Rozprawy doktorskie na temat „Metallic channel”

Polymer electrolyte membrane fuel cells (PEMFCs) have emerged as a strong and promising candidate to replace internal combustion engines (ICE) due their high efficiency, high power density and near-zero hazardous emissions. However, their commercialization waits for solutions to bring about significant cost-reductions and significant durability for given power densities. Bipolar plate (BPP) with its multi-faceted functions is one of the essential components of the PEMFC stacks. Stainless steel alloys are considered promising materials of choice for bipolar plate (BPP) applications in polymer electrolyte membrane fuel cells (PEMFC) due to their relatively low cost and commercial availability in thin sheets. Stainless steel materials build a protective passive metal oxide layer on their surface against corrosion attack. This passive layer does not demonstrate good electrical conductivity and increases interfacial electric contact resistance (ICR) between BPP and gas diffusion layer GDL in PEMFC. Lower ICR values are desired to reduce parasitic power losses and increase current density in order to improve efficiency and power density of PEMFC. This study aimed to bring about a broader understanding of manufacturing effects on the BPP contact resistance. In first stage, BPP samples manufactured with stamping and hydroforming under different process conditions were tested for their electrical contact resistance characteristics to reveal the effect of manufacturing type and conditions. As a general conclusion, stamped BPPs showed higher contact conductivity than the hydroformed BPPs. Moreover, pressure in hydroforming and geometry had significant effects on the contact resistance behavior of BPPs. Short term corrosion exposure was found to decrease the contact resistance of bipolar plates. Results also indicated that contact resistance values of uncoated stainless steel BPPs are significantly higher than the respective target set by U.S. Department of Energy. Proper coating or surface treatments were found to be necessary to satisfy the requirements. In the second stage, physical vapor deposition technique was used to coat bipolar plates with CrN, TiN and ZrN coatings at 0.1, 0.5 and 1 μm coating thicknesses. Effects of different coatings and coating thickness parameters were studied as manufactured BPPs. Interfacial contact resistance tests indicated that CrN coating increased the contact resistance of the samples. 1 µm TiN coated samples showed the best performance in terms of low ICR; however, ICR increased dramatically after short term exposure to corrosion under PEMFC working conditions. ZrN coating also improved conductivity of the SS316L BPP samples. It was found that the effect of coating material and coating thickness was significant whereas the manufacturing method and BPP channel size slightly affected the ICR of the metallic BPP samples. Finally, effect of process sequence on coated BPPs was investigated. In terms of ICR, BPP samples which were coated prior to forming exhibited similar or even better performance than coated after forming samples. Thus, continuous coating of unformed stripes, then, applying forming process seemed to be favorable and worth further investigation in the quest of making cost effective BPPs for mass production of PEMFC.

