No description available.

No description available.

Several industrial processes involve the use of silver in one form or another, two of which there are in the use of conductive adhesives within the Personnel Computer Boards manufacturing industry and as a catalyst for the epoxidation of ethylene. Both these processes could be improved significantly if the silver surface were altered morphologically. One possible way to improve either of the aforementioned technologies would be to plasma modify the silvers surface. The aim of the work described in this thesis is to produce a high surface area silver using a high frequency rf plasma, with the hope that with further research this technology may be used to modify silver powder for use in conductive adhesives or as a catalyst for the epoxidation of ethylene This thesis describes the use of high frequency continuous wave (CW) oxygen plasmas for the treatment of silver foils, in particular the effects on surface morphology of changes in rf power (15-50 W), gas pressure (0.2-0.8 mbar), exposure time (lmin-2hrs), orientation of the foil in the reactor with respect to the gas flow (perpendicular or parallel) and distance of the foil from the live electrode (0-17cm). Scanning Electron Microscopy was used to observe these morphological changes. Further to observing the surface topographies of the oxides under these various conditions, a brief, but no means complete, study of the structure of the various oxides produced at these various conditions was performed using X-ray powder diffraction. Reduction of these oxidised foils using a hydrogen plasma (CW) was investigated, with particular emphasis being placed on how rf power (15-50W), gas pressure (0.2-0.8 torr) and exposure times (lmin-2hrs) affected the extent of reduction of the oxidised foil (using X-ray Crystallograhy) and its surface morphology (using Scanning Electron Microscopy ).Pulsed oxygen plasma treatments of the silver foils and CW oxygen plasma treatment of earthed silver samples were performed to try and elucidate the mechanism behind any surface modification. From the above investigations it was found that oxygen plasma treatment of silver foils resulted in significant modification of the surfaces, and this depended on the energy density of the plasma. The optimum surface texture (in terms of apparent surface area) was observed at rf powers of 15W, gas pressures of 0.8 torr, exposure times of 30 min, and at distances of 8-9 cm from the live electrode. The oxidised foil could be fully reduced back to silver metal using a hydrogen plasma, with little change in the optimum surface morphology. The pulsing and earthing studies indicated that the formation of the new surface textures was either due to surface diffusion of a charged silver species or was caused by ion bombardment of the silver surface resulting in a facetted surface texture. The oxides observed were Ag0 and Ag(_2)0, but to what extent each was present appeared to depend on the energy density of the plasma.

No description available.

A key goal in oxide surface science is to determine the mechanism of catalytic reactions at the atomic level. Here we are ultimately interested in CO oxidation over a model automobile catalyst: Pd supported on Ce02(lll) ultrathin films. The degree of reduction (Ce3+ concentration) is thought to play an important role in determining the catalyst performance. In this work X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) is used to study the degree to which the Ce3+ concentration can be varied in Ce02(l 11) ultrathin films on Pt(lll) and Rh(lll) and how they are modified by Pd. Reflection absorption infrared spectroscopy (RAIRS) has also been used to investigate the adsorption of CO on Pd/Ce02 x/Pt(l 11). The stoichiometry of three layer Ce02_x(lll) grown on Pt(lll) has been varied between CeOi 91 and CeO!M by UHV annealing at 970 K. Pd deposition induces an additional reduction of the film, for instance one monolayer equivalent of Pd on CeOL91 changes the stoichiometry to Ce0187 This reduction, which is dependent on the initial stoichiometry, is thought to be due to charge transfer from Pd to Ce02_x CO adsorbs only in the presence of Pd, independent of the stoichiometry of the original film, without affecting the Ce3+ concentration. Reflection absorption infrared spectroscopy (RAIRS) has been used to investigate the adsorption of CO on Ce02.x-supported Pd nanoparticles at room temperature. RAIRS results show that when Ce02 x is initially grown on Pt(l 11), a small proportion of the surface remains as bare Pt sites. However, when Pd is deposited onto Ce02.x/Pt(lll), most of the Pd grows directly on top of the Ce02.x(l 11). RAIR spectra of CO adsorption on 1 ML Pd/Ce02.x/Pt(l 11) show a broad CO-Pd band, which is inconsistent with a single crystal Pd surface. However, the 5 ML and 10 ML Pd/Ce02 x/Pt(lll) spectra show vibrational bands consistent with the presence of Pd(lll) and (100) faces, suggesting the growth of Pd nanostuctures with well defined facets. XPS studies of Ce02.x films grown on Rh(l 11) have also been carried out. It has been shown that by altering the oxygen pressure during Ce deposition the Ce3+ concentration of the resultant Ce02x thin film can be altered by up to 60%, whilst still observing the same LEED pattern. The effect of dosing Pd onto CeOz.x thin films has been shown to be highly dependent on the initial oxidation state of the Ce02x thin films. On Pd deposition onto CeOL91 and Ce0184 thin films an increase in Ce3+ concentration has been observed. However, in direct contrast, a decrease in Ce3+ concentration is observed when Pd is deposited on CeOl 7l and CeO! 61 thin films. CO adsorbs only in the presence of Pd, independent of the stoichiometry of the original film, without affecting the Ce3+ concentration.

