Nanotubos de carbono de parede dupla submetidos à condições extremas de altas pressões e altas temperaturas

Abstract

In this thesis, we present a study of double wall carbon nanotubes (DWNTs) under extreme high temperature and high pressure conditions using experimental and theoretical techniques. By using theoretical calculations of structural, electronic and vibrational properties based on first principles methods and classic potentials, we have predicted the structural evolution of carbon nanotubes under pressure. High purity bundles of doublewalled carbon nanotubes were studied under high-pressure conditions by using diamond anvil cells. Pristine and Br2 doped samples were submited to pressures up to 30 GPa using paraffin oil and solid NaCl as pressure transmitting medium, and they were analyzed mainly using Raman Spectroscopy technique. We also have used Transmission Electron Microscopy (TEM) images, X-Ray Photoelectron Spectroscopy (XPS) chemical analysis and X-Ray Absorption spectroscopy (XAS) studies to support the overall interpretation of our results. The tangential (G band) and radial (RBM) modes of DWNT bundles were studied here up to a pressure limit not studied before. We have studied the inner tube behavior compared with the outer one concerning the higher stability found for DWNTs than SWNTs, which leads to higher critical pressure for the collapse than expected for individual SWNTs. The effect of including another chemical species inside the external tube (filling effect) is also discussed regarding the nanotube collapse. We have have found that structural stability increases for argon-filled and tube-filled (DWNT) nanotubes (the critical pressure for the collapse increases) while for iodine-doped SWNTs and C70@SWNTs (peapods) the structural stability descreases (critical pressure descreases). It means that inhomogeneous filling effect in the case of iodine and C70 induces strong uniaxial stress components or a strong chemical interaction that decrease the structural stability of SWNTs. Similar effects were observed for Br2-intercalated DWNTs regarding the structure collapse. However, we could follow the electronic, vibrational and structural behavior of Bromine molecules when intercalated on the 1D interstitials channels of DWNT bundles using low-frequency Raman spectra and X-Ray absorption analysis. We have observed that polyanions Brn (n=2,3,5) are lively to be present in these 1D channels even under high pressure and this hypothesis was supported by ab initio calculations. Furthermore, the vibrational properties of the polyanions are dramatically affected when the DWNT bundles collapse suggesting some kind of enhancement of chemical interaction between bromine and collapsed structures. We also present results of high-pressure and high-temperature processing of DWNTs samples. This study was performed in order to synthesize new carbon nanostructured materials using DWNTs as starting point and searching the pressure-temperature conditions to play with DWNT collapse and sp3 bonding formation between outter walls of DWNTs, which could be potentialized with temperature increasing under extreme pressure conditions. Such experiments were realized using the Paris-Edinburg cell which is suitable to process the samples to a specific p-T phasediagram points at the same time up extreme conditions (p=15 GPa,T=1800 K). New polymerized DWNTs-based structures were proposed based on Raman and XPS data and discussed by considering current knowledge using a nanotube p-T phase-diagram cons-tructed with some recent results in this research field. The possibility of synthesize several carbon structures as disordered graphite, diamond, modified nanotubes and polymerized nanotubes were discussed in terms of our results obtained by analysis of post-treated DWNT samples. Those ideas can be used as basis for designing hybrid nanostructures based on carbon nanotubes and improve their mechanical applications. In this case, the DWNTs are better candidates for engineering at nanostructured level than SWNTs through the specific individual inner/outer nanotube role: the external tube is coupled with the chemical environment (maybe with sp3 bond formation) while the screened inner tube give the mechanical support for the whole structure.