Slow-release devices retained in the rumen, are a simple method for continuous administration of bioactives to ruminant animals. To satisfy regulatory requirements and avoid waste of bioactive due to under- or over-dosing, it is advantageous to have a constant and predictable release rate. Existing intraruminal controlled-release technologies cannot easily be adapted for different bioactives or rates of release and can be influenced by the variable physiological environment in the rumen. Some existing commercial products use the pressure generated by a hydrogen gas-producing cell to extrude fluids from a syringe-like device. This technology may provide advantages for ruminal controlled-release as the gas production rate is unaffected by environment in the rumen and can be easily adjusted using electrical resistance applied to the gas cell. This technology was adapted for use in the rumen in these studies. Initial experiments identified the need for greater understanding of the rate that hydrogen is produced by the gas cell and the rate that gas diffuses through the barrel walls. Gas production rate was found to be inversely proportional to the resistance applied to the gas-producing cell. Factors affecting gas diffusion rate from the device were studied and a polymer was identified that reduced hydrogen diffusion to 5% of that for the initial components used. A relationship was developed to predict the release profile of a device. Controlled-release devices were constructed from selected materials. They released blank formulation at in vitro at a constant rate, which was within experimental variation of predicted values. Release rates from the devices used in vivo were slightly higher than predicted. The presence of rumen gases inside in vivo devices suggested that the difference may be due to inward diffusion of these gases; these may be eliminated by further study of barrel materials. Recommendations on the redesign of this technology for use as a generic intraruminal delivery system are given.

Accuracy in the effective diffusion coefficient of the gas diffusion layer (GDL)/microporous layer (MPL) is important to accurately predict the mass transport limitations for high current density operation of polymer electrolyte membrane (PEM) fuel cells. All the previous studies regarding mass transport limitations were limited to pure GDLs, and experimental analysis of the impact of the MPL on the overall diffusion in the porous GDL is still lacking. The MPL is known to provide beneficial water management properties at high current operating conditions of PEM fuel cells but its small pore sizes become a resistance in the diffusion path for mass transport to the catalyst layer. A modified Loschmidt cell with an oxygen-nitrogen mixture is used in this work to determine the effect of MPL on the effective diffusion coefficients. It is found that Knudsen effects play a dominant role in the diffusion through the MPL where pore diameters are less than 1 μm. Experimental results show that the effective diffusion coefficient of the MPL is only about 21% that of its GDL substrate and Knudsen diffusion accounts for 80% of the effective diffusion coefficient of the GDL with MPL measured in this study. No existing correlations can correlate the effective diffusion coefficient with significant Knudsen contribution.

This thesis investigates surface coatings as xenon diffusion barriers on plastic scintillators. The motivation for the work is improved radioxenon detection systems, used within the verification regime of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). One type of radioxenon detection systems used in this context is the Swedish SAUNA system. This system uses a cylindrical plastic scintillator cell to measure the beta decay from radioxenon isotopes. The detector cell also acts as a container for the xenon sample during the measurement. One problem with this setup is that part of the xenon sample diffuses into the plastic scintillator material during the measurement, resulting in residual activity left in the detector during subsequent measurements. This residual activity is here referred to as the memory effect. It is here proposed, and demonstrated, that it is possible to coat the plastic scintillator material with a transparent oxide coating, working as a xenon diffusion barrier. It is found that a 425 nm Al2O3 coating, deposited with Atomic Layer Deposition, reduces the memory effect by a factor of 1000, compared an uncoated detector. Furthermore, simulations show that the coating might also improve the light collection in the detector. Finally, the energy resolution of a coated detector is studied, and no degradation is observed. The focus of the thesis is measurements of the diffusion barrier properties of Al2O3 films of different thicknesses deposited on plastic scintillators, as well as an evaluation of the expected effect of a coating on the energy resolution of the detector. The latter is studied through light transport simulations. As a final step, a complete coated plastic scintillator cell is evaluated in terms of memory effect, efficiency and energy resolution. In addition, the xenon diffusion process in the plastic material is studied, and molecular dynamics simulations of the Xe-Al2O3 system are performed in order to investigate the reason for the need for a rather thick coating to significantly reduce the memory effect.

