Graphite general physical properties

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 packing arrangement of atoms or molecules in a crystalline solid phase is generally not unique, and for organic molecules in particular, it is common for two or more crystalline forms of the same substance to exist. The most familiar example in elemental terms is Graphite and Diamond. Both are composed entirely of the element Carbon, however their ciystal structures are very different, and so too are their physical properties. Calcium Carbonate is another common example with three polymorphic forms Calcite, Aragonite and Vaterite. 

In formulating reinforced thermoplastics, glass fibers with lengths less than 1/4 in. are generally used. Commonly, milled fibers are selected. The fibers improve the physical properties of the base resin, in particnlar the heat-deflection temperatnre. Some thermoplastics are reinforced with graphite fibers to give electromagnetic interference protection. Aramid fibers with thermoplastics resnlt in excellent wear and abrasion resistance. 

Natural graphite is classified into three general types flake (also known asp/umbago), crystalline (vein), and amorphous, varying in physical properties, appearance, chemical composition, and impurities. These differences stem from the type of precursor material (oil, coal, or other carbonaceous deposits) and the natural process by which graphite was formed. Table 10.2 summarizes the characteristics ofthesethreetypes.I lPl... 

Carbon-derived powders and particles comprise a family of synthetic materials, known under the generic term of carbon black, made by burning hydrocarbons in insufficient air. Carbon-black particles are aggregates of graphite microcrystals, each only a few unit cells in size and so small that they are generally not detectable by diffraction techniques. The physical properties of these materials are essentially determined by the nature and extent of their surface areas. 

Table 10.1 summarizes some of the sahent physical properties of diamond and graphite as they will be needed in the remainder of this chapter. We compare them with those of silicon as the closest Idn to diamond. Some of the outstanding properties of diamond such as its superior hardness, chemical inertness, and high thermal conductivity are generally known nevertheless, it is instructive to briefly review these and other diamond properties in context. 

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