Los materiales conocidos como vidrios metálicos han sido sujeto de estudio e todo el mundo desde los años 50, por el cual se ha conseguido un progreso importante en el entendimiento del comportamiento de estos materiales. Como el nombre sugiere, son aleaciones metálicas amorfas en las que no existe el orden a largo alcance. La ausencia de este tipo de orden les dota de propiedades físicas, químicas y mecánicas que son únicas comparadas con las de otros materiales metálicos convencionales.
Sin embargo, los primeros sistemas amorfos fueron obtenidos por técnicas de solidificación rápida, y requerían velocidades de enfriamiento criticas de hasta 106 K s−1. Por consiguiente, se obtenían cintas finas con un espesor limitado hasta unas decenas de micrómetro. Unos treinta a cuarenta años más tarde, una gama de aleaciones multicomponentes, las cuales requiren velocidades de enfríamiento más lentas, fueron desarrolladas, lo cual significó el nacimiento de los vidrios metálicos macizos. Entre estas aleaciones multicomponentes, las que son a base de Zr han sido protagonistas por su capacidad extraordinaria de formar vidrios. Por el mismo, se hicieron como aleaciones modelo para el estudio de propiedades fúndamentales y comportamientos característicos.
Su límite de fluencia excepcionalmente alto, cerca del límite teórico, les proporciona a los vidrios metálicos macizos un potencial para ser utilizados en aplicaciones estructurales. Desafortunadamente, la deformación plástica a temperatura ambiente occure de una manera muy localizada en bandas de cizalladura. En vez de endurecimiento mécanico, los vidrios metálicos sufren un ablandamiento al deformarlos lo cual impide una deformación plástica estable. Así que, a pesar de su límite de fluencia alto, la ruptura occurirá después de una deformación macroscópica limitada. Este mecanismo de deformación inhomogénea a temperatura ambiente limita la fiabilidad de los vídrios metálicos macizos en aplicaciones estructurales. Lógicamente, la mejora de la plasticidad de estos materiales ha sido ampliamente estudiada durante la última década.
El concepto más explorado para evitar la ruptura catastrófica ha sido probablemente el desarollo de una microestructura heterogenea con una segunda fase. Ésta segunda fase puede tener dimensiones tanto a escala micrómetra como la escala nanométrica y puede ser tanto una fase cristalina como una fase amorfa. Varias rutas han sido probadas para obtener ésta segunda fase en la matriz amorfa: añadir directamente una fase reforzante al material fundido, diseñar una composición adecuada que resulta en un material compuesto al solidificarla o precipitar la segunda fase durante un tratamiento térmico después de colar.
Por tratamientos térmicos por debajo de la temperatura de transición vidria, cambios del orden topológico y químico a corto alcance han sido observado en la literatura. El primer efecto suele deteriorar la plasticidad por relajación de la estructura amorfa. La influencia del cambio de orden químico a corto alcanze se ha estudiado en muy poco detalle. Aparte de los tratamientos térmicos, los tratamientos mecánicos pueden inducir cambios estructurales y microestructurales. El estudio del efecto de estos dos tratamientos forma la parte parte de esta tesis.
Los cambios en el orden topólogico y químico de corto alcance de vídrios metálicos a base de Zr, inducidos por tratamientos térmicos y mecánicos, han sido caracterizados por técnicas de calorimetría, difracción de rayos-X y por microscópia electrónica. Luego, la influencia de estos mismos en el comportamiento mecánico de los vídrios se ha estudiado por tests de compresión y de nanoindentación.
Sin embargo, en la primera parte de ésta tesis, se demuestra que al aplicar la técnica de nanoindentación, se debería tomar en cuenta la existencia de un "size-effect", correlacionado directamente con los cambios estructurales que occuren durante la deformación. Este "size-effect" implica que la dureza y el módulo elástico bajan al aumentar la profundidad de la indentación, similar a lo que se observa normalmente para materiales cristalinos. Durante la deformación, aumenta el volumen libre del vídrio metálico. Este crecimiento del volumen libre influirá en la respuesta del material a la nanoindentación. En particular, se observa un ablandamiento dinámico cuando se aplican cargas elevadas al material. Además, concentraciones más altas de volumen libre en el estado inicial después del colado, ocasionan un ablandamiento mayor y por consecuencia, aumentan el "size-effect".
Después, se hizo un estudio sistemático de los cambios de corto y medio alcance, inducido por tratamientos térmicos, en particular por tratamientos a baja temperatura. Aplicando varias técnicas de caracterización, como por ejemplo la calorimetría, difracción de rayos X y microscopía electrónica, cambios importantes de orden quimico han sido observados. Ya durante tratamientos cortos a baja temperatura, se formaban clústers de Cu en una matriz con un contenido de Cu reducido. Más adelante, el estudio se enfocó en la influencia de los cambios microestructurales en las propiedades mecánicas. Aunque los cambios observados eran moderados, su influencia en el comportamiento mecánico, y en particular en la plasticidad en compresión, es enorme cuando los cambios topológicos son todavía moderados (como es el caso durante el tratamiento térmico a baja temperatura). La plasticidad aumenta significativamente, lo que va en contra a la fragilización que suele ocurrir durante los tratamientos térmicos. Durante tratamientos a temperaturas más elevadas, los cambios topológicos empiezan a dominar (disminución del volumen libre) y forman un contrapeso para el efecto positivo de los cambios del orden químico a corto alcance. Posteriormente, se investigó en más detalle la influencia de los cambios de orden químico en la cristalización. Los clústeres de Cu que se forman durante el calentamiento se puede interpretar como fases embrionarias en el proceso de cristalización.
Durante los tratamientos térmicos, se dan tanto cambios de orden topológico como de orden químico, pero tienen un efecto contradictorio en la plasticidad por lo cual puede ser difícil controlarlos. Sin embargo, los cambios beneficiosos de orden químico se pueden obtener por tratamientos mecánicos de deformación severa, por ejemplo por torsión a alta presión (TAP). La deformación no causa una reducción del volumen libre sino la aumenta incluso más. Además, TAP produce una muestra maciza bastante homogenea siempre y cuando el número de revueltas sea menor, lo que es lo contrario de lo que se sabe para materiales cristalinos. Estos materiales suelen requerir más vueltas para evitar una microestructura heterogenea.
Metallic glasses have been the subject of widespread research since the 1950's with significant progress in the understanding in their behavior. As the name suggests, they are amorphous metallic alloys, i.e. with the absence of long-range order. The absence of this long-range order offers them unique physical, chemical and mechanical properties compared to conventional metallic materials.
However, the early amorphous systems were obtained typically by rapid quenching techniques, with critical cooling rates up to 106 K s−1, resulting typically in ribbons or thin foils with a thickness limited to a few tens of micrometer. About thirty to forty years later, a large range of multicomponent alloys was developed which required significant lower critical cooling rates leading to the birth of so-called bulk metallic glass (BMG). Among these multicomponent systems, Zr-based alloys have been key players with outstanding glass forming ability, which has made them to model alloys for the study of fundamental properties and characteristic behaviors.
The exceptionally high yield strength, close to the theoretical limit, and yield strain of these amorphous metallic systems in bulk offer them potential for structural applications. However, plastic deformation at room temperature occurs in a highly localized manner by the formation of a few shear bands. Instead of work hardening, metallic glasses soften upon deformation, which prevents stable plastic elongation. Although BMGs possess a high fracture strength, once yielding has set in, early failure after a small percentage of macroscopic deformation appears. This inhomogeneous deformation mechanism at ambient temperature still limits the reliability of BMGs for structural applications. Logically, the enhancement of ductility of this type of materials has been the subject of many research works in the last decade.
Probably the most explored concept to avoid catastrophic failure has been the development of a heterogeneous microstructure, with a second phase on different length scales, both crystalline and amorphous. Various routes have been tried out to obtain this second phase in the amorphous matrix: physically adding a reinforcing phase to the melt, by direct precipitation from the melt of a properly designed composition or by (partial) nano-crystallization of the glass after casting.
Upon annealing below the glass transition, changes in both topological and chemical short range order have been reported. The former is believed to deteriorate plasticity due to structural relaxation of the amorphous structure.
The effect of changes of the chemical short range order on plasticity has hardly been studied into detail. Besides annealing, deformation has been reported to induce structural and microstructural changes. These (micro-)structural changes, induced by annealing and deformation, form the main topic of the work presented in this thesis.
Topological and chemical changes in the short range order of Zr-based bulk metallic glasses upon annealing and deformation treatments have been characterized by calorimetry, X-ray diffraction and electron microscopy. The influence of these changes on the mechanical behavior of these glasses was investigated through compression tests and nanoindentation tests.
However, in a first part of this thesis, it is shown that one should be aware when applying this technique of the existence of a so-called size-effect, directly linked with the structural changes upon deformation.
A decrease of hardness and elastic modulus on the maximum penetration depth was found, similar as what is typically observed for crystalline materials. Upon deformation, free volume typically increases. Due to this increase, free volume will influence the response of the material during nanoindentation testing. In particular, a dynamic softening is observed when being plastically deformed at higher loads. Larger free volume concentrations in the as-cast state result in enhanced mechanical softening and, concomitantly, more pronounced indentation size effects.
Afterwards, a systematic study on changes on the short and medium range order upon annealing was performed, in particular at low temperatures. By means of various characterization techniques, like by calorimetry, X-ray diffraction and electron microscopy, important changes in chemical ordering were found, with the formation of Cu-clusters in a more Cu-depleted matrix, already upon low temperature annealing for a short time. In a next step, the study focussed on the influence of this altered microstructures on the mechanical properties. Although the (compositional) changes observed were moderate, their influence on the mechanical behavior, and in particular plasticity under compression, is great, when the topological changes are still moderate (low temperature annealing). Plasticity is enhanced greatly, in large contrast to the generally assumed embrittlement upon annealing. A too large increase in topological short range order (free volume decrease) counterbalances the effect induced by the chemical short range order upon high temperature annealing. Finally, the influence of these changes of chemical short-range order on the crystallization behaviour was studied in more detail. The formation of the Cu-rich clusters upon annealing can thus be understood as a very embryonic phase towards crystallization.
Upon annealing, topological (densification) and chemical ordering occur simultaneously, but these processes have a contradictory effect on plasticity and it might be difficult to control them. Therefore, it is interesting that the beneficial changes in chemical ordering can be achieved also upon high-deformation treatments, e.g. by high-pressure torsion (HPT). Deformation does not lead to the adverse reduction of free volume but even produces some more. HPT itself is able to produce a rather homogeneous bulky sample, in particular for a low amount of revolution - contrary to what is observed in crystalline materials, where more revolutions are necessary to overcome the undesired inhomogeneous microstructure.

