion impurities. 

Carbon in the structural form of diamond is the only element used industrially as a hard material. Each year about ten tons of natural diamond and about twenty tons of synthetic diamond (produced via high temperature high pressure synthesis) are marketed as hard materials. While pure diamond is transparent, a yellow tint results from the replacement of some carbon atoms by nitrogen, and a blue, yellow, or even green tint through substitution of carbon by boron atoms. Polycrystalline diamond with impurities, used as an abrasive, is often black. 

CVD-diamond coatings are polycrystsdiine, as opposed to natural and high-pressure synthetic diamond which are normally single crystals. This polycrystalline characteristic has important bearing on the general properties of the coatings as shown in Sec. 4.0. 

The free-exciton emission is strongest in diamonds with low concentrations of defects (46), but even in the best natural diamonds this luminescence is weak compared with the luminescence observed in the visible spectral region (discussed briefly in Sec. III.D). By contrast, measurements in the author s laboratory in 1995 have shown that in very high purity synthetic diamonds the free-exciton emission is strong, compared with the visible luminescence. Some polycrystalline CVD specimens examined also exhibit relatively strong edge emission, and in a few homo-epitaxial layers of CVD diamond the free-exciton luminescence is dominant. This indicates that diamond can now be manufactured with a considerably lower defect density than that found in the best natural diamonds. 

PCD (polycrystalline diamond) is obtained by sintering synthetic diamond powder in the presence of a metal binder (Co, Ni or Fe a low percentage by volume), at 1,350-1,500°C imder 5 GPa pressure. One may also sinter a layer of diamonds (0.5 mm thick) on a sintered hard metal substrate, the cobalt of the substrate thus participating in the sintering of the diamond and the adherence of the PCD on the substrate. Inserts up to 72 mm diameter and hardness of 5,000 to 8,000 HV may thus be attained. 

Cubic Phase of Boron Nitride c-BN. The cubic phase of boron nitride (c-BN) is one of the hardest materials, second only to diamond and with similar crystal structure. It is the first example of a new material theoretically predicted and then synthesized in laboratory. From automated synthesis a microcrystalline phase of cubic boron nitride is recovered at ambient conditions in a metastable state, providing the basic material for a wide range of cutting and grinding applications. Synthetic polycrystalline diamonds and nitrides are principally used as abrasives but in spite of the greater hardness of diamond, its employment as a superabrasive is limited by a relatively low chemical and thermal stability. Cubic boron nitride, on the contrary, has only half the hardness of diamond but an extremely high thermal stability and inertness. 

Boron is an acceptor in diamond with an activation energy of 0.37 eV. Natural type II b diamond is an example of a boron doped p-type diamond. There are many commercial sources of synthetic boron doped-diamond. Polycrystalline boron doped diamond films, made by CVD, have been made for electrode applications. High-pressure high-temperature diamonds with boron doping are also available. ... 

Considerable attention has been paid to possible mechanisms of formation since a firm understanding of this aspect could lead to the development of more effective synthetic routes to the individual fullerenes. It is also known that, when thin films of Cgo and C70 are laser-vaporized into a rapid stream of an inert gas, individual molecules of Ceo or C70 can themselves coalesce to form stable larger fullerenes such as Cno or C140, and higher multiples. Even more dramatically, when a sample of C o is subjected to a pressure of 20 GPa (i.e. 200 kbar), it apparently immediately transforms into polycrystalline diamond. 

In addition, electron diffraction patterns of polycrystalline diamond are similar to those of basal-plane oriented polycrystalline graphite and, when analyzing mixtures of the two, it may be difficuitto separate one pattern from the other. Unfortunately, mixed graphite-carbon-diamond aggregates eire common in natural and synthetic materials. 

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