Carbon modification processes

A characteristic feature of the carbon modifications obtained by the method developed by us is their fractal structure (Fig. 1), which manifests itself by various geometric forms. In the electrochemical cell used by us, the initiation of the benzene dehydrogenation and polycondensation process is associated with the occurrence of short local discharges at the metal electrode surface. Further development of the chain process may take place spontaneously or accompanied with individual discharges of different duration and intensity, or in arc breakdown mode. The conduction channels that appear in the dielectric medium may be due to the formation of various percolation carbon clusters. 

Modification of porous inorganic materials by carbon makes it possible to obtain porous carboniferous composites with high thermal and chemical stability and strength. To introduce carbon into pores, both gas phase pyrolysis and carbonization through thermochemical solid-phase reactions are employed. The formation of carbon structures depends on carbonization conditions process rate, precursor concentration, presence of catalyst, etc. [1-3]. Phenolic resins, polyimides, carbohydrates, condensed aromatic compounds are most widely used as polymeric and organic precursors[4-6]. 

The modification in structural characteristics of the filaments was accompanied by an appreciable difference in their respective surface areas. The filaments produced from the interaction of CO/H2 (4 1) with pure iron had a BET surface area at -196°C of 64 m2/g, whereas those obtained from the sulfur treated metal had a value of 166 m2/g. From these observations we may conclude that in addition to exerting an influence on the chemistry occurring at the metal/gas interface, sulfur also has an impact on the events surrounding the carbon deposition process taking place at the rear of the catalyst particle. 

Foamed carbon is also nongraphitizable. The cell structure of the polymer foam remains intact upon careful carbonization and densities lower than 0.1 g/cm- are obtainable. The thermal conductivity is just above that of plastic foams, but foamed carbon can be used at much higher temperatures. Its relatively low compressive strength can be increased by impregnation with pyrolytic carbon, although the thermal conductivity increases at the same time. By comparison with glassy carbon, foamed carbon is easy to work, so that the shape does not have to be established at the start of the process. Foamed carbon is corrosion resistant, as are all carbon modifications. 

THE COMPOSITIONS MODIFICATION PROCESSES BY METAL/CARBON NANOCOMPOSITES... 

More recently, the need for low hysteresis compounds has reactivated chemical modification studies. Many modification processes have been proposed functionalization (Tsubokawa and Hosoya, 1991), surface coating of carbon black by silica (Wang et al., 1997), and alumina (EP1034222 et al., 1997). 

The carbon-fiber tip modification process consists of poly-(TMHPP)Ni film deposition on the electrode surface and confirmation of deposition followed by demetalation to poly-TMHPP and confirmation of demetalation. Polymeric film is deposited from a 0.25 mM (TMHPP)Ni solution in 0.1 M sodium hydroxide by constant-potential electrolysis at 0.70 V. The number of monolayers deposited is dependent on the initial concentration of (TMHPP)Ni and the time of electrolysis. At the end of the deposition time, the electrode is immersed in 0.1 M sodium hydroxide. The presence of (poly-TMHPP)Ni film on the electrode surface is confirmed by a peak at Ep = 0.55 V, attributable to the Ni(II)/Ni(III) couple, in a differential pulse voltammetry scan. The (poly-TMHPP)Ni film is demetalated in a chemical process by placing the electrode in 0.1 M HCl. 

Considerable effort has been dedicated to die investigation of different surface modification processes of metal and carbon electrodes in the construction of electrochemical enzyme biosensors with separate redox mediator and enzyme layers, see Fig. 6.4. 

Merz, L. and Muth, O. (1996) Solubility, extraction and modification of polymers and polymer additives in supercritical carbon dioxide. Process Technol Proc. 12,373-78. Rindfleisch, F., DiNoia, T. and McHugh, M. (1996) Solubility of Polymers and Copolymers in Supercritical C02, J. Phys. Chem. 38, 15581-87. 

Modification procedures for carbon electrodes are quite diverse. For convenience, in this chapter, they are broadly divided into two parts covalent and non-covalent modifications. It is worth mentioning that this chapter is not intended to cover the entire hterature on carbon modification, instead the focus is given on basic features of each process of modification and how it changes the nature of carbon electrode surfaces. From the wide variety of applications of modified electrodes, a few selected articles are discussed in short at the end of each modification process to demonstrate its utility. 

The classical modification processes for solid homogeneous electrodes are (i) film and (ii) membrane covering, as well as (iii) adsorption, and (iv) covalent attachment immobilization of the modifier. These avenues are also open to modify carbon paste as the most popular representative of heterogeneous electrode materials. But, because of its composite character, it facilitates simpler ways of modification by direct addition of the modifier to the paste either during or after the preparation of the material. The term direct mixing was coined by Baldwin [106], though the history of modification of CPEs dates back to the mid-1960s, when Kuwana was the first who added electroactive components to the paste [107]. (In his case, however, there had been no intention to alter the CPE characteristics, and the purpose was to study the redox behavior of the adduct in a nonaqueous environment.)... 

Keita and Nadjo discovered that, whatever the oxometalate in acidic solution, setting a potential of —1.2V versus SCE at the working electrode brings about a persistent modification of the surface [125-142]. Typically, the evolution of the electrochemical behavior of a glassy carbon electrode used for such an electrolysis in the presence of [PWi204o] in 0.5 M H2SO4 are sketched in Fig. 5. As a remarkable observation, Fig. 5(b), recorded on a much less sensitive scale than that used for Fig. 4(a), shows that the voltammetric pattern is now dominated by the HER. During the modification process and after its completion, the heteropolyanion waves that are not obscured by the HER retain their initial peak current intensities. The conclusion is that the enhanced kinetics for the HER is not due primarily to the surface increase. 

Figure 8. Illustration of severe carbon corrosion process [27]. Reprinted from Carbon 35, Pittman, Jr., C. U. He, G. R. Wu, B. Gardner, S. G., Chemical modification of carbon fiber surfaces by nitric acid oxidation followed by reaction with tetraethylenepentamine, 317-331, Copyright (1977), with permission from Elsevier.

In our work we used new modifiers in order to improve polymer-fiUer interactions. The modification process was carried out in Brabender mixer. During this process the coupling agent was grafted on the polymer macromolecule and then the reaction with chemical groups on the carbon black surface took place [5]. 

The following is a modification of the process described and gives quite satisfactory results. Wash the crude mixture of benzonitrile and dibromopentane with sodium carbonate solution until the latter remains alkaline, and then with water. Distil it under reduced pressure and collect the fraction boiling up to 120°/18 mm. Dissolve this in twice its volume of light petroleum, b.p. 40-60°, which has previously been shaken with small volumes of concentrated sulphuric acid until the acid remains colourless. Shake the solution with 6 per cent, of its volume of concentrated sulphuric acid, allow to settle, and run ofi the sulphuric acid layer repeat the extraction until the acid is colourless or almost colourless. Wash successively with water, sodium carbonate solution and water, dry over anhydrous calcium chloride or calcium sulphate, and distil off the solvent. Distil the residue under diminished pressure and collect the 1 6-dibromopentane at 98- 100°/13 mm. 

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