Multiwalled carbon nanotubes (MWNTs) are shown to be ballistic conductors at room temperature, with mean free paths of the order of tens of microns. The electrical transport measurements are performed both in air and in high vacuum in the transmission electron microscope on nanotubes pointing out of a nanotube-containing fiber that contact with a liquid metal. These measurements demonstrate that metallic MWNTs are one dimensional conductor that have quantized conductance nearly 1G0 (~{!V~}(12.9 K~{ and 8~})-1). The intrinsic resistance per unit length is found to be smaller than 100 ~{ and 8~}/~{ and L~}m, indicating a mean free path l> 65 ~{ and L~}m. The nanotube-metal contact resistances are in the range from 0.1 to 1 k~{ and 8 and L~}m. Contact scattering can explain why the measured conductances are about half of the expected theoretical value of 2G0. Current-to-voltage characteristic are in accord with the electronic structure. The nanotubes can survive high current (up to 1 mA, i.e., current density on the order of 109 A/cm2). In situ electron microscopy shows that a relative large fraction of the nanotubes do not conduct (even at high bias), consistent with the existence of semiconducting nanotubes. Discrepancies with other measurements are most likely due to damage caused to the outer layer(s) of the nanotubes during processing. The measured mean free path of clean, undamaged arc-produced MWNTs is several orders of magnitude greater than that for metals, making this perhaps the most significant property of carbon nanotubes.

No description available.

M.Sc. / Radionuclides exist in the environment because of natural and human activities that are an essential part of our lives. Nuclear processing, medicinal applications (using isotopes) and electric power production by nuclear stations are few examples of human activities that result in production of radioactive waste (RAW). The nuclear power stations in our world have to store their waste in such a manner that the present and future generations are protected from harmful radiations and this is a challenge. Exposure to RAW can result in severe, diverse and irreversible consequences such as damage of the ecosystem, pollution, cancers, birth mutations, to mention just a few. Solvent extraction (SE) technique is currently used to purify large volumes of secondary liquid waste before they are released to the environment or stored. However, even after the SE purification, highly radioactive liquid waste is given off. This highly radioactive liquid waste is solidified in a glass matrix (vitrification). In an attempt to reduce the disposal of large volumes of secondary RAW generated during the purification technology, this study was initiated to investigate the possibility of using multiwalled carbon nanotubes (MWCNTs) as part of the SE technique. As the main nuclear liquid extraction processes involve tributyl phosphate (TBP), the MWCNTs were linked to TBP, polymerised to give a MWCNTs-TBP polymer that was tested in the nuclear environment. This polymer should possess good chelating properties due to the inclusion of the phosphate and should be a good absorbent as MWCNTs are promising absorbent carbon materials. To test the hypothesis of the study MWCNTs-TBP polymer was tested for uranium extraction. The MWCNTs-TBP polymer gave a zero Kd value which indicates that the adsorption capacity of the polymer to remove radionuclides from waste streams was not successful. The MWCNTs were then tested for iodine-131 extraction whereby they were compared with single walled carbon nanotubes (SWCNTs) and double walled carbon nanotubes (DWCNTs). In this test SWCNTs gave a Kd value of 81694 mL/g which proved that they can be used in nuclear waste applications.

Single wall carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs) were dispersed in mineral oil and epoxy resin. The magnetorheological properties of these dispersions were studied using a parallel plate rheometer. Strain sweeps, frequency sweeps, magneto sweeps and steady shear tests were conducted in various magnetic fields. G', G", h* and ty increased with increasing magnetic field, which was partially attributed to the increasing degree of the alignment of nanotubes in a stronger magnetic field. The SWNT/mo dispersions exhibited more pronounced magnetic field dependence than SWNT/ep and MWNT/mo counterparts due to their much lower viscosity. The alignment of SWNTs in mineral oil increased with rising nanotube concentration up to 2.5vol% but were significantly restricted at 6.41vol% due to nanotube flocculation.

