No description available.

Many powders and particulate solids are stored and handled in large quantities across various industries. These solids often encounter handling and storage difficulties that are caused by the material cohesion. The cohesive strength of a bulk material is a function of its past consolidation stress. For example, high material cohesive strength as a result from high storage stresses in a silo can cause ratholing problems during discharge. Therefore, it is essential to consider the stress-history dependence when evaluating such handling behaviour. In recent years the Discrete Element Method (DEM) has been used extensively to study the complex behaviour of granular materials. Whilst extensive DEM studies have been performed on cohesionless solids, much less work exists on modelling of cohesive solids. The commonly used DEM models to model adhesion such as the JKR, DMT and linear cohesion models have been shown to have difficulty in predicting the stress-history dependent behaviour for cohesive solids. DEM modelling of cohesive solid at individual particle level is very challenging. To apply the model at single particle level accurately would require one to determine the model parameters at particle level and consider the enormous complexity of interfacial interaction. Additionally it is computationally prohibitive to model each and every individual particle and cohesion arising from several different phenomena. In this study an adhesive elasto-plastic contact model for the mesoscopic discrete element method (DEM) with three dimensional non-spherical particles is proposed with the aim of achieving quantitative predictions of cohesive powder flowability. Simulations have been performed for uniaxial consolidation followed by unconfined compression to failure using this model. Additionally, the scaling laws necessary to produce scale independent predictions for cohesionless and cohesive solids was also investigated. The influence of DEM input parameters and model implementation have been explored to study the effect of particle (meso-scale) properties on the bulk behaviour in uniaxial test simulation. The DEM model calibration was achieved using the Edinburgh Powder Tester (EPT) – an extended uniaxial tester to measure flowability of bulk solids. The EPT produced highly repeatable flowability measurements and was shown to be a good candidate for DEM model calibration. The implemented contact model has been shown to be capable of predicting the experimental flow function (unconfined compressive strength versus the prior consolidation stress) for a limestone powder which has been selected as a reference solid in the Europe wide PARDEM research network. Contact plasticity in the model is shown to affect the flowability significantly and is thus essential for producing satisfactory computations of the behaviour of a cohesive granular material. The model predicted a linear relationship between a normalized unconfined compressive strength and the product of coordination number and solid fraction. Significantly, it has been found that contribution of adhesive force to the limiting friction has a significant effect on bulk unconfined strength. Failure to include the adhesive contribution in the calculation of the frictional resistance may lead to under-prediction of unconfined strength and incorrect failure mode. The results provide new insights and propose a micromechanical based measure for characterising the strength and flowability of cohesive granular materials. Scaling of DEM input parameters in a 3D simulation of the loading regimes in a uniaxial test indicated that whilst both normal and tangential contact stiffness (loading, unloading, and load dependent) scales linearly with radius of the particle, the adhesive forces scales with the square of the radius of the particles. This is a first step towards a mesoscopic representation of a cohesive powder that is phenomenological based to produce the key bulk characteristics of a granular solid and the results indicate that it has potential to gain considerable computational advantage for large scale DEM simulations. The contact model parameters explored include particle contact normal loading stiffness, tangential stiffness, and contact friction coefficient. The DEM model implementation parameters included numerical time step, strain rate, and boundary condition. Many useful observations have been made with significant implications for the relative importance of the DEM input parameters. Finally the calibration procedure was applied to a spray dried detergent powder and the simulation results are compared to whole spectrum of loading regime in a uniaxial experiment. The experimental and simulation results were found to be in reasonable agreement for the flow function and compression behaviour.

