The Stokes-Einstein relation is considered a benchmark in the transport of small particles in fluids, predicting an increase in diffusion with decreasing particle size. However, there is doubt about its validity at the nano-scale where some theoretical studies have predicted deviations from it. Experimental data from single nanoparticle tracking are presented in this thesis, collected using a recently-developed technique based on the optical phenomenon of caustics to detect the particle in a conventional inverted optical microscope. Experiments were performed on gold and polystyrene nanoparticles dispersed in water and glycerol-water mixtures, with viscosities ranging from 0.00008 to 0.15 Pa·s, to investigate the effect of nanoparticle size, density, concentration, and liquid viscosity on diffusion rates at a fixed temperature of 30°C. It is shown that below a critical concentration and critical size of particle diffusion falls orders of magnitude below the Stokes-Einstein prediction and it is better represented by the fractional Stokes-Einstein relation. At these experimental conditions, the diffusion coefficient was found to be constant with particle size and independent of material, but dependent on fluid viscosity. This thesis is aimed at enriching the knowledge on nanoparticle motion in simple fluid. The validity of the Stokes-Einstein diffusion at the nanoscale is addressed by experiments, within the context of simple, isotropic fluids.

Ion exchange resin (IEX) wastes from the nuclear power generation are a growing problem faced by the nuclear industry both in the UK and abroad. Vitrification of these wastes offers a route for their disposal by which these wastes can be immobilised in a durable waste-form. However, vitrification of these wastes possessed three major problems due to the anionic content of the waste, the organic content of the waste and the volatile radionuclides present in the waste which are problematic for vitrification. In order to identify routes by which these wastes may be successfully vitrified, systematic development experiments were carried out on three glass systems, an alkali borosilicate glass system, an alkali alkaline-earth glass system and a calcium silicate glass system. These development experiments identified a number of novel glass compositions which possessed improved properties in terms of reduced melting temperatures, increased capacities for sulphur species, increased capacities for reduction, amongst other improvements to physical properties. These improvements were attributed to changes to the glass forming network due to additive species. Glass compositions were waste-loaded with simulant IEX waste. ZnO containing alkali alkaline-earth silicate glass waste-forms demonstrated high levels of sulphur retention and near total caesium retention. These results were attributed to the physical properties of the novel glass compositions. Developed waste-forms were demonstrated to possess superior aqueous durability and radiation stability due to low alkali content and the presence of Fe in their composition. A mechanism was identified by which Fe acted to prevent the accumulation of ?-radiation induced defects in the waste-forms. ZnO-containing alkali alkaline-earth silicate glass compositions were shown to have a low melting temperature, high density, increased capacity for anionic species and the ability to achieve high Cs retentions.

A vibration isolator, sometimes called an isolating mount, is the device situating between the vibration source and the sensitive system preventing the transmission of undesired disturbances. The performance is measured by the force or the displacement transmissibility, both functions of frequency. A good vibration isolation system has three main properties - a low resonant peak, a large isolation range and low transmissibility at non-resonant regions. Unfortunately, these characteristics cannot be achieved simultaneously by a simple linear vibration isolation system. The thesis addresses this problem for single-degree-of-freedom (sdof) vibration isolation systems by introducing nonlinear damping and stiffness devices into the system. First, theoretical studies were carried out to rigorously reveal the benefits of the proposed nonlinear vibration isolation systems over linear ones. Next, the performance of these nonlinear systems were analysed by simulations. Then, experimental studies were conducted to verify the theoretical and simulations results. Finally, a systematic approach was developed to design the parameters of the nonlinear damping and stiffness devices in order to satisfy specific vibration isolation requirements. Many vibration isolators can be modelled as a single-degree-of-freedom mass-spring-damper system. Many researchers have attempted to enhance the vibration isolation performance by designing springs with nonlinear stiffness. Others have focused on different types of damping nonlinearities. The new vibration isolation system proposed in the thesis combines both spring and damping nonlinearities in one system to exploit the advantages of both components while avoiding their undesirable effects. The theoretical properties of this proposed nonlinear vibration isolation system were analysed rigorously using the output frequency response function (OFRF) approach, a novel and unique method recently proposed at Sheeld. The stiffness nonlinearity is already a well researched area and can readily be realised in practice. Therefore, the implementation of the proposed nonlinear vibration suppression system focused on the realisation of the nonlinear damping component using commercially available magneto-rheological (MR) dampers which provide a damping force that is dependent on a control current. With feedback control, the force-velocity relationship of an MR damper can be shaped into a designed function. This implementation has been incorporated first in a vibration isolation system by simulation, then in a physical experimental rig which has a moving mass. The simulation and experimental data not only showed the successful realisation of a damping device with a particular nonlinear damping characteristic, but also confirmed the theoretical findings on the beneficial effects of nonlinear damping on a vibration isolation application. The final part of the thesis is devoted to the practical design of the proposed vibration isolation system. Given specific transmissibility requirements at certain critical frequencies, the values of the linear parameters are first designed, then the OFRF approach is applied to determine the nonlinear parameters. This pragmatic method simplifies the design of a complicated nonlinear system, which was traditionally difficult to work with, into a step-by-step guide and, therefore, has significant potential of industrial applications. The thesis has exploited the special effects of two nonlinear components on the performance of a passive sdof vibration isolation system. With the support of theoretical, simulation and experimental studies, the newly proposed configuration has shown substantial benefits to many vibration isolation problems. The simple yet effective design and implementation has significant implications for a wide range of engineering applications such as car suspension designs and building protection against earthquakes.

