Graphite crystal characteristics

The n value changed from 1.4- to 2 with increasing HTT with a kink at 2200 C as shown in Fig. 9 Note that n = 1 for ordinary metal and graphite crystals. The characteristic feature (n > 1) can be understood if we assume that pyrolyzed POD is a two-dimensional conductor, for this T dependence has been encountered with two-dimensional organic metals (H) and graphite intercalation compounds (15). 

Chapters 2 and 3 were a review of the carbon atom and its bonding mechanisms and howthese atoms combine tofonn graphite crystals. Inthis and the next six chapters, the focus wiil be on how iarge numbers of these crystallites are combined to form synthetic (and naturai) carbon and graphite products. The various types of synthetic materials will be reviewed including their production processes, their properties and characteristics, and their present and potential applications. 

The molded graphites, reviewed in the previous chapter, are derived from precursors that graphitize readily, such as petroleum cokes and coal-tar pitch. They exhibit varying degrees of anisotropy and have characteristics and properties that, in some cases, can be very similar to those of the ideal graphite crystal. 

In most cases, the chemical properties of vitreous carbon are similar to those of the graphite crystal, reviewed in Ch. 3, Sec. 7. Since the material has low permeability, is essentially non-porous and free of surface defects, and can be made with very low impurities, its resistance to chemical attack is generally excellent and is one of its outstanding characteristics. In many instances, it is far more chemically resistant than other forms of carbon, such as molded or pyrolytic graphites. 

It is seen from Fig. 6.70 that the structures of activated carbon derived from different carbon materials are different. The activated carbon derived from coal has an obvious characteristic peak of graphite. From the crystal structure analysis of graphite, we can see that there is a hexagonal comby plane layer (A-B-A in Fig. 6.71) lattice structure formed via bonding the sp" hybrid orbit with three neighboring atoms. There is still one 2p electron in the 2p orbit in per carbon atom. These p orbits parallel each other and perpendicular to sp" hybrid orbital plane, and therefore form a big tt bond. Thus these tt electrons can move on throughout the whole carbon plane, which is similar to metallic bond. The interaction between carbon layers with horizontal structure via intermolecular force (van der Waals force) forms graphite crystal (Fig. 6.71). 

All nonmetals with a known high-thermal conductivity have either diamond-like, boron carbide, or graphite crystal structure. The fundamental characteristics for a crystal to exhibit... 

Jin x-ray monochromator. A monochromator is a large single crystal (usually graphite) that is oriented so that a very iatense reflection is directed toward the sample. AH wavelengths are absorbed by the monochromator except a small range of wavelengths used for the diffraction experiment. Usually only the characteristic radiation is used if an x-ray tube is the x-ray source. 

A number of chemical elements, mainly oxygen and carbon but also others, such as tin, phosphorus, and sulfur, occur naturally in more than one form. The various forms differ from one another in their physical properties and also, less frequently, in some of their chemical properties. The characteristic of some elements to exist in two or more modifications is known as allotropy, and the different modifications of each element are known as its allotropes. The phenomenon of allotropy is generally attributed to dissimilarities in the way the component atoms bond to each other in each allotrope either variation in the number of atoms bonded to form a molecule, as in the allotropes oxygen and ozone, or to differences in the crystal structure of solids such as graphite and diamond, the allotropes of carbon. 

The graphite lattice may show stacking faults or defects within the sheets, and, possibly, bending of the sheets (Fig. 2.25). Omission of a carbon atom (voids), or inclusions of noncarbon elements or molecules, disrupts the orderly configuration and inhibits crystallization of carbon as graphite. These impurities act as sites of local strain that directly influence crystallite size, distribution, and orientation within a sample, and in turn affect the physical and chemical characteristics of the material, especially its strength. 

Graphite is midway between a semiconductor and a metal in electrical characteristics. The resistivity is very sensitive to impurities, and therefore to crystal perfection, and is directional, being two to three times greater in the basal plane than perpendicular to the plane. Properties measured in the hexagonal plane are independent of the direction within the plane, probably... 

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