The enormous biodiversity of microalgae as well as their high photosynthetic rates can be exploited for a wide variety of applications including the production of high value chemicals, nutraceuticals, aquaculture feed, and most recently, biofuels. Regardless of the application, the productivity of the microalgae culture must be optimized in order to make the systems economically feasible. One environmental factor that greatly affects the productivity of mass cultivation systems is temperature since it can be prohibitively expensive to control in outdoor systems. Temperature affects microalgae growth rates both directly by its effect on metabolic rates, and indirectly, by changing the bioavailability of the inorganic carbon present in solution. In the first part of this research, the effects of dissolved inorganic carbon (DIC) concentration (varied by sparging CO₂-enriched air) and temperature on the growth of a model microalga species (Nannochloris sp., UTEX LB1999) were investigated in a turbidostat bioreactor. The results indicate that increasing DIC concentration yields higher microalgae growth rates up to an optimum value (around 3 mM for Nannochloris sp.) but higher concentrations actually inhibited growth. Since increasing the temperature decreases the DIC concentration for a given gas pCO₂, it is necessary to adjust the pCO₂ to maintain the target DIC concentration in the optimal range for growth. In the next phase of the research, the effect of average light intensity (Gav) and temperature on the growth rate of two microalgae species (Nannochloris sp., UTEX1999. and Phaeodactylum tricornutum, UTEX646) was investigated. Growth rates were measured over a range of average light intensities and temperatures using a turbidostat bioreactor. A multiplicative model was developed to describe growth as a function of both average light intensity and temperature. In the third phase of this research, both microalgae species were grown together to explore the effects of temperature fluctuations on the population dynamics of the co-culture. It was observed that Nannochloris was inhibited by the presence of P. tricornutum in the medium, probably due to the excretion of secondary metabolites into the medium that affected Nannochloris growth (allelopathic effects). The temperature and average light intensity model developed under monoculture conditions was modified to incorporate the allelopathic effects observed. The resulting model provided a reasonable fit to the dynamic behavior of a Nannochloris/P. tricornutum co-culture subjected to temperature variations in chemostat experiments. / text

Material properties, coupled with typical device structures of GaN-based light-emitting diode (LED) wafer give rise to Lambertian emission patterns with large beam divergence. However, this pattern may not be useful or beneficial to many applications. In some specific applications, such as spot lighting or light sources for fiber coupling, emission with narrow beam divergence is required, whereas in general lighting such as the street lamps and indoor lighting, a diffused light source rather than a point source is needed. By manipulating the optical emission of LEDs at the chip level, some performance metrics of LEDs can be enhanced and their applications can be extended into new fields rather than merely for lighting. Additionally, the need for external optics can be eliminated, thereby increasing the flexibility of design. In this thesis, five implementations are reported to achieve emission control, namely chip design, optics design, package design and system design, which are ordered according to the LED fabrication process flow. Manipulation of optical emission can be observed by comparing the proposed devices with the conventional devices, or the successful demonstration of a new application. By chip shaping via laser micromachining, a three-dimensional truncated-conic LED (TC-LED) is proposed to cut off efficiently lateral emissions from the LED sidewall, thus enhancing color uniformity from its top quantum-dot coated surface. The optical properties of TC-LED are investigated: the beam divergence is reduced by 32o and the power in the normal direction is enhanced by 21.7%. After applying quantum dots to achieve white-light emission, the top emission color uniformity is improved by 37%. By including optics on the chip level, beam divergence can be narrowed down. The hemispherical lens LED (HL-LED) with directional beam is proposed, achieving a 53.8% enhancement of fiber coupling efficiency. On top of a flip-chip-packaged TC-LED, a hemispherical BK-7 lens is capillary-bonded onto the sapphire surface. Compared with TC-LED, the divergence of HL-LED is significantly reduced by 50o. Vertically-mounted LED (vmLED) is proposed to broaden the emission pattern at the packaging level. By mounting the LED die upright to expose two large illumination surfaces instead of the traditional way of bonding the die flat down, the optical emission pattern is converted from Lambertian to a two-lobed pattern. Both the optical properties and thermal properties are investigated and it is found that there is a trade-off between the heat dissipation and light output. A sapphire-prism-mounted vmLED is further proposed to improve the heat sinking. In the last two chapters, micro-LED arrays with smaller illuminated active regions are introduced and the combination with external optics, including optical fibers and projection lens sets are used to demonstrate novel LED applications. By coupling a bi-linear micro-LED array into a fiber bundle, a portable microdisplay system is demonstrated and this comprehensive system can be used for image projection. Another application involved a linear UV-micro-LED array coupled with a projection lens set; this optical system has been demonstrated as a direct-write lithographic tool for the fabrication of polymer microlens arrays on InGaN LEDs. / published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

