A. James Clark School of Engineering
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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item CO Tolerance of Nano-architectured Pt-Mo Anode Electrocatalysts for PEM Fuel Cell Systems(2009) Hu, Jennifer Ezu; Jackson, Gregory S; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Enhanced CO tolerance of PEM fuel cell anode electrocatalysts is essential for improving the performance of PEM fuel cell systems operating with hydrocarbon reformers. This work explores the CO tolerance of PEM fuel cell membrane electrode assemblies (MEAs) fabricated with two promising nano-architectured Pt-Mo anode electrocatalysts -- Pt0.8Mo0.2 alloy and MoOx@Pt core-shell -- which demonstrated extremely high CO tolerance in previous thin-film electrode studies. By holding all other MEA components constant, polarization tests in pure H2 and H2 streams contaminated with up to 1000 ppm CO provided a basis for assessing the relative CO tolerance of the catalysts. Anode electrocatalyst stability was also investigated by operating the MEAs in 100 ppm CO over several days. Commercial and in-house fabricated MEAs with a conventional anode catalyst were used for comparison. Pt0.8Mo0.2 demonstrated the highest performance in CO, with a voltage drop of only 95 mV in 100 ppm CO at 0.5 A/cm2, compared to drops of 230 mV for PtRu and 260 mV for the core-shell electrocatalyst. However, the MoOx@Pt electrochemical performance, with its reduced Pt content, was comparable to the highly active Pt0.8Mo0.2 electrocatalyst for CO concentrations below 50 ppm on a per gram of precious metal basis, and preliminary stability studies indicate that the core-shell structure may also provide protection against detrimental Mo leaching in the acidic electrolyte. Both Mo-containing catalysts were poorly utilized, perhaps owing to residual surface contamination from the synthesis procedures, suggesting that their performance could be significantly improved with further optimization of fabrication procedures. A system-level model was also used to explore the impact of current-day and potential advances in CO tolerant electrocatalysts on the system performance of a PEM fuel cell system operating in conjunction with a hydrocarbon autothermal reformer and a preferential CO oxidation (PROx) reactor for CO clean-up. Empirical models of CO tolerance fuel cell performance were based on experimental data obtained with the Pt0.8Mo0.2 alloy tested in the experimental portion of this study. As CO tolerance was increased, system efficiencies improved due primarily at conditions where the fuel cell stack operated at high current densities, and the improvement is largely to higher fuel cell voltages and to a lesser extent to reductions in parasitic loads. Furthermore, increased fuel cell CO tolerance permitted significantly lower PROx CO selectivities and CO conversions without the significant penalties in overall system efficiency observed with the present-day CO tolerance of Pt alloy electrocatalysts.Item Self-Assembled Photoresponsive and Thermoresponsive Fluids with Tunable Rheology(2009) Kumar, Rakesh; Raghavan, Srinivasa R.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fluids whose rheological properties can be tuned by light or heat (termed as photorheological (PR) or thermorheological (TR) fluids, respectively) have attracted a lot of attention as they can be useful in numerous applications such as drug delivery, coatings, sensors, and valves for microfluidic devices. However, current formulations of these fluids suffer from several limitations: in particular, they often require synthesis of complex organic molecules by elaborate procedures, and this limits the widespread use of these fluids. In this dissertation, we seek to develop and investigate new classes of PR and TR fluids based on organic molecules that are readily available and quite inexpensive. Since no new synthesis is required, these systems could prove to be more attractive for a variety of applications. In the first part of this study, we describe a new aqueous photorheological (PR) fluid based on the zwitterionic surfactant, erucyl dimethyl amidopropyl betaine (EDAB) and the photosensitive molecule, ortho-methoxy cinnamic acid (OMCA). EDAB/OMCA fluids exhibit photogelling, i.e., a large (~ 10,000 fold) increase in viscosity upon exposure to UV radiation. We show that this photogelling is caused by the growth of long wormlike micelles in the sample. This structural change, in turn, is induced by the UV-induced isomerization of OMCA molecules from their trans to cis form. Evidence from zeta-potential studies, small-angle neutron scattering (SANS), and rheology are used to systematically reveal the molecular and microstructural mechanism for our results. In the second part of this study, we turn our attention to non-aqueous solvents and demonstrate a new class of PR fluids using such solvents. The PR effect here relies on transformations of "reverse" micellar structures formed by a well-known lipid (lecithin) in conjunction with para-coumaric acid (PCA). Lecithin/PCA fluids exhibit a substantial decrease in viscosity upon exposure to UV light (i.e., photothinning). Initially, the molecules self-assemble into long wormlike micelles, leading to highly viscoelastic fluids. Upon UV irradiation, PCA is photo-isomerized from trans to cis. This change in geometry induces a transition from long to short micelles. In turn, the solution viscosity is decreased by more than three orders of magnitude. Small-angle neutron scattering (SANS) is used to confirm the dramatic reduction in micellar length. In the last study, we report a class of aqueous fluids whose viscosity increases upon heating (i.e., thermo-thickening). These fluids are mixtures of telechelic associating polymers (HEURs) and a type of supramolecules called cyclodextrins (CDs) in water. Interestingly, we observe this behavior only with a particular type of CDs, called alpha-CDs, and not with the other common CD types, i.e., beta- and gamma-CDs. These results are explained in terms of a competition between the hydrophobic end-caps and the hydrophilic backbone of the polymer for complexation with alpha-CD molecules. We have also investigated the effect of amphiphiles (single-tailed surfactants and double-tailed lipids) on the thermo-thickening. The addition of lipids substantially enhances the thermo-thickening behavior, which is explained to be due to an enhancement of the connectivity of hydrophobic junctions by lipid vesicles.Item Radiation Transport Measurements in Methanol Pool Fires with Fourier Transform Infrared Spectroscopy(2008) Yilmaz, Aykut; Jackson, Gregory S; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Pool fires rely on radiation and conduction heat feedback from the combustion process to the liquid surface to vaporize the fuel. This coupled relationship determines the fuel burning rate and thus the fire structure and size. Radiative heat transfer is the dominant heat feedback in large pool fires. Species concentrations and temperatures have large influence on the radiative heat transfer in the fuel rich-core between the flame and the pool surface. To study radiative transport in the fuel-rich core, an experimental method was developed to measure radiative absorption through various pathlengths inside a 30 cm diameter methanol pool fire by using a Fourier Transform Infrared Spectrometer with N2 purged optical probes. The measured spectra are used to estimate species concentration profiles of methanol, CO, and CO2 in the fuel rich core by fitting predictions of a spectrally resolved radiation transport model to the measured spectra. Results show the importance of reliable temperature measurements for fitting the data and the need for further measurements to further understand the structure of fuel rich cores in pool fires.Item Micelles of Polybutadiene-b-Poly(Ethylene Oxide) in a Binary Solvent System(2008-05-01) Ploetz, Christopher D; Greer, Sandra C; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We studied the assembly behavior of a polybutadiene-b-poly(ethylene oxide) diblock copolymer in methanol, cyclohexane, and the corresponding partially miscible binary solvent system. Dynamic light scattering indicates that the copolymer forms coexisting spherical and cylindrical micelles in both of the pure solvents. In the binary solvent system, spherical micelles form in the methanol-rich phase for a wide range of temperatures. Conversely, micelles are present in the cyclohexane-rich phase only near the critical temperature. At the critical solvent composition, micelles form in the single phase region above the critical temperature. Size exclusion chromatography results for the binary solvent system show that the copolymer generally prefers the methanol-rich phase. The preference becomes more pronounced as temperature decreases.Item Design and Characterization of an Electrohydrodynamic (EHD) Micropump for Cryogenic Spot Cooling Applications(2008-04-21) Foroughi, Parisa; Ohadi, Michael M.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)High-temperature superconducting (HTSC) components are being incorporated into communication and monitoring electronic devices to increase their signal-to-noise ratio or their channel capacity. Those devices must be maintained at cryogenic temperatures to prevent the loss of their superconducting properties and retain their performance superiority. They are conventionally cooled via direct heat conduction, which leads to undesirable temperature differences among the various components being cooled. Compact micropumps capable of pumping liquid nitrogen at 77 K into liquid-cooling circuits would enable a much more compact and lightweight method of maintaining a uniform temperature across the cooling circuit. These pumps can also address the demand for delivering small doses of LN2 to particular spots in bioengineering applications.
