Powders belonging to group A of the Geldart classification have been studied in detail using the bed collapse technique. To obtain the correct properties of the powders a model is developed, which takes into account the system configuration for both one- and two-valve experiments. An experimental apparatus has been assembled and used to validate the model using glass ballotini in the size range 22-106 mum. The system allows simultaneous measurements of total bed height, using a digital camera, and pressure transients at various positions along the vertical axis. The powders have been sieved to obtain 6 size ranges and the fluidization properties have been measured. Mixtures of particles have been prepared and the effect of the size differences has been investigated. The experimental results allow the construction of plots of the void fraction vs inlet gas superficial velocity, which show a continuous transition around the minimum bubbling point, with no sudden contraction of the dense phase voidage immediately above the minimum bubbling point for all powders. The plot of 6d vs Ud yields a characteristic curve, which within the experimental uncertainty has overlapping values for systems below and above the minimum bubbling point. The minimum bubbling point was affected by the structure of the distributor plate, with a coarser sinter leading to premature bubbling. The experimental data were used to test drag force correlations reported in the literature. It was found that none of the correlations commonly used in CFD simulations describes accurately the experimental 6d and Ud measurements. Modifying the literature equations it was possible to correlate the experimental 6d and Ud curve and use this information to predict the minimum bubbling point using a CFD model recently developed at UCL. The experimental results measured in this study show a strong dependence with voidage, which has allowed the formulation of constitutive equations for the characteristic dimension of the CFD model. The resulting simple correlation of the model parameter was used to obtain a new minimum bubbling criterion and predictions are compared to an extensive database of literature values.

Electromagnetic applications have been at the forefront of technological advancements for many years. As demand for these grows, the need for materials possessing unique electromagnetic properties increases. The aim of this research is therefore to investigate the electromagnetic properties of the nanoparticle colloids in the frequency range of 1 MHz to 20 GHz, with focus on the electromagnetic absorption mechanisms at microwave frequencies. A broad range of magnetic and dielectric properties are investigated using novel, as well as conventional, theoretical and experimental techniques. A number of mechanisms are highlighted for tailoring the electromagnetic performance. Results from the CoxNii.x series, with particle sizes ranging from 25 nm to 200 nm, show particle size related dielectric and magnetic properties, which aid the optimisation of resulting properties, in addition to conventional mechanisms, which are also demonstrated in colloidal form. Further reduction in particle size to below 20 nm, leads to single magnetic domain particles, which also exhibit enhanced electromagnetic properties, as demonstrated with broadband magnetic performance achieved for Co ferrofluid with an average particle size of 5 nm. Charged particle colloids, which typically consist of particles with negative surface charge suspended in an aqueous electrolyte solution, have also been investigated. These exhibit interesting dielectric properties, which occur over a range of timescales determined by various aspects of the system. It is shown that the small nanoparticle sizes, ranging from 20 nm to 220 nm, lead to faster charge dynamics than those expected from conventional micron particle sizes, such that the loss extends to microwave frequencies. The results are fitted to various models, which are validated through experiment and used to establish the origin of the results. These are used to further optimise the electromagnetic properties achievable from nanoparticle colloids and assess their potential benefits.

