Existing growth methods produce single-walled carbon nanotubes (SWCNTs) with a range of structures and electronic properties, but many potential applications require pure nanotube samples. Density gradient ultracentrifugation (DGU) has recently emerged as a technique for sorting as-grown mixtures of single-walled nanotubes into their distinct ( n,m ) structural forms, but this approach has been limited to samples containing only a small number of nanotube structures, and has often required repeated DGU processing. For the first time, it has been shown that the use of tailored nonlinear density gradient ultracentrifugation (NDGU) can significantly improve DGU separations. This new sorting process readily separated highly polydisperse samples of SWCNTs grown by the HiPco method in a single step to give fractions enriched in any of ten different ( n,m ) species. In addition, minor variants of the method allowed separation of the minor-image isomers (enantiomers) of seven ( n,m ) species. Optimization of this new approach was aided by the development of instrumentation that spectroscopically mapped nanotube contents inside undisturbed centrifuge tubes. Besides, sorted nanotube samples enabled the discovery of novel oxygen-doped SWCNTs with remarkable photophysical properties. Modified nanotube samples were produced using mild oxidation of SWCNTs with ozone followed by a photochemical conversion step that induced well-defined changes in emissive properties. As demonstrated for a set of ten separated SWCNT ( n,m ) structures, chemically altered nanotubes possess slightly lower band gap energies with correspondingly longer photoluminescence wavelengths. Treated samples showed distinct, structure-specific near-infrared fluorescence at wavelengths 10 to 15% longer than the pristine semiconducting SWCNTs. Quantum chemical modeling suggests that dopant sites harvest light energy absorbed in undoped nanotube regions by trapping mobile excitons. The oxygen-doped SWCNTs are much easier to detect and image in biological specimen than pristine SWCNTs because they give stronger near-IR emission and do not absorb at the shifted emission wavelength. This novel modification of SWCNT properties may lead to new optical and electronic applications, as it provides a way to change optical band gaps in whole nanotubes or in selected sections.

This dissertation describes the development of a processing route for fabricating conventional and doped multi-wall carbon nanotube (MWCNT)/polymer composite films for dielectric applications. Previous research has shown that such composites are promising dielectric materials, but their commercial development has been impeded by the nanotube agglomeration in the polymer matrix and the inefficiency in forming uniform films. Moreover, the harsh fabrication treatments often disrupt the structure of the nanotubes, hence damaging their inherent electrical properties. This work presents safer routes for forming non-aqueous, surfactant free dispersions of conventional and doped MWCNTs, which can be readily mixed with polymers and formed into films through aerosol spraying. Dispersibility behaviour of in-house synthesised conventional, nitrogen doped (N-MWCNTs), and boron doped (B-MWCNTs) MWCNTs was studied in 22 organic solvents. Based on thermodynamic theories it was suggested that doping, in particular nitrogen doping, significantly reduced the surface energy of the nanotubes. This aspect was crucial to understand the dispersibility of N-MWCNTs in low surface energy solvents and to achieve dispersions with high nanotube concentrations (0.82 mg/ml). Also, a "destruction reduced sonication protocol" involving mild sonication was suggested for forming MWCNT dispersions in organic solvents. Dispersions formed using this protocol were homogeneous and showed high stability of at least 2.5 years. Furthermore, the effect of ultrasonic probes on MWCNT lengths was studied and a decrease of 96–99% for MWCNTs and 85–95% for N-MWCNTs was observed. A numerical value for the nanotube length decrease during sonication has been reported for the first time. Preliminary studies to generate dielectric films of MWCNT/perfluoro alkoxy polymer were performed using aerosol spraying. An improvement in the dielectric constant (3.56) with a low dissipation factor (0.003) was observed in 0.3 wt.% B- MWCNT/PFA composite films. Consistency in the test results from various parts of the films confirmed the uniformity of the nanotube distribution within the composite. Future work should concentrate on the effects of B-MWCNTs and N-MWCNTs at the percolation threshold due to their inherent electric properties.

