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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.
31

Localised variation of magnetic properties of grain-oriented electrical steels

Xu, Xintong January 2015 (has links)
Localised magnetic flux density, magnetising field and power loss are believed to distribute non-uniformly in grain oriented electrical steel. Understanding of the causes of their variation can help reduce the overall power loss of the material. In this investigation, magnetic domain observation was often used in the study of domain configuration and crystal orientation of the test specimens. Methods of domain observation have been studied and compared in order to select the appropriate method for different parts of the investigation and to improve the understanding of the image observed. A less destructive local loss measurement sensor has been built for the measurement of localised flux density, magnetising field and power loss. The sensor was tested and evaluated specifically for the measurement of localised magnetic power loss of the high permeability grain oriented electrical steel. The results obtained from local loss scanning measurements indicated that localised flux density and magnetising field can vary substantially in grain oriented electrical steel under AC magnetisation of 50 Hz. The variation of localised flux density has been found mainly resulted by grain misorientation and local grain arrangement. The transverse component of flux density was detected and has been found increases with increasing grain misorientation. The variation of localised magnetising field has been found mainly influenced by the localised demagnetising field due to formation of free magnetic poles at grain boundaries. It has been proved that both flux density and magnetising field have strong influence on the distribution of localised power loss. The study of the effect of domain refinement on distribution of localised flux density showed that domain refinement by means of ball scribing on one surface of grain oriented electrical steel can improve the uniformity of distribution of flux density. However, results also inferred that excessive scribing in a confined area can cause obvious uneven distribution of flux density in the direction of the specimen’s thickness.
32

Magnetoresistance and doping effects in conjugated polymer-based organic light emitting diodes

Gu, Hang January 2015 (has links)
Magnetoresistance (MR) and doping effects have been investigated in a poly(3-hexylthiophene-2,5-diyl) (P3HT) based organic light emitting diodes. In single device of fixed composition (Au/P3HT/Al as spun and processed in air), the measured MR strongly depends on the drive conditions. The magnetoconductance (MC) varies from negative to positive (-0.4% ≤ MC ≤ 0.4%) with increasing current density, depending on which microscopic mechanism dominates. The negative MC is due to bipolaron based interactions and the positive MC to triplet-polaron based interactions (as confirmed by light emission). Oxygen doping is prevalent in P3HT devices processed in air and the effect of de-doping (by annealing above the glass transition temperature) is investigated on the MC of an Au/P3HT/Al diode. De-doping reduces the current through the device under forward bias by ~3 orders of magnitude, but increases the negative (low current) MC from a maximum of -0.5% pre-annealing to -3% post-annealing. This increased negative MC is consistent with bipolaron theory predictions based on Fermi level shifts and density of states (DoS) changes due to de-doping. The decrease in current density is explained by increased injection barriers at both electrodes also resulting from de-doping. Deliberate chemical doping of P3HT is carried out using pentacene as a hole trap centre. The trapping effect of pentacene is confirmed by reproducible and significant hole mobility-pentacene concentration behaviour, as measured by dark injection (DI) transient measurements. The enhanced carrier injection resulting from the pentacene doping also leads to increased electroluminescence (EL). The resultant MC in pentacene doped devices is strongly dependent on carrier injection and can be significantly enhanced by doping, for example from -0.2% to -0.6% depending on device and drive conditions. Throughout this thesis Lorentzian and non-Lorentzian function fitting is carried out on the measured MC, although the underlying microscopic mechanisms cannot always be discerned.
33

Self-sensing permanent magnet machine

Wang, Tianhao January 2017 (has links)
This thesis looks at the saliency-based self-sensing control of permanent magnet synchronous machines (PMSM) and a novel machine configuration is proposed to improve the self-sensing performances. In recent time, PMSM drives have been steadily gaining popularity and have widespread applications in industry due to its benefits such as high power density, good dynamic performance and high efficiency. Self-sensing drives are superior to conventional drives in applications where the reliability and the cost are of important factors. Machine saliency is utilized for rotor position tracking during the start-up and the low speed operation when Back-EMF components are not detectable. For conventional PMSM machines, however, the saliency of Interior Permanent Magnet (IPM) machine is heavily affected by saturation effects under loaded operation; for the case of Surface Mounted Permanent Magnet (SMPM) machines, the saliency is not apparent and hard to detect. Hence the rotor position signals are relatively small or even undetectable at specific operation points, and these are the main challenges of PMSM drive self-sensing controlled at the low speed. Addition of a novel saliency modulation rotor end (SMRE) structure to the end of a conventional PMSM rotor to improve the self-sensing capability is proposed. The SMRE provides an additional space anisotropic to the rotor. The saliency modulation of the rotor end is electrically asynchronous with the machine`s rotating reference frame. Therefore, the machine saliency provided by SMRE is not affected by saturation effects under loaded operation when high frequency injection scheme is adopted in low speed ranges. In addition, for the medium and high speed range, the rotor position can be tracked without superposed injection as the saliency modulation can be achieved by taking the fundamental voltage as the carrier signal. A genetic algorithms (GA) optimization environment combined with the finite element analysis (FEA) enables to obtain optimized rotor end geometry for the maximum modulation signal and minimum total harmonic distortions (THD). The expected self-sensing performance is validated by a prototype machine and is compared with conventional PMSMs in experimental tests.
34

