Graphitic microcrystallites

Figure 2.4S. The model for structure in a carbon black using the graphitic microcrystallite theory (Biscoe and Warren, 1942).

This example is to illustrate that a single determination of crystallite sizes is of extremely limited value as no distinction is possible between contributions from size and contributions from strain. The unchanging graphitic microcrystallite model carmot explain these phenomena. 

Smallness of size associated with imperfect layering of defective structural units, that is the defective micro-graphene layer (and not graphitic microcrystallites). 

Sadly, this graphitic microcrystallite concept appears to have been passed down over the years (perhaps uncritically) from publication to publication. Now, it requires a careful reassessment to enable the subject of structure in non-graphitizable carbons to move on. 

Clearly, if the crystallite based on the graphite hexagonal lattice is an essential component of all carbons, then it must be present in the activated porous carbons which are so relevant to this book. This implies that the surfaces of the porosity of micro and mesoporous carbons must be the surfaces of the graphitic microcrystallites (if they exist). This aspect is not discussed in the literature. The evidence which comes from adsorption studies of carbons of increasing HTT to a maximum of >2000 °C would never support this concept. The... 

The earlier literature also uses the term "apparent crystallite size indicating that the sizes are indicative of some level of order of carbon atoms within the carbon. And that is about as far as the concept should be developed. Carbons of quite low HIT, say 500 °C, exhibit linebroadening phenomena. In fact, all carbons do this. There exists no discussion as to how a graphitic microcrystallite can appear in such a low-temperature material. It cannot. 

Certainly, there is no evidence from RMS of the existence of the graphitic microcrystallite even though this concept appears firmly to be rooted in much of the RMS literature. It would appear that this concept has simply been carried over into the RMS literature from wide-angle X-ray diffraction (WAXD) data without too much critical analysis. 

Figure 2.49. A model which summarizes concerns about the validity of the graphitic microcrystallite theory as applied to XRD studies. The existence of very limited parallelism between graphene layers accounts for the X-ray scattering. The concept of graphitic microcrystallites is not needed to explain this scattering.

Unfortunately, for studies of carbons, many of the RMS publications have assumed, uncritically, the existence of the graphitic microcrystallites. There is no independent justification for the assumption. This section therefore assesses the contributions which RMS can make to further understandings of carbon materials, in particular the isotropic porous-activated carbons of which surface structure is of major importance in adsorption studies. 

An example of this problem can be seen in a study by Manivannan et al. (1999) who examined three porous carbons (by PICA) from coconut (surface area 1000 m g" ), from a wood (surface area 1150m g ) and a coconut shell (2000 m g ). values of graphitic microcrystallites ate reported as 0.9,1.0 and 1.5 ran, and values as 3.9,2.5 and 2.7 nm, respectively. However, these authors also interpreted their Raman spectra in terms of values of 2.9, 3.1 and 2.6 nm of these graphitic raicrocrystallites and commented on the... 

This research by Dumont et al. (2002) describes the development of Raman spectra from a parent pitch to a resultant coke. The RMS spectrum of the coke originates from within one (embryonic) graphene layer which most probably forms part of a cross-linked carbon network. The proposed existence of a graphitic crystallite" composed of several grapherte layers stacked parallel to each other is not required. Hence, the frequent use of the term "graphitic microcrystallite within the RMS literature is simply an uncritical carry-over of words from WAXD studies and which unfortunately impart seriously misleading ideas. 

Equation (3.1) was developed to relate surface area to the dimensions of the graphitic microcrystallite, which has the form ... 

This model is firmly based on the acceptance of graphitic microcrystallites within the structure of these microporous carbons, as are the models of Kaneko et al. (1992a, b). The case against the acceptance of this type of model is as explained in Section 3.4.6. The conclusions are based on one interpretation of the diffraction data. They make no mention of the nature of the intercoimecting carbon and assume (without saying so directly) that it... 

Once again, this model is based on the assumption of the graphitic microcrystallite as the foundation unit of structure in activated carbon (see Section 3.4.10 for concerns about this assumption). The disks are all of a cylindrical shape and composed of stacked graphene layers with variable thicknesses dependent on the number of graphene layers within the disk. A further assumption or approximation in the design of the model is that the carbon atoms of the surfaces are of uniform and continuous density. 

Byrne and Marsh (1995) were unable to accept the concept that activated carbon possessed a structure made of graphitic microcrystallites. A major concern was the improbability of graphitic (graphene-like layers) being formed at temperatures <900 °C from organic precursors. 

No serious attempts have been made to interpret adsorption data correctly. Low values of surface areas of this series of porous carbons comes from the inappropriate use of the isotherm of nitrogen adsorption at 77 K. Incidentally, all isotherms should be made available in all publications. The low values are associated with slow-activated diffusion effects in microporosities of dimensions <0.5 nm. High values of surface area ( 1200m g ) result from multilayer adsorption in the wider of the micropores and in porosities of indefinite shape, approximating to cylinders, cones, spheres, etc. Hence, it seems unreasonable to relate such adsorption processes to monolayer adsorption on surfaces of graphitic microcrystallites. 

Figure 3.27. (a) Models of arrangements of graphitic microcrystallites in activated carbons used to explain the porosity. (Py et al., 2004). (b) A diagram of the variation of average pore dimension (Lg in nm) obtained from pore-size distributions with specific micropore volume (cm g ) obtained from isotherm data, for a selection of porous carbons (Py et al., 2004). 

The approach of Py et al. (2004) is based on two assumptions, namely that (a) carbon material is composed of graphitic microcrystallites, and that (b) it is the presentation at surfaces of these graphitic microcrystallites which controls the adsorption process. 

With this mind, the need to speculate on possible arrangements of graphitic microcrystallites to explain these properties of porosity disappears. 

The second assumption (B), that is, is the size and orientation of the graphitic microcrys-tallite which controls the adsorption process, has to be addressed very seriously. Extents and enthalpies of adsorption are massively dependent on the London Dispersion Forces (van der Waals forces) which exist within the porosity of a carbon. Such porosity is unique among solid porous materials and exhibits considerable versatility. Calculations of areas of theoretical graphitic microcrystallites, as has been reported above, with the assumption that these areas are available for adsorption, have little relevance. 

This chapter has reviewed modeling over the years and the progress that can be identified. It has been demonstrated that modelers, using modem computational systems, base their calculations on stmcture systems, usually around the graphitic microcrystallite. An objective of such modelers is the simulation of adsorption isotherms determined experimentally. Whether or not the structural models assumed for their work can be considered to be realistic is rarely a matter for discussion. Accordingly, the considerations of such modelers should not be adopted, uncritically, by those who have other interests in activated carbon. 

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