Dielectric mixtures -- electrical properties and modeling

In this paper, a review on dielectric mixtures and the importance of the numerical simulations of dielectric mixtures are presented. It stresses on the interfacial polarization observed in mixtures. It is shown that this polarization can yield different dielectric responses depending on the properties of the constituents and their concentrations. Open question on the subject are also introduced.Comment: 40 pages 12 figures, to be appear in IEEE Trans. on Dielectric

Review of literature relating to the modeling of soil temperatures based on meteorological factors

Abstracts of 72 papers, journal articles, and other publications are presented. The applicabilities of each is assessed for use in improving winterkill parameters for a winter wheat model

Hidróxidos Duplos Lamelares (HDL) como nanoreservatórios versáteis para aplicação em revestimentos multifuncionais

The objective of the present work is synthesis and characterization of inorganic nanoreservoirs based on layered double hydroxides (LDH) loaded with active species, namely corrosion inhibitors and pH indicators. The most attractive feature of LDH is the anion-exchange ability. Despite the countless studies that describe the use of LDH for applications in protective coatings for anti-corrosion applications, the study related to immobilization and consequent release of anionic species is somewhat limited. Besides, there is still a lack of systematic studies correlating the structure of nanocontainers with properties (release profiles, triggering conditions) and the corresponding effect in coatings. In this work, several methodologies were used for preparation of LDH. Structure, morphology, colloidal and textural properties of resulting nanocontainers have been characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), dynamic light scattering (DLS), thermogravimetric analysis (TG) and nitrogen adsorption-desorption isotherms. Particular attention has been paid to the release studies of active species immobilized in LDH, determination of triggering conditions, establishment of kinetic models and definition of release mechanisms by UV-Visible spectrophotometry. Additionally, the adsorption of active species in LDH was investigated to understand how the active species are immobilized during the synthesis, as well as for evaluation of its impact on the application of nanoreservoirs in multi-functional coatings. In general, the release of active species is rate-limited by diffusion with possibility of anion-exchange reaction when anionic species are presenting in the environment. However, the release profiles, extent of release and adsorption of active species are strongly dependent on the method of synthesis and on the structure and properties of the active species. In addition, the LDH compositions studied were found to release the active species in response to the presence of anions in solution (e.g. chlorides, hydroxides, carbonates), even when the active species are not intercalated into the LDH galleries, but adsorbed onto the external surface of the particles. Besides, the propensity of LDH to dissolution or reaction under extreme pH conditions, in combination with changes in the surface properties, can play a role in the agglomeration and/or aggregation of the particles, thereby affecting adsorption and release of different species. Taking into account the properties of the materials obtained, it is possible to conclude that several LDH compositions studied in this work are prospecting additives for application in multi-functional coatings. Moreover, the present work can be used as some sort of library of experimental data to support the building and validation of computational models, to aid on the prediction of the uptake and release of active species from LDH.O objetivo do presente trabalho consistiu na síntese e caracterização de nanoreservatórios inorgânicos baseados em hidróxidos duplos lamelares (HDL) com diferentes espécies ativas imobilizadas, nomeadamente inibidores de corrosão e indicadores de pH. Uma das características mais relevantes associada com o HDL é a sua capacidade de permuta aniónica. Apesar dos inúmeros trabalhos que descrevem a utilização do HDL para aplicações em revestimentos protetores contra a corrosão, o estudo relacionado com a imobilização e consequente libertação das espécies aniónicas é algo limitado. Além disso, não existem estudos sistemáticos na literatura que correlacionem a estrutura dos nanocontentores com as propriedades (perfis e condições de libertação controlada) e o efeito correspondente nos revestimentos. Neste trabalho, várias metodologias foram aplicadas para a preparação dos HDL. A estrutura, morfologia, propriedades coloidais e texturais dos nanocontentores resultantes foram caracterizadas por difração de raios-X (DRX), espectroscopia no infravermelho por transformada de Fourier (FTIR), microscopia eletrónica de varrimento (SEM), dispersão dinâmica de luz (DLS), análise termogravimétrica (TG) e isotérmicas de adsorção-desadsorção de azoto. Atenção especial foi prestada aos estudos de libertação de diferentes espécies ativas imobilizadas em HDL, nomeadamente as condições experimentais que podem levar à libertação das espécies ativas (ex.: pH, presença de sais), estabelecimento de modelos cinéticos e definição de mecanismos de libertação, recorrendo à espetrofotometria de UV-Visível. Além disso, o processo de adsorção de espécies ativas foi investigado para tentar perceber o que ocorre durante a síntese de diferentes composições de HDL bem como avaliar o impacto na aplicação prática de nanoreservatórios em revestimentos multifuncionais. Os resultados obtidos permitem concluir que, de forma geral, a libertação de espécies ativas é determinada pela difusão, com possibilidade de reação de permuta aniónica, quando espécies aniónicas estão presentes no ambiente. No entanto, os perfis de libertação, extensão da mesma e adsorção de espécies ativas em HDL dependem do método de síntese, bem como da estrutura e propriedades das espécies ativas. Igualmente relevante, é que as diferentes composições de HDL apresentam capacidade de libertar as espécies ativas imobilizadas em resposta à presença de aniões na solução (ex.: cloretos, hidróxidos, carbonatos), mesmo quando não intercaladas nas galerias do HDL, encontrando-se apenas adsorvidas na superfície externa do HDL. Além disso, a tendência dos materiais de HDL à dissolução ou reação sob condições extremas de pH, em combinação com alterações nas propriedades da superfície, tem influência no maior ou menor grau de aglomeração e/ou agregação das partículas, podendo afetar a adsorção e libertação das diferentes espécies. Atendendo às propriedades dos materiais obtidos, concluiu-se que vários HDL estudados são candidatos promissores para aplicação em revestimentos multifuncionais. Para além disso o presente trabalho pode ser usado como suporte na elaboração e validação de modelos computacionais que visam prever a captação e libertação de espécies ativas do HDL.III Quadro Comunitário de ApoioPrograma Doutoral em Ciência e Engenharia de Materiai