Magnetostriction and magnetic anisotropy in non-oriented electrical steels and stator core laminations

Somkun, Sakda January 2010 (has links)
Magnetostriction is a source of vibration and acoustic noise of electrical machines and it can be highly anisotropic even in non-oriented electrical steel. Understanding of magnetostriction under magnetisation and stress conditions present in stator core laminations can help predict the core vibration and radiated noise. Anisotropy of magnetostriction of a 0.50 mm thick non-oriented steel investigated in Epstein strips cut at angles to the rolling direction was much higher than the anisotropy of its magnetic and elastic properties because magnetostriction arises directly from magnetic domain processes. Magnetostriction of a disc sample of the 0.50 mm thick steel was measured under ID and 2D magnetisation and compared with that of a 0.35 mm thick steel with different anisotropy level. A 2D magnetostriction model and an analytical simple domain model were used to explain the experimental results. 2D magnetostriction is dependent on the magnetostrictive anisotropy and the ratios of the transverse to longitudinal magnetostriction. AC magnetostriction measured in the disc samples was larger than in the Epstein strips due to the form effect. An induction motor model core was constructed from the 0.50 mm thick steel for measurements of localised flux density and deformation. Core deformation due to Maxwell forces was calculated. Magnetostriction and specific power loss of the core material under magnetisation conditions present in the core was measured. The localised loss in the stator teeth, tooth roots and back iron differed from their average value by 52%, 19% and 36% due to the magnetic anisotropy. Magnetostriction was estimated to be about 55% and 80% of the radial deformation at the tooth root and back iron regions respectively. Stator teeth deformed asymmetrically and the magnitude of the space harmonics increased due to the magnetostrictive anisotropy. The measurement results inferred that 2D magnetostriction can be predicted from the magnetostrictive anisotropy and vice versa. Also, core deformation and vibration of large machines, where segmented stator core laminations are used, can be estimated analytically with the knowledge of 2D magnetostriction of the core material.
35

Health and safety risks in the design and construction of magnetic shielding for magnetic resonance imaging (MRI) suites

Price, Terence Raymond January 2012 (has links)
Once energised, and even when the equipment is not imaging, magnets used in magnetic resonance imaging will produce a static magnetic field that extends in three dimensions around the magnet. This static magnetic field is invisible; it is impossible to know that it is present or to be aware of it unless told. It is important to know the position and magnetic flux density of the static magnetic field because those persons having ferromagnetic material embedded within their bodies or their eyes (the result of a welding process for example), or fitted with electronic body implants, could suffer harm from the effects of the static magnetic field at relatively low levels. Those individuals fitted with some heart pacemakers could be affected at 0.5 mT. The published literature relating to magnetic resonance imaging is, by its nature, restricted to the medico-technical-academic press and does not systematically appear in publications destined for construction professionals. There is no published literature relating to the design of magnetic shielding for MRI suites as it relates to health and safety risks to those exposed to the static magnetic field during the construction, maintenance and demolition phases of a magnetic resonance imaging project. This thesis is progressive in its structure and fills gaps in knowledge by commencing with a study to determine if the requirements placed on duty holders as defined by The Construction (Design and Management) Regulations 2007 (CDM) and its antecedent regulations are understood by all those parties involved with the conception, design, construction and maintenance of an MRI suite. Several misconceptions are highlighted. A second study gave an evaluation of the availability of as-built drawings showing the position of the 0.5 mT footprint of the static magnetic field of the magnet and gives disappointing results. The third study was to assess the effectiveness of a retrofit installation of passive magnetic shielding and highlights some failings, with the fourth study to investigate if magnetic shielding had been installed to an operational MRI suite. The fifth study was to review if the client had considered the magnetic shielding design requirements of a magnet before it was installed. Finally, the sixth case study was to evaluate if there was a clear understanding by designers of the function and attributes of RF shielding and of passive magnetic shielding to a Faraday cage. Examples, by the inclusion of annotated drawings, are given. There was not a clear understanding by CDM duty-holders of responsibilities placed upon them under the CDM Regulations. The introduction of magnetic shielding into a magnetic resonance suite design can distort the symmetry of the 0.5 mT footprint of the static magnetic field, create areas of increased magnetic flux density and push parts of the 0.5 mT footprint to the outside of any designated controlled area. This will consequentially increase the risk of unscreened persons (both inside and outside the control of the employer) being exposed to the effects of the static magnetic field unless the magnitude and position of the 0.5 mT footprint is documented and disseminated to all those persons likely to come into contact with it. The incorporation of magnetic shielding as retrofit can result in leakages of magnetic flux at its joint with the finished floor and through any shielding fixing bolts. This thesis could be useful to designers in developing risk management plans for MRI suite construction, maintenance and demolition. By making a synthesis that has not been made before, this thesis makes a contribution to knowledge by addressing these issues for the first time.
36

