Graphite and Diamond

Fully conjugated cyclopolyynes, so-called cyciocarbons, constitute another class of carbon modifications besides diamond, graphite, and the recently discovered fullerenes (see section 5.6). Syntheses of these unstable rings might be possible by mild elimination, extrusion, or... 

Static Pressure Synthesis. Diamond can form direcdy from graphite at pressures of about 13 GPa (130 kbar) and higher at temperatures of about 3300—4300 K (7). No catalyst is needed. The transformation is carried out in a static high pressure apparatus in which the sample is heated by the discharge current from a capacitor. Diamond forms in a few milliseconds and is recovered in the form of polycrystalline lumps. From this work, and studies of graphite vaporization/melting, the triple point of diamond, graphite, and molten carbon is estimated to He at 13 GPa and 5000 K (Fig. 1)... 

Despite many publications on carbynes, their existence has not been universally accepted and the literature has been characterised by conflicting claims and counter claims [e.g., 27-29]. This is particularly tme of meteoritic carbynes. An interesting account of die nature of elemental carbon in interstellar dust (including diamond, graphite and carbynes) was given by Pillinger [30]. Reitmeijer [31] has re-interpreted carbyne diffraction data and has concluded that carbynes could be stratified or mixed layer carbons with variable heteroelement content (H,0,N) rather than a pure carbon allotrope. 

Dresselhaus, M. S. and Kalish, R., Ion Implantation in Diamond, Graphite and Related Materials, Springer Series in Materials Science, Vol. 22, Springer-Verlag, Berlin, 1992. 

What I hope to have added to the discussion has been a philosophical reflection on the nature of the concept of element and in particular an emphasis on elements in the sense of basic substances rather than just simple substances. The view of elements as basic substances, is one with a long history. The term is due to Fritz Paneth, the prominent twentieth century radio-chemist. This sense of the term element refers to the underlying reality that supports element-hood or is prior to the more familiar sense of an element as a simple substance. Elements as basic substances are said to have no properties as such although they act as the bearers of properties. I suppose one can think of it as a substratum for the elements. Moreover, as Paneth and before him Mendeleev among others stressed, it is elements as basic substances rather than as simple substances that are summarized by the periodic table of the elements. This notion can easily be appreciated when it is realized that carbon, for example, occurs in three main allotropes of diamond, graphite and buckminsterfullenes. But the element carbon, which takes its place in the periodic system, is none of these three simple substances but the more abstract concept of carbon as a basic substance. 

Solid carbon materials are available in a variety of crystallographic forms, typically classified as diamond, graphite, and amorphous carbon. More recently another structure of carbon was identified—namely the fullerenes which resemble a soccer ball... 

Use the phase diagram for carbon in Exercise 8.14 (a) to describe the phase transitions that carbon would undergo if compressed at a constant temperature of 2000 K from 100 atm to 1 X 106 atm (b) to rank the diamond, graphite, and liquid phases of carbon in order of increasing density. 

Carbon has an important series of allotropes diamond, graphite, and the fullerenes. 

The state of research on the two classes of acetylenic compounds described in this article, the cyclo[ ]carbons and tetraethynylethene derivatives, differs drastically. The synthesis of bulk quantities of a cyclocarbon remains a fascinating challenge in view of the expected instability of these compounds. These compounds would represent a fourth allotropic form of carbon, in addition to diamond, graphite, and the fullerenes. The full spectral characterization of macroscopic quantities of cyclo-C should provide a unique experimental calibration for the power of theoretical predictions dealing with the electronic and structural properties of conjugated n-chromophores of substantial size and number of heavy atoms. We believe that access to bulk cyclocarbon quantities will eventually be accomplished by controlled thermal or photochemical cycloreversion reactions of structurally defined, stable precursor molecules similar to those described in this review. 

Raman spectroscopy A nondestructive method for the study of the vibrational band structure of materials, which has been extensively used for the characterization of diamond, graphite, and diamond-like carbon. Raman spectroscopy is so far the most popular technique for identifying sp bonding in diamond and sp bonding in graphite and diamond-like carbon. 

It is outside the scope of this Chapter to undertake a comprehensive review of structure-property relationships for the different forms of carbon. However, a limited comparison of properties is useful for illustrating the influence of chemical bonding upon the properties of diamond, graphite and Buckminsterfullerene, Qo, Table 4. Carbynes are omitted from the comparison since insufficient is known of their properties. 

Carbon nanotubes (CNTs) constitute a nanostructured carbon material that consists of rolled up layers of sp2 hybridized carbon atoms forming a honeycomb lattice. After diamond, graphite and fullerenes, the one-dimensional tubular structure of CNTs is considered the 4th allotrope of carbon (graphene is the 5th). 