Les nanotubes de carbone mono-feuillets sont actuellement étudiés et développés du fait de leurs propriétés physiques uniques. En particulier, l'émission de photons uniques à température ambiante a été récemment publiée. Dans le but d'intégrer les nanotubes de carbone dans des dispositifs, des efforts doivent être faits du point de vue expérimental. Les nanotubes étant exclusivement constitués d'atomes de surface, leurs propriétés électroniques et optiques sont fortement influencées par leur environnement local. Par exemple, les phénomènes de scintillement et de diffusion spectrale sont observés lors d'expériences à basse température. Par ailleurs, les nanotubes sont des objets fragiles qui sont dégradés par les étapes de lithographie standard nécessaires à la réalisation de dispositifs photoniques. Notre stratégie est de synthétiser des nanostructures cœur-coquille : le nanotube de carbone constitue le cœur actif tandis qu'un polymère agit en tant que coquille protectrice. La synthèse et la caractérisation de ces nouveaux objets seront décrites en détails ainsi que leur étude en tant que candidats pour des dispositifs. Une étude de microphotoluminescence à basse température permettra de caractériser leurs propriétés en termes de scintillement et de diffusion spectrale. / The single-wall carbon nanotubes are currently studied and developed because of their unique physical properties. In particular, single-photon emission at room temperature has been recently reported. In order to integrate carbon nanotubes in devices, efforts have to be made on the material side. Nanotubes being only made of surface atoms their electronic and optical properties are strongly influenced by their local environment. For instance, blinking and spectral diffusion processes are observed in low temperature experiments. Moreover, nanotubes are fragile objects that are degraded by standard lithography processes needed to build real photonics devices. Our strategy is to synthesize core/shell nanostructures: the nanotube is the active core, while a polymer acts as a protective shell. Synthesis and characterization of these new objects will be described as well as their study as candidates for devices. Microphotoluminescence experiments at low temperature will permit to characterize blinking and spectral diffusion phenomena.

The discovery of nanotubes with unique mechanical, electrical, and thermal properties has led to their use in the development of the next generation of composite materials. However, their poor solubility and dispersion properties in any organic and aqueous solvents limits their potential applications. In order to improve their solubility, single-walled carbon nanotubes (SWNTs) were functionalized along their sidewalls with phenol groups using a 1,3-di^polar cycloaddition reaction. These phenols could be further derivatized with 2-bromoisobutyryl bromide, resulting in the attachment of atom transfer radical polymerization initiators to the sidewalls of the nanotubes. These initiators were found to be active in the polymerization of methyl methacrylate and t-butyl acrylate from the surface of the nanotubes. However, the polymerizations were not controlled, leading to the production of high molecular weight polymeric grafts with relatively large polydispersities. The resulting polymer carrying nanotubes were analyzed by IR, Raman spectroscopy, solid-state NMR, DSC, TEM, and AFM. The nanotubes functionalized with poly(methyl methacrylate) were found to be insoluble in organic solvents, such as THF and CH2CI2. However, the dispersion property of SWNTs in the polymer matrix was improved dramatically. Another monomer t-butyl acrylate (t-BuA) was also polymerized using the same macroinitiators. It was found that the SWNTs functionalized with t-BuA iii were soluble in a variety of organic solvents. The t-butyl groups of these appended polymers could also be removed to produce nanotubes functionalized with poly (acrylic acid), resulting in nanocomposites that are soluble in aqueous solutions. In addition, polystyrene (PS) which was prepared by stable free radical polymerization, was used to functionalize SWNTs through the radical coupling reaction. IR, NMR, TEM, and AFM confirmed that this polystyrene was covalently bonded to the SWNTs. It was also found that the resulting PS-SWNTs composites were quite soluble in organic solvents, such as THF and CH2C12. / Thesis / Master of Science (MSc)