Conductively filled Poly(methyl methacrylate) composites : manufacture and testing processes for EMI shielding effectiveness

Smuga, Jonathan R. January 2012 (has links)
Electromagnetic interference (EMI) is an escalating concern in the modern electronic climate. As such it has become a critical area to consider when designing and packaging electronics. With the growing volume of electronic devices available and with processor frequencies increasing, the electromagnetic environment is becoming ever more congested. The need for adequate EMI shielding has become an essential consideration. The desire for high performance combined with reductions in size, weight and manufacturing cost suggests that polymers should be ideal materials for parts such as electronic housings. Unfortunately polymers generally do not provide shielding from electromagnetic waves. The research detailed in this thesis investigates the manufacture and testing of conductively filled poly(methyl methacrylate) (PMMA) composites. Samples of PMMA resin and various electrically conductive filler materials were manufactured. The processing methods, electrical properties and electromagnetic behaviour were all investigated. Composite polymer coatings were printed with a K-Control Coater and evaluated for surface resistivity and EMI shielding effectiveness. Samples were produced with a range of filler materials including nickel, carbon, copper/aluminium and silver coated glass spheres. Shielding effectiveness values of approximately 70 dB were obtained for coatings of PMMA filled with silver coated hollow glass microspheres. Attempts were made to produce an alternaalternative filler material by electroless nickel plating of expanded graphite powder. Successful plating was achieved using conventional methods of surface sensitisation of the graphite. This however resulted in agglomerations of the powder and a loss of the desired physical properties. Alternative thermal surface treatments proved to be unsuccessful in activating the graphite surface with no nickel deposition occurring. Furthermore, electroless nickel plating techniques were successfully utilised in the development of an alternative manufacturing process for producing electrically conductive PMMA composites which contained a reduced metallic content, in relation to a more traditional production technique. Plaques were manufactured by compression moulding of nickel plated PMMA granules. These were compared against samples manufactured with nickel powder mixed in a Brabender Plasti-Corder. The electroless plating method produced samples that outperformed the comparative method and were shown to contain a reduced metallic content. Shielding effectiveness of the electroless plated granule samples achieved approximately 34 dB compared to a maximum of only 2.5 dB for the Brabender compounded samples. Outwith these areas of empirical testing a computer model was produced to simulate the electromagnetic shielding behaviour of composite materials using Comsol Multiphysics. This model appears to successfully simulate the waveguide testing apparatus. However the theoretical conductivity values as calculated from effective media theory resulted in disproportionate shielding effectiveness values obtained. Further research into the electroless plated and compression moulded PMMA composites would be beneficial in order to fully optimise the process. Equally the theoretical model would require further investigating and validating before more accurate simulations could be achieved.
37

Ανάλυση, εφαρμογή και πειραματική μελέτη μηχανικού συστήματος αιώρησης / Analysis, implementation and experimental study of mechanical levitation system

Κασιδάκης, Ευθύμιος, Λαδιάς, Νικόλαος 04 October 2011 (has links)
Σκοπός της διπλωματικής εργασίας, είναι η κατασκευή ενός κυκλώματος με ανάδραση για τον έλεγχο ενός ηλεκτρομαγνήτη με στόχο την αιώρηση ενός σταθερού μαγνητικού αντικειμένου. / The purpose of the thesis is to build a circuit with feedback in order to control a solenoid to levitate a constant magnetic object.
38

