Dissertations / Theses on the topic 'POTENTIAL CATHODE'

Demand for lithium (Li) and cobalt (Co) is on the rise, due in part to their increased use in rechargeable Li-ion batteries (RLIB). Current recycling processes that utilize chemical leaching efficiently recover in Li and Co from the cathode material in spent batteries; however, these processes are costly and emit hazardous waste into the environment. Therefore, a more sustainable process for recycling Li and Co is needed, and bioleaching may provide a solution. Fungal bioleaching has been shown in previous studies to effectively mobilize metals (Pb, Al, Mn, Cu, and Zn) from mine tailings, electronic scrap, and spent batteries with organic acids. However, little is known regarding fungal tolerance to Li and Co, and if the concentrations of organic acids excreted by fungi can effectively leach Li and Co from the cathode material. In order to address these questions, experiments were first conducted to test the Li and Co leaching efficiency with organic acids at concentrations similar to what has been previously reported in fungal cultures. The remaining experiments were performed with three fungal species: Aspergillus niger, Penicillium chrysogenum, and Penicillium simplicissimum. First, fungal biomass production, pH and organic acid excretion were examined when the fungi were grown in Czapek dox broth (CDB) or Sabouraud dextrose broth (SDB). Second, fungal biomass production and pH were examined when the fungi were grown in the presence of Li or Co. This determines tolerance of the fungi to the metals, and if fungal processes were inhibited by the metals. Third, bioleaching was performed with cathode material from RLIB in batch cultures to test the ability of organic acids excreted by A. niger to mobilize Li and Co. Three bioleaching strategies, one-step, two-step, and spent-medium leaching techniques were used to mobilize Li and Co from the cathode in RLIB. Low concentrations of organic acids similar to what is excreted by fungi have not been tested to leach Li and Co from the cathode in RLIB. Results from chemical leaching with low concentrations of organic acids in this study indicate that organic acid leaching efficiency can be increased by utilizing higher concentrations (above 50 mM) of citric or oxalic acid to mobilize Li or Co from the cathode in RLIB. Furthermore, 100 mM of citric acid or 100 mM of oxalic acid mobilized more Co or Li than mixtures of organic acids. Notably the addition of hydrogen peroxide to mixed concentrations of organic acids significantly improved mobilization of Li and Co under abiotic conditions. Different growth media may alter biomass production and potentially organic acid excretion by the three fungal species. Analysis of biomass production by A. niger and P. simplicissimum showed that differences in media composition between CDB and SDB did not affect collected biomass for each species. However, CDB cultures with P. chrysogenum had significantly less biomass than SDB cultures after 10 days of growth. Differences in growth by P. chrysogenum between CDB and SDB may be attributed to preferred nutrients and/or low pH present in SDB cultures. Biomass production by the three fungi increased up to day 10 in CDB or SDB. This result indicated that nutrients in CDB or SDB were not limiting toward fungal growth. Cultures with A. niger had the highest concentrations of organic acids (50 mM of oxalic acid), followed by cultures with P. simplicissimum (30 mM oxalic acid), and P. chrysogenum (less than 5 mM oxalic acid). Organic acids excreted by all three fungal species were detected in cultures in CDB, while only A. niger and P. chrysogenum excreted organic acids in SDB cultures. Metals such a Li or Co present in the cathode of RLIB may be toxic to fungal processes when exposed to high metal concentrations. Metal tolerance experiments indicate that biomass production by the three fungi was significantly inhibited by 100 mg/L Co compared to controls, which contained no metal. Li at a concentration of 1000 mg/L inhibited biomass production by A. niger and P. simplicissimum. However, biomass production by P. chrysogenum was not significantly inhibited by 1000 mg/L Li. I found that P. simplicissimum was the most susceptible to toxic effects of Li and Co among the three fungi. In A. niger cultures amended with 100 mg/L Li or Co, pH at day 5 was similar to control cultures of A. niger without metals (pH 3.0 – 3.4), whereas pH was significantly higher in cultures with 1000 mg/L of Li or Co (pH 7.1 – 7.3). Cultures of A. niger were exposed to the cathode material from RLIB to test the leaching efficiency of excreted organic acids after mobilizing Li and Co. In bioleaching experiments with A. niger, organic acids excreted in the presence of cathode material from RLIB were quantified at concentrations under 50 mM. At the end of bioleaching experiments with A. niger, 40 mM tartaric acid was detected and was the highest produced organic acid in bioleaching cultures. However, with conditions set in this study, organic acids excreted by A. niger mobilized only ̴7% of Co and 20% of Li when using spent medium with cathode material from RLIB. According to findings in chemical leaching experiments, concentrations of organic acids higher than 50 mM will be required in fungal cultures to increase mobilization of Li or Co from the cathode material in RLIB. Modifying growth media to include higher concentrations of sucrose will potentially increase organic acid excretion as demonstrated in previous publications. Future studies should focus on how to maximize organic acid excretion by fungi when exposed to metals found in the cathode of RLIB.

>Magister Scientiae - MSc
Spinel lithium manganese oxide (LiMn2O4), for its low cost, easy preparation and nontoxicity, is regarded as a promising cathode material for lithium-ion batteries. However, a key problem prohibiting it from large scale commercialization is its severe capacity fading during cycling. The improvement of electrochemical cycling stability is greatly attributed to the suppression of Jahn-Teller distortion (Robertson et al., 1997) at the surface of the spinel LiMn2O4 particles. These side reactions result in Mn2+ dissolution mainly at the surface of the cathode during cycling, therefore surface modification of the cathode is deemed an effective way to reduce side reactions. The utilization of a nanocomposite which comprises of metallic Cu and Au were of interest because their oxidation gives rise to a variety of catalytically active configurations which advances the electrochemical property of Li-ion battery. In this research study, an experimental strategy based on doping the LiMn2O4 with small amounts of Cu-Au nanocomposite cations for substituting the Mn3+ ions, responsible for disproportionation, was employed in order to increase conductivity, improve structural stability and cycle life during successive charge and discharge cycles. The spinel cathode material was synthesized by coprecipitation method from a reaction of lithium hydroxide and manganese acetate using 1:2 ratio. The Cu-Au nanocomposite was synthesized via a chemical reduction method using copper acetate and gold acetate in a 1:3 ratio. Powder samples of LiMxMn2O4 (M = Cu-Au nanocomposite) was prepared from a mixture of stoichiometric amounts of Cu-Au nanocomposite and LiMn2O4 precursor. The novel LiMxMn2O4 material has a larger surface area which increases the Li+ diffusion coefficient and reduces the volumetric changes and lattice stresses caused by repeated Li+ insertion and expulsion. Structural and morphological sample analysis revealed that the modified cathode material have good crystallinity and well dispersed particles. These results corroborated the electrochemical behaviour of LiMxMn2O4 examined by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The diffusion coefficients for LiMn2O4 and LiMxMn2-xO4 obtained are 1.90 x10-3 cm2 / s and 6.09 x10-3 cm2 / s respectively which proved that the Cu-Au nanocomposite with energy band gap of 2.28 eV, effectively improved the electrochemical property. The charge / discharge value obtained from integrating the area under the curve of the oxidation peak and reduction peak for LiMxMn2-xO4 was 263.16 and 153.61 mAh / g compared to 239.16 mAh / g and 120 mAh / g for LiMn2O4. It is demonstrated that the presence of Cu-Au nanocomposite reduced side reactions and effectively improved the electrochemical performance of LiMn2O4.

