Made available in DSpace on 2014-10-09T12:52:46Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:02:42Z (GMT). No. of bitstreams: 0 / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Made available in DSpace on 2014-10-09T12:52:46Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:02:42Z (GMT). No. of bitstreams: 0 / Cerâmicas porosas à base de nitreto de silício vêm sendo amplamente estudadas por possibilitarem a obtenção de corpos que aliam porosidade e resistência mecânica. Isto se deve, principalmente ao característico mecanismo de reforço in-situ que o material apresenta, obtido com o crescimento anisotrópico de grãos de Si3N4. Neste estudo foram obtidos corpos de Si3N4 com diferentes porosidades (percentual, distribuição de tamanhos, etc.), por três técnicas diferentes: gel-casting de espumas com diferentes gelificantes (albumina, ágar e gelatina), gel-casting com amido e fase sacrificial com amido. As técnicas de obtenção de cerâmicas porosas por gel-casting de espuma, utilizando ágar, gelatina ou albumina como agentes gelificantes gerou muitos resultados, produzindo alterações em termos de estrutura de poros desenvolvida (ampla faixa de porosidade e de tamanho de poros), mas devido às dificuldades encontradas para um rígido controle do processamento (estabilização e reprodutibilidade da estrutura de bolhas) e consequentemente dos corpos obtidos, o tema foi desenvolvido apenas parcialmente. Com a utilização da técnica gel-casting com amido, as amostras produzidas apresentaram maior reprodutibilidade, sendo que a porosidade obtida variou de acordo com o amido empregado: com a utilização do amido de arroz obteve-se a menor porosidade (entre 17,4% e 20,8%), enquanto com o amido de batata atingiu-se a maior porosidade (entre 23% e 36%) e com o amido de milho, chegou-se a valores intermediários de porosidade (entre 18,9% e 28,1%). As amostras obtidas foram sinterizadas a 1800 °C e caracterizadas quanto à porosidade aparente, microestrutura e resistência mecânica por ensaio de compressão, podendo-se relacionar a porosidade e o tipo iv de amido adicionado com a resistência mecânica. Utilizando-se a técnica da fase sacrificial, adicionou-se à matriz de nitreto de silício diferentes tipos de amido (de arroz, de milho ou de batata), em diferentes percentuais (20, 30 e 40 % em volume), sendo as amostras obtidas sinterizadas sob diferentes temperaturas e tempos. Estas amostras foram caracterizadas de modo semelhante às descritas anteriormente, podendo-se relacionar a porosidade e tratamento térmico com a resistência mecânica. Os resultados obtidos com as amostras com o amido usando as duas técnicas foram comparados, permitindo-se concluir que as amostras obtidas por gel-casting apresentam maior resistência mecânica quando comparadas a amostras com porosidade semelhante, produzidas por fase sacrificial. / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Extensive consumption of carbon resources has led to decreasing reserves of fossil fuels and growing concern about global warming. This dilemma has promoted a shift in the economy to develop new long-term, environmentally friendly, and sustainable sources for fuels and chemicals to replace fossil fuel-based sources. Renewable biomass is an ideal alternative, as it is abundant, and relatively cheap. Among current biofuel resources, 5-hydroxymethylfurfural (5-HMF) is a versatile intermediate between biomass-based carbohydrate chemistry and fossil fuel-based industrial organic chemistry, which can be used to synthesize a broad range of chemicals that are currently derived from fossil fuel-based resources. Carbohydrates became the preferred feedstock for high yield production of 5-HMF, and the most convenient route for the synthesis of 5-HMF is the acid-catalyzed dehydration of hexose. Within this context, a variety of processes were developed for the synthesis of 5-HMF from dehydration of fructose involving various solvents, including water, organic solvents, and biphasic systems. Likewise, a range of catalysts were employed, such as homogeneous acid catalysts and metal chlorides, which showed high catalytic activity. Heterogeneous catalysts have also been receiving attention due to their advantages such as easy recovery and recyclability. In the current research, microwave-assisted synthesis of 5-HMF by dehydration of fructose over various acidic porous catalysts, such as periodic mesoporous organosilica (PMO), carbon materials, and metal organic frameworks (MOFs), was investigated. The results showed that the obtained 5-HMF yields were satisfactory, and more importantly highlighted some of the properties of porous heterogeneous catalysts that may improve the production of 5-HMF.