Over the past century or so Drosophila has been established as an ideal model organism to study, among other things, neural computation and in particular sensory processing. In this respect there are many features that make Drosophila an ideal model organism, especially the fact that it offers a vast amount of genetic and experimental tools for manipulating and interrogating neural circuits. Whilst comprehensive models of sensory processing in Drosophila are not yet available, considerable progress has been made in recent years in modelling the early stages of sensory processing. When it comes to visual processing, accurate empirical and biophysical models of the R1-R6 photoreceptors were developed and used to characterize nonlinear processing at photoreceptor level and to demonstrate that R1-R6 photoreceptors encode phase congruency. A limitation of the latest photoreceptor models is that these do not account explicitly for the modulation of photoreceptor responses by the network of interneurones hosted in the lamina. As a consequence, these models cannot describe in a unifying way the photoreceptor response in the absence of the feedback from the downstream neurons and thus cannot be used to elucidate the role of interneurones in photoreceptor adaptation. In this thesis, electrophysiological photoreceptor recordings acquired in-vivo from wild-type and histamine defficient mutant fruit flies are used to develop and validate new comprehensive models of R1-R6 photoreceptors, which not only predict the response of these photoreceptors in wild-type and mutant fruit flies, over the entire environmental range of light intensities but also characterize explicitly the contribution of lamina neurons to photoreceptor adaptation. As a consequence, the new models provide suitable building blocks for assembling a complete model of the retina which takes into account the true connectivity between photoreceptors and downstream interneurones. A recent study has demonstrated that R1-R6 photoreceptors employ nonlinear processing to selectively encode and enhance temporal phase congruency. It has been suggested that this processing strategy achieves an optimal trade-off between the two competing goals of minimizing distortion in decoding behaviourally relevant stimuli features and minimizing the information rate, which ultimately enables more efficient downstream processing of spatio-temporal visual stimuli for edge and motion detection. Using rigorous information theoretic tools, this thesis derives and analyzes the rate-distortion characteristics associated with the linear and nonlinear transformations performed by photoreceptors on a stimulus generated by a signal source with a well defined distribution.

The ’rare earth crisis’ that occurred in 2011 ignited research into mitigating and replacing the dependence on these elements in Rare Earth Permanent Magnets (REPM) and their applications. It is the former that this body of work is based upon. A method of increasing the bulk resistivity of a REPM, Sm(Co, Fe, Cu, Zr)8.2 (henceforth referred to as Sm2(Co, Fe, Cu, Zr)17), through the addition of an electrically insulating material, CaF2, to form a rare earth composite permanent magnet is studied. The aim of increasing the bulk resistivity is to minimise the irreversible demagnetisation and performance losses that occur within REPMs due to circulating eddy currents during operation of high frequency applications, such as electric motors and drives. The project utilises an emerging sintering technology platform, Spark Plasma Sintering (SPS), to rapidly consolidate powders. The processing methodology for Sm2(Co, Fe, Cu, Zr)17 powders by the SPS technique has therefore been studied with the optimal processing parameters found for producing full dense specimens for study: 1050°C to 1100°C hold temperature, 100°C= min to 200°C= min heating rate, 51MPa hold pressure and 5 minute hold time. Sm2(Co, Fe, Cu, Zr)17 permanent magnets require a characteristic multi-stage heat treatment to precipitate a cellular nanostructure which enhances the coercivity of the permanent magnets. The optimal heat treatment for the full density SPS processed Sm2(Co, Fe, Cu, Zr)17 magnets has also been studied, with the following achieving the largest coercivities and energy products: 1. Homogenisation - 1170°C for 2 hours in an argon atmosphere. Cooled slowly in air to room temperature. 2. Ageing - 850°C for 8 hours (for largest coercivity) or 16 hours (for largest (BH)max) in argon atmosphere. 3. Slow cool (1°C=min) from 850°C to 400°C in argon atmosphere. Quench in oil to room temperature. This established processing route was then used to prototype isotropic Sm2(Co, Fe, Cu, Zr)17 and CaF2 composite magnets and study the effect of the electrically insulating phase on the microstructure, and material and magnetic properties before and after heat treatment.

