Browsing by Subject "carbon nanotube"
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Item Carbon Fillers for Actuation of Electroactive Thermoset Shape Memory Polyurethane Composites by Resistive Heating(2014-05-01) Yu, Ya-JenThe shape memory polymer (SMP) is one type of smart material with shape memory effect. SMP?s recovery can be actuated by external energy, such as heat. However, traditional direct heating limits potential applications of the SMP device. Thus, focusing on stimuli-responsive SMPs enables researchers to develop more versatile devices with SMP composites. The electroactive SMP composites incorporated with conductive fillers such as carbon black and carbon nanotubes allow shape recovery actuation by electrical resistive heating. Until now, most developments in electroactive SMPs are based on the use of a thermoplastic matrix doped with conductive fillers. Only limited reports have been made of thermoset polymers as the matrix used to synthesize an electroactive SMP composite. In this study, thermoset SMP composites with loading carbon black and carbon nanotube are made and characterized by evaluating thermomechanical behavior, measuring electrical resistivity and percolation threshold, coating with water-resistant membrane, and actuating the device with resistive heating. The electrical conductivity of thermoset SMP composites will be investigated so that a voltage-triggered or resistive-heat- triggered shape memory polymer for applications where a self-actuated polymer is necessary. The development of electroactive SMP composites makes this research advantageous for electrical resistive heating of device design in minimally invasive surgery application.Item Computational Analysis of Carbon Nanotube Networks in Multifunctional Polymer Nanocomposites(2013-09-16) Maxwell, Kevin SCarbon nanotubes (CNTs) have attracted much attention as reinforcements in polymer composite materials because of their unique mechanical, electrical, and thermal properties. The high electrical conductivity of CNTs is especially promising for use in multifunctional materials. Dispersing a small amount of CNTs in electrically insulating polymers has been shown to increase the conductivity of the material by many orders of magnitude because the high aspect ratio CNTs form percolating networks at very low volume fractions. Additionally, it has been shown that the application of mechanical strain to these nanocomposites results in a change in material resistivity, or piezoresistivity. Many experimental research e?orts have focused on optimizing this e?ect for strain and damage sensing applications, but much is still unknown about the dominant mechanisms a?ecting piezoresistivity. The objective of this work was to develop a computational model that can predict and investigate the electrical and piezoresistive properties of CNT/polymer composites. The nanocomposites were modeled as random networks of resistors in 2D and 3D in order to understand the mechanisms that a?ect the percolative, electrical, and piezoresistive performance of di?erent material systems. The model was used extensively to analyze and predict the electrical conductivity of 2D single-walled car- bon nanotube thin ?lms and 3D multi-walled carbon nanotube (MWCNT)/polymer nanocomposites. It was found that the contact resistance between individual nanotubes greatly a?ects the conductivity of 2D ?lms as well as 3D MWCNT/polymer materials. Additionally, it was shown that the electrical conductivity model could be calibrated to experimental results by adjusting the contact resistance alone. The 3D random resistor network model was also used to predict the piezoresis-tive properties for MWCNT/polymer Nano composites. The dominant mechanisms that cause the piezoresistive e?ect in these material systems were investigated, and the Poisson?s ratio of the composite was found to greatly impact the piezoresistive performance. The predictions indicated that decreasing the Poisson?s ratio of the composite leads to higher strain sensitivity, which could have implications for choosing material systems for strain sensor applications.Item Development of Advanced Nanomanufacturing: 3D Integration and High Speed Directed Self-assembly(2011-10-21) Li, HuifengDevelopment of nanoscience and nanotechnology requires rapid and robust nanomanufacturing processes to produce nanoscale materials, structures and devices. The dissertation aims to contribute to two major challenging and attractive topics in nanomanufacturing. Firstly, this research develops fabrication techniques for three dimensional (3D) structures and integrates them into functional devices and systems. Secondly, a novel process is proposed and studied for rapid and efficient manipulation of nanomaterials using a directed self-assembly process. The study begins with the development of nanoimprint lithography for nanopatterning and fabrication of 3D multilayer polymeric structures in the micro- and nano-scale, by optimizing the layer-transfer and transfer-bonding techniques. These techniques allow the integration of microfluidic and photonic systems in a single chip for achieving ultracompact lab-on-a-chip concept. To exemplify the integration capability, a monolithic fluorescence detection system is proposed and the approaches to design and fabricate the components, such as a tunable optical filter and optical antennas are addressed. The nanoimprint lithography can also be employed to prepare nanopatterned polymer structures as a template to guide the self-assembly process of nanomaterials, such as single-walled carbon nanotubes (SWNTs). By introducing the surface functionalization, electric field and ultrasonic agitation into the process, we develop a rapid and robust approach for effective placement and alignment of SWNTs. These nanomanufacturing processes are successfully developed and will provide a pathway to the full realization of the lab-on-a-chip concept and significantly contribute to the applications of nanomaterials.Item Effects of Carbon Nanotube Coating on Bubble Departure Diameter and Frequency in Pool Boiling on a Flat, Horizontal Heater(2011-08-08) Glenn, Stephen T.The effects of a carbon nanotube (CNT) coating on bubble departure diameter and frequency in pool boiling experiments was investigated and compared to those on a bare silicon wafer. The pool boiling experiments were performed at liquid subcooling of 10 degrees