Philosophiae Doctor - PhD / Nanocrystalline silicon (nc-Si:H) is an interesting type of silicon with superior electrical properties that are more stable compared to amorphous silicon (a-Si:H). Silicon nitride (SiNₓ) thin films are currently the dielectric widely applied in the microelectronics industry and are also effective antireflective and passivating layers for multicrystalline silicon solar cells. Research into the synthesis and characterization of nc-Si:H and SiNₓ thin films is vital from a renewable energy aspect. In this thesis we investigated the film growth mechanisms and the filament stability during the hot-wire chemical vapour deposition (HWCVD) of nc-Si:H and SiNₓ thin films. During the HWCVD of nc-Si:H, electron backscatter diffraction (EBSD) revealed that the tantalum (Ta) filament aged to consists of a recrystallized Ta-core with Ta-rich silicides at the hotter centre regions and Si-rich Ta-silicides at the cooler ends nearer to the electrical contacts. The growth of nc-Si:H by HWCVD is controlled by surface reactions before and beyond the transition from a-Si:H to nc-Si:H. During the transition, the diffusion of hydrogen (H) within the film is proposed to be the reaction controlling step. The deposition pressure influenced the structural, mechanical and optical properties of nc-Si:H films mostly when the film thickness is below 250 nm. The film stress, optical band gap, refractive index and crystalline volume fraction approached similar values at longer deposition times irrespective of the deposition pressure. Filament degradation occurred during the HWCVD of SiNₓ thin films from low total flow rate SiH₄ / ammonia (NH₃) / H₂ gas mixture. Similar to the HWCVD of nc-Si:H, the Ta-core recrystallized and silicides formed around the perimeter. However, Tanitrides formed within the filament bulk. The extent of nitride and silicide formation, porosity and cracks were all enhanced at the hotter centre regions, where filament failure eventually occurred. We also applied HWCVD to deposit transparent, low reflective and hydrogen containing SiNₓ thin films at total gas flow rates less than 31 sccm with NH₃ flow rates as low as 3 sccm. Fluctuations within the SiNₓ thin film growth rates were attributed to the depletion of growth species (Si, N, and H) from the ambient and their incorporation within the filament during its degradation.

Radar backscatter intensity data from the ERS-1 and ERS-2 scatterometers are compared with daily rainfall data in two areas in the Free State province of South Africa. Knowledge of the relation between daily rainfall data and ERS C-band scatterometer data for a specific area can be useful to make reliable soil moisture measurements. The assumption is made that an increase in rainfall will lead to higher radar backscatter data values. This is based on the fact that moisture increases the dielectric properties of surfaces. This leads to higher backscatter intensities when incident radar energy is reflected back to the sensor. Various techniques are used to study the relationship between daily rainfall data and ERS scattrerometer data. It includes correlations, interpolations, visual interpretations, statistical analysis, and a simple model. Weak positive correlations were found between radar and rainfall data in arid areas. This is supported by literature regarding the Sahel. No correlation was found in agricultural areas receiving more rainfall. Vegetation also increases radar backscatter intensities, even in the absence of rain. There is thus a relationship between rainfall and radar data but it is more visible in arid areas and over longer periods of time. / Dissertation (MA)--University of Pretoria, 2008. / Geography, Geoinformatics and Meteorology / MA / Unrestricted

Ultrasound imaging is a ubiquitous part of the modern medical diagnostics toolbox. It has widespread applications to many areas of medicine, including angiology, cardiology, nephrology, urology, and obstetrics. It is often preferred over other imaging modalities, such as x-ray computed tomography (CAT) and magnetic resonance imaging (MRI) because it is non-invasive, non-ionizing, inexpensive, and has excellent penetration depth in the body. The design, optimization, and manufacturing of ultrasound transducers used in ultrasound imaging is a challenging engineering problem. Faced with a variety of different imaging environments, ultrasound transducers must often be optimized for performance in very specific applications. This is especially true for catheter-based solutions, such as intracardiac and intravascular ultrasound, where imaging performance is strongly dependent on the strength of backscatter from tissue due to significant limitations in device size, electronics, and signal-to-noise ratio. Currently, there is a need for the accurate and fast simulation of the imaging process used in ultrasound imaging, including the ability to capture the effects of backscatter from a variety of different tissues. This thesis discusses the development of simulation tools for the quantitative simulation of tissue backscatter and blood motion from acoustic fields coupled to spatial array transducers, based on an application of the Rayleigh speckle model to the linear systems model for acoustic diffraction from spatial array transducers. These simulation tools have potential applications in the field of medical ultrasonics, with particular attention to the areas of transducer design and optimization, beamforming and array processing, and image reconstruction. We demonstrate how the simulation tools developed here can be used to characterize array imaging performance and to investigate reconstruction performance of common flow algorithms for Doppler ultrasound imaging.

