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<p>The pore-size dependence of liquid diffusivity in mesopores has been a controversial topic. It is especially meaningful in dye-sensitized solar cells (DSSCs) because the triiodide ion diffusivity is closely related to the cell performance. By applying electrochemical measurements, the pore-size dependence of ion diffusivity in DSSCs was investigated based on TiO₂ thin films of variable pore diameters. The alternation of pore-size was achieved by the epitaxial growth of TiO₂ after TiCl₄ post-treatments. From the trend of normalized diffusivities, the respective valid regimes of pore-size dependent and independent diffusion were determined, which were separated by the transition point located at 5-7 nm. In addition, my results have showed that the DSSC fabrication processes, e.g., dye loading, TiCl₄ post-treatment will not lead to the transition of diffusion behaviors. Furthermore, the unexpected drop of diffusivity after one TiCl₄ treatment is attributed to the involvement of surface diffusion in untreated TiO₂ matrix.</p> / Master of Applied Science (MASc)

The main aim of the work reported is the design of proof-of-concept of at point-of-use hydrogen peroxide electrogeneration from air. The experimental work discussed within this thesis explores five major areas: the kinetics of electrocatalysis, ion-pairing, change of solvent media, the electrode surface modication by a redox mediator, and the electrochemical reduction of oxygen within enhanced mass transport systems. The electrocatalytic rates and mass transport of two oxygen reduction redox meditors, viz. anthraquinone and methyl viologen, are studied in aqueous solutions. The investigation is facilitated through the use of a boron-doped diamond electrode, allowing the catalytic response to be clearly delineated from that of the direct oxygen reduction process. The use of simulation software is highlighted in combination with experimental voltammograms to extract kinetic data. Specifically, the voltammetric features, such as the `reverse' peak and the `split waves', are given particular attention. Consequently, it is possible to deconvolute the electrocatalytic reaction mechanisms. The reactivity of the viologen radical cation is comparable to the semiquinone radical anion in aqueous solution ((4.8~6)x10^9 M^-1 s^-1), but over a far wider pH range (pH 2.5 - pH 8.5). The change of local proton concentration, and sequential electron transfers play key roles here. Moreover, the reduced reactivity of semiquinone is observed upon formation of ion-pairs with tetrabutylammonium cations in alkaline solutions. The electro-reduction of oxygen and its mediated pathways are also investigated in non-aqueous media; in particular the thermodynamics, the kinetics, and mass transport involved in these processes. Through a variable temperature study in electrolytic acetonitrile solution, the oxygen dissolution is quantitatively shown to be an endothermic process. Moreover, the diffusion coeficients and concentration of oxygen upon change of acetonitrile mole fraction is also explored in water-acetonitrile mixtures. The rates of bimolecular reactions are extracted from simulation programs, involving semiquinone in anhydrous acetonitrile and viologen radical cation in ethanol, and show a 3 - 4 orders of magnitude reduction compared to that in aqueous solution. Although the solubility of oxygen is ca. 6 - 8 times larger in non-aqueous solvents, the much reduced homogeneous rates limit the electrogeneration of hydrogen peroxide in pure organic media. Novel surface modification methodologies for graphitic surfaces with covalently attached anthraquinonyl groups are studied and characterised. The anthraquinonyl-modified carbon surfaces show much reduced overpotentials required for oxygen reduction. In the final chapter, utilising the new surface modification methodology and novel designs, two gravity-feed flow cells for electrochemical reduction of oxygen in aqueous solutions are proposed and characterised, one based upon the tubular electrode geometry. The other exhibits much enhanced current conversion by using a porous reticulated vitreous carbon electrode. The latter may provide a prototype hydrodynamic system to produce dilute hydrogen peroxide solution at point-of-use.

