Stable processes, diamond synthesis

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. 

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 first industrialized process for the synthesis of diamond employed the HPHT method, which mimics nature in that thermodynamically stable conditions are used for growth [19]. In HPHT, typically a synthesis capsule containing graphite, seed crystals, and a metal solvent is compressed to tens of thousands... 

Asmussin, J., and D. K. Reinhard. 2002. Diamond Films Handbook. New York Marcel Dekker. Detailed information on the science, processes, and application of low-pressure diamond deposition, which has been developed over the past two decades to synthesize a form of diamond that is metastable with respect to graphite, as distinguished from the high-pressure synthesis of diamond that is the stable form of carbon. Available online on CRCnetBASE. 

The Clapeyron equation can be applied to substances under extreme conditions of temperature and pressure, because it can estimate the conditions of phase transitions—and therefore the stable phase of a compound—at other than standard conditions. Such conditions might exist, say, at the center of a gas giant planet like Saturn or Jupiter. Or, extreme conditions might be applied in various industrial or synthetic processes. Consider the synthesis of diamonds, which normally occurs deep within the earth (or so it is thought). The phase transition from the stable phase of carbon, graphite, to the unstable phase, diamond, is a viable target for the Clapeyron equation, even though the two phases are solids. 

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