Formation of diamond

DeCarli P S and Jamieson J C 1961 Formation of diamond by explosive shook Science 133 1821... 

The standard enthalpy of formation of diamond is therefore reported as AHt°(C, diamond) = + 1.9 kj-mol l. Values for a selection of other substances are listed in Table 6.5 and Appendix 2A. 

Some elements exist in more than one form under standard conditions. For example, carbon can exist as either graphite or diamond, as shown in Figure 5.16. Graphite is defined as the standard state of carbon. Therefore, the standard enthalpy of formation of graphite carbon is 0 kj/mol. The standard enthalpy of formation of diamond is 1.9 kj/mol. Another example is oxygen, 02(g). Oxygen also exists in the form of ozone,... 

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... 

Since thermodynamics deals with systems at equilibrium, time is not a thermodynamic coordinate. One can calculate, for example, that if benzene(equilibrium with hydrogen(g) and carbon(s) at 298.15 K, then there would be very little benzene present since the equilibrium constant for the formation of benzene is 1.67 x 10-22. The equilibrium constant for the formation of diamond(s) from carbon(s, graphite) at 298.15 K is 0.310 that is, graphite is more stable than diamond. As a final example, the equilibrium constant for the following reaction at 298.15 K is 2.24 x 10-37 ... 

From Table 7-1, the formation of diamond from graphite (the standard state of carbon) is accompanied by a positive AH of 1.88kJ/mol at 25°C. From Problem 16.1(f), AS for the same process is negative. Since 25°C is not the transition temperature, the process is not a reversible one. In fact, it is not even a spontaneous irreversible process, and (16-2) does not apply with the inequality sign. On the contrary, the opposite process, the conversion of diamond to graphite at 1 atm, is thermodynamically spontaneous. The AS for this process would obey (16-2) with the inequality sign. This means that diamonds are NOT forever The term spontaneous does not cover the speed... 

A.A.Bochechka, V.G.Gargin. The effect of liquid phase on compacting diamond powders under thermobaric conditions//in Physico-Chemical Processes at the phase boundary in diamond synthesis and formation of diamond-containing composites [in Russian], ISM AN Ukrainy, 1993. 

Towards the end of the 18 century a British chemist, Smithson Tennant, showed that diamonds are composed of nothing but carbon a discovery that gave a more scientific direction to synthesis efforts. By the beginning of the 19 century, it was known that carbonaceous materials, heat and pressure are required for diamond formation. Finally, success in artificial synthesis was achieved in the middle of the 20 century by two routes the High Pressure High Temperature (HPHT) route leading to the formation of diamond grit and the Low Pressure... 

The heats of formation of diamond and graphite are very close, since although in diamond each atom has four a bonds and in graphite each atom forms only three a bonds and one weaker n bond, in the latter there is an increase of stability owing to resonance of a similar type to that occurring in benzene. 

Ravindran TR, Badding JV (2002) Ultraviolet Raman analysis of the formation of diamond from Cgo. Sol St Commun 121 391... 

Bulanova G. P. (1995) The formation of diamond. Diamond Exploration into the 21st Century. J. Geoch. Explor. 53(1-3), 1-23. 

Deines P., Viljoen E., and Harris J. W. (2001) Implications of the carbon isotope and mineral inclusion record for the formation of diamonds in the mantle underlying a mobile belt Venetia, South Africa. Geochim. Cosmochim. Acta 65(5), 813-838. 

Kumar M. D. S., Akaishi M., and Yamaoka S. (2000) Formation of diamond from supercritical H2O-CO2 fluid at high pressure and high temperature. J. Cryst. Growth 213, 203 - 206. 

Pal yanov Y. N., Sokol A. G., Borzdov Y. M., and Khokhryakow A. F. (2002a) Fluid-bearing alkaline-carbonate melts as the medium for the formation of diamonds in the Earth s mantle an experimental study. Lithos 60, 145-159. 

It is of most intrigue that unlike previous nucleation models, this model assumes a formation of diamond nuclei or nucleation sites inside the P-SiC layer, while the p-SiC layer concurrently plays a role of basal lattice for diamond epitaxial growth like in the precedent models. The second point of intrigue is the fact that the exposure and survival of the diamond nuclei or nucleation sites are made possible by a subtle balance of etching rates of Si, P-SiC, diamond, and other forms of carbon. This is consistent with the fact that HOD films can be formed only when the substrate was pretreated by proper BEN conditions. 

C. Cagniard de la Tour claimed to have synthesized diamond from a solution. His crystals turned out to be aluminum and magnesium oxide. Many other claims and attempts were made but until the discovery of diamond in kimberlite, the role of high pressure and high temperature (HPHT) in the formation of diamond was not known. Diamond can be formed at great depths in the earth because of HPHT it is the dominant phase of carbon while at low pressures like those attained in a typical laboratory experiment, graphite is the dominant phase (Fig. 4). 

The emphasis of most studies on nucleation and growth of diamond has been placed on the heterogeneous formation of diamond particles and the crystallization and deposition of diamond films on substrate surfaces. Only a limited number of experiments have been conducted to achieve the homogeneous nucleation of diamond in the gas phase at atmospheric and subatmospheric pressures. However, there is evidence that, at least in some cases, diamond can be nucleated homogeneously in the nas phase.I2i H i]... 

Figure 4. Schematic diagram showing the effect of oxygen on diamond nucleation and growth on scratched Ni foil substrates oxygen suppresses the formation of diamond nucleation sites on the pre-deposited graphite layer.l (Reproduced with permission.)...

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