Browsing by Subject "Porous"
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Item Creation and In Vivo evaluation of a porous electrode for pacing in a coronary vein: an assessment of the potential for improved electrical performance and chronic stability of coronary venous pacing leads.(2009-05) Koop, Brendan EarlyIn this work, a porous electrode was fabricated and evaluated in a chronic animal study on a coronary venous pacing lead in order to assess its potential for mitigating chronic lead dislodgements and reducing the characteristic rise in pacing thresholds after implant, both of which being important issues that impact safety and efficacy of implanted cardiac resynchronization therapy systems. Eight test leads were assembled with a porous tip electrode with an average pore size of approximately 30 micrometers, created via a novel fabrication method, and eight control leads were assembled with a standard solid tip electrode design. Both groups were created without steroid-eluting collars and without a capacitive coating on the tip electrodes in order to isolate the affects of electrode porosity. Leads were implanted in canines, and electrical data and x-rays of lead position were taken regularly throughout the 60-day study. Tissue histology was performed for each lead. Significantly lower (p<0.05) mean rise in pacing threshold after implant was observed at day 3 and day 21 for the test group leads (with porous electrodes) as compared to the control group leads. Despite the higher surface area of the porous tip electrodes, pacing impedance was not statistically different between the groups throughout the study, a result likely due to decreased chronic inflammatory response at the surface of porous electrodes. The test group had no lead retractions after day 3 as determined by inspection of x-ray radiographs, while 3-6 (of 8) control group leads retracted after day 3, a result likely due to anchoring of the lead tip due to observed tissue growth into porous electrodes. Mean fibrous capsule thickness at pre-defined measurement points on the tip electrode was not statistically different between the groups, which correlates with the nearly equal mean pacing thresholds for the groups at day 60. The lack of lead retractions for the test groups leads after day 3 is a promising result which should be investigated further, along with investigations of lead extraction force and further electrical data evaluations, using larger sample sizes and more challenging implant conditions.Item Mixed convection in horizontal fluid-superposed porous layers(2013-08) Dixon, John MarkMixed convection in horizontal fluid-superposed porous layers is studied in the following work. Much research has been done in the field of natural, mixed, and forced convection in a porous layer. Several studies have investigated natural and forced convection in a two-domain system that includes a porous and a fluid layer, but mixed convection has not been addressed. This problem can be found in many natural and engineering applications. Some examples include beach sand, human lungs, bread, gravel, soil, rock, packed bed reactors, fiberglass insulation, thermal energy storage systems, electronic cooling, crude oil extraction, nuclear reactors, and the list goes on. The present study is motivated by the wide range of applications and seeks to fill the gap in the literature regarding mixed convection. The problem considers a long, narrow channel that is partially filled with a porous layer and has a fluid layer above the porous layer. The channel is partially heated on the bottom and cross flow along the length of the channel is added in varying degrees. The problem is studied at a fundamental level, with the governing equations being derived, non-dimensionalized, discretized, and solved numerically. The two layers are treated as a single domain and the porosity is used as a switching parameter, causing the governing equations to transition from an extended form of the Darcy-Brinkman-Forchheimer equation in the porous layer to the Navier-Stokes equations in the fluid layer. This method avoids the need for interfacial boundary conditions to be explicitly defined at the interface between the two domains. Several dimensionless numbers are varied and their effects on the overall Nusselt number of the system are documented. The parameters varied include the Peclet number, the Rayleigh number, the porous layer height ratio, the Darcy number, the Prandtl number, and the conductivity ratio between the solid and fluid phases. In addition, the impact of the various additional terms in the extended form of Darcy's law is investigated and documented as well. The conductivity ratio, Darcy number, porous layer height ratio, Rayleigh number, and Peclet number all have a strong effect on the overall Nusselt number of the system, while the Prandtl number, the Brinkman term, the Forchheimer term, and the convective terms have a negligible effect. A critical Peclet number was observed, where the Nusselt number is a minimum, and was shown to be proportional to the Rayleigh-Darcy number and inversely proportional to the porous layer height ratio. A critical porous layer height ratio was also found, where the Nusselt number is a minimum, and was shown to be proportional to the Rayleigh-Darcy number and inversely proportional to the Peclet number. The streamlines capture the transition from the natural convection regime to the forced convection regime. In the transition region the flow patterns have characteristics of both domains. The isotherms capture the plume flow and show the influence of the cross flow on the shape and character of the plume. An experimental apparatus is designed in order to collect data over a similar range of parameters as explored numerically. The average error between the numerical and experimental results is 30%, with a peak of 67%. The numerical results show good agreement with the experimental data within the bounds of uncertainty. The experimental results confirm the presence of a critical Peclet number. However, they do not show the same trends at intermediate porous layer height ratios. The effect of the porous layer height ratio, η=h_p⁄H, on the Nusselt number is shown to be small in the range of η = 0.5 to η = 1 and large in the range of η = 0 to η = 0.5. Also, the transition to the forced convection regime occurs earlier