Electrolyte additives are generally used in commercial Li-ion cells to improve capacity retention and calendar life. Although it is apparent that electrolyte additives play an important role, the details of how they work are poorly understood. In order to be able to distinguish the effect of an additive on the positive or negative electrodes, an experimental method has been developed based on electrochemical impedance spectroscopy of symmetric cells constructed from electrodes of disassembled full cells similar to the method described by previous workers. This technique proved to be useful and showed that the effects of additives on both electrodes depend strongly on their concentration. It also showed that in some cases, when two additives are introduced in the same cell, both additives contribute to the formation of the surface layer of both electrodes. In other cases, each additive controls the formation of the surface layer of only one electrode.

Biomaterials have been used for more than a century in the human body to improve body functions and replace damaged tissues. Currently approved and commonly used metallic biomaterials such as, stainless steel, titanium, cobalt chromium and other alloys have been found to have adverse effects leading in some cases, to mechanical failure and rejection of the implant. The physical or chemical nature of the degradation products of some implants initiates an adverse foreign body reaction in the tissue. Some metallic implants remain as permanent fixtures, whereas others such as plates, screws and pins used to secure serious fractures are removed by a second surgical procedure after the tissue has healed sufficiently. However, repeat surgical procedures increase the cost of health care and the possibility of patient morbidity. This study focuses on the development of magnesium based biodegradable alloys/metal matrix composites (MMCs) for orthopedic and cardiovascular applications. The Mg alloys/MMCs possessed good mechanical properties and biocompatible properties. Nine different compositions of Mg alloys/MMCs were manufactured and surface treated. Their degradation behavior, ion leaching, wettability, morphology, cytotoxicity and mechanical properties were determined. Alloying with Zn, Ca, HA and Gd and surface treatment resulted in improved mechanical properties, corrosion resistance, reduced cytotoxicity, lower pH and hydrogen evolution. Anodization resulted in the formation of a distinct oxide layer (thickness 5-10 μm) as compared with that produced on mechanically polished samples (~20-50 nm) under ambient conditions. It is envisaged that the findings of this research will introduce a new class of Mg based biodegradable alloys/MMCs and the emergence of innovative cardiovascular and orthopedic implant devices.

Per-Fluoro-Sulphonic-Acid (PFSA) ionomers have been singled out as the preferable ionomers for making the Polymer Electrolyte Membrane Fuel Cells (PEMFC) membranes owing to their extensive intrinsic chemical stability and super sulfonic acid strength which is core to the PEMFC proton conductivity. This thesis presents a deeper analysis into these PFSA ionomer membrane electrode assemblies (MEA), presenting an electrochemical-analytical comparative analysis of the two basic types, which are the Long-Side-Chain (LSC) Nafion® and the ShortSide-Chain (SSC) Aquivion® ionomer MEA with emphasis on performance and durability which are currently not well understood. In particular, electrochemical circuit models and semiempirical models were employed to enable distinguishable comparative analysis. Also, in this thesis, we present a further probe into the effect of ionomer ink making processes, critically investigating the effect of the High Share Dispersion (HSD) process on both the Nafion® and Aquivion® ionomer membrane electrode assembly (MEA). The findings in this research provides a valuable insight into the performance and durability of PFSA ionomer membrane under various application criteria. The effect of operating parameters and accelerated stress testing (AST) on the PFSA ionomers was determined using electrochemical impedance spectroscopy (EIS) and electronic circuit model (ECM) analysis. The result of this study, shows that the ionomer ink making process for Nafion® and Aquivion® MEAs are not transferrable. Analysis of the PEMFC performance upon application of the high shear dispersion (HSD) process showed that Nafion® MEA had a 10.47% increase in voltage while the Aquivion® MEA had a 2.53% decrease in voltage at current density of 1.14A/cm2 . Also, upon accelerated stress testing, the Nafion® showed a 10.49% increase in its voltage while the Aquivion® on the other hand had a 7.16% decrease in voltage at 0.66A/cm2 . Thus indicating the HSD process enhances the performance of the Nafion® MEA and inhibits the performance of the Aquivion® MEA.

