Pyrolytic graphite structure

Figure 56 Photograph of pyrolytic graphite structure on a graphite substrate (Unicam Analytical Systems Ltd.)...

Graphite is commonly produced by CVD and is often referred to as pyrolytic graphite. It is an aggregate of graphite crystallites, which have dimensions (L ) that may reach several hundred nm. It has a turbostratic structure, usually with many warped basal planes, lattice defects, and crystallite imperfections. Within the aggregate, the crystallites have various degrees of orientation. When they are essentially parallel to each other, the nature and the properties of the deposit closely match that of the ideal graphite crystal. 

The deposition of pyrolytic graphite in a fluidized bed is used in the production of biomedical components such as heart valves, ] and in the coating of uranium- and thorium-carbides nuclear-fuel particles for high temperature gas-cooled reactors, for the purpose of containing the products of nuclear fission.fl" The carbon is obtained from the decomposition of propane (CgHg) or propylene (CgHg) at 1350°C, or of methane (CH4) at 1800°C. Its structure is usually isotropic (see Ch. 4). 

In a subsequent study (S9), isotherms of bromine on pyrolytic graphite showed the presence of several phases C4 Br (n = 2 to 5). X-ray studies confirmed these to be stages 2 to 5, respectively. At intermediate concentrations, X-ray patterns showed mixtures of higher and lower stages. The density and configuration of intercalated bromine molecules were believed to be the same in all stages. Other structural types... 

It has been recently demonstrated that the simplest of the cobalt porphyrins (Co porphine) adsorbed on a pyrolytic graphite electrode is also an efficient electrocatalyst for reduction of 02 into 1120.376 The catalytic activity was attributed to the spontaneous aggregation of the complex on the electrode surface to produce a structure in which the cobalt-cobalt separation is small enough to bridge and activate 02 molecules. The stability of the catalyst is quite poor and largely improved by using porphyrin rings with mew-substitu-tion.377-380 Flowever, as the size of the mew-substituents increases the four-electron reduction efficiency decreases. 

An example is represented by the reduction of cytochrome c using a pyrolytic graphite electrode suitably prepared. In fact, as schematised in Figure 6, this type of carbon material (briefly alluded to in Chapter 3, Section 1.1) has a solid state structure fairly close to the ideal structure of graphite. 

It is interesting to note that the plastocyanin from the alga Anabaena variabilis has a similar structure to that isolated from poplar, while having a different amino acid sequence.58 This results in an overall positive charge of +2. Thus, as shown in Figure 32a, it affords a wellshaped response for the Cu(II)/Cu(I) reduction E° = + 0.32 V vs. NHE) using a perpendicular pyrolytic graphite electrode, without the presence of additives. 

Carbon, Carbides, and Nitrides. Carbon (graphite) is a good thermal and electrical conductor. It is not easily wetted by chemical action, which is an important consideration for corrosion resistance. As an important structural material at high temperature, pyrolytic graphite has shown a strength of 280 MPa (40,600 psi). It tends to oxidize at high temperatures, but can be used up to 2760°C for short periods in neutral or reducing conditions. The use of new composite materials made of carbon fibers is expected, especially in the field of aerospace structure. When heated under... 

Electron Diffraction and Electron Microscopy. A limited amount of information regarding graphite structure has been obtained by the use of electron beams. Grisdale (27) has measured the degree of orientation using electron diffraction methods on films of pyrolytic carbon deposited on a silica surface under a variety of conditions. Oxidation of the graphite causes an increase in the degree of orientation. 

Figure 10.3 Structure of highly ordered pyrolytic graphite.

Cleavage of bulk crystals to expose clean, defined lattice planes is possible for brittle substances. Some materials, like mica or highly oriented pyrolytic graphite (HOPG) that exhibit a layered structure, are readily cleaved by just peeling off some layers or using a razor blade. For other materials the so-called double-wedge technique can be used (Fig. 8.8). 

V. Brabec and G. Dry hurst, Electrochemical behaviour of natural and biosynthetic polynucleotides at the pyrolytic graphite electrode. A new probe for studies of polynucleotide structure and reactions, J. Electroanal. Chem., 89 (1978) 161-173. 

---