Celsius and 20 degrees Celsius using PF-5060 as the test fluid and at atmospheric pressure. High-speed digital image acquisition techniques were used to perform hydrodynamic measurements. Boiling curves obtained from the experiments showed that the CNT coating enhanced critical heat flux (CHF) by 63% at 10 degrees Celsius subcooling. The CHF condition was not measured for the CNT sample at 20 degrees Celsius subcooling. Boiling incipience superheat for the CNT-coated surface is shown to be much lower than predicted by Hsu's hypothesis. It is proposed that bubble nucleation occurs within irregularities at the surface of the CNT coating. The irregularities could provide larger cavities than are available between individual nanotubes of the CNT coating. Measurements from high-speed imaging showed that the average bubble departing from the CNT coating in the nucleate boiling regime (excluding the much larger bubbles observed near CHF) was about 75% smaller (0.26 mm versus 1.01 mm)and had a departure frequency that was about 70% higher (50.46 Hz versus 30.10 Hz). The reduction in departure diameter is explained as a change in the configuration of the contact line, although further study is required. The increase in frequency is a consequence of the smaller bubbles, which require less time to grow. It is suggested that nucleation site density for the CNT coating must drastically increase to compensate for the smaller departure diameters if the rate of vapor creation is similar to or greater than that of a bare silicon surface.Item Micromechanics modeling of the multifunctional nature of carbon nanotube-polymer nanocomposites(2009-06-02) Seidel, Gary DonThe present work provides a micromechanics approach based on the generalized self-consistent composite cylinders method as a non-Eshelby approach towards for assessing the impact of carbon nanotubes on the multi-functional nature of nanocom-posites in which they are a constituent. Emphasis is placed on the e?ective elastic properties as well as electrical and thermal conductivities of nanocomposites con-sisting of randomly oriented single walled carbon nanotubes in epoxy. The e?ective elastic properties of aligned, as well as clustered and well-dispersed nanotubes in epoxy are discussed in the context of nanotube bundles using both the generalized self-consistent composite cylinders method as well as using computational microme-chanics techniques. In addition, interphase regions are introduced into the composite cylinders assemblages to account for the varying degrees of load transfer between nanotubes and the epoxy as a result of functionalization or lack thereof. Model pre-dictions for randomly oriented nanotubes both with and without interphase regions are compared to measured data from the literature with emphasis placed on assessing the bounds of the e?ective nanocomposite properties based on the uncertainty in the model input parameters. The generalized self-consistent composite cylinders model is also applied to model the electrical and thermal conductivity of carbon nanotube-epoxy nanocomposites. Recent experimental observations of the electrical conductivity of carbon nanotube polymer composites have identi?ed extremely low percolation limits as well as a per-ceived double percolation behavior. Explanations for the extremely low percolation limit for the electrical conductivity of these nanocomposites have included both the creation of conductive networks of nanotubes within the matrix and quantum e?ects such as electron hopping or tunneling. Measurements of the thermal conductivity have also shown a strong dependence on nanoscale e?ects. However, in contrast, these nanoscale e?ects strongly limit the ability of the nanotubes to increase the thermal conductivity of the nanocomposite due to the formation of an interfacial thermal resistance layer between the nanotubes and the surrounding polymer. As such, emphasis is placed here on the incorporation of nanoscale e?ects, such as elec-tron hopping and interfacial thermal resistance, into the generalized self-consistent composite cylinder micromechanics model.Item Theoretical Investigations on Nanoporpus Materials and Ionic Liquids for Energy Storage(2012-02-14) Mani Biswas, MousumiIn the current context of rapidly depleting petroleum resources and growing environmental concerns, it is important to develop materials to harvest and store energy from renewable and sustainable sources. Hydrogen has the potential to be an alternative energy source, since it has higher energy content than petroleum. However, since hydrogen has very low volumetric energy density, hence it is important to design nano porous materials which can efficiently store large volumes of hydrogen gas by adsorption. In this regard carbon nanotube and Metal Organic Framework (MOFs) based materials are worth studying. Ionic liquids (IL) are potential electrolytes that can improve energy storage capacity and safety in Li ion batteries. Therefore it is important to understand IL's thermodynamic and transport properties, especially when it is in contact with electrode surface and mixed with Li salt, as happens in the battery application. This dissertation presents computation and simulation based studies on: 1. Hydrogen storage in carbon nanotube scaffold. 2. Mechanical property and stability of various nanoporous Metal Organic Frameworks. 3. Thermodynamic and transport properties of [BMIM][BF4] ionic liquid in bulk, in Li Salt mixture, on graphite surface and under nanoconfinement. In the first study, we report the effects of carbon nanotube diameter, tube chirality, tube spacer distance, tube functionalization and presence of Li on hydrogen sorption capacity and thermodynamics at different temperature and pressure. In the second one, we observe high pressure induced structural transformation of 6 isoreticular MOFs: IRMOF-1. IRMOF-3, IRMOF-6, IRMOF-8, IRMOF-10 and IRMOF-14, explore the deformation mechanism and effect of Hydrogen inside crystal lattice. In the third study, we observe the equilibrium thermodynamic and transport properties of [BMIM][BF4] ionic liquid. The temperature dependence of ion diffusion, conductivity, dielectric constant, dipole relaxation time and viscosity have been observed and found similar behavior to those of supercooled liquid. The ion diffusion on graphite surfaces and under nanoconfinement was found to be higher compared to those in bulk.