Chemical and Biomolecular Engineering Theses and Dissertations

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End date: 2009Subject: search.filters.subject.Engineering, Materials Science

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    NANOSTRUCTURED THIN FILM POLYMER ELECTROLYTES FOR FLEXIBLE BATTERY APPLICATIONS
    (2009) Ghosh, Ayan; Kofinas, Peter; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    In recent years, the interest in polymeric batteries has increased dramatically. With the advent of lithium ion batteries being used in cell phones and laptop computers, the search for an all solid state battery has continued. Current configurations have a liquid or gel electrolyte along with a separator between the anode and cathode. This leads to problems with electrolyte loss and decreased performance over time. The highly reactive nature of these electrolytes necessitates the use of protective enclosures which add to the size and bulk of the battery. Polymer electrolytes are more compliant than conventional inorganic glass or ceramic electrolytes. The goal of this work was to design and investigate novel nanoscale polymer electrolyte flexible thin films based on the self-assembly of block copolymers. Block copolymers were synthesized, consisting of a larger PEO block and a smaller block consisting of random copolymer of methyl methacrylate (MMA) and the lithium salt of methacrylic acid (MAALi). The diblock copolymer [PEO-b-(PMMA-ran-PMAALi)] with added lithium bis(oxalato)borate, LiBC4O8 (LiBOB) salt (in the molar ratio ethylene oxide:LiBOB = 3:1) was used to form flexible translucent films which exhibited nearly two orders of magnitude greater conductivity than that shown by traditional high molecular weight PEO homopolymer electrolytes, in the absence of ceramic fillers and similar additives. The presence of the smaller second block and the plasticizing effect of the bulky lithium salt were shown to effectively reduce the crystallinity of the solid electrolyte, resulting in improved ion transporting behavior. The tailored solid self-assembled diblock copolymer electrolyte matrix also exhibits an exceptionally high lithium-ion transference number of 0.9, compared to a value between 0.2 and 0.5, shown by typical polymer-lithium salt materials. The electrolyte material also has a wide electrochemical stability window and excellent interfacial behavior with lithium metal electrode. The combination of these properties make electrolyte membranes composed of the diblock copolymer PEO-b-(PMMA-ran-PMAALi) and LiBOB salt, viable electrolyte candidates for flexible lithium ion based energy conversion/storage devices.
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    Combinatorial Experiments Using a Spatially Programmable Chemical Vapor Deposition System
    (2007-05-02) Sreenvivasan, Ramaswamy; Adomaitis, Raymond; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    A CVD reactor concept featuring a segmented design allows individual regions of a wafer to be exposed to different precursor concentrations simultaneously during a run resulting in different thickness profiles on the wafer and a thickness gradient at the boundaries between segment regions. Different recipes were cycled through each of the segments in a sequence of deposition experiments to develop a model relating precursor concentration to film thickness in each segment region. As a demonstration of spatial programmability, the system was re-programmed using this model to produce uniform thickness amongst the segments; inter-segment uniformity approaching 0.48 % (thickness standard deviation) was demonstrated. In a subsequent study, segmented CVD reactor designs enabling spatial control of across-wafer gas phase composition were evaluated for depositing graded films suitable for combinatorial studies. Specifically two reactor designs were evaluated with experiments and response surface model (RSM) based analysis to quantify the reactor performance in terms of film thickness uniformity, sensitivity to adjustable reactor operating conditions, range of thickness over which uniformity could be achieved and each reactor's ability to control the thickness gradient across the wafer surface. Design features distinguishing the two reactor systems and their influence on gradient control versus deposition rate performance are summarized. Response Surface (RS) models relating wafer state properties to process recipes are shown to be effective tools to quantify, qualify and compare different reactor designs.
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    Synthesis, Characterization, and Bio-conjugation of Silicon Nanocrystal
    (2006-12-21) choi, jonghoon; Wang, Nam Sun; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Bulk silicon is unable to emit fluorescence because of its indirect band gap nature. However, it has been reported that nano-structured silicon can be fluorescent when properly excited. Silicon nanocrystals (SN) have gathered scientific attention not only because of their uncertain fluorescence origin, but also because of their advantages over fluorescent dyes. This thesis summarizes studies on the synthesis, characterization and bio applications of fluorescent SN. Size control of SN provided size dependent tuning of the fluorescence. Higher quantum yield and narrower size distribution of SN have been achieved. HrTEM, SANS, light scattering, photoluminescence and optical transmission spectroscopy on the size measurements of SN helped determine the average size accurately. Fuctionalization of surface prepares silicon nanocrystals as a platform for bio conjugating applications.
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    Synthesis and Characterization of Diblock Copolymer Templated Iron Oxide Nanoparticles
    (2005-07-26) Akcora, Pinar; Kofinas, Peter; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Templating ordered assemblies of magnetic oxide nanoparticles within self-assembled diblock copolymers of varying morphologies is an important problem with a wide applicability such as in electromagnetics, optical devices, metal catalysts, medicine and biology. In this thesis, the effects of different polymer structures on particle ordering and resultant magnetic properties have been investigated using various microstructure and magnetic characterization tools. Ring-opening metathesis polymerization (ROMP) of norbornene and functionalized norbornene monomers has been used to synthesize diblock copolymers of narrow polydispersities using Grubbs' catalyst. These block copolymers can be used as templates to form inorganic nanoparticles. In this research, the structural and physical understanding of the inorganic-copolymer system was studied by small-angle neutron and x-ray scattering techniques and transmission electron microscopy. Synthesis of $\gamma$-Fe$_2$O$_3$ nanoparticles has been achieved within novel block copolymers of (norbornene)-b-(deuterated norbornene dicarboxylic) acid and (norbornene methanol)-(norbornene dicarboxylic acid). The polymer morphologies were controlled by varying the volume fractions of the constituent blocks. The pure norbornene based diblock copolymer morphologies were demonstrated by electron microscopy for the first time. Spherical, cylindrical and lamellar morphologies of these novel diblock copolymers were reported. The block ratios of the synthesized polymers were determined using gel permeation chromatography - light scattering, elemental analysis and UV-VIS spectroscopy. Solution phase doping and submersion of thin films in metal salt solutions were employed as metal doping methods and the observed nanoparticle structures were compared to those of the undoped copolymer morphologies. This project reports on the types of templating structures and dispersion of the nanoparticles. The effects of particle interactions on the microphase separation and magnetic properties were also investigated. The knowledge gained from understanding the templating mechanism in block copolymer / iron oxide nanocomposites can be applied to other similar systems for a variety of biological and catalyst applications.
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    Production of High Surface Area Silica Particles by Acoustodispersion Precipitation
    (2004-07-28) Matthews, Jermey Norman Augustus; Harris, Michael T; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    A continuous Acoustodispersion Precipitation Reaction (APR) flow process was applied to the production of high surface area silica particles. An ultrasonic reactor was used to atomize and disperse silica gel into a drying chamber for further solidification. Upon particle synthesis, the resulting powder was analyzed The effect of sodium silicate concentration, volumetric flow rate through the ultrasonic nozzle, and drying temperature on particle diameter and specific surface area (Asp) was observed. A mathematical equation was derived using the Box-Behnken statistical design. SEM imaging was employed in measuring the particle size and observing the morphology, and BET theory was exploited in collecting surface area data. SEM images showed porous and spherical silica particles. The number average diameter of the particles ranged from 13 - 55 microns. Most of the Asp values ranged from 100 300 m2/g. The highest Asp achieved was 342 m2/g and the lowest was 19.9 m2/g.