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In recent years considerable research has been done in the area of "nanofluids". Nanofluids are colloidal suspensions of nanometer size metallic or oxide particles in a base fluid such as water, ethylene glycol. Nanofluids show enhanced heat transfer characteristics compared to the base fluid. The thermal transport properties of nanofluids depend on various parameters e.g. interfacial resistance, Brownian motion of particles, liquid layering at the solid-liquid interface and clustering of nanoparticles. In this work atomic scale simulation has been used to study possible mechanisms affecting the heat transfer characteristics of nanofluids. Molecular dynamics simulation for a single silica nanoparticle surrounded by water molecules has been performed. Periodic boundary condition has been used in all three directions. The effect of nanoparticle size and temperature of system on the thermal conductivity of nanofluids has been studied. It was found that as the size of nanoparticle decreases thermal conductivity of nanofluid increases. This is partially due to the fact that as the diameter of nanoparticle decreases from micrometer to nanometer its surface area to volume ratio increases by a factor of 103. Since heat transfer between the fluid and the nanoparticle takes place at the surface this enhanced surface area gives higher thermal conductivity for smaller particles. Thermal conductivity enhancement is also due to the accumulation of water molecules near the particle surface and the lattice vibration of the nanoparticle. The phonon transfer through the second layer allows the nanofluid thermal conductivity to increase by 23%-27% compared to the base fluid water for 2% concentration of nanosilica.

The current work is part of the EPSRC MUZIC project, which established the collaboration among several universities to carry out a multidiscipline study on the breakaway oxidation of zirconium alloys. The overall goal of the project is to further understand the mechanisms of the oxidation and breakaway process of zirconium alloys. This thesis describes the nano/micro-structural study and nano-analysis of the corroded zirconium alloys using up-to-date TEM and 3D focused ion beam (FIB) slicing and reconstruction techniques. The work mainly focused on the characterization of ZIRLO. The oxide morphology in general comprises an inner columnar layer and an outer equiaxed layer, except for a post-second transition oxide grown on a Zr-Nb-Ti test alloy with a very poor corrosion resistance, which exhibits generally only equiaxed grains throughout the whole oxide scale. Detailed investigation reveals oxides in a slower oxidation stage exhibit better developed columnar grain structure. All the oxides, independent of different corrosion stages and alloy types, contain predominantly monoclinic oxide and a small amount of tetragonal oxide. Defects at different length scales were examined. In stead of a sudden burst of crack nucleation at the kinetic transition, a gradual introduction of cracks parallel to the metal/oxide interface throughout the pre-transition stage is found, suggesting no direction correlation between the formation of cracks and the transition. Besides cracks, the oxide also contains different forms of nano-porosity: isolated pores of 1-3 nm or interconnected pores at grain boundaries. The density of interconnected porosity, especially those along the oxide growth direction, increases towards the oxide surface, evolving over time. It is suggested that the kinetic transition is related to the development of an interconnected porosity down to the metal/oxide interface, providing easy pathways for the transportation of oxidation species. The metal-oxide interface has a wavy morphology both in the micrometer and nanometer scale. The roughness develops to a maximum just before the first kinetic transition. An intermediate suboxide layer with complex 3D morphology between the bulk oxide and the metal substrate is found. Quantitative EELS analysis shows the composition of this layer to be 40-50 at. % oxygen. The suboxide appears to develop in thickness with increasing oxidation time for the pre-transition oxides, while is very thin or absent in the post-, and post-second transition oxides. In the suboxide region, multiple phases including α-Zr, ω-Zr, tetragonal oxide and a phase with an unidentified structure were found, suggesting different structures can coexist in the suboxide layer. Second-phase particles (SSPs) of β-Nb and hexagonal Zr(Fe,Nb)₂ types were found in ZIRLO samples and FCC Zr(Fe,Cr)₂ was the predominant type in Zircaloy-4. The SPPs showed delayed oxidation compared to surrounding Zr. In ZIRLO, those containing high Fe contents were found to be oxidized and transform into an amorphous state much earlier than β-Nb. Hydrides of different types (γ, σ and ε) were observed in the metal and metal/oxide region for both Zircaloy-4 and ZIRLO samples. A higher density of hydrides was seen in post-transition oxides of ZIRLO than in pre-transition oxides.

