PHYSICAL PROPERTIES OF GRAPHITE

The properties of the ideal graphite material, that is a material that most closely corresponds to an infinitely large graphite crystal, are reviewed in this section. Such a material does not exist in the real world and the properties given below are either calculated or based on the actual properties of graphite crystals closely approaching this ideal structure. 

As already mentioned and as will be seen in later chapters, a wide range of materials comes under the heading of carbon or graphite eind these materials often have properties that are much different from those of the ideal graphite crystal. Obviously it is necessary to define the material accurately when speaking of the properties of carbon or graphite . 

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Experimental results are presented that show that high doses of electron radiation combined with thermal cycling can significantly change the mechanical and physical properties of graphite fiber-reinforced polymer-matrix composites. Polymeric materials examined have included 121 °C and 177°C cure epoxies, polyimide, amorphous thermoplastic, and semicrystalline thermoplastics. Composite panels fabricated and tested included four-ply unidirectional, four-ply [0,90, 90,0] and eight-ply quasi-isotropic [0/ 45/90]s. Test specimens with fiber orientations of [10] and [45] were cut from the unidirectional panels to determine shear properties. Mechanical and physical property tests were conducted at cold (-157°C), room (24°C) and elevated (121°C) temperatures. 

W. N. Reynolds, Physical Properties of Graphite, Elsevier Publishing Co. Inc., New York, 1968. 

The sheets, however, are 3.4 A apart, a much greater distance. This is as if the valence bonds acted wholly within the separate sheets, and only Van der Waals forces, or similar weak forces, held the sheets together and were not able to pull them very close together. This impression is made stronger by the physical properties of graphite. It is a very soft material... 

Table 8.2. Some physical properties of graphite and diamond.

Graphite is used as a lubricant and as the lead in pencils. The enormous differences in physical properties of graphite and diamond—both of which are pure carbon—arise from differences in their three-dimensional structure and bonding. 

Figure 2.16 Rhombohedral unit cell structure of graphite. Source Reprinted from Reynolds WN, The Physical Properties of Graphite, Elsevier, 1968.

Table 6. Some Physical Properties of Graphite Cloth Laminates Based on Resin E...

The physical properties of graphite eire summarized in Table 3.1. It should be stressed that to obtain accurate measurements of the properties of materials much above 3000 K is a trying task. In the case of graphite, many of these measurements are based on carbon-arc experiments which are difficult to perform and interpret. The results must be viewed accordingly and some of these results and conclusions are still controversial. 

Figure 12.11 (a) A diamond. The physical properties of a diamond include a density of 3.5 g/cm lack of color, and extreme hardness, (b) Graphite. The physical properties of graphite include a density of 2.3 g/cm, black in color, and relative softness. These two forms of carbon are physically very different. 

Figure 12.13 A particulate-level model of graphite. Each carbon atom is bonded to three other carbon atoms in a flat sheet. Very weak chemical forces loosely hold the sheets together until an external force peels them apart. This bonding arrangement leads to the characteristic physical properties of graphite at the macroscopic level. 

It is apparent in Figure 10.1a that graphite has two distinct surfaces present, the basal plane and the edge sites. Furthermore, the physical properties of graphite are highly anisotropic because of this crystallographic structure. For instance, the electrical conductivity in the direction parallel to the basal plane is about 100 times higher than in the perpendicular direction. 

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