2.2 eV luminescence in GaN

The yellow Luminescence in GaN centered at 2.2 eV has been studied in various epitaxial layers grown by MOVPE on sapphire and by the sandwich sublimation method on 6H-SiC substrates. The photoluminescence and optically detected magnetic resonance results can be consistently explained by a recombination model involving shallow donors and deep donors.

Catalytic decomposition of formic acid using supported metal nanoparticles

Upgrade of hydrogen to valuable fuel is a central topic in modern research due to its high availability and low price. For the difficulties in hydrogen storage, different pathways are still under investigation. A promising way is in the liquid-phase chemical hydrogen storage materials, because they can lead to greener transformation processes with the on line development of hydrogen for fuel cells. The aim of my work was the optimization of catalysts for the decomposition of formic acid made by sol immobilisation method (a typical colloidal method). Formic acid was selected because of the following features: it is a versatile renewable reagent for green synthesis studies. The first aim of my research was the synthesis and optimisation of Pd nanoparticles by sol-immobilisation to achieve better catalytic performances and investigate the effect of particle size, oxidation state, role of stabiliser and nature of the support. Palladium was chosen because it is a well-known active metal for the catalytic decomposition of formic acid. Noble metal nanoparticles of palladium were immobilized on carbon charcoal and on titania. In the second part the catalytic performance of the “homemade” catalyst Pd/C to a commercial Pd/C and the effect of different monometallic and bimetallic systems (AuxPdy) in the catalytic formic acid decomposition was investigated. The training period for the production of this work was carried out at the University of Cardiff (Group of Dr. N. Dimitratos).

Multiphase modeling of melting / solidification with high density variations using XFEM

La modélisation de la cryolite, utilisée dans la fabrication de l’aluminium, implique plusieurs défis, notament la présence de discontinuités dans la solution et l’inclusion de la difference de densité entre les phases solide et liquide. Pour surmonter ces défis, plusieurs éléments novateurs ont été développés dans cette thèse. En premier lieu, le problème du changement de phase, communément appelé problème de Stefan, a été résolu en deux dimensions en utilisant la méthode des éléments finis étendue. Une formulation utilisant un multiplicateur de Lagrange stable spécialement développée et une interpolation enrichie a été utilisée pour imposer la température de fusion à l’interface. La vitesse de l’interface est déterminée par le saut dans le flux de chaleur à travers l’interface et a été calculée en utilisant la solution du multiplicateur de Lagrange. En second lieu, les effets convectifs ont été inclus par la résolution des équations de Stokes dans la phase liquide en utilisant la méthode des éléments finis étendue aussi. Troisièmement, le changement de densité entre les phases solide et liquide, généralement négligé dans la littérature, a été pris en compte par l’ajout d’une condition aux limites de vitesse non nulle à l’interface solide-liquide pour respecter la conservation de la masse dans le système. Des problèmes analytiques et numériques ont été résolus pour valider les divers composants du modèle et le système d’équations couplés. Les solutions aux problèmes numériques ont été comparées aux solutions obtenues avec l’algorithme de déplacement de maillage de Comsol. Ces comparaisons démontrent que le modèle par éléments finis étendue reproduit correctement le problème de changement phase avec densités variables.

