No description available.

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The magnetic properties, crystal and magnetic structures of Nd5 SixGe4-x were investigated by ac susceptibility and high resolution neutron powder diffraction. The magnetic and crystalline phase diagrams were derived. Four distinct structures exist in the Nd 5SixGe4-x system: Gd5Ge 4-type [O(II)], Gd5Si2Ge2-type (M), Gd5Si4-type [O(I)], and Zr5Si4-type (T). The refinements of the neutron powder diffraction patterns revealed that the Nd5SixGe4-x compounds exhibit mixed ferro-antiferromagnetic structures. The ac susceptibility measurements showed that the magnetic ordering temperature of Nd5SixGe 4-x increases slightly with increasing silicon content, except that it increases by a factor of 2 in the orthorhombic Gd5Si 4-type [O(I)] phase region. The abrupt change of the magnetic ordering temperature between x = 2.25 and x = 2.5, where the monoclinic Gd5Si 2Ge2-type (M) structure changes to the orthorhombic Gd 5Si4-type [O(I)] structure, suggested that a first order magnetostructural transition likely takes place in this narrow composition range (2.25 < x < 2.5). The investigation of Nd5Si2.335 Ge1.665 revealed that Nd5Si2.335Ge 1.665 adopts the Gd5Si2Ge2-type (M) structure and undergoes a first order magnetostructural transition from the paramagnetic-monoclinic Gd5Si2Ge2-type (M) structure to the orthorhombic Gd5Si4-type [O(I)] structure upon cooling. The T1-T1 bonds increases by ∼1 A when the the Gd 5Si4-type [O(I)] structure (10 K) transforms to the Gd 5Si2Ge2-type (M) structure (140 K). The giant magnetocaloric effect is not observed in Nd5Si2.335Ge 1.665 probably due to the co-existence of the M phase and the O(I) phase. The maximum magnetic entropy change in Nd5Si2.335Ge 1.665 is 7.3 J/kg K for magnetic field change from 0 to 7 Tesla, which is similar to that obtained in Nd5Si1.5Ge2.5, the neighboring phase O(I).

No description available.

No description available.

No description available.

Permanent magnets are today widely used in electrical machines of all sorts. With their increase in popularity, the amount of research has increased as well. In the wind power project at Uppsala University permanent magnet synchronous generators have been studied for over a decade. However, a tool for studying demagnetization has not been available. This Ph.D. thesis covers the development of a simulation model in a commercial finite element method software capable of studying demagnetization. Further, the model is also capable of simulating the connected electrical circuit of the generator. The simulation model has continuously been developed throughout the project. The simulation model showed good agreement compared to experiment, see paper IV, and has in paper III and V successfully been utilized in case studies. The main focus of these case studies has been different types of short-circuit faults in the electrical system of the generator, at normal or at an elevated temperature. Paper I includes a case study with the latest version of the model capable of handling multiple short-circuits events, which was not possible in earlier versions of the simulation model. The influence of the electrical system on the working point ripple of the permanent magnets was evaluated in paper II. In paper III and VI, an evaluation study of the possibility of creating a generator with an interchangeable rotor is presented.  A Neodymium-Iron-Boron (Nd-Fe-B) rotor was exchanged for a ferrite rotor with the electrical properties almost maintained.

