Graphite 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. 

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... 

A quite different set of dynamic high-pressure techniques are based on the use of chemical or nuclear explosions to produce transient shock waves of high peak pressure but short duration. With such methods, one can often penetrate the high-T, P regions where kinetic barriers become unimportant and a catalyst is unnecessary. However, the same kinetics that allows facile conversion of graphite to diamonds as the shock front arrives also allows the facile back-conversion as the shock wave passes. As a pioneer of shock-wave diamond synthesis remarked ruefully, We were millionaires for one microsecond [B. J. Alder and C. S. Christian. Phys. Rev. Lett. 7, 367 (1961) B. J. Alder, in W. Paul and D. M. Warschauer (eds). Solids under Pressure (McGraw-Hill, New York, 1963), p. 385]. 

Figure 15.6 The (graphite + diamond) phase diagram, including the pressure-temperature region for diamond synthesis with ferrous metals and their alloys as solvent catalysts. Reproduced with permission from H. M. Strong, Early Diamond Making at General Electric , Am. J. Phys., 57, 794-802 (1989). Published by the American Association of Physics Teachers.

The technical synthesis of graphite, diamond and a variety of other forms of sp2 carbons (Fig. 3) is described in a review [39] and is not covered here. As the unintended formation of carbon in deactivation processes and the modification of primary carbon surfaces during chemical treatment (in catalytic service and during oxidative reactivation) and their chemical properties arc frequent problems encountered in catalytic carbon chemistry, it seems appropriate to discuss some general mechanistic ideas which mostly stem from the analysis of homogeneous combustion processes (flame chemistry) and from controlled-atmosphcre electron microscopy. 

Refs. [i] Pierson HO (1993) Handbook of carbon, graphite, diamond, and fullerenes. Noyes Publications, New Jersey [ii] Yoshimura S, Chang RPH (eds) (1998) Supercarbon. Synthesis, properties and applications. Springer, Berlin [iii] Kinoshita K (1988) Carbon. Electrochemical and physicochemical properties. Wiley, New York... 

Let us illustrate this with the diamond synthesis as an example. It is common knowledge that the graphite to diamond phase transformation is only possible at ultrahigh pressures and temperatures. However, it has become habitual in recent years to synthesize diamond whiskers and fine diamond films under far from extreme conditions. 

Figure 7 (a) Phase diagram of carbon. (Reproduced by permission ofElsevier from F.P. Bundy, P/ty /ca, 1989, A156,169) (b) Aportion of the phase diagram including the melting line of pure Ni, the Ni-graphite eutectic, and the diamond synthesis zone. (Reproduced by permission of Decker from F.P. Bimdy. )... 

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. 

Russian group of Derjaguin et al., by irradiating ultrafine graphite powder with a CO laser beam, and was later confirmed by Roy et al. Various other researchers have reported diamond synthesis by similar techniques (e.g., 24-27). Diamond formation by laser ablation of graphite too has been claimed (e.g., 28, 29). 

Among the most attractive tasks for a plasma chemist is the possibility of preparation of diamond, a high temperature and high pressure modification of carbon. The conventional synthesis of diamond is performed in its stability region at a pressure of several ten kilobars and several thousand degrees Kelvin, using suitable solvent catalysts in order to overcome the kinetic barrier for the transition from sp to sp hybridization of C-atoms. Without a catalyst the graphite diamond transition occurs only at pressures of several hundred kilobars... 

Figure 1. P T diagram of the low-pressure, low-temperature labile equilibriums of carbon solution 1 = graphite-diamond equilibrium line, 2 = glassy carbon-diamond transition line, 3 = range of pneumatolytic hydrothermal processes, 4=oxidative corrosion of diamond, 5 = anticipated area of diamond hydrosynthesis, 6 and 7 = diamond synthesis from glassy carbon precursors, 8 = low-pressure, low-temperature hydrothermal homoepitaxy of diamond. Reproduced from [15] with permission from A. Szymanski.

The idea underlying the dynamic synthesis of nanometer-sized diamonds essentially consists of providing the pressure and tanperature needed to drive the graphite-diamond-phase transition by means of a shock (explosive) wave. Since both the pressure and temperature in a shock wave are characteristic of the region of thermodynamic stability of diamond, the time of synthesis is necessarily very short (as a rule, a few fractions of a microsecond), and, hence, the diamond crystallites produced usually remain nanometer sized. 

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