Recuperação de componentes de metal duro por projeção térmica

The main objective of this work is the development of a hardmetal components (WC-6%Co) recovery method by thermal deposition process. The thermal deposition technique used was HVOF (high velocity oxygen-fuel). The HVOF enables depositions of thick coatings (100-500 µm) with low porosity levels, high hardness and excellent adhesion. Before deposition, hardmetal samples with different geometries (plates and cylinders) were finished in order to have different roughness. The influence of these parameters in adhesion was studied. After this step, different re-sintering temperatures were used, in order to determine which one allows to obtain the maxima densification, elements distribution and metallurgical bonding. The re-sintering promotes the densification of the coating, with an increase of its hardness and metallurgical bonding formation. The inclusion of an intermetallic layer was tested along with different layer parameters. In liquid phase sintering (1383 and 1455 ºC) a complete densification of the coating occurred, while a bonding between the substrate and the coating only partially happened. The results of SEM/EDS show low levels of porosity and a complete and uniform distribution of the elements of the alloy. The cylindrical samples without intermetallic layer showed the lowest level of porosity and best metallurgical bonding. When the substrate surface was polished (Ra = 0.05 mm) lower levels of porosity and greater metallurgical bonding were found for both geometries. Taking into account the results obtained in this study, we can conclude that the implementation of this process is appropriate for cylindrical components with a polished surface. In these components the intermetallic layer is unnecessary and punctual defects like pores can be repaired with this process.

Growth and Characterization of III-V Semiconductor Materials for MOCVD Reactor Qualification and Process Control

Metalorganic chemical vapor deposition is examined as a technique for growing compound semiconductor structures. Material analysis techniques for characterizing the quality and properties of compound semiconductor material are explained and data from recent commissioning work on a newly installed reactor at the University of Illinois is presented.

In situ Lithium and Boron isotope determinations in mica, pyroxene, and serpentine by LA-MC-ICP-MS

Boron and Li are light, incompatible elements that preferentially partition into the liquid phase, whether melt or aqueous fluid, and thus are useful for tracking fluid-related processes in rocks. Most of the Li isotopic data presently available on subduction-related rocks are from whole-rock analyses; and the B isotopic analyses of subduction material have been carried out either on whole-rocks or in-situ on an accessory phase, such as tourmaline. The new method presented here couples an ESI New Wave UP-193-FX ArF* (193 nm) excimer laser-ablation microscope with a Neptune Plus (Thermo Scientific) MC-ICP-MS aiming to measure both Li and B isotopes in situ with good spatial resolution (metamorphic minerals are commonly chemically zoned, and whole-rock analyses lose this detail). The data thus obtained are compared with SIMS analyses on the same mineral samples for B, and with MC-ICP-MS analyses on whole-rock or mineral separates from the same sample for Li. Additionally, data acquired on tourmaline standards were compared to SIMS values. The results show that for B concentrations above 5 μg/g, the data obtained by LA-MC-ICP-MS and by SIMS are identical within error, for mica (phengitic muscovite), pyroxene (jadeite), serpentine (antigorite), and tourmaline. For Li concentrations above 10 μg/g, the data obtained by LA-MC-ICP-MS and by MC-ICP-MS are also identical, within error, for mica (phengitic muscovite), and pyroxene (jadeite). However, analyses of tourmaline standards have shown significant differences with reference values, so LA-MC-ICP-MS does not yet appear to be an appropriate method to analyze Li isotopes in tourmalines. Thus, LA-MC-ICP-MS is a suitable method to measure Li and B isotopes with good spatial resolution in major rock-forming silicates from subduction-related rocks where concentrations exceed 10 μg/g and 5 μg/g, respectively, with an error on individual measurements equal to or less than previously used methods, but obtainable in a significantly shorter amount of time. The external reproducibility is ± 2.88 to 3.31 ‰ for B and ± 1.50 to 1.75 for Li, which is lower than or equal to the variations encountered within a given chemically zoned sample (up to 10 ‰ of variation within a given natural sample).

Cryopreservation of avocado (Persea Americana Mill.) using somatic embryos

Avocado genetic resources are currently maintained in the form of field repositories at great cost and risk of natural disasters, pest and diseases. Cryopreservation offers a necessary, complimentary method that is safe, cost-effective and long-term. However, long-term maintenance and regeneration of plantlets from avocado somatic embryos has been a major barrier in the development of new avocado cultivars. In this study, two protocols for vitrification-based cryopreservation of avocado somatic embryos were investigated. Globular somatic embryos of two avocado cultivars were tested, revealing cultivar-dependent differences in desiccation tolerance and subsequent freezing resistance, possibly attributed to their size and culture age. A two-step regeneration system, involving an intermediate liquid phase step between subcultures in solid medium, significantly enhanced shoot development from somatic embryo tissue. This work will add considerable value towards cryopreservation of avocado somatic embryos for germplasm conservation and the generation of new and improved avocado cultivars.