Accuracy in the effective diffusion coefficient of the gas diffusion layer (GDL)/microporous layer (MPL) is important to accurately predict the mass transport limitations for high current density operation of polymer electrolyte membrane (PEM) fuel cells. All the previous studies regarding mass transport limitations were limited to pure GDLs, and experimental analysis of the impact of the MPL on the overall diffusion in the porous GDL is still lacking. The MPL is known to provide beneficial water management properties at high current operating conditions of PEM fuel cells but its small pore sizes become a resistance in the diffusion path for mass transport to the catalyst layer. A modified Loschmidt cell with an oxygen-nitrogen mixture is used in this work to determine the effect of MPL on the effective diffusion coefficients. It is found that Knudsen effects play a dominant role in the diffusion through the MPL where pore diameters are less than 1 μm. Experimental results show that the effective diffusion coefficient of the MPL is only about 21% that of its GDL substrate and Knudsen diffusion accounts for 80% of the effective diffusion coefficient of the GDL with MPL measured in this study. No existing correlations can correlate the effective diffusion coefficient with significant Knudsen contribution.

Afin de favoriser la commercialisation à grande échelle des piles à combustible PEMFC, de nombreuses études sont menées dans le but de réduire les coûts et d'augmenter la durée de vie tout en améliorant les performances et de comprendre les phénomènes physiques mis en jeu. Cette étude se concentre sur le développement de couches de diffusion pour un fonctionnement à faible humidité relative, en particulier sur l'influence de la structure du support de diffusion sur les performances. Grâce à un procédé de fabrication différent de ceux utilisés pour les produits actuellement commercialisés, la structure des supports est plus aisément modifiée. Ainsi, selon les paramètres de fabrication choisis, les propriétés physico-chimiques associées donnent accès à des niveaux de performances différents. Le modèle d'analyse mis en place permet d'expliquer ces différences observées. Par conséquent, il permet de relier les paramètres de fabrication aux propriétés physiques et aux performances en pile. / To help PEMFC development and large scale commercialization, several studies deal with reducing costs and increasing durability while trying to improve performances and to understand physical phenomena involved. This study focuses on developing gas diffusion media which operates at low relative humidity, more particularly it deals with the influence of the structure of gas diffusion media on performances. Thanks to a process different from those used for currently commercialized gas diffusion layers, the structure of the media is more easily modified. According to the manufacturing parameters chosen, different physical and chemical properties will be obtained and thus different performances. A model is used to help analyze these differences and consequently allow the link between manufacturing parameters and physical properties and performances.

Une pile à membrane échangeuse de protons (PEMFC) est un dispositif convertissant l’hydrogène en électricité grâce à une réaction électrochimique appelé électrolyse inverse. Comme chaque pile à combustible ou batterie, les PEMFC sont composées d’une série de couches. Nous nous intéressons à la couche de diffusion (GDL) du côté de la cathode. La GDL est constituée de fibres de carbone traitées pour être hydrophobes. Elle peut être vue comme un milieu poreux mince avec une taille moyenne de pores de quelques dizaines de microns. Une question clé dans ce système est la gestion de l'eau produite par la réaction. Dans ce contexte, le principal objectif de la thèse est le développement d'un outil numérique visant à simuler la formation de l'eau liquide dans la GDL. Une approche réseau de pores est utilisée. Nous nous concentrons sur un scénario où l’eau liquide se forme par condensation dans la GDL. Les comparaisons entre simulations et expériences effectuées grâce à un dispositif microfluidique bidimensionnel, sont d'abord présentées pour différentes conditions de mouillabilité, de distributions de température et d'humidité relative à l’entrée, afin de valider le modèle. Une étude de sensibilité est alors effectuée afin de mieux caractériser les paramètres contrôlant l'invasion de l'eau. Enfin, les simulations sont comparées à des distributions d’eau obtenues in-situ par micro-tomographie à rayons X, ainsi que des distributions expérimentales de la littérature obtenues par imagerie neutronique. / A Proton Exchange Membrane Fuel Cell (PEMFC) is a device converting hydrogen into electricity thanks to an electrochemical reaction called reverse electrolysis. Like every fuel cell or battery, PEMFCs are made of a series of layers. We are interested in the gas diffusion layer (GDL) on the cathode side. The GDL is made of carbon fibers treated hydrophobic. It can be seen as a thin porous medium with a mean pore size of few tens of microns. A key question in this system is the management of the water produced by the reaction. In this context, the main objective of the thesis is the development of a numerical tool aiming at simulating the liquid water formation within the GDL. A pore network approach is used. We concentrate on a scenario where liquid water forms in the GDL by condensation. Comparisons between simulations and experiments performed with a two-dimensional microfluidic device are first presented for different wettability conditions, temperature distributions and inlet relative humidity in order to validate the model. A sensitivity study is then performed to better characterize the parameters controlling the water invasion. Finally, simulations are compared with in situ experimental water distributions obtained by X-ray micro-tomography as well as with experimental distributions from the literature obtained by neutron imaging.