The Rolls-Royce Integrated Planar Solid Oxide Fuel Cell (IP-SOFC) features anode, electrolyte and cathode layers of 5-20 pm in thickness, connected in series by highly-conductive precious-metal based current collecting layers of ~10 mum thickness, which are screen-printed and sintered upon a porous substrate. Replacement of the palladium- based cathode current collector material is desirable for increasing the economic and environmental potential of the IP-SOFC system, due to low resource availability, high cost and environmental degradation caused by mining of platinum-group metals. The electrically conductive ABO3 perovskite-structured lanthanum transition-metal oxide ceramics were identified as potential cathode current collector materials, and lanthanum nickel ferrite materials, of compositions LaNi0.6Fe0.4O3 and LaNi0.5Fe0.5O3, were selected for investigation based on the combined favourable properties of electrical conductivity, phase stability and compatible coefficient of thermal expansion with other cell materials. Both compositions were found to be reactive towards the IP-SOFC cathode materials, lanthanum strontium manganite (LSM) and yttria-stabilised zirconia (YSZ), and the lower conductivity of LaNi0.5Fe0.5O3 compared with LaNi0.6Fe0.4O3 meant a thicker layer would be required to meet the conductivity requirements, which negates the advantage of its more suitable coefficient of thermal expansion. It was found that a LaNi0.6Fe0.4O3 layer of ~80 mum was adequate to meet the conductivity target, and could be applied by a single stencil-print and sintered at 1125°C, which is compatible with the screen-printing and firing production line, although the manufacturing method requires optimisation to eliminate layer defects. In addition it is believed that the material can offer significant economic and environmental advantages over the present palladium-based cathode current collector. However the reaction of LaNi0.6Fe0.4O3 (LNF) with LSI'I was found to critically compromise its use in conjunction with an LSM-based cathode. Efforts to incorporate an LNF-based cathode also failed due to reaction of LNF with YSZ and gadolinium-doped ceria (CGO), and it must be concluded that the reactivity of LNF with common solid oxide fuel cell materials severely limits its potential to be used as a cathode current collector layer in the IP-SOFC.

La2Mo2-yWyO9 (y = 1.0 to 2.0) oxides were synthesized by conventional solid state route and studied by XRD, TC-XRD and DTA. A phase diagram of the series was proposed. The thermodynamically stable phases at room temperature are: for 1.0≤ y ≤1.2 a cubic β-La2Mo2O9 type solid solution, for 1.3≤ y ≤1.575 a biphasic mixture of β-La2Mo2O9 type + α-La2W2O9 type phases, and for 1.6≤ y ≤2.0 a triclinic α-La2W2O9 type solid solution. Inhomogeneous distribution of W is suspected in the biphasic samples. It is clear that the compounds above y =1.2 are not suitable for SOFC applications.Cationic diffusion studies were performed using SIMS on La2Mo2O9 (LMO)/La0.8Sr0.2MnO3-δ (LSM) annealed couples. Rod shaped LaMnO3 grains were observed on LMO pellet and SrMoO4 type phases were seen to be growing on LSM pellet. Hypotheses for possible reaction mechanisms are presented. Bulk diffusion coefficients of Sr and Mn in LMO and of Mo in LSM are extrapolated to be around 1x10-20 cm2.s-1 and 1x10-15 cm2.s-1, respectively, at 800oC. Similar diffusion studies were performed by depositing Mn and Sr cation rich solutions on LMO pellets and Mo rich solution on LSM pellet. Mn solution was observed to be forming, upon annealing, LaMnO3 single crystals on the surface of the LMO pellet. Mo in LSM and Sr in LMO diffusion coefficients appear to be much higher than in LMO/LSM couple experiments, namely around 1-2x10-10cm2.s-1 at 1150°C. Because of the reactivity, LMO/LSM couple is not desirable for SOFC applications, unless an appropriate buffer layer separates them.The stability of LMO and W-LMO was studied under reductive atmospheres. Successive structural changes from LMO to La7Mo7O30 (7730), an amorphous reduced phase La2Mo2O7-δ, and partial decomposition to metallic Mo were observed as a function of oxygen loss. The pO2 stability domain of La2Mo2-yWyO9 did not appear to change with W content, but the reduction kinetics varied with y. At reverse, the stability limit of the 7730 phase was found to be dependent on W content. The amorphous reduced phase can accommodate a wide range of oxygen stoichiometry (7-δ from 6.69 to 6.20), but its stability vs. pO2 is questioned. Resistivity measurements performed on a low compacity crack-free amorphous La2Mo2O7-δ sample showed significant increase in the conductivity (> 1 S.cm-1 at 1000 K) relative to La2Mo2O9, with a pseudo activation energy 0.255eV. It is postulated that n-type electronic conductivity arises from partial reduction of hexavalent Mo6+ to a mixture of Mo3+ and Mo4+.

This thesis is concerned with kinetic characterisation of theinsertion compound LiMn2O4, which is used as positive electrodematerial in rechargeable lithium batteries. Three different typesof electrode configurations have been investigated, namely singleparticles, thin films and composite electrodes. Differentelectrochemical techniques, i.e. linear sweep voltammetry (LSV),electrochemical impedance spectroscopy (EIS), potential step, andgalvanostatic experiments were applied under various experimentalconditions. The majority of the experimental data were analysedby relevant mathematical models used for describing the reactionsteps of insertion compounds.

It was concluded that a model based on interfacialcharge-transfer, solid-phase diffusion and an external iR-dropcould be fairly well fitted to LSV data measured on a singleelectrode system over a narrow range of sweep rates. However, itwas also found that the fitted parameter values vary greatly withthe characteristic length and the sweep rate. This indicates thatthe physical description used is too simple for explaining theelectrochemical responses measured over a large range of chargeand discharge rates.

EIS was found to be a well-suited technique for separatingtime constants for different physical processes in the insertionand extraction reaction. It was demonstrated that the impedanceresponse is strongly dependent on the current collector used.According to the literature, reasonable values of theexchange-current density and solid-phase diffusion coefficientwere determined for various states-of-discharge, temperatures andelectrolyte compositions. Experiments were carried out in bothliquid and gel electrolytes. A method which improves thedistinction between the time constants related to thematerial’s intrinsic properties and possible porous effectsis presented. The method was applied to composite electrodes.This method utilises, in addition to the impedance responsemeasured in front of the electrode, also the impedance measuredat the backside of the electrode.

Finally, the kinetics of a composite electrode was alsoinvestigated by in situ X-ray diffraction (in situ XRD) incombination with galvanostatic and potentiostatic experiments. Noevidence of lithium concentration gradients could be observedfrom XRD data, even at the highest rate applied (i.e. ~6C), thusexcluding solid-phase diffusion and also phase-boundary movement,as described by Fick’s law, as the ratelimiting step.

Key words:linear sweep voltammetry, electrochemicalimpedance spectroscopy, potential step, in situ X-raydiffraction, microelectrodes, electrode kinetics, LiMn2O4cathode, rechargeable lithium batteries

A thin wire Ag/AgCl reference electrode was prepared using 50 μm Ag wire in dilute FeCl3. The wire was embedded beneath the polyurethane topcoat of two sacrificial coating systems to monitor their corrosion potential. This is the first report of a reference electrode embedded between organic coating layers to monitor substrate health. The embedded reference electrode (ERE) successfully monitored the corrosion potential of Mg primer on AA 2024-T3 for 800 days of constant immersion in dilute Harrison’s solution. Zn primer on steel had low accuracy in comparison. This is in part due to short circuiting by Zn oxidation products, which are much more conductive than Mg corrosion products. Data interpretation was improved through statistical analysis. On average, ERE corrosion potentials are 0.1 to 0.2 V and 0.2 to 0.3 V more positive than a saturated calomel electrode (SCE) in solution for AA 2024-T3 and steel coating systems, respectively. Further research may confirm that ERE obtains corrosion potential information not possible by an exterior, conventional reference electrode. The ERE is stable under polarization. AA 2024-T3 was polarized to -0.95 V vs ERE to emulate controlled potential cathodic protection (CPCP) applications. Polarizations of -0.75 V vs ERE are recommended for future experiments to minimize cathodic delamination. The ERE was utilized to analyze coating mixtures of lithium carbonate, magnesium nitrate, and Mg metal on AA2024-T3. Corrosion potential, low frequency impedance by electrochemical impedance spectroscopy (EIS), and noise resistance by electrochemical noise method (ENM) were reported. Coating performance ranking is consistent with standard electrochemical characterization and visual analyses. The results suggest anti-corrosion resistance superior to a standard Mg primer following 1600 hours of B117 salt spray. Both lithium carbonate and magnesium nitrate are necessary to achieve corrosion protection. Unique corrosion protective coatings for aluminum could be designed through continued mixture optimization. The Ag wire ERE has been utilized for the characterization and ranking of experimental coatings on metal substrates. Structural health monitoring and corrosion potential feedback of cathodic protection systems are additional uses. There is some indication that CPCP may be applied by ERE to control the substrate polarization for an organic coating system.