Valorização de resíduos provenientes da atividade no setor avícola: aplicação do processo de compostagem

O setor avícola, como setor em constante crescimento está associado à produção de elevadas quantidades de resíduos sólidos orgânicos. A crescente taxa de produção de resíduos avícolas leva à necessidade de lhes dar um destino adequado podendo a valorização orgânica, nomeadamente a compostagem, ser uma opção. O presente estudo pretende dar um contributo para o projeto de uma unidade de compostagem numa empresa portuguesa do setor, com valorização dos resíduos produzidos pela mesma, avaliando o potencial de aplicação desta operação de tratamento de resíduos e a qualidade do produto final. Como primeiro passo foi realizada uma caracterização dos resíduos a utilizar de forma a perceber a sua possível influência no processo de degradação. Os ensaios de compostagem foram realizados à escala laboratorial, numa gama de temperatura de 50 a 55ᵒC, utilizando como substrato base uma mistura dos resíduos avícolas, de acordo com os seus quantitativos de produção. Vários potenciais agentes estruturantes foram também testados, como adição ao substrato. A monitorização do processo foi feita com base em análises das fases gasosa (gases de exaustão), sólida (substrato em decomposição) e líquida (lixiviado). A avaliação do processo foi ainda complementada com a caracterização dos produtos finais obtidos, os quais foram ainda sujeitos a testes de fitotoxicidade. Foram observados graus de conversão de matéria orgânica relativamente altos (40-50%), comprovando a aplicabilidade desta operação de tratamento de resíduos. Embora a diferença entre ensaios tenha sido pouco notória, a utilização de uma combinação de cama de aviário e mato destroçado e compostado aparenta ser ligeiramente mais eficiente, enquanto a utilização de casca de eucalipto produziu os piores resultados. Perdas de humidade e de azoto foram as duas situações mais críticas observadas para os vários ensaios, tendo influência tanto sobre o processo como sobre o produto final. Os compostos obtidos revelam baixa qualidade, exibindo valores elevados de pH e condutividade elétrica e elevadas concentrações de metais pesados, tais como cobre e zinco. Supõe-se que as características adversas verificadas estejam associadas à aplicação de quantidades demasiado elevadas de cinza nos substratos, levando à necessidade de redução deste material; estudos adicionais seriam necessários a fim de avaliar as possibilidades de ajuste das quantidades a utilizar, dos diferentes materiais, com o intuito de produzir um produto final com qualidade, respeitando os requisitos legais.

Dinâmica do fósforo em cultivo heterotrófico e produção de compostos celulares por cianobactéria integrado a biorrefinarias agroindustriais

