Diamond-graphite transformation

The Kinetic Barrier. Aithough thermodynamicaiiy feasible at relatively low pressure and temperature, the transformation graphite-diamond faces a considerable kinetic barrier since the rate of transformation apparently decreases with increasing pressure. This kinetic consideration supersedes the orable thermodynamic conditions and it was found experimentally that very high pressure and temperature (>130 kb and >3300 IQ were necessary in order for the direct graphite-diamond transformation to proceed at any observable rate.f l These conditions are very difficult and costly to achieve. Fortunately, it is possible to bypass this kinetic barrier by the solvent-catalyst reaction. 

The rate of graphite-diamond transformation goes up as pressure (P) and temperature (T) are increased. 

There is a wealth of information on this thermodynamic transformation, e.g. in most textbooks of physical chemistry. The site http //chemistry.about.comAibrary/weekly/ aa071601a.htm has copious links, while the short Web page http //members.tripod. com/graphiteboy/Graphite Diamond.htm cites a few nice details... 

From a thermodynamic point of view, the transformation of graphite is accessible with the available experimental apparatuses, but it is kinetically impossible. Geological times, hundreds of years, are required for spontaneous formation of diamond in appropriate conditions, and kinetic factors prevent the observation of the reaction in any practical time scale. H. T. Hall has demonstrated that for graphite diamond conversion, carbon-carbon bonds must be broken in a solvent and on December 1954 realized the first synthesis of diamond, at approximately 2000 K and 10 GPa, in molten troilite (FeS) solvent, using a belt-type high-pressure-high-temperature apparatus [516-519]. Since then, many substances, minerals, and transition metals, in particular, have been... 

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. 

Rgure 2.3 Phase diagram for carbon. The letters indicated correspond to various transformations between graphite, diamond, and lonsdaleite. (Reprinted from Ref. [31] with permission from Elsevier.)... 

Show that the volume change for the transformation graphite —> diamond is negative. 

Summarizing reactions (3) to (8), one obtains a thermodynamically strongly favored net reaction (9) for the graphite-to-diamond transformation, driven by the input of atomic hydrogen ... 

The solvent action of nickel is shown in Fig. 12.2. When a nickel-graphite mixture is held at the temperature and pressure found in the cross-hatched area, the transformation graphite-diamond will occur. The calculated nickel-carbon phase diagram at 65 kbar is shown in Fig. 12.3. Other elemental solvents are iron and cobalt.i i However, the most common... 

Fullerene-Diamond Transformation. The rapid compression of Cqo powder, to more than 150 atm in less than a second, caused a collapse of the fullerenes and the formation of a shining and transparent material which was identified as a polycrystalline diamond in an amorphous carbon matrix.O Thus the fullerenes are the first known phase of carbon that transforms into diamond at room temperature. Graphite also transforms into diamond but only at high temperatures and pressures (see Ch. 12, Sec. 3.0). 

In this process, diamond forms from graphite without a catalyst. The refractory nature of carbon demands a fairly high temperature (2500—3000 K) for sufficient atomic mobiUty for the transformation, and the high temperature in turn demands a high pressure (above 12 GPa 120 kbar) for diamond stabihty. The combination of high temperature and pressure may be achieved statically or dynamically. During the course of experimentation on this process a new form of diamond with a hexagonal (wurtzitic) stmcture was discovered (25). 

Static Pressure Synthesis. Diamond can form direcdy from graphite at pressures of about 13 GPa (130 kbar) and higher at temperatures of about 3300—4300 K (7). No catalyst is needed. The transformation is carried out in a static high pressure apparatus in which the sample is heated by the discharge current from a capacitor. Diamond forms in a few milliseconds and is recovered in the form of polycrystalline lumps. From this work, and studies of graphite vaporization/melting, the triple point of diamond, graphite, and molten carbon is estimated to He at 13 GPa and 5000 K (Fig. 1)... 

Elucidation of the phase relationships between the different forms of carbon is a difficult field of study because of the very high temperatures and pressures that must be applied. However, the subject is one of great technical importance because of the need to understand methods for transforming graphite and disordered forms of carbon into diamond. The diagram has been revised and reviewed at regular intervals [59-61] and a simplified form of the most recent diagram for carbon [62] is in Fig. 5. 

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