Yes / Owing to their small size, good wettability, uniform dispersion ability and high thermal properties, the nickel-plated carbon nanotubes (Ni-CNTs) with different aspect ratios are used to reinforce reactive powder concrete (RPC) through modifying the nano/micro- structural units of concrete. Incorporating only 0.075 vol% of Ni-CNTs (0.03 vol% of CNTs) can significantly increase mechanical properties of RPC. The enhancement effect on compressive strength caused by the incorporation of Ni-CNTs with aspect ratio of 1000 reaches 26.8%/23.0 MPa, mainly benefiting from the high polymerization C-S-H gels, low porosity, and refined pore structure. The 33.5%/1.92 MPa increases of flexural strength can be attributed to the decrease of large pore, original cracks, molar ratio of CaO to SiO2, and gel water content when Ni-CNTs with aspect ratio of 125 are added. Ni-CNTs with aspect ratio of 1500 have the largest utilization rate of being pulled-out, resulting from the improvement of dispersibility and the pining effect of nickel coating and then leading to the increased toughness. Therefore, incorporating Ni-CNTs can fundamentally modify the nano/micro- scale structural nature of RPC, providing a bottom-up approach for controlling the properties of RPC. / Funding supported from the National Science Foundation of China (51908103 and 51978127) and the China Postdoctoral Science Foundation (2019M651116).

published_or_final_version / abstract / Physics / Master / Master of Philosophy

published_or_final_version / Physics / Master / Master of Philosophy

published_or_final_version / Chemistry / Master / Master of Philosophy

Carbon nanotubes have now been a subject of intense research for approaching two decades. Although a short time relative to most conventional materials, much hype about the intrinsic properties of this material has now been substantiated by experiment. The results are conclusive that carbon nanotubes are truly phenomenal materials with highly desirable mechanical, electrical and thermal properties. Furthermore, multiwall carbon nanotubes (MWNTs) have emerged as the most economically viable and abundant form of carbon nanotubes, and therefore the most likely candidate for application. The key materials engineering challenge remains in effectively transferring their properties to macro-scale materials in the form of composites. It is here that research merges with application. This dissertation has therefore been directed to focus on carbon nanotube composites in an applied sense. Here, the state of the art is reviewed, and experimental results of carefully selected composite systems, studied in detail for (1) mechanical, (2) electrical and (3) thermal properties, are presented and discussed. In terms of mechanical properties, the effects of MWNTs for augmentation of the tensile properties of PAN-based carbon fiber, and fatigue performance of poly(methyl methacrylate) are investigated and reported. In MWNT composite PAN-based carbon fiber, the formation of an ordered interphase layer sheathing the nanotubes was observed in fracture surfaces, which indicated a clear importance of their function to template the growth of carbon formation in the PAN-based matrix fiber. These structures open up a route to nano-scale tailorability of the crystallographic morphology of the composite fibers. Large improvements in fatigue performance were observed in MWNT/PMMA composites compared to MWNT/chopped carbon fiber composites, and attributed to the nanometer scale dimensions of the MWNTs enabling them to mitigate submicron damage such as polymer crazing. In terms of electrical and thermal properties, MWNT/epoxy composites were superior to MWNT/carbon black composites. Furthermore, extremely large improvements in the thermal conductivity of epoxy were observed for epoxy-infiltrated aligned MWNT arrays. The alignment of the MWNTs was shown to play a dominant role in enabling the improvement. Finally, these results, in concert with the literature are discussed in terms of the application of carbon nanotubes in engineering materials.