O objetivo desta tese foi avaliar a dinâmica do fósforo em cultivo heterotrófico e produção de compostos celulares por Aphanothece microscopica Nägeli visando avaliar a perspectiva de implementação de uma biorrefinaria microalgal. Desta forma, foi avaliado o comportamento do micro-organismo em estudo no cultivo heterotrófico, utilizando como meio de cultivo o efluente de laticínios. O trabalho foi desenvolvido em 3 etapas. Em um primeiro momento foi avaliada a influência da temperatura (20 e 30°C) e os valores máximos e mínimos de nutrientes, em especial do fósforo dissolvido reativo (PDR), disponíveis no efluente de laticínio, na remoção de nutrientes. Os resultados demonstraram que a concentração inicial de fósforo dissolvido e a temperatura exerceram influência no crescimento celular e na eficiência de remoção de nutrientes. Em termos de otimização de processo os cultivos conduzidos a 20°C e maiores concentrações de PDR (5,5 mg.L-1 ) no efluente de laticínio, foram os mais eficientes na conversão de poluentes em biomassa e remoção de nutrientes. A segunda etapa foi desenvolvida com o objetivo de avaliar a dinâmica de distribuição de fósforo na fase líquida e sólida do reator heterotrófico, quando o efluente de laticínio foi tratado pela Aphanothece microscopica Nägeli, a 20°C e nas máximas concentrações de fósforo dissolvido encontradas no efluente. Foi demonstrado que as formas fosforadas na fase líquida do reator se caracterizam pela predominância da fração dissolvida em comparação à particulada e por apresentar como fração predominante a de fósforo orgânico. No que se refere à fase sólida, ficou demonstrado que a Aphanothece microscopica Nägeli, quando cultivada heterotroficamente apresenta 3,8 vezes mais fósforo que o requerido para o crescimento celular. Ficando demonstrado ainda que a remoção biológica de fósforo por Aphanothece microscopica Nägeli pode resultar em substanciais aportes financeiros para as estações de tratamento de efluentes. Uma terceira etapa foi desenvolvida, a qual teve como objetivo avaliar a estimativa de produção de compostos celulares por Aphanothece microscopica Nägeli, a partir do efluente de laticínio, bem como o efeito da redução de temperatura de cultivo no teor de lipídios , no momento em que é obtida a máxima concentração deste componente celular, nas condições otimizadas.Foi obtido na fase logarítmica de crescimento, concentrações de 41,8 % de proteinas, carboidratos 28,5 %, lipídios 10,4 % e minerais 10,8 %. O maior teor de lipídio registrado a 20°C correspondeu a biomassa analisada na fase logarítmica.Com a redução da temperatura para 5°C por um período de 30 h é possível obter concentrações de lipídios 2,4 vezes superior ao registrado na fase logarítmica a 20 °C. No entanto, não foram indicadas diferenças significativas (p≤0,05) em função da temperatura entre as concentrações de lipídios obtidas para a biomassa a 10°C em 40 h. O perfil de ácidos graxos da biomassa gerada a 20°C, apresentou como ácidos graxos majoritários, os ácidos graxos: palmítico, oléico, γ-linolênico, palmitoleico e esteárico, resultando um aumento na concentração de ácidos graxos saturados as espensa dos insaturados, quando a temperatura é reduzida. Em paralelo,um reator heterotrófico descontinuo foi definido, ficando demonstrado que a extrapolação da operação em batelada para contínua requer um biorreator heterotrófico com volume útil de trabalho de 240,51 m 3 , permitindo tratar 950 m3 diários de efluente, gerando 11,8 kg.d-1 de biomassa útil para produção de compostos celulares por Aphanothece microscopica Nägeli, visando à simultânea remoção de matéria orgânica, nitrogênio total e fósforo total, gerando insumos que podem suportar a implementação de uma biorrefinaria microalgal.

Desenvolvimento de uma Metodologia para a Seletividade cinética aplicada ao processo de hidroconversão do petróleo, ocorrendo em reator gás-líquido

Este trabalho tem como objetivo desenvolver uma metodologia de seletividade cinética, para os pseudocomponentes do petróleo em escoamento gás-liquido em colunas de bolhas usando a Fluidodinâmica Computacional (CFD). Uma geometria cilíndrica de 2,5m de altura e 0,162m de diâmetro foi usada tanto na validação fluidodinâmica com base em dados experimentais da literatura, como na análise cinética do reator operando em dois modos distintos em relação a fase líquida: batelada e contínuo. Todos os casos de estudo operam em regime heterogêneo de escoamento, com velocidade superficial do gás igual a 8 cm/s e diâmetro médio de bolhas de 6 mm. O modelo fluidodinâmico validado apresentou boa concordância com os dados experimentais, sendo empregado como base para a implementação do modelo cinético de rede de Krishna e Saxena (1989). A análise da hidroconversão foi realizada a 371ºC, e os resultados mostraram o comportamento esperado para o processo reativo estudado, definindo-se os tempos (batelada) e posições axiais (contínuo) de coleta ideal para os pseudocomponentes leves. Em síntese, ressaltase o uso da ferramenta CFD no entendimento, desenvolvimento e otimização de processos.