Les zéolites, aluminosilicates cristallisés microporeux, sont largement utilisés en raffinage, en pétrochimie et en conversion de la biomasse. En raison du faible diamètre des micropores, limitations diffusionnelles et effets de confinement peuvent favoriser la formation de sous-Produits non désirés. L'introduction de mésopores par désalumination et/ou désilication ("zéolites hiérarchisées") peut diminuer ces phénomènes. Cependant, les mécanismes ces réactions restent méconnus à l'échelle moléculaire. Par calculs quantiques périodiques, au niveau de la théorie de la fonctionnelle de la densité (DFT) et selon une approche hybride QM/QM, nous avons pu mettre en évidence l'importance de l'attaque de la molécule d'eau sur l'atome d'aluminium, qui se fait en anti par rapport au site acide de Brønsted. Des structures d'Al penta ou tetra coordinées ont aussi été suggérées expérimentalement comme précurseurs de la désalumination. Malgré une forte hétérogénéité structurale des sites T, l'élucidation des chemins réactionnels et les énergies d’activation des étapes d’hydrolyse des liaisons Al-O (70-100 kJ/mol) dans les systèmes zéolitiques investigués (MOR, FAU, MFI, CHA) nous a permis d'établir des corrélations du type Brønsted-Evans-Polanyi. Ces corrélations permettent d'estimer et prédire des énergies d'activation par le biais de la thermodynamique, donne ainsi une prédiction aisée des sites T sensibles à la désalumination. Un autre facteur clé pour la compréhension de la désalumination est l'effet de confinement sur l'espèce aluminique extra-Réseau générée (EFAL), exercé par les cavités. Finalement nous avons pu montrer que le chemin réactionnel de désalumination et désilication, consécutif ou simultané, , est thermodynamiquement plus favorable qu'une simple désalumination ce qui est en accord avec les propositions mécanistiques de la littérature sur la genèse de mésopores par démétallation. / Zeolites are crystalline microporous aluminosilicates widely used in refining, petrochemistry and biomass conversion. However, diffusion limitation and confinement effect can promote the formation of undesired products. The introduction of mesopores by dealumination and/or desilication ("hierarchical zeolites") is a possible solution widely used experimentally. Nevertheless, the mechanisms of these demetallation reactions are poorly described at the molecular scale. We determine the mechanisms of the formation of extraframework Al species (EFAL) for zeotypes MOR, FAU, MFI and CHA occurring during the dealumination process, possibly associated with desilication. First-Principles periodic density functional theory (DFT) and hybrid QM/QM calculations have been employed in order to analyze full reaction paths leading to extraframework species and to quantify the activation energies of the determining steps. It has been demonstrated that the initiation of an Al-O(H) bond break takes place via water adsorption on the Al atom in anti-Position to the Brønsted acid site, via a penta- or tetra-Coordinated Al species. Such species are shown to be at the initiation of the Al dislodgement from the zeolitic framework. Despite a strong structural heterogeneity of T sites, we determined Brønsted-Evans-Polanyi (BEP) relationships for the entire dealumination pathway. Moreover, it is shown that not only the initiation and propagation mechanisms are primordial for the understanding of an Al extraction, but also the confinement effect on EFAL species within the zeolites cavities. Finally, from the energy profile of combined dealumination/desilication pathways, we show that it is thermodynamically favoured to extract extraframework Si species (EFSI) in the course of dealumination.

