Diamond natural deposits

The contribution of petroleum of abiotic origin to the natural deposits is a topic of enormous importance and actuality. The geologic conditions of the Earth s upper mantle can be easily reproduced by using the diamond anvil cell in combination with different heating methods. The first experimental study of an abiotic hydrocarbons synthesis was performed by Kenney et al. [471]. The... 

The diamond is found in natural deposits in many parts of the world. Also, it can be synthesized from graphite or other carbonaceous materials. Graphite can be converted to diamond under high temperatures (about 1,400°C) and very high pressure (in the range 4,000-5,000 atm) in the presence of a metal catalyst such as iron or nickel. Presence of trace impurities can impart different coloration to diamonds. For example, introducing trace boron or nitrogen causes blue or yellow coloration. 

Natural deposits were the sole source of diamond until the mid-1950s as there was no synthesis of artificial material available-despite a variety of approaches. 

A Figure 12.35 Atomic force microscope image of thin diamond films deposited on silicon. The image is about 1 ju.m along each edge in the plane of the films. The polycrystalline nature of the film is evident. The mean surface roughness (mean distance between peaks and troughs) is on the order of 30 nm. 

Crystalline carbon exists in natural deposits in three crystalline modifications a and 8-graphite and diamond. Synthetic graphites contain only the a-form, from which the (3 can be made by mechanical working. This introduces the possibility of the transformation a p during a hardness test. Localized transformation from diamond to a metallic carbon 8-graphite could also be considered, but in this case a radical rearrangement of covalent bond hybridization would be required from sp to sp + p such that time would be a problem. 

K. Okano, S. Koizumi, S.R.P. Silva, G.A.J. Amaratunga, Low-threshold cold cathodes made of nitrogen-doped chemical-vt mur-deposited diamond. Nature 381(6578), 140-141 (1996)... 

Diamonds also occur in meteorites, probably as a result of high pressures produced dynamically by impact (10,11). The shock or explosive mode of synthesis is a viable process for fine diamond powders of both the cubic and hexagonal (lonsdaleite) polymorphs (12) naturally or otherwise. Some diamonds in space appear to have formed by processes more closely related to the low pressure chemical vapor deposition processes described later (see... 

In addition to C onions, C atoms condense into various kinds of chemically bonded forms, and they are known to have excellent physical properties depending on the bonding nature. This means that research and applications not only in the materials science but also in other scientific fields are expected. At JAERI, the optimum growth conditions have been successfully obtained for the preparation of high-quality Cgo, diamondlike carbon, and nanocrystalline diamond by means of ion-beam-assisted deposition [80-82]. The susceptibility of Ni/Cgo thin films to thermal treatment, the formation of nanocrystalline diamond and nanotubes due to codeposition of Co and Ceo, and the surface modification of glassy... 

FIGURE 3.10 (a) Chemical-vapour deposition reactor (b) cross section of a 100 pm-thick CVD diamond film grown by DC arc jet. The columnar nature of the growth is evident, as is the increase in film quality and grain size with growth time. (Courtesy of Dr. Paul May and Prof. Mike Ashfold, Bristol University.)... 

The rounded forms of natural diamond crystals are commonly observed in crystals occurring both in alluvial deposits (secondary deposits) and in mother rocks... 

For anodic processes BDD electrodes offer another very promising feature. Because of the highly reactive nature of the intermediates formed on the diamond surface, no electrode fouling by coating of byproducts or carbonization is found. Eventually formed deposits on the diamond layer will be mineralized to small... 

Fortuitously, just as India s supply of diamond was running out, deposits were discovered in Brazil and just as Brazil s were nearly exhausted, huge supplies were found in Africa. Today the African continent is the major supplier of natural diamond to the world. Most gem diamonds come from the coastal deposits in South West Africa whereas Zaire and Angola mines yield industrial quality diamonds. Sizable... 

Several researchers have reported successful deposition of homoepitaxial diamond films on natural as well as HPHT diamond substrates (e.g., 101-104). 

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