One of the main trend in the development of high power electric machines (motors, generators) is to replace the magnetic components with superconducting wires, this inevitably leads to a critical requirement from the industry (Converteam) to operate power devices at cryogenic temperatures. However, the current understanding of the behaviour power devices at cryogenic temperatures is limited, especially below the liquid nitrogen temperature of 77 K. This is a problem since most of the superconducting wires operate at temperatures below 77 K. Furthermore, it is uncertain which device type is better, if at all suited to cryogenic operation. In order to answer this, a thorough analysis of the known cryogenic behaviour of all the generic power devices was performed, including the physical behaviour of silicon at cryogenic temperatures. It is concluded that the power MOSFET is the best likely candidate for cryogenic operation. To understand the cryogenic behaviour of silicon power MOSFETs especially between the temperatures of 20 K and 100 K, a cryogenic measurement system was built to characterise different types of power MOSFETs. All the measured power MOSFETs exhibited large improvement in on-state resistance down to 50 K and non-linear degradation of breakdown voltages with lower temperatures. Various behaviour was observed below 50 K including carrier freeze-out, electric field dependent ionisation of free charge carriers and large variations in on-state resistance between identical devices. Several power Schottky diodes were also characterised and all exhibited merged PiN Schottky diode behaviour at cryogenic temperatures. Non-silicon devices such as silicon carbide power MOSFETs and gallium nitride HEMTs were also measured. Silicon carbide exhibited no improvements at cryogenic temperatures, whereas gallium nitride HEMTs may prove to be the best power device to be utilised in future cryogenic applications. Since unusual behaviour was observed in power MOSFETs below 50 K, an attempt was made to explain these phenomena using theoretical equations of semiconductor physics and analytical models of power MOSFETs. The author suggested that careful control of the dopant concentration at the accumulation region below the oxide gate is required to improve the power MOSFET operations below 50 K. Moreover, the super-junction power MOSFETs could be optimised for better cryogenic operation. It is the intention of this work to demonstrate the benefits of power MOSFET cryogenic operation in a realistic industrial application. A demonstration model was designed and simulated, this circuit uses a back-to-back power MOSFETs configuration to control the freewheeling current flowing through a high temperature superconducting coil. The electrical and thermal design of the model has been described, simulated and presented in this work.

With the development of mass spectrometry (MS) instruments starting in the late 19th century, more and more research emphasis has been put on MS related subjects, especially the instrumentation and its applications. Instrumentation research has led modern mass spectrometers into a new era where the MS performance, such as resolving power and mass accuracy, is close to its theoretical limit. Such advanced performance releases more opportunities for scientists to conduct analytical research that could not be performed before. This thesis reviews general MS history and some of the important milestones, followed by introductions to ion cyclotron resonance (ICR) technique and quadrupole operation. Existing electronic designs, such as Fourier-transform ion cyclotron resonance (FT-ICR) preamplifiers (for ion signal detection) and radio-frequency (RF) oscillators (for ion transportation/filtering) are reviewed. Then the potential scope for improvement is discussed. Two new FT-ICR preamplifiers are reported; both preamplifiers operate at room temperature. The first preamplifier uses an operational amplifier (op amp) in a transimpedance configuration. When a 18-k feedback resistor is used, this preamplifier delivers a transimpedance of about 85 dB , and an input current noise spectral density of around 1 pA/ p Hz. The total power consumption of this circuit is around 310 mW when tested on the bench. This preamplifier has a bandwidth of fi3 kHz to 10 MHz, which corresponds to the mass-to-charge ratio, m/z, of approximately 18 to 61k at 12 T for FT-ICR MS. The transimpedance and the bandwidth can be adjusted by replacing passive components such as the feedback resistor and capacitor. The feedback and bandwidth limitation of the circuit is also discussed. When using an 0402 type surface mount resistor, the maximum possible transimpedance, without sacrificing its bandwidth, is approximated to 5.3 M . Under this condition, the preamplifier is estimated to be able to detect ~110 charges. The second preamplifier employs a single-transistor design using a different feedback arrangement, a T-shaped feedback network. Such a feedback system allows ~100-fold less feedback resistance at a given transimpedance, hence preserving bandwidth, which is beneficial to applications demanding high gain. The single-transistor preamplifier yields a low power consumption of ~5.7 mW, and a transimpedance of 80 dB in the frequency range between 1 kHz and 1 MHz (m/z of around 180 to 180k for a 12-T FT-ICR system). In trading noise performance for higher transimpedance, an alternative preamplifier design has also been presented with a transimpedance of 120 dB in the same frequency range. The previously reported room-temperature FT-ICR preamplifier had a voltage gain of about 25, a bandwidth of around 1 MHz when bench tested, and a voltage noise spectral density of ~7.4 nV/ p Hz. The bandwidth performance when connecting this preamplifier to an ICR cell has not been reported. However, from the transimpedance theory, the transimpedance preamplifiers reported in this work will have a bandwidth wider by a factor of the open-loop gain of the amplifier. In a separate development, an oscillator is proposed as a power supply for a quadrupole mass filter in a mass spectrometer system. It targets a stabilized output frequency, and a feedback control for output amplitude stabilization. The newly designed circuit has a very stable output frequency at 1 MHz, with a frequency tolerance of 15 ppm specified by the crystal oscillator datasheet. Within this circuit, an automatic gain control (AGC) unit is built for output amplitude stabilisation. A new transformer design is also proposed. The dimension of the quadrupole being used as a mass filter will be determined in the future. This circuit (in particular the transformer and the quadrupole connection/mounting device) will be finalised after the design of the quadrupole. Finally, this thesis concludes with a discussion between the gain and the noise performance of an FT-ICR preamplifier. A brief analysis about the correlation between the gain, cyclotron frequency, and input capacitance is performed. Future work is also suggested for extending this research.

