Thesis (Master)--İzmir Institute Of Technology, İzmir, 2005. / Keywords: superconductivity, magnesium diboride, composites, superconductor. Includes bibliographical references (leaves. 56-61).

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It is well established that the extended range V(I) transition behavior in filamentary superconducting composites depends strongly on the processing conditions of the composite. Based on previous work, a model proposed by Warnes et al predicts that the second derivative of the V(I) transition is related to a more fundamental property the critical current distribution in the composite. The present work consolidates the said model by showing that in spite of the V(I) transition being dependent on the matrix resistance, the second derivative is relatively independent. Also, the second derivative V(I) curve has been correlated to the critical current distributions and area distributions in monofilament and multifilamentary superconductors, using image analysis techniques. The work provides a better understanding of the critical current distributions in composite superconductors and can be an effective tool for measuring conductor quality more precisely. In addition to this, image analysis techniques have been used to study the effect of processing on the structure of wires - and have been related to the predictions of a model of inplane stresses in composites. / Graduation date: 1996

Measurement of the copper to superconductor ratio (Cu/SC) in composite superconducting wires is necessary for process control, quality assurance, and characterization of the final wire properties. A comparison has been made of three techniques for measuring the Cu/SC ratio on a set of commercial wires produced for the Superconducting SuperCollider (SSC) Laboratory. The simplest and most straightforward technique, chemical etching, was found to display the best reproducibility, while the electrical resistivity technique shows the most variation and sensitivity to measurement errors, as well as being the most difficult to perform. The image analysis technique is fast and fairly reproducible, and is capable of providing much more information on the wire parameters than either of the other techniques. / Graduation date: 1993

Thesis (Ph.D.)--University of the Witwatersrand, Science Faculty (Physics), 2000. / Percolation phenomena are studied in a series of composites, each with a cellular structure (small conductor particles embedded on the surfaces of large insulator particles). The DC and AC conductivities, l/f noise and magnetic properties (in some series) are measured in the systems consisting of Graphite, Graphite-Boron Nitride, Carbon Black, Niobium Carbide, Nickel and Magnetite (Fe304) as the conducting components with Talc-wax (Talc powder coated with 4% wax by volume) being the common insulating component. Compressed discs of 26mm diameter and about 3mm thickness (with various conductor volume fractions covering both the insulating and conducting region) were made from the respective powders at a pressure of 380MPa and all measurements were taken in the axial (pressure) direction. The conductivity (σm) and dielectric constant (εm) of percolation systems obey the equations: σm = σc( ɸ - ɸc)t for ɸ >ɸc; σm = σi( ɸc - ɸ-s and εm = εi( ɸc - ɸ-s' for ɸ < ɸc; outside of the crossover region given by ɸc± (δdc ~=(σi/σc)1/(t+s). Here ɸc is the critical volume fraction of the conductor (with conductivity σ = σc) and cri is the conductivity of the insulator, t and s are the conductivity exponents in the conducting and insulating regions respectively and S’ is the dielectric exponent. The values of s and t are obtained by fitting the DC conductivity results to the combined Percolation or the two exponent phenomenological equations. Both universal and non-universal values of the sand t exponents were obtained. The dielectric exponent S’, obtained from the low frequency AC measurements, is found to be frequency-dependent. The real part of the dielectric constant of the systems, has been studied as a function of the volume fraction (ɸ) of the conducting component. In systems where it is measurable beyond the DC percolation threshold, the dielectric constant has a peak at ɸ > ɸ, which differs from key predictions of the original Percolation Theory. This behaviour of the dielectric constant can be qualitatively modeled by the phenomenological two exponent equation given in Chapter two of this thesis. Even better fits to the data are obtained when the same equation is used in conjunction with ideas from Balberg's extensions to the Random Void model (Balberg 1998a and 1998b). At high frequency and closer to the percolation threshold, the AC conductivity and dielectric constant follow the power laws: σm( ɸ,שּׂ) ~ שּׂX and εm( ɸ,שּׂ) ~ שּׂ-Y respectively. In some of the systems studied, the x and y exponents do not sum up to unity as expected from the relation x + y = 1. Furthermore, the exponent q obtained from שּׂ x σm( ɸ,O)q in all but the Graphite-containing systems is greater than 1, which agrees with the inter-cluster model prediction (q = (s + t)/t). The Niobium Carbide system is the first to give an experimental q exponent greater than the value calculated from the measured DC s and t exponents. l/f or flicker noise (Sv) on the conducting side (ɸ > ɸc) of some of the systems has been measured, which gives the exponents k and w from the well-established relationships Sv/V2 = D(ɸ - ɸc)-k and Sv/V2 = KRw. V is the DC voltage across the sample with resistance R while D and K are constants. A change in the value of the exponent k and w has been observed with k taking the values kl ~ 0.92 - 5.30 close to ɸc and k2 ~ 2.55 - 3.65 further into the conducting region. Values of WI range from 0.36 -1.1 and W2 ~ 1.2 - 1.4. These values of ware generally well within the limits of the noise exponents proposed by Balberg (1998a and 1998b) for the Random Void model. The t exponents calculated from k2 and W2 (using t = k/w) are self-consistent with the t values from DC conductivity measurements. Magnetic measurements in two of the systems (Fe304 and Nickel) show unexpected behaviour of the coercive field and remnant magnetisation plotted as a function of magnetic volume fraction. Fitting the permeability results to the two exponent phenomenological equation gives t values much smaller than the corresponding DC conductivity exponents. A substantial amount of data was obtained and analysed as part of this thesis. Experimental results, mostly in the form of exponents obtained from the various scaling laws of Percolation Theory, are presented in tabular form throughout the relevant chapters. The results have been tested against various models and compare with previous studies. While there is some agreement with previous work, there are some serious discrepancies between the present work and some aspects of the standard or original Percolation Theory, for example the dielectric constant behaviour with conductor volume fraction close to but above ɸc. New results have also emerged from the present work. This includes the change in the noise exponent k with (ɸ - ɸc), the variation of the dielectric exponent s' with frequency and some DC scaling results from the Fe304 system. The present work has dealt with some intriguing aspects of Percolation Theory in real continuum composites and hopefully opened avenues for further theoretical and experimental research. / AC 2016

