The Carbon Phase Diagram

The carbon phase diagram is shown in Fig. 2.20.i l Another expression of the T-P phase diagram, showing the calculated total vapor pressure of carbon, is shown in Fig. 3.7 of Ch. 3. 

Carbon vaporizes at 4800 K at a pressure of 1000 atmospheres, which is the area where diamond is stable. The high-pressure conversion of diamond from graphite occurs at temperatures of approximately 3000 K and pressures above 125 kbars (in the absence of catalyst) and will be reviewed in Ch. 12. 

It was established that diamond is an allotrope of carbon by Tennant in 1797 [1]. This led to many attempts to crystallize diamond using various carbonaceous starting materials, but it was not until about a century and a half later that successful synthesis was proven, as referred to in Section 1.1.3. The first clear success was by a Swedish group at the ASEA Company in February 1953 [2]. This was followed by the General Electric Company in December 1954 [3]. The early attempts and the subsequent successes are well reviewed [4—6]. 

Natural diamond has been used by man since at least biblical times, not only as a gem but also, due to its extreme properties relative to other materials, as an abrasive and even as a medicine in crushed form, a panacea for all ills . It is not intended here to enter the debate concerning the crystallization of natural diamond. Some general 

A region of solvent/catalyst-based synthesis of diamond from graphite using  

Aj commercially used transition metal solvent/catalyst [16] 

A3 nonmetallic solvents, mainly metal salts, silicates [22-28] 

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Equation of State Applied to the Carbon Phase Diagram. 

Figure 8, the carbon phase diagram, forms a basis for discussing the processes involved. Ideal graphite has a density of 2.2 and diamond, 3.52, so 1 ml of graphite becomes 0.63 ml of diamond, a relatively large change. Diamond is favored to form at pressures and temperatures where it is stable, but the carbon atoms must be in the proper environment, particularly at the milder conditions. 

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 Carbon Phase Diagram Revisited Carbon Melting . 342... 

FIGURE 16.2 The proposed extreme disordered amorphous carbon field defined by low-pressure diamond melting and melting superheated graphite in the previously modified [27,68] high-tcmperature/low-pressure region of the carbon phase diagram. 

Theoretically at least, diamond is not forever graphite would be better qualified. However, in all fairness, the rate of the diamond-graphite conversion is infinitesimally small at ordinary temperatures and, for all practical purposes, diamond is stable, as evidenced by the presence of natural diamonds in some alluvial deposits which were formed overa billion years ago and have not changed since. The carbon phase diagram, illustrated in Fig. 2.20 of Ch. 2, shows the relationship between these two allotropes of carbon. 

The theoretical bases of the mechanism of diamond crystal formation and graphi-tization (transition of carbon of the diamond phase to other nondiamond forms) in the expansion of the explosion products are given in the published papers [4,5], which present the carbon phase diagrams and consider the fundamental principles of the existence of carbon in a phase. 

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