Graphene has been shown to be very powerful as a transducer in many biosensor applications due to its high sensitivity. This enables smaller surfaces and therefore less material consumption when producing sensors and concequently cheaper and more portable sensors compared to the commercially available sensors today. The electrical properties of graphene are very sensitive to gas exposure why presence of molecules or small changes in concentration could easily be detected when using graphene as a sensing layer. Graphene is sensitive towards many molecules and in order to detect and possibly identify gas molecules the surface needs to be functionalized. The intention of this project was to use nanoparticles (NPs) to further increase sensitivity and specificity towards selected molecules and also enable biofunctionalization of the NPs, and by that tune the electrical properties of the graphene. This study proposes the use of Fe3O4 and TiO2 NPs to enable sensitive detection of volatile gases and possibly further functionalization of the NPs using biomolecules as a detecting agent in a liquid-phasebiosensor application. The interaction between graphene and NPs have been investigated using several surface charactarization methods and electrical measurements for detection of gaseous molecules and also molecules in a liquid solution. The characterizing methods used are XPS, AFM with surface-potential mapping and Raman spectroscopy with reflectance mapping in order to investigate the NPs interaction with the graphene surface. Sensors where manufactured for gas-phase detection of CO, formaldehyde, benzene and NH3 specifically and display differences in sensitivity and behavior of the Fe3O4 and TiO2 NPs respectively. For liquid measurements the difference in behavior in two buffers was investigated using an in-house flow-cell setup. The surface charecterizing measurements indicated that just a small difference could be found between the two NPs, however a significant change in sensor response could be detected as a function of coverage. The liquid and gas-phase measurements rendered information on differences in sensitivity between the NPs and between analytes where TiO2 showed a higher level of sensitivity towards most of the gases investigated. Both Fe3O4 and TiO2 NP coated graphene showed capability to detect formaldehyde and benzene down to 50 ppb and 5 ppb respectively. The sensitive gas detection could help protecting individuals being exposed to a hazardous level of volatile gases if concentrations increase rapidly or at a long term exposure with lower concentrations, improving saftey and health where these gases are present.

Cette thèse visait à mieux comprendre la structure, les propriétés électrochimiques, la surface et les propriétés de transport des phases spinelles haut potentiel LiNi0. 5Mn1. 5O4 sous forme de poudre et de films minces, matériaux de cathode potentiel pour les batteries Li-ion. L'effet des paramètres de synthèse sur la stœchiométrie en oxygène et sur les phases formées a été examiné. Nous avons proposé une transformation de phase de type peritectoid pour expliquer la formation de phases de type NaCl. Les films minces de LiNi0. 5Mn1. 5O4 ont été déposés par Ablation Laser Pulsée. L'impact de la pression et de la température de dépôt sur la microstructure, la morphologie et les propriétés électrochimiques a été étudié. Le coefficient apparent de diffusion du lithium dans ces phases a été estimé à 1~2 ×10−12 cm2 s–1. Nous avons de plus mis en évidence l'effet bénéfique sur les retentions de capacité d'enrobages à base d'aluminium déposé par la méthode ALD et traité thermiquement. Enfin, les propriétés de transport des films mince de LiMn1. 5Ni0. 5O4-δ ont été mesurées. La stœchiométrie en oxygène apparaît comme le facteur principal qui contrôle la conductivité électronique
The thesis objectives were to obtain a deeper understanding of the structure, electrochemical properties, surface modification, and transport properties of high voltage spinel LiNi0. 5Mn1. 5O4 powders and thin films as cathode material in lithium ion battery. The effect of synthesis parameters on oxygen deficiency and the formed phases were investigated. We proposed a peritectoid phase transition to explain the presence of some rock salt impurities. LiNi0. 5Mn1. 5O4 thin films were deposited by Pulsed Laser Deposition (PLD) method. The effect of deposition pressure and temperature on microstructure, morphology, and electrochemical properties was investigated. The apparent lithium diffusion coefficient was estimated to a value of 1~2 ×10−12 cm2 s–1. We evidenced the beneficial effect of ALD prepared Al2O3-based coating when an annealing treatment was performed at 600oC. We propose that this is linked with the formation of LiAlx(Ni0. 5Mn1. 5)yO4 lithium ion conductor thus allowing improving the capacity retention. The transport properties of LiMn1. 5Ni0. 5O4-δ thin film via PLD were measured. It is demonstrated that the oxygen stoichiometry is the main factor for controlling the electronic conductivity

Les piles à combustible à membrane échangeuse de protons sont des dispositifs de conversion de l’énergie propre et efficace. Les gammes de puissance accessibles permettent leur utilisation dans le domaine de transport et des applications stationnaires. Il existe deux verrous technologiques à lever pour le déploiement de la cathode:i) Diminution de la quantité de platine dans le catalyseur.ii) l'amélioration de la stabilité du support de catalyseur à haut potentiel. Dans ce travail, nous présentons deux stratégies qui permettent de faire face à ces problématiques et améliorer les performances et la durabilité des cathodes: développer de nouveaux électrocatalyseurs à très faible quantité de platine et des supports à base de matériaux résistants à la corrosion.Afin de réduire la quantité de platine dans le catalyseur, nous avons développé des nanostructures fines de platine, qui permettent une exploitation électrocatalytique maximale et une quantité minimale de métal noble. Pour atteindre cet objectif, nous avons utilisé une méthode électrochimique basée sur le dépôt sous potentiel et le déplacement galvanique. Les nanostructures fines déposées sur le substrat modèle ont été caractérisées électrochimiquement ainsi que par des techniques microscopiques et d'analyse élémentaire.Pour réaliser un support résistant à la corrosion, notre approche a consisté en le remplacement du carbone noir conventionnel par un matériau conducteur d’oxyde d'étain dopé. Les matériaux à base de SnO2 ont démontré leur efficacité comme support électrochimique stable mais également efficace pour la réaction de réduction de l'oxygène. Dans cette étude, l’oxyde d’étain dopé au tantale a été préparé par électrofilage suivi d'une étape de calcination, permettent ainsi d’obtenir des fibres de morphologie tubulaires. Ces fibres ont été utilisées comme support de nanoparticules de platine préparées par la méthode de polyol assistée par micro-ondes, puis caractérisées pour leurs propriétés physico-chimiques et électrocatalytiques. En particulier, la stabilité aux cyclage en potentiel a été évaluée par analyse électrochimique ex situ. La possibilité d’associer l'électrocatalyseur à surface étendue avec les supports résistants à la corrosion pour obtenir des cathodes actives et durables est en cours
Proton exchange membrane fuel cells are clean and efficient energy converters. Their accessible power ranges allow their use in the field of transport or stationary applications. Two main challenges concern the cathode deployment:i) The reduction of the amount of low abundant platinum group metal in the catalyst.ii) The enhancement of stability of the catalyst support at high voltage.In this work we present two strategies to address these challenges and improve performance and durability of the cathodes: developing novel ultra-low loaded platinum electrocatalysts and corrosion resistant support materials.To reduce noble metal amount in the catalyst, we developed platinum thin films, which allow maximal electrocatalytic exploitation thus minimal loading. For that, we have used electrochemical methods based on under-potential deposition and galvanic displacement. The thin structures deposited on model substrates were characterized by electrochemical, elemental analysis and microscopy techniques.To prepare corrosion resistant supports, our strategy was the replacement of conventional carbon black with a doped conducting tin oxide. SnO2-based materials have been demonstrated as electrochemical stable supports also promoting platinum activity for the oxygen reduction reaction. In this work, tantalum-doped tin oxide was prepared by electrospinning followed by calcination, leading to a fiber-in-tube morphology. This support was catalyzed with platinum nanoparticles prepared by a microwave-assisted polyol method, and characterized for their physico-chemical and electrocatalytic properties. In particular, stability to voltage cycling was evaluated by ex situ electrochemical analysis.The possibility to associate the extended surface electrocatalyst with the corrosion resistant supports to obtain active and durable cathodes is in progress