This thesis presents a study on several reliability issues for LED lighting systems. Firstly, a full survey on exsiting LED ballast has been conducted, and critical design challenges are classified on power level basis (low/ middle/ high power). Specifically, reliability issues have been highlighted, and three major factors have been stipulated: issue of electrolytic capacitor; issue of current imbalance in parallel LED strings; issue of LED junction temperature. The information revealed in the whole survey provide important design criteria for existing LED system designs and guidance for further research directions by pointing out the critical design problems. Two possible solutions for Electrolytic-Capacitor-Less LED Ballasts are proposed regarding the first reliability issue. A series of novel passive LED ballasts are proposed. They are found to be suitable for outdoor applications, such as street lighting applications, where the ability to withstand extreme weather conditions are of major concern. When compared with those in switched mode power supplies, these passive ballasts have good power factor performance and comparatively high efficiency. In addition, an active solution has been developed for indoor applications. Its circuit topology is derived from existing differential inverter topologies and inherits same merits such as simple structure, reduced size, and low cost. Self-configurable current-mirror techniques have been derived and developed afterwards to cope with the current imbalance issue for system with parallel LED strings. In contrast with traditional current sharing methods (either linear type or switched type), the proposed techniques offer a simple solution without the need of independent current references, complicated controllers and auxiliary power supplies. These features are favored by outdoor applications and such re-configurable mirror circuits are originally designed for passive LED ballast as post-current regulators. The techniques are further extended with the ability to tolerate possible circuit failure, such as short circuit and open circuit fault. Then, a new non-contact method for the measurement of both junction-to-case thermal resistance and junction temperature in a LED device has been proposed, with respect to the third reliability issue. Traditionally the direct measurement of junction temperature in LED is not easy without the help of sophisticated methods such as laser or expensive equipment like TeraLED Transient Thermal Tester system. In contrast, the proposed method requires only the external LED power, luminous flux, and heatsink temperature information. The method can be further adopted as a simple tool by engineers to check the internal temperature states in a practical LED system for regulation and evaluation purpose. Finally, a thermal design methodology has been developedfor an LED street lamp system powered by a weakly regulated renewable small power grid. It has been successfully incorporated in the proposed passive LED ballast, such that the LED system can provide the least output luminous fluctuation against line voltage variation. It is envisaged that, with the proposed design methodology, such lighting system will not only provide high reliability, with potential lifetime exceeding 10 years, but with a proven feature of reduced light fluctuation, furthermore, it is found that the passive LED system can act as a smart load and has the potential of reducing the energy storage requirement for smart grids. These merits are attractive to public lighting systems powered by future renewable power grids. / published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

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In this paper, we report on an increase in emission intensity of up to 10 nW/mm2 that has been realized with a new novel two junction, diagonal avalanche control, and minority carrier injection silicon complementary metal–oxide–semiconductor (CMOS) light emitting device (LED). The device utilizes a four-terminal configuration with two embedded shallow nþp junctions in a p substrate. One junction is kept in deep-avalanche and light-emitting mode, while the other junction is forward biased and minority carrier electrons are injected into the avalanching junction. The device has been realized using standard 0.35 mm CMOS design rules and fabrication technology and operates at 9V in the current range 0.1– 3 mA. The optical output power is about one order of magnitude higher for previous single-junction nþp light-emitting devices while the emission intensity is about two orders of magnitude higher than for single-junction devices. The optical output is about three orders of magnitude higher than the low-frequency detectivity limit of silicon p–i–n detectors of comparable dimensions. The realized characteristics may enable diverse optoelectronic applications in standard-CMOS-silicon-technology-based integrated circuitry.

The objectives of this research programme were to examine the influence of manufacturing and processing parameters on the environmental stability of aqueous acrylic based latices and to also extend the programme to an assessment of the behaviour of various stabiliser types suitable to such materials through either end-group modification or direct in-situ addition. During the research programme a number of physical and chemical techniques were assessed to routinely characterise and monitor the photochemical and thermal degradation of acrylic latices. These were based on emulsion polymerised formulations of methyl methacrylate and butyl acrylate. In the first instance, acrylic latices based on different formulations were chosen as homopolymers and copolymers. Here the nature of impurities and oxidation products generated during various stages of their manufacture have to be characterised and inter-related to their influence on subsequent environmental degradation. This involved the use of reflectance FT-IR spectroscopy to show functional group changes together with colourimetric U.V. analysis to determine photochemical generation of hydroperoxides. The early chemical changes and their subsequent influence on the physical and chemical properties of the latices during the later stages of environmental degradation were found to exhibit a close inter-relationship. In this regard the formation and the manufacturing temperatures and periods were crucial. Thermal methods of analysis were also used to characterise differences in the properties of the latices before and during thermal and photochemical oxidation. The nature of the residual persulphate initiator and the compositions of the latices in terms of end group modification were important parameters. De-esterification and hydroperoxide formation were found to be important processes during latex degradation while the use of low levels of co-monomer addition. such as methacrylic acicL to the emulsion reaction gave latices with improved light stability. Subsequent aspects of the research programme involved a detailed investigation into the behaviour and performance of various stabiliser types and formulations. Thus, while coreactive hindered piperidine stabilisers was found to be effective, the incorporation of simple terminal dialkyl acrylamide/methacrylaroide groups were also found to be effective. The efficiency of dialkylamide groups operating through a sacrificial mechanism is discussed. The influence of these processing operations on subsequent stabiliser activity provides valuable information on the mode of action of these stabilisers in such complex media. Further work involved a study into the effect of fluorinated methacrylates used in emulsion polymerised formulations. Here, the presence of residual starting materials in the monomer was found to have a detrimental effect in terms of photostability. Subsequent analysis of acrylic resins, based on solution polymerised formulations of methyl methacrylate and fluorinated methacrylates, was undertaken. The influence of reactive stabiliser types on these resins was important and to improve stabiliser perfonnance, synergistic stabiliser formulations were studied. This project will provide important fundamental information on the behaviour of active chromophores produced during the manufacturing process and hopefully resuh in the development of monitoring methodologies to establish paint quality.

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published_or_final_version / abstract / Physics / Master / Master of Philosophy