State-of-Health (SoH), a specified measure of stability, is a critical parameter for determining the safe operating area of a battery cell and battery packs to avoid abuse and prevent failure and accidents. A series of experiments were performed to evaluate the performance of a 4P1S battery array using electrochemical impedance spectroscopy to identify key frequencies that may describe battery state of health at any state of charge. Using a large sample number of cells, the state of health frequency, fSoH, for these LiFePO4 26650 cells is found to be 158 Hz. Four experiments were performed to evaluate the lifetime in different configurations: single-cell at 1C (2.6A), single-cell at 10C (26A), four cells in parallel at 10C (ideal match), and four cells in parallel (manufacturer match). The lifetime for each experiment set degraded substantially, with the final parallel series reaching end of life at 400 cycles, a 75.32% reduction in life compared to operating solo. Analysis of the fSoH data for these cells revealed a change in imaginary impedance at the critical frequency that corresponded to changes in the capacity and current data, supporting the development of a single-frequency diagnostic tool. An electrochemical model of the battery was generated, and it indicated the anode material was aging faster than the SEI layer, the opposite of normal cell degradation. A post-mortem analysis of cells from three configurations (baseline, single-cell, and parallel-cell) supported the modeling, as physical damage to the copper current collector in the anode was visible in the parallel-connected cell. / Ph. D.

Biosensors monitor physiological activities for diagnosis and treatment of disease. Molecularly imprinted polymers (MIPs) are a viable synthetic approach for molecular recognition in biosensing. For biosensing purposes, the most important properties in MIP optimization are sensitivity and selectivity towards a desired analyte. This study aims to optimize MIP sensitivity and selectivity by varying the amount and type of cross-linker used in the synthesis of cortisol and melatonin. The four cross-linkers tested were trimethylpropane trimethacrylate (TRIM), ethyleneglycodimethacrylate (EGDMA), divinylbenzene (DVB), and pentaerythritol triacrylate (PETRA). Based on literature, the following ratios were used for the template molecule to functional monomer to cross-linker in MIP synthesis: for EGDMA cross-linked polymers, 1:6:30; for TRIM and PETRA cross-linked polymers, 1:8:8, 1:6:3, and 1:8:35; for DVB cross-linked polymers, 1:6:30, 1:4:16, and 4:1:60. The polymers were ground and washed, then suspended in a polyvinyl matrix which was spin-coated onto an organic electrochemical transducer (OECT). The device performance was evaluated using electrochemical impedance spectroscopy. For each device, the impedance was measured in electrolyte solutions containing target molecules in concentrations ranging from 1 pM to 100 uM. The impedance was plotted against the analyte concentration to give the sensing slope, which is a measurement for the binding affinity of the polymer. For a device to be considered sensitive, its sensing slope should be greater than its non-imprinted counterpart by a factor above the error margin (+/- 1.79). Of the devices tested, CM1835T (highly cross-linked with TRIM) showed sensitivity towards cortisol, but lacks selectivity towards cortisol over its structural analog, estradiol. Of the melatonin selective polymers, MM163T (low cross-linking with TRIM), MM1630D, and MM4160D (both highly cross-linked with DVB) all showed promising results in sensitivity to melatonin. Overall, the results indicate that high degrees of cross-linking in MIPs improve sensitivity for large, rigid, non-aromatic molecules such as cortisol; however there is no correlation between selectivity and the degree of cross-linking. Meanwhile, divinylbenzene as a cross-linker improves sensitivity and selectivity towards aromatic analytes such as melatonin and estradiol. This study could be improved upon by further characterization of imprinted and non-imprinted polymers, investigation of molecular dynamics, and optimization of devices.

No description available.

No description available.

