Amorphous Carbon orientation

Fig. 2. Cables of parallel SWNTs thal have self-assembled during oxidative cleanup of arc-produced soot composed of randomly oriented SWNTs imbedded in amorphous carbon. Note the large cable consisting of several tens of SWNTs, triple and single strand tubes bent without kinks, and another bent cable consisting of 6 to 8 SWNTs.

As reported elsewhere [22], similar to those found on other catalysts, the forms of carbon materials deposited on Fe-loading zeolite molecular sieves are carbon nanotube, carbon nanofiber and amorphous carbon. One obvious phenomenon of the carbon nanotubes formed on Fe/NaY or Fe/SiHMS catalysts is that almost all tips of these tubes are open, indicating the interaction between catalyst particles and supports is strong [23]. On the other hand, the optimal formation time of carbon nanotubes on Fe/SiHMS is longer than that on Fe/NaY. However, the size of carbon nanotubes is easily adjusted and the growth direction of carbon nanotubes on the former is more oriented than on the latter. 

As indicated previously, the so-called amorphous varieties of carbon embrace numerous common materials. It is perhaps improper to designate all these materials as truly amorphous. Some of these materials contain less well-defined crystalline structures comparable to that of graphite but with a much less well-ordered and regular orientation of carbon atoms. Nevertheless, these materials have long been known as forms of amorphous carbon. 

Since the nanocrystallites were separated from the substrate with a layer of amorphous carbon, one might think that the ordered arrangement of the nanocrystallites was due to an auto-orientation mechanism, which operated at all stages of the growth of the ordered nanocrystalline forms of the films of the zirconia-based solid electrolyte. 

A number of complications lead to differences in experimental results. Aside from possible catalytic influences of impurities in the carbon, there are different structural types of carbon that have been studied. These include amorphous carbon (typically randomly oriented graphite crystals)... 

Basically, the effect of the surface nanotexture on the strength of metal-carbon bonding may occur as a result of epitaxy or interdiffusion of atoms in the contact region of a metal crystallite and carbon support. However, information concerning these aspects of the metal-carbon interaction is scarce. Graphite-supported Pd and Pt crystallites are oriented their 202 for Pd [19] and 111 or 110 for Pt [20-22] planes parallel to the basal plane of graphite substrate, but this epitaxial interaction is relatively weak [19-21,23]. In contrast, Pd particles supported on amorphous carbons are in random orientation [19,25]. Hence, heterogeneous support surfaces comprise structurally different sites for metal-particle stabilization. 

Figure 9.34 Characteristic spectra of carbon in different structures (a) highly oriented graphite (b) polycrystalline graphite (c) amorphous carbon and (d) diamond-like carbon. (Reproduced with permission from G. Turrell and J. Corset, Raman Microscopy, Developments and Applications, Academic Press, Harcourt Brace Company, London. 1996 Elsevier B.V.)...

The mean size of the crystallites of linear-chain carbon films calculated from the width of their electron diffraction maximums depends on the film thickness (Table 11.1). As can be seen from Table 11.1 the mean crystal size of the films is comparable with the film thickness if that thickness is about 4.0 nm. In this case, the films become highly oriented and there are sharp reflections in their diffraction pattern (Figure 11.1(a)). If the thickness of the films increases to 16.0 nm, the orientation still takes place. In films with a thickness greater than 60 nm amorphization is observed, with the formation of inter-chain bounds. This is clearly seen by increasing the intensity of diffraction maximums corresponding to the three-dimensional amorphous carbon phase and finally by the complete disappearance of diffraction maximums corresponding to the sp phase. 

Fig. 6.14. Ion beam incidence angular dependence of the liquid crystal pretilt angle (3 and the molecular tilt angle 7 of the polymer segment distribution at the film surface for polyimide (top) and amorphous carbon (bottom). As predicted by the alignment model the liquid crystal pretilt angle / follows the molecular tilt angle 7. The line is a fit to y 0) using a model that assumes finite, but different cross sections for breaking of phenyl rings oriented along or perpendicular to the ion beam direction [35].

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