Properties of pyrolytic graphite

The properties of pyrolytic graphite are summanzed in Table 7.2. The values listed were collected from the manufacturer s data sheets.P i i The spread in value may represent slightly different materials and differences in the degree of graphitization. It may also reflect variations in the test methods for instance, the measurement of mechanical properties may vary widely depending on the sample geometry and the test method (see Ch. 5, Sec. 2.1). 

Mechanical Properties. The mechanical properties of pyrolytic graphite are like those of the ideal graphite crystal In the sense that they show a marked increase with increasing temperature as shown in Fig. 7 9 [iij[26i Above 2600°C, the strength drops sharply. 

Electrical Properties. The electrical properties of pyrolytic graphite also reflect the anisotropy of the material and there is a considerable difference between the resistivity in the ab and the c directions. Pyrolytic graphite is considered to be a good electrical conductor in the ab directions, and an insulator in the c direction. Its electrical resistivity varies with temperature as shown in Fig. 7.11. 

At the present, CVD diamond is produced in the form of coatings. In this respect, it is similar to pyrolytic graphite (see Ch. 7, Sec. 1.3). These coatings can alter the surface properties of a given substrate as shown in Table 13.2. Thistable may be compared to Table 7.1 of Ch. 7 in which the critical properties of pyrolytic graphite coatings are listed. 

Deposition, Structure, and Properties of Pyrolytic Carbon, J. C. Bokros The Thermal Conductivity of Graphite, B. T. Kelly... 

Pyrolytic graphite is a highly anisotropic material, which means that its physical properties are different in different crystal directions. In a parallel direction to the tube surface the heat conductivity of pyrolytic graphite is about 300 times higher than in the vertical direction to the surface. This means that heating along the tube is rapid, whereas heat conduction outwards is poor. 

The properties of pyrolytic carbon and graphite are given in Table 2.7. The pyrolytic carbon is much harder, stronger, less permeable to gases and can be polished. Consequently,... 

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. 

Table 7.1 summarizes the surface properties that may be obtained or modified by the use of pyrolytic graphite coatings. 

Electrical Resistivity. Like the thermal properties, the electrical resistivity of carbon fibers, measured along the axis, is similar to that of pyrolytic graphite in the ab direction and approximately an order of magnitude higher than metal conductors such as aluminum or copper, as shown in Table 8.11. 

Mechanical Properties. The hexagonal symmetry of a graphite crystal causes the elastic properties to be transversely isotropic ia the layer plane only five independent constants are necessary to define the complete set. The self-consistent set of elastic constants given ia Table 2 has been measured ia air at room temperature for highly ordered pyrolytic graphite (20). With the exception of these values are expected to be representative of... 

Wei and Robbins [10] have reviewed much of the work performed on the thermal physical properties of CBCF. Fhe emissivity parallel to the fibers was 0.8 over the temperature range from 1000 to 1800 °C. This value is higher than the emissivity of c-direction pyrolytic graphite (0.5-0.6), but is close to values for graphite and dense carbon-carbon composite (0.8-0.95). 

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. 

Savinova ER, Lebedeva NP, Simonov PA, Kryukova GN. 2000. Electrocatalytic properties of platinum anchored to the surface of highly oriented pyrolytic graphite. Russ J Electrochem. 36 (9) 952-959. 

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