This investigation determined that severe corrosion of steel can occur in the submerged portions of reinforced concrete structures in marine environments. Field studies of decommissioned pilings from actual bridges revealed multiple instances of strong corrosion localization, showing appreciable local loss of steel cross-section. Quantitative understanding of the phenomenon and its causes was developed and articulated in the form of a predictive model. The predictive model output was consistent with both the corrosion rate estimates and the extent of corrosion localization observed in the field observations. The most likely explanation for the observed phenomena that emerged from the understanding and modeling is that cathodic reaction rates under oxygen diffusional limitation that are negligible in cases of uniform corrosion can nevertheless support substantial corrosion rates if the corrosion becomes localized. A dynamic evolution form of the model was created based on the proposition that much of the steel in the submerged concrete zone remained in the passive condition given cathodic prevention that resulted from favorable macrocell coupling with regions of the steel that had experienced corrosion first. The model output also matched observations from the field, supporting the plausibility of the proposed scenario. The modeling also projected that corrosion in the submerged zone could be virtually eliminated via the use of sacrificial anode cathodic protection; the rate of corrosion damage progression in the low elevation zone above water could also be significantly reduced. Continuation work should be conducted to define an alternative to the prevalent limit-state i.e., visible external cracks and spalls, for submerged reinforced concrete structures. Work should also be conducted to determine the possible structural consequences of this form of corrosion and to assess the technical feasibility and cost/benefit aspects of incorporating protective anodes in new pile construction.

Ce travail de thèse a pour but d'explorer de nouveaux matériaux de type structural Tavorite et de revisiter certains déjà bien connus. Dans un premier temps, les synthèses de compositions ciblées ont été réalisées selon des procédures variées (voies tout solide, hydrothermale, céramique assistée par sol-gel, broyage mécanique) afin de stabiliser d'éventuelles phases métastables et d'ajuster la microstructure impactant fortement les performances électrochimiques de tels matériaux polyanioniques. Ces matériaux ont ensuite été décrits en profondeur, dans leurs états originaux, depuis leurs structures moyennes, grâce aux techniques de diffraction (diffraction des rayons X sur poudres ou sur monocristaux et diffraction des neutrons) jusqu'aux environnements locaux, en utilisant des techniques de spectroscopie (résonance magnétique nucléaire à l'état solide, absorption des rayons X, infra-rouge et Raman). Par la suite, les diagrammes de phases et les processus d'oxydoréduction impliqués pendant l'activité électrochimique des matériaux ont été étudiés grâce à des techniques operando (diffraction et absorption des rayons X). La compréhension des mécanismes impliqués pendant le cyclage permet de mettre en évidence les raisons de leurs limitations électrochimiques : La synthèse de nouveaux matériaux (composition, structure, microstructure) peut maintenant être développée afin de contrepasser ces limitations et de tendre vers de meilleures performances
This PhD work aims at exploring new Tavorite-type materials and at revisiting some of the well-known ones. The syntheses of targeted compositions were firstly performed using various ways (all solid state, hydrothermal, sol-gel assisted ceramic, ball milling) in order to stabilize eventual metastable phases and tune the microstructure impacting strongly the electrochemical performances of such polyanionic compounds. The materials were then described in-depth, at the pristine state, from their average long range structures, thanks to diffraction techniques (powder X-rays, single crystal X-rays and neutrons diffraction), to their local environments, using spectroscopy techniques (solid state Nuclear Magnetic Resonance, X-rays Absorption Spectroscopy, Infra-Red and/or Raman). Thereafter, the phase diagrams and the redox processes involved during electrochemical operation of the materials were investigated thanks to operando techniques (SXRPD and XAS). The in-depth understanding of the mechanisms involved during cycling allows to highlight the reasons of their electrochemical limitations: the synthesis of new materials (composition, structure and microstructure) can now be developed to overcome these limitations and tend toward better performance

The polarization performance of two types of commercial galvanic point anodes for protection of reinforced steel around patch repairs was investigated. Experiments included measurement of the polarization history of the anode under constant current impressed by galvanostatic circuits and in reinforced concrete slabs. The tests revealed, for both types of anodes, a potential-current function (PF) indicating relatively little anodic polarization from an open circuit potential at low current levels, followed by an abrupt increase in potential as the current approached an apparent terminal value. Aging of the anodes was manifested by a continually decreasing current output in the concrete tests, and by increasingly more positive potentials in the galvanostatic tests. Those changes reflected an evolution of the PF generally toward more positive open circuit potentials and, more importantly, to the onset of elevated polarized potentials at increasingly lower current levels. There was considerable variability among the performance of replicate units of a given anode type. Modest to poor steel polarization levels were achieved in the test yard slabs. Modeling of a generic patch configuration was implemented with a one-dimensional approximation. The model calculated the throwing distance that could be achieved by a given number of anodes per unit perimeter of the patch, concrete thickness, concrete resistivity, amount of steel and amount of polarization needed for cathodic prevention. The model projections and aging information suggest that anode performance in likely application scenarios may seriously degrade after only a few years of operation, even if a relatively optimistic 100 mV corrosion prevention criterion were assumed. Less conservative criteria have been proposed in the literature but are yet to be substantiated. Other investigations suggest a significantly more conservative corrosion prevention may apply instead. The latter case would question the ability of the point anodes to provide adequate corrosion prevention.

Three mathematical models, two of primary alkaline battery cathode discharge, and one of primary alkaline battery discharge, are developed, presented, solved and investigated in this thesis. The primary aim of this work is to improve our understanding of the complex, interrelated and nonlinear processes that occur within primary alkaline batteries during discharge. We use perturbation techniques and Laplace transforms to analyse and simplify an existing model of primary alkaline battery cathode under galvanostatic discharge. The process highlights key phenomena, and removes those phenomena that have very little effect on discharge from the model. We find that electrolyte variation within Electrolytic Manganese Dioxide (EMD) particles is negligible, but proton diffusion within EMD crystals is important. The simplification process results in a significant reduction in the number of model equations, and greatly decreases the computational overhead of the numerical simulation software. In addition, the model results based on this simplified framework compare well with available experimental data. The second model of the primary alkaline battery cathode discharge simulates step potential electrochemical spectroscopy discharges, and is used to improve our understanding of the multi-reaction nature of the reduction of EMD. We find that a single-reaction framework is able to simulate multi-reaction behaviour through the use of a nonlinear ion-ion interaction term. The third model simulates the full primary alkaline battery system, and accounts for the precipitation of zinc oxide within the separator (and other regions), and subsequent internal short circuit through this phase. It was found that an internal short circuit is created at the beginning of discharge, and this self-discharge may be exacerbated by discharging the cell intermittently. We find that using a thicker separator paper is a very effective way of minimising self-discharge behaviour. The equations describing the three models are solved numerically in MATLABR, using three pieces of numerical simulation software. They provide a flexible and powerful set of primary alkaline battery discharge prediction tools, that leverage the simplified model framework, allowing them to be easily run on a desktop PC.