Skin wound healing is an urgent problem in clinical treatment, in particular, with a military context. Although significant advances have been made in treating skin wounds, traditional methods face several challenges, e.g., limited donor skin tissue for transplants and inflammation over the period of long term healing. To address these challenges, in this study we present a method to fabricate Poly (ethylene-co-vinyl alcohol) (EVOH) nanofibres encapsulated with the Ag nanoparticle, using the electro-spinning technique. The manufacturing process of nanofibres by electro-spinning is the subject of the present research. Electro-spinning is a process which produces nanofibres through the electrically charged jet of a polymer solution. While the principle has long been understood, the process of forming them still remains difficult to control. In its simplest form, the technique consists of a pipette to hold the polymer solution, two electrodes and a DC voltage supply over a 10 KV range. The polymer dropping from the tip of the pipette is drawn into a jet which is electrically charged and spun into fine fibres by the electronic field. An appropriate combination of the control parameters, such as the charge voltage, density and viscosity of the polymer solution and the travel distance of the jet, etc. will lead to the production of fibres with diameters in the range of 10-7~8 meters. The fibres can then be collected on the surface of a grounded target. regulating three main parameters, namely, a concentrated EVOH solution, the electric voltage and the distance between the injection needle tip (high voltage point) and the fibre collector. Ag was added to the nanofibres to offer long term anti-inflammation properties by the slow release of Ag nanoparticles through the gradual degradation of the EVOH nanofibre. The method developed here could lead to new dressing materials for the treatment of skin wounds. The thin EVOH nanofibre sheets obtained from electro-spinning were tested in uniaxial tension for their mechanical properties, with a view to the possibilities of using them as wound dressings. It was found that the sheets show a mild hardening behaviour with extensive elongation and necking before failure in multiple fractures at random locations. The failure is not simply fibre breakage. Due to the random orientation of the continuing fibres in the sheet, detachment, shear, straightening and twinning. etc., among the fibres all occur at the same time to different extents. The Young’s modulus and the yield stress (at 0.4~0.5% proof strains) are predominately affected by the diameters of the fibres. The latter are largely insensitive to strain rate over the range tested.

The most-discussed pairing symmetry in Sr₂RuO₄ is chiral p-wave, pₓ ± p[sub]y, whose degeneracy is protected by the lattice symmetry. When the lattice symmetry is lowered by the application of a symmetry-breaking field, the degeneracy can be lifted, potentially leading to a splitting of the superconducting transition. To lift the degeneracy, the symmetry breaking field used in this study is uniaxial stress. Uniaxial stress generated by a piezo-electric actuator can continuously tune the electronic structure and in situ lower the tetragonal symmetry in Sr₂RuO₄. Previous studies of magnetic susceptibility and resistivity under uniaxial stress have revealed that there is a strong peak in T[sub]c when the stress is applied along the a-axis of Sr₂RuO₄. In addition, it has been proposed that the peak in T[sub]c coincides with a van Hove singularity in the band structure, and measurements of Hc₂ at the maximum T[sub]c indicate the possibility of an even parity condensate for Sr₂RuO₄ at the peak in Tc. In this thesis, the heat capacity approach is used to study the thermodynamic behavior of Sr₂RuO₄ under uniaxial stress applied along the crystallographic a-axis of Sr₂RuO₄. The first thermodynamic evidence for the peak in T[sub]c is obtained, proving that is a bulk property. However, the experimental data show no clear evidence for splitting of the superconducting transition; only one phase transition can be identified within the experimental resolution. The results impose strong constraints on the existence of a second phase transition, i.e. the size of the second heat capacity jump would be small or the second T[sub]c would have to be very close to the first transition. In addition to these results, I will present heat capacity data from the normal state of Sr₂RuO₄. The experimental results indicate that there is an enhancement of specific heat at the peak in T[sub]c, consistent with the existence of the van Hove singularity. The possibility of even parity superconductivity at the maximum T[sub]c has also been investigated. However, the heat capacity measurements are shown to be relatively insensitive to such a change, so it has not been possible to obtain strong and unambiguous evidence for whether it takes place or not.

There are many factors which contribute to fatigue failure in HMA. While studies have been made with respect to binder aging, little has been done to investigate the effect of aging on the fatigue failure of asphalt mixtures. The lack of an effective and efficient method of testing field samples has contributed to this deficiency. This study focused on the development of a method for preparing and testing field samples in direct tension. This methodology was then be employed in combination with the VEC and RDT* tests to investigate several factors that affect fatigue in HMA. Particular emphasis was placed on the role of aging in the fatigue process. A method of testing field samples in direct tension was successfully developed. Results from the VEC and RDT* tests performed on several field samples collected from across the state of Texas were analyzed. US 277 field sample results were compared to laboratory mixed and compacted (LMLC) sample results as well as results obtained from extracted binder testing. Findings show that oxidative aging has an impact on the stiffness and performance of HMA. Chip seal surface treatments can extend the life of the pavement, but their affects are found primarily at the surface. Two additional field sites were tested, analyzed, and compared to LMLC results. These comparisons verified the effects of aging and show that a relationship between LMLC samples and field samples can be developed. Modulus values for one month of artificial aging of LMLC samples is equivalent to 10.5 months of aging in the field. Finally, 21 Texas sites used for the study were analyzed and a multivariate linear regression was performed to determine the factors that play the most significant role in the aging process. A linear regression model was constructed to determine the number of loads to failure from fatigue cracking due, primarily, to aging.

Light propogation in uniaxial crystal was discussed. We used Maxvel equations for electromagnetic waves in vectorial form and general environment equations for liquid crystals to deduced Frenel equation. We make study of tetragonal singony planar class liquid crystal.

