Thesis (Ph. D.)--University of Wisconsin--Madison, 1955. / Typescript. Includes abstract and vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 90-92).

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

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This work presents microfabricated microelectrodes, capable of quantitative analysis in molten salt (MS). MSs are an electrolytic medium of growing interest, especially in the area of nuclear reprocessing. However, designing sensors for a MS-based nuclear reprocessing system is a challenge, owing to the usually corrosive nature and high operating temperatures (typically 450 - 500◦C) of MS. Microelectrodes are well placed as sensors, with numerous advantages over macro-scale electrodes. As a consequence, there have been previous attempts to utilise microelectrodes inMS. However, these have not been successful and all have suffered disadvantages inherent in traditional microelectrode manufacturing. The microelectrodes presented in this work were produced using standard microfabrication techniques and characterised in MS. An analysis of failure mechanisms guided a systematic study of material combinations. This resulted in a sensor, which is capable of delivering quantifiable electrochemistry in MS. However, the lifetime and yield of the sensor were determined to only be 46% and 1.4 hours respectively. Further investigation of the microelectrode failure mechanisms guided several layout changes to the microelectrode design. By reducing critical area, where defects or pinholes could form, these resulted in improvements in performance. This increased the yield to 65%, while the average lifetime increased up to 45 hours. Test structures were designed to investigate the causes of the continued microelectrode failures and identified shorting between the electrode metal and silicon substrate. This suggests the existence of defects in the underlying insulator are the cause of the 35% of microelectrodes which never functioned. Separate test structures suggested the lifetimes of the microelectrodes could also be improved by removing the need for a metal adhesion layer. Tantalum has been suggested as a replacement electrode metal and a proof of concept study demonstrated the feasibility of employing thin film tantalum as an electrode metal in LKE. Using this technology as a platform, several proof-of-concept microelectrode designs are also presented: liquid microelectrodes, microelectrode arrays, and a nanoelectrode. These are targeted at specific sensing applications, and provide an expanded spectrum of measurements in MS.

PART A The syntheses, spectroscopic and electrochemical characterisation of a series of titanium and tantalum phthalocyanine complexes are reported. The complexes are unsubstituted and substituted at either the peripheral or non-peripheral positions with sulphonates, aryloxy, arylthio or amino groups. The complexes mostly exhibit Qbands in the near-infrared region as well as interesting properties in different solvents. The interaction of differently sulphonated titanium and tantalum phthalocyanine complexes with methyl viologen (MV[superscript 2+]), and hence the stoichiometry and association constants are evaluated. Detailed photophysicochemical properties of the complexes were investigated and are for the first time presented with fluorescence lifetimes easily obtained from fluorescence quenching studies. The transformation of 1-hexene photocatalysed by aryloxy- and arylthio-appended complexes is also presented for the first time. The electrochemical properties of the complexes are unknown and are thus presented. Cyclic (CV) and square wave (SWV) voltammetries, chronocoulometry and spectroelectrochemistry are employed in the study of the complexes. Two oneelectron reductions and a simultaneous 4-electron reduction are observed for the unsubstituted Cl[subscript 3]TaPc. Reduction occurs first at the metal followed by ring-based processes. The tetra- and octa-substituted derivatives however exhibit peculiar electrochemical behaviour where a multi-electron transfer process occurs for complexes bearing certain substituents. For all complexes, the first two reductions are metal-based followed by ring-based processes. A comparative study of the electrocatalytic activities of the complexes towards the oxidation of nitrite is also investigated. The complexes are immobilised onto a glassy carbon electrode either by drop-dry or electropolymerisation methods. All the modified electrodes exhibit improved electrocatalytic oxidation of nitrite than the unmodified electrodes by a twoelectron mechanism producing nitrate ions. Catalytic currents are enhanced and nitrite overpotential reduced to ~ 0.60 V. Kinetic parameters are determined for all complexes and a mechanism is proposed. PART B: The syntheses and electrochemical characterisation of chromium and titanium complexes for the selective trimerisation of ethylene to 1-hexene are presented. The synthesis of the chromium complex requires simple steps while tedious steps are used for the air-sensitive titanium complex. The spectroscopic interaction of the chromium complex with the co-catalyst methylaluminoxane is investigated. The complexes are characterised by electrochemical methods such as cyclic voltammetry and spectroelectrochemistry.

