Graphite thermodynamic stability

The discovery of what appears to be a thermodynamic threshold governing the intercalation of graphite by fluoro anions, MF,, has required the evaluation of the thermodynamic stability of a number of such species. Since germanium tetrafluoride and fluorine are intercalated, in combination, by graphite to form both GeFj" and GeFs ", the first and second fluoride ion affinities of that molecule are each of interest. Evaluation of the fluoride ion affinity of boron trifluoride by Altshuller yielded a value of-71 kcal moF. This has been accepted by several authors as the basis for other fluoride ion affinities and electron affinities. Sharpe, however, has preferred a value of -91 kcal moF, based upon the data of Bills and Cotton. Although this latter value is in harmony with other fluoride ion affinities and electron affinities, its confirmation was clearly desirable to provide a firm basis for correction of affinities based upon the lower value. This paper describes the studies that have provided these fluoride ion affinities. 

Returning now to the graphite laser vaporisation results in fig. 1, we should first explain why we think it is appropriate to concentrate on kinetic, rather than thermodynamic, stability. In fact there are two main reasons for this, one of which is physical and the other theoretical. 

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. 

It is worth to mention that under the same conditions (1540 °C, N2) leading to surface crystallization via gas phase processes, Kleebe et al. were able to show by TEM and XRD that, within an amorphous SiCN matrix, globular inclusions also exist where crystallization has taken place and where crystallites of graphite, silicon carbide, as well as silicon nitride were found side by side. However, the thermodynamic stability of this system vanishes at higher temperatures [162]. At T = 1600 °C silicon nitride has disappeared and, e.g., the Si NMR spectrum reveals only [SiC4] (and no longer [SiN4] which had been detectable at T < 1500 °C.)... 

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. 

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. 

It has been suggested that in practical non-aqueous lithium battery systems the anode (Li or graphite) is always covered by a surface layer named the solid electrolyte interphase (SEI), 1-3 run thick, which is instantly formed by the reaction of the metal with the electrolyte. This film, which acts as an interphase between the metal and the solution, has the properties of a solid electrolyte. This layer has a corrosive effect and grows with the cycling life of the battery [52], Thermodynamic stability of a lithium cell requires the electrochemical potentials of electrodes a and Ec located within the energetic window of the electrolyte, which contrains the cell voltage Eq of th electrochemical ceU to ... 

Fig. 14.22 Electronic band diagrams of Li-ion batteries (a) graphite//LiCo02 and (b) LTO//LFP ceils. a and Eq represent the Fermi level of anode and cathode respectively. g is the electrolytic window that ensures the thermodynamic stability, while Ea > l and Ec < h requires a kinetic stability by the formation of an SEI layer...

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