The electron physics in the collisionless magnetic reconnection is studied using data from the Polar spacecraft. Among the types of discontinuities in space plasmas, the Electron Diffusion Region (EDR) at the center of the reconnection has the theoretically unique properties that its thickness is of order of the electron gyroradius, and in such a region electrons are demagnetized with a non-gyrotropic pressure tensor. These unusual properties of EDRs reflect the expected violation of guiding center theory for electrons and are exploited in this thesis. We use four dimensionless, diagnostic, single spacecraft observables derived from theoretical properties of EDRs to locate them. They are electron agyrotropy, out-of-interconnection-plane electron Mach number, and dimensionless thresholds for electric field strengths parallel and perpendicular to the magnetic field. These observables are constructible using the electron density, bulk velocity, pressure tensor, and the electromagnetic field data. With a 3-year survey using particle data from a slower version of Hydra's moment producing system, M3, the vast preponderance of these dimensionless parameters are below unity, which is consistent with the theoretical expectations for most space plasmas being strongly magnetized. The unusual outliers with the demagnetization parameters over unity (<1%) in the distribution are geophysically distributed near the magnetopause within 8-9 Re shells and collected as potential reconnection sites, although a number of other possibilities are also considered in this thesis such as data processing anomalies, systematic effects of data acquisition and aliasing. It is shown that plasma particle data with the highest time resolution possible are needed to improve the time aliasing issues, and to sense the rapidly changing and short scale current structure like the EDR. We use a recently developed algorithm G3/T1, which reduces the aliasing time of the 3D analysis of products of the Polar Hydra Hot Plasma Analyzer by a factor of 12. With this new technique, we have found that among these outliers some demagnetization signatures are ameliorated by higher time cadences and the ones caused by time aliasing effects are ruled out. The moment recoveries of G3/T1 at a 2.3 cadence are in excellent agreement with input distribution models over a considerably wide range of density, Mach number, electron anisotropy, and agyrotropy, provided that a suitable accurate inventory can be made in advance for the bulk velocity of these distributions. The 2 candidate reconnection events analyzed in this thesis by G3/T1 processing techniques demonstrate: (1) strong out-of-interconnection-plane electron flows along the separatrices also observed in 2D PIC simulations; (2) significant electron agyrotropy enhancements framing high thermal Mach number flow, proving excellent consistency with the agyrotropy islands predicted by PIC simulations of asymmetric reconnection geometry; and (3) measured thermal electron gyroscale current channels in patterns that are supported by PIC simulation models as resolved examples of EDRs with direct measures of the electron demagnetization.

In a world where money often is the main controlling factor, everything that can be tends to be more and more optimized. Regarding electrical machines, developers have always had the goal to make them better. The latest trend is to make machines as efficient as possible, which calls for accurate simulation models where different designs can be tested and evaluated. The finite element method is probably the most popular approach since it makes it possible to, in an easy and accurate way, get numerical solutions to a variety of physics problems with complex geometries and non-linear materials. This licentiate thesis includes two different projects in which finite element methods have had a central roll. In the first project, the goal was to develop a simulation model to be able to predict demagnetization of permanent magnets. It is of great importance to be able to predict if a permanent magnet will be demagnetized or not in a certain situation. In the worst case, the permanent magnets will be completely destroyed and the machine will be completely useless. However, it is more probable that the permanent magnets will not be completely destroyed and that the machine still will be functional but not as good as before. In a time where money is more important than ever, the utilization has to be as high as possible. In this study the demagnetization risk for different rotor geometries in a 12 kW direct driven permanent magnet synchronous generator was studied with a proprietary finite element method simulation model. The demagnetization study of the different rotor geometries and magnet grades showed that here is no risk for the permanent magnets in the rotor as it is designed today to be demagnetized. The project also included experimental verification of the simulation model. The simulation model was compared with experiments and the results showed good agreement. The second project treated the redesign of the rotor in the generator previously mentioned. The goal was to redesign the surface mounted NdFeB rotor to use a field concentrating design with ferrite permanent magnets instead. The motivation was that the price on NdFeB magnets has fluctuated a lot the last few years as well as to see if it was physically possible to fit a ferrite rotor in the same space as the NdFeB rotor. A new rotor design with ferrite permanent magnets was presented together with an electromagnetic and a mechanical design.

A determinação das principais propriedades magnéticas de um ímã permanente geralmente requer sistemas de medição sofisticados e custosos. Este trabalho analisa e propõe um sistema de caracterização de ímãs permanentes de Terras Raras à temperatura ambiente de baixo custo e de implementação mais simples e rápida. Ele consiste de um circuito magnético formado pelo ímã permanente sob caracterização, uma culatra de aço, um entreferro variável e uma sonda de efeito Hall com seu respectivo gaussímetro para a medição da intensidade do campo magnético no entreferro. Como introdução se apresenta o estado da arte da caracterização dos ímãs permanentes, conceitos básicos de magnetismo e em relação aos ímãs permanentes, sua história, os principais tipos, sua importância e suas aplicações principais. Os resultados teóricos e experimentais são comparados a resultados certificados, validando o método como proposto. / Determining the main magnetic properties of a permanent magnet usually requires sophisticated and expensive measurement setups. This work analyses and proposes a low-cost system to characterize Rare Earth permanent magnets at ambient temperature with easy and fast implementation. It consists of a magnetic circuit with the permanent magnet under test, appropriate pole pieces, a variable air gap and a Hall Effect probe with its respective Gaussmeter. As an introduction, it is presented the state of the art of permanent magnets characterization, basic concepts of magnetism and its history, the main types of permanent magnets, their importance and applications. The theoretical and experimental results are compared to certified ones, validating so the method as proposed.