The increasing human population, industrialization, urbanisation and climate change challenges have resulted in an increased demand for already scarce water resources. This has left the agricultural sector with less water for production. Sustainable water management strategies would therefore require accurate determination of water-use. In agriculture, water-use can best be determined from total evaporation which is the loss of water from soil and vegetation to the atmosphere. Accurate quantification of total evaporation from vegetation would require a thorough understanding of water transport processes between vegetation and the atmosphere, especially in a water-scarce country like South Africa. Several methods for estimating total evaporation have been developed and are in use today. Some of the common methods used today are: the Bowen ratio energy balance, eddy covariance, scintillometry, flux variance and surface renewal. However, various methods have advantages and disadvantages. Considerations include the cost of equipment and level of skill required for use of some of the methods. A number of methods involve indirect or direct estimation of sensible heat flux then calculating latent energy flux and hence total evaporation as a residual of the shortened energy balance equation. The main objective of this study is to determine the effects of grassland management practices on the energy balance components as well as on the surface radiation balance. Eddy covariance and surface renewal methods were employed to investigate the effects of grassland management practices (mowing and burning) on the micrometeorology of naturally occurring grassland. A 4.5-ha grassland site (Ukulinga, Pietermaritzburg, South Africa) was divided into two halves: one area was initially mowed (cut-grass site) to a height of 0.1 m while the other was not mowed (tall-grass site). The tall-grass site was later treated by burning and hence referred to as the burnt-grass site. Two eddy covariance systems were deployed, one at each of the cut-grass and the tall-grass sites. The systems each comprised a three-dimensional sonic anemometer to measure high frequency sonic temperature, orthogonal wind speeds and directions and the eddy covariance sensible heat flux (W m-2). Latent energy flux, from which total evaporation was then determined, was calculated as a residual from the shortened energy balance equation from measurements of sensible heat flux, net irradiance and soil heat flux assuming closure is met. Other microclimatic measurements of soil water content, soil temperature, surface reflection coefficient and reflected solar irradiance were performed, the latter with a four-component net radiometer. An automatic weather station was also set up at the research site for continuous measurements of solar irradiance, air temperature, relative humidity, wind speed and direction and rainfall. Water vapour pressure and grass reference evaporation were also determined online. Energy fluxes from the tall-grass site were measured from March to June 2008. Greater total evaporation rates (2.27 mm day-1) were observed at the beginning of the experiment (March). As winter approached most of the energy balance components showed a constant decreasing trend and the average total evaporation rates for May and June were 1.03 and 0.62 mm day-1, respectively. The tall-grass site had consistently lower soil temperatures that changed diurnally when compared to the cut-grass site. The soil water content at both sites showed no significant differences. Most of the energy balance components were similar between the two sites and changed diurnally. Although there were small differences observed between other energy balance components, for example, latent energy flux was slightly greater for the tall-grass site than for the cut-grass site. The tall-grass site had more basal cover and this may have contributed to the differences in temperature regimes observed between the two sites. However, the plants growing at the cut-grass site showed more vigour than the ones at the tall-grass site as spring approached. Burning of a mixed grassland surface caused significant changes to most of the optical properties and energy fluxes of the surface. Following burning, the soil temperature was elevated to noticeable levels due to removal of basal cover by burning. The surface reflection coefficient measured before and after the burn also presented a remarkable change. The surface reflection coefficient was significantly reduced after the burn but a progressive increase was observed as the burnt grass recovered after the spell of spring rains. The energy fluxes: net irradiance, latent energy flux and soil heat flux also increased following the burn but the latent energy flux was reduced as transpiration was effectively eliminated by the burning of all actively transpiring leaves. As a result, the main process that contributed towards latent energy flux was soil evaporation. An ideal surface renewal analysis model based on two air temperature structure functions was used to estimate sensible heat flux over natural grassland treated by mowing. Two air temperature lag times r (0.4 and 0.8 s) were used when computing the air temperature structure functions