Thesis (PhD) -- Stellenbosch University, 2014.
ENGLISH ABSTRACT: Currently the reduction of the levelised cost of electricity (LCOE) is the main goal of concentrating solar power (CSP) research. Central to a cost reduction strategy proposed by the American Department of Energy is the use of advanced power cycles like supercritical steam Rankine cycles to increase the efficiency of the CSP plant. A supercritical steam cycle requires source temperatures in excess of 620°C, which is above the maximum storage temperature of the current two-tank molten nitrate salt storage, which stores thermal energy at 565°C. Metallic phase change materials (PCM) can store thermal energy at higher temperatures, and do not have the drawbacks of salt based PCMs. A thermal energy storage (TES) concept is developed that uses both metallic PCMs and liquid metal heat transfer fluids (HTF). The concept was proposed in two iterations, one where steam is generated directly from the PCM – direct steam generation (DSG), and another where a separate liquid metal/water heat exchanger is used – indirect steam generation, (ISG). Eutectic aluminium-silicon alloy (AlSi12) was selected as the ideal metallic PCM for research, and eutectic sodium-potassium alloy (NaK) as the most suitable heat transfer fluid. Thermal energy storage in PCMs results in moving boundary heat transfer problems, which has design implications. The heat transfer analysis of the heat transfer surfaces is significantly simplified if quasi-steady state heat transfer analysis can be assumed, and this is true if the Stefan condition is met. To validate the simplifying assumptions and to prove the concept, a prototype heat storage unit was built. During testing, it was shown that the simplifying assumptions are valid, and that the prototype worked, validating the concept. Unfortunately unexpected corrosion issues limited the experimental work, but highlighted an important aspect of metallic PCM TES. Liquid aluminium based alloys are highly corrosive to most materials and this is a topic for future investigation. To demonstrate the practicality of the concept and to come to terms with the control strategy of both proposed concepts, a storage unit was designed for a 100 MW power plant with 15 hours of thermal storage. Only AlSi12 was used in the design, limiting the power cycle to a subcritical power block. This demonstrated some practicalities about the concept and shed some light on control issues regarding the DSG concept. A techno-economic evaluation of metallic PCM storage concluded that metallic PCMs can be used in conjunction with liquid metal heat transfer fluids to achieve high temperature storage and it should be economically viable if the corrosion issues of aluminium alloys can be resolved. The use of advanced power cycles, metallic PCM storage and liquid metal heat transfer is only merited if significant reduction in LCOE in the whole plant is achieved and only forms part of the solution. Cascading of multiple PCMs across a range of temperatures is required to minimize entropy generation. Two-tank molten salt storage can also be used in conjunction with cascaded metallic PCM storage to minimize cost, but this also needs further investigation.
AFRIKAANSE OPSOMMING: Tans is die minimering van die gemiddelde leeftydkoste van elektrisiteit (GLVE) die hoofdoel van gekonsentreerde son-energie navorsing. In die kosteverminderingsplan wat voorgestel is deur die Amerikaanse Departement van Energie, word die gebruik van gevorderde kragsiklusse aanbeveel. 'n Superkritiese stoom-siklus vereis bron temperature hoër as 620 °C, wat bo die 565 °C maksimum stoor temperatuur van die huidige twee-tenk gesmelte nitraatsout termiese energiestoor (TES) is. Metaal fase veranderingsmateriale (FVMe) kan termiese energie stoor by hoër temperature, en het nie die nadele van soutgebaseerde FVMe nie. ʼn TES konsep word ontwikkel wat gebruik maak van metaal FVM en vloeibare metaal warmteoordrag vloeistof. Die konsep is voorgestel in twee iterasies; een waar stoom direk gegenereer word uit die FVM (direkte stoomopwekking (DSO)), en 'n ander waar 'n afsonderlike vloeibare metaal/water warmteruiler gebruik word (indirekte stoomopwekking (ISO)). Eutektiese aluminium-silikon allooi (AlSi12) is gekies as die mees geskikte metaal FVM vir navorsingsdoeleindes, en eutektiese natrium – kalium allooi (NaK) as die mees geskikte warmteoordrag vloeistof. Termiese energie stoor in FVMe lei tot bewegende grens warmteoordrag berekeninge, wat ontwerps-implikasies het. Die warmteoordrag ontleding van die warmteruilers word aansienlik vereenvoudig indien kwasi-bestendige toestand warmteoordrag ontledings gebruik kan word en dit is geldig indien daar aan die Stefan toestand voldoen word. Om vereenvoudigende aannames te bevestig en om die konsep te bewys is 'n prototipe warmte stoor eenheid gebou. Gedurende toetse is daar bewys dat die vereenvoudigende aannames geldig is, dat die prototipe werk en dien as ʼn bevestiging van die konsep. Ongelukkig het onverwagte korrosie die eksperimentele werk kortgeknip, maar dit het klem op 'n belangrike aspek van metaal FVM TES geplaas. Vloeibare aluminium allooie is hoogs korrosief en dit is 'n onderwerp vir toekomstige navorsing. Om die praktiese uitvoerbaarheid van die konsep te demonstreer en om die beheerstrategie van beide voorgestelde konsepte te bevestig is 'n stoor-eenheid ontwerp vir 'n 100 MW kragstasie met 15 uur van 'n TES. Slegs AlSi12 is gebruik in die ontwerp, wat die kragsiklus beperk het tot 'n subkritiese stoomsiklus. Dit het praktiese aspekte van die konsep onderteken, en beheerkwessies rakende die DSO konsep in die kollig geplaas. In 'n tegno-ekonomiese analise van metaal FVM TES word die gevolgtrekking gemaak dat metaal FVMe gebruik kan word in samewerking met 'n vloeibare metaal warmteoordrag vloeistof om hoë temperatuur stoor moontlik te maak en dat dit ekonomies lewensvatbaar is indien die korrosie kwessies van aluminium allooi opgelos kan word. Die gebruik van gevorderde kragsiklusse, metaal FVM stoor en vloeibare metaal warmteoordrag word net geregverdig indien beduidende vermindering in GLVE van die hele kragsentrale bereik is, en dit vorm slegs 'n deel van die oplossing. ʼn Kaskade van verskeie FVMe oor 'n reeks van temperature word vereis om entropie generasie te minimeer. Twee-tenk gesmelte soutstoor kan ook gebruik word in samewerking met kaskade metaal FVM stoor om koste te verminder, maar dit moet ook verder ondersoek word.

Deployment of high temperature (>500 °C) thermal energy storage in solar power plants will make solar power more cost competitive and pave the way towards a sustainable future. In this research, a unique metallic encapsulation has been presented for thermal energy storage at high temperatures, capable of operation in aerobic conditions. This goal was achieved by employing low cost materials like carbon steel. The research work presents the unique encapsulation procedure adopted, as well as various coatings evaluated and optimized for corrosion protection. Experimental testing favored the use of 150 μm of nickel on carbon steel for corrosion protection in these conditions. These metallic encapsulations survived several thermal cycles at temperatures from 580 °C to 680 °C with one encapsulation surviving for 1700 thermal cycles.

Purpose: The purpose of this thesis is to investigate Stena Metall’s values and how they are communicated within the organization.