Development and implementation of the generalized continuum model for transport in porous media

Fluid flow phenomena in porous media have always attracted a lot of attention of scientists and engineers. Attempts to quantify the average transport in homogeneous media with a simple partial differential equation with constant coefficients disclosed significant inconsistencies comparing to experiments. Modern numerical simulations of porous networks confirmed that those inconsistencies are systematic and not caused by the observation error. The error appeared as a result of the, so called, anomalous or non-Fickian transport, which was in contrast to the normal regime, described by the Fick’s laws. The problem has been addressed through the introduction of more complex and substantial models to describe the phenomena. Although, these new approaches have resolved the problem of quantification, they have raised another question for researchers and engineers, how to choose the most suitable approach and, if it is possible, to parametrize the modeling choice at all. The models general lack of physical consistency makes it difficult to distinguish the model parameters. This leaves judging of suitability to the general accuracy of quantification only, which is often not the most important criterion. In other words, the model parameters are typically estimated by fitting the model to the experimental data, and are often not related to the real properties of the medium. Therefore, a model is often chosen a priory, based only on the experience of the researcher. In this work, we address the problem of model selection by introducing a new model: the Generalized Continuum Transport model. This model transforms into existing models at certain limits and, therefore, constrains the modeling choice through the introduction of the parameter space. It is shown that the Generalized Continuum Transport model limits to the advection-dispersion equation, the Continuous Time Random Walk, the Multi-Rate Mass Transfer and the Multiple-Porosity models, when corresponding configurations of the parameter space are applied. The model’s accuracy is studied by quantifying the breakthrough curves obtained from a fine scale porous network modeldemonstrating significant appearance of anomalous transport phenomena. The results show that the error of quantification is smaller than the error of the existing models. It is discussed that the parameters of the Generalized Continuum Transport model are related to the physical properties of porous media. Finally, it is presented that the parameter space of GCT can be constrained and related to the transport phenomena studied. Hence, the limits of GCT are controlled by the transport complexity and the desired accuracy and the modeling choice can be parametrized

Advanced eddy-current methods for quantitative NDE

The objectives of this dissertation were to devise and develop advanced eddy-current methods for quantitative NDE. The techniques used include time-domain methods (pulsed eddy current), frequency-domain methods (swept-frequency eddy current), and the photoinductive imaging method that combines eddy-current and laser-based thermal-wave techniques. We first developed theoretical models to predict the pulsed eddy current signal and showed this technique can be used to characterize metallic coatings on metal substrates. A feature-based rapid inversion method was developed to determine the conductivity and thickness of the coating simultaneously. In the second work, we studied the fundamentals of eddy current interactions with magnetic metals using swept-frequency eddy current method. We have found that the eddy current response of well-annealed, demagnetized commercially-pure nickel is dominated by a thin region at the sample\u27s surface that has a very significantly reduced permeability--i.e., a surface dead-layer. This dead layer may be due to the presence of surface damage. We calculated the impedance of the coil based on the hypothesized single layer structure and found excellent quantitative agreement between the model and experiment. These results may have important consequences for many aspects of the interaction of low frequency electromagnetic fields with magnetically soft metals. In the third work, we developed theoretical calculations and practical measurement methods using both swept-frequency eddy current and pulsed eddy current methods for determining the thickness, conductivity, and permeability of metallic coatings on metal substrates for the case when either coating, metal, or both are ferromagnetic. This work paves the way for development of new, quantitative methods to characterize surface layers on ferrous materials, such as depth of case hardening. In the fourth work, we applied the photoinductive imaging technique to characterize corner cracks on the surface around a bolt hole. The photoinductive signals reflect the geometrical shape of the triangular and rectangular electrical-discharge-machined (EDM) notches as well as real fatigue cracks. The results show promise for using this technique to characterize the shape, depth, and length of corner cracks. The capability of the photoinductive imaging technique is demonstrated in this work