A redução de peso é uma questão tecnológica fundamental para a indústria aeroespacial, uma vez que diminui o consumo de combustível, resultando em redução de custos e contribuindo para a redução da emissão de gases de efeito estufa. Devido à relação favorável entre resistência mecânica e peso, as ligas de alumínio de elevada resistência mecânica contribuem favoravelmente para este aspecto. Entretanto, como estas ligas são dificilmente soldáveis pelos processos tradicionais envolvendo fusão, o procedimento de junção utilizado em aeronaves é a rebitagem, resultando em ganho de peso. O processo de soldagem por fricção (friction stir welding -FSW), desenvolvido no início dos anos noventa pelo The Welding Institute (TWI) do Reino Unido, se constituiu em um grande avanço para a soldagem das ligas de alumínio utilizadas na indústria aeroespacial, pois permite a produção de soldas mais confiáveis e virtualmente livres de defeitos. Entretanto, o aquecimento das peças e a deformação mecânica durante a FSW geram zonas com diferentes características microestruturais que, de acordo com a literatura, apresentam resistências à corrosão diferentes. Por oferecerem elevada resolução lateral, as técnicas eletroquímicas locais são úteis para elucidar as diferenças de reatividade local de eletrodos heterogêneos, como no caso de metais soldados. No presente trabalho técnicas eletroquímicas locais foram empregadas para caracterização da resistência à corrosão em meio contendo cloreto das diferentes zonas geradas pela soldagem de topo da liga de alumínio 2024-T3 por FSW, comparando-a com a exibida pelo metal base. O estudo foi complementado com a caracterização microestrutural destas regiões e também por ensaios macroscópicos de corrosão. Os resultados dos procedimentos de caracterização microestrutural confirmaram que a FSW provoca modificações na microestrutura das regiões afetadas pelo processo, principalmente no que concerne à distribuição das nanopartículas precipitadas durante o envelhecimento natural da liga. Por sua vez, os resultados dos ensaios macroscópicos de corrosão e eletroquímicos locais mostraram-se concordantes na determinação da região mais sensível à corrosão, que foi verificada como sendo as zonas termicamente afetada (Heat Affected Zone - HAZ) e termomecanicamente afetada (Thermomechanically Affected Zone - TMAZ) do lado do avanço da ferramenta de soldagem, mostrando também que as regiões afetadas pelo processo de soldagem apresentam resistência à corrosão inferior à do metal base. Através do uso da espectroscopia de impedância eletroquímica local (Local Electrochemical Impedance Spectroscopy - LEIS) foi evidenciado que o acoplamento galvânico entre as diferentes zonas geradas durante o processo de soldagem não desempenha um papel relevante na aceleração do processo corrosivo, o que está em desacordo com os resultados publicados em diversos estudos realizados com esta liga soldada por FSW. O trabalho apresenta ainda uma contribuição teórica original demonstrando que medidas de ângulo de contato e de espectroscopia de impedância eletroquímica em uma gota séssil podem ser usadas simultaneamente para a determinação da capacitância da dupla camada elétrica. As previsões do modelo teórico foram confirmadas tanto através de resultados obtidos com um sistema modelo como também em determinações realizadas nas diferentes regiões geradas pela soldagem por FSW da liga 2024-T3. / Weight reduction is a fundamental technological issue for the aerospace industry, as it decreases the fuel consumption, resulting in reduced both costs and greenhouse gases emission. Due to the favorable relation between strength and weight, high strength aluminum alloys favorably contribute to this aspect, but they remain difficult to weld by conventional processes involving fusion, and, therefore, the junction procedure used in aircraft is riveting, resulting in weight gain. The friction stir welding (FSW) process, developed in the early nineties by the \"The Welding Institute\" (TWI), United Kingdom, is a major breakthrough for the welding of aluminum alloys as it allows the production of more reliable and virtually defect-free welds. However, the heating of the parts and the mechanical deformation during FSW generate zones with different microstructures with different corrosion resistances. As they offer high lateral resolution, local electrochemical techniques are useful for elucidating differences in local reactivity of heterogeneous electrodes, as the case of welded metals. In the present work, local electrochemical techniques were employed to characterize the corrosion resistance in chloride environment of the different zones generated by butt welding the 2024-T3 aluminum alloy by FSW, and to compare this response with that displayed by the base metal. The study was complemented with the microstructural characterization of these regions and also by macroscopic corrosion tests. The results of the microstructural characterization confirmed that FSW causes changes in the microstructure of the regions affected by the process, especially with regard to the distribution of the precipitated nanoparticles during the natural aging of the alloy. The results of the macroscopic corrosion and of the local electrochemical tests showed good agreement in the determination of the most sensitive regions to corrosion, which were found to be the heat affected (HAZ) and the thermomechanically affected (TMAZ) zones of the advancing side of the weld tool. They also showed that the regions affected by the welding procedure have a lower corrosion resistance than the base metal. By using Local Electrochemical Impedance Spectroscopy (LEIS), it was shown that the galvanic coupling between the different areas generated during the welding process does not need to be taken into account in the description of the corrosion process, which is at odds with the results published in several studies of this alloy welded by FSW. The work also present an original theoretical contribution, demonstrating that contact-angle measurements and electrochemical impedance spectroscopy in a sessile drop can be used simultaneously to determine the capacity of the interface. The theoretical model predictions were confirmed by the experimental results obtained both with a model system and in the different regions generated by FSW of aluminum alloy 2024-T3.