The evolution of rhyolitic lava from effusion to cessation of activity is poorly understood. Recent lava dome eruptions at Unzen, Colima, Chaiten and Soufrière Hills have vastly increased our knowledge on the changes in behaviour of active domes. However, in ancient domes, little knowledge of the evolution of individual extrusion events exists. Instead, internal structures and facies variations can be used to assess the mechanisms of eruption. Rhyolitic magma rising in a conduit vesiculates and undergoes shear, such that lava erupting at the surface will be a mix of glass and sheared vesicles that form a permeable network, and with or without phenocryst or microlites. This foam will undergo compression from overburden in the shallow conduit and lava dome, forcing the vesicles to close and affecting the permeable network. High temperature, uniaxial compression experiments on crystal-rich and crystal-poor lavas have quantified the evolution of porosity and permeability in such environments. The deformation mechanisms involved in uniaxial deformation are viscous deformation and cracking. Crack production is controlled by strain rate and crystallinity, as strain is localised in crystals in crystal rich lavas. In crystal poor lavas, high strain rates result in long cracks that drastically increase permeability at low strain. Numerous and small cracks in crystal rich lavas allow the permeable network to remain open (although at a lower permeability than undeformed samples) while the porosity decreases. Flow bands result from shear movement within the conduit. Upon extrusion, these bands will become modified from movement of lava, and can therefore be used to reconstruct styles of eruption. Both Ngongotaha and Ruawahia domes, from Rotorua caldera and Okataina caldera complex (OCC) respectively, show complex flow banding that can be traced to elongated or aligned vents. The northernmost lobe at Ngongotaha exhibits a fan-like distribution of flow bands that are interpreted as resulting from an initial lava flow from a N – S trending fissure. This flow then transitioned into intrusion of obsidian sheets directly above the conduit, bound by wide breccia zones which show vertical movement of the sheets. Progressive intrusions then forced the sheets laterally, forming a sequence of sheets and breccia zones. At Ruawahia, the flow bands show two types of eruption; long flow lobes with ramp structures, and smaller spiny lobes which show vertical movement and possible spine extrusion. The difference is likely due to palaeotopography, as a large pyroclastic cone would have confined the small domes, while the flow lobes were unconfined and able to flow down slope. The vents at Ruawahia are aligned in a NE – SW orientation. Both domes are suggested to have formed from the intrusion of a dyke. The orientations of the alignment or elongation of vents at Ngongotaha and Ruawahia can be attributed to the overall regional structure of the Taupo Volcanic Zone (TVZ). At Ngongotaha, the N – S trending elongated vent is suggested to be controlled by a N – S trending caldera collapse structure at Rotorua caldera. The rest of the lobes at Ngongotaha, as well as other domes at Rotorua caldera, are controlled by the NE – SW trending extensional regional structure or a NW – SE trending basement structure. The collapse of Rotorua caldera, and geometry of the deformation margin, are related to the interplay of these structures. At Ruawahia, the NE – SW trending vent zone is parallel to the regional extension across the OCC, as shown by the orientation of intrusion of the 1886AD dyke through the Tarawera dome complex. The NE – SW trending regional structures observed at both Rotorua caldera and Okataina caldera complex are very similar to each other, but differ from extension within the Taupo rift to the south. Lava domes, such as Ngongotaha, that are controlled by this structure show that the ‘kink’ in the extension across Okataina caldera complex was active across Rotorua caldera during the collapse at 240 ka, and possibly earlier. This study shows the evolution of dyke-fed lava domes during eruption, and the control of regional structures in the location and timing of eruption. These findings improve our knowledge of the evolution of porosity and permeability in a compacting lava dome, as well as of the structures of Rotorua caldera, the longevity of volcanic activity at dormant calderas and the hazard potential of dyke-fed lava domes.

Three different groups of materials were chosen to investigate the uniaxial compaction of particulate solids. Dentritic and cubic sodium chloride were chosen as plastically deforming, dicalcium phosphcte and sugar as fragmentary and styrocell, homopolymer and copolyrinier as non-compactable materials. The uniaxial compaction of the materials was continuously followed by measurement of the applied force, the force transmitted radially to the die wall and the upper punch displacement. The data obtained was presented in the form of Mohr circles, stress pathways (shear-mean compaction stress planes) and a three dimensional representation in mean compaction stress, shear stress and volume change. The yield loci evaluated from Mohr circles and shear-mean compaction stress relationships of compactable and non-compactable materials were found to be similar in shape. The unloading stress profiles were however more informative. All unloading shear-mean compaction stress curves of the compactable materials cross the mean compaction stress axis to give negative values of shear stress and reach a minimum value of ^t_min' which was material and compaction pressure dependent. The unloading curves of non-compactable materials gave approximately zero shear. The parameters evaluated from the characteristic stress profiles were correlated to the tensile strength and hardness of compacts. Mathematical expressions have been proposed for the shear-mean compaction stress relationships of the materials investigated. The materials were characterised before and after compaction in terms of specific surface area, porosity and mechanical strength of compacts with compaction pressure.

