京都大学 / 新制・課程博士 / 博士(工学) / 甲第23196号 / 工博第4840号 / 新制||工||1756(附属図書館) / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 辻 伸泰, 教授 田中 功, 教授 乾 晴行 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Sulfide minerals are a major source of metals; however, certain sulfide minerals, such as pyrite and pyrrhotite, are less desirable. Froth flotation is a commonly used separation technique, which requires the use of several reagents to float and depress different sulfide minerals. Xanthate, a thiol collector, has gained immense usage in sulfide minerals flotation. However, some sulfides are naturally hydrophobic and may float without a collector. Iron sulfides, such as pyrite and pyrrhotite, are few of the most abundant minerals, yet economically insignificant. Their existence with other sulfide minerals leads to an inefficient separation process as well as environmental problems, such as acid mine drainage during mining and processing and SO2 emissions during smelting process. A part of the present study is focused on understanding their behavior, which leads to undesired flotation and difficulties in separation. The major reasons for the undesired flotation are attributed to the collectorless hydrophobicity and the activation with heavy metal ions. To better understand the collectorless hydrophobicity of pyrite, Electrochemical Impedance Spectroscopy (EIS) of freshly fractured pyrite electrodes was used to study the oxidation and reduction of the mineral. The EIS results showed that the rate of reaction increases with oxidation and reduction. At moderate oxidizing potentials, the rate of reaction is too slow to replenish hydrophilic iron species leaving hydrophobic sulfur species on the surface. However, at higher potentials, iron species are replaced fast enough to depress its flotation. Effects of pH and polishing were also explored using EIS. Besides collectorless hydrophobicity, the activation of pyrrhotite with nickel ions and interaction with xanthate ions makes the separation more difficult. DETA and SO2 are commonly used as pyrrhotite depressants; however, the mechanism is not very well understood. Contact angle measurements, cyclic voltammetry and Tafel studies have been used to elucidate the depressing action of DETA and SO2. It was observed that DETA and SO2 complement each other in maintaining lower pulp potentials and removing polysulfides. DETA also helps in deactivating pyrrhotite. Therefore, the combined use of DETA and SO2 leads to the inhibition of both the collectorless flotation and the adsorption of xanthate. The adsorption of xanthate on sulfide minerals is a mixed-potential mechanism, i.e., the anodic oxidation of xanthate requires a cathodic counterpart. Normally, the cathodic reaction is provided by the reduction of oxygen. However, oxygen can be replaced by other oxidants. Ferric ions are normally present in the flotation pulp. Their source could be either iron from the grinding circuit or the ore itself. The galvanic studies were carried out to test the possibility of using ferric ions as oxidants and positive results were obtained. Tafel studies were carried out to measure the activation energies for the adsorption of ethylxanthate on several sulfide minerals. Pyrite, pyrrhotite (pure and nickel activated), chalcocite and covellite were studied in 10-4 M ethylxanthate solution at pH 6.8 at temperatures in the range of 22 – 30 0C. The Tafel studies showed that xanthate adsorbs as dixanthogen (X2) on pyrite and pyrrhotite, nickel dixanthate (NiX2) on nickel-activated pyrrhotite and cuprous xanthate (CuX) on both chalcocite and covellite. However, the mechanism for xanthate adsorption on each mineral is different. The free energy of reaction estimated from the activation energies are in good agreement with thermodynamically calculated ones. / Ph. D.