Background: In the Fall of 2008, the world was shocked by the worst financial crisis in decades. The crisis had deep effects on the Swedish economy, and many companies suffered heavily. The recycling and environmental service company Stena Metall experienced their first negative result in 30 years, and 900 employees had to leave the company. An action program, including an altered culture was established with the purpose to adapt the operations to the new business environmental conditions. Values, which are a part of the organizational culture, were decided to be an important part of the change. To implement these in the entire organization a well-structured communication process is needed. Within these subjects; values and communication, a qualitative study at Stena Metall has been conducted.

Method: To fulfill the purpose, a qualitative method has been used. Thirteen interviews were conducted to collect data from different levels of the organization. The interviews were designed differently based on the employee’s level of responsibility in the organization. The theoretical framework used when analyzing the empirical material includes earlier research in the areas of culture, with emphasis on values, and communication.

Conclusion: Two set of values have been identified, core values and aspiration values. The findings indicate a gap in the communicational process at Stena Metall. Part of the new information communicated about Stena Metall´s values is lost on its way from the management to the lower levels in the organization. The perception of what the values mean, both core and aspiration values, differs depending on level in the organization.

Toxic elements are present in polluted water from mines, industrial outlets, storm water etc. Wetland plants take up toxic elements and increase the pH of the medium. In this thesis was investigated how the shoots of submerged plants and roots of emergent plants affected the pH of the surrounding water in the presence of free toxic ions. The aim was to clarify the mechanisms by which these plants change the surrounding water pH in the presence of toxic ions. The influence of Elodea canadensis shoots on the pH of the surrounding water was studied in the presence of cadmium (Cd) at low initial pH (4-5). The involvement of photosynthetic activity in the pH changes was investigated in the presence and absence of Cd. The cytosolic, vacuolar and apoplasmic pH changes as well as cytosolic Cd changes in E. canadensis were monitored. The influence of Eriophorum angustifolium roots on the pH of the surrounding water was investigated in the presence of a combination of Cd, copper, lead, zinc and arsenic at low initial pH (3.5). Eriophorum angustifolium root exudates were analyzed for organic acids. Elodea canadensis shoots increased the pH of the surrounding water, an effect more pronounced with increasing Cd levels and/or increasing plant biomass and increased plant Cd uptake. The pH increase in the presence of free Cd ions was not due to photosynthesis or proton uptake across the plasmalemma or tonoplast. Cadmium was initially sequestered in the apoplasm of E. canadensis and caused its acidosis. Eriophorum angustifolium roots increased the surrounding water pH and this effect was enhanced in the presence of arsenic and metals. This pH increase was found to depend partly on the release of oxalic acid, formic acid and succinic acid by the plants. In conclusion, E. canadensis shoots and E. angustifolium roots were found to increase the low initial pH of the surrounding water. The pH modulation by these species was enhanced by low levels of free toxic ions in the surrounding water.
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 3: Submitted. Paper 4: Manuscript.

