Ph.D. (Physics) / Compounds of rare-earth (RE) elements (Ce, Yb, Eu) as well as actinide element U with partially filled 4f or 5f shells have been receiving considerable attention in the field of low temperature studies. This stems from the diversity of magnetic ground state exhibited by the compounds. These range from non-magnetic, metallic, ferromagnetic or antiferromagnetic ordered state. The nature of the ground state depends on the balance between the on-site Kondo interaction, which mostly favours a local non-magnetic singlet and also the indirect Rudderman-Kittel-Kasuya- Yosida (RKKY) exchange interactions. The interactions of the localized f moments with their environment result in the crystal electric field (CEF) effects at the rare-earth (RE) ion site which splits the free-ion ground-state multiplet. The competing energy scales of CEF splitting, Kondo and RKKY interaction cause the large variety of exceptional phenomena observed in rare earth systems, such as heavy-fermion (HF) behaviour, intermediate valence, unconventional superconductivity, and quantum criticality. Generally, the low-temperature properties of RE systems depend sensitively on the position of the undisturbed 4f state with respect to the Fermi level. Thus, the hybridization between the 4f and conduction-electron states may give rise to either an enhanced density of states (DOS) near the Fermi level in HF metals or to the formation of a gap at Fermi level in Kondo insulators. Recent considerable research interest in strongly correlated electron systems (SCES) studies has been the study of intermetallic compounds close to magnetic instability and the introduction of non-Fermi-liquid (NFL) state. NFL behaviour in f-electron systems is characterized by special power laws and logarithmic divergences in temperature of the physical properties of materials at low temperatures...

Manganese spinel cathodes LiMn₂O₄ offer the advantage of a strong, edge-shared octahedral framework with fast, 3-dimensional Li⁺-ion conduction. To better understand the safety of these materials, the thermal stability characteristics of spinel oxide and oxyfluoride cathodes Li[subscript 1.1]Mn[subscript 1.9-y]M[subscript y]O₄[subscript-z]F[subscript z] (M = Ni and Al, 0 ≤ y ≤ 0.3, and 0 ≤ z ≤ 0.2) have been investigated systematically. The thermal characteristics are assessed in terms of the onset temperature and reaction enthalpy for the exothermic reaction. The thermal stability increases with decreasing lithium content in the cathode in the charged state. High-voltage spinel cathodes LiMn[subscript 1.5]Ni[subscript 0.5]O₄ are promising candidates for electric vehicles and stationary storage of electricity produced by renewable energies due to their high power capability. However, widespread adoption of this high-voltage spinel cathode is hampered by severe capacity fade resulting from aggressive reaction with the electrolyte to form a thick solid-electrolyte interphase (SEI) layer. The synthesis conditions of the co-precipitation method are found to influence the microstructure and morphology through nucleation and growth of crystals in solution. Two samples prepared by similar wet-chemical routes have been characterized by microscopy and electrochemical methods to determine the role of microstructure and morphology on the electrochemical performance. It is found that the surface crystal planes play a key role in the capacity retention and rate performance. In order to achieve consistent electrochemical properties essential for the commercialization of the high-voltage spinel cathode LiMn[subscript1.5]Ni[subscript 0.5]O₄, the relationship between cation ordering, presence of impurity phase, and particle morphology must be elucidated. Accordingly, comparison of the stoichiometric LiMn[subscript1.5]Ni[subscript 0.5]O₄ cathodes with a Mn/Ni ratio of 3.0 prepared by different methods having varying morphologies and degrees of cation ordering is presented. It is found that although an increase in the degree of cation ordering decreases the rate capability, the crystallographic planes in contact with the electrolyte have a dominant effect on the electrochemical properties. To examine the effect of cation substitution on morphology, an investigation of the nucleation and growth of doped co-precipitated mixed-metal hydroxide precursor particles and the resulting stabilization of preferred crystallographic surface planes in the final spinel samples are presented. It is found that doping with certain cations stabilizes the growth of low-energy (111) surface planes, facilitating a long cycle life and fast high-rate performance. With an aim to develop a better understanding of the factors influencing the electrochemical properties, a systematic investigation of LiMn[subscript 1.5]Ni[subscript0.5-x]M[subscript x]O₄ (M = Cu and Zn and x = 0.08 and 0.16), in which Ni²⁺ ions are substituted by divalent Cu2+ and Zn2+ ions, is presented. It is found that although both Zn and Cu are divalent with ionic radii similar to that of Ni2+, they behave quite differently with respect to cation ordering and site occupancy, and higher levels of doping leads to distinct differences in cycling and rate performances. / text