online. The surface renewal sensible heat fluxes were computed using an iteration process in Excel. The fluxes, obtained using an iterative procedure, were calibrated to determine the surface renewal weighting factor (a) and then validated against the eddy covariance method using different data sets for unstable conditions during 2008. The latent energy flux was computed as a residual from the shortened energy balance equation. The surface renewal weighting factor was determined for each of the two heights and two lag times for each measurement height (z) above the soil surface. The a values obtained during the surface renewal calibration period (day of year 223 to 242, 2008) ranged from 1.90 to 2.26 for measurement height 0.7 m and r = 0.4 and 0.8 s. For a measurement height of 1.2 m and r = 0.4 and 0.8 s, a values of 0.71 and 1.01 were obtained, respectively. Good agreement between surface renewal sensible heat flux and eddy covariance sensible heat flux was obtained at a height of 1.2 m using a = 0.71 and a lag time of 0.4 s. Total evaporation for the surface renewal method was compared against the eddy covariance method. The surface renewal method, for a height of 1.2 m and a lag time of 0.4 s, yielded 1.67 mm while the eddy covariance method yielded 1.57 mm for a typical cloudless day. For the same day for a measurement height of 1.2 m and a lag time of 0.8 s, eddy covariance and surface renewal methods yielded 1.57 and 1.10 mm, respectively. For a lag time of 0.4 s, the surface renewal method overestimated total evaporation by 0.10 mm while for a lag time of 0.8 s, the total evaporation was underestimated by 0.47 mm. As a result, the surface renewal method performed better for z = 1.2 m and a lag time of 0.4 s. The eddy covariance method gave reliable sensible heat fluxes throughout the experiment and this allowed a comparison of fluxes across all treatment areas to be achieved. The short-term analysis of the surface renewal method also gave reliable energy fluxes after calibration. Compared to the eddy covariance method, the surface renewal method is more attractive in the sense that it is easy to operate and use and it is relatively cheap. However, the surface renewal method requires calibration and validation against a standard method such as the eddy covariance method. This study showed that grassland management practices had a considerable effect on surface radiation and energy balance of the mowed and burnt treatment sites. Total evaporation was mainly controlled by the available energy flux, rainfall and grassland surface structure. High total evaporation values were observed during summer when net irradiance was at its highest and grass growth at its peak. Low total evaporation values were observed in winter (dry atmospheric conditions) when net irradiance was at its lowest and most vegetation was dormant. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2011.

Recent studies suggest that integration of photovoltaic panels with green roofs may improve the performance of both. While vegetation may provide a benefit by reducing the net radiation load on the underside of the photovoltaic (PV) panels, it may also affect convective cooling of panels, and consequently, panel efficiency. Both effects likely diminish with the height of the PV panel above the roof, although placing PV panels too close to the vegetation increases the risk of the plants growing over the edges of, and shading the PV panel. There is a gap in the literature with respect to evaluating these competing effects. The present study aims to fill this gap. Experiments were conducted over a two-month period during summer using two identical PV panels within an array of rooftop-mounted panels. These experiments were performed at two heights (18 cm and 24 cm) using three roofing types: white, black and green (vegetated). Results showed that the mean power output of the system in which the PV panel was mounted above a green roof was 1.2% and 0.8% higher than that of the PV-black roof and the PV-white roof at the 18 cm height. At the 24 cm height, the benefit of the green roof was slightly diminished with power output for the PV panel above a green roof being 1.0% and 0.7% higher than the black and white roof experiments, respectively. These power output results were consistent with measured variations in mean panel surface temperatures; the green roof systems were generally cooler by 1.5˚C to 3˚C. The panel surface mean heat transfer coefficients for the PV-green roof were generally 10 to 23% higher than for the white and black roof configurations, suggesting a mixing benefit associated with the roughness of the plant canopy. As expected, the results indicate that the best PV panel performance is obtained by locating the PV panel above a green roof. However, the relative benefits of the roof energy balance diminish with distance between the PV panel and the roof. Moreover, the results of this study showed that the mean power output of the PV panel above the white roof was 0.7% and 0.44% higher than that of the PV panel above the black roof at 18 cm and 24 cm heights, respectively. The results of the power output differences in all the experiments were statistically significant at the 95% confidence interval (P < 0.01).