Els efectes del canvi climàtic incideixen directament en les poblacions de microorganismes fotòtrofs dels tapissos microbians, produint alteracions en altres paràmetres ambientals com és l’increment de la temperatura, que pot provocar sequeres i fins i tot la desertització d’aquests ecosistemes, així com altres efectes en l’osmolaritat de les cèl·lules, degut a l’augment de la salinitat. Els microorganismes esmentats són molt abundants en els tapissos microbians, principalment els cianobacteris i les microalgues, que a més de ser els principals estabilitzadors d’aquests ecosistemes, al mateix temps es troben exposats a diferents condicions d’estrès. La majoria dels estudis que es realitzen per valorar l’impacte que tenen sobre els microorganismes condicions ambientals tan variables utilitzen cultius axènics que provenen de microorganismes aïllats de l’ambient natural o bé de col·leccions de cultius; res més lluny de la realitat, ja que en els ambients naturals els microorganismes fotòtrofs estableixen associacions estables amb microorganismes heteròtrofs i, a vegades, amb altres fotòtrofs. A més, hi ha pocs estudis que analitzin, en aquestes condicions i a nivell individual, els efectes dels paràmetres ambientals o de la pol·lució per metalls en aquestes associacions. La manca de metodologies que poden aplicar-se per esbrinar aquests efectes en un microorganisme en concret, quan està associat amb un altre de manera selectiva, in vivo, de manera ràpida i sense l’ús de cap tipus de tinció, és, per tant, un repte a la hora d’analitzar les possibilitats que tenen aquests microorganismes enfront a canvis tan dràstics. En aquest treball, s’ha intentat solucionar aquesta problemàtica mitjançant l’optimització de diferents tècniques que tenen com a base el microscopi làser confocal, considerant la principal característica dels cianobacteris i les microalgues, que és la d’emetre fluorescència natural. La clorofil·la a és el pigment majoritari d’aquests microorganismes i s’ha utilitzat amb anterioritat com a bioindicador, especialment en estudis realitzats en metalls per altres membre del grup. Així mateix, una problemàtica important és esbrinar el paper que juguen aquests microorganismes en la resistència davant canvis sobtats de les condicions naturals o antropogèniques. En aquest sentit, també s’ha accentuat l’interès en les anomenades cèl·lules dorments i en l’estudi de cèl·lules viables i no viables. Per aquest motiu, en aquest treball s’ha posat a punt una nova metodologia que utilitza el microscopi làser confocal i dos làsers específics i que ha permès determinar el percentatge d’aquestes cèl·lules en mostres exposades a diferents condicions d’estrès. Els microscopis electrònics de rastreig i transmissió, ambdós acoblats a un detector d’energia dispersiva de raigs X, juntament amb el microscopi de transmissió de raig X del sincrotró ALBA, s’han aplicat en mostres sotmeses a varis factors d’estrès per avaluar els canvis morfològics en cèl·lules senceres i en seccions ultrafines, així com en estudis de captació de metalls extra- i intracel·lularment. Els objectius del present treball s’han centrat en l’aplicació combinada de totes aquestes metodologies en dos consorcis de microorganismes: Scenedesmus sp. DE2009 i Geitlerinema sp. DE2011. L’efecte de la llum i la salinitat (com a paràmetres ambientals), així com l’impacte del plom, coure i crom (com a contaminants), s’ha estudiat àmpliament en cèl·lules individuals d’ambdós microorganismes. Finalment, aquesta tesi està estructurada en diferents capítols. El Capítol 3 correspon amb els articles publicats (un d’ells en revisió); així doncs, els resultats obtinguts de les investigacions realitzades s’exposen en les Seccions 3.1, 3.2 i 3.3 i es discuteixen globalment en el Capítol 4.
Los efectos del cambio climático inciden directamente en las poblaciones de microorganismos fototróficos de los tapices microbianos, produciendo alteraciones en otros parámetros ambientales como es el incremento de la temperatura, que puede provocar sequías y hasta la desertización de estos ecosistemas, así como otros efectos en la osmolaridad de las células, debido al aumento de la salinidad. Dichos microorganismos son muy abundantes en los tapices microbianos, principalmente las cianobacterias y las microalgas, que además de ser los principales estabilizadores de estos ecosistemas, a veces se encuentran expuestos a diferentes condiciones de estrés. La mayoría de los estudios, que se realizan para valorar los efectos que tienen sobre los microorganismos condiciones ambientales tan variables, utilizan cultivos axénicos que provienen de microorganismos aislados del ambiente natural o bien de colecciones de cultivos; nada más lejos de la realidad, ya que en los ambientes naturales los microorganismos fototróficos establecen asociaciones estables con microorganismos heterotróficos y, a veces, con otros fototróficos. Además, hay pocos estudios que analicen, en estas condiciones y a nivel individual, los efectos de los parámetros ambientales o de la contaminación por metales en estas asociaciones. La falta de metodologías que pueden aplicarse para averiguar estos efectos en un microorganismo en concreto, cuando está asociado con otro de manera selectiva, in vivo, de manera rápida y sin el uso de ningún tipo de tinción, es, por tanto, un reto a la hora de analizar las posibilidades que tienen estos microorganismos frente a cambios tan drásticos. En este trabajo, se ha intentado solucionar dicha problemática mediante la optimización de distintas técnicas que tienen como base el microscopio láser confocal, considerando la característica principal de las cianobacterias y las microalgas, que es la de emitir fluorescencia natural. La clorofila a es el pigmento mayoritario de estos microorganismos y se ha utilizado con anterioridad como bioindicador, especialmente en estudios realizados en metales por otros miembros del grupo. Asimismo, una problemática importante es valorar el papel que juegan estos microorganismos en la resistencia ante cambios repentinos de las condiciones naturales o antropogénicas. En este sentido, también se ha incrementado el interés por las denominadas células durmientes y en el estudio de células viables y no viables. Por este motivo, en este trabajo se ha puesto a punto una nueva metodología que utiliza el microscopio láser confocal y dos láseres específicos y que ha permitido determinar el porcentaje de estas células en muestras expuestas a diferentes condiciones de estrés. Los microscopios electrónicos de rastreo y transmisión, ambos acoplados a un detector de energía dispersiva de rayos X, junto con el microscopio de transmisión de rayos X del sincrotrón ALBA, se han aplicado en muestras expuestas a varios factores de estrés para evaluar los cambios morfológicos en células enteras y en secciones ultrafinas, así como para los estudios de captación de metales extra- e intracelularmente. Los objetivos de este trabajo se han centrado en la aplicación combinada de todas estas metodologías en dos consorcios de microorganismos: Scenedesmus sp. DE2009 y Geitlerinema sp. DE2011. El efecto de la luz y la salinidad (como parámetros ambientales), así como el impacto de metales como plomo, cobre y cromo (como contaminantes), se ha estudiado ampliamente en células individuales de ambos microorganismos. Finalmente, esta tesis está estructurada en distintos capítulos. El Capítulo 3 corresponde con los artículos publicados (uno de ellos en revisión); así pues, los resultados obtenidos de las investigaciones realizadas se exponen en las Secciones 3.1, 3.2 y 3.3 y se discuten globalmente en el Capítulo 4.
The effects of climate change directly affect the populations of phototrophic microorganisms in microbial mats, causing alterations in other environmental parameters such as the increase in temperature. This in turn causes drought and sometimes the desertification of these ecosystems, as well as affecting the osmolarity of cells due to the increase in salinity. The microorganisms referred to are highly abundant in microbial mats; principally, they are cyanobacteria and microalgae, which, apart from being the main stabilisers of these ecosystems, are exposed to distinct stress conditions at the same time. Most studies carried out to assess the impact on microorganisms of such variable environmental conditions use axenic cultures that come from microorganisms isolated from the natural environment or else from culture collections. Nothing could be further from reality, since—in natural environments—phototrophic microorganisms establish stable associations with heterotrophic bacteria and, at times, with other phototrophs. In addition, very few studies analyse (in these conditions and at individual level) the effects of environmental parameters or of metal pollution in these associations. The lack of methodologies that can be applied to ascertain these effects in a specific microorganism, when this is selectively associated with another, in vivo, swiftly and without using any type of staining, is therefore a challenge in analysing the possibilities of these microorganisms when facing such drastic changes. In the current work, an attempt has been made to solve this problem by means of the optimisation of distinct techniques, based on confocal laser microscopy and centring on the main characteristic of cyanobacteria and microalgae, which is the emission of natural fluorescence. Chlorophyll a is the majority pigment in these microorganisms and has previously been used as a bioindicator, in studies carried out with metals by other members of the group. Determining the role played by these microorganisms in the resistance to sudden changes in natural or anthropogenic conditions is a further, and important, issue. In addition, in this respect, the interest in dormant cells and in the study of viable and non-viable cells has increased. For this reason, a new methodology has been developed in this work; using a confocal laser microscope and two specific lasers, this has allowed us to ascertain the percentage of these cells in samples exposed to distinct stress conditions. The electronic scanning and transmission microscopes, both coupled to an X-ray dispersive energy detector, jointly with the X-ray transmission microscope from the ALBA synchrotron, have been used in samples prepared to evaluate morphological changes due to stress factors both in complete cells and in ultrafine sections, as well as in studies of extra- and intracellular metal extraction. The objectives of this work centre on the combined application of all these methodologies on two consortia of microorganisms: Scenedesmus sp. DE2009 and Geitlerinema sp. DE2011. The effect of light and salinity (as environmental parameters), in addition to the impact of lead, copper and chromium (as pollutants) was studied extensively in individual cells for both microorganisms. Finally, this thesis is organised into distinct chapters. The Chapter 3 correspond to the articles that have already been published (one of them currently under review); thus, the results obtained from the research carried out are therefore presented in Sections 3.1, 3.2 and 3.3 and are discussed globally in Chapter 4.