Conselho Nacional de Desenvolvimento Científico e Tecnológico / Given the difficulties of determining the relative humidity (RH) in rooms used for the storage of apples accurately, the aim of this study was to calibrate the ventilated evaporimeter, an instrument used to measure the water evaporation that occurs due to the relative humidity and temperature, as well as evaluating if the mass loss improves gas diffusion and contributes to the quality maintenance of Royal Gala and Galaxy apples during storage and enables the use of a high CO2 partial pressure. This work also aimed to evaluate if there are any differences in the levels of mass loss between apples that were placed in distinct positions in the controlled atmosphere storage room. In order to do this, four experiments were performed: the first aimed to calibrate the ventilated evaporimeter in 85, 90, 95 and 100% of RH at a temperature of 1ºC. In the second experiment, the treatments were consisted by the period and level of mass loss (1 or 3%) in Royal Gala apple stored in controlled atmosphere at a temperature of 1ºC. For the third experiment, carried out with Galaxy apple, treatments were conducted with a high CO2 (3.0 kPa) and a low O2 (0.4 kPa) partial pressure associated with 3% of mass loss, aside from two treatments with a respiratory quotient (RQ) of 1.5, at 1ºC. For the fourth experiment, apple samples were placed in different positions in a commercial apple storage room in order to evaluate the mass loss. It was possible to obtain an average value of evaporation, in cm day-1, at a temperature of 1ºC, in different relative humidities through the calibration of the evaporimeter and the daily determination of water evaporation. The mass loss, both initial and linear, during the storage period, improved gas diffusion in the pulp of Royal Gala apple and reduced cracking, flesh breakdown and mealiness. The mass loss in Galaxy apple also improved the gas diffusion, which enabled the use of 3.0 kPa of CO2, however, with an RQ 1.5, a high CO2 partial pressure cannot be used. Mass loss occurred heterogeneously in the commercial storage room, being higher at the bottom and on the front of the storage room. / Diante das dificuldades de determinação precisa da umidade relativa em câmaras de armazenamento de maçãs, este trabalho teve como objetivo calibrar o evaporímetro ventilado, equipamento para quantificar a evaporação de água que ocorre em função da umidade relativa e temperatura, além de avaliar se a perda de massa aumenta a difusão de gases e contribui para manutenção da qualidade de maçãs Royal Gala e Galaxy e possibilita a utilização de pressão parcial de CO2 mais elevada em atmosfera controlada. O trabalho também objetivou avaliar se existe diferença de níveis de perda de massa em diferentes posições de uma câmara comercial de armazenamento de maçãs. Para isso foram realizados quatro experimentos: no primeiro buscou-se calibrar o evaporímetro ventilado nos níveis de UR de 85, 90, 95 e 100% na temperatura de 1ºC. No segundo experimento os tratamentos constituíram-se de período e nível de perda de massa (1 ou 3%) em maçã Royal Gala em atmosfera controlada na temperatura de 1ºC. No terceiro experimento, com maçãs Galaxy , os tratamentos foram com pressão parcial de CO2 (3,0 kPa) e O2 (0,4 kPa) associado com 3% de perda de massa, além de dois tratamentos com Quociente Respiratório (QR) de 1,5, todos em 1ºC. Num quarto experimento, os tratamentos foram amostras de maçãs expostas em diferentes posições de uma câmara comercial para avaliar a perda de massa. Por meio da calibração do evaporímetro foi possível, através da determinação da evaporação diária de água, obter um valor de evaporação médio, em cm dia-1, para temperatura de 1°C em diferentes umidades relativas. A perda de massa, tanto inicial quanto linear durante o período de armazenamento, aumentou a difusão de gases na polpa de maçã Royal Gala , reduziu rachadura, degenerescência e polpa farinácea. Em maçã Galaxy a perda de massa também aumentou a difusão de gases, o que possibilitou a utilização de 3,0 kPa de CO2, entretanto, com QR 1,5 o alto CO2 não pôde ser utilizado. A perda de massa ocorreu de forma heterogênea na câmara comercial de armazenamento, sendo mais elevada na parte inferior e na frente da câmara.