No / Hemp concrete is an attractive alternative to traditional materials used in building construction. It has a very low environmental impact, and it is characterized by high thermal insulation. Hemp aggregate particles are parallelepiped in shape and can be organized in a plurality of ways to create a considerable proportion of open pores with a complex connectivity pattern, the acoustical properties of which have never been examined systematically. Therefore this paper is focused on the fundamental understanding of the relations between the particle shape and size distribution, pore size distribution, and the acoustical properties of the resultant porous material mixture. The sound absorption and the transmission loss of various hemp aggregates is characterized using laboratory experiments and three theoretical models. These models are used to relate the particle size distribution to the pore size distribution. It is shown that the shape of particles and particle size control the pore size distribution and tortuosity in shiv. These properties in turn relate directly to the observed acoustical behavior.

The discoveries of materials property-property correlations usually require prior knowledge or serendipity, the process of which can be time-consuming, costly, and labour-intensive. On the other hand, artificial neural networks (ANNs) are intelligent and scalable modelling techniques that have been used extensively to predict properties from materials’ composition or processing parameters, but are seldom used in exploring materials property-property correlations. The work presented in this thesis has employed ANNs combinatorial searches to explore the correlations of different materials properties, through which, ‘known’ correlations are verified, and ‘unknown’ correlations are revealed. An evaluation criterion is proposed and demonstrated to be useful in identifying nontrivial correlations. The work has also extended the application of ANNs in the fields of data corrections, property predictions and identifications of variables’ contributions. A systematic ANN protocol has been developed and tested against the known correlating equations of elastic properties and the experimental data, and is found to be reliable and effective to correct suspect data in a complicated situation where no prior knowledge exists. Moreover, the hardness increments of pure metals due to HPT are accurately predicted from shear modulus, melting temperature and Burgers vector. The first two variables are identified to have the largest impacts on hardening. Finally, a combined ANN-SR (symbolic regression) method is proposed to yield parsimonious correlating equations by ruling out redundant variables through the partial derivatives method and the connection weight approach, which are based on the analysis of the ANNs weight vectors. By applying this method, two simple equations that are at least as accurate as other models in providing a rapid estimation of the enthalpies of vaporization for compounds are obtained.

è estuda a cinética de oxidação de filmes finos de cobre e os mecanismos envolvidos em sua passivação através de; implamtação iônica, formação de ligas e utilização de uma camada protetora. / Oxidation kinetics and its mechanisms has been studied for thin Copper films deposited by PVD on glossy carbon (UDAC) and Si wafers.

The effects of reactive metal particles on the microstructure and mechanical properties of epoxy-based composites are investigated in this work. To examine these effects castings of epoxy reinforced with 20-40 vol.% Al and 0-10 vol.% Ni were prepared, while varying the aluminum particle size from 5 to 50 microns and holding the nickel particle size constant at 50 microns. In total eight composite materials were produced, possessing unique microstructures. The microstructure is quantitatively characterized and correlated with the composite constitutive response determined from quasi-static and dynamic compressive loading conditions at strain-rates from 1e-4 to 5e3 /s. Microstructures from each composite and at each strain rate were analyzed to determine the amount of particle strain as a function of bulk strain and strain rate. Using computational simulations of representative microstructures of select composites, the epoxy matrix-metallic particle and particle-particle interactions at the mesoscale under dynamic compressive loading conditions were further examined. From computational simulation data, the stress and strain localization effects were characterized at the mesoscale and the bulk mechanical behavior was decomposed into the individual contributions of the constituent phases. The particle strain and computational analysis provided a greater understanding of the mechanisms associated with particle deformation and stress transfer between phases, and their influence on the overall mechanical response of polymer matrix composites reinforced with metallic particles. The highly heterogeneous composite microstructure and the high contrasting properties of the individual constituents were found to drive localized deformations that are often more pronounced than those in the bulk material. The strain rate behavior of epoxy is shown to cause a strain rate dependent deformation response of reinforcement particle phases that are typically strain rate independent. Additionally, the epoxy matrix strength behavior was found to have a higher dependence on strain rate due to the presence of metal particle fillers. Discrepancies between experimental and simulation mechanical behavior results and these findings indicate a need for epoxy constitutive models to incorporate effects of particle reinforcement on the mechanical behavior.