Les récifs coralliens se sont largement dégradés au cours de ces dernières décennies sous l’influence des activités anthropiques. Parmi ces perturbations, l’augmentation des concentrations métalliques concerne de nombreux récifs à travers le monde (e.g. grande barrière australienne, Costa Rica, Mer Rouge, Nouvelle-Calédonie). A ces stress locaux vient s’ajouter le changement climatique, et plus particulièrement l’élévation des températures et l’acidification des océans, auxquelles les coraux doivent désormais faire face. Dans ce contexte particulier, les objectifs de cette thèse ont donc été (1) de déterminer l’effet sur la physiologie corallienne, des principaux métaux présents dans les sédiments latéritiques (fer, manganèse, nickel et cobalt), à des concentrations représentatives de celles mesurées sur le littoral calédonien, et (2) de définir leurs rôles potentiels dans ce contexte de changement climatique. Les résultats obtenus ont montré que les métaux ont des effets très différents sur le métabolisme corallien. Alors qu’à température ambiante un apport en nickel ou en manganèse stimule le métabolisme de l’hôte et la photosynthèse des symbiotes, à l’inverse, un enrichissement en cobalt inhibe la calcification et devient même toxique dès 1.0 μg L-1 pour les symbiotes et l’hôte. De même, malgré son importance dans les processus photosynthétiques, un apport en fer diminue la densité en Symbiodinium des tissus et inhibe la calcification. En période de stress thermique, le manganèse augmente la tolérance des coraux à cette hausse de température, probablement en stimulant leurs défenses antioxydantes, tandis que le nickel aggrave ces effets en diminuant encore davantage leur croissance. Ces travaux constituent une étape importante vers une meilleure compréhension de la réaction des coraux aux enrichissements métalliques et permettraient d’expliquer, dans une certaine mesure, la sensibilité des espèces coralliennes aux changements climatiques
Coral reefs have largely degraded in recent decades under the influence of human activities. Among those disturbances, the increase in metal concentrations affects many reefs worldwide (e.g. Australian Great Barrier Reef, Costa Rica, Red Sea, New Caledonia). Furthermore, reefs have now to face climate change, and more particularly temperature increase and ocean acidification. In this context, the aims of my thesis were to (1) determine the effects of the main metals present in lateritic sediments (iron, manganese, nickel and cobalt) on coral physiology using concentrations representative of those measured along the New Caledonian coastline, and to (2) define their potential roles in this context of climate change. My results showed that metals affect coral metabolism in different ways. While at ambient temperature, a nickel or manganese enrichment stimulates host metabolism and symbiont photosynthesis, conversely a cobalt enrichment inhibits calcification and becomes even toxic for the host and symbionts, from 1 μg L-1. Despite its importance in photosynthetic processes, an iron enrichment induces a decrease in Symbiodinium densities and an inhibition of calcification rates. Under thermal stress, manganese enhances coral tolerance to temperature increase, likely by stimulating their antioxidant defenses, while nickel worsens its effects by decreasing even more their growth. These works represent an important step towards a better understanding of coral responses to metal enrichment and would explain, to some extent, species susceptibility to climate change

A flexible Metal–Organic Framework Zn4O(BenzTB)3/2 (DUT-13) was obtained by combination of a tetratopic linker and Zn4O6+ as connector. The material has a corundum topology and shows the highest pore volume among flexible MOFs
Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich

A flexible Metal–Organic Framework Zn4O(BenzTB)3/2 (DUT-13) was obtained by combination of a tetratopic linker and Zn4O6+ as connector. The material has a corundum topology and shows the highest pore volume among flexible MOFs.
Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.

In dieser Arbeit wird das Wachstum von dünnen quasi-kristallinen Ge-Sb-Te (GST) Schichten mittels Molekularstrahlepitaxie demonstriert, die zu einer geordneten Konfiguration von intrinsischen Kristallgitterfehlstellen führen. Es wird gezeigt, wie es eine Strukturanalyse basierend auf Röntgenstrahlbeugungssimulationen, Dichtefunktionaltheorie und Transmissionselektronenmikroskopie ermöglicht, eine eindeutige Beurteilung der Kristallgitterlückenanordnung in den GST-Proben vorzunehmen. Das Verständnis für die Ordnungsprozesse der Gitterfehlstellen erlaubt eine gezielte Einstellung des Ordnungsgrades selbst, der mit der Zusammensetzung und der Kristallphase des Materials in Zusammenhang steht. Auf dieser Basis wurde ein Phasendiagramm mit verschiedenen Wachstumsfenstern für GST erstellt. Des Weiteren wird gezeigt, dass man eine hohe Ordnung der Gitterfehlstellen in GST auch durch Ausheizprozesse und anhand von Femtosekunden-gepulster Laserkristallisation von amorphem Material erhält, das zuvor auf einem als Kristallisationsgrundlage dienenden Substrat abgeschiedenen wurde. Diese Erkenntnis ist bemerkenswert, da sie zeigt, dass sich kristalline GST Schichten mit geordneten Kristallgitterlücken durch verschiedene Herstellungsprozesse realisieren lassen. Darüber hinaus wurde das Wachstum von GeTe/Sb2Te3 Übergittern durchgeführt, deren Struktur die von GST mit geordneten Gitterfehlstellen widerspiegelt. Die Möglichkeit den Grad der Gitterfehlstellenordung in GST gezielt zu manipulieren wurde mit einer Studie der Transporteigenschaften kombiniert. Die Anwendung von großflächigen Charakterisierungsmethoden wie XRD, Raman und IR-Spektroskopie, erlaubte die Bestimmung der Phase und des Fehlstellenordnungsgrades von GST und zeigte eindeutig, dass die Fehlstellenordnung für den Metall-Isolator-Übergang (MIT) verantwortlich ist. Insbesondere wird durch das Vergleichen von XRD-Messungen mit elektrischen Messungen gezeigt, dass der Übergang von isolierend zu leitend erfolgt, sobald eine Ordnung der Kristallgitterlücken einsetzt. Dieses Phänomen tritt in der kubischen Kristallphase auf, wenn Gitterfehlstellen in GST von einem ungeordneten in einen geordneten Zustand übergehen. Im zweiten Teil des Kapitels wird eine Kombination aus FIR- und Raman-Spektroskopie zur Untersuchung der Vibrationsmoden und des Ladungsträgerverhaltens in der amorphen und der kristallinen Phase angewendet, um Aktivierungsenergien für die Elektronenleitung, sowohl für die kubische, als auch für die trigonale Kristallphase von GST zu bestimmen. Hier ist es wichtig zu erwähnen, dass, in Übereinstimmung mit Ergebnissen aus anderen Untersuchungen, das Auftauchen eines MIT beim Übergang zwischen der ungeordneten und der geordneten kubischen Phase beobachtet wurde. Schlussendlich wurden verschiedene sogenannte Pump/Probe Technik, bei der man das Material mit dem Laser anregt und die Röntgenstrahlung oder Terahertz (THz)-spektroskopie als Sonde nutzt, angewandt. Dies dient um ultra-schnelle Dynamiken zu erfassen, die zum Verständnis der Umschaltmechanismen nötig sind. Die Empfindlichkeit der THz-Messungen hinsichtlich der Leitfähigkeit, sowohl in GST, als auch in GeTe/Sb2Te3 Übergittern zeigte, dass die nicht-thermische Natur der Übergitterumschaltprozesse mit Grenzflächeneffekten zusammenhängt und . Der Ablauf wird mit beeindruckender geringer Laser-Fluenz erreicht. Dieses Ergebnis stimmt mit Berichten aus der Literatur überein, in denen ein Kristall-zu Kristallwechsel von auf Übergittern basierenden Speicherzellen für effizienter gehalten wird als GST Schmelzen, was zu einen ultra-schwachen Energieverbrauch führt.
The growth by molecular beam epitaxy of Ge-Sb-Te (GST) alloys resulting in quasi-single-crystalline films with ordered configuration of intrinsic vacancies is demonstrated. It is shown how a structural characterization based on transmission electron microscopy, X-ray diffraction and density functional theory, allowed to unequivocally assess the vacancy ordering in GST samples, which was so far only predicted. The understanding of the ordering process enabled the realization of a fine tuning of the ordering degree itself, which is linked to composition and crystalline phase. A phase diagram with the different growth windows for GST is obtained. High degree of vacancy ordering in GST is also obtained through annealing and via femtosecond-pulsed laser crystallization of amorphous material deposited on a crystalline substrate, which acts as a template for the crystallization. This finding is remarkable as it demonstrates that it is possible to create a crystalline GST with ordered vacancies by using different fabrication procedures. Growth and structural characterization of GeTe/Sb2Te3 superlattices is also obtained. Their structure resembles that of ordered GST, with exception of the Sb and Ge layers stacking sequence. The possibility to tune the degree of vacancy ordering in GST has been combined with a study of its transport properties. Employing global characterization methods such as XRD, Raman and Far-Infrared spectroscopy, the phase and ordering degree of the GST was assessed, and unequivocally demonstrated that vacancy ordering in GST drives the metal-insulator transition (MIT). In particular, first it is shown that by comparing electrical measurements to XRD, the transition from insulating to metallic behavior is obtained as soon as vacancies start to order. This phenomenon occurs within the cubic phase, when GST evolves from disordered to ordered. In the second part of the chapter, a combination of Far-Infrared and Raman spectroscopy is employed to investigate vibrational modes and the carrier behavior in amorphous and crystalline phases, enabling to extract activation energies for the electron conduction for both cubic and trigonal GST phases. Most important, a MIT is clearly identified to occur at the onset of the transition between the disordered and the ordered cubic phase, consistently with the electrical study. Finally, pump/probe schemes based on optical-pump/X-ray absorption and Terahertz (THz) spectroscopy-probes have been employed to access ultrafast dynamics necessary for the understanding of switching mechanisms. The sensitivity of THz-probe to conductivity in both GST and GeTe/Sb2Te3 superlattices showed that the non-thermal nature of switching in superlattices is related to interface effects, and can be triggered by employing up to one order less laser fluences if compared to GST. Such result agrees with literature, in which a crystal to crystal switching of superlattice based memory cells is expected to be more efficient than GST melting, therefore enabling ultra-low energy consumption.

