11 research outputs found

    Time-resolved magnetic flux and AC-current distributions in superconducting yttrium barium copper oxide thin films and multifilaments

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    Time-resolved magneto-optical imaging (TRMOI) technique allows dynamic ac transport measurements on superconductors. The high time and spatial resolutions of the measurements also offer good quantitative data analysis of the MO images. YBa2Cu 3O7-delta (YBCO) was discovered as a high-temperature superconductor (HTSC) which has wide applications due to its high critical temperature of Tc = 91 K, and high critical current density Jc in the order of 106-7 Acm-2. Many of the applications require high ac current load and a high magnetic field. We study the interaction behavior of YBCO thin films in an ac transport current and a dc magnetic field by the TRMOI technique.;In this dissertation, I first introduce the applications of high-temperature superconductors with focus on YBCO and describe the advantages of the TRMOI technique we developed over other methods to map the magnetic flux distribution of superconductors. The theories to understand the magnetic properties of HTSC are presented, followed by theoretical models. I also introduce a newly developed finite elemental method (FEM) simulation which is proved to be a better theoretical guideline to our data analysis. The TRMOI experimental setup and the procedures are discussed in detail. I show step-by-step the calibration of light intensity profiles averaged from MO images to determine magnetic field distribution, and a numerical inversion of the Biot-Savart law to calculate the current density distributions.;The current density evolution in YBCO thin films is studied by TRMOI as a function of the phase of an ac current applied simultaneously with a perpendicular dc magnetic field. The measurements show that an ac current enables the vortex matter in YBCO thin films to reorganize into two coexisting steady states of driven vortex motion with different characteristics. to study the transport current effects in YBCO thin films, we present a new empirical method to separate the total current distribution into a circulating shielding current and a transport current.;Furthermore, we performed TRMOI measurements on multifilamentary YBCO thin films with six superconducting filaments. Several sets of measurements with different experimental parameters are compared to find optimized measurements especially fitting the TRMOI technique for best quantitative results. The integrated transport current in the optimized measurements agrees fairly well with the current we applied. Nearly half of the transport current flows in the most outer two filaments while the rest of the current flows roughly evenly in the inner four filaments. Comparing with the FEM simulation results, the multifilamentary film shows higher critical current than the single bridged TBCO thin film Finger-like inhomogeneous flux penetration patterns are observed in the TRMOI study of YBCO coated conductors in ac current regime. A quantitative analysis of the images show how the grain boundary network affects the overall behavior of the flux and current density evolution

    Analysis of loss mechanisms in superconducting windings for rotating electric generators

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    Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1981.MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING.Vita.Includes bibliographical references.by Joseph Vito Minervini.Ph.D

    AMSAHTS 1990: Advances in Materials Science and Applications of High Temperature Superconductors

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    This publication is comprised of abstracts for oral and poster presentations scheduled for AMSAHTS '90. The conference focused on understanding high temperature superconductivity with special emphasis on materials issues and applications. AMSAHTS 90, highlighted the state of the art in fundamental understanding of the nature of high-Tc superconductivity (HTSC) as well as the chemistry, structure, properties, processing and stability of HTSC oxides. As a special feature of the conference, space applications of HTSC were discussed by NASA and Navy specialists

    Contactless excitation for electric machines: high temperature superconducting flux pumps

