Carbon diamond synthesis

Other Industrial Applications. High pressures are used industrially for many other specialized appHcations. Apart from mechanical uses in which hydrauhc pressure is used to supply power or to generate Hquid jets for mining minerals or cutting metal sheets and fabrics, most of these other operations are batch processes. Eor example, metallurgical appHcations include isostatic compaction, hot isostatic compaction (HIP), and the hydrostatic extmsion of metals. Other appHcations such as the hydrothermal synthesis of quartz (see Silica, synthetic quartz crystals), or the synthesis of industrial diamonds involve changing the phase of a substance under pressure. In the case of the synthesis of diamonds, conditions of 6 GPa (870,000 psi) and 1500°C are used (see Carbon, diamond, synthetic). 

In the attempt at diamond synthesis (4), much unsuccesshil effort was devoted to processes that deposited carbon at low, graphite-stable pressures. Many chemical reactions Hberating free carbon were studied at pressures then available. New high pressure apparatus was painstakingly buHt, tested, analy2ed, rebuilt, and sometimes discarded. It was generally beheved that diamond would be more likely to form at thermodynamically stable pressures. 

The refractory nature of carbon indicated that temperatures of about 2000 K and pressures of 5—10 GPa (50—100 kbar) might suffice. Months after operating pressures of ca 7 GPa (70 kbar) had been attained, a reproducible diamond synthesis called the cataly2ed diamond synthesis was finally achieved. 

Plasma Synthesis The use of plasma methods has lead to a new range of materials having unique properties. An example is the family of amorphous elemental hydrides (eg cr-C H Of -Si H or-P H) which contain a variable proportion of H from almost zero to 50 atomic %. The carbon films, known variously as "hard carbon", "diamond-like carbon", " a-carbon" etc (9 ) - These layers are of considerable interest because of their optical and abrasion-resistant properties etc (Table I). The properties of these Gr-carbon films, can be tailored by modifying the plasma parameters. 

Recently, diamond synthesis has been successfully performed under high-temperature, high-pressure conditions in a system using kimberlite powder, various carbonates, sulphates or water as the solvent [13], [14]. Higher pressure and temperature conditions are required in a non-metallic solution than in a metallic solution, and the crystals obtained are mainly simple octahedral, differing from those observed in crystals grown from metallic solutions. Crystals synthesized in a non-metallic solution show the same characteristics as natural diamond Tracht. These observations indicate that the solvent components have a definitive effect upon surface reconstruction, and thus on the morphology of the crystals. 

FIGURE 8 Carbon phase diagram showing diamond synthesis regions. 

Around the same time a similar technique was independendy developed whereby micrometer sized diamond crystallites were grown (161). What is required in essence for the low pressure diamond synthesis is a source of carbon (typically a hydrocarbon gas), hydrogen, and a temperature above 2000°C to convert molecular hydrogen to its atomic state. 

Nagano, T. and Shibata, N. (1993), Diamond synthesis by microwave-plasma chemical vapor deposition using CH3CI and CH2C12 as carbon source. Part 1 Regular papers and short notes and review papers Jpn. J. Appl. Phys., 32(11A) 5067-5071. 

Figure 7 (a) Phase diagram of carbon. (Reproduced by permission ofElsevier from F.P. Bundy, P/ty /ca, 1989, A156,169) (b) Aportion of the phase diagram including the melting line of pure Ni, the Ni-graphite eutectic, and the diamond synthesis zone. (Reproduced by permission of Decker from F.P. Bimdy. )... 

Another approach to diamond synthesis involves energetic ion or laser beams to produce local areas where, for short duration, carbon atoms are subjected to pressure and temperature conditions that reach into the diamond stable region of the carbon phase diagram. After rapid quenching to ambient temperature and pressure, diamond thus formed remains metastable with respect to graphite. Attempts to deposit diamond by such techniques have been only moderately successful. 

Today there are several other ways of diamond synthesis besides the HPHT method. For example, it is possible to utilize the pressure of a shock-wave generated in an explosion. This process mostly yields powdery products with particle sizes in the range of micrometers (1 mm at max.) that may be employed for industrial purposes as well. Moreover, very small diamonds (5-20 nm) can be made by reacting explosives in confined containers. Diamond films are produced on various substrates by chemical vapor deposition (CVD method using methane as a carbon source. Detonation synthesis and vapor deposition will be described in detail in Chapters 5 and 6. 