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Neste trabalho investigou-se a influência do método de preparação nas propriedades estruturais e eletrocatalíticas do sistema RuO2/Ta2O5. Durante a investigação sistemática, as propriedades mecânicas e morfológicas dos eletrodos foram determinadas usando ânodos de composição nominal Ti/RuO2-Ta2O5 (Ru:Ta - 10 % - 90 % Ru variando em passos de 10 % atômico) preparados pelo método dos precursores poliméricos (MPP), na qual as soluções precursoras foram preparadas pela dissolução de RuCl3.xH2O - Aldrich e Ta(OC2H5)5 - ACROS em etilenoglicol e ácido cítrico. Ânodos de composição nominal Ti/RuO2-Ta2O5 (Ru:Ta = 30:70; 50:50 e 80:20 % atômico) também foram preparados pelo método tradicional (MT), a partir da mistura das razões molares adequadas dos sais de cloretos precursores (RuCl3.xH2O e TaCl5 ( Aldrich) em isopropanol (Aldrich). Para ambos os métodos, a decomposição térmica foi executada a 450 oC por 1h sob fluxo de 5 dm3 min-1 de O2. Análises da composição e da morfologia da camada de óxidos foram feitas antes e após o teste acelerado de vida (TAV), por meio das técnicas de EDX (Energia dispersiva de raios X) e MEV (Microscopia eletrônica de varredura), respectivamente. A microscopia mostrou estrutura de \"barro-rachado\" para maioria dos filmes. As análises de EDX evidenciaram uma ligeira redução do conteúdo do Ru após o TAV. Dados de DRX (difração de raios X) mostraram forma rutílica para o RuO2 e ortorrômbica para Ta2O5. Os voltamogramas cíclicos deste sistema em meio ácido (0,5 mol dm-3 de H2SO4) apresentaram um comportamento característico da dupla Ru(III) / Ru(IV). Em baixos sobrepotenciais, o coeficiente de Tafel, b, apresenta valores médios de 40 mV. A atividade eletrocatalítica apresentou uma forte dependência em relação ao conteúdo de RuO2 no filme. O teste acelerado de vida (TAV), executado em condições de operação drásticas (densidade de corrente de 750 mA cm-2 T = 80 oC em 0,5 mol dm-3 H2SO4), mostrou uma melhora na estabilidade do revestimento em comparação ao método tradicional, MT (~ 100 %). Os dados de VC na presença de Etanol e Acetaldeído mostraram que os eletrodos preparados via MPP apresentam melhor desempenho eletrocatalítico do que os eletrodos preparados via MT. O estudo frente à oxidação de Etanol mostrou alta reatividade com respeito á formação de Ácido Acético e CO2 em baixa concentração de Etanol (5 mmol dm-3). Por outro lado, o aumento da concentração de Etanol (15 - 1000 mmol dm-3) favorece a formação de Acetaldeido (1 - 3%) e, conseqüentemente, uma diminui na formação de Ácido Acético e CO2. / In this work, the influence of the preparation method on the structural and electrochemical properties of the RuO2/Ta2O5 system was investigated. A systematic investigation of the structural, morphological, electrochemical, and electrocatalytic properties for O2 evolution from acidic solutions was carried out using anodes of nominal composition Ti/RuO2-Ta2O5 (Ru:Ta - 10 - 90 % Ru step 10 atom %), prepared by the polymeric precursor method (PPM), in which the solutions were prepared by mixing citric acid (Merck) in Ethylene glycol (Merck) with RuCl3.xH2O (Aldrich) and Ta(OC2H5)5 (ACROS). Anodes of nominal composition Ti/RuO2-Ta2O5 (Ru:Ta = 30:70; 50:50 and 80:20 atom %) were also prepared by the standard method (SM) from the appropriate molar ratios of the precursors mixtures (RuCl3.xH2O and TaCl5 - Aldrich) in alcoholic solutions (isopropanol - Aldrich). For both methods, thermal decomposition was carried out under at 450 oC for 1 h - O2-flux of 5 dm3 min-1. Analyses of the composition and the morphology of the oxide layer were done before and after the accelerated life tests (ALT), through EDX (Energy dispersive X-rays) and SEM (Scanning electron microscopy), respectively. SEM results gave evidence of typical mud-flat-cracking morphology for the majority of the films. EDX analyses even after total deactivation showed that there is reduction in the content of Ru in the coating. Structure data were obtained by XRD (X-rays diffraction), and they showed the rutile-type struture for RuO2 and the orthorhombic one for Ta2O5. Cyclic voltammograms of this system where registered in acidic solutions (0.5 mol dm-3 of H2SO4). The observed peak was attributed to the solid-state redox transition of Ru(III)/Ru(IV). At low overpotentials, the Tafel slope (b) displayed average values of 40 mV. The electrocatalytic activity showed a strong dependence on the content of RuO2 in the film. The stability of the electrodes was evaluated by ALT, performed at 0.5 mol dm-3 in aqueous H2SO4 (750 mA cm-2 and T = 80 oC). Results showed a performance that is 100 % better than that obtained with the electrodes prepared by SM. In the presence of an organic compound (Ethanol and Acetaldehyde), the VC and Tafel results showed that electrodes prepared by PPM are electrocatalytically better than those obtained by SM. The studies on Ethanol and Acetaldehyde oxidation showed high reactivity for Acid Acetic and CO2 formation at low Ethanol concentration (5 mmol dm-3). On the other hand, the increase in Ethanol concentration (15 - 1000 mmol dm-3) favors Acetaldehyde formation (1 - 3 %) and, therefore, the formation of Acid Acetic and CO2 is lower.

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