This study reports on the physical and optical characterization of scandium oxide thin films. Thin films of scandium oxide, 20-40 nm thick, were deposited on silicon wafers, quartz slides, and silicon photodiodes by reactively sputtering scandium in an oxygen environment. These samples were characterized using ellipsometry, high-resolution transmission electron microscopy, scanning transmission electron microscopy, and energy dispersive x-ray analysis. A 28.46 nm thick scandium oxide thin film was measured in the Extreme Ultraviolet (EUV) from 2.7 to 50 nm (459.3 to 24.8 eV) using synchrotron radiation at the Advanced Light Source Beamline 6.3.2 at the Lawrence Berkeley National Laboratory. In these measurements, a new method for data collection was used, in which the reflection and transmission data were collected simultaneously. Analysis of the EUV reflection and transmission data was performed using a front-side reflection, matrix-multiplication technique, which is novel among EUV analytical practice. During data analysis, a new weighting scheme was used, named "adaptive weighting". This analysis provides the first experimentally determined optical constants n and k for scandium oxide thin films from 4.5-30 nm. Also, the positions of the L2 and L3 electronic transitions of scandium oxide have been measured, at 3.069 and 3.101 nm (404.0 and 399.9 eV), respectively, while the measurements near the M transition suggest it to be at approximately 31.5 nm (39.4 eV). Comparing the electronic transition positions of scandium oxide to those of scandium show that the oxidation of scandium shifts the positions to lower energies. For L2 the shift is about 1.8 eV, for L3 the shift is about 1.4 eV, and for M the shift is about 1.9 eV. The binding energies of scandium oxide are greater than those of scandium, as is expected for an oxide compared to its parent metal. This trend in the shift of the transition positions is unexpected, and warrants further investigation.

Les matériaux absorbants des ondes électromagnétiques sont des éléments importants pour l'évaluation de nombreux systèmes électroniques militaires mais également civils. Ces matériaux sont utilisés, par exemple, pour la réduction des interférences électromagnétiques (EMI) dans divers composants sans fils, la réduction de la surface équivalente radar (SER) ou comme absorbants à l'intérieur des chambres de mesures. C’est cette dernière application qui est visée par les travaux de cette thèse. L’objectif de mes travaux de thèse est d’optimiser des matériaux absorbants utilisés dans les chambres anéchoïques. La géométrie et la composition du matériau absorbant sont les deux paramètres qui influencent la capacité d’absorption de l’onde électromagnétique par un matériau. Ce seront donc les deux pistes d’optimisation explorés durant cette thèse. Notre but est d’obtenir les absorbants présentant les plus faibles coefficients de réflexion et de transmission, soit une absorption élevée, ceci dans une large bande de fréquence. / The electromagnetic absorber materials are important elements for evaluating various electronic and civil systems. These materials are used, for example, for minimizing electromagnetic interferences (EMI) in different wireless components, for minimizing the radar cross section (RCS) or for usage in anechoic chambers. The latter application is the targeted work in this thesis. The objective of this work is to optimize the absorber materials used in anechoic chambers. The geometry and the material composition are the two parameters that influence the absorption of the electromagnetic wave inside the material itself. This are the two topics of optimization explored in this thesis. Our objective is to obtain material absorbers that have low reflection and transmission coefficients and high absorption in a large frequency band.

Recently, remarkable advances have been made in the understanding of micro/nanoscale energy transport, opening new opportunities in various areas such as thermal management, data storage, and energy conversion. This dissertation focuses on thermally-sensed nanotopography using a heated silicon microcantilever and near-field thermophotovoltaic (TPV) energy conversion system. A heated microcantilever is a functionalized atomic force microscope (AFM) cantilever that has a small resistive heater integrated at the free end. Besides its capability of increasing the heater temperature over 1,000 K, the resistance of a heated cantilever is a very sensitive function of temperature, suggesting that the heated cantilever can be used as a highly sensitive thermal metrology tool. The first part of the dissertation discusses the thermal characterization of the heated microcantilever for its usage as a thermal sensor in various conditions. Particularly, the use of heated cantilevers for tapping-mode topography imaging will be presented, along with the recent experimental results on the thermal interaction between the cantilever and substrate. In the second part of the dissertation, the so-called near-field TPV device is introduced. This new type of energy conversion system utilizes the significant enhancement of radiative energy transport due to photon tunneling and surface polaritons. Investigation of surface and bulk polaritons in a multilayered structure reveals that radiative properties are significantly affected by polariton excitations. The dissertation then addresses the rigorous performance analysis of the near-field TPV system and a novel design of a near-field TPV device.