Kuznetsov, V.L. and Butenko, Y.V. (2005) Nanodiamond Graphitization and Properties of Onion-like Carbon, in Synthesis, Properties and Applications of Ultrananocrystalline Diamond, NATO Science Series, vol. 192 (eds D.M. Gruen,... 

Figure 1. Carbon phase diagram with temperature and pressure ranges corresponding to various diamond synthesis processes, as indicated by the shaded areas. (Reproduced with permission from Ref 3, American Chemical Society, 1989)...

C. Chen, and T. Hong, The role of hydrogen in diamond synthesis from carbon dioxide-hydrocarbon gases by microwave plasma chemical vapor deposition, Sutf. Coat Technol, 54-55(l-3) 368-373 (1992)... 

The influence of the initial C/H/0 composition of the carbon compounds used for non-equilibrium diamond synthesis on the composition of the deposited films is usually illustratedby the so-called Bachmann triangle, showninFig. 9-45. The Bachmanntriangle summarizes experimental data and indicates the C/H/0 compositions corresponding to deposition of non-diamond films, diamond films, or no deposition at all (which corresponds to domination of etching over deposition). Deposition of diamond films from the gas phase is typically performed at pressures of 10-100 Torr in a mixture of hydrogen with about 1-5% methane. The plasma-chemical conversion of carbon compounds can lead not only to the formation of diamond films but also to so-called diamond-like carbon (DLC). DLC films are... 

Diamond will be etched by caustic alkalis and some oxidizing salts, for example by NaClO and KOCl at 380°C [41], and K and Na nitrates above 400°C. Diamond reacts with metals, which form carbides (e.g. W, Ta, Ti, and Zr) or which dissolve carbon (e.g. Fe, Co, Mn, Ni, and Cr) [42]. Detailed knowledge of the temperatures of reactions with metals is reviewed in the context of experiments with metals for the catalysis of diamond synthesis [43],... 

Figure 1. P T diagram of the low-pressure, low-temperature labile equilibriums of carbon solution 1 = graphite-diamond equilibrium line, 2 = glassy carbon-diamond transition line, 3 = range of pneumatolytic hydrothermal processes, 4=oxidative corrosion of diamond, 5 = anticipated area of diamond hydrosynthesis, 6 and 7 = diamond synthesis from glassy carbon precursors, 8 = low-pressure, low-temperature hydrothermal homoepitaxy of diamond. Reproduced from [15] with permission from A. Szymanski.

The discussion of hydrothermal diamond synthesis is divided into two sections, dealing with synthesis from C-H-0 liquids and synthesis based on decomposition of silicon carbide, respectively. Both start with thermodynamic calculations in order to demonstrate the theoretical possibility of carbon formation before the experimental findings are summarized. Naturally, equilibrium calculations do not consider kinetic limitations. 

In many instances diamond synthesis in the C-H-O system is related to diamond formation in nature as discussed in part 2. It was shown that precipitation of diamond can occur in the CO2/CH4 system [38]. No reagent external to the C-H-O system should be required for carbon precipitation while high water fugacities may exist at the site of diamond precipitation. 

Successful hydrothermal diamond synthesis was carried out in autoclaves filled with a specially prepared carbon enriched water solution , the composition of which was not disclosed [15,45,46]. The carbon precursor should be fine-grained diamond, vitreous carbon or emulsion of crude oil and water [29]. The presence of free radical catalysts was mentioned and paragenetic crystallization of quartz needles and diamond indicate the presence of silicon [45]. The synthesis was described as a sol/gel colloidal process working in the range 200-600°C and 100-200 MPa. Healing and joining of diamond crystals was reported. After 21 days at 400°C and 170 MPa, thin colorless films of polycrystalline diamond were obtained on (111) surfaces of seed crystals (Fig. 3c). With a reported size of 15-40 pm, these are the largest diamond crystals from hydrothermal experiments. 

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