Computational modelling of turbulent magnetohydrodynamic flows

Wilson, Dean Robert January 2016 (has links)
The study of magnetohydrodynamics unifies the fields of fluid mechanics and electrodynamics to describe the interactions between magnetic fields and electrically conducting fluids. Flows described by magnetohydrodynamics form a significant aspect in a wide range of engineering applications, from the liquid metal blankets designed to surround and remove heat from nuclear fusion reactors, to the delivery and guidance of nanoparticles in magnetic targeted drug delivery. The ability to optimize these, and other, processes is increasingly reliant on the accuracy and stability of the numerical models used to predict such flows. This thesis addresses this by providing a detailed assessment on the performance of two electromagnetically extended Reynolds-averaged Navier-Stokes models through computations of a number of electromagnetically influenced simple channel and Rayleigh-Bènard convective flows. The models tested were the low-Re k-ε linear eddy-viscosity model of Launder and Sharma (1974), with electromagnetic modifications as proposed by Kenjereš and Hanjalić (2000), and the low-Re stress-transport model of Hanjalić and Jakirlić (1993), with electromagnetic modifications as proposed by Kenjereš and Hanjalić (2004). First, a one-dimensional fully-developed turbulent channel flow was considered over a range of Reynolds and Hartmann numbers with a magnetic field applied in both wall-normal and streamwise directions. Results showed that contributions from the electromagnetic modifications were modest and, whilst both models inherently captured some of the reduction in mean strain that a wall-normal field imposed, results from the stress-transport model were consistently superior for both magnetic field directions. Then, three-dimensional time-dependent Rayleigh-Bènard convection was considered for two different Prandtl numbers, two different magnetic field directions and over a range of Hartmann numbers. Results revealed that, at sufficiently high magnetic field strengths, a dramatic reorganization of the flow structure is predicted to occur. The vertical magnetic field led to a larger number of thinner, more cylindrical plumes whilst the horizontal magnetic field caused a striking realignment of the roll cells' axes with the magnetic field lines. This was in agreement with both existing numerical simulations and physical intuition. The superior performance of the modified stress-transport model in both flows was attributed to both its ability to provide better representation of stress generation and other processes, and its ability to accommodate the electromagnetic modifications in a more natural, and exact, fashion. The results demonstrate the capabilities of the stress-transport approach in modelling MHD flows that are relevant to industry and offer potential for those wishing to control flow structure or levels of turbulence without recourse to mechanical means.
39

Investigating magnetism and superconductivity using high magnetic fields

Ghannadzadeh, Saman January 2014 (has links)
This thesis investigates a number of transition-metal coordination polymers and iron-pnictide superconductors through the use of high magnetic fields, low temperatures, and on occasion, high pressures. The thesis will begin by describing my development of the proximity detector dynamic susceptometer, a novel technique that can be used for magnetometery and transport measurements in high magnetic fields. This technique is highly compact and has no moving parts, making it suitable for use in pressure cells, hence opening the way for a variety of new experiments. Through high-field magnetometery and other measurements, I will demonstrate that the pressure can be used to directly control the magnetic properties of the polymeric magnet CuF<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>(pyrazine). In particular, I observe a transition from quasi-two-dimensional to quasi-one-dimensional antiferromagnetism at 9~kbar, driven by the rotation of the Jahn-Teller axis. I will then present a series of measurements on two coordination polymers, showing how a small chemical difference can lead to drastically different magnetic properties. I show that [Cu(pyrazine)H<sub>2</sub>O(glycine)<sub>2</sub>]ClO<sub>4</sub> is an excellent spin-chain, while the sister compound [Cu(pyrazine)(glycine)]ClO<sub>4</sub> is a dimerised material that shows a spin-gap and is disordered down to very low temperatures, but then undergoes a field-induced phase transition to an ordered phase. I will also describe a series of pulsed-field measurements of the upper critical field of the iron-based superconductors NaFe<sub>1-x</sub>Co<sub>x</sub>As across the whole of the doping phase diagram. It is shown that paramagnetic pair-breaking effects dominate the critical field when the field is parallel to the crystal planes. In the parent compound the paramagnetic limit is equal to that expected from BCS theory, but becomes significantly enhanced above the BCS limit upon doping. It is shown that the multi-band nature of the superconductivity leads to a convex curvature in the evolution of the critical field as the temperature is reduced.
40

Application of magnetic torque on the bacterial flagellar motor

Lim, Ren Chong January 2015 (has links)
There is a strong need to develop a mechanical method to apply external torque to the bacterial flagellar motor. Such a method will allow us to probe the behaviour of the motor at a range of different speeds under different external conditions. In this thesis, I explored various methods to deliver torque at the single-molecule level, in particular the use of angular optical trapping and magnetic tweezers. I have identified rutile particles as suitable handles for use in angular optical trapping due to their high birefringence. Further progress was not achieved using angular optical trapping due to the lack of a suitable method to attach birefringent particles to the bacterial flagellar motor. On the other hand, I was able to make further progress using magnetic tweezers. A highly-reproducible and high-yielding magnetic bead assay was developed along with electromagnets capable of generating fast-rotating magnetic fields at magnitudes on the order of tens of mT. Using the system of delivering magnetic torque developed, I was able to stall and rotate the motor forward at speeds up to 220 Hz and in the reverse direction. Stalling experiments carried out on the motor revealed the stator mechanosensing depends on torque and not rotation. Signatures of stators dropping out at low load experiments further confirm the load dependence of stators.

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