Cognitive radio has been proposed as a promising solution to the conflict between the spectrum scarcity and spectrum under-utilization. As the demand increases for wireless communication services, cognitive radio technology attracts huge attention from both commercial industries and academic researches. The purpose of this thesis is to provide an analytical evaluation of the cognitive radio system performance while taking into consideration of some realistic conditions. Several problems are investigated in this thesis. First, by adopting a dynamic primary user traffic model with one primary user occupancy status change and exponentially distributed channel holding times, its effect on the cognitive radio system performance is evaluated. In the evaluation, the sensing-throughput tradeoff of the cognitive radio is used as the examination criteria, while energy detection is applied during the spectrum sensing. The thesis then takes the investigation further by establishing a primary user multiple changes traffic model which considers multiple primary user occupancy status changes and any reasonable channel holding time distributions. The effect of the primary user multiple changes traffic on the spectrum sensing performance is investigated while the channel holding times are assumed to be exponential, Gamma, Erlang and log-normal distributed. The analytical evaluation of cognitive radio is also carried out from the secondary user transmission perspective, where the performance of the adaptive modulation in cognitive radio system is investigated. The effect of the cognitive radio distinctive features on the performance of both the adaptive continuous rate scheme and the adaptive discrete rate scheme of the adaptive modulation are examined. The BER performance and the link spectral efficiency performance are derived for both schemes. A novel frame structure where the spectrum sensing is performed by using the recovered received secondary frames is also evaluated in this thesis. A realistic scenario which considers the secondary user signal decoding errors is examined for the novel structure, while an ideal upper bound performance is given when the decoding process is assumed perfect. By extending the system to include multiple consecutive secondary frames, the performance of the novel structure is compared to the performance of the traditional frame structure proposed by the IEEE 802.22 WRAN standard. The effect of the primary user multiple changes traffic is also examined for the novel structure. Several major findings are made from the analytical evaluations presented in this thesis. Through numerical examinations, it was shown that, first, the dynamic primary user traffic degrades the performance of cognitive radio systems. Second, the degree of the performance degradation of the cognitive radio systems is related to the number of primary user status changes and the primary user traffic intensity. Different primary user channel holding times distributions also lead to different sensitivities of the system performance to the primary user traffic. Third, cognitive radio distinctive features degrades the performance of the adaptive modulation. When the novel structure is applied for cognitive radio, a higher secondary achievable throughput can be obtained with a limited saturation threshold.

