THE GRAPHITIZATION PROCESS

During the graphitization process, the crystal size (both the Lc and La values) increases from about 5nm at 1200°C to the order of lOnm at 2000°C and to more than lOOnm at higher graphitization temperatures. 

Surface area and porosity also change as the graphitization process pro-ceeds and physical adsorption of gases and liquids onto different carbons has been of major interest for many years. It is not the purpose of this article to discuss the wealth of data that exists except in so far as it pertains to the present topic. In this connection, it should be pointed out that surface area and porosity of carbons can be altered significantly by various treat-... 

A review of the graphitization of carbons, containing 183 references, has been published.20 The influence of doping by boron on the graphitization of carbons has been the subject of an exhaustive investigation 21 a new mechanism for the graphitization process has been developed.21... 

Materials with carbon on the surface used as column packings for chromatography include carbosils, which are prepared mainly by pyrolysis of aliphatic alcohols [92], aromatic hydrocarbons [93], chloroalkanes [94], and other organic compounds on the surface of silica gel. The carbon deposit obtained under the standard pyrolysis conditions is amorphous and possesses different physicochemical properties than graphitized carbon blacks. The transformation of such amorphous carbon deposit to graphitized carbon black on a silica gel surface is not possible because of the high temperature of the graphitization process (ca.3000 K). 

Figure 7.15 Various steps in the graphitization process as a function of heat treatment temperature Tht Source Reprinted with permission from Goma J, Oberiin A, Thin solid films, 65, 221, 1980. Copyright 1980, Elsevier.

Kimura [37] selected three kinds of thermosetting resins—furan, diphenylether-formaldehyde and polyimide resins—as matrix precursors to fabricate carbon fiber reinforced carbon composites (C/C composites). After heat treatment at 2000-3000°C, the graphitization process of the matrix was examined by optical microscopy and X-ray diffraction. In the C/C composite derived from a polyimide, the graphite structure was not as well developed as the others. This retarded development is attributed to less internal stress between fibers and matrix as well as to less stretching of the matrix. 

Sample Analysis and Interpreting the Results AMS reports results in Modern, which is a defined unit of C/C isotope concentration equal to 1.18 X 10 C/C. Conversion of this number to a meaningful result requires careful inventory of the carbon sources in the sample. For the simplest case in which a sample contains sufficient carbon so as not to require the addition of carrier carbon, this conversion is facile. In many experiments, it is necessary to add carrier to make up the total amount of carbon necessary for the graphitization process to occur. In these experiments, the fraction Modern and the carbon mass of both the sample and the carrier must be considered in the calculation. Correctly accounting for all sources of carbon in the analyzed sample is essential, as a low estimate of carbon will artificially increase the apparent quantity of tracer, and an overestimate of carbon will deflate the apparent amount of tracer present. 

Fillers. The filler is usually selected from carbon materials that graphitize readily. As mentioned in Ch. 4, such materials are generally cokes, also known in industry as "soft fillers . They graphitize rapidly above 2700°C (the graphitization process is described in Sec. 2.4 below). Other major fillers are synthetic graphite from recycled electrodes, natural graphite, and carbon black (see Ch. 10). 

Even though dgQ2, Lc, and La are not adequate quantitative measures of the graphitization process, it is tempting to establish correlations among some of these parameters. One of the first correlations, abusively generalized and too often used and accepted without verification, is a relation between Pi and dgQ2 [69,70]. 

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