The objective of this PhD thesis was the study of friction in carbon nanotubes by analytical methods and molecular dynamics simulations. The goal of this research was to characterize the properties of friction in nanotubes and from a more general point of view the understanding of the microscopic origin of friction. Indeed, the relative simplicity of the system allows us to interpret more easily the physical phenomenon observed than in larger systems. In order to achieve this goal, non-equilibrium statistical mechanics permitted first to develop models based on Langevin equations describing the dynamics of rotation and translation in double walled nanotubes. The molecular dynamics simulations then permitted to validate these analytical models, and thus to study general properties of friction such as the dependence on area of contact, temperature and the geometry of the nanotubes. The results obtained shows that the friction increases linearly with the sliding velocity or the angular velocity until very high values beyond that non-linearities appear enhancing dissipation. In the linear regime, it is shown that the proportionality factor between the dynamic friction force and the velocity is given by the time integral of the autocorrelation function of the restoring force for the sliding friction and of the torque for the rotational friction. Furthermore, a novel resonant friction phenomenon increasing significantly dissipation was observed for the sliding motion in certain types of nanotubes. The effect arises at sliding velocities corresponding to certain vibrational modes of the nanotubes. When the dynamics is described by the linear friction in velocity, the empirical law stating that friction is proportional to the area of contact is very well verified thanks to the molecular dynamics simulations. On the other hand, friction increases with temperature. The fact that friction increases as well with the area of contact as the temperature can be easily interpreted. Indeed, if the temperature is large enough so that the electronic effects can be negligible, dissipation is only due to the phonons. Indeed, it is the phonons who give the sliding or rotation energy to the other degrees of freedom until thermodynamic equilibrium is achieved. Thus, if the temperature increases, the coupling between the phonons and the rotational or translational motions increases, as well as friction. In the same manner, when the area of contact increases, the number of available phonons to transport energy increases, explaining thus the increase of the friction force.

Carbon nanotubes have been found to have extraordinary properties, such as ballistic electrical conductivity, extremely high thermal conductivity and they can be metallic or semiconducting with a wide range of band gaps. There are however several issues that have to be solved before these properties can be fully utilised. One of these issues is that the nanotube growth temperature must be lowered in order to make the synthesis compatible with the fabrication processes used in electronics. The whole environment is heated to temperatures typically higher than 500 °C in the standard growth techniques whereas only a very localised area is heated in the technique developed here. This technique thus provides a way around the temperature issue. In the method developed here, the catalyst is deposited on top of a small metal (molybdenum) wire on the substrate. The high temperature required for nanotube growth is then reached by Joule heating by sending a current through the metal wire. This process eliminates the furnace which is used in conventional chemical vapour deposition and localises the high temperature to a very small and controlled area of the sample. Consequently, this technique is compatible with the semiconductor technology used today. Another advantage of this technique is that, since no furnace is required, a small growth chamber, which fits under a microscope, can be used. This allows in situ studies of the growth by optical microscopy and by Raman spectroscopy. By changing the carbon precursor, single- or multiwalled nanotubes can be grown. This can be important when producing devices since single-walled nanotubes predominantly are semiconducting whereas multi-walled mainly are metallic. The multi-walled nanotubes grow in a rapid and concerted process. This growth was monitored through an optical microscope. It was found that the thickness of the support layer and especially the catalyst are even more crucial parameters for nanotube growth using this local heating technique than in conventional processes. The activation energy could be extracted and was found to be 1.1-1.3 eV. The carbon nanotube growth was investigated by in situ Raman spectroscopy. The growth evolution could be well described by a model using the initial growth rate and the catalyst lifetime as parameters. The process was found to be limited by the mass transport of the carbon precursor. It was found that the molybdenum wire creates an additional pathway for the carbon cycle from gas to nanotube formation. The Raman spectra were studied at elevated temperatures. A decrease in intensity and a shift towards lower wavenumbers with increasing temperature was observed for the Stokes signal. It was found that the laser used for the Raman measurements could heat the nanotubes to high temperatures without any other heat source. Vertically aligned arrays of nanotubes were grown by conventional CVD. These arrays were actuated by applying a DC voltage between them. An effective Young's modulus of the arrays was found to be similar to that of rubber, which is orders of magnitude lower than for individual nanotubes. The capacitance between the arrays was measured to be tens of fF with a tunability of over 20%.

<p>In this study we report on the effect of the deposition parameters on the morphology and structural properties of CNTs, synthesized by means of the hot-wire chemical vapour deposition technique. SEM, Raman and XRD results show that the optimum deposition conditions for the HWCVD synthesis of aligned MWCNTs, with diameters between 50 and 150 nm and lengths in the micrometer range are: Furnace temperature of 500 &ordm / C, deposition pressure between 150 and 200 Torr, methane/hydrogen dilution of 0.67 and a substrateto- filament distance of 10 cm.</p>