No description available.

No description available.

This thesis develops a hardware circuit implementation of a novel algorithm for reducing a SRM drives input current ripple or equivalently to improve the SRM drives input power quality. The algorithm requires the SRMs phase current to follow a trapezoidal trajectory relative to the rotors position with the magnitude of the current dependent on the desired average torque. This thesis deals with the generation of the required current command that is the input to a separate analog current regulator that forces the SRMs current to follow the generated current command. The final circuit design must be capable of operating at 200C to be part of a high temperature aircraft actuator. In this thesis, room temperature hardware is used to emulate and verify the high temperature design. Both a high temperature microcontroller based design and a high temperature gate array based design are considered with the high temperature gate array based design being chosen. Ultimately, a standard room temperature Xilinx FPGA is chosen to emulate the high temperature gate array. The FPGA is programmed using Verilog HDL and the code is downloaded into the chip using Xilinx ISE software. The experimentally generated output is validated by comparing it with simulation results from a detailed Simulink model of the complete drive system.

This thesis describes investigations into the application of high temperature superconductoR_s(HTS) for microwave devices. The ultimate aim of this research was to produce HTS components that would demonstrate the commercial potential of their use in spacecraft such as satellites or in base stations for terrestrial cellular communication systems. During the course of this research the surface resistance (R_s) of HTS thick and thin films deposited onto different substrates has been measured by a variety of techniques. These have been compared and contrasted with each other. There has been a particular emphasis upon the use of dielectric resonators as characterization tools and this thesis describes their use to measure the R_sof films and the loss tangents of the materials used in them. Consideration has been given to losses in the walls of the enclosures that surround dielectric resonators which is an area that has been neglected in previous work. Many of the films characterized had R_s values lower than normal conductoR_sat the same temperature and frequency including a thick film which had an R_sof 2.7mΩ±8.0% at 77K and scaled to 10 GHz which represents the second lowest result achieved in a thick film to date. Using such high quality films a number of devices have been made including microstrip resonatoR_soperating at 1.7-8.4 GHz, 5.58 GHz, 23 GHz and 24.75 GHz dielectric resonatoR_sand filteR_swith centre frequencies from 5.58-13 GHz. Such devices have shown performance enhancements of up to 700% over their normal conductor equivalents. Benefits have been delivered in light, low volume packages which is of particular advantage to the satellite communications industry. Finally, there has been a demonstration, for the first time, of a tunable HTS thin film filter.

No description available.

No description available.

In this thesis, we study the effects of doping in two-dimensional quantum antiferromagnets. We consider cases where the undoped parent compound is a Mott insulator with long-range antiferromagnetic order and focus on the low-doping situations. The limit of localized impurities is studied in a system consisting of a host magnet and two additional weakly coupled spins. We derive the effective Hamiltonian describing the interaction between these impurities as a function of their distance and show that it exhibits xyz anisotropy, leading to NMR and EPR line broadening. We calculate the magnetization disturbance in the host magnet induced by a single impurity and find that it always enhances Neel order. Relaxing the localization constraint, we investigate the single-hole dynamics of the t-J model on the honeycomb lattice. Using exact diagonalizations, series expansion and the self-consistent Born approximation, we calculate the quasi-particle dispersion, bandwidth and residues and compare our findings with the well-established results for the square lattice. Similar to the latter case, we find an almost flat band along the edges of the magnetic Brillouin zone and well-defined hole pockets around the corners. The most important part of this thesis is devoted to the magnetic properties of lightly doped La2-xSrxCuO4, the simplest and by far most studied cuprate superconductor. Starting from the undoped parent compound, we calculate the spin-wave spectrum and the spin-flop transitions in a uniform magnetic field at zero temperature. We then consider the low-doping regime and derive the effective field theory describing the spin dynamics in insulating La2-xSrxCuO4, x ≤ 0.055, at low temperature. The spin structure resulting from the spiral solution of the extended t-J model, obtained by taking into account the Coulomb trapping of holes by Sr ions, is confined in the copper-oxide planes. Our solution explains why the incommensurate structure is directed along the orthorhombic b axis and allows us to calculate the positions and shapes of the neutron scattering peaks numerically. These results are in perfect agreement with experimental data. We also show that topological defects (spin vortex-antivortex pairs) are an intrinsic property of the spin-glass ground state.

Thesis (Ph.D.)--McMaster University, 2006. / Supervisor: Catherine Kallin. Includes bibliographical references (p. 168-177).

Thesis (Ph.D.)--McMaster University (Canada), 1996. / Source: Dissertation Abstracts International, Volume: 57-10, Section: B, page: 6315. Adviser: J. P. Carbotte.

Thesis (Ph. D.)--University of Adelaide, Dept. of Chemical Engineering, 1963. / [Typewritten]. A thesis submitted for the degree of Doctor of Philosophy in the Faculty of Engineering of the University of Adelaide. Includes bibliographical references.