Many elements can give rise to more than one elementary substance. These may be substances containing assemblages of the same mono- or poly-atomic unit but arranged differently in the solid state (as with tin), or they may be assemblages of different polyatomic units (as with carbon, which forms diamond, graphite and the fullerenes, and with sulfur and oxygen). These different forms of the element are referred to as allotropes. Their common nomenclature is essentially trivial, but attempts have been made to develop systematic nomenclatures, especially for crystalline materials. These attempts are not wholly satisfactory. 

Elementary carbon exists as the allotropes diamond, graphite and the recently characterized series of cluster molecules known as fullerenes, e.g. 

It exists in two cryst forms diamond graphite and in various("amorphous ) forms, such as carbonblacks (acetylyne black, lampblack, etc) and charcoal. Soot Sc coke are impure amorphous carbon... 

Diamond, graphite, and fullerene are elemental forms of carbon which have very different properties but which are all solids at room temperature. Looking at the 3D models of diamond, graphite, and fullerene in eChapter 19.6, what intermolecular forces do you expect to find in each ... 

Three forms of carbon are diamond, graphite and Buckminsterfullerene. 

Until 1985, the only known elemental forms of carbon were diamond, graphite, and amorphous carbon. Then Kroto et al. announced the discovery of C6o, a spherical arrangement of carbon atoms in hexagons and pentagons, as shown in Figure 17.7. They called this form Buckminsterfullerene after the architect Buckminster Fuller, who developed the geodesic dome. The name for this type of carbon molecule has since been shortened to fullerene, but it is commonly called a buckyball. Since this first discovery, it has been found that fullerenes can be made in quantity from electrical arcs between graphite electrodes. About 75% of... 

Organic substances such as methane, naphthalene, and sucrose, and inorganic substances such as iodine, sulfur trioxide, carbon dioxide, and ice are molecular solids. Salts such as sodium chloride, potassium nitrate, and magnesium sulfate have ionic bonding structures. All metal elements, such as copper, silver, and iron, have metallic bonds. Examples of covalent network solids are diamond, graphite, and silicon dioxide. 

Amorphous carbon is a general term that covers non-crystalline forms of carbon such as coal, coke, charcoal, carbon black (soot), activated carbon, vitreous carbon, glassy carbon, carbon fiber, carbon nanotubes, and carbon onions, which are important materials and widely used in industry. The arrangements of the carbon atoms in amorphous carbon are different from those in diamond, graphite, and fullerenes, but the bond types of carbon atoms are the same as in these three crystalline allotropes. Most forms of amorphous carbon consist of graphite scraps in irregularly packing. 

Different forms of an element with different properties. For example, diamond, graphite, and fullerenes are different allotropic forms of elemental carbon, (p. 737)... 

There are three common forms of carbon diamond, graphite, and amorphous carbon. All three are important for electrochemical applications. The ideal graphite structure consists of layers of carbon atoms arranged in hexagonal rings. Metallic properties are exhibited in the direction parallel to the layer planes, while semiconducting properties are exhibited in a perpendicular direction. Because of the anisotropic structure of graphite, it indeed possesses anisotropic properties. 

In the solid state, three allotropes of carbon (diamond, graphite, and fullerene Fig. 1) are well-established. Synthetic approaches to other carbon allotropes, including poly-ynes, cvclo[ ]carbons and other carbon networks have been surveyed.1 The chemistry of fullerenes, the so-called third form of carbon, and of the closely related carbon nanotubes, has been extensively detailed.2 4 The molecule C2o may be the smallest fullerene, although other structures have been suggested (Fig. 2).5 It is highly reactive and has been produced in only miniscule amounts in a mass spectrometer from a highly brominated dodecahedrane. 

Diamond, graphite, and the fullerenes differ in their physical and chemical properties because of differences in the arrangement and bonding of the carbon atoms. Diamond is the densest (3.51 vs 2.22 and 1.72 g cm-3 for graphite and Cw, respectively), but graphite is more stable than diamond, by 2.9 kJ mol-1 at 300 K and 1 atm pressure it is considerably more stable than the fullerenes (see later). From the densities it follows that to transform graphite into diamond, pressure must be applied, and from the thermodynamic properties of the two allotropes it can be estimated that they would be in equilibrium at 300 K under a pressure of —15,000 atm. Of course, equilibrium is attained extremely slowly at this temperature, and this property allows the diamond structure to persist under ordinary conditions. 

Very large molecules, containing billions and billions of atoms, are called macromolecules. Diamond, graphite, and silica (sand) are examples (Figure 5.12). Formulas for macromolecules cannot state the number of atoms of each element... 

---