Amphiphilic catalysts for the treatment of oily wastewaters

The main objective of this study was to obtain an effective catalyst for removal of diazo dye - Sudan IV by Catalytic Wet Peroxide Oxidation (CWPO). For this purpose liquid phase treatment was used to increase the basicity of activated carbon surface favoring the adsorption of organic pollutants. Modified activated carbon catalysts were used in different types of experiments: 1) decomposition of H2O2 in aquatic media, 2) decomposition of H2O2 in organic media, 3) adsorption of Sudan IV, 4) Sudan IV removal by CWPO. As the result of all of these experiments the most effective catalyst was obtained and discussed. It was not observed removal of Sudan IV from biphasic system by CWPO. The obtained results in some cases show a slight increase in concentration of Sudan IV, which may be ascribed to experimental errors. Different factors could be the reason of those errors. For example, the high volatility properties of organic media used in experiments should be taken into account during experiments. Under settled reaction temperature the decrease of cyclohexane volume during experiment could give rise in Sudan IV concentration. The initial concentration of model diazo dye also should be reviewed more detailed for CWPO experiments. Despite of these experimental errors the behavior of our catalysts in different media was observed.

Resistência ao choque térmico de carbeto de silício sinterizado via fase líquida

Tese (Doutorado em Tecnologia Nuclear)

Two-State Thermodynamics of Supercooled Water

Water has been called the “most studied and least understood” of all liquids, and upon supercooling its behavior becomes even more anomalous. One particularly fruitful hypothesis posits a liquid-liquid critical point terminating a line of liquid-liquid phase transitions that lies just beyond the reach of experiment. Underlying this hypothesis is the conjecture that there is a competition between two distinct hydrogen-bonding structures of liquid water, one associated with high density and entropy and the other with low density and entropy. The competition between these structures is hypothesized to lead at very low temperatures to a phase transition between a phase rich in the high-density structure and one rich in the low-density structure. Equations of state based on this conjecture have given an excellent account of the thermodynamic properties of supercooled water. In this thesis, I extend that line of research. I treat supercooled aqueous solutions and anomalous behavior of the thermal conductivity of supercooled water. I also address supercooled water at negative pressures, leading to a framework for a coherent understanding of the thermodynamics of water at low temperatures. I supplement analysis of experimental results with data from the TIP4P/2005 model of water, and include an extensive analysis of the thermodynamics of this model.

Caracterização de compósitos Nb-20%cu obtidos por moagem de alta energia e sinterizados por fase líquida

In this work, was studied the formation of a composite of the refractory metal niobium with copper, through the process of high-energy milling and liquid phase sintering. The HEM can be used to synthesize composite powders with high homogeneity and fine size particle distribution. It may also produce the solid solubility in immiscible systems such as Nb-Cu, or extend the solubility of systems with limited solubility. Therefore, in the immiscible system Cu-Nb, the high-energy milling was successfully used to obtain the composite powder particles. Initially, the formation of composite particles during the HEM and the effect of preparation technique on the microstructure of the material was evaluated. Four loads of Nb and Cu powders containing 20%wt Cu were synthesized by MAE in a planetary type ball mill under different periods of grinding. The influence of grinding time on the metal particles is evaluated during the process by the withdrawal of samples at intermediate times of milling. After compaction under different forces, the samples were sintered in a vacuum furnace. The liquid phase sintering of these samples prepared by HEM produced a homogeneous and fine grained. The composite particles forming the sintered samples are the addition of a hard phase (Nb) with a high melting point, and a ductile phase (Cu) with low melting point and high thermal and electrical conductivities. Based on these properties, the Nb-Cu system is a potential material for many applications, such as electrical contacts, welding electrodes, coils for generating high magnetic fields, heat sinks and microwave absorbers, which are coupled to electronic devices. The characterization techniques used in this study, were laser granulometry, used to evaluate the homogeneity and particle size, and the X-ray diffraction, in the phase identification and to analyze the crystalline structure of the powders during milling. The morphology and dispersion of the phases in the composite powder particles, as well the microstructures of the sintered samples, were observed by scanning electron microscopy (SEM). Subsequently, the sintered samples are evaluated for density and densification. And finally, they were characterized by techniques of measuring the electrical conductivity and microhardness, whose properties are analyzed as a function of the parameters for obtaining the composite