This study focuses on the processing, structure, and properties of gel spun polyacrylonitrile (PAN) and polyacrylonitrile/carbon nanotube (PAN/CNT) carbon fibers. Gel spun PAN based carbon fibers are manufactured beginning with a study of PAN dissolution in an organic solvent (dimethylformamide, DMF). Homogeneity of the PAN/DMF solution is determined through dynamic shear rheology, and the slope of the Han Plot (log G’ vs log G’’). Solutions were then extruded into gel spun fibers using a 100 filament fiber spinning apparatus in a class 1000 cleanroom. Fibers were then subjected to fiber drawing, stabilization, and carbonization, to convert the PAN precursor fiber into carbon fiber. Carbon fiber tensile strength was shown to scale with the homogeneity of the PAN/DMF solution, as determined by the slope of the log G’ vs log G’’ plot. After the development of the understanding between the homogeneity of the PAN/DMF solutions on the gel spun PAN based carbon fiber tensile properties, the effect of altering the fiber spinning processing conditions on the gel spun PAN based carbon fiber structure and properties is pursued. Cross-sectional shape of the gel spun PAN precursor fiber, characterized with a stereomicroscope, was found to become more circular in cross-section as the gelation bath temperature was increased, the amount of solvent in the gelation bath was increased, and when the solvent was switched from DMF to dimethylacetamide (DMAc). Gel spun fibers were then subjected to fiber drawing, stabilization, and carbonization to manufacture the carbon fiber. Carbon fibers were characterized to determine single filament tensile properties and fiber structure using wide-angle x-ray diffraction (WAXD) and high resolution transmission electron microscopy (HRTEM). It was found that the carbon fiber tensile properties decreased as the carbon fiber circularity increased, as a result of the differences in microstructure of the carbon fiber that result from differences in fiber spinning conditions. In the second half of this study, the addition of CNT into the PAN precursor and carbon fiber is investigated. CNT addition occurs during the solution processing phase, prior to gel spinning. As a first study, Raman spectroscopy is employed to investigate the bundling behavior of the CNT after gel spinning and drawing of the PAN/CNT fibers. By monitoring the peak intensity of the (12,1) chirality in the as-received CNT powder, and in differently processed PAN/CNT fibers, the quality of CNT dispersion can be quickly monitored. PAN/CNT fibers were then subject to single filament straining, with Raman spectra collected as a function of PAN/CNT filament strain. As a result of the PAN/CNT strain, stress induced G’ Raman band shifts were observed in the CNT, indicating successful stress transfer from the surrounding PAN matrix to the dispersed CNT. Utilization of the shear lag theory allows for the calculation of the interfacial shear strength between the PAN and incorporated CNT, which is found to increase as the quality of CNT (higher aspect ratio, increased graphitic perfection, and reduced impurity content), quality of CNT dispersion, and fiber drawing increase. PAN/CNT fibers were then subjected to stabilization and carbonization for the manufacture of gel spun PAN/CNT based carbon fibers. These fibers were then characterized to investigate the effect of CNT incorporation on the structure and properties of the carbonized fibers. The gel spun PAN/CNT based carbon fibers were compared to commercially produced T300 (Toray) and IM7 (Hexcel) carbon fibers, and gel spun PAN based carbon fiber. Fiber structure was determined from WAXD and HRTEM. Carbon fibers properties investigated include tensile properties, and electrical and thermal conductivity. PAN/CNT based carbon fibers exhibited a 103% increase in room temperature thermal conductivity as compared to commercially available IM7, and a 24% increase in electrical conductivity as compared to IM7. These studies provide a further understanding of the processing, structure, property relationships in PAN and PAN/CNT based carbon fibers, beginning at the solution processing phase. Through the manufacture of more homogeneous PAN/DMF solutions and investigations of the fiber spinning process, gel spun PAN based carbon fibers with a tensile strength and modulus of 5.8 GPa and 375 GPa, respectively, were successfully manufactured in a continuous carbonization facility. Gel spun PAN/CNT based carbon fibers exhibit room temperature electrical and thermal conductivities as high as 74.2 kS/m and 33.5 W/m-K.

published_or_final_version / abstract / Chemistry / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Mechanical Engineering / Master / Master of Philosophy

No description available.