abstract: This work aimed to characterize and optimize the variables that influence the Gas Diffusion Layer (GDL) preparation using design of experiment (DOE) approach. In the process of GDL preparation, the quantity of carbon support and Teflon were found to have significant influence on the Proton Exchange Membrane Fuel Cell (PEMFC). Characterization methods like surface roughness, wetting characteristics, microstructure surface morphology, pore size distribution, thermal conductivity of GDLs were examined using laser interferometer, Goniometer, SEM, porosimetry and thermal conductivity analyzer respectively. The GDLs were evaluated in single cell PEMFC under various operating conditions of temperature and relative humidity (RH) using air as oxidant. Electrodes were prepared with different PUREBLACK® and poly-tetrafluoroethylene (PTFE) content in the diffusion layer and maintaining catalytic layer with a Pt-loading (0.4 mg cm-2). In the study, a 73.16 wt.% level of PB and 34 wt.% level of PTFE was the optimal compositions for GDL at 70 °C for 70% RH under air atmosphere. For most electrochemical processes the oxygen reduction is very vita reaction. Pt loading in the electrocatalyst contributes towards the total cost of electrochemical devices. Reducing the Pt loading in electrocatalysts with high efficiency is important for the development of fuel cell technologies. To this end, this thesis work reports the approach to lower down the Pt loading in electrocatalyst based on N-doped carbon nanotubes derived from Zeolitic Imidazolate Frameworks (ZIF-67) for oxygen reduction. This electrocatalyst perform with higher electrocatalytic activity and stability for oxygen reduction in fuel cell testing. The electrochemical properties are mainly due to the synergistic effect from N-doped carbon nanotubes derived from ZIF and Pt loading. The strategy with low Pt loading forecasts in emerging highly active and less expensive electrocatalysts in electrochemical energy devices. This thesis focuses on: (i) methods to obtain greater power density by optimizing content of wet-proofing agent (PTFE) and fine-grained, hydrophobic, microporous layer (MPL); (ii) modeling full factorial analysis of PEMFC for evaluation with experimental results and predicting further improvements in performance; (iii) methods to obtain high levels of performance with low Pt loading electrodes based on N-doped carbon nanotubes derived from ZIF-67 and Pt. / Dissertation/Thesis / Masters Thesis Mechanical Engineering 2016