New Catalysts for the Renewable Production of Monomers for Bioplastics

Terephthalic acid (PTA) is one of the monomers used for the synthesis of the polyester, polyethylene terephthalate (PET), that is used for the large-scale manufacture of synthetic fibers and plastic bottles. PTA is largely produced from the liquid-phase oxidation of petroleum-derived p-xylene (PX). However, there are now ongoing worldwide efforts exploring alternative routes for producing PTA from renewable, biomass resources.

In this thesis, I present a new route to PTA starting from the biomass-derived platform chemical, 5-hydroxymethylfurfural (HMF). This route utilizes new, selective Diels-Alder-dehydration reactions involving ethylene and is advantageous over the previously proposed Diels-Alder-dehydration route to PTA from HMF via 2,5-dimethylfuran (DMF) since the H₂ reduction of HMF to DMF is avoided. Specifically, oxidized derivatives of HMF are reacted as is, or after etherification-esterification with methanol, with ethylene over solid Lewis acid catalysts that do not contain strong Brønsted acids in order to synthesize intermediates of PTA and its equally important diester, dimethyl terephthalate (DMT). The partially oxidized HMF, 5-(hydroxymethyl)furoic acid (HMFA) is reacted with high pressure ethylene over a pure-silica molecular sieve catalyst containing framework tin (Sn-Beta) to produce the Diels-Alder-dehydration product, 4-(hydroxymethyl)benzoic acid (HMBA), with ~30% selectivity at ~20% yield. If HMFA is protected with methanol to form methyl 5-(methoxymethyl)furan-2-carboxylate (MMFC), MMFC can react with ethylene in the presence of a pure-silica molecular sieve containing framework zirconium (Zr-Beta) to produce methyl 4-(methoxymethyl)benzenecarboxylate (MMBC) with >70% selectivity at >20% yield. HMBA and MMBC can then be oxidized to produce PTA and DMT, respectively. When Lewis acid containing mesoporous silica (MCM-41) and amorphous silica, or Brønsted acid containing zeolites (Al-Beta), are used as catalysts, a significant decrease in selectivity/yield of the Diels-Alder-dehydration product is observed.

An investigation to elucidate the reaction network and side products in the conversion of MMFC to MMBC was performed, and the main side products are found to be methyl 4-formylcyclohexa-1,3-diene-1-carboxylate and the ethylene Diels-Alder adduct of this cyclohexadiene. These products presumably form by a different dehydration pathway of the MMFC/ethylene Diels-Alder adduct and should be included when determining the overall selectivity to PTA or DMT since, like MMBC, these compounds are precursors to PTA or DMT.

Fundamental physical and chemical information on the ethylene Diels-Alder-dehydration reactions catalyzed by the Lewis acid-containing molecular sieves was obtained. Madon-Boudart experiments using Zr-Beta as catalyst show that the reaction rates are limited by chemical kinetics only (physical transport limitations are not present), all the Zr⁴⁺ centers are incorporated into the framework of the molecular sieve, and the whole molecular sieve crystal is accessible for catalysis. Apparent activation energies using Zr-Beta are low, suggesting that the overall activation energy of the system may be determined by a collection of terms and is not the true activation energy of a single chemical step.