Com base em um modelo fenomenológico e a partir de dados experimentais obtidos numa planta piloto, foi obtida uma correlação entre o desempenho de uma torre de resfriamento em função das principais variáveis de processo: fluxos mássicos do gás e da água pela torre, e temperatura de entrada da água. Os resultados apresentaram boa consistência, comparados com os da literatura. A metodologia desenvolvida pode, com relativa facilidade, ser aplicada para torres de resfriamento industriais, pois se baseia em medidas de variáveis, factíveis em termos práticos. Efetuou-se também um estudo paralelo com base em modelagem e simulações matemáticas do comportamento de uma torre de resfriamento de água em condições severas, com temperatura da água de alimentação superior a 50°C. Constatou-se que o coeficiente de transporte de massa na torre de resfriamento aparentemente não é afetado. / Cooling towers are widely used in many industrial and utility plants and its thermal performance is of vital importance. In the present work, using a phenomenological model and by experiments carried on over a pilot installation, the mass transfer coefficient dependence of air and water flow rates and inlet cooling water temperature is determined. The approach proposed may be useful in addition for characterization of industrial cooling towers since it depends on temperature and flow rate measurement usually available in typical plants. A parallel study concerning high mass transfer rate theory is accomplished. Through mathematical modeling and simulations based on this study no influence is detected on the mass transfer coefficient in the cooling tower, operating under harsh conditions with inlet water temperature up to 90°C.

Centenas de compostos orgânicos são emitidos para a atmosfera por uma variedade de fontes biogênicas e antrópicas. Essa mistura complexa causa preocupação devido ao impacto que esses compostos podem provocar na saúde e no ambiente. Apesar de constituírem 10-70% da massa do aerossol atmosférico, a caracterização dos compostos orgânicos particulados permanece ainda deficitária. A contribuição das principais fontes de emissão para a poluição a nível regional pode ser diagnosticada através da utilização de marcadores moleculares específicos. De acordo com esta necessidade, o objetivo do presente trabalho é a caracterização de compostos orgânicos do aerossol atmosférico coletado em sítios de cidades da América do Sul. As amostragens do material particulado (MP10) foram realizadas em duas áreas em Bogotá na Colômbia, uma de influência industrial (BOI) e outra veicular (BOV) em 2007, e em áreas urbanas em São Paulo no Brasil em 2007 (SPA 07) e 2008 (SPA 08), e em Buenos Aires (BAI) na Argentina em 2008. Diferentes classes de compostos orgânicos foram determinadas: n-alcanos e hidrocarbonetos policíclicos aromáticos (HPA) por cromatografia a gás com detecção por ionização em chama (CG-DIC), n-alcanais e n-alcanonas por cromatografia a gás com detecção por espectrometria de massas (CG/EM). A composição e concentração do aerossol atmosférico foram afetadas substancialmente pelo transporte de massas provenientes de regiões com queima de biomassa e pelas condições meteorológicas locais. Em SPA 08, BOI e BAI foram encontradas as maiores concentrações de MP10 onde as amostragens foram realizadas em época de seca, favorecendo a acumulação dos poluentes. A constituição orgânica do MP10 nos sítios estudados reflete a incorporação nas amostras de marcadores moleculares biogênicos como n-alcanos com número ímpar de carbonos. Dentre os traçadores antrópicos, foram identificadas a presença de pristano e fitano, a mistura complexa não-resolvida (MCNR) e HPA de queima de combustíveis fósseis. Em SPA 08 foi identificada a presença do reteno, um traçador da queima de biomassa. As maiores concentrações de n-alcanais e n-alcanonas foram observadas no sítio BOV. Os n-alcanais (< C20) identificados nos sítios deste trabalho podem ser originados a partir da oxidação de alcanos e alcenos, e as n-alcanonas (< C20) podem ter como origem atividades antrópicas, processos oxidativos na atmosfera ou atuação microbiana sobre outros compostos. / Hundreds of organic compounds are emitted to the atmosphere by biogenic and anthropogenic sources. This complex mixture generates concern due to the impact of these compounds on health and environment. In spite of accounting for 10-70% of the atmospheric aerosol mass, particulate-phase organic compounds are not yet well characterized. The contribution of the major emission sources to regional particulate pollution can be diagnosed by using specific molecular markers. According to this necessity, the present work has as objective the characterization of organic compounds from atmospheric aerosol collected in sites of South American cities. The particulate matter (PM10) sampling were accomplished in two areas in Bogota (Colombia), one with industrial (BOI) and other with vehicular (BOV) influence in 2007, and in urban areas in Sao Paulo (Brazil) in 2007 (SPA 07) and 2008 (SPA 08), and in Buenos Aires (BAI), Argentina, in 2008. Different organic classes were determined: n-alkanes and polycyclic aromatic hydrocarbons (PAH) by gas chromatograph with flame ionization detection (GC-FID), n-alkanals and n-alkanones by gas chromatograph with mass spectrometer detection (CG/MS). The aerosol atmospheric composition and concentration were substantially affected by mass transport from biomass burning regions and by the local meteorological conditions. In SPA 08, BOI and BAI were found the highest PM10 concentrations where the samplings were accomplished in dry period, contributing for the pollutants accumulation. The PM10 organic composition at studied sites reflects the incorporation from biogenic molecular markers in the samples like n-alkanes with odd carbon number. Among the anthropogenic tracers, were identified the presence of pristane and phytane, the unresolved complex mixture (UCM) and PAH from fossil fuel combustion. In SPA 08 was observed the presence of retene, a biomass burning tracer. Higher concentration from n-alkanals and n-alkanones were observed at the BOV site. The n-alkanals (< C20) identified at sites from this work may be originated from oxidation of alkanes and alkenes, and n-alkanones (< C20) are derived from anthropogenic activity, atmospheric oxidative processes or microbial action over other compounds.

