Pressure-temperature conditions, diamond

It was argued that the most celebrated violation of thermodynamics is the fact that CVD (chemical vapor deposition) diamond forms under thermodynamically unstable pressure - temperature conditions. A careful analysis of thermodynamic data led to the conclusion that certain equilibria are taking part in the process, so that the process no longer appears as a thermodynamic paradox [18]. The key idea was to assume that CVD diamond formation is a chemical process consisting in accretion of polymantane macromolecules. Thus, violations of thermodynamic principles could be avoided. 

The very complex nature of the macroscopic and microscopic structures as they affect strength and the behavior of diamond abrasive/particles still requires extensive work to elucidate fully. However, from a crystallization point of view, gaining control over the crystallization behavior is the key to the production of optimal diamond abrasives. This, of course, may be achieved by choice and manipulation of the pressure/temperature conditions, source carbon structure and solvent/catalyst metal type, leading to control over nucleation and growth rates. 

In the case of the graphite-to-diamond transformation, thermodynamic results predict that graphite is the stable allotrope at a fixed temperature at all pressures below the transition pressure and that diamond is the stable aUotrope at all pressures above the transition pressure. But diamond is not converted to graphite at low pressures for kinetic reasons. Similarly, at conditions at which diamond is the thermodynamically stable phase, diamond can be obtained from graphite only in a narrow temperature range just below the transition temperature, and then only with a catalyst or at a pressure sufficiently high that the transition temperature is about 2000 K. 

A table of crystal structures for the elements can be found in Table 1.11 (excluding the Lanthanide and Actinide series). Some elements can have multiple crystal structures, depending on temperature and pressure. This phenomenon is called allotropy and is very common in elemental metals (see Table 1.12). It is not unusual for close-packed crystals to transform from one stacking sequence to the other, simply through a shift in one of the layers of atoms. Other common allotropes include carbon (graphite at ambient conditions, diamond at high pressures and temperature), pure iron (BCC at room temperature, FCC at 912°C and back to BCC at 1394°C), and titanium (HCP to BCC at 882°C). 

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 9.14. Spiral growth layers observed on (a) 100 and (b) 111 faces of diamond crystals synthesized under high-pressure and high-temperature conditions.

It is well known that graphite is the stable form of carbon at ambient conditions, and studies have been undertaken to find pressure and temperature conditions where diamond becomes stable, and as a consequence, graphite would convert spontaneously to diamond. The process is fraught with difficulty, and for many years the conversion was not successful. The... 

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. 

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. 

The research effort directed towards artificial synthesis of diamond under low pressure low temperature conditions led to the... 

Akaishi M., Kumar M. D. S., Kanda H., and Yamaoka S. (2000) Formation process of diamond from supercritical H2O-CO2 fluid under high pressure and high temperature conditions. Diam. Relat. Mater. 9, 1945—1950. 

Unlike other thin film deposition processes, conditions for diamond CVD have three unique features (i) high substrate temperature typically at 700-1200 °C, (ii) high gas pressure P at 20-150Torr (lTorr= 133.3 Pa), and (iii) low methane (CH4) concentration of usually 1-5% with respect to the dilution gas, hydrogen (H2). A standard temperature for diamond growth, monitored by an optical pyrometer without emissivity correction, is 800 °C. It is, however, considered that the surface temperature of the specimen exposed to the plasma is actually higher. Under these conditions, at least more than 95% of the deposited film can be crystalline diamond,... 

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