One of the main objectives of the present study was to develop an electrohydrodynamic (EHD) ion-drag micropump with LN2 as the working liquid. EHD ion-drag pumping phenomenon refers to liquid motion caused by an interaction between electric and hydrodynamic fields in a dielectric liquid.
To investigate the effect of each design parameter on the performance of the micropump, several prototypes with four distinct designs were fabricated and packaged. The designs included a variety of emitter shapes, inter-electrode spacings, electrode-pair spacings, and channel heights. The micropumps were tested at different DC voltages ranging from 0 to 2.5 kV. Two test rigs with novel measurement techniques were also designed, built, and calibrated to measure the generated static pressure head, electric current, and flow rate with an acceptable level of accuracy.
The relationships between pressure/current (P-I) and pressure/voltage (P-V) for various designs were investigated experimentally. The results showed good agreement with the general analytical trends reported for EHD pumping in the literature. The experimental results also demonstrated that electrode geometry and gaps are effective in determining the pressure onset voltage. The results also show that a maximum static pressure head of 160 Pa at 1400 V is achievable for a design with a combination of a 50-μm emitter-collector gap, a 200-μm electrode-pair gap, and a saw-tooth shaped emitter/flat collector.Item SELF-ASSEMBLY OF AMPHIPHILIC MOLECULES IN ORGANIC LIQUIDS(2007-08-06) Tung, Shih-Huang; Raghavan, Srinivasa R.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Amphiphilic molecules are well-known for their ability to self-assemble in water to form structures such as micelles and vesicles. In comparison, much less is known about amphiphilic self-assembly in nonpolar organic liquids. Such "reverse" self assembly can produce many of the counterparts to structures found in water. In this dissertation, we focus on the formation and dynamics of such reverse structures. We seek to obtain fundamental insight into the driving forces for reverse self-assembly processes. Three specific types of reverse structures are studied: (a) reverse wormlike micelles, i.e., long, flexible micellar chains; (b) reverse vesicles, i.e., hollow containers enclosed by reverse bilayers; and (c) organogel networks. While our focus is on the fundamentals, we note that reverse structures can be useful in a variety of applications ranging from drug delivery, controlled release, hosts for enzymatic reactions, and templates for nanomaterials synthesis. In the first part of this study, we describe a new route for forming reverse wormlike micelles in nonpolar organic liquids. This route involves the addition of trace amounts of a bile salt to solutions of the phospholipid, lecithin. We show that bile salts, due to their unique "facially amphiphilic" structure, can promote the aggregation of lecithin molecules into these reverse micellar chains. The resulting samples are viscoelastic and show interesting rheological properties. Unusual trends are seen in the temperature dependence of their rheology, which indicates the importance of hydrogen-bonding interactions in the formation of these micelles. Another remarkable feature of their rheology is the presence of strain-stiffening, where the material becomes stiffer at high deformations. Strain-stiffening has been seen before for elastic gels of biopolymers; here, we demonstrate the same properties for viscoelastic micellar solutions. The second reverse aggregate we deal with is the reverse vesicle. We present a new route for forming stable unilamellar reverse vesicles, and this involves mixing short- and long-chain lipids (lecithins) with a trace of sodium chloride. The ratio of the short to long-chain lipid controls the type and size of self-assembled structure formed, and as this ratio is increased, a transition from reverse micelles to vesicles occurs. The structural changes can be explained in terms of molecular geometry, with the sodium chloride acting as a "glue" in binding lipid headgroups together through electrostatic interactions. The final part of this dissertation focuses on organogels. The two-tailed anionic surfactant, AOT, is well-known to form spherical reverse micelles in organic solvents. We have found that trace amounts (e.g., less than 1 mM) of the dihydroxy bile salt, sodium deoxycholate (SDC) can transform these dilute micellar solutions into self-supporting, transparent organogels. The structure and rheology of these organogels is reminiscent of the self-assembled networks formed by proteins such as actin in water. The organogels are based on networks of long, rigid, cylindrical filaments, with SDC molecules stacked together in the filament core.Item Functionalization of Nanoparticles for Biological Applications(2005-12-09) Koh, Isaac; Ehrman, Sheryl H.