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Block copolymer-derived nanoporous materials are featured with microstructures defined by the microphase separation of constituent blocks, enabling various applications in energy storage. Dictated by the molecular weights and volume fractions of constituent blocks, the microphase separation forms nanoscale microstructures of 1-100 nm. Selective removal of a sacrificial phase produces nanopores with tailored pore width, continuity, and tortuosity. The remaining phase customizes the properties of resulting nanoporous materials, including specific surface area, electrical conductivity/insulation, and mechanical performance. Therefore, block copolymer-derived porous materials are felicitous for use in high-performance energy storage. This dissertation presents the utilization of block copolymers to derive nanoporous materials: i) high-modulus polyimide separators for lithium-metal batteries, and ii) high-surface-area carbon electrodes for fast-charging zinc-ion batteries. In lithium-metal batteries, the dendritic growth of lithium leads to deteriorating performance and severe safety concerns. Suppressing lithium dendrites is imperative to guarantee both high performance and safe cycling. Mesoporous polyimide separators are promising for dendrite suppression: i) the mesopores are smaller than the width of lithium dendrites, preventing lithium dendrites from penetrating the separator. ii) The high-modulus polyimide ceases the growth of lithium dendrites. Herein, this dissertation reports a mesoporous polyimide separator produced by thermalizing polylactide-b-polyimide-b-polylactide at 280 °C. The mesoporous polyimide separator exhibits a median pore width of 21 nm and a storage modulus of 1.8 GPa. When serving as a dendrite-suppressing separator in lithium-metal batteries, the mesoporous polyimide separator enables safe cycling for 500 hours at a current density of 4 mA/cm2. In zinc-ion batteries, developing cathodes compatible with fast charging remains a challenge. Conventional MnO2 gravel cathodes suffer from low electrical conductivity and slow ion (de-)insertion, resulting in poor recharging performance. In this dissertation, porous carbon fiber (PCF) supported MnO2 (PCF@MnO2), comprising nanometer-thick MnO2 deposited on block copolymer-derived PCF, serves as a fast-charging cathode. The high electrical conductivity of PCF and fast ion (de-)insertion in nanometer-thick MnO2 both contribute to a high rate capability. The PCF@MnO2 cathode, with a MnO2 loading of 59.1 wt%, achieves a MnO2-based specific capacity of 326 and 184 mAh/g at a current density of 0.1 and 1.0 A/g, respectively. This dissertation investigates approaches to utilizing block copolymers-derived nanoporous materials for high-performance energy storage. Those approaches are envisaged to inspire the design of block copolymer-derived nanoporous materials, and advance the development of "beyond Li-ion" energy storage. / Doctor of Philosophy / When we talk with friends on mobile phones, accomplish works on laptops, drive back home and see family's smiling faces under lamplights, we must have noticed that our daily life significantly relies on electrical energy. Although being predominantly employed in today's rechargeable energy storage, lithium-ion batteries using graphite anodes have approached their theoretical energy limits. We are expecting better-performance batteries for a more convenient life: to fully charge our phones faster, to use our laptops for a longer time, and to drive our electric cars for a further distance. Lithium-metal batteries and aqueous zinc-ion batteries stand out for "beyond lithium-ion" energy storage because they deliver more energy and charge faster. The commercialization of lithium-metal batteries and zinc-ion batteries may benefit from revolutionary porous materials derived from block copolymers. On one hand, lithium-metal batteries employ metallic lithium anodes, storing about 10 times of energy compared to equal-weight graphite anodes and allowing faster charging rates. However, the lithium-metal anodes grow needle-shaped dendrites during cycling. Those lithium dendrites traverse the battery separator through its large pores, causing internal short circuits and even fire hazards. Suppressing lithium dendrites is imperative for safe lithium-metal batteries. Stiff separators with small pores can suppress lithium dendrites. The small pores prevent lithium dendrites from traversing, and the stiff separators cease the dendritic growth. This dissertation introduces a dendrite-suppressing separator derived from block copolymers comprising stiff polyimide blocks and vulnerable blocks. When those block copolymers form films, the vulnerable blocks spontaneously disperse as a network embedded in the polyimide. Then, the vulnerable blocks are removed at elevated temperatures to create interconnected small pores. This porous polyimide separator suppresses lithium dendrites to allow safe cycling for 500 hours, surpassing today's separators which encounter short circuits within 60 hours. On the other hand, zinc-ion batteries require fast-charging cathodes for high charging rates. A fast-charging cathode demands both good electrical conductivity and fast ion insertion. Herein, this dissertation reports a porous carbon fiber supported MnO2 cathode. The block copolymers comprise a polyacrylonitrile block and a vulnerable block. The vulnerable blocks form a network dispersing in the polyacrylonitrile fibers. At elevated temperatures, polyacrylonitrile is converted to graphitic carbon fibers, and the vulnerable network decomposes to create interconnected pores. The porous carbon fibers afford a large surface area, allowing a high loading of MnO2 to deposit as nanometer-thick sheaths. The resulting cathode combines good electrical conductivity of porous carbon fibers and the fast ion insertion in thin MnO2 sheaths, therefore, exhibiting superior fast-charging performance. This dissertation reports the methods of using block copolymers to produce porous materials for high-performance batteries. We envisage those methods to inspire the design of block copolymer-derived porous materials, and advance the development of high-performance energy storage for a more convenient life.