When an electronic system is confined in one or more dimensions to a length scale comparable to the de Broglie wavelength, quantum confinement occurs. In metallic quantum thin films grown on semiconductor substrates, such confinement occurs between the vacuum-solid and the solid-solid interfaces, which results in the formation of distinctive quantum well states (QWS). Due to this confinement, many physical phenomena occurring in the thin metal system are totally different from the bulk system, which makes the study of quantum thin films interesting and important. In this thesis, quantum thin film studies, mainly based on the Pb/Si(111) system, were performed utilizing low-temperature scanning tunneling microscopy/spectroscopy (STM/STS) with a focus on three main aspects: phase relationship, surface reactivity, and coherent coupling. The Pb/Si(111) system is chosen due to its unique phase matching between the Fermi wavelength and the lattice spacing along [111], leading to a bi-layer quantum oscillation in many physical properties, including the surface energy and the work function. Surprisingly, STM/STS measurement revealed that quantum oscillations of work function and surface energy have identical phase, in contrast to a theoretically predicted 1/4 wavelength phase shift in the phase relationship. Here, a new solution to this puzzle is provided. Furthermore, it is found out that the oxidation rate of Pb/Si(111) system is greatly enhanced in the presence of atomic scale catalyst -- Cs substitutional atoms, while the reactivity to CO is saturated after the initial enhanced nucleation. Finally, by inserting thin Ag layers in between Pb/Si(111) system, the coherent coupling of double quantum wells (a Pb quantum well and a Ag quantum well) are probed, where combined QWS features are observed by STS measurement. The growth mechanism of these heterostructures -- Pb/Ag/Si(111) -- is also investigated. / text

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The mechanical behavior of discrete atoms has been studied with molecular dynamics whose computational time is proportional to the square of the number of atoms, O(N²). Recently, a faster algorithm, Atomic-scale Finite Element Method (AFEM) with computational time proportional to the number of atoms, O(N), had been developed. The main idea of AFEM, compared with conventional finite element method is to replace nodes with atoms and elements with electric forces between atoms. When interpreting a non-linear system, it is necessary to use an iteration scheme. A simulation of molecular dynamics based on the Verlet's method was conducted in order to validate AFEM in one dimension. The speed of AFEM was investigated in one and two dimensional atomic systems. The results showed that the computational time of AFEM is approximately proportional to the number of atoms, and the absolute computation time appears to be small. The frameworks of AFEM not only for multi-body potential but also pair potential are presented. Finally, AFEM was applied to analyze and interpret the mechanical behavior of a carbon nanotube and a quartz. The buckling behavior of carbon nanotube showed a good agreement with the results illustrated in the original literature. / Master of Science

Properties of aggregates are affected by their morphological characteristics, including shape factors, angularity and texture. These morphological characteristics influence the aggregate's mutual interactions and strengths of bonds between the aggregates and the binder. The interactions between aggregates and bond strengths between the aggregate and the binder are vital to rheological properties, related to workability and friction resistance of mixtures. As a consequence, quantification of the aggregate's morphological characteristics is essential for better quality control and performance improvement of aggregates. With advancement of hardware and software, the computation capability has reached the stage to rapidly quantify morphological characteristics at multiple scales using digital imaging techniques. Various computational algorithms have been developed, including Hough transform, Fourier transform, and wavelet analysis, etc. Among the aforementioned computational algorithms, Fourier transform has been implemented in various areas by representing the original image/signal in the spatial domain as a summation of representing functions of varying magnitudes, frequencies and phases in the frequency domain. This dissertation is dedicated to developing the two-dimensional Fourier transform (FFT2) method using the Fourier Transform Interferometry (FTI) system that is capable to quantify aggregate morphological characteristics at different scales. In this dissertation, FFT2 method is adopted to quantify angularity and texture of aggregates based on surface coordinates acquired from digital images in the FTI system. This is followed by a comprehensive review on prevalent aggregate imaging techniques for the quantification of aggregate morphological characteristics, including the second generation of Aggregate Image Measurement System (AIMS II), University of Illinois Aggregate Image Analyzer (UIAIA), the FTI system, etc. Recommendations are made on the usage of aggregate imaging system in the measurements of morphological parameters that are interested. After that, the influence of parent rock, crushing, and abrasion/polishing on aggregate morphological characteristics are evaluated. Atomic-scale roughness is calculated for crystal structures of five representative minerals in four types of minerals (i.e., α-quartz for quartzite/granite/gravel/aplite, dolomite for dolomite, calcite for limestone, haematite and magnetite for iron ore); roughness ranking at atomic-scale is further compared with surface texture ranking at macroscale based on measurement results using the FTI system and AIMS II. Morphological characteristics of aggregates before and after crushing test and micro-deval test are measured to quantitatively evaluate the influences of the crushing process and the abrasion/polishing process on morphological characteristics of aggregates, respectively. / Ph. D.

