Thesis (Ph. D.)--University of Hong Kong, 2006.

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<p>The development of highly formable aluminum alloy sheets is of great interest to the automotive industry, because they provide a lightweight alternative to steel sheet for structural panels. Finding ways to improve the formability of Al alloys is the main subject of the present investigation. This issue is tackled from two angles. First, a possibility of fabricating a two-phase material containing newly discovered ductile intermetallic compounds is considered. The Al-La-Mg system is thermodynamically optimized accompanied with a differential thermal analysis (DTA) experiment to validate the optimization results. A new approach is introduced to deal with the incompatibility of phase models in binary Al-La and La-Mg systems. This approach is successfully applied to the Laves and B2 phases in the binary La-Mg system. A utilization of the thermodynamic description of the Al-La-Mg system to model solidification at low and high cooling rates shows that it is impossible to fabricate such a two-phase material by casting.</p> <p>Second, the effect of small additions of cerium and lanthanum on Fe-bearing intermetallics in a wrought heat-treatable Al alloy is examined. Fe-containing intermetallics are known to deteriorate the formability of Al alloys by acting as void nucleation sites. It is found that in alloys containing 0.1-0.2 wt. % of lanthanum, the fraction of less harmful Chinese script particles is pronouncedly higher than that in the reference alloy. In addition to this advantage, much smaller grains are seen in the alloy with 0.2 wt. % La. Despite similarities between La and Ce, the latter metal neither modifies the microstructure nor noticeably affects the gram size. Hot rolling and solutionizing nullifies the beneficial effect of small La additions resulting in no improvement in the formability of the alloy.</p> <p>In order to understand how lanthanum affects the phase portrait of the alloy, a socalled direct thermal analysis experiment is performed. Solidification paths are derived for slowly cooled alloys by coupling the results of this investigation with microstructural observations. The likelihood of two modification mechanisms is speculated using these solidification paths.</p> / Thesis / Doctor of Philosophy (PhD)

A first-generation LaB6 heaterless hollow cathode with a flat-plate anode is experimentally investigated. The cathode is characterized using krypton as propellant at varying flow rates, discharge currents and cathode-anode distances. Voltage probes, used to make direct voltage measurements in the ignition circuit, are the only diagnostic tool used experimentally. A plasma model is used to infer plasma parameters in the cathode emitter region. The cathode characterization results are consistent with those obtained during previous investigations of 1 A-class LaB6 hollow cathode with krypton. A peak-to-peak anode voltage criterion is used to identify the discharge modes and the occurrence of mode transition. Fourier analysis of the keeper and anode voltage waveforms carried out to study the discharge mode behavior reveals resonant frequencies ranging from 40 to 150 kHz. Lastly, post-test visual observations of the cathode components show signs of emitter poisoning and keeper erosion. / Master of Science / Recent years have seen rapid growth in the development of both stand-alone satellites and satellite constellations. A critical component of these satellites is the on-board propulsion system, which is responsible for controlling their orientation with respect to the object of interest and keeping the spacecraft in the assigned orbit. Generally, electric propulsion systems are used for this purpose. These types of propulsion systems use electrical power to change the velocity of satellite, providing a small thrust for a long duration of time as compared to chemical propulsion systems. Certain types of electric thrusters utilize a hollow cathode device as an electron source to start-off and support the thruster operation. In this research, a non-conventional hollow cathode for low power applications is developed and tested. The main characteristic of the developed cathode is the heaterless configuration, which eliminates the heater module used in conventional cathodes to enable the cathode to reach its operational temperature. The absence of a heater reduces the complexity of the cathode and the electrical power system. The cathode utilizes an electron emitter material which is insensitive to impurities and air exposure. Additionally, unlike typical electric thrusters which use xenon as the fuel, this cathode uses krypton which is similar to xenon but is less expensive. The presented work includes an overview of electric propulsion and the hollow cathode operation, followed by a detailed discussion of the heaterless hollow cathode design, the experimental setup and the test results. Several noteworthy findings regarding cathode operation are included as well. This research shows that the non-conventional heaterless hollow cathode and its operation with krypton have the potential to improve the overall thruster performance by reducing the weight and the cost, thus contributing to an integral aspect of satellite on-board propulsion.

published_or_final_version / abstract / Chemistry / Doctoral / Doctor of Philosophy

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

No description available.

No description available.