Thesis (MSc)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Fluid flow through porous media plays an important role in a variety of contexts of which filtration is one. Filtration efficiency of fibrous filters depends on the micro-structural characterization of these porous materials and is reflected in the permeability there-of. Compression of fibrous porous media has a significant effect on the permeability. Experimental data indicate that the permeability varies generally with more than an order of magnitude over the narrow porosity range in which the compression takes place. Relative to the amount of experimental studies regarding this phenomenon, there is a scarcity of geometric models in the literature that can account for the effect of compression on the permeability of a fibrous porous medium. Within the context of existing geometric porescale models based on rectangular geometry, a new model is presented and an existing model improved to predict the effect of one-dimensional compression in the streamwise direction. In addition, without compromising on a commitment to mathematical simplicity, empirical data of a non-woven fibrous porous medium was used to highlight the effect of model geometry on its predictive capability. Different mathematical expressions for the relationship between compression and porosity were considered. The permeability is expressed explicitly in terms of the fibre diameter and the compression fraction and implicitly in terms of the porosity. The porosity is incorporated through the relationship between the linear dimensions of the geometric model. The general applicability of the model(s) was validated by making use of data on airflow through a soft fibrous porous material as well as through glass and nylon fibres. The permeability predictions fall within the same order of magnitude as the experimental data. Given the mathematical simplicity of the model(s), the prediction capability is satisfactory. Attention is drawn to assumptions made and model restrictions within the analytical modelling procedure. A general predictive equation is presented for the permeability prediction in which a solid distribution factor is introduced. The proposed models serve as basis for further adaptation and refinement towards prediction capability. / AFRIKAANSE OPSOMMING: Vloei van vloeistowwe deur poreuse media speel ’n belangrike rol in ’n verskeidenheid kontekste waarvan filtrasie een is. Die filtrasie doeltreffendheid van vesel filters hang af van die mikro-strukturele karakterisering van hierdie poreuse materiale en word gereflekteer in die permeabiliteit. Kompressie van veselagtige poreuse media het ’n beduidende effek op die permeabiliteit. Eksperimentele data dui aan dat die verandering in permeabiliteit gewoonlik oor meer as ’n orde grootte strek oor die klein porositeitsinterval waarin die kompressie plaasvind. Relatief tot die aantal eksperimentele studies rakende hierdie verskynsel, is daar ’n tekort aan geometriese modelle in die literatuur wat die effek van kompressie op die permeabiliteit van veselagtige poreuse media in ag kan neem. Binne die konteks van bestaande geometriese kanaal-skaal modelle gebasseer op reghoekige geometrie, is ’n nuwe model voorgestel en ’n bestaande model verbeter om die effek van een-dimensionele kompressie in die stroomsgewyse rigting te voorspel. Sonder om die verbintenis tot wiskundige eenvoud prys te gee, is empiriese data van ’n nie-geweefde veselagtige poreuse medium gebruik om die effek van die geometrie van ’n model op sy voorspellingsvermo¨e uit te lig. Verskillende wiskundige uitdrukkings is oorweeg vir die verband tussen kompressie en porositeit. Die permeabiliteit is eksplisiet uitgedruk in terme van die veseldiameter en die kompressie breukdeel en implisiet in terme van die porositeit. Die porositeit is ge-inkorporeer deur die verhouding tussen die lineêre dimensies van die geometriese model. Die algemene toepaslikheid van die model(le) is gestaaf deur gebruik te maak van data oor lugvloei deur ’n sagte veselagtige poreuse materiaal sowel as deur glas en nylon vesels. Die voorspellings van die permeabiliteit val binne dieselfde groote orde as die eksperimentele data. Gegee die wiskundige eenvoud van die model(le), is die voorspellingsvermoë bevredigend. Aandag is gevestig op aannames wat gemaak is en modelbeperkings binne die analitiese modellerings prosedure. ’n Algemene voorspellingsvergelyking is voorgestel vir die voorspelling van die permeabiliteit waarin ’n vaste stof distribusie faktor geinkorporeer is. Die voorgestelde modelle dien as basis vir verdere aanpassing en verfyning van voorspellingsvermoë.

