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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Mono- and diamine directed synthesis of microporous materials and their characterisation by diffraction techniques

Stephenson, Richard Alexander January 2003 (has links)
No description available.

High field dielectric properties of hard PZT ceramics

Stevenson, Paul John January 1998 (has links)
At the present time there is an increasing demand for higher acoustic and electric power densities in piezoelectric transducers. The preferred material for such acoustic sources is hard (acceptor doped) lead zirconate titanate (PZT) ceramic due to a combination of desirable properties including high piezoelectric coupling factors, high ~, low electrical losses and good temperature stability. Four commercially available materials from two independent suppliers have been characterised; PZ26 and PZ28, obtained from Ferroperm Ltd, and PC4 and PC8 obtained from Morgan Matroc. The use of Mn as the acceptor dopant was found for the F erroperm materials, and Fe for the Morgan Matroc materials. A finer grain size was also observed in the Morgan Matroc materials by comparison with those from Ferroperm. Various dielectric properties of each material were investigated as a function of field amplitude, ageing time after poling and with regard to the effect of grain size and temperature. A 'threshold field' type of behaviour was observed for all materials followed by a linear increase in permittivity with field level. Such behaviour is related to the Rayleigh law, first introduced to model the behaviour of ferromagnetic materials. The most encouraging results were observed for the Morgan Matroc material with the PC8 exhibiting the most stable performance under high drive conditions. Linear plots of the real versus imaginary parts of the dielectric permittivity were observed for all materials as a function of field amplitude or ageing time an related to a ferroelectric domain wall translation mechanism. The corresponding values of tan Odw, the loss associated with domain wall translation were compared. Evidence was also found for intrinsic and reversible domain wall vibration contributions to the permittivity in the 'low field' region, while partial domain switching was observed on approaching the coercive field Ee. Field-driven de-ageing studies of poled and unpoled PZ26 ceramICS revealed a gradual reduction in the effective internal bias field Ei with time. For poled materials the reduction followed a logarithmic time law, whereas for unpoled materials an exponential decrease was observed.

Combined stability of conical shells

Ifayefunmi, Olawale Friday January 2011 (has links)
This thesis is concerned with the study of the elastic-plastic buckling of short and relatively thick conical shells subjected to combined loading, i.e., axial compression and external pressure acting simultaneously. This is both numerical and experimental study. Within the context of numerical study, a nonlinear finite element calculations were carried out in order to obtain: (i) the failure loads of cones under axial compression only, external pressure only and under combined loading, (ii) the spread of plastic strain and the effect of strain hardening of the material on failure loads, and (iii) the sensitivity of buckling loads to initial geometric imperfections or to structural defects under various loading conditions. The thesis provides results of extensive FE calculations. An experimental programme involved tests on thirteen conical specimens CNC machined with integral top and bottom flanges from 252mm diameter steel billet. The specimens were made from mild steel material with average yield stress of 230.6 MPa, Young's modulus of 21 0490 MPa and Poisson' s ratio of 0.28l. Prior to tests, the existing test rig had to be significantly modified and instrumented in order to accommodate independent/combined loadings. The test procedure has been developed and successfully implemented. Two models were subjected to axial compression, with further two subjected to external pressure. The remaining nine cones were subjected to combined action of axial compression and external pressure. Experimental results were compared with predictions of failure loads obtained from the existing design codes. For the case of axial compression an extension of the design rules is outlined in order to widen the range of applicability. For the case of external pressure, the test data compared well with the theoretical work by Esslinger and Van Impe, [40]. At the same time the test data highlighted how inadequate estimates of the load carrying capacity are given by the design codes. The case of combined loading, i.e., axial compression and external pressure is only covered by ASME code case 2286-2, [157], and experimental data does not exist. The current study provides the first and much needed test data. The thesis also looks into the concept of equivalent cylinder. Numerical results point out to the fact that this approach is unsuitable for combined stability scenario (axial compression and external pressure). Experimental data is also compared with predictions given by the Finite Element calculations. Details about various approaches to modeling material properties, shape, wall thickness distribution, and boundary conditions are discussed. The quality of FE models is assessed by comparing the FE predictions of the load carrying capacity with the test data.

Innovation of a piezoelectric touch trigger probe

Lewis, Rhodri Wyn Charles January 2012 (has links)
No description available.

Disordered microstructures and anomalous mechanical properties

Horrigan, Emma January 2009 (has links)
This study concerns the effects of structural heterogeneity upon mechanical properties, particularly on negative Poisson's ratio. Two separate methods were used to generate two-dimensional honeycombs optimised for a series of material properties, in particular large negative Poisson's ratio and high stiffness. The properties and heterogeneity of crumpled and recycled materials were compared.

Design-construction incorporating sustainable materials from environmentally valuable ecosystems : a model for practice developed through site-specific case studies

Willey, Tim Vincent January 2004 (has links)
No description available.

Synthesis, characterization and applications of novel nanomaterials

Jin, Yizheng January 2005 (has links)
No description available.

