Graphite to diamond

DeCarli and Jamieson [61D01] 1961 graphite to diamond the key work... 

At 25°C and 1 atm, graphite is the stable form of carbon. Diamond, in principle, should slowly transform to graphite under ordinary conditions. Fortunately for the owners of diamond rings, this transition occurs at zero rate unless the diamond is heated to about 1500°C, at which temperature the conversion occurs rapidly. For understandable reasons, no one has ever become very excited over the commercial possibilities of this process. The more difficult task of converting graphite to diamond has aroused much greater enthusiasm. 

Considering that so little energy is required to convert graphite to diamond (recall Problem 8, Chapter 7), how do you account for the great difficulty found in the industrial process for accomplishing this ... 

AH = 2.9 kj mol-1 at 300 K and 1 atm, there is no low-energy pathway for the transformation, so the process is difficult to carry out. However, synthetic diamonds are produced on a large scale at high temperature and pressure (3000 K and 125kbar). The conversion of graphite to diamonds is catalyzed by several metals (i.e., chromium, iron, and platinum) that are in the liquid state. It is believed that... 

Heating graphite at the same time as compressing it under enormous pressure will yield diamond. The energy needed to convert 1 mol of graphite to diamond is 2.4 kJmol-1. We say the enthalpy of formation AHt for the diamond is +2.4 kJ mol-1 because graphite is the standard state of carbon. 

Graphite sulfate, 12 777 Graphite-to-diamond direct process, 8 535-538... 

Synthetic camphor, 24 540 Synthetic compounds, as plant growth regulators, 13 39-56 Synthetic crude oil, 13 640 Synthetic cyclic molecules, 24 35 Synthetic diamond, 3 530-543 catalyzed synthesis, 3 531-535 crystal growth, 3 535 crystal morphology, 3 534-535 crystal structure, 3 537-538 direct graphite-to-diamond process, 3 535-538... 

A short review on the development of laser heating in special applications under pressure has been published by Bassett (2001). A heating system to be used, with either ruby or YAG laser, under pressure in a diamond anvil cell has been described. Graphite to diamond and several silicate phase transformations have been studied. 

In practice, the primary objective of chemical thermodynamics is to estabhsh a criterion for determining the feasibility or spontaneity of a given physical or chemical transformation. For example, we may be interested in a criterion for determining the feasibility of a spontaneous transformation from one phase to another, such as the conversion of graphite to diamond, or the spontaneous direction of a metabohc reaction that occurs in a cell. On the basis of the first and second laws of thermod5m-amics, which are expressed in terms of Gibbs s functions, several additional theoretical concepts and mathematical functions have been developed that provide a powerful approach to the solution of these questions. 

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. 

The enthalpy change for the transition from graphite to diamond is an essential item of information in calculation of the conditions for the geological and industrial production of diamonds. 

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]. 

Akin to the alchemist s dream of converting lead into gold has been the desire to convert graphite to diamond from the simple chemical process. 

The successful conversion of graphite to diamond involves crystallizing the diamond from a liquid melt. The solvent most often used is nickel metal, or alloys of nickel with other ferrous metals. The reason for this success can be seen by referring to Figure 15.7, the binary (solid + liquid) phase diagram for (nickel + carbon).u8 We note from the figure that (Ni + C) forms a simple... 

Figure 4.16. A hypothetical scheme for conversion of 3IP graphite to diamond by compression.

From the data in Appendix B, graphite is seen to be the stable form of carbon at 298 K and atmospheric pressure. The densities of graphite and diamond are 2.25 g/cm3 and 3.52 g/cm3, respectively. Calculate the pressure to which graphite must be raised at 298 K in order for a diamond to become the stable form of carbon. Is this a viable method for converting graphite to diamond Explain your answer. 

It must be emphasized again that even reactions that are unlikely are not necessarily impossible, especially under different conditions. Changing the conditions (i.e., temperature and pressure) may cause the reaction to become feasible because AG may be negative under a different set of conditions. This is especially true for reactions where AG is only slightly positive. For example, the transformation of graphite to diamond can be shown as... 

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. 

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