Other Graphitization Processes

The Other graphitization processes transfer the heat indirectly to the carbon articles, which makes continuous operation easier. 

Heating can be carried out by induction in which the furnace is surrounded by an induction coil and the dimensions of the article and the coil have to be precisely correlated with one another. Alternatively heating takes place by radiation, which is more flexible. 

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Fig. 5.28 The principle behind the ab initio calculation of heat of formation (enthalpy of formation) using an isodesmic reaction. Methanol and hydrogen are (conceptually) made from methane and water (other isodesmic reactions could be used) the 0 K enthalpy input for this is the ab initio energy difference between the products and reactants. Graphite, hydrogen and oxygen are converted into methane and water and into methanol and hydrogen, with input of the appropriate heats of formation. The heat of formation of methanol at 0 K follows from equating the heat of formation of methanol with the sum of the energy inputs for the other two processes. The diagram is not meant to imply that methanol necessarily lies above its elements in enthalpy...

The past decade has led to the detection of new carbon allotropes such as fullerenes26 and carbon nanotubes,27 28 in which the presence of five-mem-bered rings allows planar polycyclic aromatic hydrocarbons to fold into bent structures. One notes at the same time that these structures are not objects of controlled chemical synthesis but result from unse-lective physical processes such as laser ablation or discharge in a light arc.29 It should be noted, on the other hand, that, e.g., pyrolytic graphitization processes, incomplete combustion of hydrocarbon precursors yielding carbon black, and carbon fibers30 are all related to mechanisms of benzene formation and fusion to polycyclic aromatic hydrocarbons. 

There are good reasons to suppose that a similar phenomenon of fluidization of nanoparticles of the same catalytically active metals may happen to some other catalytic processes when they are accompanied by the formation of graphitized carbon. Examples of such processes are catalytic pyrolysis of methane... 

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). 

For other irreversible processes it is possible to reverse the electrode reaction but only at much increased potentials. Hydroxylamines undergo irreversible reduction to amines. At the potential of the reduction wave, the rate of the back reaction (reoxidation of the amine) is negligible. However at much more anodic (positive) potentials a solution of the amine can undergo oxidation. This oxidation is also an irreversible process and at these more anodic potentials rereduction of the hydroxylamine back to amine has become negligible. Thus the cathodic wave (reduction of hydroxylamine to amine) and anodic wave (reoxidation of the amine) occur at widely separated potentials. At each wave the appropriate back reaction rate is negligible. In both cases the potential of the wave is determined by the initial species only and not by the electrode product. (The oxidation of the amine in fact usually occurs at potentials too anodic to be achieved at a dropping mercury electrode and requires a graphite electrode to be seen.)... 

Invention describes the use of plasticizer to dissolve resin. A solution of resin is mixed with graphite to form a mass, which, after the process of calendering, forms a film or a sheet. The formed material is subjected to a temperature sirfficiently high to remove the plasticizer by evaporation (250°C). After the plasticizer (diallyl phthalate) is removed a sheet is subjected to temperatures between 500 and 1000°C imder nitrogen atmosphere which decomposes the resin and leaves a fully carbonaceous diaphragm. Other similar processes were discussed in Chapters 11 and 13. In many cases, the plasticizer is also removed by extraction with a suitable solvent. 

Our previous papers [15,16] and the current work show that die imprinting of mesophase pitch particles with colloidal silica is an efficient technique to prepare mesoporous carbons with uniform spherical pores as well as carbons with bimodal pore size distributions. These carbons exhibit negligible amount of micropores, which can be further eliminated during graphitization process. If micropores are need, they can be created by controlled oxidation analogous to that used in the preparation of activated carbon fibers. The possibility of tailoring the size of uniform spherical mesopores is of great importance for catalysis, adsorption and other advanced applications such as die manufacture of hi -quaiity electrochemical double-layer capacitors, fuel cells and lidiium batteries. 

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. 

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