Mechanical behavior of Nb-Ti superconducting composites with copper matrices have been studied experimentally and theoretically. Experimental work includes extensive measurements of Cu/Nb-Ti composite system. Techniques for fine fiber testing and composite wire measurement have been developed. Experimental parameters examined in this research include geometry, hardness, Young's modulus, Poisson's ratio, yield strength and ultimate strength. Three theoretical models have been developed to study the mechanics of the Cu/Nb-Ti composite system. The influence of several design parameters on the mechanics of the Nb-Ti composites was studied and provides some insight on superconducting composite design for improvements in processing and performance. The mechanical behavior of the Cu/Nb-Ti composite system are found to be functions of geometry, composition and processing. Geometry of Nb-Ti superconducting composites is different from most engineering composites and there are two factors affecting sample geometry: the variability of fiber geometry and the placement of fibers within the composite. The strength distribution of Nb-Ti fibers is closely related to the distribution of fiber geometry and the composite strength increases as the scatter of fiber strength decreases. Heat treatment reduces the hardness of the bulk copper dramatically. The first heat treatment increases the strength and hardness of the Nb-Ti fibers, further heat treatments reduce the strength and hardness while increasing Young's modulus of the fibers. As the extent of cold work increases, the strength of Nb-Ti fibers and that of the composite wires increases. Cold work effects on the Young's modulus of the composites and the Nb-Ti fibers are not significant. For a constant global Cu/SC ratio, the lower the local Cu/SC ratio, the lower the micro-in-plane stresses. From this point of view, the fibers should be packed as close as possible to one another. For a constant local Cu/SC ratio, when the inner radius of the Nb-Ti assembly increases (the fibers are packed further from center), the macro-in-plane stresses increase. From this point of view, the fibers should be packed as close to the center of the wire as possible. For a constant geometry, the higher the difference between E[subscript f] and E[subscript m], the lower the in-plane stresses, and the higher the difference of the Poisson's ratio between the components, the higher the in-plane stresses. / Graduation date: 1995

