Amorphous carbon impurities

This process is very simple and requires low temperatures. However, the MWCNTs predominantly contain a large number of structural defects as well as amorphous carbon impurities. Furthermore, a significant part of the salt remains encapsulated within the CNTs. 

Fig. 11 Baseline-corrected MIR spectrum of KCso and Na2C6o compared to the spectrum of Ceo-The line positions characteristic of neutral Cso and of molecules are shown. Amorphous carbon impurity in the KCgo sample is denoted by an asterisk...

A precise description of the SWNT sorbent is also problematic. In theory, nanotubes should form perfecdy ordered hexagonal bundles, giving a structure as well-defined as zeolites. In practice, nanotubes contain significant quantities of metal catalyst particles, amorphous carbon impurities, and geometric and chemical defects in the nanotubes themselves. Thus, the accurate modeling of gas adsorption on SWNTs is a challenge at all levels. Nevertheless, statistical... 

Purification of DWCNTs, the smallest member of the M WCNT family, was realized using treatments in air at temperatures between 350°C and 550°C [159]. Air oxidation of MWCNTs between 450°C and 750°C was shown not only to successfully ranove amorphous carbon impurities and enable the opening of CNT caps, but may also lead to the introduction of defects to the wall structure [66,153]. Several studies have identified temperatures around 400°C to be optimal for removing amorphous carbon from MWCNT samples without damaging the tube structure [28,160]. Osswald et al. proposed that rapid oxidation at higher temperatures (=550°C), referred to as flash oxidation, may be a suitable alternative to conventional purification methods, as it yields similar purification levels, but simultaneously introduces a certain number of wall defects that are necessary for subsequent functionalization. Several groups have scaled their oxidation experiments to the bulk level and demonstrated that gas-phase oxidation is a suitable technique for industrial CNT purification [67,68],... 

An alternative to acid-based chemical purifications is thermal annealing. This strategy has been reported to lead to the removal of most amorphous carbon impurities, as well as of residual catalyst particles. It has even been reported to increase the graphitic perfection of the CNTs. Formation of graphitic shells and other undesirable nanoparticles, however, may be promoted. Note that thermal annealing can also be used in combination with acid-based chemical treatments. ... 

Carbon based nanomaterials are known to have good mechanical strength and are highly biocompatible as well. These materials are highly conducting and the amorphous carbon impurities sticking from the synthesis time to their walls facilitate these materials with great mechanical... 

Defects in arc-grown nanotubes place limitations on their utility. Since defects appear to arise predominantly due to sintering of adjacent nanotubes in the high temperature of the arc, it seemed sensible to try to reduce the extent of sintering by cooling the cathode better[2]. The most vivid assay for the extent of sintering is the oxidative heat purification treatment of Ebbesen and coworkers[7], in which amorphous carbon and shorter nanoparticles are etched away before nanotubes are substantially shortened. Since, as we proposed, most of the nanoparticle impurities orig-... 

A colorless gel formed which was isolated by vacuum evaporation of the volatiles. The resulting colorless glassy solid was pyrolyzed in vacuo at 900°C for 24 hours in a quartz tube and the evolved volatiles identified as NH3 and NH4CI. The remaining solid was briefly (2 hours) heated in air at 1200°C in order to remove minor carbon impurities and to improve crystallinity. This solid was then treated at room temperature with 40% aqueous HF to remove boric acid and silica formed in small quantities. The solid obtained at 900°C was identified as boron nitride however, the majority of the material was amorphous. After treatment at 1200°C, white crystalline boron-nitride was obtained in about 55% yield. 

Variation of the content of impurities in the different CNT preparations [21] offers additional challenges in the accurate and consistent assessment of CNT toxicity. As-produced CNTs generally contain high amounts of catalytic metal particles, such as iron and nickel, used as precursors in their synthesis. The cytotoxicity of high concentrations of these metals is well known [35, 36], mainly due to oxidative stress and induction of inflammatory processes generated by catalytic reactions at the metal particle surface [37]. Another very important contaminant is amorphous carbon, which exhibits comparable biological effects to carbon black or relevant ambient air particles. 

Hou et al. [73] considered small "carbon islands" as the main hydrogen-adsorption sites in an MWNT. The hydrogen-storage capacity of a CNT varies widely and the reason for such a variation is not clear, possibly caused by the impurity such as metal catalysts or amorphous carbon. It is not clear yet how the metallic catalyst particles, which are used during the preparation of nanotube samples, affect the hydrogen-storage capacity of nanotubes. 

CNTs can be chemically functionalized to achieve good dispersion in polymer/ CNT composites and strong interface adhesion (Gao et al., 2004). CNTs can be assembled as ropes or bundles, and there are some catalyst residuals, bucky onions, spheroidal fullerenes, amorphous carbon, polyhedron graphite nanoparticles, and other forms of impurities during the growth process of CNTs. 

It exists in two cryst forms diamond graphite and in various("amorphous ) forms, such as carbonblacks (acetylyne black, lampblack, etc) and charcoal. Soot Sc coke are impure amorphous carbon... 

The smaller diameter carbon nanotubes are known to be less stable than their larger diameter counterparts and tend to oxidize at lower temperatures. Amorphous carbon and carbon nanotubes with defects undergo combustion at lower temperatures, in Figure 4, we show the thermogravimetric analysis (TGA) curves of undoped as well as N- and B-doped DWNTs. The decomposition temperatures of all these doped DWNTs are comparable to but slightly lower that the decomposition temperature of pure DWNTs. Derivative TGA curves also shows the same trend. The slight increase in mass at high temperature may be due to the small metallic impurity. 

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)... 

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