Physical properties and structural stability of carbon nanotubes under extreme conditions

Sammanfattning: Carbon nanotubes (CNTs) have attracted an immense attention of the research community since reporting on this system by S. Ijima in 1991. A "single-walled" CNT (SWCNT) can be considered as a rolled-up single-layer graphene - a one atom-thick layer of carbon atoms arranged in a hexagonal lattice. This cylindrical object being just about 1 nm in diameter and up to a few centimeters long can be considered as a quasi-one-dimensional system. Several nanotubes "inserted" one into another build a so-called multi-walled CNT. CNTs exhibit outstanding mechanical, thermal and electronic properties which make this material a promising candidate for numerous applications - reinforced composite materials, nano-electronics, molecular sensors and drug delivery systems to name just a few. CNTs possess tensile strength 10 and 5 times higher than that of steel and Kevlar, respectively, that creates a great prospective for their use as reinforcing units in materials subjected to high-impact dynamic loads/stress (bullet-proof jackets, for example). Nonetheless, to date there are no reports on experimental study of CNTs behavior at extreme dynamic loads which may substantiate such prospective. In addition, several theoretical predictions indicate a possibility of CNTs transformation into new structural forms at extreme pressures. The goal of this work is a systematic study of structural properties and exploration possibility of synthesis of new materials from CNTs under extreme pressures/stress. In a set of experiments purified SWCNTs were subjected to high dynamic (shock) pressures up to 52 GPa. Recovered from each pressure step sample was characterized by High Resolution Transmission Electron Microscopy (HRTEM) and Raman spectroscopy. We observed a gradual increase of defects concentration on the CNT surface with pressure along with shortening and "un-zipping" of the tubes and an onset of the complete CNT destruction at 26 GPa shock which sets-up a limit for certain practical applications of this kind of material. Further increase of the dynamic load to 35 and 52 GPa led to CNT transformation into a mixture of disordered sp²/sp³- bonded carbon atoms with nano-sized graphene clusters. No CNT polymerization or coalescence was observed contrary to some theoretical predictions. For comparison, we conducted a separate experiment on the same CNT material under static compression up to 36 GPa in a diamond anvil cell (DAC). The system evolution was monitored in-situ during the high-pressure run using Raman spectroscopy. Examination of the material recovered from high pressure revealed that certain fraction of the CNTs survived exposure to 36 GPa though similar damages were introduced to the nanotubes as in the shock experiments evidenced by the Raman spectra. This result testifies a substantial difference in the processes of CNT destruction by dynamic vs static compression. A separate set of experiments in DACs was aimed at in-situ monitoring of the Raman spectra (in particular G-band) during pressure evolution and establishing the level ofstatic pressure which causes a complete destruction of SWCNTs from the same batch as used in similar experiments at the dynamic compression. Pressure dependence of G-band, G(p), exhibited several peculiarities at approximately 15, 45 and 60 GPa which we associate with collapse of large (1.2 nm) and small (∼1 nm) diameter CNTs, and an onset of nanotubes transformation to a new phase respectively. Raman spectra of the sample recovered after 58 GPa static compression exhibit no RBM signal, large G-band broadening and high D/G peak intensity ratio that testifies for CNT destruction. Pressure increase to 100 GPa resulted in a substantial altering of Raman spectrum of the recovered sample - appearance of characteristic features of highly disordered sp²-and sp³-bonded carbons which may stem from interlinked nano-sized graphene clusters. Change of CNTs structure results in the altering of their electronic properties thus structure evolution of the CNTs with pressure may be followed by monitoring electrical resistance change with pressure. In a series of experiments we conducted in-situ electrical resistance (R) measurements of the SWCNTs under static pressures up to 45 GPa (temperature range 293 - 395 K) in a conductive DAC. Isobaric temperature dependence of the resistance indicated that the nanotube sample is comprised predominantly of semiconducting CNTs. A set of anomalies observed in R(p) at room temperature we interpret as a sequential, diameter-dependent collapse of the CNTs. Raman characterization of the samples after the pressure cycling confirmed reversibility of these structural transitions for at least certain CNT species accompanied by a substantial increase of CNT defects density. No indication of nanotubes polymerization was observed. Although thermal conductivity of individual CNTs is excellent (5 times better than that of copper) heat conduction becomes far less efficient in "conventional" system, i.e. when the tubes form bundles/ropes which may lead to a risk of CNT destruction by overheating. Therefore probing CNTs response to extreme heat (temperature) is important both for testing capabilities of the nanotube material and developing methods of its proper characterization. We followed temporal evolution of the Raman spectra of bundled SWCNTs exposed to high laser irradiance in both air and argon atmosphere. Temperature threshold for CNT destruction in air appeared to be lower than that in Ar, the fact indicating importance of the CNTs oxidation for their structural integrity. We show that primary damage occurs in resonant with excitation laser CNTs which act as photon energy absorbers. We show that smaller diameter and metallic nanotubes are less stable to high irradiance/heat flux than their large diameter/semiconducting counterparts. Remarkably, some small diameter, non-resonant CNTs were destroyed indirectly, i.e. via overheating induced by neighbor CNTs in resonance (photon absorbers). We demonstrate the importance of laser heating effects on Raman characterization of nanotubes. Even though carbon nanotubes exhibit susceptibility to extreme pressure/stress and high laser irradiance/overheating their potential for use in very demanding applications is not yet challenged: for example SWCNT destruction under dynamic compression occurs at pressure exceeding 20 times the typical threshold levels in ballistic impact. Cold compression of nanotubes also opens up perspectives of synthesis of new carbon phases with superior mechanical properties.

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