This dissertation focuses on the fundamental understanding of liquid dynamics confined in polymer membranes. Such knowledge guides the development of better polymer membranes for practical applications and contributes to the general understanding of confined liquid dynamics in various nanoporous materials. First, we investigate the membrane transport by experimental measurements on a PFSA membrane and computer modeling of the confined liquid molecules. We probe the nano-scale environment in the ionomer membrane by determining the activation energy of diffusion. We notice two structural features of the PFSA membrane that dominate membrane transport. At relatively high hydrations, the nano-scale phase-separation creates bulk-like water in the ionomer membrane and prompts fast transport of mobile species. At relatively low hydrations, the nanoconfinement of the polymer matrix leads to the ordering of confined water and prompts a high energy barrier for transport. We then delve deeper into the confinement effect by molecular modeling of various nanoconfining geometries, including carbon nanotubes, parallel graphene sheets, and parallel rigid rods. We notice retarded water dynamics under hydrophobic confinement regardless of the geometry. We further investigate the confined water by determining the residence time of water around water, which evaluates the timescale of associations between water molecules. We learn that a decreasing confinement size prompts longer associations among water molecules. Further, we propose that the prolonged associations are responsible for the retarded water dynamics under hydrophobic confinement. Next, we turn our attention to the effect of interactions between mobile species (mostly water molecules) and a confining surface. In ionomer membranes, interactions between mobile species and the ionic groups dominate the water-surface interactions. We start by looking at water-ion interactions in bulk solutions. Using solutions at varying concentrations, we notice a temperature-concentration superposition behavior from diffusion coefficients of water molecules and ions in the solutions in both experimental and computational results. Observation of this superposition behavior in bulk solutions is unprecedented. The temperature-concentration superposition parallels the well-known time-temperature superposition. We are able to extract the offset of reciprocal temperature, which fits well to a Williams-Landel-Ferry type equation. The temperature-concentration superposition points to the new perspective that the effect of ions on water dynamics can be similar to the effect of lowering temperature. We further investigate the effect of ions by modeling ions/charges onto confining geometries. Remarkably, we reveal that the presence of ions can break the ordered water structure induced by confinement. The hydrophobic confinement prompts the ordering of water molecules, which leads to slower diffusion and higher activation energy. The presence of ions/charges on the confining surface has multiple effects on the dynamics of confined water. First, the ions associate strongly with neighboring water molecules while breaking the hydrogen-bonding network between water molecules. Second, the disruption of the hydrogen-bonding network leads to decreased activation energy of diffusion and enhanced water mobility. At relatively high ion density, the water-ion interactions overcome the structure-breaking effect and lead to retarded water diffusion. Overall, the studies presented in this dissertation augment our understanding of water transport in nanostructures by revealing the rich behavior of liquid-water dynamics under both hydrophobic and ionic confinement. / Doctor of Philosophy / Polymer separations membranes contribute to important applications such as fuel cells and water desalination. Optimizing the separation ability of polymer membranes improves their practical performance. The transport property of a polymer membrane depends on its nanoscale and microscale structures. This dissertation focuses on the nanoscale structure-transport relations in ionic polymer membranes. We utilize nuclear magnetic resonance techniques and molecular dynamics simulations to probe the transport properties. We investigate the effects of membrane nanostructure and water-ion interactions on the dynamics of confined water. Such knowledge not only guides the development of high-performance membranes but also contributes to the fundamental understanding of liquid dynamics in nanoporous materials.

No / In rehydroxylation (RHX) dating, the activation energy of the rehydroxylation reaction is required first in the estimate of a material's effective lifetime temperature (ELT), and second to correct the RHX rate constant obtained at a given measurement temperature to that at the ELT. Measurement of the activation energy is thus integral to the RHX methodology. In this paper, we report a temperature-step method for the measurement of activation energy and develop fully the underlying theoretical basis. In contrast to obtaining the activation energy from a series of separate experiments (each of which requires the sample to be dehydroxylated prior to measuring the RHX rate constant), the temperature-step method not only requires a single dehydroxylation at 500°C but also eliminates repeated acquisition of Stage I data, which are not required for dating purposes. Since the first temperature step is set to correspond to the temperature at which a dating determination is carried out, the measurement of rate constants at higher temperatures simply becomes an extension of dating. Consequently, the logistics of obtaining the activation energy of rehydroxylation are greatly simplified.

Redox hopping is the dominant charge transport mechanism in many catalyst-modified metal-organic frameworks (MOFs). Previous studies have shown that ion diffusion is the rate-determining step of redox hopping, but the realization regarding to the fundamental mechanism of redox hopping in MOF is still infantile. In this dissertation, we will discuss the redox hopping process in MOFs from multiple perspectives, including how to use the Scholz model to analyze the coefficients in redox hopping, the influence of the type of carrier, the influence of electrolyte concentration, and the influence of temperature on redox hopping, so as to try to reveal the mechanism of the redox hopping process and make some constructive suggestions for the future design and application of MOF based on this topic. / Doctor of Philosophy / Redox hopping is a keyway that electron moves in many special materials called metal-organic frameworks (MOFs) used as catalysts. While we know that the movement of ion is always the bottleneck in MOF because of their huge volume when compared to tiny electrons, there's still a lot to learn about the basic details of how redox hopping works in MOFs. This thesis explores redox hopping in MOFs from various angles. It looks at how we can use a specific model (the Scholz model) to understand this process better and examines the effects of different factors like the type of carrier (electron or hole), the concentration and type of the electrolyte, and the temperature. The goal is to uncover the fundamental workings of redox hopping and gives helpful ideas or principles to guide the design and applications of MOFs in the future.

