Stability of Carbon-Based Nanotubes and Nanofibers

the above-mentioned discussion testifies that the problems of thermal stability are also peculiar to ID structure. As is well known, the laws of physics are the same for everyone. We can only hope that these problems will not be as strong as for polycrystaUine materials. Basing on the discussions presented, one can conclude that prospects of ID structures used for elaboration of devices intended for work at higher temperature (such as conductometric gas sensors) should be estimated more realistically. Otherwise, in the future one s disappointment could be very intense, as in the case of low-temperature superconductivity. 

On the basis of research carried out it was concluded that oxidation at lower temperature takes place due to higher curvature and associated lattice strain (Joshi et al. 1990 Yao et al. 1998 Singh et al. 2010). Bond curvature has been found to affect the oxidation of single-walled CNTs (Miyata et al. 2007). Therefore, nanotubes with smaller diameters, due to higher curvature strain, are oxidized at lower temperature. Defects and derivatization moiety in nanotube walls can also lower the thermal stability (Arepalli et al. 2004). Therefore, a higher oxidation temperature is always associated with purer, less defective samples. 

It was established that catalysts present in CNTs also strongly affect thermal stability of CNTs in air. Active metal particles present in the nanotube samples catalyze carbon oxidation, so the amount of metal impurity in the sample can have a considerable influence on the thermal stability. For example, Zhou et al. (2001) found that if the oxidation of as-synthesized CNTs, which contained traces of catalyst (Fe), was quite rapid and homogeneous at 350 °C due to the catalytic effect, the purified CNTs had negligible weight loss, even after annealing at 460 °C. Furthermore, the presence of Fe obscured the dependence of oxidative stability on tube diameter as discussed earlier. After removing the Fe, all tubes were more appropriate for observing diameter-dependent oxidative stability. Li et al. (2011) have found that the presence of cobalt catalysts dramatically decreases the thermal stability of CNT/peroxide-curable methyl phenyl silicone gum composites as well. This means that the presence of uncontrolled impurities in CNTs can be one of the reasons for reduced reproducibility of sensor parameters. This conclusion is confirmed by results obtained by Boccaleri et al. (2006) and Zhou et al. (2001) (see Fig. 21.5). 

Ajayan PM, Ebbesen TW, Ichihashi T, lijima S, Tanigaki K, Hiura H (1993) Opening carbon nanotubes with oxygen and implications for filling. Nature 362 522-525 

Andrews R, Jacques D, Anthony 1, Chen B, Meier MS, Selegue IP (2002) Thermogravimetric analysis of the oxidation of multiwalled carbon nanotubes evidence for the role of defect sites in carbon nanotube chemistry. Nano Lett 2 615-619 

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