A special application of optical wireless data transfer, namely on-line monitoring and diagnostic of rotors in turbines and engines, has been considered in this thesis. In this application, to maintain line of sight, i.e. data transfer, between a sensor placed on a rotating component inside the turbine and a monitoring point placed in a fixed position outside the turbine, a periodic fast fading channel is generated, which gives the transceivers more flexibility regarding their mounting location. The communication in such a channel is affected by the intermittency and variation of the signal power, which produces a unique channel condition that influences the performance of the optical transceiver. To investigate the channel condition and the error rate of the periodic fast fading channel with signal fluctuation, a model is developed to simulate the optical channel by considering the variation of signal power as a result of the change in the relative position of the photodiode with respect to the Lambertian radiation pattern of the LED, in a simplified linear geometry. The error rate is estimated using the Saddlepoint approximation on a specific threshold strategy. The results show that the channel can afford the sensor data transmission and the performance can be improved by modifying several parameters, such as geometrical distance, transmitter power and load resistor. Compared to a normal channel, a higher load resistor on the photodiode front end has the advantage of decreasing the noise level and increasing the data capacity in the fast fading channel. The analysis of the automatic gain control amplifier indicates that a higher load resistor needs a lower loop gain and from the model of the Transimpedance amplifier (TIA), the bandwidth extension from the amplifier is more significant for a higher resistor. In addition to the theoretical model, an experimental setup is built to emulate the channel in practice. The degree of similarity between the experimental setup and the theoretical model of the channel is estimated from the comparison of the generated communication windows. Since it has been found that differences exist in the duration of the communication window and the variation of the signal power, scaling factors to ensure their compatibility have been derived. Transceiver hardware which implemented the modelled functionality has been developed and a protocol to establish the communication with the required error rate has been proposed. Using the hardware implementation, a detection method for both rising and falling edges of the signal pulses and a threshold strategy have been demonstrated. The device power consumption is also estimated. What is more, the electromagnetic environment of a squirrel cage motor is simulated using the finite element method to investigate the interference and the possibility of providing power to the IR communication devices using power scavenging. In the conclusion, the key findings of the thesis are summarised. A solution is proposed for sensor data transfer using an optical channel for rotor monitoring applications, which involves the design of the IR transceiver, the implementation of the developed protocol and the power consumption estimation.

This work describes the development of a semiconductor switch to replace an electromechanical contactor as used within the electrical power distribution system of the More Electric Aircraft (MEA; a project begun in the 1990‟s by the United States Air Force). The MEA is safety critical and therefore requires highest reliability components and systems, but subsequent to a short circuit load fault the electro-mechanical contactor switch often welds shut. This risk is increased when using high discharge energy lithium ion dc batteries. Predominately the semiconductor switch controls inductive loads and is required to safely turn off current of up to 10 times the nominal level during sporadic load fault events. The switch requires the lowest static loss (lowest on state resistance), but also the lowest dynamic loss (losses due to the switching event). Presently, unipolar devices provide the lowest dynamic loss, but bipolar devices provide the lowest static loss. One possible solution is use of a Metal Oxide Semiconductor Field Effect Transistor (MOSFET), the area of which is sized to suit the fault current, but at relatively high cost in terms of silicon area. The resultant area is typically achieved by several die connected in parallel, unfortunately, such a solution suffers from current share imbalance and the potential of cascade die failure. The use of a parallel combination of unipolar and bipolar device types (MOSFET and Insulated Gate Bipolar Transistors, IGBTs) to form a hybrid appears to offer the potential to reduce the silicon area, and static loss, whilst reducing the impact of the increased dynamic losses of the IGBT. Unfortunately, this goal requires optimised gate timing of the resultant hybrid which proves challenging if the load current is to be shared appropriately during fault switching in order to prevent failure. Some form of single MOS (Metal Oxide Semiconductor) gated integrated hybrid device with self biased bipolar injection is therefore required to ensure highest reliability through a non latching design which offers lowest losses under all conditions and achieves an even temperature distribution. In this work the novel concept of the integrated hybrid device has been investigated at a low Blocking Voltage (BV) rating of 100 V, using computer simulation. The three terminal hybrid silicon DMOS (Double diffused Metal Oxide Semiconductor) device utilises a novel merged Schottky p-type injector to provide self biased entry into a reduced static loss bipolar state in the event of high fault current. The device achieves a specific on state resistance, R(ON,SP) = 1.16 mΩcm2 in bipolar mode (with BV=84 V), that is below the silicon limit line and requires half the area of a traditional unipolar MOSFET to conduct fault current. During comparative standard unclamped inductive switching trials, the hybrid device provides a self clamping action which enables increased inductive energy switching (higher inductance and/or higher load current), relative to that achieved by either the MOSFET or IGBT. The hybrid conducting in bipolar mode switches an inductive load off much faster than that typically achieved by an IGBT (toff =20 ns, in comparison to typically >10 μs for an IGBT). This results in a low turn off energy for the hybrid (1.26*10-4 J/cm2) as compared to that of the IGBT (8.72*10-3 J/cm2). The hybrid dynamic performance is enhanced by the action of the merged Schottky contact which, unlike the IGBT, acts to limit the emitter base voltage (VEB) of the internal PNP Bipolar Junction Transistor, BJT (the integral PNP BJT is otherwise a shared feature with the IGBT). The self biased bipolar activation is achieved at a forward bias (VAK) =1.3 V at temperature (T)= 300 K. The device is latch up free across the operational temperature range of T=233 K to 400 K. A viable charge balanced structure to increase the BV rating to approximately 600 V is also proposed. The resulting performance of the single gated, self biased, hybrid, utilising a novel merged Schottky/P type injector, could lead to a new class of rugged MOS gated power switching devices in silicon and potentially silicon carbide.