A diminuição nos custos de fabricação das SOFC e também a flexibilização do combustível utilizado em seu abastecimento são obstáculos que, uma vez transpostos, podem possibilitar a utilização em larga escala das SOFC. A melhoria no desempenho dos componentes das SOFC pode fazer esse dispositivo trabalhar em temperaturas mais baixas, facilitando a operação do dispositivo e aumentando sua vida útil. Nesse sentido, é útil o desenvolvimento de métodos de fabricação simples e de baixo custo de componentes de SOFC, que atualmente são obtidos na forma de filmes finos cerâmicos. Assim, é proposto o desenvolvimento de uma nova técnica baseada na reação de combustão para síntese de pós e a aerografia, no intuito de, por via úmida, obter filmes finos porosos de composições cerâmicas. Foram realizadas deposições de manganitas de lantânio dopadas com estrôncio e cromo (LSM e LSCM) e óxido de cério dopado com lantânio (LCO) sobre substratos de zircônia estabilizada com ítria (YSZ) e de aço AISI 430, com o propósito de verificar a viabilidade do método de combustão de solução aspergida (CSA) proposto neste trabalho. Embora vários dos filmes fabricados tenham ficado descontínuos ou com uma quantidade excessiva de trincas, alguns resultados foram satisfatórios. Alguns filmes de LSM depositados sobre YSZ apresentaram tamanho de grão reduzido, da ordem de algumas dezenas de nanômetros, e uma porosidade fina e interconectada que levou o material a apresentar um bom desempenho eletroquímico, visando sua aplicação como cátodo de SOFC. A partir das suas propriedades eletroquímicas e, principalmente, pela energia de ativação dos processos do eletrodo, verificou-se que a microestrutura conferida pelo método de CSA proporcionou uma melhoria no desempenho da LSM. Os filmes de LSM obtidos nesse trabalho apresentaram energias de ativação inferiores a 1,26 eV, enquanto na literatura são encontrados valores mais altos para meia-células semelhantes. / Lowering fabrication costs of SOFC’s and also their fuel flexibility are obstacles that, once transposed, can make possible the mass utilization of such devices. The improvement on the performance of SOFC’s components can allow these devices to work in lower temperatures, facilitating their operation and increasing their lifespan. In this sense, is very useful the development of simple and cheap fabrication techniques of SOFC’s components, which are nowadays obtained in the form of thin ceramic films. Thus, the development of a new fabrication method is proposed, based on the solution combustion synthesis reaction and airbrush painting, in order to obtain, by a wet chemical route, thin and porous ceramic films. Depositions of strontium and cobalt doped lanthanum manganites (LSM and LSCM) and lanthanum doped cerium oxide (LCO) were carried out over yttria-stabilized zirconia (YSZ) and AISI 430 steel with the intent of verifying the viability of the Airbrushed Solution Combustion (ASC) method, proposed in this work. Although several films fabricated by ASC were discontinuous or excessively cracked, some results were satisfactory. Some LSM films deposited over YSZ showed reduced grain size, in the order of tens of nanometers, and a fine interconnected porosity, which led the material to present good electrochemical performance, aiming its application as a SOFC’s cathode. From their electrochemical properties and, mainly, through activation energy of the electrode processes, it was seeing that the microstructure acquired with the ASC method enhanced the overall performance of LSM. The LSM films obtained in this work showed activation energies below 1,26 eV, while in the literature the values are higher than this for similar half-cells.

The aim of the study was to investigate the crystallization kinetics and solidification behaviour of Fe60Co8Mo5Zr10W2B15 bulk glass forming alloy. The solidification behaviour in near-equilibrium and non-equilibrium cooling conditions was studied. The eutectic and peritectic reactions were found to exist in the solidification sequence of the alloy. The bulk metallic glass formation was achieved by using two methods: quenching from the liquid state and quenching from the semi-state. Scanning electron microscopy, x-ray diffraction and thermal analysis techniques were utilized in the characterization of the samples produced throughout the study. The choice of the starting material and the alloy preparation method was found to be effective in the amorphous phase formation. The critical cooling rate was calculated as 5.35 K/s by using the so-called Barandiaran and Colmenero method which was found to be comparable to the best glass former known to date. The isothermal crystallization kinetics of the alloy was studied at temperatures chosen in the supercooled liquid region and above the first crystallization temperature. The activation energies for glass transition and crystallization events were determined by using different analytical methods such as Kissinger and Ozawa methods. The magnetic properties of the alloy in the annealed, amorphous and as-cast states were characterized by using a vibrating sample magnetometer. The alloy was found to have soft magnetic properties in all states, however the annealed specimen was found to have less magnetic energy loss as compared to the others.

