The More Electric Aircraft is a technological trend in modern aerospace industry to increasingly use electrical power on board the aircraft in place of mechanical, hydraulic and pneumatic power to drive aircraft subsystems. This brings major changes to the aircraft electrical system, increasing the complexity of the network topology together with stability and power quality issues. Shunt active power filters are a viable solution for power quality enhancement, in order to comply with the standard recommendations. The aircraft electrical system is characterized by variable supply frequency in the range 360-900Hz, hence the harmonic components occur at high and variable frequencies, compared to the terrestrial 50/60Hz systems. In this kind of system, fast and accurate algorithms for the detection of the reference signal for the active filter control and robust high-bandwidth control techniques are needed, in order for the active filter to perform the harmonic elimination successfully. In this thesis, two novel algorithms are proposed. The first algorithm is a frequency and harmonic detection technique, particularly suitable for tracking the variable supply frequency and the harmonic components of voltages and currents in the aircraft electrical system. Complete identication of the reference signal for the active filter control is possible when applying this technique. The second algorithm is a control technique based on the use of multiple rotating reference frames. Only the measurement of the voltage at the Point of Common Coupling and the active filter output current are needed, hence no current sensors are required on the distorting loads. Both the techniques have been validated by means of simulation and experimental analysis. The results show that the proposed methods are effective for a successful harmonic compensation by means of active shunt filters, in the More Electric Aircraft environment.

The purpose of this work is to investigate the design and implementation of a 7.5kVA Matrix Converter-based power supply for aircraft applications (GPU Ground Power Unit). A Matlab/Simulink as well as SABER simulation analysis of the candidate Matrix Converter system is provided. The design and implementation of the Matrix Converter is described, with particular attention to the strict requirements of the given power supply application. This AC-AC system is proposed as an effective replacement for the conventional AC-DC-AC system which employs a two-step power conversion. The Matrix Converter is an attractive topology of power converter for power supply applications where factors such as the absence of electrolytic capacitors, the potentiality of increasing power density, reducing size and weight and good input power quality are fundamental. An improved control structure is proposed. This structure employs an ABC reference frame implementation comprising at the Repetitive Control strategy combined with a traditional tracking controller in order to attenuate or eliminate the unwanted harmonic distortion in the output voltage waveform of the Matrix Converter and to compensate for the steady-state error. The system with the proposed control was initially fully analyzed and verified by simulation. The analysis of the input and output waveforms identified the constraints that need to be satisfied to ensure successful operation of the converter. Finally, to demonstrate both the Matrix Converter concept and the control strategy proposed, a 7.5kVA prototype of the proposed system was constructed and tested in Nottingham PEMC laboratory. The experimental results obtained confirmed the expectations from the simulation study and the validity of the power converter and control design.

Inverse modelling methods are receiving significant interest, due to their simplicity and ease of use in the design of modern microwave components. This study investigates and further develops the technique of numerical time-reversal, in the context of automated component design, for modelling metal waveguide devices. The thesis demonstrates that time-reversal methods suffer from temporal truncation, evanescent wave decay and significant computational resource requirements and will investigate different methods to solve these problems. In order to reduce the runtime, the use of Prony’s method for temporal extrapolation of a discrete waveform is proposed. Lossy materials are investigated, with particular attention given to the loss of modal content from the reverse model due to material loss present in the forward phase of the time-reversal process. The memory and time requirements of a successful time-reversal design simulation are significant. Temporal, spatial and modal filtering are used to minimise the computational demands of time-reversal. Further, in order to accelerate convergence of the time-reversal design process, a number of linear acceleration methods are developed, notably successive over relaxation, conjugate gradients and generalised minimal residual. A convergence acceleration factor of two is achieved. It is shown that local evanescent content around optimised scattering elements is not always captured by the time-reversal process, and is dependant upon the component order, numerical sampling and machine precision. Internal mirrors are developed which capture the fast decaying fields around the metal features of a designed component and further increase the accuracy and speed of the time-reversal convergence. Their use for higher order component design is shown to be paramount in achieving convergence. Further, combined with the linear acceleration methods, the capture of local evanescent content is shown to greatly improve the viability of the time-reversal technique to practical microwave component design. The time-reversal methodology is implemented using the numerical transmission-line modelling (TLM) method for transverse magnetic polarisation in two-dimensions. A brief examination of the three-dimensional time-reversal using the symmetrical condensed TLM node is also given.

