High power electromagnetic pulses are of great importance in a variety of applications such as transient radar, investigations of the effect of strong radio-frequency impulses on electronic systems and modem bio-medical technology. In response to the current trend, a simple, compact, and portable electromagnetic pulse (EMP) radiating source has been developed, based on pulsed transformer technology and capable of producing nanosecond rise-time pulses at voltages exceeding 0.5 MY. For this type of application pulsed transformer technology offers a number of significant advantages over the use of a Marx generator, e.g. design simplicity, compactness and cost effectiveness. The transformer is operated in a dual resonance mode to achieve a high energy transfer efficiency, and although the output voltage inevitably has a slower rise-time than that of a Marx generator, this can be improved by the use of a pulse forming line in conjunction with a fast spark-gap switch. The transformer design is best achieved using a filamentary modeling technique, that takes full account of bulk skin and proximity effects and accurately predicts the self and mutual inductances and winding resistances of the transformer. One main objective of the present research was to achieve a high-average radiated power, for which the radiator has to be operated at a high pulse repetition frequency (pRF), with the key component for achieving this being the spark-gap switch in the primary circuit of the pulsed transformer. Normally a spark-gap switch has a recovery time of about ten milliseconds, and a PRF above 100 Hz is difficult to achieve unless certain special techniques are employed. As the aim of the present study is to develop a compact system, the use of a pump for providing a fluid flow between the electrodes of the spark gap is ruled out, and a novel spark-gap switch was therefore developed based on the principle of corona-stabilization. For simplicity, an omnidirectional dipole-type structure was used as a transmitting antenna. Radiated electric field measurements were performed using a time-derivative sensor, with data being collected by a suitable fast digitizing oscilloscope. Post-numerical processing of the collected data was necessary to remove the ground reflected wave effect. Measurements of the radiated electric field at 10 m from the radiating element indicated a peak amplitude of 12.4 kV/m. Much of the work detailed in the thesis has already been presented in peer reviewed journals and at prestigious international conferences.

This thesis considers wearable antennas that are useful for body-centric communication systems. Novel wearable printed monopoles with flexible neoprene substrates and drapable conductive elements have been designed, synthesized and measured with respect to their on-body performance. Starting with a comprehensive literature review of wearable antennas this work contains an introduction to wearable antenna designs, flexible materials for wearable antenna fabrication, human body models and the impacts of the human body on the efficiency of small wearable antennas. Definitions of material effective and total conductivity, the calculations of antenna Q and mutual couplings between antennas and the human body using the Method of Moment (MoM) are presented. Four types of flexible printed monopoles have been designed and measured. They are two single band monopoles for ISM (433.05 434.79 MHz) service, one multiband monopole for GSM 900 (890 960 MHz), DCS (1710 1880 MHz), PCS (1850 1990 MHz), UMTS (1920 2170 MHz), and WLAN2.4GHz (2400-2484MHz) frequency bands and a UWB band antenna (3.1-10.6GHz) respectively. Effects of the ground plane dimensions on printed monopoles are illustrated first by changing the dimensions thereof and subsequently by adding wing structures. The new designs yield improved impedance match for printed monopoles. It also shows how meander lines can used to miniaturize antennas and add additional resonances. Models of the human body were created in Microstripes, a 3D electromagnetic (EM) simulator, to analysis the impacts of the human body on the performance of the wearable antennas mentioned above.

Energy harvesting; that is, harvesting small amounts of energy from environmental sources such as solar, air flow or vibrations using small-scale (≈1cm 3 ) devices, offers the prospect of powering portable electronic devices such as GPS receivers and mobile phones, and sensing devices used in remote applications: wireless sensor nodes, without the use of batteries. Numerous studies have shown that power densities of energy harvesting devices can be hundreds of µW; however the literature also reveals that power requirements of many electronic devices are in the mW range. Therefore, a key challenge for the successful deployment of energy harvesting technology remains, in many cases, the provision of adequate power. This thesis aims to address this challenge by investigating two methods of enhancing the power output of a piezoelectric-based vibration energy harvesting device. Cont/d.

Active ow control can be used to improve the aerodynamic e ciency of passenger aircraft, road tra c and wind turbines amongst other applications. This work describes the fabrication of an ultra-compact microvalve that has been designed as part of an active aerodynamic ow control system that generates airjets of a scale and velocity that have been shown to have desirable e ects on the macroscopic air ow. The design of the microvalve is based on criteria speci ed by the requirements of active ow control and the piezoelectric bimorph actuator which opens and closes the valve outlet. Cont/d.