Under consideration is the finite-size scaling of two thermomechanical responses of random heterogeneous materials. Stochastic mechanics is applied here to the modeling of heterogeneous materials in order to construct the constitutive relations. Such relations (e.g. Hooke's Law in elasticity or Fourier's Law in heat transfer) are well-established under spatial homogeneity assumption of continuum mechanics, where the Representative Volume Element (RVE) is the fundamental concept. The key question is what is the size L of RVE? According to the separation of scales assumption, L must be bounded according to d<L<<LMacro where d is the microscale (or average size of heterogeneity), and LMacro is the macroscale of a continuum mechanics problem. Statistically, for spatially ergodic heterogeneous materials, when the mesoscale is equal to or bigger than the scale of the RVE, the elements of the material can be considered homogenized. In order to attain the said homogenization, two conditions must be satisfied: (a) the microstructure's statistics must be spatially homogeneous and ergodic; and (b) the material's effective constitutive response must be the same under uniform boundary conditions of essential (Dirichlet) and natural (Neumann) types. / In the first part of this work, the finite-size scaling trend to RVE of the Darcy law for Stokesian flow is studied for the case of random porous media, without invoking any periodic structure assumptions, but only assuming the microstructure's statistics to be spatially homogeneous and ergodic. By analogy to the existing methodology in thermomechanics of solid random media, the Hill-Mandel condition for the Darcy flow velocity and pressure gradient fields was first formulated. Under uniform essential and natural boundary conditions, two variational principles are developed based on minimum potential energy and complementary energy. Then, the partitioning method was applied, leading to scale dependent hierarchies on effective (RVE level) permeability. The proof shows that the ensemble average of permeability has an upper bound under essential boundary conditions and a lower bound under uniform natural boundary conditions. / To quantitatively assess the scaling convergence towards the RVE, these hierarchical trends were numerically obtained for various porosities of random disk systems, where the disk centers were generated by a planar Poisson process with inhibition. Overall, the results showed that the higher the density of random disks---or, equivalently, the narrower the micro-channels in the system---the smaller the size of RVE pertaining to the Darcy law. / In the second part of this work, the finite-size scaling of effective thermoelastic properties of random microstructures were considered from Statistical to Representative Volume Element (RVE). Similarly, under the assumption that the microstructure's statistics are spatially homogeneous and ergodic, the SVE is set-up on a mesoscale, i.e. any scale finite relative to the microstructural length scale. The Hill condition generalized to thermoelasticity dictates uniform essential and natural boundary conditions, which, with the help of two variational principles, led to scale dependent hierarchies of mesoscale bounds on effective (RVE level) properties: thermal expansion strain coefficient and stress coefficient, effective stiffness, and specific heats. Due to the presence of a non-quadratic term in the energy formulas, the mesoscale bounds for the thermal expansion are more complicated than those for the stiffness tensor and the heat capacity. To quantitatively assess the scaling trend towards the RVE, the hierarchies are computed for a planar matrix-inclusion composite, with inclusions (of circular disk shape) located at points of a planar, hard-core Poisson point field. Overall, while the RVE is attained exactly on scales infinitely large relative to microscale, depending on the microstructural parameters, the random fluctuations in the SVE response become very weak on scales an order of magnitude larger than the microscale, thus already approximating the RVE. / Based on the above studies, further work on homogenization of heterogeneous materials is outlined at the end of the thesis. / Keywords: Representative Volume Element (RVE), heterogeneous media, permeability, thermal expansion, mesoscale, microstructure.

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A detailed design was developed of an apparatus to measure moisture transfer in porous materials. The apparatus is to be used to collect data to aid in the development of mathematical models which accurately describe this phenomena. The apparatus consists of dual environmental chambers between which a specimen material is sealed. The temperature of each chamber is controlled separately allowing nonisothermal test conditions. The relative humidity is maintained without the use of saturated salt solutions. The moisture transfer rate is measured by periodically weighing a desiccant column used to absorb moisture as result of diffusion across the specimen. The apparatus was built and used to verify a heat transfer model written to predict its thermal characteristics. The chamber temperature capabilities are 5°C to 60°C with up to a 20°C temperature difference across the specimen. The relative humidity limits are based on the heat transfer into or out of the system. High relative humidities (75 to 85 percent) are possible at chamber temperatures close to ambient, but decrease sharply at the extremely high or low temperatures and during nonisothermal operation. The apparatus maintains a constant temperature within ±0.4°C of the setpoint when subjected to varying ambient temperatures. The spatial temperature variation close to the sample (within 25 mm) is within approximately ​​​​±1°C of the average chamber temperature. The relative humidity can be manually controlled to within ±.7 percent RH. Automated control, complicated by a response lag, was within ±1 percent RH. / Master of Science