When solid particles of different types are mixed together, a random distribution of the components is rarely produced and deterioration of the mixture can occur on subsequent handling. Among the microscopic processes responsible, one important mechanism for free-flowing materials is thought to be interparticle percolation, the drainage of particles through the interstices between larger ones. If the larger particles are stationary this is called spontaneous percolation, whereas if it is produced by shear strain the term strain-induced percolation is used. Here a quantitative evaluation of both and some consequences are described. A practical application of spontaneous percolation has been the design and construction of a new static mixer or distributor, consisting of rows of angle bars mounted horizontally in a vertical channel. Material fed to the top of a unit bounces off the bars and is distributed across the channel. Two mixers were built; one dispersed material in one lateral direction only and could be used for feeding material onto a belt or distributing seed from a moving vehicle. The other produced a two-dimensional dispersion and would be useful in distributing material flowing into hoppers or whenever a good mixture were required. Optimisation of the design was investigated using a computer program which simulated the motion of a spherical particle as it fell through such a mixer. Design data was deduced from the record of the position of the particle. The mixers were not suitable for use with fine materials. Interpretation of experimental results from this equipment requires suitable statistical indices and two were developed here. One related the variance of sample compositions to the number of particles fed to the mixer by assuming that the distributions of material were ordered. The second, using the correlation coefficient between samples, related the variance to the sample size in those situations where two orthogonal processes are in operation. Both techniques are generally applicable to fields other than that of powder mixing. On the theoretical side, an existing model of spontaneous percolation for inelastic materials has been extended and improved. The original form did not account for the motion of a particle between collisions with bulk particles but this has now been included. An entirely new semi-empirical model for partly elastic materials has also been proposed. Both predict percolation velocities which agree with experimental data. In order to extend earlier experimental studies on strain-induced percolation, a simple shear cell was modified by installing a hydraulic drive which enabled the cell to be driven at a constant speed. Advantages of the use of such a cell include the possibility of detecting a percolating particle on entry to and exit from the bed and the constant strain throughout the material. Reliable and accurate readings of residence times of percolating particles were recorded and percolation velocities and both lateral and axial diffusion coefficients were calculated. These were functions of the relative particle size and density, the material properties of the percolating particle and bed conditions such as strain rate and normal stress Denser and softer particles percolated faster. Decreasing the diameter ratio between percolating and bulk particles from 0.67 to 0.27 caused a twenty-five fold increase in the percolation rate. The dependence of this rate on particle diameter was interpreted using statistical mechanics. The percolation rate has been shown to reach a constant value as the strain rate increases, in contrast to the deductions drawn in earlier work by Scott, whose procedure has been proved to be unsound.

