Changes in the thermogravimetrically determined oxidation behaviors of CVD-grown multiwalled carbon nanotubes with varying synthesis conditions are examined. Catalyst type and synthesis temperature are found to have a measurable impact upon nanotube stability, suggesting differing levels of crystalline perfection in the resulting nanotubes. The results provide evidence showing the catalytic effects of nanotube catalyst particles and their oxides upon the oxidation of nanotube carbon and graphite. The significance of thermogravimetric analysis as a characterization tool for carbon nanotubes is discussed.

Carbon nanotubes (CNTs) have been a subject of great interest partially due to their potential for exceptional material properties. Improvements in synthesis methods have facilitated the production of ultralong CNT mats, with lengths in the millimeter range. The increased length of these ultralong mats has, in return, opened the way to greater flexibility to probe their mechanical response. In this work, mats of dense, well-aligned, multiwalled carbon nanotubes were grown with a vapor-phase chemical vapor deposition technique using ferrocene and benzene as reactants, and subsequently tested in both tension and compression using two methods, in a thermomechanical analyzer and in situ inside a scanning electron microscope. In compression, measured stiffness was very low, due to buckling of the nanotubes. In tension, the nanotube mats behaved considerably stiffer; however, they were still more compliant than expected for nanotubes (∼1TPa). Analysis of both the growth method used and the nanotube mat fracture surface suggests that the mats grown in this method are not composed of continuous nanotubes and their strengths actually closely match those of woven nanotube yarns and ropes.

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Oxidation rates are measured and kinetic parameters determined by isothermal analysis for several length ranges of as-grown and processed chemical-vapor-deposition-generated multiwalled carbon nanotubes within a range of 700-900 K. The activation energy barrier is found to be within the range calculated previously by others and follows comparable progression patterns for similar length tubes. The activation energy and frequency factor are not observed to change with the synthesis method, but do increase gradually as oxidation progresses in longer nanotube arrays or peak and decline in shorter arrays. The results also confirm that the oxidation of carbon nanotubes need not originate in the nanotube caps, but may originate in other areas of increased reactivity, such as wall defects. The behavior of the kinetic parameters as oxidation progresses is indicative of the consumption of defective structures followed by the coalescence of etched holes, suggesting lower defect densities in longer carbon nanotubes.