This thesis provides a new explanation for the behaviour of radar backscatter of forests using vegetation structure models from the field of macroecology. The forests modelled in this work are produced using allometry-based ecological models with backscatter derived from the parameterisation of a radiative transfer model. This work is produced as a series of papers, each portraying the importance of macroecology in defining the forest radar response. Each contribution does so by incorporating structural and dynamic effects of forest growth using one of two allometric models to expose variations in backscatter as a response to vertical and horizontal forest profiles. The major findings of these studies concern the origin of backscatter saturation effects from forest SAR surveys. In each work the importance of transition from Rayleigh to Optical scattering, combined with the scaling effects of forest structure, is emphasised. These findings are administered through evidence including the transition’s emergence as the region of dominant backscatter in a vertical profile (according to a dominant canopy scattering layer), also through the existence of a two trend backscatter relationship with volume in the shape of the typical “saturation curve” (in the absence of additional attenuating factors). The importance of scattering regime change is also demonstrated through the relationships with volume, basal area and thinning. This work’s findings are reinforced by the examination of the relationships between forest height and volume, as collective values, providing evidence to suggest the non-uniqueness of volume-toheight relationships. Each of the studies refer to growing forest communities not single trees, so that unlike typical studies of radar remote sensing of forests the impact of the macroecological structural aspects are more explicit. This study emphasises the importance of the overall forest structure in producing SAR backscatter and how backscatter is not solely influenced by electrical properties of scatteres or the singular aspects of a tree but also by the collective forest parameters defining a dynamically changing forest.

Observations of the bottom boundary layer on the inner West Florida Shelf were made with a downward looking pulse coherent acoustic Doppler profiler throughout the passage of Tropical Storm Isadore during September 2002. The storm passed through the Gulf of Mexico roughly 780 km offshore of the Florida study site. Significant wave heights ranged from 0 m to 2.5 m within a span of eight days. The excellent, non-invasive, 5 cm resolution of the near bed (bottom meter) mean flows were used to estimate bed shear velocity and bottom roughness using the standard log-layer approach. A unique opportunity to examine boundary layer structure was provided by the high-resolution data. Calculated friction velocity due to currents (u*c) and apparent bottom roughness (z0) reduced considerably when velocity measurements closer to the bed were emphasized. This observation may be indicative of segmentation within the bottom boundary layer and has implications for common practices of estimating bed shear stress measurements from distances greater than a few tens of centimeters above the bed. Acoustic backscatter strength was used as a proxy for sediment suspension in the water column revealing no relationship between current parameters and sediment resuspension during the ten-day data set. Wave effects were included following the work of Grant and Madsen and others with strong relationships between wave and wave-current parameters and the ABS as a proxy for sediment resuspension evident.

Depuis quelques années le nouveau paradigme d'internet des objets, qui se traduit par la connexion des objets de la vie quotidienne à internet grâce à des dispositifs électroniques intégrés, est devenu plus qu'un simple concept et a commencé à entrer dans la réalité. Il en est ainsi des systèmes d'identification par radiofréquence (RFID), qui font partie des technologies disponibles. Lorsque le coût et la taille des tags deviennent des exigences prépondérantes, l'adoption de tags (semi-)passifs fondés sur la modulation d'un signal rétrodiffusé représente une solution attractive. En outre, l'utilisation de techniques ULB apporte des avantages propres à favoriser le remplacement d'une partie des systèmes RFID actuels qui souffrent de certains défauts tels qu'une trop forte consommation ou une faible robustesse en présence d'interférence. La thèse de doctorat se place dans ce contexte: elle a pour objectif l'étude des systèmes ULB RFID semi-passifs et se situe dans le cadre du projet européen SELECT, qui vise à développer et à évaluer un tel système. La prise de conscience qu'il était fondamental de comprendre les mécanismes électromagnétiques intrinsèques à la communication lecteur-tag, préalablement à la conception du système, s'est traduite par la mise en oeuvre d'une collaboration franco-italienne dans le cadre d'une cotutelle de thèse. C'est donc au départ autour d'une analyse fine et d'une modélisation des aspects antennaires et du canal radio entre le lecteur et le tag que la 1re partie du travail s'est construite. La 2nde partie a exploité ces travaux pour l'élaboration des schémas de modulation et de codage du lecteur et du tag pour les systèmes semi-passifs.