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The preparation and characterisation of conductive nanowires and nanoropes on DNA templates is the focus of this work. This work is motivated by the search for alternative bottom-up approaches to nanoscale electronics. The structures and composition of prepared nanowires were determined by Fourier transform infrared (FTIR) spectroscopy, ultraviolet-visible (UV-Vis) absorption spectroscopy, X-ray diffraction (XRD) and X-ray photoelectron (XPS) spectroscopy. The morphology and physical properties were examined by atomic force microscopy (AFM), electrostatic force microscopy (EFM), conductive atomic force microscopy (C-AFM) and two-terminal current-voltage (I-V) measurements using microelectrodes fabricated by photolithography techniques. Polypyrrole (PPy) nanoropes were formed by chemical polymerization of pyrrole on a DNA template. The diameter of these 'nanoropes' was between 5–30 nm. At room temperature, the conductivity of a PPy-DNA nanorope is confirmed by C-AFM, while the temperature dependence of the conductivity was observed to follow a simple Arrhenius behaviour with a characteristic temperature of T0 = 4000K. Silver (Ag) nanoparticle chains were grown along λ-DNA templates using Tollens' reagent under mild conditions (50 ºC, 10 min). UV–Vis spectroscopy of these nanowires exhibited an absorption band at 400-440 nm due to the Ag plasmon. The DNA-templated Ag wires were found to be coated with an oxidized shell. Unlike DNA-templated polypyrrole, these nanowires were often rough. Using metal-binding functionality (alkynyl) introduced into DNA-templated polymer nanowires by chemical modification of the monomer, the morphology of the wire was improved on this hybrid template. Finally, DNA was used as a template for the growth of cuprous oxide (Cu2O) nanowires using Benedict's reagent and ascorbic acid as reducing agent at room temperature. AFM showed that these nanowires are uniform and continuous (diameters of 5-30 nm and lengths of 5-16 μm). C-AFM revealed that the average resistance of Cu2O–DNA nanowires was in the range from 0.13-0.18 MΩ depending on deflection setpoints, which suggests the material is doped. In summary, a range of inorganic and organic materials can be templated on DNA in the form of nanowires and various methods for measuring the conductivity of these nanowires have been established.

This thesis reports the design and implementation of different experimental systems to study electron-dynamics in two-dimensional electron gases (2DEGs) at microwave and terahertz (THz) frequencies. Both time and frequency domain techniques have been used to study the transport properties of electrons in GaAs/Al- GaAs heterostructures. First of all a free-space time-domain spectroscopy (TDS) polarisation sensitive detection system capable of measuring the full polarization state of a propagating THz electric field has been demonstrated. Polarization- sensitive systems provide additional information on material characterization that is inaccessible through conventional techniques. The polarization sensitivity of the THz-TDS system has been evaluated using a 0.5mm thick LiNbO3 sample. The ability to measure the rotation in polarisation caused by a sample allows measurement of direction-dependent material properties, such as the electrical conductivity of low-dimensional semiconductors displaying the quantum Hall effect, or the di-electric permitivity in anisotropic material systems. The newly designed system has been used to evaluate the magneto-conductivity of a 2DEG up to 1.4 THz and 6T. Two separate continuous-wave polarisation-sensitive free-space systems, operating at microwave and THz frequencies, have also been demonstrated. Both systems have been used to study the magneto-conductivity of a 2DEG at microwave (75{110 GHz) and THz (2.6{3.2 THz) frequencies up to a magnetic field of 8T. The continuous-wave microwave system has also been used to study the phenomenon of edge magneto-plasmons in a 2DEG. The work presented in this thesis provides a valuable groundwork to study the electron-dynamics in 2DEGs over a wide range of frequencies (both microwave and THz) and for a wide range of experimental conditions.

Thermal management is one of the main issues which must be overcome in order to maintain the continued reduction of feature sizes in silicon microelectronics. The new materials and device architectures which are used to achieve better performance have had a detrimental effect on thermal management due to higher thermal resistances and additional thermal interfaces. Current thermal models, based on diffusive continuum flow, are inaccurate at nanoscale dimensions and a model which considers the microscopic nature of heat generation and heat tansport must be used. On the microscale, heat transport can be described by the Boltzmann transport equation for phonons. The complexity and quantity of the phonon anharmonic interactions make a direct numerical solution difficult without numerous approximations. The Monte Carlo technique has been established as an accurate approach to the modelling of charge transport in semiconductors and the application of this method to phonon transport is a logical choice. There has been little reported work on Monte Carlo simulations of phonon transport. This thesis examines previous work on non-momentum conservation Monte Carlo simulations, exposing the shortcomings of this approach and attempting to rectify them with algorithmic changes as well as the use of relaxation times calculated using second and third order elastic constants. These changes result in an improvement of the equilibrium simulation as well as producing more realistic thermal gradients for both large and small temperature differences at steady state. The improved techniques are applied to nanowire geometries and found to provide the correct trends. The work is extended further to produce a momentum conservation Monte Carlo simulation and used to simulate the anharmonic three-phonon processes (both absorption and emission type) for acoustic phonon modes in silicon. Phonon-phonon absorption events are performed by selecting a partner phonon from within the same real space cell that satisfies momentum and energy conservation. In previous derivations of analytical approximations for phonon lifetimes and thermal conductivities, it has been difficult to determine the relative contribution of Normal and Umklapp processes in phonon-phonon interactions; this information can be extracted directly from the Monte Carlo simulation. Some success is achieved with this method at low temperatures and it is shown that with higher computational power, this simulation could provide a very accurate model for heat transport.

No description available.

No description available.