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Functionalization of metal oxide nanoparticles enables their use in biological applications via hybridization of biological molecules and modification of surface properties. This Ph. D research is aimed at increasing knowledge of the process of metal oxide nanoparticle functionalization for biological applications. The achievements presented in this dissertation can be divided into three categories: i) a fluorescence-based quantitative evaluation of surface coverage and bio-activity of antibodies immobilized on magnetic nanoparticles (MNPs), ii) differential functionalization of SiO2/TiO2 mixed nanoparticles via preferential binding of phosphonic acids to TiO2 and subsequent trimethyl silyl group binding to the remaining surface, and iii) X-ray scattering (XRS)-mediated detection of peak shifts of a biological substrate, Escherichia coli (E. coli), as a function of applied magnetic field strength and magnetic nanoparticle concentration in a cell growth medium. In a study of MNP surface modification, quantitative evaluation of anti-mouse IgG binding on MNPs and bioactivity on MNPs was conducted via fluorescence assays. Nanosize -Fe2O3 particles were hybridized with anti-mouse IgG via silane chemistry with 3-aminopropyltriethoxy silane and glutaraldehyde activation. A chemisorption isotherm via fluorescence assays demonstrated that immobilization of anti-mouse IgG can be stoichiometrically controlled with the surface coverage at saturation corresponding to 36% of the theoretical limit. The immobilized anti-IgG retains ~50% of its bioactivity at saturation. Differential functionalization of SiO2/TiO2 mixed nanoparticles was demonstrated via aqueous-phase preferential binding of phosphonic acids to TiO2 and subsequent binding of trimethyl silyl group to the remaining surface. SiO2/TiO2 mixed nanoparticles with three different mole ratios of Si/Ti together with pure SiO2 and TiO2 nanoparticles were used in comparative XPS study of differential functionalization. Differential functionalization of metal oxide-metal oxide mixed nanoparticles demonstrated herein adds a route to multifunctional nanoparticles. An In situ XRS study of E. coli in applied magnetic fields up to 423 mT was performed. Two peaks, a sharp peak at q = 0.528 Å-1 (1.189 nm) and a diffuse peak at q = 0.612 Å-1 (1.027 nm), were detected in XRS of MNP-absent E. coli culture. The presence of SiO2/-Fe2O3 MNPs at 40 mg/L in E. coli growth medium changes the sharp peak to the lower side of q as a function of applied magnetic field strength, while the position of the diffuse peak is invariable. 362 mT was found to be a critical magnetic field strength, at which the sharp peak disappears. This study demonstrates magnetic field-assisted interactions between E. coli cell membranes and MNPs.Item Coil-to-Helix Transition of Poly(ethylene oxide) in Solution(2004-07-30) Alessi, Michael Louis; Greer, Sandra C.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Poly(ethylene oxide) (PEO) is a simple polymer with repeating units [- C - C - O -] soluble in organic and aqueous solvents. The carbon atoms are hydrophobic; the oxygen atoms are hydrophilic and participate in hydrogen bonding. In all solvents in which PEO has previously been studied, PEO forms a coil in solution. Neutron scattering studies of PEO in isobutyric acid show that PEO undergoes a coil-to-rod transition in a solution of isobutyric acid (IBA). The stiffening is seen to progress smoothly with the addition of IBA, from a coil in D2O to a rod in pure deuterated-IBA. In addition to a solvent driven transition, a reversible rod-to-coil transition was seen to occur as a function of temperature, between 55 and 60 oC. Polarimetry experiments show that the rod formed by the PEO in solution is actually a helix, the conformation that PEO has in the solid state. It is also shown that, through the use of chiral impurities and temperature, the direction of the helix can be affected, allowing polymer folding to be influenced on a molecular level.