This thesis aims to characterise the structural, adhesion and release properties of polymeric microcapsules which are used ina griculture for t he controlled delivery of pesticides to crops. An adaptation of atomic force microscopy (AFM) known as chemical force microscopy (CFM) has been used to investigate the adhesion characteristics of functionalised polyurea microcapsules to surfaces. The adhesion properties of microcapsules (attached to AFM cantilevers) to self-assembled monolayers (SAMs) of alkyl thiols on gold have been investigated at the single capsule level, and as a function of polymer wall surface functionality. Measurements have been made to OHterminated, CH3-terminated and mixed CH3: COOH-terminated SAMs. The importance of microcapsule topography and elasticity on adhesion measurements has been discussed. The surface pK1I2 of a capsule with a surface modified with a sulfonateterminated molecule has been estimated using force titration methodology. The adhesion properties of functionalised microcapsules to the leaf cuticle of Prunus laurocerasus have been examined, highlighting areas of hydrophilicity on the surface of the leaf. The release properties of a ctive ingredients from p olyurea microcapsules into solution using confocal laser scanning microscopy (CLSM) have been investigated. Preliminary studies obtained the optimum species to use inside the microcapsule and the solution into which the species should be released. The release characteristics 0 f six varieties of microcapsule, differing in the wall thickness and cross linking density were examined and from the release profiles obtained, the product of the mass transfer coefficient and solubility coefficient could be calculated. The high resolution imaging capabilities of the confocal microscope have been demonstrated, providing detail on the structure of the leaf of the Prunus Laurocerasus plant and t he caterpillars Heliothis virescens and P lutella xylostella, two of t he most common pests, which the microcapsules of interest are aimed at controlling. The release properties to leaf surfaces of two types of polyurea microcapsules, which have different release mechanisms, were investigated using CLSM. Detailed release studies to model surfaces showed that one type of capsule (lambda cyhalothrin) released in the dry state and the other (emamectin benzoate) did not. A release rate was determined in the former case. A technique has been developed which is capable of visualising the release and diffusion of a species (Cd2+) from a hemisphere. Voltammetry has been used in conjunction with CLSM to monitor quantitatively and visually the preconcentration and release of Cd2+ into and away from a mercury hemisphere UME. The fluorescent indicator used was Calcium Green-5N, which is a cadmium-sensitive fluorophore that fluoresces upon binding Cd2+and can thus be detected by CLSM. A delay was observed between the release of C d2+ from t he hemisphere (determined voltammetrically) and binding to the fluorophore (visualised using CLSM).

Les surfaces texturées sont devenues, ces dernières années, des substrats de choix pour de nombreuses applications. En effet, la texturation des surfaces, de l'échelle nanométrique à l'échelle microscopique, permet d'accroître les propriétés d'adhérence ou de renforcer la résistance mécanique intrinsèque. Dans ce travail de thèse nous proposons une étude sur l'influence des textures chimiques sur le comportement tribologique, adhésif et sur la mouillabilité des substrats. Dans le premier chapitre, on propose une stratégie de micro-texturation des surfaces basées sur la technique de microcontact-printing et le greffage de chaînes de polymères de géométries/formes contrôlées. En outre, on a notamment étudié de manière approfondie la mouillabilité des surfaces texturées afin de comprendre les effets de diminution de taille des textures sur le comportement adhésif. Dans les expériences d'adhésion et frottement, un dispositif de type JKR (pour Johnson, Kendall et Roberts), une machine de frottement et une FFM ont été utilisés permettant d'observer le contact entre une sphère élastique et une pointe rigide avec un plan texturé tout en contrôlant la force entre les surfaces. En outre l'utilisation de ces différentes approches ne nous a pas finalement donné des explications satisfaisantes sur les mécanismes agissant sur les phénomènes interfaciaux. De ce fait, l'utilisation du démouillage de films minces de PS et de PDMS sur des surfaces texturées nous as permis de suivre l'évolution de l'instabilité du bourrelet à l'interface. Enfin, dans le dernier chapitre, nous avons étudier les différents aspects prédominants des phénomènes interfaciaux sur des surfaces homogènes / Micro and Nanoscale surface patterns are considered as potential templates and building blocks for Micro/nanotechnology. As for materials in general, these micro /nano-scale surface structures have been of increasing research interest in recent years, due to their unique properties. They are expected to exhibit novel and significantly improved physical, chemical, mechanical and other properties, as well as to offer opportunities for manifestation of new phenomena and processes. In the present PhD work, we propose a multiple scale analysis of the adhesion, friction and wetting behaviors for different patterned interfaces. In a first chapter, we developed a general methodology to design well-defined surfaces combining micro-contact printing (µCP), self-assembled monolayers (SAMs) and polymer grafting techniques. Then we study the wettability of a patterned solid surface. Where, the stick-slip regime, and the effect of the patterning at the mesoscale was investigated. Furthermore, we concentrate on the dependence of adhesion and friction between a polymer and a rigid tip on the composition of the patterned substrates using a JKR, FFM and friction machines. Intriguingly, the uses of these approaches did not provide us with a clear answer to our bewilderment. Therefore, in the third chapter, we adopted the approach of the dewetting of thin polymer film on top of patterned surfaces. We study the impact of the solid/liquid boundary condition on the evolution of the rim instability during the course of dewetting. The last chapter details the investigation of the predominant aspect between the chemistry introduced on the surface and the mechanical proprieties of the substrate