Les transducteurs ultrasonores capacitifs micro-usinés (cMUT : capacitive Micromachined Ultrasound Transducers) apparaissent, au vu de leur maturité croissante, comme une alternative de plus en plus viable à la technologie piézoélectrique. Caractérisés par une large bande passante et une large directivité, ces transducteurs sont des solutions intéressantes pour le développement de sondes ultrasonores « exotiques » dont les spécifications sont difficilement atteignables en technologie piézoélectrique. C'est dans ce contexte et fort de l'expérience acquise par notre laboratoire sur cette technologie pendant plus d'une dizaine d'années, que s'est inscrit ce travail de thèse. L'originalité du travail rapporté ici est d'aller de l'analyse du comportement général des barrettes cMUT jusqu'à un exemple précis de conception de sonde cMUT pour l'évaluation du tissu osseux. Des outils de modélisation précis et rapides, basés sur l'introduction de conditions de périodicité, ont été développés. Plusieurs modèles ont ainsi été mis en place afin d'adapter la stratégie de modélisation à la topologie du dispositif cMUT à modéliser : cellule isolée, colonne de cellules, matrice de cellules et élément de barrette. Ces modèles ont permis d'étudier le comportement des éléments de barrette cMUT et d'améliorer notre connaissance sur les mécanismes physiques mis en jeu. De cette façon, l'origine des effets de baffle, problème récurrent du comportement des barrettes cMUT, a clairement été interprété par l'intermédiaire d'une analyse modale. Des solutions ont ainsi été identifiées et proposées afin d'optimiser le comportement des barrettes cMUT, de façon à réduire la présence des effets de baffle et à augmenter leur bande passante. Le développement d'une barrette cMUT dédiée à l'évaluation du tissu osseux est présenté dans sa totalité, afin d'illustrer les différents aspects liés à la conception d'une sonde de cette technologie. Un travail original de caractérisation a été réalisé sur cette barrette, afin d'estimer l'homogénéité inter-cellules à l'échelle de l'élément et l'homogénéité inter-éléments à l'échelle de la barrette. Enfin, une confrontation a été réalisée avec une sonde PZT de même topologie sur plusieurs fantômes osseux. Il a ainsi été démontré que la sonde cMUT permettait la détection d'un plus grand nombre de modes guidés, et par conséquent, une meilleure évaluation du tissu osseux. / Following recent advances, the capacitive Micromachined Ultrasound Transducers (cMUT) technology seems to be a good alternative to the piezoelectric technology. For specific applications, the requirements and specifications of the probe are sometimes difficult to obtain with the traditional PZT technology. The cMUT technology, with both large bandwidth and angular directivity, can be an interesting way to overcome these limitations. This PhD has been carried out in this context, in a laboratory which has nearly 10 years of experience in the field of cMUT technology. The originality of the work sustained in this PhD is that it covers the cMUT technology, from general aspects dealing of modeling and characterization up to a complete example of cMUT-based probe applied to the assessment of cortical bone. Fast and accurate modeling tools, based on periodicity conditions, have been developed. Several models have been proposed to match the modeling strategy to the topology of the cMUT array : isolated cell, columns of cells, 2-D matrix of cells and array element. These models have been used to analyze the cMUT array behavior and to understand how mutual couplings between cMUTs impact the response of one element. Origins of the baffle effect, well-known as a recurrent problem in cMUT probe, have been explained using an original method based on the normal mode decomposition of the radiated pressure field. Thus, solutions have been identified and tested to optimize the cMUT frequency response, i.e. to increase the bandwidth, and to suppress parasitic disturbances linked to baffle effect in the electroacoustic response. The development of a dedicated cMUT array for the assessment of bone tissue is accurately detailed in the manuscript, including description of the design rules, fabrication steps and packaging procedure. An original characterization work has been carried out in order to check the device homogeneity, first from cell to cell and then from element to element. Finally, a comparison with a PZT probe with the same topology has been performed on bone mimicking phantom. Nice results has been obtained, showing that cMUT probe allows detecting higher number of guided modes in the cortical shell, and consequently, improving the cortical bone assessment.