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No description available.

This research was primarily focussed on finding novel silver brazing alloys. The first alloy development method used to achieve this goal was a CALPHAD-based approach. This was used to predict melting temperatures and the intermetallic content of alloys. The method was developed and then applied to a commonly used silver brazing alloy. An alternative method was developed based on High Entropy Alloys (HEAs).

The international market for hip and knee joint arthroplasty is expected to continue to grow into the foreseeable future as people live longer and expect a higher quality of life. Therefore, designing and production of a material with higher mechanical reliability is targeted by the international market. Ceramics are the latest materials in this regard, which offer the potential for the lowest wear rate and therefore the longest lifespan. The strength and toughness of ceramic prostheses have been greatly enhanced through the emergence of a zirconia toughened alumina (ZTA) composite. However, there remains a clear drive to reduce fracture rates through yet more reliable mechanical properties. This research focuses on processing of a new ceramic composite with fine, well dispersed and high-density microstructure and also evaluating the effect of grain refinement on wear resistance. Furthermore, it seeks to further investigate the contribution of ternary and quaternary metal oxides, namely SrO and Cr2O3, to wear behaviour of this nano-composite. Therefore, nano-ceramic composites of zirconia toughened alumina (ZTA) and ZTA containing SrO and Cr2O3 additives (ZTA-additives) were formed using Spark Plasma Sintering (SPS) and were optimised, in terms of mechanical properties and grain size. The prepared specimens were almost the same, in terms of grain size, but differed in constituent components. In addition to these specimens, a commercial sample called BIOLOX® delta (with the same composition, but coarser grain size) was also investigated. All materials were subjected to lubricated reciprocating sliding wear testing. To define the operative lubrication regime for each load, the Stribeck curve was plotted for each individual specimen. The results obtained from this curve, along with AFM and SEM images, revealed the operation of full fluid film and mixed lubrication in the ZTA composite and the operation of mixed lubrication in the ZTA-additives and commercial specimens. Upon applying a load of 32 N, the specific wear rate of ZTA presented an abrupt increase of two orders of magnitude, but for the other two specimens, only a small change was observed. This highlighted the effect of the ternary oxide, Cr2O3, on changing the chemistry of the material and therefore the behaviour of the surface. The addition clearly had a beneficial effect on the transition point from mild to severe wear, which was shifted to higher loads. The specific wear rates, SEM and AFM images of the worn surfaces revealed that the commercial specimen was mid transition to severe wear at 32 N load, while the ZTA-additives composite did not show any sign of transition. This was probably due to the smaller grain size of the ZTA-additives, compared to the commercial specimen. The grain size refinement would decrease the thermal mismatch stresses and slip length and therefore dislocation accumulation. In all three materials tested at 8N, TEM results exhibited limited dislocation activity in the surface alumina grains. This was in line with the specific wear rate and the features of the worn surfaces. The polymorphic phase transformation in zirconia grains was observed only in the commercial specimen, which was consistent with the results obtained from Raman spectroscopy. This was probably due to the smaller grain size of zirconia in the ZTA and the ZTA-additives compared to the commercial specimen. The SEM and AFM images presented contradictory results about the effect of platelet grains on the wear resistance of the composite. However, the constructive effect of these grains on the fracture toughness was observed in this study, as it was demonstrated by other researchers. The AFM, SEM and TEM results suggested that the dominant wear mechanism pre-transition was tribochemical wear for all materials. This led to the presence of a thin (nm) tribo-layer which may have affected the coefficient of friction (COF) and the specific wear rate.

PGSm was synthesized, and characterized chemically and mechanically. Porous PGSm was photocured into 3D foams. PGSm was printed via microstereolithography into nerve guidance conduits and tested for use in peripheral nerve repair in vitro, ex vivo and in vivo. Porous PGSm conduits were developed and tested ex vivo, with an intended use for larger gap injuries. Porous PGSm was developed into tunable microparticles and explored for use as a cartilage tissue-engineering scaffold. The polyHIPE was also developed as an in vitro neuronal model and a scaffold for ESCs. Results show the material was developed into a photocurable polymer, capable of being 3D printed into highly accurate NGCs. PGSm conduits performed well overall and regeneration into the distal stump was witnessed in vivo. Techniques were developed to photocure reproducible porous polyHIPE conduits, with promising initial in vitro/ex vivo results. Porous microparticles were seen to allow for the development of cartilage like tissue in vitro. Porous PGSm was used for neuronal models and stem cell scaffolds. In summary the developed PGSm is useful for simple and complex scaffolds for soft tissue engineering.