The spontaneous formation of supramolecular assemblies has been viewed as a potential route to the creation of functional nano-scale architectures for a number applications in electronics. In this thesis a number of assemblies formed from molecular constituents deposited from the solution phase have been studied. The structures formed by two carboxylic acid derivatives on the highly oriented pyrolytic graphite (HOPG) surface from nonanoic acid solutions are presented. Quaterphenyl-tetracarboxylic acid (QPTC) molecules are observed to form a supramolecular network where all the constituents lay parallel to one another on the surface. The network is stabilised by four carboxylic acid dimer bonds per molecule in addition to admolecule-substrate interactions. Terphenyl-tetracarboxylic acid (TPTC) molecules form a much more complex structure with individuals orientating themselves in one of three directions to form a network with hexagonal symmetry but no translational order. To characterise such an unusual supramolecular morphology we introduce a rhombus tiling representation of the network where each molecule is schematically replaced with a lozenge rhombus producing a tiling. Such tilings have been studied previously in the literature and utilising this we are able to determine that the morphology is stabilised by entropic contributions to the free energy. In addition to this we present the tip-induced manipulation of the TPTC supramolecular network. The manipulation is performed by imaging the structure within a specific voltage bias range resulting the TPTC molecules reordering into a close packed structure. Returning the voltage to that conventional used for imaging causes the network to relax back into the open structure although with a different morphology. We then discuss the changes induced in these supramolecular networks when additional molecular species are introduced to the system. First, coronene and perylene are separately codeposited with QPTC resulting in the formation of a hexagonally ordered network with coronene or perylene located at the vertices of six QPTC molecules. This new structure is observed to form even when QPTC is deposited first. Second, the adsorption of coronene into the porous TPTC network is presented. When the TPTC network forms before the introduction of coronene we note little effect on the network morphology. However, when the molecules are mixed in the solution phase and deposited simultaneously we observe the non-uniform adsorption of coronene into the TPTC structure. At higher coronene concentrations we note the network forms with a different morphology shifted towards a more ordered state suggesting that when the molecules are deposited sequentially the system is kinetically trapped in the originally formed structure. We then present a series of studies of molecular adsorption on the Au (111) surface. First, hexaazatrinaphthylene (HATNA) molecules are observed to form stable supramolecular structures when deposited from ethanol solutions. A core hydrogen bonding junction is identified. The network switches between two domain orientations and we identify a linear defect where the two domains meet. Second, we report the adsorption of Tri( 4-bromophenyl) benzene (TBPB) on the Au (111) surface. TBPB forms three different structures at room temperature. When samples are heated during the deposition stage we observe the covalent coupling of pairs of molecules to form dimers. This reaction is confirmed by ToFSIMS experiments. The substrate is confirmed to play a significant role in the coupling process as subsequent experiments on HOPG failed to yield dimer formation. Finally we demonstrate the potential of a UHV-prepared sample by templating the adsorption of adamantanethiols. Finally, we demonstrate the adsorption of a solubilised derivative of perylene tetracarboxylic dianhydride (PTCDA). PTCDA molecules have poor solubility in most solvents commonly used for liquid deposition. The addition of alkane chains attached to the sides of the perylene core promotes the solubility of the molecule in these solvents whilst leaving the anhydride functionality intact. Deposition is performed from 1-phenyloctance solutions on HOPG. The molecules form an ordered structure characterised by a single molecule unit cell. The results presented in this thesis show that the understanding of supramolecular networks has progressed to the point where changes in the morphology can be induced via a variety of processes.

Smart power supply grids may be required to link future energy production and consumers. Multilevel converters are a building block for smart grids. There are several structures of multilevel converters, for example the Neutral Point Clamped (NPC), the Flying Capacitor Circuit and the Cascaded H-Bridge (CHB) converter. The modular structure of the CHB multilevel converter makes it one of the best options for smart grids. Using modular converter structures reduces production and maintenance costs. Implementation of efficient and fast controllers for multilevel converters requires accurate measurement of the voltages and currents for the system feedback loops. Knowledge of the DC link voltages is necessary to construct voltage control loops. In a typical CHB multilevel converter there are many DC links which means that a lot of voltage transducers maybe required. Voltage transducers at medium voltage are not easy to implement and add to system cost. This thesis presents an efficient way to observe the DC link voltages and hence eliminate the cost associated with voltage transducers. A “Sliding Mode Observer (SMO) using the Equivalent Control Method” has been chosen because of its robustness against system uncertainties. Simulation and practical work has been performed on a three-phase, three-cell multilevel converter to validate the use of this observer.

