Harnessing the Properties of Carbon Nanotubes to Increase Applicability
Open Access
- Author:
- Rotella, Christopher
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 13, 2019
- Committee Members:
- Mauricio Terrones, Dissertation Advisor/Co-Advisor
Mauricio Terrones, Committee Chair/Co-Chair
Cristina Rosa, Outside Member
Joshua Alexander Robinson, Committee Member
James Hansell Adair, Committee Member
John C Mauro, Program Head/Chair - Keywords:
- Carbon Nanotubes
Chemical Doping
Increasing applicability
Biomedical applications
Li-ion Batteries - Abstract:
- Carbon nanotubes have been an interest in the field of materials research since their identification in 1991. They boast robust mechanical, electrical, thermal, and chemical properties that make them desirable for a wide range of applications. A main feature that is studied heavily in this work is their acceptance of many different chemical dopants. Dopants such as B, N, Si, and S have been utilized in previous works, all showing unique changes to the physical properties of CNTs, allowing them to be tailored to different applications. This thesis seeks to further increase the applicability of CNTs through various methods, for both broad and specific applications. Nitrogen and silicon doping and their affect on both single- and multi-walled carbon nanotubes is studied in the effort of controlling their growth parameters. By introducing a growth aid in the form of ethanol, heights of vertically aligned N-doped multi-walled tubes were extended over 200% from heights with no growth aid. This is done through the extension of the catalytic activity time by etching amorphous carbon that deposits. Results suggest that the density and size of the catalyst particles plays a large factor in the concentration of growth aid required to effectively extend growth times. Silicon doping of single-walled tubes was achieved experimentally through the use of the precursor methoxytrimethylsilane inside the precursor solution of an aerosol-assisted chemical vapor deposition system. It is reported here that higher concentrations of the silicon precursor (>0.15wt% in solution) have an adverse effect of the growth of CNTs, effectively poisoning the catalyst. At lower concentrations, however, the silicon precursor contributes to a decrease in average tube diameter and an increase in overall lattice disorder, observed through the use of Raman spectroscopy. Fitting electrical resistance measurements as a function of temperature to Mott’s variable range hopping models allowed us to determine the mechanism behind electron conductance between nanotubes in bundles. The data suggests that electrons move in a 3-dimensional hopping pattern for bundles of pristine single-walled nanotubes, confirming theoretical previous research. For samples with small concentrations of silicon precursor present, the electron hopping changes to a mix between 2- and 3-dimensional. This indicates that the silicon adatoms act as a scattering point for electrons, directing them to neighboring tubes with a dimensionality factor of two. Further theoretical models were used to study the effect of single or multiple silicon adatoms on neighboring single-walled tubes and their effect on conductance between those tubes. This data suggests that the inclusion of any silicon dopants increases the conductance between the tubes, compared to pristine tubes. Interconnected networks of multi-walled tubes were formed through the use of “welding” and annealing. The networks were formed through first cross-aligning sheets of CNTs and ensuring physical contact between them. Following this, Ar was bubbled through ethanol and introduced to the sample inside a tube furnace. The ethanol deposited carbon on the nanotube networks, effectively welding them together. Subsequent tubes increased dramatically in diameter from the original tubes, though the outer walls lacked the crystallinity found in the original tubes. Through an anneal at 2200℃ the overall crystallinity was increased, measured through Raman spectroscopy, high-resolution transmission electron microscopy, and fast Fourier transform. Mechanical testing was performed on the original, welded, and annealed samples to compare tensile strength. The welding process improved the tensile strength by 38%, and the further annealing increased the tensile strength by 66% of the original value. By pre-patterning the iron catalyst used to grow CNTs onto the substrate through the use of photolithography, patterned arrays of vertically aligned CNTs can be formed. These arrays were then utilized as filters to trap viruses through the devices’ encapsulation in a polymer mold. The CNT-STEP (size tunable enrichment platform) devices were utilized in this work towards the trapping and understanding of the tomato-spotted wilt virus (TSWV). TSWV is a virus that infects upwards of 1000 different plants across the globe and is responsible for massive monetary losses in damages annually. The problems containing the virus stem from the fast nature at which it is able to be spread to neighboring plants, the long time in between inoculation and symptoms (2 weeks), and its inability to be detected prior to symptom onset. Our CNT-STEP devices were utilized to trap the TSWV viruses with the arrays of aligned tubes, allowing larger and smaller contaminants to be removed from the sample. This process effectively hyper-concentrates the viruses, allowing conventional detection methods to be able to detect the viruses. Experimental data shows that the devices were able to effectively trap the viruses, but further processing is required to optimize the system in order to achieve enhancement (where previously undetectable low concentrations are hyper-concentrated in devices in order to be effectively detected). Interconnected networks of single-walled tubes were used as a backbone to support an active battery material (LNMC) to create flexible lithium-ion battery cathodes. Carbon nanotubes are able to effectively replace the metal current collectors and the polymer binders commonly used in Li-ion battery electrodes. By replacing these components with CNTs, the surface area of functional material is increased, while decreasing the weight. Novel methods were utilized to aerosolize powdered LNMC while simultaneously growing and depositing single-walled tubes to create an even dispersion. The CNTs provide mechanical support to the structure, while also being the main contributor to electrical conductivity in the sample. The weight percentage of CNTs in the sample is able to be tuned, with more flexible and mechanically robust samples having >5 wt% CNTs. The ideal samples were grown with between 1-3 wt % CNTs, as higher CNT content made samples increasingly difficult to process, due to inherent adhesion to all surfaces. Simultaneous tensile and resistance measurements were performed to compare growth parameters. Samples were able to withstand 10% elongation at break, but lower stresses than the materials they look to replace. Samples showed overall decreases in resistivity after multiple cycles of elongation and relaxation, indicating small amounts of disorder in the network that are eliminated through stretching. Overall the samples showed excellent promise for use in flexible battery electrodes. Through the use of chemical doping, and strict growth parameter control, the applicability of CNTs can be widely increased.