Indiana University-Purdue University Indianapolis (IUPUI) / Since 1990s, rechargeable Li-ion batteries have been widely used in consumer electronics such as cell phones, global positioning systems (GPS), personnel digital assistants (PDA), digital cameras, and laptop computers. Recently Li-ion batteries received considerable attention as a major power source for electric vehicles. However, significant technical challenges still exist for widely deploying Li-ion batteries in electric vehicles. For instance, the energy density of Li-ion batteries is not high enough to support a long-distance commute. The Li-ion batteries used for the Nissan Leaf and Chevy Volt only can support 50 – 100 miles per charge. The cost of Li-ion battery packs in electric vehicles is still high. The battery pack for the Chevy Volt costs about $8,000, and the larger one in the Nissan Leaf costs about $12,000. To address these problems, new Li-ion battery electrode materials with high energy density and low cost should be developed. Among Li-ion battery cathode materials, vanadium pentoxide, V2O5, is one of the earliest oxides studied as a cathode for Li-ion batteries because of its low cost, abundance, easy synthesis, and high energy density. However, its practical reversible capacity has been limited due to its irreversible structural change when Li insertion is more than x = 1. Tremendous efforts have been made over the last twenty years to improve the phase reversibility of LixV2O5 (e.g., 0 ≤ x ≤ 2) because of vanadium pentoxides’ potential use as high capacity cathodes in Li-ion batteries. In this thesis, a new strategy was studied to develop vanadium pentoxide cathode materials with improved phase reversibility. The first study is to synthesize vanadium oxide cathodes via a new chemical route – creating a phase transformation from the vanadium sulfide to oxide. The β-Na0.33V2O5 was prepared via a new method of chemical synthesis, involving the chemical transformation of NaVS2 via heat-treatment at 600 °C in atmospheric air. The β-Na0.33V2O5 particles were well crystalized and rod-shaped, measuring 7–15 μm long and 1–3 μm wide with the formation of the crystal defects on the surface of the particles. In contrast to previous reports contained in the literature, Na ions were extracted, without any structural collapse, from the β -Na0.33V2O5 structure and replaced with Li ions during cycling of the cell in the voltage range, 1.5 V to 4.5 V. This eventually resulted in a fully reversible Li intercalation into the LixV2O5 structure when 0.0 ≤ x ≤ 2.0. The second study is to apply the synthesis method to LiVS2 for the synthesis of β׳-LixV2O5 for use as a high performance cathode. The synthesis method is based on the heat treatment of the pure LiVS2 in atmospheric air. By employing this method of synthesis, well-crystalized, rod-shaped β׳-LixV2O5 particles 20 – 30 μm in length and 3 – 6 μm in width were obtained. Moreover, the surface of β׳-LixV2O5 particles was found to be coated by an amorphous vanadium oxysulfide film (~20 nm in thickness). In contrast to a low temperature vanadium pentoxide phase (LixV2O5), the electrochemical intercalation of lithium into the β׳-LixV2O5 was fully reversible where 0.0 < x < 2.0, and it delivered a capacity of 310 mAh/g at a current rate of 0.07 C between 1.5 V and 4 V. Good capacity retention of more than 88% was also observed after 50 cycles even at a higher current rate of 2 C. The third study is the investigation of NaVS2 as a cathode intercalation material for sodium ion batteries. We have shown that reversible electrochemical deintercalation of x ~ 1.0 Na per formula unit of NaxVS2, corresponding to a capacity of ~200 mAh/g, is possible. And a stable capacity of ~120 mAh/g after 30 cycles was observed. These studies show that the new chemical synthesis route for creating a phase transformation from the vanadium sulfide to oxide by heat treatment in air is a promising method for preparing vanadium oxide cathode material with high reversibility. Although this sample shows a relatively low voltage range compared with other cathodes such as LiCoO2 (3.8 V) and LiFePO4 (3.4 V), the large capacity of this sample is quite attractive in terms of increasing energy density in Li-ion batteries. Also, NaVS2 could be a promising cathode material for sodium ion batteries.