The need for cleaner and more efficient alternative energy sources is becoming urgent as concerns mount about climate change wrought by greenhouse gas emissions. Solid oxide fuel cells (SOFCs) are one of the most efficient options if the goal is to reduce emissions while still operating on fossil energy resources. One of the foremost problems in SOFCs that causes efficiency loss is the polarization resistance associated with the oxygen reduction reaction(ORR) at the cathodes. Hence, improving the cathode design will greatly enhance the overall performance of SOFCs. Lanthanum nickelate, La2NiO4+δ (LNO), is a mixed ionic and electronic conductor that has competitive surface oxygen exchange and transport properties and excellent electrical conductivity compared to perovskite-type oxides. This makes it an excellent candidate for solid oxide fuel cell (SOFC) applications. It has been previously shown that composites of LNO with Sm0.2Ce0.8O2-δ (SDC20) as cathode materials lead to higher performance than standalone LNO. However, in contact with lanthanide-doped ceria, LNO decomposes resulting in free NiO and ceria with higher lanthanide dopant concentration. In this study, the aforementioned instability of LNO has been addressed by compositional tailoring of LNO: lanthanide doped ceria (LnxCe1-xO2,LnDC)composite. By increasing the lanthanide dopant concentration in the ceria phase close to its solubility limit, the LNO phase has been stabilized in the LNO:LnDC composites. Electrical conductivity of the composites as a function of LNO volume fraction and temperature has been measured, and analyzed using a resistive network model which allows the identification of a percolation threshold for the LNO phase. The thermomechanical compatibility of these composites has been investigated with SOFC systems through measurement of the coefficients of thermal expansion. LNO:LDC40 composites containing LNO lower than 50 vol%and higher than 40 vol% were identified as being suitable to incorporate into full button cell configuration from the standpoint of thermomechanical stability and adequate electrical conductivity. Proof-of-concept performance comparison for SOFC button cells manufactured using LNO: La0.4Ce0.6O2-δ composite to the conventional composite cathode materials has also been provided. This thermodynamics-based phase stabilization strategy can be applied to a wider range of materials in the same crystallographic family, thus providing the SOFC community with alternate material options for high performance devices.

Orientador: Joni Augusto Cirelli / Banca: Raquel Scarel Caminaga / Banca: Willian Fernando Zambuzzi / Resumo: Com a evolução no desenvolvimento de biomateriais, a utilização de matrizes tridimensionais (scaffolds), construídas a partir de impressão tridimensional (3DP), via SLS (selective laser sinterization) e por Extrusão de Filamentos, vêm ganhando bastante destaque no ramo da engenharia tecidual óssea. A possibilidade de impressão de modelos 3D, baseado em modelo virtual prévio, com forma, tamanho, e porosidade altamente controlados, assemelha o material ao osso perdido, favorecendo a reconstrução de defeitos ósseos em substituição aos autoenxentos, considerados "padrão ouro". O objetivo deste trabalho foi caracterizar físico-química e biologicamente scaffolds à base de Poli(3-hidroxibutirato) (PHB), confeccionados por impressão 3D via SLS, revestidos com celulose bacteriana (CB), funcionalizados ou não com apatitas (HA) e/ou peptídeo de crescimento osteogênico (osteogenic growth peptide - OGP) (Estudo 1), assim como matrizes de Poli(hidroxibutirato-co-valerato) (PHBV) e PHBV-apatita radiopaca dopada com Lantânio (PHBV-La20OAP) confeccionadas por Extrusão de Filamentos (Estudo 2), com finalidade de regeneração óssea. Os resultados de caracterização físico-química por meio da MEV/EDS demonstraram que os scaffolds produzidos apresentaram composição química e arquitetura (forma e porosidade) adequadas. A resistência mecânica nos scaffolds de PHB não sofreu alteração significativa após o revestimento com CB; porém, teve redução com a adição de apatita. O lantânio promoveu um aumento ... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: With the evolution in the development of biomaterials, the use of three-dimensional matrices (scaffolds), constructed from three-dimensional printing (3DP) via SLS (selective laser sinterization) and Filament Extrusion are gaining a lot of attention in the field of bone tissue engineering. The possibility to print 3D models based on previous virtual models with highly controlled shape, size and porosity, makes the material similar to the lost bone and favours the reconstruction of bone defects in the replacement of autografts considered "gold standard". The aim of this study was to physicochemically and biologically characterize Poly (3-hydroxybutyrate) (PHB) scaffolds, made by 3D printing via SLS coated with bacterial cellulose, functionalized or not with apatite (HA) and / or osteogenic growth peptide- (OGP) (Study 1) and matrices of Poly (hydroxybutyrate-co-valerate) (PHBV) and PHBV-La containing radiopaque apatite (PHBV-La20OAP) made by filament extrusion (Study 2) for bone regeneration. Results from physicochemical characterization by SEM/EDS demonstrated that the all produced scaffolds presented appropriate chemical composition and architecture (shape and porosity). The mechanical strength of the PHB scaffolds was not significantly affected my CB coating but was reduced after apatite addition. The La allowed an increase in the elasticity modulus of PHBV matrices. Experiments for analysis of the biological behaviour of the scaffolds showed that they allowed the maintenan... (Complete abstract click electronic access below) / Mestre