Windward Passage, which separates the islands of Cuba and Hispaniola, has been recognized as an important inflow channel to the Caribbean Sea for nearly a century. Despite this fact, few direct measurements of the volume transport through the passage exist. In an effort to gain a more comprehensive understanding of the variability, structure, and mean transport associated with flow through Windward Passage, the University of Miami?s Rosenstiel School of Marine and Atmospheric Science (RSMAS) and the National Oceanic and Atmospheric Administration (NOAA)?s Atlantic Oceanographic and Meteorological Laboratory (AOML) conducted a targeted research study of the passage and surrounding region from October 2003 through February 2005. The project deployed a moored current meter array across the passage and conducted four regional hydrographic surveys. Velocity sections collected across Windward Passage during the four cruises from lowered and hull-mounted acoustic Doppler current profilers show a highly variable field dominated by small-scale eddy features and other areas of locally-intensified flow. However, when integrated horizontally across the passage, the resulting transport-per-unit-depth profiles reveal a remarkably robust vertical shear structure. A net inflow of surface and thermocline waters was observed over the four cruises. Beneath these layers, a persistent outflow of intermediate water was found, intensified along the east side of the passage. Deep inflow, just above the sill depth maximum (1680 m), was observed on cruise #1 and, based on data from the moored current meter record, was determined to be a regular flow feature. Together, project velocity sections and water mass analyses of Windward and surrounding passages suggest that Surface Water (SFC), Subtropical Underwater (SUW), and Central Water (CW) primarily arrive at Windward Passage from the east via the Hispaniola Basin. A majority of SFC and SUW enters the Cayman Basin through Windward Passage, while the arriving CW bifurcates, with slightly more than half bypassing the passage and continuing westward north of Cuba. An intermediate water outflow pathway from the Cayman to the Hispaniola Basin via Windward Passage was also observed. Much of this outflow possessed a salinity signature characteristic of upstream inflow regions immediately to the east and south of the Lesser Antilles. Total Windward Passage transport, calculated from the four ship surveys, was found to be an inflow of 3.0 ±2.8 Sverdrups (1 Sv ≡ 10^6 m^3 s^-1). Data from the 16-month moored current meter array yielded a larger mean inflow of 5.0 ±1.6 Sv. These numbers are lower than previous estimates based on regional passage transport differences, and suggest that more transport may be entering the Florida Current system through passages in the Bahamas (the Northwest Providence and Old Bahama Channels) than previously thought, with proportionately less flow entering the system through the Caribbean Sea.