碩士
淡江大學
電機工程學系碩士班
94
Radio wave is easily effected by obstacles in indoor environments. The obstacles are walls, ceilings, furniture in indoor environment. These obstacles will cause multiple reflection and diffraction of the radio waves and the phenomenon is called as multi-path effect. Due to this multi-path effect, the intersymbol inference (ISI) which increases the bit error rate and outage probability of the communication system occurred. As a result, the quality of communication becomes worse. The bit error rate (BER) performance for ultra-wide band (UWB) indoor communication with impact of metallic furniture is investigated. The impulse responses of different indoor environments for any transmitter and receiver location are computed by shooting and bouncing ray/image and inverse Fourier transform techniques. By using the impulse responses of these multipath channels, the BER performance for binary pulse amplitude modulation (BPAM) impulse radio UWB communication system are calculated. Numerical results have shown that the multi-path effect by the metallic cabinets is an important factor for BER performance. Also the outage probability for the UWB multi-path environment with metallic cabinets is more serious (about 18%) than with wooden cabinets. Finally, it is worth noting that in these cases the present work provides not only comparative information but also quantitative information on the performance reduction.

Ultrafine-grained metals produced by severe plastic deformation processes are distinguished with many outstanding mechanical properties such as a combination of high strength and ductility or improved fatigue behavior and thus can be used to meet the high strength requirements of the metal forming industry. Although several severe plastic deformation processes have been developed over the last three decades, the industrial utilization of such materials is still in an early stage. Production of bulk UFG materials is still expensive. Therefore, the few applications are limited to the sports goods, biomedical parts or sputtering targets where the price of the parts mostly doesn’t play a major role in the buying decision. To overcome the efficiency problems of current SPD methods, a new process, called “Equal Channel Angular Swaging” has been proposed which is based upon the combination of the conventional ECAP and the incremental bulk metal forming method of infeed rotary swaging. In the current study, firstly, mechanics of the process is investigated by using slip line field approach. It is determined that unlike conventional ECAP, due to the kinematics of the process, the friction forces help the deformation by drawing the samples into the forming operation. Accordingly, the loads in the feeding direction are relatively low. Therefore, ECAS has a great potential as a continuous and so economical SPD process. Secondly, in order to validate the slip line solution, a tool system has been developed. By the development, a thermo-mechanical coupled finite element simulation approach is utilized to investigate the effect of the geometrical parameters on material flow and temperature formation as well as process loads. It is determined that both, channel length and outer corner radius has a significant effect on the process. Accordingly, a middle channel length of 15 mm and an outer corner radius of 5 mm have been selected for the tool system of the model experiments. First experiments with the developed tool system prove the feasibility of ECAS process. The deformation of round bars from two different materials, commercially pure copper and low carbon steel was possible. Moreover, the deformation on the samples follows predominantly a simple shear pattern. Developed thermo-mechanical coupled finite element models are capable of representing the deformation as well as the process loads required to form the materials. However, due to the highly idealized assumptions by the slip line field approach, defined analytical formulas overestimate the real process forces. Nonetheless, the analytical theory is capable of representing the load correlations. The materials deformed by ECAS process are distinguished by an increased tensile strength compared to their as-received state. Moreover, microstructural investigations reveal that an average grain size under 1 µm can be achieved even after a single pass with the developed SPD process. Although model experiments prove the feasibility of ECAS and the process loads, especially in the feeding direction are low which demonstrate the potential for a continuous and economical processing, the investigated feed rate of 1 mm/s isn’t appropriate for a wide commercial implementation. Therefore, in the last step of investigations, the effect of the most important process parameters of friction, temperature, feeding speed and feeding type is investigated by means of finite element simulations. These simulations reveal that a discontinuous feeding is inevitable for the speed-up of ECAS process. Moreover, an active cooling is definitely necessary to keep the temperatures at acceptable levels and to ensure an efficient grain refinement.

Techniques and technologies to transfer electromagnetic energy through sub-wavelength channels have been researched extensively in the past few years because their application in different areas such as sub-wavelength imaging, telecommunication, increasing the storage capacity, and confinement and transmission of electromagnetic energy. Common ways of achieving such transmission includes exciting surface plasmon polaritons on both sides of the cannel or using double negative metamaterials. Recently a mechanism to squeeze the electromagnetic energy through sub-wavelength channels using materials with extremely small permittivity was introduced. Such materials may be found naturally at some limited frequencies in the infrared and optical frequency ranges, but they are commonly fabricated for a desired frequency as engineered metamaterials by by embedding metallic inclusions in a dielectric medium. The main problem with the engineered materials is that they have relatively large losses at their low permittivity frequency. In this thesis,I have presented a novel structure consisting of arrays of metallic wires that can be used to squeeze electromagnetic energy through sub-wavelength channels and junctions with negligible loss. The theory of transmission through such array is derived and design methods to tune the transmission frequency is provided. The structure is also tested numerically and experimentally in several geometries and results are compared with previous methods.

