Length and the Oxidation Kinetics of Chemical-Vapor-Deposition-Generated Multiwalled Carbon Nanotubes

Abstract

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.

In Situ Synthesis of Multiwalled Carbon Nanotubes over LaNiO3 as Support of Cobalt Nanoclusters Catalyst for Catalytic Applications

“…Thermogravimetric analysis can provide information regarding the carbonaceous impurity content, residual catalyst content, and the crystalline defect density of the nanotubes because of the differences in the reactivity of the various forms of carbon causing preferential burn off at different temperatures and the defects in nanotube walls leading to a lower-temperature oxidation and gasification of the carbon. , Thermogravimetric analysis was performed to study the oxidation behavior of CNTs with a heating rate of 15 °C min –1 up to 800 °C in 50% O 2 –N 2 (Figure ). An initial weight loss from ambient temperature to 200 °C for both the crude and purified samples is less than 0.5 wt %, which is ascribed to the removal of physisorbed water.…”

Microwave Makes Carbon Nanotubes Less Defective

“…TGA has been used widely to study the oxidative stability of CNTs so as to evaluate CNT quality, that is, defect density and purity. 31,32,56,57 In our study, TGA measurements were carried out in air, using a heating rate of 10 °C min Ϫ1 , up to 900 °C. For the control VACNTs, the temperature where the derivative weight loss peaks is ϳ675 °C, consistent with the reported values for defective (raw, unannealed) CNTs.…”

“…Our postulation can be verified by thermogravimetric analysis (TGA) of the VACNTs (Figure ). TGA has been used widely to study the oxidative stability of CNTs so as to evaluate CNT quality, that is, defect density and purity. ,,, In our study, TGA measurements were carried out in air, using a heating rate of 10 °C min −1 , up to 900 °C. For the control VACNTs, the temperature where the derivative weight loss peaks is ∼675 °C, consistent with the reported values for defective (raw, unannealed) CNTs. ,, VFM (500 W) treatments of 1 and 3 min shift the peak temperature up to ∼ 730 °C and ∼740 °C, respectively, indicating an effective improvement in oxidative stability, comparable to high-temperature annealed CNTs .…”

Diameter dependence of oxidative stability in multiwalled carbon nanotubes: Role of defects and effect of vacuum annealing

“…23 More recent study has shown that the oxidation of CNTs need not originate in the nanotube caps, but may originate in the other areas of increased reactivity, such as wall defects. 24 However, no systematic studies on the effect of diameter and the role of defects and strain on the oxidative stability of MWNTs have been reported in the literature.…”