The availability of large amounts of data generated by high-throughput computing and experimentation has generated interest in the application of machine learning techniques to materials science. Machine learning of materials behavior requires the use of feature vectors that capture compositional or structural information influence a target property. We present methods for assessing the similarity of compositions, substructures, and crystal structures. Similarity measures are important for the classification and clustering of data points, allowing for the organization of data and the prediction of materials properties. / Engineering and Applied Sciences

è estuda a cinética de oxidação de filmes finos de cobre e os mecanismos envolvidos em sua passivação através de; implamtação iônica, formação de ligas e utilização de uma camada protetora. / Oxidation kinetics and its mechanisms has been studied for thin Copper films deposited by PVD on glossy carbon (UDAC) and Si wafers.

No description available.

Functionally graded materials (FGMs) are composites that have continuously varying material properties, which eliminate undesirable stress concentrations that might otherwise occur in layered composites. The concept of inhomogeneously varying properties is observed in nature; examples include bones, teeth, shells and timber. Modern engineering applications of FGMs include thermal barrier coatings, wear-resistant coatings, biomedical implants and MEMS devices. Syntactic foams, particle filled nano-composites are examples of inhomogeneous materials of current interest. Analyses and experiments available in the literature have focused on characterizing the inhomogeneous material modulus and density variations. Common techniques employed are nano-indentation and wave propagation studies. There are a few fracture mechanics analyses and experiments available in the literature; most of which are devoted to measuring the fracture toughness of graded materials. A few fracture analyses of graded materials are devoted to deriving asymptotic stress, strain and displacement fields around stationary and steadily growing cracks in inhomogeneous materials. Only a few studies exist that deal with understanding the effect of material property inhomogeneity on the spontaneous crack propagation. In the present thesis the effect of material property inhomogeneity on the dynamic fracture mechanics of cracks in FGMs is described. Numerical analysis of the elastodynamic initial boundary value problem is performed using a spectral scheme. Spectral scheme is a special numerical technique developed to simulate spontaneous, planar crack propagation in a variety of materials. The method is numerically efficient as it can be implemented on parallel machines with ease. The numerical scheme is versatile and can handle any state-and rate-dependent traction-separation laws (cohesive zone models) or frictional laws. Spectral scheme has successfully been used in simulating intersonic crack propagation, earthquake slip dynamics and also direct silicon wafer bonding process used in realizing 3D MEMS structures. In the present work, the spectral formulation accounts for the inhomogeneous variation in the material wave speeds in the medium. The effect of inhomogeneity on spontaneous crack propagation due to in-plane mixed-mode loading is also addressed here. A parametric study has been performed by varying the inhomogeneity length scales independently in the top and bottom half-spaces. The effect of inhomogeneity in shear wave speed on the dynamic stress intensity factors (SIFs) of a crack propagating in a quasi-steady-state along the interface between the two functionally graded half-spaces is studied. A symmetric hardening FGM offers the maximum fracture resistance, while the fracture resistance is minimum for a symmetric softening FGM. Our simulation shows that increasing the inhomogeneity in the wave speed leads to eliminate the overshoot in the dynamic stress intensity factor. The magnitude of the steady-state (long-time) SIF increases indicating an increase in the fracture resistance. The effect of the inhomogeneous wave speed on the mode-3 crack propagation characteristics is demonstrated by taking snapshots of the crack opening at a time interval. The magnitude of the crack sliding displacement is found to increase with increase in the inhomogeneity. The effect of the material property inhomogeneity on the mode-1 crack propagation is simulated to track the crack opening displacements. The inhomogeneity is assumed to be symmetric about the weak-plane. Our spectral scheme developed here for functionally graded material with exponential variation in the material properties is capable of simulating independent bimaterial combinations. When the graded material becomes progressively stiffer and denser (hardening), the crack opening displacement in reduced, indicating an increase in the fracture resistance. On the other hand, for the softening FGMs the crack opening displacement increases indicating a reduction in fracture toughness. It is noted that the cohesive fracture resistance on the weak-plane remains same in all the FGMs.