Key steps towards carbon nanotube-based conductors
Barrera, Enrique V.
Doctor of Philosophy
Making a robust carbon nanotube-based conductor as a replacement of copper in electricity grids can initiate a paradigm shift in energy transmission. This dissertation identifies four fundamental factors for making carbon nanotube-based conductors as functionalization, dispersion, concentration and processing. These four factors are discussed in detail by studying four separate systems: nanotube/epoxy composites, nanotube/porous medium density polyethylene (MDPE) composites, nanotube/high density polyethylene (HDPE) composites and pure nanotube cables. In nanotube/epoxy composites, homogeneous dispersion of nanotubes and a strong interface between nanotubes and epoxy matrix were simultaneously achieved through the development of a novel nanotube functionalization. While the degree of functionalization was high, the process was non-destructive to the mechanical properties of the nanotubes. In addition, the functional groups constructed covalent bonds with the epoxy matrix and also made dispersing the nanotubes much easier. As a result, the composites reinforced by the functionalized nanotubes had better mechanical properties than the samples reinforced by the raw nanotubes. In nanotube/porous MDPE composites, the degree of nanotube dispersion reached a level of 1 micron for nanotube agglomerate size within the matrix. This successful dispersion was primarily attributed to creating the porous MDPE. The pore size was tuned to be as small as 1 micron so that the sub-micron long HiPco nanotubes could easily penetrate into the matrix. The nanotube/porous MDPE composites obtained enhancement both in mechanical strength and electrical conductivity compared to the control samples. In nanotube/HDPE composites, the nanotube conducting networks were studied. Conductivity of the composites with the loading ratio at the percolation threshold was not sufficiently high for conductor applications. Nanotube/HDPE composite wires with higher loading ratios up to 40 wt% were prepared. Key factors for improving the formation of the conducting networks were identified. Through optimization in processing, maximum conductivity of âˆ¼10 3 S/m was achieved. Pure nanotube cables were prepared by a solid spinning procedure, which showed the potential to make macroscopic cables of various length and thickness. The pure nanotube cables circumvented the bottleneck in improving conductivity for composite systems, in which polymer in-between the nanotubes caused high contact resistance. The pure nanotube cables reached conductivity as high as âˆ¼10 6 S/m. Through iodine doping, conductivity further was enhanced so that the specific conductivity of the doped cables exceeded that of metals such as copper. As a result of applying the knowledge learned from study of the four fundamental factors, a macroscopic carbon-nanotube cable was created. It reached an unprecedented conductivity as high as âˆ¼10 7 S/m. Mechanically it was more robust than steel, but with 1/6 the weight. This advanced nanotube-based conductor can have a wide spectrum of applications such as transmission lines and low dimensional connecting wires.