Today, many of the communication systems are operated in several different bandwidths. Till now, the common solution to get a multi bandwidth transmitter/receiver is to insert several fixed microwave devices with different bandwidth to the required standard. This increases the size and power consumption of trans/receiver. One solution is to develop electronically tunable microwave devices. By replacing several fixed microwave devices with a single electronically tunable device, the size and power consumptions of the transmitter/receiver can be significantly reduced. The tunable devices are usually implemented by high permittivity tunable materials that exhibit a change of dielectric constant with respect to a DC electric field. In this work, two different types of tunable materials: ferroelectric Barium Strontium Titanate (BST) Oxide and pyrochloe Bismuth Zinc Niobate Oxide (BZN) thin films are investigated. A simple and cost effective chemical solution deposition (CSD) method has been used to prepare the thin films. In addition, two different types of microwave devices (coplanar waveguide and metal insulated metal capacitors) were fabricated to measure the microwave dielectric properties of BST and BZN thin films. The maximum errors in the measured dielectric constants are 24% due to the calibration errors. To improve the dielectric properties of BST thin films, acceptor ions such as lithium (Li) and cobalt (Co) were doped into BST thin films. According to the measured results, the Li doped BST thin film exhibits an increase of dielectric constant and a decrease of dielectric loss, which makes it highly attractive for implementing microwave device. In contrast, the BZN thin films exhibit little dielectric tunability (3.0%) even when a large DC electric field (500kV/cm) is applied. These results demonstrate that ferroelectric BST thin films are still the only practical materials for implementing tunable microwave devices due to its high tunability.

In the Physics-of-Failure based approach to reliability design and assessment, which aims to relate lifetime to the identified root-cause of the potential failures, the development of effective failure mechanism models is a crucial task. The extent and rate of wire bond degradation depends on both the magnitude and duration of exposure to the loads. In the existing physics-of-failure models for wire bond interconnects, lifetime is accounted for by loading amplitude alone and is usually derived based on a regular thermal cycle of a known duration. They are not ready to predict life of arbitrary mission profiles and the effects of time at temperature on the wear-out behaviour are not addressed, leading to substantial errors for thermal cycling regimes with high peak temperatures. In this thesis, a new lifetime prediction model for wire bonds is proposed based on some phenomenological considerations, which accounts not only for the damage accumulation processes but also the damage removal phenomena during thermal exposure. The methodology discards the usual cycle-dependent modelling methodology and is instead based on a time domain representation so that it can more accurately reflect the observed temperature-time effects and related phenomena. This new time-based presentation allows estimation on the bonding interface damage condition at regular time intervals through a damage based crack propagation model which includes the effect of temperature and time dependent material properties. Meanwhile, bond degradation state is indicated in the form of crack growth and shear force reduction that are predicted by the total interface damage as a function of time. The model is calibrated and validated by the experimental results from wire material characterization tests and accelerated thermal cycling which demonstrates the advantages over the cycle-based methodologies.

