Thermodynamic stability, diamond

If 14 or more carbons are present, the product may be diamantane or a substituted diamantane. These reactions are successful because of the high thermodynamic stability of adamantane, diamantane, and similar diamond-like molecules. The most stable of a set of C H isomers (called the stabilomer) will be the end product if the reaction reaches equilibrium. Best yields are obtained by the use of sludge ... 

In addition to thin-film electrodes, compact diamond single crystals grown at high temperature and high pressure have become the object of electrochemical study in recent years. These so-called HTHP crystals can be obtained by crystallization from a carbon solution in a metal melt (e.g., based on the Ni-Fe-Mn system) at /arranges that correspond to the conditions of thermodynamic stability of diamond. These crystals can be also doped with boron in the course of growth. 

Mechanical properties, electrical properties, thermodynamic stability, surface chemical activity, and other important parameters can all be discussed relative to the structure of the carbon network, composed of both aromatic layers and 3D-arranged (diamond-like) phases. 

To determine the diamond equilibrium solubility in the metal melts in a range of its thermodynamic stability we also used a special method which was originally developed early. Based on the data obtained we estimated carbon supersaturations in the melts [4,5]. 

Jiang H.M., Hwang N.M., Theory of the charged cluster formation in the low pressure synthesis of diamond Part II. Free energy function and thermodynamic stability. J. Materials Res., 13(12) (1998) 3536-3549. 

Note There are three allotropes of carbon graphite, diamond, and buckminsterfullerene (Cgo) the latter, discovered in 1985, is composed of soccer-ball-shaped molecules. The thermodynamic stability of buckminsterfullerene has not yet been determined. The validity of its inclusion on the C phase diagram is, therefore, uncertain. (Metastable phases, such as supercooled water, do not appear on phase diagrams.) The crystal structure is face-centered cubic with Ceo molecules at the corners and faces of a cubic unit cell. The unit cell is shown below. 

Figure 8. Phase diagram of the Gay-Berne model with the original and the most-studied parameterization (k = 3, k = 5, fi = 2, v = 1) in the density-temperature plane as obtained from computer simulations. Filled diamonds mark simulation results the phase boundaries away from these points are drawn as a guide only. The domains of the thermodynamic stability of the isotropic (/), nematic (N), and smectic (SB) phases are shown. The liquid-vapor critical point is denoted by C. Two-phase regions are shaded. (Reproduced from Ref. 104.)...

The presence of small diamond particles has been confirmed independently in all cases using several methods of analysis. These recent findings became primarily possible due to the development of new analytical techniques that allow the detection of diamond in small quantities and with particle sizes down to a few nanometers. The formation of diamond particles in the range of the thermodynamic stability of graphite can be explained either by kinetic factors, or by a higher stability of small diamond particles compared to graphite, which has been controversially discussed [18]. 

The relative thermodynamic stability of graphite versus diamond provides a classic illustration of the interplay between thermodynamics and kinetics. Graphite and diamond are both polymorphs (same composition but different phases) of carbon. At room temperature and pressure, thermodynamics tells us that diamond is less stable than graphite—in other words, there is an energetic driving force favoring the transformation of diamond into graphite. So, are diamonds forever Thermodynamics... 

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. 

An analysis of the above pnbUcations indicates a wide range of experimental conditions (laser parameters, target, and type of Uqnid) tested to determine the combination favorable for formation of diamond structures by PLA. It is believed that this process evolves formation of the so-called laser plume or a cloud of reaction products consisting of the evaporated substrate material and, partially, the surrounding liquid. These evaporated substances form bubbles inside the liquid. As the amount of the evaporated material increases, the bubbles expand and, as the pressure and temperature reach a certain critical combination, they collapse. At the collapse of the bubbles, the temperature and pressure may reach in the range of thermodynamical stability of diamond. 

Like all dynamic methods, the detonation-based synthesis of ND is basically a version of classical synthesis adapted to the high-pressure and temperature conditions providing the required thermodynamic stability of diamond. It is the short time during which these conditions have to be maintained in the shock wave in dynamic synthesis (not over a few microseconds) that is the main factor determining the size of the diamond nanocrystals fabricated by such methods. 

The shell is formed in the process of reverse diamond-to-graphite transition in the concluding stages of detonation synthesis, after the shock wave has passed and the pressure has dropped below the limit of thermodynamic stability of diamond. The thickness of the shell is largely determined by the conditions of the DND synthesis and in the course of DND isolation from detonation carbon, the thickness of the shell decreases. In the strongest regimes of i p -oxidation, the shell can be ranoved completely, except for separate single-layer 5p2 caj-, Q j islands, which result, as shown by calculations, from natural reconstruction of the free surface of diamond nanoparticles. 

Hydrogen-driven CVD processes are not necessarily the only type. In a nitrogen environment, diamonds are also formed (21). Supposition that diamond can be synthesized only under conditions of its thermodynamic stability is an old concept that has passed away. The diamond-graphite borderline, known as Berman-Simon extrapolation, is not an exclusion to growth of diamond at lower pressures. 

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