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    With the intensification of global warming and climate change, the pace of transformation to a neutral-emission society is accelerating. In various sectors, electrification has become the absolute tendency to promote such a movement, where electric machines play an important role in the current power generation system. It is widely convinced that electric machines with very high power density are essential for future applications, which, however, can be hardly achieved by conventional technologies. Owing to the maturation of the second generation (2G) high temperature superconducting (HTS) technologies, it has been recognized that superconducting machine could be a competitive candidate to realize the vision. One significant obstacle that hinders the implementation of superconducting machines is how to provide the required magnetic fields, or in other words, how to energise them appropriately. Conventional direct injection is not suitable for HTS machines, because the current leads would bridge ambident temperature to the cryogenic environment, which can impose considerable heat load on the system and increase the operational cost. Thus, an efficient energisation method is demanded by HTS machines. As an emerging technology that can accumulate substantial flux in a closed loop without any physical contact, HTS flux pumps have been proposed as a promising solution. Among the existing developed HTS flux pumps, rotary HTS flux pumps, or so-called HTS dynamo, can output non-zero time-averaged DC voltage and charge the rest of the circuit if a closed loop has been formed. This type of flux pump is often employed together with HTS coils, where the HTS coils can potentially work in the persistent current mode, and act like electromagnets with a considerable magnetic field, having a wide range of applications in industry. The output characteristics of rotary HTS flux pumps have been extensively explored through experiments and finite element method (FEM) simulations, yet the work on constructing statistical models as an alternative approach to capture key characteristics has not been studied. In this thesis, a 2D FEM program has been developed to model the operation of rotary HTS flux pumps and evaluate the effects of different factors on the output voltage through parameter sweeping and analysis of variance. Typical design considerations, including the operating frequency, air gap, HTS tape width, and remanent flux density have been investigated, in particular, the bilateral effect of HTS tape width has been discovered and explained by looking at the averaged integration of the electric field over the HTS tape. Based on the data obtained from various simulations, regression analysis has been conducted through a collection of machine learning methods. It has been demonstrated that the output voltage of a rotary HTS flux pump can be obtained promptly with satisfactory accuracy via Gaussian process regression, aiming to provide a novel approach for future research and a powerful design tool for industrial applications using rotary HTS flux pumps. To enhance the applicability of the proposed statistical models, an updated FEM program has been built to take more parameters into account. The newly added parameters, namely the rotor radius and the width of permanent magnet, together with formerly included ones, should have covered all the key design parameters for a rotary HTS flux pump. Based on data collected from the FEM model, a well-trained semi-deep neural network (DNN) model with a back-propagation algorithm has been put forward and validated. The proposed DNN model is capable of quantifying the output voltage of a rotary HTS flux pump instantly with an overall accuracy of 98% with respect to the simulated values with all design parameters explicitly specified. The model possesses a powerful ability to characterize the output behaviour of rotary HTS flux pumps by integrating all design parameters, and the output characteristics of rotary HTS flux pumps have been successfully demonstrated and visualized using this model. Compared to conventional time-consuming FEM-based numerical models, the proposed DNN model has the advantages of fast learning, accurate computation, as well as strong programmability. Therefore, the DNN model can greatly facilitate the design and optimization process for rotary HTS flux pumps. An executable application has been developed accordingly based on the DNN model, which is believed to provide a useful tool for learners and designers of rotary HTS flux pumps. A new variant inspired by the working principles of rotary HTS flux pumps has been proposed and termed as stationary wave HTS flux pumps. The superiority of this type is that it has a simple structure without any moving components, and it utilises a controllable current-driven electromagnet to provide the required magnetic field. It has been demonstrated that the origin of the output voltage is determined by the asymmetric distribution of the dynamic resistance in the HTS tape, for which the electromagnet must be placed at such a position that its central line is not aligned with that of the HTS tape. A numerical model has been built to simulate the operation of a stationary wave HTS flux pump, based on which the output characteristics and dynamic resistance against various parameters have been investigated. Besides, accurate and reliable statistical models have been proposed to predict the open circuit voltage and effective dynamic resistance by adapting the previously developed machine learning techniques. The work presented in this PhD thesis can bring more insight into HTS flux pumps as an emerging promising contactless energisation technology, and the proposed statistical models can be particularly useful for the design and optimization of such devices

    Second International Symposium on Magnetic Suspension Technology, part 2

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    In order to examine the state of technology of all areas of magnetic suspension and to review related recent developments in sensors and controls approaches, superconducting magnet technology, and design/implementation practices, the 2nd International Symposium on Magnetic Suspension Technology was held at the Westin Hotel in Seattle, WA, on 11-13 Aug. 1993. The symposium included 18 technical sessions in which 44 papers were presented. The technical sessions covered the areas of bearings, bearing modelling, controls, vibration isolation, micromachines, superconductivity, wind tunnel magnetic suspension systems, magnetically levitated trains (MAGLEV), rotating machinery and energy storage, and applications. A list of attendees appears at the end of the document

    Pertanika Journal of Science & Technology

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    Batch-fabrication of novel nanoprobes for SPM

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    A micromachining method has been developed for fabricating 20µm tall silicon atomic force tips with flat tops less than 2µm wide suitable for defining nanosensors upon, and with low aspect ratio sides suitable for defining electrical connections to the sensor. Methods have been developed to allow flat substrate processing techniques to be applied to such non-planar micromachined substrates. This has necessitated the development of a novel resist-coating technique and the use of defocused electron-beam lithography. Methods for through-wafer alignment by electron-beam lithography and accurate alignment to the tips using micromachined alignment markers have also had to be developed. The fabrication process has been designed to enable a wide variety of sub-micron sensors to be defined on the atomic force probes, with little additional development beyond that of : sensors themselves. This flexibility has enabled very different sensors meant for very different scanning probe microscopy techniques to be designed without significant redevelopment of the underlying fabrication process. The main restrictions on the type of sensor that can be used are the physical dimensions of the sensor, the number of alignment levels necessary, the degree of alignment accuracy required and the choice of sensor materials. However, within these constraints it has been found that probes optimised for scanning near-field optical microscopy (SNOM), scanning thermal microscopy, modulation differential scanning calorimetry (MDSC) and scanning Hall-probe microscopy can be fabricated. For the SNOM probes three methods for fabricating sub-l00nm diameter apertures have been developed, analysed and compared with each other to evaluate both the process latitude. and, the size and reproducibility of apertures that can be fabricated, as a function of electron beam dose, pattern shape and size, and metallisation material and thickness. Two methods, both utilising multilayer 'resist' schemes have been found suitable for this purpose, one based on conventional electron-beam lithography with PMMA and a new dry etching process for titanium, and the other based on a novel electron-beam lithography technique utilising cross-linked PMMA for lifting off nichrome. A simple analytical model has also been developed for these probes allowing the effects of changes in the sensor design parameters on the light throughput to be compared qualitatively, if not quantitatively. For the scanning thermal probes a method for lifting-off sub-l00nm, thin-film thermocouple sensors on silicon tips without the loss of electrical continuity has been developed. For the MDSC probes, a similar method has been developed for defining thermal resistors. A method has also been presented for fabricating sensors for scanning Hall-probe microscopy based on an evaporated germanium sensing layer. This has been found to require annealing and optimisation of sensor design and geometry to reduce sensor resistance to acceptable levels

    Fully Discrete Scheme for Bean's Critical-state Model with Temperature Effects in Superconductivity

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