Made available in DSpace on 2016-06-02T20:15:22Z (GMT). No. of bitstreams: 1 3378.pdf: 3991405 bytes, checksum: c6b2048415c68c99866303d7c910e05d (MD5) Previous issue date: 2010-08-15 / Universidade Federal de Minas Gerais / Nowadays, the optimized magnetoelectric coupling in particulate composites is based on the optimization of the processing techniques, which support an integrity interface between the phases, and on the novel combinations of the ferroelectric and ferromagnetic phases. In this context, this work suggests the synthesis and characterization of particulate magnetoelectric composites of different ferroelectric systems, of compositions in the morphotropic phase boundary (MPB)- since they show a maximization of the piezoelectric coefficients- coupled with nickel or cobalt ferrites- since they show great magnetoestrictive coefficients. As ferroelectric phases, the lead zirconate titanate, or Pb(Zr0,53Tio,47)O3 (of simple perovskite structure), the lead titanate modified magneion niobate titanate, or 0,675Pb(Mg1/3Nb2/3)- 0,325PbTiO3 (of complex perovskite structure) and the lead barium niobate, or Pb0,61Ba0,39Nb2O6 (of tungsten bronze structure), where choosen. The ferroelectric/ferrite system composites were obtained in the molar ratio 80/20, by the conventional solid state and oxide mixing technique. The ceramic bodies were densified in different conditions, as a function of the time and temperature, by the conventional sintering process, or by the uniaxial hot pressing process. The relationship between the kind/combination of the phases, sintering techniques, and processing parameters, with the densification degree and the phases integrity was determined by structural, microestructural, physical and electrical characterization of the ceramic composites (with attention in those composed by nickel ferrite). As the main result, a processing protocol (validated only for the hot pressed composites, since the conventional sintering process generated conductive ceramics), independent of the constituent phases, which allows the synthesis adequate composites for research and application in the industry, were novelty determined. This protocol were also tested with significant results for composites based on cobalt ferrites. For all the cases of optimized processing conditions, dielectric, magnetic and magnetoelectric characterizations locking for the influences of the ferroelectric and/or magnetic phases in the multiferroic properties were performed. The values of magnetoelectric coefficients (xME) were close or higher than the most of the common particulate composites, and the PMN-PT/NFO shown the highest ratio dxME/dH of the studied materials. A general picture of the relationship between the magnetoelectric response and the research parameters, in this work, is presented, proving the relationship between the high density and integrity of the phases and the maximization of the magnetoelectric coupling. / Atualmente, a otimização do acoplamento magnetoelétrico em materiais compósitos particulados baseia-se na busca por técnicas de processamento que favoreçam uma interface íntegra entre as fases, e por novas combinações composicionais de fases ferroelétricas e magnéticas. Nesse contexto, este trabalho propõe a síntese e caracterização de compósitos magnetoelétricos particulados de diferentes sistemas ferroelétricos, com composições na região de contorno de fase morfotrópico (CFM) por apresentarem maximização dos coeficientes piezoelétricos nessa região combinados com ferritas de níquel ou cobalto por apresentarem relativamente altos coeficientes magnetostrictivos entre as ferritas. Para as fases ferroelétricas, as composições escolhidas foram o titanato zirconato de chumbo, ou Pb(Zr0,53Tio,47)O3 (estrutura perovskita simples), o niobato de magnésio modificado com titanato de chumbo, ou 0,675Pb(Mg1/3Nb2/3)-0,325PbTiO3 (perovskita complexa) e o niobato de bário e chumbo, ou Pb0,61Ba0,39Nb2O6 (estrutura tungstênio bronze). Compósitos dos sistemas ferroelétrico/ferrita foram obtidos na razão molar de 80/20, utilizando o método convencional de mistura de óxidos. Para densificação dos corpos cerâmicos, em diferentes condições de temperatura/tempo, foram utilizadas as técnicas de sinterização convencional e sinterização assistida por pressão (ou no caso, prensagem uniaxial a quente). A relação entre tipo/combinação de fases, técnicas de sinterização, e parâmetros de processamento, com o grau de densificação e de integridade entre as fases foi determinada a partir da caracterização estrutural, microestrutural, física e elétrica dos corpos cerâmicos compósitos (particularmente para aqueles com ferrita de níquel). Como resultado, foi determinado pela primeira vez, um protocolo de processamento (validado apenas para a sinterização assistida por pressão, já que a convencional gerou corpos cerâmicos condutivos) que, independentemente das fases constituintes, possibilita a obtenção de compósitos adequados para investigação e aplicação como multiferróicos magnetoelétricos. Tal protocolo foi testado e bem sucedido para o caso dos compósitos com ferrita de cobalto. Para todos os casos de condições de processamento otimizadas, foram realizadas a caracterização dielétrica, magnética e magnetoelétrica, para a investigação da influência da fase ferroelétrica e/ou magnética nas propriedades multiferróicas. Os valores de coeficiente magnetoelétrico (xME) encontrados foram próximos ou superiores aqueles da maioria dos compósitos particulados já conhecidos, com a combinação PMN-PT/NFO, apresentando a maior razão dxME /dH entre os compósitos estudados. Um quadro geral da dependência da resposta magnetoelétrica em função dos principais parâmetros utilizados para a investigação, neste trabalho, é apresentado comprovando a estreita relação entre alta densidade e integridade de fases com a maximização do acoplamento magnetoelétrico.