The ecological effects and mechanistic efficiency of chemical oil spill countermeasures must be evaluated prior to their ethical application during real spill response scenarios. Equally important is the ability to monitor the effectiveness of any spill response in real time, permitting informed response management. In-situ sensors are key components of such event based monitoring and continuous monitoring programs. This project investigates crude oil toxicity as a particulate suspension, suitability of in-situ instrumentation to measure crude oil suspensions, and the applicability of using acoustic backscatter to measure suspended solids and sub-surface oil droplet suspension concentrations. The ecological effects to inter- and sub-tidal sediment dwelling organisms exposed to crude oil, both treated with a chemical dispersant and un-treated, was evaluated. Elevated toxicity, expressed as percent mortality and reduced luminescence, and oil concentrations were observed in inter-tidal sediments receiving oil only treatments compared to oil-plus-dispersant treatments. Sub-tidal sediments showed heterogeneous distribution of crude oil with elevated amphipod mortality compared to no oil controls suggesting an oil-sediment aggregation mechanism. A separate laboratory scale study found that the soluble crude oil fractions were responsible for the observed mortality in pelagic species while the more dominant oil droplet fractions were relatively non-toxic. Subsequent studies focused on the in-situ detection of crude oil and particle suspensions in aqueous environments. The first showed that both in-situ fluorescence spectroscopy and Laser In-Situ Scattering Transmissometry (LISST) can effectively measure crude oil concentrations in aqueous environments. The applicability of the LISST implies that crude oil in an aqueous medium can be measured as a particle suspension. Acoustic backscatter (ABS) was investigated for its applicability as a surrogate measurement technology for aqueous particle suspensions. This study showed a log linear correlation between ABS and volume concentration (VC) over a variable particle size distribution. This correlation is due to the dependency of both ABS and VC to the particle size distribution. Log-linear ABS responses to oil-droplet suspension volume concentrations were also demonstrated. However, the inability to reproduce response factors suggests that more work is required to produce viable calibrations that may be used for sub-surface oil plume detection.

Since the inception of the backscatter-radio technology, this field has continually evolved. As a result, this technology is used for a multitude of applications like personnel identification, logistics and assets management and military purposes etc. Radio Frequency Identification (RFID) technology works in several ISM-frequency bands. This work pertains to the design and development of an RF tag that uses 5.8 GHz ISM band for backscatter. This frequency band has many inherent advantages like higher gain antennas, smaller sized tags, increased immunity to conductive object losses, and larger RF bandwidth. The objective of this research is design and prototype development of an RF tag capable of sensing acceleration, angular motion, and shock experienced by an object on which it is installed. The sensed information is modulated onto an incident continuous wave (CW) and backscattered to the reader. Literature research suggested that such work has not been done previously using an RFID platform. The challenges include integration of the sensor, antenna and other electronics to efficiently backscatter the information to the receiver, designing a suitable planar antenna, realtime backscattering of the sensed information, and low power consumption. As a further step, it is required to design and integrate two antennas on RF tag to simultaneously backscatter the same information; and to measure and compare its effect with single antenna tag. The sensed impact/shock and rotational movement information from the inertial sensors (accelerometer and gyroscope) was backscattered instantly and displayed on the custom developed graphical user interface. The development of GUI was not part of this project and was developed by another lab member. RF Tags with single and dual antenna configurations were designed and tested. It was observed that by increasing the number of antennas higher read range can be achieved. Moreover, by doubling the antennas the radar cross-section for the tag was approximately doubled.

Estimating with precision coastal marine properties such as primary production, particulate and dissolved carbon, and red tide concentrations is a challenging but important part of marine research. It benefits not only the local communities, but also provides an important input to various global biogeochemical modeling efforts. Due to the complexity of coastal environments resulting from temporal variability of tidal and riverine influences, it is useful to develop and deploy an automated sensor network that provides real-time feedback. It can be used to validate remote sensing models to retrieve in-water constituents, and provide calibration and validation for atmospheric correction of satellite sensors. For turbid waters, satellite observations in the infrared part of the spectrum can not be used to estimate atmospheric aerosol concentration because the water is not black as is found for clearer waters. This research contribution introduces a modeling effort for a turbid coastal harbor area using a semi-analytical hyperspectral remote sensing algorithm for Case 2 waters to process data from the Autonomous Marine Optical System (AMOS). Retrieved results are then compared with field sample measurements showing satisfactory closure between measurements and theory. A time series of AMOS data over a one-month time span is examined, revealing significant variations in biological activity. A sensitivity analysis of the model is performed to expose the limitations and possible improvements to AMOS measurements in the future.

The ability to accurately predict dose from electron backscatter created by internal lead shielding utilized during various superficial electron beam treatments (EBT), such as lip carcinoma, is required to avoid the possibility of an overdose. Methods for predicting this dose include the use of empirical equations or physically measuring the electron backscatter factor (EBF) and upstream electron backscatter intensity (EBI). The EBF and upstream EBI are defined as the ratio of dose at, or upstream, from the shielding interface with and without the shielding present respectively. The accuracy of these equations for the local treatment machines was recognised as an area that required verification; in addition the ability of XiO's electron Monte Carlo (eMC) treatment planning algorithm to handle lead interfaces was examined. A Monte Carlo simulation using the EGSnrc package of a Siemens Artiste Linac was developed for 6, 9, 12, and 15 MeV electron energies and was verified against physical measurements to within an accuracy of 2 % and 2 mm. Electron backscatter dose distributions were predicated using the MC model, Gafchromic film, and XiO eMC, which when compared showed that XiO's eMC could not accurately calculate dose at the lead interface. Several MC simulations of lead interfaces at different depths, corresponding to energies of 0.2-14 MeV at the interfaces, were used to validate the accuracy of the equations, with the results concluding that the equation could not accurately predict EBF and EBI values, especially at low energies. From this data, an equation was derived to allow estimation of the EBF and upstream EBI, which agreed to within 1.3 % for the EBF values and can predict the upstream EBI to a clinically acceptable level for all energies.