Browsing by Subject "electrical conductivity"
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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 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 Soil Salinity Abatement Following Hurricane Ike(2012-10-19) Mueller, RyanIn September 2008 Hurricane Ike hit the Texas Gulf Coast with a force stronger than the category 2 storm at which it was rated. With a 3.8 m (12.5 ft) storm surge, the agricultural industry in the area was devastated. The goal of this research was to determine the length of time required to reduce the salt levels brought by the storm surge to near pre-hurricane levels. To do this, four sets of samples were taken across two years and analyzed for salinity using the saturated paste extract method. The initial salt levels in November 2008 had an electrical conductivity (ECe) of the inundated soils as high as 26.7 dS/m. Fifty-four percent of the soils sampled in the 0-15 cm horizons and 9% in the 15-30 cm horizons of the edge area had an ECe >= 4 dS/m. In the surge area 79% of the soils sampled in the 0-15 cm horizons and 30% in the 15-30 cm horizons had an ECe >= 4 dS/m. In April 2009, 38% of the soils sampled in the 0-15 cm horizons and 13% in the 15-30 cm horizons of the edge area had an ECe >= 4 dS/m. In the surge area 71% of the soils sampled in the 0-15 cm horizons and 39% in the 15-30 cm horizons had an ECe >= 4 dS/m. By December 2009, none of the soils sampled in the edge area had an ECe >= 4 dS/m. In the surge area 21% of the soils sampled in the 0-15 cm horizons and 33% in the 15-30 cm horizons had an ECe >= 4 dS/m. By October 2010, all soils sampled had leached sufficient salts to be classified as non-saline to very slightly saline soils. Utilizing the November 2008 data set, 28 random samples were selected for exchangeable Na percent (ESP) in order to develop the ESP-SAR (Na adsorption ratio) predictive equation, ESP= 1.19(SAR)^0.82. The SAR-ESP relationship is statistically significant (95% confidence level), with a correlation coefficient of 0.964 (df=26).