No abstract

Carbon nanotubes (CNT) were produced by catalytic chemical vapor deposition using, as carbon source, a mixture of hydrocarbons and hydrogen that simulates the effluent gases from pyrolysis of polyethylene (PE). An Fe/Al 3 O 3 catalyst was used in a range of temperatures from 600 ºC to 800 ºC.Multi-wall carbon nanotubes of 20 nm in diameter and length on the order of microns were obtained. Higher yields were observed at 650 ºC, where no prior catalyst reduction was necessary. TEM, XRD and Raman spectrometry show a higher crystalline quality at 750 ºC, although the balance yield-quality indicates that 650 ºC is a satisfactory temperature for producing CNTs at a reasonable cost, since no extra hydrogen is necessary for the process. In addition to this, the effluent gas from the process can be further used for energy production.

Different studies in the literature indicate the effectiveness of CNTs as reinforcing materials in cement–matrix composites due to their high mechanical strength. Nevertheless, their incorporation into cement presents some difficulties due to their tendency to agglomerate, yielding a non-homogeneous dispersion in the paste mix that results in a poor cement–CNTs interaction. This makes the surface modification of the CNTs by introducing functional groups on the surface necessary. In this study, three different treatments for incorporating polar oxygen functional groups onto the surface of carbon nanotubes have been carried out, with the objective of evaluating the influence of the type and oxidation degree on the mechanical and electrical properties and in strain-sensing function of cement pastes containing CNTs. One treatment is in liquid phase (surface oxidation with HNO3/H2SO4), the second is in gas phase (O3 treatment at 25 and 160 °C), and a third is a combination of gas-phase O3 treatment plus NaOH liquid phase. The electrical conductivity of cement pastes increased with O3- and O3-NaOH-treated CNTs with respect to non-treated ones. Furthermore, the oxygen functionalization treatments clearly improve the strain sensing performance of the CNT-cement pastes, particularly in terms of the accuracy of the linear correlation between the resistance and the stress, as well as the increase in the gage factor from 28 to 65. Additionally, the incorporation of either non-functionalized or functionalized CNTs did not produce any significant modification of the mechanical properties of CNTs. Therefore, the functionalization of CNTs favours the de-agglomeration of CNTs in the cement matrix and consequently, the electrical conductivity, without affecting the mechanical behaviour.

 The viability of the bamboo-type carbon nanotubes production from pyrolysis gas by a simple process of catalytic chemical vapor deposition is demonstrated.  The pyrolyzed polymer polyamide 6.6 provided the necessary carbon, nitrogen and hydrogen.  Good quality nanotubes were obtained with 5 to 20 layers and 20 nm in diameter.  The best results were obtained from pyrolysis at 900 ºC and growth at 750 ºC.