In Pressurised Water Reactors, the reactor pressure vessel (RPV) is considered a life limiting component due to the degradation of its mechanical properties. Nano-scale Cu-enriched precipitates are known to cause embrittlement in the form of increases in hardness and the ductile-to-brittle transition temperature. The effect of irradiation on the RPV is the dominant contributor to this embrittlement. This is due to the increased mobility of Cu from the high number of vacancies, and the matrix damage providing many heterogeneous nucleation sites. However, there are also thermal effects which may be difficult to separate from the irradiation effects. To understand the contribution of the long term thermal ageing to RPV embrittlement a series of weld and plate materials containing systematic variations of Ni and Cu has been thermally aged for times up to 100,000 hrs at 330 degrees C, 365 degrees C and 405 degrees C. Microstructural characterisation using Atom Probe Tomography has been performed. Complimentary Monte-Carlo simulations have been used to investigate the early stages of formation of Cu-enriched precipitates. Thermal ageing produces a high number density of nano-scale Cu-enriched precipitates. These nanometre precipitates have a Ni-Mn-Si rich interface which was found to be wider with increased precipitate size, lower ageing temperature and higher bulk Ni content. This interface reduces the interfacial energy of the Cu-enriched precipitates through a combination of the minimising of unfavourable Fe-Cu bonds and reduction in lattice strain. The matrix Cu levels after ageing for 90,000 - 100,000 hrs were found to be around 0.06 - 0.07 at.%, close to the expected solubility limits for Cu in Fe. The Fe content of the precipitates has been characterised and found to be higher at lower ageing temperature and for smaller precipitate sizes. Cu precipitation and solute segregation at dislocations were observed, particularly in the SG steels aged at higher temperatures where the supersaturation is lowest. Movies were produced from the Kinetic Monte-Carlo modelling (see accompanying DVD) and along with other analyses indicated that sub-critical Ni and Mn clusters may be active in the formation of Cu-enriched precipitates. This mechanism appears to occur in both the high and low Ni steels. Thus, the higher number density of larger precipitates observed with increased bulk Ni content is thought to be a consequence of the greater number of sub-critical Ni-Mn clusters providing more nucleation sites. A small number of grain boundaries were examined. Segregation of many solute species to them was observed, which is thought to result from a multi-element co-segregation process. Ni-Mn-Si precipitates were observed at grain boundaries and dislocations in the high Ni steels with high and low Cu levels. These are consistent with similar phases in found in irradiated high Ni steels. In the high Cu steels these particles were much larger and associated with Cu-enriched precipitates.

This work examines various light metal borohydrides, particularly those formed from group II metals, with the aim of understanding their fundamental physical properties and improving their hydrogen storage ability. The structure of a new phase (γ) of Mg(BH₄)₂ is reported and the decomposition is fully characterized in a combination of diffraction and thermogravimetric studies. The bulk properties of γ-Mg(BH₄)₂ are compared to those of an SiO₂ isostructure and probed by various neutron scattering techniques. Negative thermal expansion is observed at low temperatures and the material absorbs up to 1.5 moles of hydrogen gas to form one of the most gravimetrically hydrogen-dense materials ever reported. The structural evolution of Ca(BH₄)₂ under different synthetic conditions and external influences (e.g. temperature) is studied up until the material decomposes. The effects of various additives on group II metal borohydrides are also examined and the influence of each is justified by observing subtle structural changes in the mixed system via in situ synchrotron X-ray powder diffraction and ¹¹B NMR measurements.