<p>The major part of the present work, deals with the reduction kinetics of Cu₂O powder and a Cu₂O-WO₃ powder mixture by hydrogen gas, studied by ThermoGravimetric Analysis (TGA). The reduction experiments were carried out both isothermally and non-isothermally on thin powder beds over different temperature intervals. During the experiments, the reductant gas flow rate was kept just above the starvation rate for the reaction to ensure that chemical reaction was the rate-controlling step. The activation energy for the reactions was evaluated from isothermal as well as non-isothermal reduction experiments.</p><p>In the case of the reduction of Cu₂O, the impact of the stability of the copper oxide on the activation energy for hydrogen reduction under identical experimental conditions is discussed. A closer investigation of additions of Ni or NiO to Cu₂O did not have a perceptible effect on the kinetics of reduction.</p><p>In the case of the reduction of the Cu₂O-WO₃mixture, the reaction mechanism was found to be affected in the temperature range 923-973 K, which is attributed to the reaction/transformation in the starting oxide mixture. At lower temperatures, Cu₂O was found to be preferentially reduced in the early stages, followed by the reduction of the tungsten oxide. At higher temperatures, the reduction kinetics was strongly affected by the formation of a complex oxide from the starting materials. It was found that the Cu₂O-WO₃mixture underwent a reaction/transformation which could explain the observed kinetic behavior.</p><p>The composition and microstructures of both the starting material and the reaction products were analyzed by X-ray diffraction (XRD) as well as by microprobe analysis. vi Kinetic studies of reduction indicated that, the mechanism changes significantly at 923 K and the product formed had unusual properties. The structural studies performed by XRD indicated that, at 923 K, Cu dissolved in W forming a metastable solid solution, in amorphous/nanocrystalline state. The samples produced at higher as well as lower temperatures, on the other hand, showed the presence two phases, pure W and pure Cu. The SEM results were in conformity with the XRD analysis and confirmed the formation of W/Cu alloy. TEM analysis results confirmed the above observations and showed that the particle sizes was about 20 nm.</p><p>The structure of the W/Cu alloy produced in the present work was compared with those for pure copper produced from Cu2O produced by hydrogen reduction under similar conditions. It indicated that the presence of W hinders the coalescence of Cu particles and the alloy retains its nano-grain structure. The present results open up an interesting process route towards the production of intermetallic phases and composite materials under optimized conditions.</p>

[pt] O presente trabalho, apresenta uma análise do impacto das variáveis, temperatura, tipo de atmosfera e composição dos materiais ferrosos e carbonosos, sobre a cinética da auto- redução, em dois tipos de briquetes auto-redutores, na faixa de temperatura de 1000 à 1300 graus Celsius. É apresentado um breve histórico da ciência de redução dos óxidos de ferro, além das características relevantes dos principais processos de auto-redução e uma análise dos principais trabalhos correlatos disponíveis na literatura, procurando evidenciar os aspectos termodinâmicos e cinéticos destes estudos. São discutidos detalhes do aparato disponível, o procedimento experimental, a caracterização das amostras, e os resultados obtidos. A partir da análise dos resultados, foi determinando a energia de ativação aparente (E0) igual à 177,10 kJ/mol e o fator de freqüência pré-exponencial da equação de Arrehnius (Constante da taxa 0) igual à 0,97x10-3 s-1. Foi observado que aumentos na temperatura de teste, diminuição na vazão de N2 e uso de atmosfera de CO, melhoraram significativamente a cinética de redução dos briquetes auto- redutores, aumentando os graus de conversão obtidos. Os resultados confirmaram que a reação de Boudouard se apresenta como a etapa controladora do processo até 1200 graus Celsius C, quando o controle passa a ser misto, sofrendo também a influência da reação química de redução dos óxidos de ferro. A importância destes resultados e observações experimentais para o desenvolvimento e projeto dos processos emergentes de auto-redução são destacados. / [en] The present work, analyzes the impact of the key variables, temperature, reduction atmosphere and composition of the ferrous and carbonaceous materials, on the kinetics of self- reducing briquettes, for two types of samples, over the temperature range 1000 - 1300 Celsius Degree. A brief history of the ironmaking science is presented, covering the most relevant features of the main self-reduction processes and a survey of the main published researches on the same subject relating to thermodynamic and kinetic aspects. The experimental procedure, details of the apparatus used, the experimental parameters, characterization of the samples and the results are described. Based on the results obtained, the kinetic parameters were evaluated determining the apparent activation energy (E0) as 177,10 kJ/mol and the pre-exponential frequency factor of the Arrehnius equation (ê0) as 0,97x10-3 s-1. It is observed that, increasing the temperature, decreasing the inert gas flow (N2) and using CO atmosphere, improves significantly the kinetics of reduction of self-reducing briquettes, raising the rate of iron oxide reduction. The results confirm that the Boudouard reaction is the rate limiting step of the overall reaction, up to 1200 Celsius Degree, when a mixed control starts, where the influence of the iron oxides reduction shall be also considered. The significance of this experimental results and observations to the design and the development of an innovative self-reduction smelting process are highlighted.

