Graphitic order

Carbonaceous materials with varying degree of graphitic order are the most common commercial anodes in secondary lithium-ion batteries. Among carbon-based materials, natural graphite is the most promising anode... 

C. Graphite ordering increases with temperature. Formation of microcrystalline graphite and millimeter-size graphite crystals occurs above the eutectic point in the Fe/Fe3C system. 

The carbon layers of carbon black rearrange to a graphitic order, beginning at the particle surface at temperatures above 1200 °C. At 3000 °C, graphite crystallites are formed and the carbon black particles assume polyhedral shape. 

As a typical example, Figure 12.15 shows the Raman spectra of an unfilled ethylene-propylene-diene rubber (EPDM). The Raman spectra of pure MWNTs, pure CB and of a EPDM / MWNTs composite are also given. The D, G and G bands are respectively located at 1348, 1577 and 2684 cm-1 in the Raman spectrum of the multiwall carbon nanotubes. The Raman spectrum of pure carbon black (CB) remains dominated by the bands associated with the D and G modes at 1354 and 1589 cm1 respectively, even when the carbons do not have particular graphiting ordering (Figure 12.11). This fact has been widely discussed by Robertson (84) and Filik (85). Amorphous carbons are mixtures of sp3 (as in diamond) and sp2 (as in graphite) hybridised carbon. The it bonds formed by the sp2 carbons being more polarisable than the a bonds formed by the sp3 carbons, the authors conclude that the Raman spectrum is dominated by the sp2 sites. 

With increasing temperature above 2273 K, the probability of turbostratic disorder decreases and the fraction of the stacked layer with graphitic order increases hence the reversible capacity also increases. Figure 26.12 shows the variation of reversible capacity of carbons in region III with the probability of turbostratic disorder F), emphasizing the fact that the reversible capacity decreases with increasing turbostratic disorder with a relation of 372(1 - P). Thermal treatments of soft carbons at temperatures greater than 2673 K form... 

Chlorination of FesC at temperatures of 400°C and above results in the formation of carbon and solid or gaseous iron chlorides. Three temperature regimes have been defined. Amorphous carbon is formed at temperatures of 400 - 500°C. Flakes and ribbons of nanocrystalline graphite form at 600 -1100°C. Graphite ordering increases with temperature. Formation of microcrystalline graphite and millimeter-size graphite crystals occurs above the eutectic point in the Fe/FcsC system. 

Figure 18.4 Low-pressure nitrogen adsorption isotherms of various nonmicroporous carbon blacks (CB) with different graphitic order and of microporous CB. (Adsorption data taken from Refs [[39], [40], and [50]] for the graphitized, the thermal CB, and the furnace, respectively.)...

The position and intensity of the monolayer formation peak depend on the graphitic order of the surface. As an example, the nitrogen adsorption isotherms... 

It was discussed above that for nonporous carbon materials the position of the monolayer formation peak depends on the graphitic order of the surface. In principle, this can also be used to study the graphitic order of the surface of porous carbons. The APD of OMCs synthesized at 900°C and above showed a monolayer formation peak. As these peaks are relatively wide, the APD data were fitted to a Gauss-Lorentzian function (Fig. 18.8). With increasing synthesis temperature, the monolayer formation peak became more pronounced... 

Of the three XPS parameters discussed, the width of the graphite peak was found to be the most suitable parameter for the graphitic order of the OMC surface [12]. The correlation between the width of the XPS peak and the position of the monolayer formation peak is presented in Fig. 18.10. This figure also includes a data point for a nonporous graphitized CB. The correlation between... 

Figure 18.10 Correlation between the graphitic order of the external surface (as determined by X-ray photoelectron spectroscopy (XPS)) and the surface of the mesopores (as determined by nitrogen adsorption).

XPS and APD data is very good, confirming that the APD is indeed a suitable method for the determination of the graphitic order of an internal surface. This information is very important because the graphitic order can strongly influence the interaction between the carbon surface and adsorbents. It might also be crucial for the anchoring of active sites such as metal clusters in OMCs [46]. 

Values for the specific surface area of OMC and other porous carbons can be obtained from the APD. As mentioned above, in this data treatment method, no assumptions about the pore geometry are made. Thus, a possible source for error is eliminated. However, the APD of the carbon sample should show a monolayer formation peak. Thus, the surface of the carbon material has to have a certain graphitic order. This is, for example, the case for graphitized and furnace CB. For these samples, the completion of the monolayer formation is clearly indicated by a minimum in the APD at the high adsorption potential end of the monolayer formation peak, located at adsorption potentials of 2.3 and 2.8 kj/mol, respectively (Fig. 18.7). The amount of nitrogen adsorbed corresponds to the monolayer, from which the specific surface area can be calculated. The surface areas calculated by the APD and the BET method differ by 7% and 15%, respectively (Table 18.2). For activated carbons, the difference between the surface areas obtained by APD and the DFT method is usually less than 10% [39]. 

For some samples, the APD method cannot be applied to calculate the specific surface area. Thermal CB, for example, has a lower graphitic order than the other CB discussed above. In the APD of thermal black, monolayer formation is only indicated by a shoulder (Fig. 18.7). Thus, the end of the monolayer formation cannot be determined with the required precision, making a mean-ingfiil determination of the surface area impossible. It can be summarized that as long as the carbon samples have a sufficient graphitic order, from the APD a reasonable estimate for the specific surface area can be obtained. 

Characteristics of Carbonized Materlais. After carbonization, the residual material is essentially all carbon. However, its structure has little graphitic order and consists of an aggregate of small crystallites, each formed of a few graphite layer planes with some degree of parallelism and usually with many imperfections. These crystallites are generally randomly oriented as described in Ch. 3, Sec. 2.0 and shown in Fig. 3.4. 

High-temperature peaks (600°C) in the TPO profiles indicate the presence of relatively low-reactive carbon deposits. Highly ordered structures in the deposits, having pregraphitic or graphitic order, would reduce oxidation reactivity compared with amorphous carbon with no apparent structural order [21]. Of the vegetable oils examined in this study, no deposits of this type were observed based on the TPO profiles. 

---