碩士
國立勤益科技大學
機械工程系
98
Nowadays, ultra fine channels and ribs cannot be formed on the commonly used graphite bipolar plates owing to low strength and brittleness of the graphite material. Therefore, this study used the stainless steel plate, SUS316L, with 1mm thick stainless, as the material of the bipolar plates, and these metallic bipolar plates are with reaction area of 20mm×20mm, height and depth of 0.8mm and ribs widths of 0.2mm, 0.4mm and 0.6mm respectively (open area rate of 85.36％, 70.24％, 58.19％,respectively). These formed metallic bipolar plates were assembled with MEA and subsequent cell performance experiments were performed to discussed the affections between the dimensions of the ribs and the channels and the performance of the proton exchange membrane fuel cell (PEMFC). The experimental results show that the metallic bipolar plate with ribs width of 0.4 mm has the optimum cell performance. The current density of the fuel cell with the metallic bipolar plate with rib width of 0.4 mm was 34.21% higher than the current density of the fuel cell with the metallic bipolar plate with rib width of 0.6mm while the voltage was 0.499V. Although, the metallic bipolar plate with 0.2mm rib width comprising the largest reaction area between fuel and MEA, the excessive open area rate reduced the effective electron transmitting, and increased internal resistance. Therefore, the performance of the metallic bipolar plate with 0.2 mm rib width is lower than the one with 0.4 mm rib width. On the other hand, the flow rate of the gas also affects the performance of the cells. When the flow rate of the gas was set at 60cc/min, better cell performances were found in all of the three cells with different rib and channel dimensions of the metallic bipolar plates. There were no significant performance differences between these three fuel cells when the flow rate of the gas was increased to 80cc/min because sufficient fuel was supplied for reaction when the flow rate was at 60cc/min. When the flow rate of the gas was set at 40cc/min, concentration polarization occurred in the fuel cells with bipolar plates with 0.6 and 0.4 mm ribs width owing to the short channel length with insufficient fuel flow. Better performance was found in the fuel cell with bipolar plate with 0.2mm rib width because of the longer length of the channel and the sufficient time for reaction.

碩士
國立臺灣科技大學
機械工程系
101
Bipolar plate is an important component of the fuel cell. Because there is no suitable fabrication process for mass-production of bipolar plate, the cost of portable fuel cell is still too high now a days. In this study, the rubber pad forming process was used to fabricate the micro-channels on metallic bipolar plate and the effect on process parameters of the rubber pad forming were analyzed. Polyurethane rubbers were used for the rubber pads, and SUS316L stainless steel sheets with a thickness of 0.1mm were tested in the experiment. Firstly, finite element analysis (FE, Ansys CFX software) was used to analyze the efficiency of fuel cell by geometric factor of channels numerically, in order to figure out the influence of velocity and pressure on channel depth, channel width, rib width. Secondly, finite element analysis (FE, Abaqus / Standard software) was also used to analyze the rubber pad forming process numerically. Finally, the experimental and numerical results showed a good agreement in this study. Furthermore, an optimization design of micro-channels for fuel cell was developed under rubber pad forming process.

A self-terminated electrochemical method was used to fabricate microscopic-scale contacts between two Au electrodes in a microfluidic channel. The conductance of contacts varies in a stepwise fashion showing quantization near the integer multiples of the conductance quantum ( ). The mechanism works by a pressure-driven flow parallel to a pair of Au electrodes with a gap on the order of micron in an electrolyte of HCl. When applying a bias voltage between two electrodes, metal atoms are etched off the anode and dissolved into the electrolyte as metal ions, which are then deposited onto the cathode. Consequently, the gap decreases to the atomic scale and then completely closes as the two electrodes form a contact. The electrochemical fabrication approach introduces large variance in the formation and location of individual junctions. Understanding and controlling this process will enable the precise positioning of reproducible geometries into nano-electronic devices.

The microstructure, and phase selections of La₆₆Al₁₄(Cu, Ni)₂₀ alloy were studied by Bridgman solidifications, and composite materials of dendrites in amorphous matrix or micro- and nano- sized eutectic matrix were formed with different cooling rates. The volume fraction of the dendrite phase reaches a maximum at the cooling rate of about 15 K/s, the secondary dendrite arm spacing λ₂ decreases from 4.3 µm to 0.6 µm with the increasing of cooling rate R, and obeys the equation of λ₂R⁰.⁵⁷=1.74µm(K/s)R⁰.⁵⁷. The compression strength, as well as the elastic strain limit of the dendrite/amorphous matrix composite are 600 MPa, and 2.3%, respectively. Improved ductility was observed for the dendrite amorphous matrix composites with more dendrite phase by slow cooling rate.
Singapore-MIT Alliance (SMA)

Additive manufacturing can bring several advantages in tooling applications especially hot working tooling as high pressure die casting. Printing of conformal cooling channels can lead to improved cooling and faster solidification, which, in turn, can possibly result in better quality of the cast part. However, few studies on advantages of additive manufactured tools in high pressure die casting are published.The aim of this study was to investigate and quantify the effect of conformal cooling on microstructure and mechanical properties of high pressure die cast aluminum alloy. Two tools each consisting of two die inserts were produced with and without conformal channels using additive manufacturing. Both tools were used in die casting of aluminum alloy. Aluminum specimens were then characterized microstructurally in light optical microscope for secondary arm spacing measurements and subjected to tensile and hardness testing. Cooling behavior of different inserts was studied with a thermal camera and by monitoring the temperature change of cooling oil during casting. Surface roughness of die inserts was measured with profilometer before and after casting.Thermal imaging of temperature as a function of time and temperature change of oil during casting cycle indicated that conformal insert had faster cooling and lower temperature compared to conventional insert. However, thermal imaging of temperature after each shot in a certain point of time showed higher maximum and minimum temperature on conformal die surface but no significant difference in normalized temperature gradient compared to the conventional insert.The average secondary dendrite arm spacing values were fairly similar for samples from conventional and conformal inserts, while more specimens from conventional insert demonstrated coarser structure. Slower cooling in conventional insert could result in the coarser secondary dendrite arm spacing.Tensile strength and hardness testing revealed no significant difference in mechanical properties of the specimens cast in conventional and conformal die inserts. However, reduced deviations in hardness was observed for samples cast with conformal insert. This is in agreement with secondary dendrite arm spacing measurements indicating improved cooling with conformal insert.Surface roughness measurement showed small wear of the inserts. More castings are needed to observe a possible difference in wear between the conventional and conformal inserts.Small observed differences in cooling rate and secondary arm spacing did not result in evident difference in mechanical properties of the aluminum alloy but the variation in properties were reduced for samples cast with conformal cooling. Future work may include more accurate measurement of cooling behavior with a thermocouple printed into the die insert, casting of thicker specimen for porosity evaluation and fatigue testing and longer casting series to evaluate the influence of conformal cooling on tool wear.