A redução de peso é uma questão tecnológica fundamental para a indústria aeroespacial, uma vez que diminui o consumo de combustível, resultando em redução de custos e contribuindo para a redução da emissão de gases de efeito estufa. Devido à relação favorável entre resistência mecânica e peso, as ligas de alumínio de elevada resistência mecânica contribuem favoravelmente para este aspecto. Entretanto, como estas ligas são dificilmente soldáveis pelos processos tradicionais envolvendo fusão, o procedimento de junção utilizado em aeronaves é a rebitagem, resultando em ganho de peso. O processo de soldagem por fricção (friction stir welding -FSW), desenvolvido no início dos anos noventa pelo The Welding Institute (TWI) do Reino Unido, se constituiu em um grande avanço para a soldagem das ligas de alumínio utilizadas na indústria aeroespacial, pois permite a produção de soldas mais confiáveis e virtualmente livres de defeitos. Entretanto, o aquecimento das peças e a deformação mecânica durante a FSW geram zonas com diferentes características microestruturais que, de acordo com a literatura, apresentam resistências à corrosão diferentes. Por oferecerem elevada resolução lateral, as técnicas eletroquímicas locais são úteis para elucidar as diferenças de reatividade local de eletrodos heterogêneos, como no caso de metais soldados. No presente trabalho técnicas eletroquímicas locais foram empregadas para caracterização da resistência à corrosão em meio contendo cloreto das diferentes zonas geradas pela soldagem de topo da liga de alumínio 2024-T3 por FSW, comparando-a com a exibida pelo metal base. O estudo foi complementado com a caracterização microestrutural destas regiões e também por ensaios macroscópicos de corrosão. Os resultados dos procedimentos de caracterização microestrutural confirmaram que a FSW provoca modificações na microestrutura das regiões afetadas pelo processo, principalmente no que concerne à distribuição das nanopartículas precipitadas durante o envelhecimento natural da liga. Por sua vez, os resultados dos ensaios macroscópicos de corrosão e eletroquímicos locais mostraram-se concordantes na determinação da região mais sensível à corrosão, que foi verificada como sendo as zonas termicamente afetada (Heat Affected Zone - HAZ) e termomecanicamente afetada (Thermomechanically Affected Zone - TMAZ) do lado do avanço da ferramenta de soldagem, mostrando também que as regiões afetadas pelo processo de soldagem apresentam resistência à corrosão inferior à do metal base. Através do uso da espectroscopia de impedância eletroquímica local (Local Electrochemical Impedance Spectroscopy - LEIS) foi evidenciado que o acoplamento galvânico entre as diferentes zonas geradas durante o processo de soldagem não desempenha um papel relevante na aceleração do processo corrosivo, o que está em desacordo com os resultados publicados em diversos estudos realizados com esta liga soldada por FSW. O trabalho apresenta ainda uma contribuição teórica original demonstrando que medidas de ângulo de contato e de espectroscopia de impedância eletroquímica em uma gota séssil podem ser usadas simultaneamente para a determinação da capacitância da dupla camada elétrica. As previsões do modelo teórico foram confirmadas tanto através de resultados obtidos com um sistema modelo como também em determinações realizadas nas diferentes regiões geradas pela soldagem por FSW da liga 2024-T3. / Weight reduction is a fundamental technological issue for the aerospace industry, as it decreases the fuel consumption, resulting in reduced both costs and greenhouse gases emission. Due to the favorable relation between strength and weight, high strength aluminum alloys favorably contribute to this aspect, but they remain difficult to weld by conventional processes involving fusion, and, therefore, the junction procedure used in aircraft is riveting, resulting in weight gain. The friction stir welding (FSW) process, developed in the early nineties by the \"The Welding Institute\" (TWI), United Kingdom, is a major breakthrough for the welding of aluminum alloys as it allows the production of more reliable and virtually defect-free welds. However, the heating of the parts and the mechanical deformation during FSW generate zones with different microstructures with different corrosion resistances. As they offer high lateral resolution, local electrochemical techniques are useful for elucidating differences in local reactivity of heterogeneous electrodes, as the case of welded metals. In the present work, local electrochemical techniques were employed to characterize the corrosion resistance in chloride environment of the different zones generated by butt welding the 2024-T3 aluminum alloy by FSW, and to compare this response with that displayed by the base metal. The study was complemented with the microstructural characterization of these regions and also by macroscopic corrosion tests. The results of the microstructural characterization confirmed that FSW causes changes in the microstructure of the regions affected by the process, especially with regard to the distribution of the precipitated nanoparticles during the natural aging of the alloy. The results of the macroscopic corrosion and of the local electrochemical tests showed good agreement in the determination of the most sensitive regions to corrosion, which were found to be the heat affected (HAZ) and the thermomechanically affected (TMAZ) zones of the advancing side of the weld tool. They also showed that the regions affected by the welding procedure have a lower corrosion resistance than the base metal. By using Local Electrochemical Impedance Spectroscopy (LEIS), it was shown that the galvanic coupling between the different areas generated during the welding process does not need to be taken into account in the description of the corrosion process, which is at odds with the results published in several studies of this alloy welded by FSW. The work also present an original theoretical contribution, demonstrating that contact-angle measurements and electrochemical impedance spectroscopy in a sessile drop can be used simultaneously to determine the capacity of the interface. The theoretical model predictions were confirmed by the experimental results obtained both with a model system and in the different regions generated by FSW of aluminum alloy 2024-T3.