A surface science and thin film approach to the development of functional materials

Naughton, James January 2011 (has links)
Nanoscale properties are becoming increasingly important for the successful utilisation of modern materials and devices, where surfaces or interfaces often largely define functionality. It is critical to understand surface behaviour and its impact on properties for the development of new and existing materials. Current surface science and thin-film techniques have been utilised and developed to investigate promising functional materials. The surface chemistry of Au/Pd surface alloys was examined for the selective oxidation (selox) of crotyl alcohol to crotonaldehyde. X-ray Photoelectron Spectroscopy (XPS) and Thermal Desorption Spectroscopy (TDS) have been used to elucidate the reaction pathway for the alcohol and the main products. Au moderates the surface reaction by ‘turning off' the main decomposition pathway of the aldehyde. The amount of surface Au has a critical role in determining the selectivity. To mimic 'true' selox, the influence of co-adsorbed O over the Au/Pd alloys was also investigated. O plays an important role in aiding desorption of the aldehyde. The exact nature and role of Au and O in moderating the reaction over Pd(111) requires further investigation. A key area of technological interest is field of spintronics, whereby the spin and charge of electrons are exploited for electronics. Thin-films of Fe3O4, a promising spin injector, were grown on a variety of substrates to characterise the magnetic/magnetoresistive (MR) properties. The Magnetorefractive Effect (MRE), an optical technique, was used for non-contact measurements of the MR. Despite no difference in the MR, the MRE shows a thickness dependence, which is partly due to the large skin-depth associated with Fe3O4. Modelling the complex dielectric function for Fe3O4/MgO allowed simulations to be carried out of the IR and MRE spectra. With further work, the model may be used to determine film thickness, examine the conductivity mechanisms and their influence on the MR/MRE.

A virtual grain structure representation system for micromechanics simulations

Zhang, Pan January 2012 (has links)
Representing a grain structure within a combined finite element computer aided engineering environment is essential for micromechanics simulations. Methods are required to effectively generate high-fidelity virtual grain structures for accurate studies. A high-fidelity virtual grain structure means a statistically equivalent structure in conjunction with desired grain size distribution features, and must be represented with realistic grain morphology. A family of controlled Poisson Voronoi tessellation (CPVT) models have been developed in this work for systematically generating virtual grain structures with the aforementioned properties. Three tasks have been accomplished in the development of the CPVT models: (i) defining the grain structure’s regularity that specifies the uniformity of a tessellation as well as deriving a control parameter based on the regularity; (ii) modelling the mapping from a grain structure’s regularity to its grain size distribution; and (iii) establishing the relation between a set of physical parameters and a distribution function. A one-gamma distribution function is used to describe a grain size distribution characteristic and a group of four physical parameters are employed to represent the metallographic measurements of a grain size distribution property. Mathematical proofs of the uniqueness of the determination of the distribution parameter from the proposed set of physical parameters have been studied, and an efficient numerical procedure is provided for computing the distribution parameter. Based on the general scheme, two- and three-dimensional CPVT models have been formulated, which respectively define the quantities of regularity and control parameters, and model the mapping between regularity and grain size distribution. For the 2D-CPVT model, statistical tests have been carried out to validate the accuracy and robustness of regularity and grain size distribution control. In addition, micrographs with different grain size distribution features are employed to examine the capability of the 2D-CPVT model to generate virtual grain structures that meet physical measurements. A crystal plasticity finite element (CPFE) simulation of plane strain uniaxial tension has been performed to show the effect of grain size distribution on local strain distribution. For the 3D-CPVT model, a set of CPFE analyses of micro-pillar compression have been run and the effects of both regularity and grain size on deformation responses investigated. Further to this, a multi-zone scheme is proposed for the CPVT models to generate virtual gradient grain structures. In conjunction with the CPVT model that controls the seed generating process within individual zones, the multi-zone CPVT model has been developed by incorporating a novel mechanism of controlling the seed generation for grains spanning different zones. This model has the flexibility of generating various gradient grain structures and the natural morphology for interfacial grains between adjacent zones. Both of the 2D- and 3D-CPVT models are capable of generating a virtual grain structure with a mean grain size gradient for the grain structure domain and grain size distribution control for individual zones. A true gradient grain structure, two simulated gradient grain structure, and a true gradient grain structure with an elongated zone have been used to examine the capability of the multi-zone CPVT model. To facilitate the CPFE analyses of inter-granular crack initiation and evolution using the cohesive zone models, a Voronoi tessellation model with non-zero thickness cohesive zone representation was developed. A grain boundary offsetting algorithm is proposed to efficiently produce the cohesive boundaries for a Voronoi tessellation. The most challenging issue of automatically meshing multiple junctions with quadrilateral elements has been resolved and a rule-based method is presented to perform the automatically partitioning of cohesive zone junctions, including data representation, edge event processing and cut-trim operations. In order to demonstrate the novelty of the proposed cohesive zone modelling and junction partitioning schemes, the CPFE simulations of plane strain uniaxial tension and three point bending have been studied. A software system, VGRAIN, was developed to implement the proposed virtual grain structure modelling methods. Via user-friendly interfaces and the well-organised functional modules a virtual grain structure can be automatically generated to a very large-scale with the desired grain morphology and grain size properties. As a pre-processing grain structure representation system, VGRAIN is also capable of defining crystallographic orientations and mechanical constants for a generated grain structure. A set of additional functions has also been developed for users to study a generated grain structure and verify the feasibility of the generated case for their simulation requirements. A well-built grain structure model in VGRAIN can be easily exported into the commercial FE/CAE platform, e.g. ABAQUS and DEFORM, via script input, whereby the VGRAIN system is seamlessly integrated into CPFE modelling and simulation processing.

A study of the containment of high speed projectiles

Zaid, A. I. O. January 1972 (has links)
No description available.

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