for the numerical results than it does for the experimental results. This points towards future research opportunities that focus on the lower range of porous layer height ratio values.Item Synthesis, characterization, and applications of porous and hierarchically-porous silica nanostructures(2014-10) Swindlehurst, Garrett RichardSilicate nanostructures can be broadly defined as any material primarily composed of silicon dioxide and having one or more dimensions smaller than 100 nm. Silica is formed of SiO4 tetrahedra connected at their vertices, and the way in which these tetrahedra can be arranged leads to materials classified as amorphous or crystalline, depending on the degree of long-range order in the structure. Due to the complexity of tetrahedral connectivity that is possible, pores can be formed in silicas with length scales ranging from a few angstroms to tens of nanometers. Some microporous silicates exist in nature, but many other porous silicas of considerable importance to chemical engineering are synthetic. One important class of these synthetic porous silicates is the zeolites, which contain pores on the size of angstroms and therefore can act as molecular sieves. In this dissertation, methods for the synthesis and characterization of "zero-dimensional" silica nanoparticles, "two-dimensional" zeolite nanosheets, and "three-dimensional" mesoporous silicas and zeolites are presented. Applications for these materials in catalytic and adsorption processes are also explored. Many of these nanostructured silicates contain hierarchical pore structure with different characteristic pore sizes existing in the materials. One particularly studied material, the self-pillared pentasil (SPP) zeolite, contains both the microporosity of traditional zeolites and mesoporosity resulting from its crystal growth mechanism. Hierarchical pore networks can significantly improve intraparticle mass transfer for interacting chemical species, offering great performance gain in the considered applications.Item Synthesis, characterization, and applications of porous and hierarchically-porous silica nanostructures(2014-10) Swindlehurst, Garrett RichardSilicate nanostructures can be broadly defined as any material primarily composed of silicon dioxide and having one or more dimensions smaller than 100 nm. Silica is formed of SiO4 tetrahedra connected at their vertices, and the way in which these tetrahedra can be arranged leads to materials classified as amorphous or crystalline, depending on the degree of long-range order in the structure. Due to the complexity of tetrahedral connectivity that is possible, pores can be formed in silicas with length scales ranging from a few angstroms to tens of nanometers. Some microporous silicates exist in nature, but many other porous silicas of considerable importance to chemical engineering are synthetic. One important class of these synthetic porous silicates is the zeolites, which contain pores on the size of angstroms and therefore can act as molecular sieves. In this dissertation, methods for the synthesis and characterization of "zero-dimensional" silica nanoparticles, "two-dimensional" zeolite nanosheets, and "three-dimensional" mesoporous silicas and zeolites are presented. Applications for these materials in catalytic and adsorption processes are also explored. Many of these nanostructured silicates contain hierarchical pore structure with different characteristic pore sizes existing in the materials. One particularly studied material, the self-pillared pentasil (SPP) zeolite, contains both the microporosity of traditional zeolites and mesoporosity resulting from its crystal growth mechanism. Hierarchical pore networks can significantly improve intraparticle mass transfer for interacting chemical species, offering great performance gain in the considered applications.Item The utilization of templated porous electrodes in electrochemical applications(2013-09) Fierke, Melissa AnnThe unifying theme within this work is three-dimensionally ordered macroporous (3DOM) carbon. This material consists of an ordered array of pores surrounded by a skeleton of amorphous carbon with nanometer-scale dimensions. 3DOM carbon offers several advantages that make it ideal for use in electrochemical applications. It has a high surface area, an interconnected pore structure, it is electrically conductive and chemically inert, the surface chemistry can be modified and characterized using slight modifications of well-established techniques, and robust monoliths can be produced. Here, 3DOM carbon was utilized in three distinct electrochemical applications. A three-dimensional interpenetrating lithium ion battery with a 3DOM carbon anode and a mixed vanadia/ruthenia cathode was investigated. Optimization of the synthesis of the polymeric separator layer and the ruthenia component of the cathode were carried out. The synthesis conditions and post-synthesis treatment greatly affect the degree of ruthenia deposition within the porous structure and the extent of hydration of the product. An ion-selective electrode system with 3DOM carbon as the solid contact was developed. 3DOM carbon was covered with an ionophore-based sensing membrane, allowing for selective detection of K+ or Ag+. This system exhibited very low detection limits (4.3 ppt for Ag+), unprecedented electrode stability, and little-to-no response to common interferents (such as carbon dioxide and light). The reasons for this excellent performance were investigated using a variety of characterization methods (with an emphasis on electrochemical techniques). The high surface area and low concentration of surface functional groups on 3DOM carbon are important factors. A receptor-based sensor for explosives detection was also developed. The pore walls of 3DOM carbon were modified with a receptor for 2,4-dinitrotoluene (DNT) using a series of chemical and electrochemical modification steps. Only 3DOM carbon that had been modified with the receptor exhibited a response to the presence of DNT. This selective detection of DNT was also possible in the presence of interfering molecules. However, the high capacitance of the 3DOM carbon led to poor limits when using cyclic voltammetry as the detection method. When square wave voltammetry was used, which eliminates the capacitive currents, much improved detection limits (10 μM) were achieved.