Master of Science / Department of Chemistry / Jun Li / This study is focused on exploring the mechanisms of DNA dynamics at carbon electrode surfaces under a strong electric field for the development of novel DNA hybridization sensors. Oligonucleotides with FAM6 attached at the distal end are covalently tethered on the carbon electrode surface. The fluorescence emission from the FAM6 is strongly quenched in close proximity to the electrode surface. The modulation to the fluorescence intensity is correlated with the reversible reorientation of the negatively charged DNA molecules under the electric field within the electric double layer. The orientation dynamics are apparently determined by the interplay of the electropotential, salt concentration, and stiffness of the DNA molecules. We have observed that dsDNAs switch with fast dynamics (in < 0.05 second) followed by relaxation at a slower rate (in > 0.1 second) when the electric field is altered by stepping the electropotential to a more positive or negative value. The DNA reorientation exhibits strong dependence on the PBS buffer concentration and electric double layer thickness. A preliminary calculation based on dipole-surface energy transfer theory indicates that the critical distance between FAM6 and glassy carbon surface is 10.95 nm. In connection with the fluorescence study, the effect of DNA hybridization on electrochemical impedance spectroscopy (EIS) has also been investigated by two methods in an attempt to develop a fast electronic detection method. First, EIS at high AC amplitude (141 mV rms) with DNA-modified glassy carbon electrodes before and after target DNA hybridization have shown notable change at high frequencies, likely related to the DNA reorientation processes. Second, reversible EIS detection of DNA hybridization has been demonstrated with patterned regular carbon nanofiber arrays at normal AC amplitude (10 mV rms). The combination of these two methods will be explored in future studies. The effects of the electric field on surface-tethered molecular beacons (MBs) have also been studied with fluorescence spectroscopy. An increase in fluorescence at negative bias is observed accompanying the opening of the MB stem, which leads to larger separation between fluorophore and quencher. At positive bias, the rehybridization of the MB stem leads to a decrease in fluorescence intensity.