by Hon Wan Man = 鎂和硼化鎂複合材料的研究 / 韓韻文. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2006. / Includes bibliographical references. / Text in English; abstracts in English and Chinese. / by Hon Wan Man = Mei he peng hua mei fu he cai liao de yan jiu / Han Yunwen. / Abstract --- p.i / 摘要 --- p.iii / Acknowledgements --- p.v / Table of contents --- p.vi / List of tables --- p.x / List of figures --- p.xi / Chapter Chapter 1 --- Introduction --- p.1-1 / Chapter 1.1. --- Background --- p.1-1 / Chapter 1.1.1. --- Conventional and unconventional superconducting materials --- p.1-1 / Chapter 1.1.2. --- Type I and type II superconductors --- p.1-2 / Chapter 1.1.3. --- Critical Temperature and Magnetic Properties (M-H Loops) --- p.1-4 / Chapter 1.2. --- Magnesium Diboride MgB2 --- p.1-6 / Chapter 1.2.1. --- Introduction --- p.1-6 / Chapter 1.2.2. --- Potential application and recent work of MgB2 --- p.1-6 / Chapter 1.2.2.1. --- Thin films --- p.1-7 / Chapter 1.2.2.2. --- Wires and tapes --- p.1-8 / Chapter 1.2.2.3. --- Powders and single crystal --- p.1-8 / Chapter 1.2.3. --- Factors affecting critical temperature in MgB2 --- p.1-9 / Chapter 1.2.3.1. --- Critical temperature versus lattice constants --- p.1-9 / Chapter 1.2.3.2. --- Critical temperature versus pressure --- p.1-9 / Chapter 1.3. --- Mg-based metal matrix composites (Mg-MMCs) --- p.1-10 / Chapter 1.4. --- Objectives and approaches --- p.1-11 / Chapter 1.5. --- Thesis layout --- p.1-12 / Chapter 1.6. --- References --- p.1-13 / Figures --- p.1-16 / Tables --- p.1-20 / Chapter Chapter 2 --- Methodology and instrumentation --- p.2-1 / Chapter 2.1. --- Introduction --- p.2-1 / Chapter 2.2. --- Experimental procedures --- p.2-1 / Chapter 2.3. --- Samples fabrication --- p.2-2 / Chapter 2.3.1. --- Powder metallurgy method (P/M) --- p.2-2 / Chapter 2.3.2. --- Argon atmosphere tube furnace heat treatment --- p.2-3 / Chapter 2.4. --- Characterization --- p.2-3 / Chapter 2.4.1. --- Differential thermal analyzer (DTA) --- p.2-3 / Chapter 2.4.2. --- X-ray powder diffractometry (XRD) --- p.2-4 / Chapter 2.4.3. --- Hot mounting and polishing --- p.2-4 / Chapter 2.4.4. --- Scanning electron microscopy (SEM) --- p.2-5 / Chapter 2.4.5. --- Transmission electron microscopy (TEM) --- p.2-5 / Chapter 2.4.6. --- Vibrating sample magnetometer (VSM) --- p.2-6 / Chapter 2.5. --- References --- p.2-8 / Figures --- p.2-9 / Chapter Chapter 3 --- Effects of sintering temperature on Mg-MgB2 composites --- p.3-1 / Chapter 3.1. --- Introduction --- p.3-1 / Chapter 3.2. --- Experimental results --- p.3-1 / Chapter 3.2.1. --- DTA and XRD analyses --- p.3-1 / Chapter 3.2.2. --- Microstructures --- p.3-2 / Chapter 3.2.2.1. --- Green sample --- p.3-2 / Chapter 3.2.2.2. --- Sample sintered at 550°C --- p.3-3 / Chapter 3.2.2.3. --- Sample sintered at 600°C --- p.3-4 / Chapter 3.2.2.4. --- Sample sintered at 700°C --- p.3-4 / Chapter 3.2.2.5. --- Hexagonal platelets --- p.3-5 / Chapter 3.2.3. --- Superconducting behaviors --- p.3-5 / Chapter 3.2.3.1. --- Critical temperature (Tc) comparison --- p.3-5 / Chapter 3.2.3.2. --- Magnetization loops (M-H loops) --- p.3-6 / Chapter 3.3. --- Discussions --- p.3-7 / Chapter 3.4. --- Conclusions --- p.3-9 / Chapter 3.5. --- References --- p.3-10 / Figures --- p.3-11 / Tables --- p.3-20 / Chapter Chapter 4 --- Effects of composition on Mg-MgB2 composites --- p.4-1 / Chapter 4.1. --- Introduction --- p.4-1 / Chapter 4.2. --- Experimental results --- p.4-1 / Chapter 4.2.1. --- XRD results --- p.4-1 / Chapter 4.2.2. --- Microstructures --- p.4-2 / Chapter 4.2.2.1 --- Mg-0 wt % B (Pure Mg) sintered at 650。C --- p.4-2 / Chapter 4.2.2.2 --- Mg-47 wt % B sintered at 650°C --- p.4-2 / Chapter 4.2.2.3 --- Amount of B in Mg sample --- p.4-3 / Chapter 4.2.2.3.1. --- Overview of Mg-5 to 40 wt % B --- p.4-3 / Chapter 4.2.2.3.2. --- MgB2 phase in different composition (Mg-5 to 47 wt %B) --- p.4-4 / Chapter 4.2.2.3.3. --- MgO phase in different composition (Mg-0 to 30 wt % B) --- p.4-4 / Chapter 4.2.3. --- VSM results (Critical temperature Tc comparison) --- p.4-5 / Chapter 4.3. --- Discussions --- p.4-6 / Chapter 4.4. --- Conclusions --- p.4-8 / Chapter 4.5. --- References --- p.4-10 / Figures --- p.4-11 / Table --- p.4-18 / Chapter Chapter 5 --- Growth Mechanisms --- p.5-1 / Chapter 5.1. --- Introduction --- p.5-1 / Chapter 5.2. --- Brief summary of SEM result --- p.5-1 / Chapter 5.3. --- Growth Mechanism of MgB2 --- p.5-2 / Chapter 5.4. --- Comparison of MgB2 grain size by XRD result --- p.5-7 / Chapter 5.5. --- Stoichiometric Ratio of MgB2 in different temperature --- p.5-7 / Chapter 5.6. --- Growth of the MgB2 platelets --- p.5-8 / Chapter 5.7. --- Conclusions --- p.5-10 / Chapter 5.8. --- References --- p.5-11 / Figures --- p.5-12 / Table --- p.5-15 / Chapter Chapter 6 --- Conclusions and Future Works --- p.6-1 / Chapter 6.1. --- Conclusions --- p.6-1 / Chapter 6.2. --- Future works --- p.6-2 / Chapter 6.3. --- References --- p.6-4

Graduation date: 1995

Thesis (M.Eng.). / Written for the Dept. of Mining and Materials Engineering. Title from title page of PDF (viewed 2009/06/17). Includes bibliographical references.