Photonic crystals (PhCs) are periodic dielectric structures that exhibit a photonic band gap; a range of wavelengths for which light propagation is forbidden. 2D PhCs exhibit most of the properties as their three dimension counterparts with a compatibility with standard semiconductor processing techniques such as epitaxial growth, electron beam lithography, Plasma deposition/etching and electromechanical lapping/polishing. Indium Phosphide (InP) is the material of choice for photonic devices especially when it comes to realization of coherent light source at 1.55 μm wavelength. Precise engineering of the nanostructures in the PhC lattice offers novel ways to confine, guide and control light in phonic integrated circuits (PICs). Strong confinement of light in PhCs offer novel opportunities in many areas of physics and engineering. Dry etching, a necessary process step in PhC device manufacturing, is known to introduce damage in the etched material. Process induced damage and its impact on the electrical and optical properties of PhCs depends on the etched material, the etching technique and process parameters. We have demonstrated a novel post-etch process based on so-called mass-transport (MT) technology for the first time on InP-based PhCs that has significantly improved side-wall verticality of etched PhC holes. A statistical analysis performed on several devices fabricated by MT process technology shows a great deal of improvement in the reliability of optical transmission characteristics which is very promising for achieving high optical quality in PhC components. Several PhC devices were manufactured using MT technology. Broad enough PhC waveguides that operate in the mono/multi-mode regime are interesting for coarse wavelength de-multiplexing. The fundamental mode and higher order mode interaction creates mini-stop band (MSB) in the dispersion diagram where the higher order mode has a lower group velocity which can be considered as slow light regime. In this thesis work, the phenomena of MSBs and its impact on transmission properties have been evaluated. We have proposed and demonstrated a method that enables spectral tuning with sub-nanometer accuracy which is based on the transmission MSB. Along the same lines most of the thesis work relates to broad enough PhC guides that operated in the multimode regime. Temperature tuning experiments on these waveguides reveals a clear red-shift with a gradient of dλ/dT=0.1 nm/˚C. MSBs in these waveguides have been studied by varying the width in incremental amounts. Analogous to semiconductors heterostructures, photonic heterostructures are composed of two photonic crystals with different band-gaps obtained either by changing the air-fill factor or by the lattice constant. Juxtaposing two PhC and the use of heterostructures in waveguide geometry has been experimentally investigated in this thesis work. In particular, in multimode line defect waveguides the “internal” MSB effect brings a new dimension in single junction-type photonic crystal waveguide (JPCW) and heterostructure W3 (HW3) for fundamental physics and applications. We have also fabricated an ultra-compact polarization beam splitter (PBS) realized by combining a multimode waveguide with internal PhC. MSBs in heterostructure waveguides have shown interesting applications such as designable band-pass flat-top filters, and resonance-like filters with high transmission. In the course of this work, InGaAsP suspended membrane technology was developed. An H2 cavity with a linewidth of ~0.4 nm, corresponding to a Q value of ~3675 has been shown. InGaAsP PhC membrane is an ideal platform to study coupled quantum well/dot-nanocavity system. / <p>QC 20120831</p>

Polymer electrolyte fuel cells (PEFC) convert chemicalenergy into electrical energy with higher efficiency thaninternal combustion engines. They are particularly suited fortransportation applications or portable devices owing to theirhigh power density and low operating temperature. The latter ishowever detrimental to the kinetics of electrochemicalreactions and in particular to the reduction of oxygen at thecathode. The latter reaction requires enhancing by the verybest catalyst, today platinum. Even so, the cathode isresponsible for the main loss of voltage in the cell. Moreover,the scarce and expensive nature of platinum craves theoptimisation of its use. The purpose of this thesis was to better understand thefunctioning of the porous cathode in the PEFC. This wasachieved by developing physical models to predict the responseof the cathode to steady-state polarisation, currentinterruption (CI) and electrochemical impedance spectroscopy(EIS), and by comparing these results to experimental ones. Themodels account for the kinetics of the oxygen reduction as wellas for the transport of the reactants throughout the cathode,i.e. diffusion of gases and proton migration. The agglomeratestructure was assumed for the description of the internalstructure of the cathode. The electrochemical experiments wereperformed on electrodes having a surface of 0.5 cm2 using alaboratory fuel cell. The response of the cathode to various electrodecompositions, thickness, oxygen pressure and relative humiditywas experimentally investigated with steady-state polarisation,EIS and CI techniques. It is shown that a content in thecathode of 35-43 wt % of Nafion, the polymer electrolyte, gavethe best performance. Such cathodes display a doubling of theapparent Tafel slope at high current density. In this region,the current is proportional to the cathode thickness and to theoxygen pressure, which, according to the agglomerate model,corresponds to limitation by oxygen diffusion in theagglomerates. The same analysis was made using EIS. Moreover,experimental results showed that the Tafel slope increases fordecreasing relative humidity. For Nafion contents lower than 35wt %, the cathode becomes limited by proton migration too. ForNafion contents larger than 40 wt %, the cathode performance athigh current density decreases again owing to an additionalmass transport. The latter is believed to be oxygen diffusionthroughout the cathode. The activity for oxygen reduction ofcatalysts based on iron acetate adsorbed on a carbon powder andpyrolysed at 900°C in ammonia atmosphere was alsoinvestigated. It was shown that the choice of carbon has atremendous effect. The best catalysts were, on a weight basis,as active as platinum. <b>Keywords:</b>polymer electrolyte fuel cell, cathode, masstransport, porous electrode, modelling, agglomerate model,electrochemical impedance spectroscopy, current interrupt,transient techniques, non-noble catalysts

