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 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. Carbon nanotubes 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 with 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 revealed CNT transformation into a mixture of disordered sp2/sp3- bonded carbon atoms with nanosized 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 as evidenced by the Raman spectra. This result testifies for a substantial difference in the processes of CNT destruction by dynamic vs static compression. Change of CNTs structure results in the altering 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 from 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 this structural transition for at least certain CNT species accompanied by a substantial increase of CNT defects density. No indication of nanotubes polymerization was observed.

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