Neste trabalho foi estudado o comportamento eletroquímico de materiais à base de carbono Printex 6L, sem e com a adição de compostos orgânicos da classe das quinonas (2-terc-butil-9,10-antraquinona (TBA) e 2-etil-9,10-antraquinona (EA)) para a produção de peróxido de hidrogênio (H2O2) a partir da reação de redução do oxigênio gasoso (O2). Na primeira etapa, foi utilizada a técnica de microcamada porosa depositada sobre um eletrodo de disco/anel rotatório, sendo que a partir dos resultados obtidos foram confeccionados eletrodos de difusão gasosa (EDG) para a eletrogeração de H2O2. Os melhores resultados utilizando a microcamada porosa foram para os materiais com a adição dos modificadores, sendo que o material com 1,0% (m/m) de TBA na demonstrou ser o mais eficiente na geração de peróxido de hidrogênio, apresentando eficiência 20% maior comparado ao Printex 6L sem modificador. Com o eletrodo de difusão gasosa confeccionado com o composto orgânico escolhido, na melhor porcentagem de adição mássica de modificador, obteve-se a concentração de 301 mg L-1, sendo que com o eletrodo confeccionado com Printex 6L sem modificador obteve-se a concentração de 175 mg L-1, sob as mesmas condições experimentais. A eficiência cinética também apresentou os mesmos resultados quanto à eficiência dos materiais escolhidos, sendo de 5,94 mg L-1 min-1 para o material com 1,0% de TBA, no potencial de -1,0 V (vs. ECS), e de 3,05 mg L-1 min-1 para o eletrodo de difusão gasosa sem modificador, no potencial de -0,8 V (vs. ECS). / In this work, the electrochemistry behavior of the materials prepared with Printex 6L, with and without addition of organic compounds of the class of quinones, being the compounds: 2-tert-butyl-9,10-anthraquinone (TBA) and 2-ethyl-9,10-anthraquinone (EA). These materials were used to promote the electrogeneration of hydrogen peroxide through the oxygen reduction reaction. In the first phase, it was used the technique of porous microlayer deposited on the rotating ring/disk electrode, and after has been confectioned gas diffusion electrodes (GDE). The best results using the porous microlayer were for the materials with addition of modifiers, and the material with 1.0% (m/m) of 2-terc-butyl-9,10-anthraquinone was demonstrated to be the most efficient in generating hydrogen peroxide, presenting an efficiency 20% higher when compared to Printex 6L without the modifier. The gas diffusion electrode made with the chosen organic compound, in the best massic percentage of modifier, obtained the concentration of 301 mg L-1, and the electrode made with Printex 6L without the modifier obtained the maximum concentration of 175 mg L-1, under the same experimental conditions. The kinect efficiency also demonstrated the same results regarding the efficiency of the chosen materials, which means 5.94 mg L-1 min-1 for the material with 1.0% of 2-terc-butyl-9,10-anthraquinone, in the potential of -1.0 V(vs. SCE), and 3.05 mg L-1 min-1 for the gas diffusion electrode without the modifier, in the potential of -0.8 V (vs. SCE).

A dual phase gas diffusion-FIA system containing a tubular PTFE-membrane was studied as a mean of producing gas samples for routine 15N/14N isotopic ratio mass spectrometry. The method is based on Rittenberg's reaction; the ammonium sample is injected into a liquid alkaline stream containing hypobromite and the N2 gas produced in the reaction diffuses across a PTFE-membrane into a helium carrier stream which carries it to the detector. Initially here, the use of a tubular microporous PTFE-membrane as a device for the preconcentration of samples in aqueous solutions was investigated. The performance of such a membrane was studied under a variety of operating conditions. A qualitative model of the membrane mechanism was developed based on the diffusion transport of vapour away from the contained liquid surface through the connected pore space. The dispersion undergone by the sample in the GD-FIA system containing this preconcentration unit was also studied and this FIA system was applied as a practical device for the determination and speciation of aluminium in a river water sample. The procedure for generating nitrogen gas involved optimisation of the system parameters including the oxidation reaction step and the production on-line of the chemicals used. The nitrogen gas was generated easily and rapidly, allowing a sample throughput capability of the order of 20 h-1. The system was applied to the determination of total nitrogen content in agricultural sample prepared by the Kjeldahl digestion. The method offered precision and accuracy comparable to those of the standard distillationtitration procedure. Isotope ratios were determined with good precision and means for obtaining accuracy comparable with established techniques were developed. It was also shown that the DPGD-FIA system can be readily adapted to enable different forms of nitrogen e. g. N02-, N03- and NH4+ to be determined.