STRUCTURE OF PYROLYTIC GRAPHITE

Figure 2.29 Effect of substrate defect on deposited structure of pyrolytic graphite. Source Reprinted with permission from Campbell J, Sherwood EM, High Temperature Materials and Technology, John Wiley, New York, 1967. Copyright 1967, The Electrochemical Society, Inc.

Columnar Structure. The columnar structure of pyrolytic graphite is shown in Fig. 7.4. The crystallites are deposited with the basal planes (ab... 

Figure 7.6. Effect of gas-phase nucleated impurities on deposited structure of pyrolytic graphite.l ...

Laminar Structure. The laminar structure of pyrolytic graphite consists of essentially parallel layers (or concentric shells if deposited on a particle or fiber). It is shown in Fig. 7.7. 

Figure 7.7. Laminar structure of pyrolytic graphite, deposited on carbon filament (Photograph courtesy of Jack Chin, La Costa, CA.)...

Hardness. Being composed of minute crystallites with essentially random orientation, isotropic pyrolytic carbon lacks the easy interlayer slippage which is characteristic of the well-oriented laminar or columnar structures of pyrolytic graphite. As a result, it is considerably harder. This makes it easy to polish and the material can be given a high gloss. The wear resistance is usually superior to that of the columnar and laminar deposits of vitreous carbon. PS)... 

The structure of pyrolytic graphite also accounts for the fact that this material can exhibit tensile strengths of 20,000 psi in the a-b direction, while in the c direction the tensile strength is about 1500 psi. As with other graphites, the pyroljrtic form displays higher strengths at elevated temperatures, does not melt under normal pressures and sublimes above 3500°C. 

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). 

X-ray diffraction analysis of the samples is performed on a DRON-4 apparatus with Cr Ka radiation. As a monohromator, we applied a crystal of pyrolytic graphite. The carbon structure morphology is investigated with a REM-200 electron microscope. The infrared spectra of the optical transmission of the pressed sample tablet in KBr are measured on a Specord M80 spectrophotometer. 

Carbons exhibit a low electrocatalytic activity for the hydrogen electrode reaction (HER). Structural characteristics have significant electrocatalytic effects on the HER as changes from 2 X 10 to 2.5 x 10 A/cm on going from the basal plane to the side face of pyrolytic graphite. On glassy carbon, the HER overpotential decreases as the pretreatment temperature is increased. This thermal treatment leads to stmctural and chemical transformations from carbonization, precrystaUization, and to graphitization. 

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

Recently, a method was described for the real-time measurement of growth rates and feedback control of three-dimensional laser assisted chemical vapor deposition [11]. This method allows the accurate reproduction of high quality films, fibers, and three-dimensional structures. High aspect ratio axisymmetric forms of desired shape and microstructure were grown from vapor phase precursors by this method. Three-dimensional rods, cones, hyperboloids, and spheroids of pyrolytic graphite, nickel, iron, and nickel-iron superalloys were obtained from ethylene, nickel tetracarbonyl, iron pentacarbonyl, and mixtures of nickel and iron carbonyls, respectively. 

Blackman LC, Saunders G, Ubbelohde AR, The structure and mechanism of pyrolytic graphite, Proc Roy Soc, A264 19, 1961. 

Because of its random structure, vitreous carbon has properties that are essentially isotropic. It has low density and a uniform structure which is generally free of defects. Its hardness, specific strength, and modulus are high. Its properties (as carbonized and after heat-treatment to 3000°C) are summarized in Table 6.2.1 i The table includes the properties of a typical molded graphite and of pyrolytic graphite for comparison (see Chs. 5 and 7). The mechanical properties of vitreous carbon are generally higher and the thermal conductivity lower than those of other forms of carbon. 

Thermal Expansion. The thermal expansion of pyrolytic graphite, like that of the ideal graphite crystal, has a marked anisotropy. It is low in the ab directions (lower than most materials) but an order of magnitude higher in the c direction. The effect of temperature is shown in Fig. 7.10. Such a large anisotropy may lead to structural problems such as delamination between planes, especially in thick sections or when the material is deposited around a sharp bend. 

Unlike the well-ordered parallel planes of pyrolytic graphite which closely match the structure of the graphite crystal, the structure of PAN-based carbon fibers is essentially turbostratic and is composed of small two-dimensional fibrils or ribbons. These are already present in the precursor and are preferentially aligned parallel to the axis of the fiber. The structure may also include lamellas (small, flat plates) and is probably a combination of both fibrils and lamellas. ... 

The formation of pyrolytic graphite directly from a hydrocarbon gas results in a structure which is distinctly different from other commercial forms of this element. While polycrystalline graphites consist of at least two solid phases (binder and filler) and are porous to varying degrees, pyrolytic graphite consists of a single phase (no binder) and is essentially impervious to gases. 

With this layered structure, the thermal and electrical conductivity of pyrolytic graphite is very high parallel to the planes but much lower in the direction perpendicular to the planes. This directionality is so great that a well-ordered pyrolytic graphite may have a thermal conductivity equal to copper in the planar (a-b) direction, while it is essentially an insulator in the direction perpendicular to the planes (the c direction). Advantage is taken of this anisotropy when pyrolytic graphite is employed for rocket nozzles, missile nose cones and other applications where conductive anisotropy is desirable. 

The commercially available forms of pyrolytic graphite include plate stock, tubes and free-standing shapes. In addition, pyrolytic graphite is used commercially to densify porous structures such as carbon felts, fabrics, composites of carbon/graphite yams and that can withstand the deposition temperature and are compatible with pyrolytic graphite. It also is used to coat conventional (bulk) graphites where increased resistance to oxidation and chemical attack are desired. 

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