The effect of various organically modified silica (ormosil) nano-particle additions on the mechanical properties of a polyester resin has been investigated. For materials produced on such a fine scale, surface properties are known to dominate the manner in which they behave. This thesis presents results from two complementary areas of study: surface analysis and mechanical testing. The surface properties of the nano-particles and the interactions of the nano-particles with a polyester and adsorbed water have been investigated. These analyses have led to the development of a model which shows that the small organic groups grafted to the silica surface (methyl, ethyl and vinyl) are able to pack comparatively densely at the surface, effectively forming a continuous monolayer. This layer is sufficiently thick to prevent interaction of retained silanol groups with the polyester resin. When the silica is modified with phenyl functionality, however, the larger size leads to a more dispersed organic coverage that cannot be considered as a complete monolayer. Hence this layer reduces, but will not completely prevent interaction of the matrix with retained silanols of the silica. The particles have been dispersed in a polyester resin successfully. The dispersion process is an important step in producing viable nano-composites. Mechanical testing of such nano-composites has found a significant improvement in the toughness properties of the phenyl ormosil modified polyester, compared with the unmodified resin, whilst the other modified polyesters show smaller improvements. When considered with the surface analysis investigation, it is argued that the improvement is a result of a reduction of the strength of the interface (with reference to a commercial nano-silica) between the particles and the matrix. The phenyl ormosil is more strongly bonded than the other ormosils. Whilst these other ormosils are able to contribute to toughness through a crack pinning mechanism, the phenyl ormosil absorbs energy through debonding and promotes plastic deformation in the matrix, around and between particles, mechanisms which lead to a greater toughness enhancement.

This thesis presents the development of the experimental techniques required to study microstructure in powder samples at high pressure through the use of 2-d data. A description is given of the upgrade and installation of new experimental apparatus constructed to permit routine investigation of microstructure in samples under pressure. The experimental techniques have been applied to the modelling of preferred orientation in powders at high pressure. A demonstration of the qualitative information about preferred orientation in powders available from a 2-d detector is given. Details are presented of a quantitative model applied to image-plate data based on preferred orientation corrections commonly used in Rietvald refinement programs. It is shown that this quantitative model is not completely adequate for describing the extremely strong preferred orientation often found in powder samples at high-pressure and a new more general model based on a commonly used model of texture used by the texture analysis community is presented and shown to work well. The structural pressure dependence of the cinnabar phase of HgS is presented. Strong preferred orientation can be induced in HgS at high pressure, it is shown that, by using the preferred orientation modelling techniques presented in this thesis, it has been possible to de-correlate the effect of preferred orientation from the refined structural parameters.

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Particle formation in turbulent flows arises in a large range of industrial and environmental processes. Examples include the formation of soot particles in combustion engines, the production of nanoparticles via flame synthesis or liquid-phase reactors, or the evolution of aerosol particles in urban/non-urban environments. Simulations' can significantly improve the understanding and design/control of these processes but several difficulties arise when trying to model the underlying physics. These are mainly rooted in both the requirements of appropriate description of the particle's polydispersity and dynamics, as well as of the random fluctuations of the turbulence. In the preliminary part of this work, the turbulent effects on the particle's dynamics are discussed, showing how correlations of various orders appear in the main equation that describes them, the Population Balance Equation (PBE). The thesis elaborates a new approach for the simulation of particle formation in inhomogeneous turbulent flows, the PBE-PDF method, whose concept was recently introduced by [105]. An equation for the joint pdf of the number density and various scalars (such as the chemical species kinetically involved) is derived from fundamental equations that describe the class of considered problems. An algorithm for the numerical solution of the pdf equation is developed, based on Monte Carlo simulations. The method is discussed both at the level of the general concept of the stochastic simulations and at the level of the specific requirements of the PBE-PDF approach. The structure of the implementation is discussed in detail. Starting from empirical or theoretical kinetics, the PBE-PDF is applied to two different turbulent processes involving particle nucleation and growth. The first case considered involves the precipitation of barium sulphate crystals in a tubular reactor, while the second one involves dibutyl phthalate condensation in a free jet. For both cases the method allows for clear analysis and understanding of the particulate phase evolution, emphasis being put on the turbulent effects. Turbulence is found to have significant impact on the overall processes by spatially redistributing the intensity of the particles' mechanisms. Both processes were selected because previously studied experimentally [5][63], which allows the comparison of computed particles' size distribution with measured data. For the crystallization process the simulations' results can be directly compared, showing excellent agreements. Identified uncertainties in the experimental methods are discussed for the aerosol process, even if the computed distributions still show good agreement with the measured ones, particularly for intermediate molar fractions values.

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