Laser Chemical Vapor Deposition (LCVD) has been shown to have great potential for the manufacture of small, complex, two or three dimensional metal and ceramic parts. One of the most promising applications of the technology is in the fabrication of an integrated dispenser cathode assembly. This application requires the deposition of a boron nitridemolybdenum composite structure. In order to realize this structure, work was done to improve the control and understanding of the LCVD process and to determine experimental conditions conducive to the growth of the required materials. A series of carbon fiber and line deposition studies were used to characterize processshape relationships and study the kinetics of carbon LCVD. These studies provided a foundation for the fabrication of the first high aspect ratio multilayered LCVD wall structures. The kinetics studies enabled the formulation of an advanced computational model in the FLUENT CFD package for studying energy transport, mass and momentum transport, and species transport within a forced flow LCVD environment. The model was applied to two different material systems and used to quantify deposition rates and identify ratelimiting regimes. A computational thermalstructural model was also developed using the ANSYS software package to study the thermal stress state within an LCVD deposit during growth. Georgia Techs LCVD system was modified and used to characterize both boron nitride and molybdenum deposition independently. The focus was on understanding the relations among process parameters and deposit shape. Boron nitride was deposited using a B3N3H6-N2 mixture and growth was characterized by sporadic nucleation followed by rapid bulk growth. Molybdenum was deposited from the MoCl5-H2 system and showed slow, but stable growth. Each material was used to grow both fibers and lines. The fabrication of a boron nitridemolybdenum composite was also demonstrated. In sum, this work served to both advance the general science of Laser Chemical Vapor Deposition and to elucidate the practicality of fabricating ceramicmetal composites using the process.

Models of the fate and transport of river plumes and the bacteria they carry into lakes are developed. They are needed to enable informed decisions about beach closures to avoid economic losses, and to help design water intakes and operate combined sewer overflow schemes to obviate exposure of the public to potential pathogens. This study advances our understanding of river plumes dynamics in coastal waters by means of field studies and numerical techniques. Extensive field measurements were carried out in the swimming seasons of 2006 and 2007 on the Grand River plume as it enters Lake Michigan. They included simultaneous aerial photography, measurements of lake physical properties, the addition of artificial tracers to track the plume, and bacterial sampling. Our observed results show more flow classes than included in previous studies (e.g. CORMIX). Onshore wind can have a significant effect on the plume and whether it impacts the shoreline. A new classification scheme based on the relative magnitude of plume-crossflow length scale and Richardson number based on the wind speed is devised. Previous studies on lateral spreading are complemented with a new relationship in the near field. The plume thickness decreased rapidly with distance from the river mouth and a new non-dimensional relationship to predict thickness is developed. Empirical near field models for surface buoyant plumes are reviewed and a near field trajectory and dilution model for large aspect ratio surface discharge channels is devised. Bacterial reductions due to dilution were generally small (less than 10:1) up to 4.5 km from the river mouth. E. coli decay rates were significantly affected by solar radiation and ranged from 0.2 to 2.2 day-1 which were within the range of previous studies in Lake Michigan. Total coliform survived longer than E. coli suggesting different die-off mechanisms. Mathematical models of the bacterial transport are developed that employ a nested modeling scheme to represent the 3D hydrodynamic processes of surface river discharges in the Great Lakes. A particle tracking model is used that provides the capability to track a decaying tracer and better quantify mixing due to turbulent diffusion. Particle tracking models have considerable advantages over gradient diffusion models in simulating bacterial behavior nearshore that results in an improved representation of bacteria diffusion, decay and transport. Due to the complexity and wide variation of the time and length scale of the hydrodynamic and turbulent processes in the near field (where plume mixing is dominated by initial momentum and buoyancy) and far field (where plume mixing is dominated by ambient turbulence), a coupling technique is adapted. The far field random walk particle tracking model incorporates the empirical near field model. It simulates the transport, diffusion and decay of bacteria as discrete particles and employs the near field output as the source and transports the particles based on ambient currents predicted by the 3D hydrodynamic model. The coupled model improves dilution predictions in the near field. The new techniques advance our knowledge of the nearshore fate and transport of bacteria in the Great Lakes and can be ultimately applied to the NOAA Great Lakes Coastal Forecasting System to provide a reliable prediction tool for bacterial transport in recreational waters.