Made available in DSpace on 2014-06-11T19:28:20Z (GMT). No. of bitstreams: 0 Previous issue date: 2011-01-18Bitstream added on 2014-06-13T18:57:24Z : No. of bitstreams: 1 bukvic_g_me_bauru.pdf: 10769904 bytes, checksum: 62e0300f452d06ec5fe503cb8b56d8d5 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / A usinagem de cerâmicas avançadas geralmente é feita com o objetivo de corrigir as deformações após a etapa de sinterização, conferir precisão dimensional e qualidade superficial. Para cumprir esta tarefa é utilizada, principalmente, a retificação com rebolo de diamante sob baixas taxas de remoção de material. O torneamento com ferramenta de diamante pode ser aplicado para a maioria das cerâmicas, mas esbarram em condições limites de remoção de material, como profundidade de corte nanamétricas, que se ultrapassadas mudam o regime de remoção de dúctil para frágil. Uma alternativa é a usinagem da peça no estado verde, a qual apresenta fácil usinagem com a diminuição da introdução de defeitos prejudiciais à resistência mecânica. Após a sinterização invariavelmente ocorrem distorções devido à distribuição heterogênea de gradientes de densidade que se localizam nas porções mais externas do compactado. Na tentativa de remover estes gradientes, o presente trabalho analisou a influência da remoção de sobrematerial no estado verde na distorção da peça após a sinterização. Corpos-de-prova de alumina, com 99,8% de pureza, foram torneados com sobremateriais de 1,0, 2,0 3,0, 4,0 e 5,0 mm por uma ferramenta de metal duro sob condições constantes de usinagem. Além da distorção, outras variáveis de saída foram analisadas, com desgaste de ferramenta, força de corte (Fc) e rugosidade média aritmética (Ra) do compactado em verde e sinterizado. Os resultados apontam uma redução da distorção da peça sinterizada de até 81,4% com o maior valor de remoção de sobrematerial em verde e a influência do desgaste da ferramenta na rugosidade da peça em verde e na correspondente peça sinterizada / Advanced ceramics machining is generally done with the objective of correcting distortions in pieces after sintering. In order to comply with this task grinding with a diamond grinding wheel is mainly used with low material removal rates. Turning can be done diamong tool to most of the ceramics, but is runs into very narrow material removal limit conditions, as nanometric cutting depths, which if exceeded change the removal system from ductile to brittle. An alternative is machining the piece on the green state, which presents easy machining without the introduction of harmful defects to mechanical resitance. However, after sintering there are invariably distortions caused by the heterogeneous distribution of density gradients that are located in the most outlying portions of the compacted. In an attempt to remove these gradients, this study examined the influence of different allowance values removed and the corresponding distortion after sintering. Alumina speciments are used for this procedure, with 99,8% purity, from which allowances of 1.0,2.0,3.0,4.0, 5.0 mm are removed by turning with a cermented carbine tool under constant machining conditions. Besides distortion, other output variables were analyzed, such as tool wear, cutting force and surface roughness of the green ceramics and the sintered. The results presents a reduction of the distortion of the sintered piece of 81,4% with the greater allowance removal values and the influence of tool wear on the green machining surface roughness and on the corresponding sintered piece