Graphite structural forms

Fig. 4. Onion-like graphitic particles formed by three concentric layers (C o, C240, Cs4o) polyhedral (marked P) and spherical (marked S) structures. For clarity, only a half pan of each shell is shown.

Figure 1 shows some of the typical structural parameters of CNT generated in our laboratory. On the average, we can think that an MWCNT is a 1 micron long structure formed by about 10-15 concentric graphitic cylinders, the external one with a diameter of 12 nm, and the innermost tube has a diameter of about 2 nm. Hence, these tubes can be used as templates, the generated enclosed wires would have dimensions of a few nm in diameter when filled, or a few tens of nm when CNTs are covered with materials. 

The formation of fullerenes and CNTs has also been affected by their environmental atmosphere [22] and, in particular, a hydrogen atmosphere plays an important role in forming graphitic structures of multi-walled CNTs (MWCNTs) in the form of buckybundles [24]. Intercalation into MWCNTs has been difficult or impossible, because there is no space for intercalants to enter into a Russian-doll-type structure of the nanotubes. However, the buckybundles formed in the hydrogen arc discharge were found to be successfully intercalated with potassium and ferric chloride (FeCl3) without breaking the... 

CVD graphite can have several structural forms columnar, laminar, or isotropic. The columnar form is shown in Fig. 7.2. The crystallites are deposited with the basal planes (ab directions) essentially parallel to the deposition surface. Their structure tends to be columnar (cone-like) as a result of uninterrupted grain growth toward the reactant source. 

Fig. 3 shows the Raman spectra of the MWNT samples as a flmction of helium pressure. The peaks around 1280 cm", called the D-mode, are Imown to be attributed la amorphous carbons and defects of nanotubes, whereas the pe around 1600 cm", called the G-mode, are known to be due to the graphitic structure of carbon atoms. The G-mode of produced MWNTs was shifted to a lower wave number region (1595 cm" ) by the strain of the forming tube [6]. The intensity of MWNTs synftiesized under 250 Torr was lower than at other pressure. And the ratio of the G-mode to the D-mode was the hi t at pressure of 500 Torr. The highest purity of MWNTs was obtained when the pressure of helium is 500 Torr. 

Boron nitride may be obtained in three primary crystalline modifications (2) a, j3, and y. The most commonly encountered a form has a graphitic structure (hexagonal cell, a = 2.504 A, c = 6.661 A). For many years, this modification has been prepared from combinations of cheap boron and nitrogen containing reagents, e.g. B(0H)3 and (NH2)C0, B(0H)3, C and N2 or KBH4 and NH4C1 (3-5). More... 

Partly disoriented layers of graphite were formed when each of these faces was dosed with sufficient benzene and annealed at temperatures between 375° and 425°C. At higher temperatures, the graphitic and other structures broke down and carbon diffused into the bulk of the metal crystal. 

It should be noted here, that not only the (chemical and morphological) composition of the protective layers at the basal plane surfaces and prismatic surfaces is different, but that these layers also have completely different functions. At the prismatic surfaces, lithium ion transport into/ffom the graphite structure takes place by intercalation/de-intercalation. Here the formed protective layers of electrolyte decomposition products have to act as SEI, i.e., as transport medium for lithium cations. Those protective layers, which have been formed on/at the basal plane surfaces, where no lithium ion transport into/from the graphite structure takes place, have no SEI function. However, these non-SEI layers still protect these anode sites from further reduction reactions with the electrolyte. 

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