Three mathematical models, two of primary alkaline battery cathode discharge, and one of primary alkaline battery discharge, are developed, presented, solved and investigated in this thesis. The primary aim of this work is to improve our understanding of the complex, interrelated and nonlinear processes that occur within primary alkaline batteries during discharge. We use perturbation techniques and Laplace transforms to analyse and simplify an existing model of primary alkaline battery cathode under galvanostatic discharge. The process highlights key phenomena, and removes those phenomena that have very little effect on discharge from the model. We find that electrolyte variation within Electrolytic Manganese Dioxide (EMD) particles is negligible, but proton diffusion within EMD crystals is important. The simplification process results in a significant reduction in the number of model equations, and greatly decreases the computational overhead of the numerical simulation software. In addition, the model results based on this simplified framework compare well with available experimental data. The second model of the primary alkaline battery cathode discharge simulates step potential electrochemical spectroscopy discharges, and is used to improve our understanding of the multi-reaction nature of the reduction of EMD. We find that a single-reaction framework is able to simulate multi-reaction behaviour through the use of a nonlinear ion-ion interaction term. The third model simulates the full primary alkaline battery system, and accounts for the precipitation of zinc oxide within the separator (and other regions), and subsequent internal short circuit through this phase. It was found that an internal short circuit is created at the beginning of discharge, and this self-discharge may be exacerbated by discharging the cell intermittently. We find that using a thicker separator paper is a very effective way of minimising self-discharge behaviour. The equations describing the three models are solved numerically in MATLABR, using three pieces of numerical simulation software. They provide a flexible and powerful set of primary alkaline battery discharge prediction tools, that leverage the simplified model framework, allowing them to be easily run on a desktop PC.

Des revêtements fer-silicium-carbone ont été réalisés par deux techniques différentes (projection d'alliages fondus et pulvérisation cathodique), sur des substrats de fer, fonte et verre et pour des teneurs variables en silicium et carbone. L'analyse structurale par diffraction de rayons X (en mode thêta/2 thêta et en incidence rasante) et par réflectométrie et l'emploi de fentes de Sollers ont conduit à une caractérisation détaillée et précise des revêtements (épaisseur texture, présence des différentes phases). L'étude des propriétés électrochimiques reposant sur la mesure du potentiel de corrosion et la résistance à la polarisation et sur la détermination de courbes de polarisation complètes a permis de se faire une représentation de l'ensemble des processus électrochimiques. Les résultats obtenus conduisent à une meilleure compréhension de l'influence du substrat et de celle de la composition et de l'épaisseur des revêtements considérés, sur leurs propriétés structurales et électrochimiques qui, dans certains cas, s'avèrent être très différentes des propriétés du matériau massif

碩士
國立臺灣科技大學
化學工程系
95
The main objective of this study is to investigate mixed-potential type NO2 gas sensor for intermediate temperature (500-700℃). The study was focused on the sensing performance of different cathodic materials Pr0.7Sr0.3BO3 ( B = Mn、Co、Cr、Fe ). The sensing electrode was made by sol-gel method to form nano particles. The nano particles were then screen-printed on the YSZ electrolyte. The electrode was analyzed by SEM, BET, XRD and finally tested in the assembled NO2 sesnor. The experimental variables include calcined temperature, sensing temperature, gas concentration and flow rate, electrode concentration. SEM and BET analyses showed that the particle size increased with increasing calcined temperature, and the particle size was not uniform at higher temperatures. Sensing performance analyses revealed that the sensing response was found to be linearly dependent on the NO2 concentration at lower concentrations and logarithmically dependent at higher concentrations. The response also decreased with increasing O2 concentration on a logarithmic scale. The electrode calcined at 1100℃ gave the highest response. The sensing response decreased with increasing sensing temperature, mainly due to the higher catalytic activities of both O2 anodic and NO2 cathodic reactions. Among all the electrodes investigated, Pr0.7Sr0.3CrO3 has the highest response, followed by Pr0.7Sr0.3MnO3. Nevertheless, the recovery time of Pr0.7Sr0.3MnO3 was faster than that of Pr0.7Sr0.3CrO3. In overall, Pr0.7Sr0.3MnO3 gave better sensing performance. The sensing response was also found to be increased with lower electrode concentration and higher sensing gas flow rate.

碩士
國立臺灣科技大學
化學工程系
104
Unlike the conventional milling method of preparing glasses and glass-ceramic, we implement a different way to make the sulfite glasses. In this study, we use propylamine as a solvent to dissolve the starting materials of Na2S and P2S5. Through the dissolution process, the obtained powder achieves composition homogeneity at the molecular level. Then we press the powder with uniaxial force(469 MPa) to form glass. The X-ray diffraction patterns of xNa2S-(100-x)P2S5 (x=75,80) show two broad background diffractions at low angle and a few unknown peaks of low intensity which indicate they are amorphous. After heating the glassy sample of 75Na2S-25P2S5 at 270 oC, the sharp peaks in X-ray diffraction pattern emerge, matching the pattern of cubic Na3PS4 crystal. It seems that crystallization of sulfide glass is easy at such a high sodium content. In addition to glasses and glass-ceramic electrolyte, we also use solid-state reaction to synthesize NaFe(SO4)2 and NaCrO2. They are candidate compositions of sodium battery cathode. After measuring electrical impedance of xNa2S-(100-x)P2S5 (x=75,80) and 75Na2S-25P2S5 which has been heated at 270 oC, we compare the sodium ion conductivity of the three solid electrolytes at different temperature. The ion conductivity of 75Na2S-25P2S5 is measured 4.8 x 10-7 S cm-1 at 30 oC and 1.9 x 10-3 S cm-1 at 200 oC in the heating ramp at a 20 oC interval. In the cooling ramp at a 10 oC interval, the ion conductivity is slight higher, measured 2.0 x 10-6 S cm-1 at 30 oC and 1.3 x 10-3 S cm-1 at 200 oC. For the composition of high sodium content, 80Na2S-20P2S5, the ion conductivity is measured 1.2 x 10-7 S cm-1 at 33 oC and 1.6 x 10-3 S cm-1 at 200 oC in the heating ramp. And the ion conductivity is 4.3 x 10-7 S cm-1 at 33 oC and 1.5 x 10-3 S cm-1 at 200 oC in the cooling ramp. On the ion conductivity of Na3PS4, the ion conductivity is 7.3 x 10-6 S cm-1 at 33 oC and 2.1 x 10-3 S cm-1 at 200 oC in the heating ramp. And the ion conductivity is 2.3 x 10-6 S cm-1 at 33 oC and 8.4 x 10-4 S cm-1 at 200 oC in the cooling ramp. On the ion conductivity of NaCrO2, the ion conductivity is 3.8 x 10-9 S cm-1 at 35 oC and 1.8 x 10-7 S cm-1 at 140 oC in the heating ramp. And the ion conductivity of NaFe(SO4)2 is 2 x 10-9 S cm-1 at 100 oC and 3.6 x 10-8 S cm-1 at 160 oC in the heating ramp.