Orientador: Mônica Pinto Barbosa / Banca: Cassio Roberto Macedo Maia / Banca: Oswaldo Cascudo Matos / Resumo: O concreto auto-adensável (CAA) é um material que representa um dos maiores avanços na tecnologia do concreto das últimas décadas. O desenvolvimento do CAA propiciou eficiência e melhora nas condições de trabalho em canteiro de obras e na indústria de pré-moldados. Do ponto de vista reológico, o CAA é uma mistura fluida que proporciona diferenças de comportamento quando comparado ao concreto convencional. Neste contexto, a proposta desta pesquisa foi estudar, num primeiro plano, as características reológicas nas fases de pasta, argamassa e concreto do CAA no estado fresco e seu comportamento no estado endurecido. Para isso, optouse por empregar a metodologia de Repette e Melo (2005), que considera a resistência à compressão como ponto de partida para a composição do traço do CAA e que estuda os aspectos reológicos envolvidos nas diferentes fases de sua dosagem. Os materiais empregados para estudo de dosagem foram o fíler basáltico como adição, areia média, brita 19 mm, aditivo superplastificante e dois tipos de cimento. Foram elaborados dois concretos, com cimentos distintos, ambos com mesma classe de resistência igual a 40 MPa. Num segundo plano, foi realizado um estudo da avaliação da resistência à compressão desses concretos quando submetidos à cura térmica a vapor, variando-se as temperaturas de cura (entre 65oC e 80oC), assim como o patamar isotérmico de cura em 4 h,6 h e 8 h para cada temperatura. A avaliação das propriedades mecânicas dos concretos foi realizada empregando o Método da Maturidade. No emprego da maturidade foram utilizados as funções de Nurse e Saul e a proposta por Freiesleben-Hansen e Pedersen (FHP). As análises comparativas foram realizadas em função do tipo de cimento empregado, temperatura de cura e tempo de patamar isotérmico / Abstract: The self-compacting concrete (SCC) is a material that represents one of the greatest advances in concrete technology in recent decades. The development of SCC has resulted in improved efficiency and working conditions at the construction site and the precast industry. Rheological point of view, the SCC is a fluid mix that provides behavior differences when compared to conventional concrete. In this context, the proposal of this research was to study, in the foreground, the rheological phases of paste, mortar and concrete of SCC in the fresh state and its behavior in the hardened state. For this, we chose to employ the methodology Repette e Melo (2005), which considers the compressive strength as a starting point for the composition of the trace of SCC and studying the rheological aspects involved in the different stages of their dosage. The materials used to study the dosage was basalt fillers such as addition, medium sand, gravel 19 mm, superplasticizer additive and two types of cement. We prepared two concretes with different cements, both with the same strength class of 40 MPa. In the background, a study assessing the compressive strength of concrete when subjected to steam curing, varying the curing temperatures (between 65oC and 80oC) as well as the level of isothermal cure at 4 h, 6 h and 8 h for each temperature. The evaluation of mechanical properties of concrete was carried out using the Maturity Method. Employment of maturity was used to Nurse e Saul function, as well as function proposed by Freiesleben-Hansen and Pedersen (FHP). Comparative analysis were performed according to the type of cement used, curing temperature and isothermal plateau / Mestre