Diamond synthesis under high pressure

This experimental protocol is also valid for most syntheses under high pressure, for example, the synthesis of diamond or cubic boron nitride in the laboratory or in industry. In the latter case, the high-pressure cell must be much larger (50 mm inner diameter) and it requires large belt-type equipment and a more powerful hydraulic press. 

It has been shown that fullerenes or their derivatives can exhibit very interesting chemical, electrical, magnetic, and mechanical properties. Besides, for example, the superconductivity, which has been experimentally verified for the alkali metal fullerides M3C60 with M = K or Rb [77], fullerenes can serve as a starting material for diamond synthesis [78,79] and may exhibit high hardness themselves under high pressure conditions [80]. 

The synthesis of a novel 7-813X4 phase with a cubic spinel structure (see Figure 2.1c) was carried out under high pressure (15 G Pa) and at temperatures above 1920 ° C in a laser-heated diamond cell [22]. Today, many different processing techniques for y-813X4 synthesis are available, the most common being the diamond anvil cell (DAC) synthesis (G. 8erghiou, et al., unpublished results), the multianvil pressure apparatus (MAP) synthesis [23], and the shock synthesis [24]. 

Hence the volume of a mixture of heterogenous molecules interacting by the vdW mechanism exceeds the additive value, and according to the Le Chatelier principle such mixture under high pressure will be separated. It follows that the diamond cluster size depends on the separation extent of components of the detonation cloud. In this context, it seemed reasonable to perform a detonation synthesis of c-BN under similar conditions (in a large chamber). No attempt in this direction has been... 

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). 

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. 

A quite different set of dynamic high-pressure techniques are based on the use of chemical or nuclear explosions to produce transient shock waves of high peak pressure but short duration. With such methods, one can often penetrate the high-T, P regions where kinetic barriers become unimportant and a catalyst is unnecessary. However, the same kinetics that allows facile conversion of graphite to diamonds as the shock front arrives also allows the facile back-conversion as the shock wave passes. As a pioneer of shock-wave diamond synthesis remarked ruefully, We were millionaires for one microsecond [B. J. Alder and C. S. Christian. Phys. Rev. Lett. 7, 367 (1961) B. J. Alder, in W. Paul and D. M. Warschauer (eds). Solids under Pressure (McGraw-Hill, New York, 1963), p. 385]. 

One of such unique coatings is Diamond Like Carbon (DLC). The conventional synthesis of synthetic diamonds requires extremely high temperatures and pressures. By PECVD, Diamond Like Carbon is created under mild conditions by the decomposition of methane in H2/CH4 mixture. The applications of DLC are numerous coatings for cutting tools, optical fibres, electronic devices for reading magnetic tapes, or even protective coatings in chemical reactors. 

Diamond is metastable under normal conditions and only becomes the more stable form of carbon at pressures above 16 kbar. The synthesis of diamonds from graphite therefore requires high pressures and, to increase the rate of reaction, high temperatures. The processes used are either diffusion-controlled (so-called catalytic process) or diffusion-less. 

The synthesis of diamond and cubic boron nitride has strongly motivated improvements in the development of high-pressure equipment and increased the interest in these materials, which have exceptional properties. Single crystals are required for optical and electronic applications. Consequently, specific crystal-growth processes have been set up under very high-pressure conditions. The principle is similar to that described, at lower pressures, for the preparation of single crystals of a-Si02. 

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