Tese de mestrado em Química Tecnológica, apresentada à Universidade de Lisboa, através da Faculdade de Ciências, 2013
Neste trabalho foram sintetizados e caracterizados materiais que possam funcionar como cátodos nas baterias de ião-Litío. Foram estudados dois tipos de materiais, do tipo orgânico e inorgânico. Primeiramente procedeu-se ao estudo dos materiais de natureza orgânica e dentro deste tipo de materiais foram escolhidos os polímeros condutores, mais concretamente o poli(3,4-etilenodioxitiofeno) doravante designado por PEDOT, dopado com poliestirenosulfonato doravante designado por PSS. Foi assim possível obter, por via electroquímica, eléctrodos modificados com filmes de PEDOT-PSS que foram os materiais estudados neste trabalho. Os filmes de PEDOT-PSS foram sintetizados por electrodeposição num eléctrodo de platina a partir de soluções aquosas do monómero (10 mM EDOT) e do dopante (0.1 M PSS). A electropolimerização foi efectuada através de dois métodos: electrodeposição em modo galvanostático, onde foi aplicada uma corrente constante (ig=0.2 mA.cm-2) durante um certo período de tempo (t=275 s) e pelo modo potenciodinâmico, onde se efectuaram diversos varrimentos de potencial (80 ciclos) dentro de uma gama de potenciais (-0.7 a 0.8 V vs SCE) a uma velocidade de varrimento de =50 mV.s-1. As condições de electropolimerização foram previamente optimizadas e seleccionadas de modo a obter eléctrodos modificados por filmes de PEDOT-PSS estáveis e homogéneos e com electroactividade semelhante para os dois modos de crescimento. Os filmes obtidos foram caracterizados por voltametria cíclica (CV), Microgravimetria (EQCM), Deflexão de Raio Laser/Técnica de Efeito de Miragem (PBD) e por Elipsometria (ELL). O processo de dopagem/dedopagem dos eléctrodos modificados foi estudado por voltametria cíclica em soluções orgâncias (CH3CN) de LiClO4 (0,1 M) comprovando-se que os filmes eram caracterizados por uma elevada electroactividade e estabilidade durante os processos de conversão redox. Ainda que numa bateria de ião Lítio se tenha que recorrer inevitavelmente a solventes orgânicos (dada a reactividade do lítio em meio aquoso), o comportamento dos eléctrodos modificados foi ainda avaliado em meio aquoso com o mesmo electrólito (0,1 M LiClO4) de forma a contribuir para o esclarecimento do efeito do solvente nos processos de transferência de massa decorrentes da conversão redox do polímero. Acresce ainda o facto de o lítio apresentar uma massa molar bastante baixa, o que se traduz numa dificuldade acrescida de detecção dos seus fluxos por electrogravimetria quando não solvatado, como acontece em meio orgânico. O fenómeno da transferência de massa que tem lugar durante a conversão redox dos filmes dos polímeros modificados de PEDOT-PSS foi investigado por EQCM e PDB. A informação combinada dos dados obtidos por estas técnicas permitiu distinguir os fluxos de massa do solvente dos fluxos iónicos, podendo ser resolvidos individualmente. Os dados revelaram uma dopagem pseudo-catiónica não ideal visto que o anião perclorato também participa no processo de conversão redox, sendo a extensão relativa de cada contribuição claramente dependente do modo electroquímico utilizado na síntese do polímero. A espessura do filme sintetizado galvanostaticamente e as suas propriedades ópticas foram avaliadas utilizando a técnica de Elipsometria. Concluiu-se que o filme de PEDOT-PSS é um filme poroso composto por duas camadas, tendo-se estimado uma espessura total de 203 nm (interna (88 nm) e externa (115 nm)) Como materiais de interesse para cátodo de natureza inorgânica, os que têm mostrado mais aplicabilidade e interesse de estudo são os compostos de intercalação de Lítio, ou seja, que já contêm lítio na sua constituição. O escolhido para este trabalho foi o composto de intercalação de Lítio Ferro Silicato (Li2FeSiO4), contendo também Silício e o Ferro que são dos elementos mais abundantes da crosta terrestre (2º e 4º respectivamente). Pretendendo-se desenvolver um método de síntese altamente eficiente através de percursores baratos e abundantes e recorrendo a material de laboratório simples, de modo a minimizar os custos de produção, foram estudados e desenvolvidos três métodos distintos de preparação dos compostos: síntese hidrotérmica (HTS), reacção de estado sólido (SS) e de combustão (CM). Tendo em atenção o descrito anteriormente, para o primeiro método os reagentes utilizados foram o LiOH, SiO2 e FeCl2•4H2O em diferentes razões estequiométricas, enquanto que para os outros dois métodos foi usado o Li2CO3, SiO2 e FeC2O4•2H2O com a adição de ácido cítrico no método CM. Estes métodos sofreram várias modificações de optimização tendo sido possível obter o composto através do método hidrotérmico, apesar de ainda conter alguma impurezas como o Fe2SiO4 e Li2SiO3. Este foi então o composto utilizado para os estudos de caracterização. A caracterização foi feita através das técnicas de Análise Termogravimétrica (TGA/DSC), Difracção de Raios-X de Pós (XRD), Microscopia Electrónica de Varrimento (SEM) e da realização de ensaios-teste de pilhas de botão de ião-Lítio utilizando este material como cátodo. Para obter a temperatura de calcinação do composto para os métodos de SS e CM foi usada a técnica de TGA/DSC que comprovou a decomposição dos precursores até 710 ºC, onde tem lugar a última reacção endotérmica admitindo-se que é referente à formação do Lítio Ferro Silicato que se pretende obter. Deste modo, as temperaturas de calcinação utilizadas foram superiores a esta temperatura. A estrutura destes compostos foi estudada através dos difractogramas obtidos por XRD e comprovou-se que tem uma estrutura ortorrômbica com grupo espacial Pmn21 tal como já tinha sido demonstrado noutros trabalhos presentes na literatura. Também foi possível comprovar que os compostos obtidos eram pouco cristalinos, o que não é desejável para a finalidade do composto uma vez que esse facto compromete a sua condutividade electrónica e iónica. A topografia dos materiais obtidos foi caracterizada por SEM onde se verificou que há a formação de aglomerados com 7 a 10 μm de tamanho, de partículas com, aproximadamente, 250 nm de tamanho, que está dentro da gama de tamanhos desejados (50 a 250 nm). Após esta caracterização, o desempenho electroquímico foi avaliado, utilizando células de botão construídas no laboratório com este material de cátodo e lítio metálico como ânodo. Ainda que promissores, os resultados mostram que a falta de cristalinidade dos materiais sintetizados comprometem as capacidades e eficiência das baterias, apesar de se ter obtido uma eficiência de 80% a partir do 2ºciclo de descarga da bateria.
In this work the mass transfer phenomena taking place during the redox conversion of PEDOT-PSS films is investigated by microgravimetry (EQCM) and Probe Beam Deflection (PBD). Polymeric films were galvanostatically and potentiodynamically synthesized onto platinum electrodes from aqueous solutions. The doping/undoping processes of the modified electrodes were studied by cyclic voltammetry in organic (CH3CN) and aqueous solutions of LiClO4. The combined information of the PBD and EQCM allows distinguishing the solvent mass fluxes from that of the ions that can be resolved individually. The data revealed a non-ideal pseudo-cationic doping since the perchlorate anion also participates in the conversion process, being the relative extension of each contribution clearly dependent on the electrochemical mode used in the polymer synthesis. Also the thickness of the galvanostatically grown film and its optical properties were computed using the Ellipsometry technique. It was concluded that the PEDOT-PSS coating is a porous film composed by two layers (internal and external) with a total thickness of 203 nm. The other cathode material of interest studied in this work was lithium iron silicate (Li2FeSiO4). Three cost effective synthetic methods were developed to produce lithium iron silicate starting with low cost precursors and basic laboratory equipment. The material was synthesized using a hydrothermal, solid-state and combustion method synthesis. The structure and morphology of the obtained materials were characterized by XRD and SEM. Afterwards the electrochemical performance was evaluated, using coin cells. Even promising the results suffer from the lack of crystallinity of the synthesised materials that compromises the capacities and efficiency of the batteries. Nevertheless, an interesting efficiency of 80% was obtain after the 2nd cycle of discharge of the battery.