Over the last decade there has been a growing increase in research in the field of vibrational energy harvesting - devices which convert ambient vibrational energy into electrical energy. The major application area for such devices is as power sources for wireless sensors, thereby replacing currently used batteries which suffer from a finite lifespan and pose environmental issues during disposal. The vast majority of designs are cantilever beams comprising of piezoelectric layers having coverage identical to the substrate layer. It is evident from the literature that rudimentary work has been performed on design optimisation, with reliable and extensive parametric studies on geometry, especially piezoelectric layer coverage, being overlooked. As a result of this, outcomes from previous research are yet to be seen in designs for practical applications. In this work a versatile linear model is developed which can accurately predict the performance of cantilever piezoelectric energy harvesters. An integral part of the model uses a transfer matrix method to accommodate the difference in structural dynamics of both uniform and non-uniform structures with model validation provided through extensive experimental work. The linear model developed is used to carry out parametric studies on the geometry of three distinct energy harvester cases thereby providing comprehensive knowledge on key variables and geometrical changes which can improve performance. In one of the cases examined, an improvement in performance of over 100% is predicted by solely altering piezoelectric layer coverage. However, the load resistance, i.e. electrical condition, has a significant effect on the trends in generated power which led to work directed toward harvester optimisation in a more realistic electrical scenario. Investigation on harvester geometry whilst utilising an electrical scenario comprising of an energy storage medium is undertaken in this work. The developed model ensures the effects of electro-mechanical coupling remain and provides a solid basis from which users can readily apply model extensions through inclusion of further electrical components to resemble practical circuitry. Theoretically, for all examined case studies, improvements in performance were realised through alterations to piezoelectric layer dimensions with the most notable result indicating an improvement of over 200% during optimisation of piezoelectric layer length. In conjunction to theoretical findings, outcomes of extensive experimental work are provided in order to highlight the accuracy and reliability of the presented theoretical models in both electrical scenarios. Variation in mechanical damping magnitude plays a pivotal role throughout experimental testing and is one key factor in explaining why devices comprising of shorter piezoelectric layers have high performance. A methodology behind unbiased design comparisons is also provided in this work, and involves comparing devices with identical fundamental frequencies. The reasoning behind this approach is to allow for each device to perform as efficiently as possible in the same excitation scenario. Systematic alterations to multiple geometric parameters are used to achieve this. Geometric parameters such as the substrate thickness are observed to provide adequate frequency control. Using this approach, performance improvements from adjustments to piezoelectric coverage still remain. The occurrence of non-linearity in piezoelectric materials is a widely known phenomena and so lastly, a more robust model is provided which incorporates material and geometric non-linearity. This model is useful in determining dynamical responses of uniform and non-uniform piezoelectric energy harvesters when subjected to moderate-to-high acceleration levels. A thorough validation of the theoretical model is achieved using extensive experimental data obtained from a range of samples. For the harvester composition tested in this work, the occurrence of mild non-linearity at base acceleration levels as low as 1 meter per second squared is witnessed with softening behaviour causing the resonant frequency to decrease with base acceleration. In order to avoid reduced efficiency in the final application, the prediction of possible frequency shifts is vital during the design process.

The work contained within this thesis seeks to address the dielectric and magnetic properties of bismuth substituted barium hexaferrite (composition BaBixFe12-xO19 where x = 0.0, 0.2, 0.5, 0.8, 1.0 and 1.5) across a broad frequency spectrum. This material is potentially of interest in antenna applications (specifically a dielectric resonator antenna) where high permittivity and permeability materials can be exploited to physically minimize the size of antenna devices. At low frequencies (20Hz - 3MHz) LCR meter based analysis was used to investigate the capacitance and inductance of the materials. The highest measured permittivity at 1MHz was Er = 86.18 - 9.910j, tanδ = 0.11. High frequency (45MHz - 20GHz) permittivity was investigated using a vector network analyser (VNA) and coaxial probe. The highest recorded permittivity at 2.45GHz was Er = 14.69 - 1.664j, tanδ = 0:08. A link between substitution level and permittivity and substitution level and inductance was made at either 1MHz or 2.45GHz. The homogeneity of the samples was explored using a near- field permittivity sensor. This showed small localized variations of permittivity across the surface but not significant enough to adversely effect bulk measurements. A dielectric resonator antenna was fabricated using a bismuth substituted barium hexaferrite sample. Resonant frequencies were identified at 6.6, 9.55 and 13.6GHz and radiation patterns for the system showed agreement with published theoretical results. This confirmed the suitability of the material in this application as a high permittivity material with relatively low loss. This thesis contributes to scientific knowledge by characterising a broad range of bismuth composition materials across a wide frequency range, investigating the link between permittivity/inductance and doping level and presenting full results. The material has also been characterised using a near-field permittivity technique and used to fabricated a dielectric resonator antenna which have not previously been undertaken.

Many problems in electrical engineering are associated with the way in which electric and magnetic fields propagate and distribute themselves in various media. Maxwell's equations provide a concise description for the interaction of fields with themselves and with the various boundaries of a problem. Therefore, a numerical procedure for the solution of these equations is an important consideration. This thesis shows how a general three-dimensional medium may be represented by an interconnection of continuous ideal two-wire transmission-lines made up of generalised two-dimensional nodes which are introduced in the earlier chapters. It is then shown how this model may be used for the numerical solution of the electric and magnetic vector fields within the medium. This is the TLM method of numerical analysis. A universal three-dimensional computer program based on the method is also introduced. This program has been written in only 110 lines of FORTRAN including the subroutines. The ease of application, versatility and accuracy of the TLM method is demonstrated by analysing a wide variety of microwave resonators using this program. The surface mode phenomenon of microstrip is also investigated.