Parallel stacked carbon sheets

Figure 9. Organization of carbon sheets in nanocrystalhne carbons. Case (A) represent bent sheets which are stacked concentrically and randomly. In the high-resolution TEM of fullerene black filamentous structures from carbon macrocycles which did not react to fullerene molecules can also be seen (weak contrasts). Case (B) shows the arrangement of stacks of planar graphene units. Only few of these stacks are oriented with the o-axis parallel to the electron beam and can be imaged as illustrated in the sketch, the majority are randomly orientated and give hence an amorphous contrast. Case (C) is the same as (B) with the addition of covalent bonding interactions between the stacks. These bonds are invisible by electron microscopy but influence the graph-iti/ability of the carbon.

On the basis of the preceding discussion, we proposed the following formation mechanism of OMCs [166], At OOO C, the products of benzene pyrolysis are known to be polycyclic aromatic hydrocarbon species, such as biphenyl, m-, p-, and o-terphenyls, and anthracene [155]. Such species first adsorb on the pore surfaces of SBA-15. Subsequently, further aromatization and carbonization of the adsorbed species takes place to form the first layer of carbon film [167,168], A layer-by-layer stacking then follows to create stacked graphene sheets. Thns, the favored direction of stacked graphene sheets is parallel to the long axis of the nanorods. Carbon deposition in the pores of the mesoporous silica template will stop when the pore size becomes smaller than the kinetic diameter of the benzene... 

Powder X-ray diffraction and SAXS were employed here to explore the microstructure of hard carbon samples with high capacities. Powder X-ray diffraction measurements were made on all the samples listed in Table 4. We concentrate here on sample BrlOOO, shown in Fig. 27. A weak and broad (002) Bragg peak (near 22°) is observed. Well formed (100) (at about 43.3°) and (110) (near 80°) peaks are also seen. The sample is predominantly made up of graphene sheets with a lateral extension of about 20-30A (referring to Table 2, applying the Scherrer equation to the (100) peaks). These layers are not stacked in a parallel fashion, and therefore, there must be small pores or voids between them. We used SAXS to probe these pores. 

Oberlin and coworkers [42, 90] (Fig. 2.7c) used a crumpled sheets of paper model to describe the nanotexture of porous carbons. Figure 2.19 depicts this model and includes an enlarged sketch of pores in sucrose-based carbons [91]. In Fig. 2.19a, each zone of molecular orientation is represented by a shaded area inside of which each individual stack is shown either as a rectangular unit or as two parallel segments depending on whether they are observed in perspective... 

The plane electrodes are separated by isolating spacers, which may lead to the formation of parallel flow channels. In any case, the electrodes are plane sheets which can be replaced and thus made out of any plain material, e.g. nickel, lead, glassy carbon or graphite. Recent technolo cal developments made at the Institute of Microtechniques, Mainz [6, 7], have led to the construction of versatile microchannel electrochemical reactors. Indeed, the pressure can be elevated to up to 35 bar and the electrodes can be stacked in order to increase the overall electrode area. Moreover, polymer electrolyte membranes can be inserted, separating anodic and cathodic compartments if necessary, and finally heat exchangers may be integrated. 

Fig. 4 shows an HRTEM overview of the fresh nanotube material. The comparative HRTEM characterization of the fresh and tested catalysts, shown in Fig. 5, indicates that some modifications of the carbon nanotube surface occurred during the reaction (Fig. 5, a and b). Fig. 5, a shows that the walls of the carbon nanotubes before the reaction consisted of two layers, an inner layer formed by conical graphene sheets and a thick outer layer of amorphous carbon parallel to the tube axis. After 20 hours time on stream in the ODH, the stacks of the inner, conical graphene layers can now be recognized well. The thick, outer... 

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