Lithium iron phosphate (LiFePO4) is a promising cathode material for lithium-ion batteries. In the past few years many improvements have led to consistent cycling capabilities, even at high rates. LiFePO4 is being commercialized as a cathode material in batteries for power tools, and is a serious candidate for the future batteries of hybrid-electric or electric vehicles. It can also be commercialized for other applications requiring a low-cost and safe battery, but its low intrinsic electrical conductivity and low Li-ion diffusion are two major disadvantages. Many groups have shown that battery performance can be enhanced by addition of carbon, during synthesis or post-synthesis carbon coating through various techniques to improve electrical conductivity. Simplification or even minimization of carbon-coating methods is one area of improvement which could help to reduce cost and increase efficiency. These carbon additives can cause multiple effects on purity, crystallinity and the electrochemical performance of the final cathode material (LiFePO4) and therefore makes it difficult to optimise the quantity and specific type of carbon that needs to be added during the synthesis of LiFePO4. All synthetic procedures reported in the literature, however, show that carbon is always present in some form in the final product. In this thesis study, in order to evaluate the effect of various carbon additives unambiguously, a novel one-step co-precipitation method was developed for synthesis of carbon-free LiFePO4. A series of LiFePO4/Carbon composites were prepared where the composites were synthesised at 550, 650 and 750°C containing 5, 10 or 20 wt% carbons. Two forms of carbon additives were tested; single wall carbon nanotubes (SWCNT) and carbon black (CB). These carbons were added at one of two different stages; (1) during pre-synthesis, mixed with the LiFePO4 precursors, or (2) in post-synthesis, during the electrode preparation. This approach helped to investigate the effect that the carbon type, carbon content, mode of mixing (pre synthesis or post synthesis) and temperature have on the electrochemical performance of the active component. The topic of electron conductivity and Li-ion diffusion LiFePO4 is also very relevant, especially since this material is now touted as an important high-rate capability cathode. To investigate these effects, cyclic voltammetry, charge-discharge and electrochemical impedance spectroscopy measurements were performed. It was found that the cell discharge capacity, rate capacity and electronic conductivity of the electrode depended on the type of carbon used. The use of a 5 wt. % loading of SWCNTs as conductive additive to LiFePO4 composites prepared at 750 °C was found to improve the electrochemical performance of cells compared to cells containing CB additives. The LiFePO4 with 5 wt. % SWCNTs mixed pre-synthesis and then synthesised at 750 °C demonstrates a smaller resistance to charge-transfer (RCT = 59Ω) and good kinetic behaviour (2.9 x 10-8 cm2/s), and has the highest specific capacity (93 mAh/g and 48 mAh/g at C/20 and C/5 respectively) than any other sample except for the one with 10 wt% SWCNT. The latter demonstrates slightly improved specific capacity at C/20 (94 mAh/g) and better Li-ion kinetic behaviour (3.3 x 10-7 cm2/s) but a worse specific capacity at C/5 (46 mAh/g), probably because the charge-transfer resistance is significantly higher (RCT = 239 Ω). Therefore, the optimisation of cell performance involves optimisation of Li-ion and electron transport and the charge transfer at the electrode/electrolyte interface. Therefore, it is important to note that the material synthesised according to the novel, single-step, co-precipitation procedure described in this thesis can be applied to many other LiFePO4-carbon composite cathode materials, to compare and evaluate the effect of various carbon additives on the electrochemical performance of cathode materials.

The present thesis is the result of a research period at the Institute of Space and Astronautical Science of the Japanese Aerospace Exploration Agency, ISAS/JAXA within Funaki Laboratory of the Department of Space Flight Systems that followed the path of plume plasma diagnostics for space electric propulsion drives. During the experimental studies two high-current hollow cathodes and an innovative prototype of a low-voltage fully external discharge plasma thruster (XPT) had their plasma plumes diagnosed using electrostatic probes and retarding potential analyser (RPA). A Hall thruster and hollow cathode plume is defined as an unmagnetised quasi-neutral plasma which is mainly formed of neutral particles, electrons, singly and doubly charged ions. Plasma diagnostic techniques provide information through practical observations in order to fully understand the dynamics of the aforementioned plume components, the physical processes taking place within the plume and their effects on the spacecraft, for instance. Mastering these aspects of the plasma plume of space electric propulsion drives bolster the design processes, leading to highly efficient devices. Firstly, the introduction provides insights on the fundamental principles of hollow cathodes and Hall thrusters and a brief presentation of the plasma diagnostic techniques used during the research: single and double Langmuir probes, emissive probes and retarding potential analyser. Then, the fundamental plume diagnostics principles are depicted in an exhaustive way, departing from classical plasma kinetic theory, energy distribution functions and ending with an overview on the theory of charge collection by cylindrical probes. Subsequently, peculiarities of various analysis techniques are exposed for the Langmuir probes, emissive probes and RPA, with an emphasis on their strengths and demerits. The experimental setups for the cathodes and XPT plume diagnostic procedures are then outlined. The experimental logic, setup and electrical diagrams as well as a presentation of each probe design and manufacturing details are extensively discussed. The hollow cathodes experimental results are exposed with a discourse that aims of overviewing the difference between the various data analysis methods applied for the raw data. A discussion ensued based on the results in order to effectively identify mechanisms that produced the observed plasma parameters distributions. For the first time, the plume of a fully external discharge plasma thruster was diagnosed utilising double Langmuir probes.  The thesis highlights the main results obtained for the XPT far-field plume plasma diagnostics. The experimental findings for both thruster centreline positions and 2D plume maps for several axial distances away from the anode plate offer a ground basis for future measurements, a comparison term and a database to support ongoing computational codes. The results are discussed and related to the thruster performances data obtained during previous experiments. The thesis includes consistency analyses between the experimental data and the numerical simulation results and the uncertainties in measured plasma parameters associated with each data analysis procedure are evaluated for each data set. Last, the conclusions underline the main aspects of the research and further work on the previously mentioned plasma diagnostic techniques for hollow cathodes and XPT is suggested.
La présente thèse est le résultat d'une période de recherche à l'Institut des Sciences Spatiales et Astronautiques de l'Agence Spatiale Japonaise, ISAS / JAXA qui a suivi la voie des diagnostics du plasma de la plume de propulseurs électriques spatiaux. Au cours des études expérimentales, deux cathodes creuses à fort courant et un prototype innovant d'un propulseur basse tension à décharge externe de plasma (XPT) avaient leurs faisceaux de plasma diagnostiqués en utilisant des sondes électrostatiques et un analyseur à potentiel retardé. La plume d’un propulseur à effet Hall et d’une cathode creuse est définie comme un plasma quasi-neutre non-magnétisé qui est principalement formé de particules neutres, d’électrons, d’ions monovalents et bivalents. Les techniques de diagnostic du plasma fournissent des informations, via des observations pratiques, afin de bien comprendre la dynamique des composants de la plume mentionnés ci-dessus, les processus physiques qui se déroulent dans la plume et leurs effets sur une sonde spatiale, par exemple. La maîtrise de ces aspects du plasma de la plume généré par les propulseurs électriques spatiaux renforce les processus de conception de ce type de propulsion, ce qui conduit à des dispositifs hautement efficaces. Tout d'abord, l'introduction donne un aperçu sur les principes fondamentaux de cathodes creuses et de propulseurs à effet Hall, et une brève présentation des techniques de diagnostic du plasma utilisées lors de la recherche : sondes de Langmuir simples et doubles, des sondes émissives et d’analyseur à potentiel retardé. Ensuite, les principes fondamentaux de diagnostic de la plume sont représentés de manière exhaustive, d’abord la théorie cinétique classique du plasma, les fonctions de distribution en énergie et pour terminer une vue d'ensemble de la théorie de la collecte de charge par des sondes cylindriques. Par la suite, les particularités des diverses techniques d'analyse sont exposées pour les sondes de Langmuir, les sondes émissives et RPA, en mettant l'accent sur leurs avantages et leurs inconvénients. Les montages expérimentaux pour les procédures de diagnostic de la plume-plasma de cathodes et du XPT sont ensuite décrits. La logique expérimentale, les schémas électriques ainsi qu'une présentation de la conception et de la fabrication de chaque sonde sont largement discutés. Les résultats expérimentaux pour les cathodes creuses sont exposés de façon à présenter la différence entre plusieurs méthodes d'analyse de données appliquées aux données brutes. Une discussion s’ensuit, basée sur les résultats afin d'identifier efficacement les mécanismes qui ont produits les propriétés électroniques observées. Pour la première fois, la plume d'un propulseur à décharge externe de plasma a été diagnostiquée en utilisant des sondes de Langmuir doubles. La thèse met en évidence les principaux résultats obtenus pour le diagnostic en champ lointain de la plume-plasma du XPT. Les résultats expérimentaux pour les positions sur l'axe du propulseur et le cartes 2D de la plume pour plusieurs distances axiales loin de l’anode offrent une base pour de futures mesures, un terme de comparaison et une base de données pour appuyer les codes numériques. Les résultats sont discutés et sont rapportés aux données de performances du propulseur obtenus lors des essais précédents. La thèse comprend des analyses de la cohérence entre les données expérimentales et les résultats de simulation numérique, et les incertitudes des paramètres mesurés du plasma associées à chaque procédure d'analyse des données sont évaluées pour chaque ensemble de données. Enfin, les conclusions soulignent les principaux aspects de la recherche et une poursuite des travaux sur les techniques de diagnostic de plasma pour les cathodes creuses et le XPT est suggérée.