Promising techniques such as micro-stereolithography (MSL) are opening up practical potential for exploiting new ideas for specialized polymer-based Micro-Electromechanical systems (MEMS) through small-batch production. As the field matures and grows, substantial research and commercial development demands better understanding of mechanical properties of MEMS materials to fully explore the potential of this technology. Bulk properties derived from conventional testing of large specimens (at 10 mm order) cannot be trusted. However, small-scale specimens (less than 1 mm) introduce major challenges, such as handling and mounting. The aim of this study was to contribute towards an improved understanding of the mechanical properties of the polymers (MSL materials) with a strong emphasis on developing new metrology. It proposed and described a special form of test-rig and compatible special MSL specimen design. A uniaxial tensile approach was chosen, partly because it offered simpler uncertainty models. The prototype used deadweight loading through a notch flexure, which acted both as a spring in parallel sharing the same displacement with the specimen and as a linear guideway. The specimen was integrally fabricated with large clamping regions and support bars released by cutting. Stiffly constrained mounting and loading surfaces were used to clamp MSL specimens to the flexure, protecting them against parasitic motions during the test in combination. Strain was measured through an elongation measurement by high-sensitivity capacitive micrometry, knowing the specimen dimensions. Verification tests on the clamping conditions showed no significant evidence of sudden slip or creep. MSL specimens were fabricated by a projection-based Envisiontec Perfactory system using a commercial acrylate-based R11 resin. Substantial shrinkage and curl distortion had been observed, which greatly reduced the fabrication accuracy of the MSL specimens. Specimens with different UV exposures and different sizes were fabricated and tested for better understanding of the MSL fabrication process. Typically, Young’s Modulus was a little smaller than expected and certainly dependent on both size and process parameters (in the region studied).

Chemical communication plays vital role in the mediating the behaviour of an organism living in the “odour space”. The mechanisms by which odours are generated and detected by the organism has evolved over thousands of years and thus the potential advantages of translating this system into a fully functional communication system has opened new avenues in the area of multi-disciplinary research. This formed the basis of the Biosynthetic Infochemical Communications project – iCHEM whose central aim was to develop a new class of communication technology based on the biosynthesis pathways of the moth, S. littoralis. This novel infochemical communication system would consist of a “chemoemitter” unit which would generate a precise mix of infochemicals which after travelling through the odour space would be detected by a complementary tuned detector – the “chemoreceiver” unit comprising of a ligand specific detection element and an associated biophysical model functioning similar to the antennal lobe neuron of the moth. This combined novel system will have the capability of communicating by the help of chemicals only, in the vapour or liquid phase. For the work presented in this thesis, the novel concept of infochemical communication has been examined in the vapour and liquid phase by employing piezoelectric microsensors. This has been achieved and demonstrated throughout the thesis by employing chemo-specific acoustic wave microsensors. For vapour phase assessment, quartz crystal microbalance, were coated with different organic polymer coatings and incorporated in a prototype infochemical communication system detecting encoded volatiles. For liquid phase assessment, shear horizontal surface acoustic wave (SH-SAW) microsensors were specifically designed and immobilised within Sf9 insect cells. This GPCR based whole cell biosensing system was then employed to detect ligand specific activations thus acting as a precursor to the development of a fully functionalised OR based signalling system, thus contributing to the growing field of communication and labelling technology.