碩士
國立中央大學
材料科學與工程研究所
104
With a framework of alternating layers between perovskite and rock-salt structure, Ruddlesden-Popper (RP) oxides, La(n+1)NinO3n+1+δ (n = 1,2 and 3, LNO) accommodate a range of hyper stoichiometry (i.e., denoted as δ) to be considered as a potential candidate of cathodes used in the solid oxide fuel cells (SOFC) operated at lower-temperature. In the present work, combustion process was investigated to prepare the precursors of Lan+1Nin(1-x)ConxO3n+1+δ (n =1, 2, 3) via mixing glycine with the a solution of La, Ni and Co nitrates with specific concentration. The precursors resulted from combustion were calcinated to powders. Examination through scanning electron microscope (SEM), the powders depicted a porous morphology and they belonged to mixed crystals of perovskite and rock-salt resultant from by x-ray diffractometer (XRD). Both crystal structure and surface morphology of the powders change and were determined by the pH values of the nitrate solutions in the process of their mixing with glycine. The XRD of the Co-doped La4Ni3O10+δ (LNO3) displayed a small shift of character peaks to higher angle. This fact implied a lattice contraction with Co-doping. Thermal gravimetric analysis (TGA) concluded that powder LNO2 containing the highest concentration of oxygen vacancies (i.e., △δ = -0.114). The powders were then formulated into pastes, which were screen-printed on a piece of home-made electrolyte of proton-transferred solid oxide fuel cell (P-SOFC) supported with NiO-anode. After drying, the single cell was subject to electrochemical tests. The electrochemical behavior of the cathodes made of different powders various in n-values were compared to find out the best RP structure of Lan+1Nin(1-x)ConxO3n+1+δ (n =1, 2, 3). The measurement of electrical conductivity by means of four-point probe indicated the cathode made of La4Ni2.1Co0.9O10+δ (LNO3Co0.2) powder depicted the highest conductivity (i.e., 140.5 S / cm) in air at 400℃. The measurements of Power density by means of polarization curve indicated the cell the highest power density (i.e., 14.4 mW/cm2) at 700 ℃. The measurements of Polarization resistance and Ohmic resistance by means of Eelectrochemical impedence spectrum (EIS) analysis indicated the ohmic resistance of cell (i.e., 17.6 Ω/cm2) and polarization resistance of cell (i.e., 2.04Ω/cm2) at 700 ℃.

Extensive research has shown that electrochemistry is one of the factors that govern the flotation of sulfide minerals. Flotation is often adversely affected by uncontrolled oxidation, which is also an electrochemical process. The interest in pyrrhotite recovery arose after observing that there is a substantial loss of PGM due to the depression of pyrrhotite and the subsequent loss of any PGMs associated with it. The first part of this study focuses on the influence of chemical composition and crystal structure on the electrochemical behaviour of pyrrhotite in a 0.05 M Na2B4O7 solution. Rest potential and polarisation resistance measurements, as well as anodic polarisation diagrams, showed that the magnetic 4C type pyrrhotite is anodically more reactive than the non-magnetic 6C type pyrrhotite. It was also shown in cathodic polarisation diagrams that the non-magnetic 6C type pyrrhotite is a better substrate for oxygen reduction and is less susceptible to oxidation. ToF-SIMS showed the formation of an oxide layer on the pyrrhotite surface after oxidation. In the second part of this work, the influence of galvanic interactions on the electrochemical behaviour of pyrrhotite in contact with pentlandite was investigated. It was observed that, under oxygen-saturated conditions, as the amount of pentlandite increases, the reactivity towards oxidation of the mixed mineral system is reduced. Impedance measurements showed a decrease in capacitance values, indicating the formation of a continuous oxide layer on the surface and an increase in oxide layer thickness with decreasing pentlandite content. Anodic polarisation diagrams showed that under oxygen-deficient conditions and in the low potential region, pentlandite behaves as an inert material and does not have an influence on the oxidation behaviour of pyrrhotite. Hence, the anodic activities of the different magnetic 4C type pyrrhotites from Sudbury Gertrude, Phoenix and Russia were compared. It was shown that the oxidation reactivity decreased in the following order: Sudbury Gertrude magnetic 4C pyrrhotite > Phoenix magnetic 4C pyrrhotite > Russian magnetic 4C pyrrhotite; it also varied according to location. In the transpassive region, higher anodic currents were observed on the mixed samples because both pentlandite and pyrrhotite reacts. The reactivity increased in the order: pure pyrrhotite (Russia) < medium-pentlandite (Sudbury Gertrude) < high-pentlandite (Phoenix). In the presence of potassium ethyl xanthate, there was no change in the initial anodic reactivities of the different pyrrhotites. The anodic polarisation diagrams of the pure and mixed samples showed a reduction in the maximum anodic peak current, suggesting the presence of xanthate on the surface, which hinders oxidation of the mineral surface. In addition, the influence of cleaning of oxidised pyrrhotite with gaseous carbon dioxide was studied, using electrochemical and microflotation measurements. Electrochemical measurements indicated that CO2 treatment resulted in depassivation of the oxidised surfaces; this was supported by ToF-SIMS measurements that demonstrated a reduction in the oxide layer thickness after CO2 treatment. Anodic polarisation diagrams showed a higher anodic peak current, indicating that the surface is more reactive. Gaseous carbon dioxide conditioning of oxidised pyrrhotite resulted in improved flotation response of pyrrhotite with the aid of copper activation and higher air flow rate. Copyright
Dissertation (MEng)--University of Pretoria, 2010.
Materials Science and Metallurgical Engineering
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