This thesis analyses the behaviour of electromagnetic flowmeters in which the flowrate of a suitably conducting liquid is measured by means of the e.m.f. induced between two electrodes by the motion in a magnetic field. The distortion of the field by the induced currents is assumed negligible, thus excluding the more highly conducting liquid metals, but the method is well-known to be suitable for certain liquid metals, water-based liquids and blood. Attention is concentrated entirely on the flowmetering device itself and not on any associated electronics. The idea of the contribution of each element of fluid moving with velocity v in a magnetic field B to the total electrode e.m.f. is first put on a sound mathematical basis by the introduction of a weight vector W = B∧∇G where ∇G is a vector entirely determined by the flowmeter and electrode geometry. The contribution is W.v per unit volume. The ultimate aim of the flowmeter designer is to make the meter sensitivity (the electrode e.m.f. divided by the flowrate) entirely independent of the flow pattern, i.e. of v. The behaviour of a flowmeter is specified entirely by W and the necessary condition on W to achieve this sensitivity independence (an ideal meter) is found to be curl W=0. It is shown that there is no magnetic field that will achieve this if point electrodes are used. The condition on W weakens progressively as more assumptions are made about the flow pattern and weight functions suitable for different types of velocity profile are defined. The effects of different types of electrode are also discussed. This basic theory is then applied, first to a class of ideal uniform field flowmeters with transverse line electrodes. These meters can, moreover, be used as valves and flowmeters simultaneously. Their performance has been experimentally checked. Secondly the theory of long flowmeters (in which all quantities are assumed invariant in the flow direction) is exhaustively discussed. Thirdly the practically important case of circular meters with non-conducting walls, point electrodes and short magnetic fields is examined. In view of the difficulty of manufacturing a magnetic field that may be mathematically desirable, this problem is by-passed by first choosing a technologically simple method of producding magnetic fields for these meters using coils specified by certain parameters and then analysing the relevant weight-function in terms of these parameters. Design tables for the case of rectilinear axisymmetric flow are presented (one specimen), and experimental tests described which check the performance of designs produced from these tables. A different method of producing the magnetic field is suggested which saves power consumption and enables the field to be more accurately designed, but to which the tables mentioned above still apply. Finally some outstanding problems are discussed.

This thesis states a new device and links configuration design for optical wireless communications system applied to indoor use, which is aimed to create a reliable ‘blocking-free’ communication connections. This work firstly investigates the essential characteristics of optical indoor channels, and manages a number of simulations based on a model of scenario for the channel response research. Modulation technique also is discussed and selected by studying and simulating the various modulation schemes for optical wireless communications system design, and the 4-PPM modulation technique in the end is selected as the suitable scheme for this work. Space-time Coding for optical wireless communications is proposed associated with the 4-PPM modulation scheme, which is orthogonal for OMIMO system signal arrangement. The whole scheme of device design and configuration links is described, and the work procedure is also illustrated by an implementation of demonstrator for experiments.

Despite the profound implication of blogs for marketing communications, there are very few theoretical and empirical studies on this emerging social phenomenon. This study is one of the first to investigate how blog users engage with blogs. Specifically it looks at the relationship between perceived interactivity and attitude towards blogs and between perceived interactivity and sponsored posts on blogs. It also looks at how personal factors and blog characteristic factors influences consumers' attitude towards blogs and sponsored posts on blogs. Key literature was reviewed, following which a conceptual framework was developed which was partly based on the advertising hierarchy of effects model and incorporating adaptations of the concepts of need for cognition, technology acceptance model as well as the concepts of perceived interactivity, perceived credibility, attitude towards Websites and avoidance of advertising. 427 completed responses were received out of which 408 responses were used for the initial analysis. After removal of outliers, 399 responses were used for the final analysis. Confirmatory factor analysis was used to refine the constructs and to test for convergent validity, discriminant validity and construct reliability. Structural equation modelling was used to look at the various fit measures and also to empirically test the proposed hypotheses. The results indicate that the higher the interactivity of blogs, the more likely it was for blog users to have a more favourable perception of sponsored posts on the blogs. On the other hand, perceived interactivity was found to be a highly significant and important negative predictor of the avoidance of sponsored posts on blogs. This means that a positive attitude towards blogs does not directly translate into a positive attitude towards sponsored posts on blogs. Using the new typology of mediations and non-mediations as given by Zhao et al. (2010), the importance of the attitude towards blogs construct was also highlighted as it was found to have a mediating effect between perceived interactivity and avoidance of sponsored posts on blogs as well as between perceived credibility and avoidance of sponsored posts on blog constructs. The study also revealed some serious issues with old and widely accepted constructs, specifically, the need for cognition and the technology acceptance model. Moreover, the results show that the need for cognition of blog users; the perceived interactivity and perceived credibility of the blog (and the blogger) are positively related to the attitude towards blogs.