The recombination rate coefficient for molecular helium ions has been measured in a pulsed afterglow at 1.86 Torr as a function of electron temperature and electron density without making a priori assumptions about the functional dependence. The concentrations of the molecular ions and electrons were measured and the source terms for the molecular ions were included in the rate equation.

Ce travail porte principalement sur la caractérisation des propriétés de transport de l'oxygène dans deux cobaltites, Ba2Co9O14 et Ca3Co4O9+δ, matériaux prometteurs comme cathode de pile à combustible à oxyde solide ou anode d’électrolyseur haute-température. Une grosse partie du travail a concerné la mise en place de la mesure de profils de diffusion de l'oxygène par échange isotopique et analyse SIMS. L'étude a ainsi démontré que ces deux matériaux sont des conducteurs mixtes ionique/électronique.Alors que les paramètres de transport mesurés sur Ba2Co9O14sont relativement faibles, les phases dérivées de Ca3Co4O9+δ présentent des coefficients d'échange en surface du même ordre de grandeur que ceux des matériaux de cathode les plus performants aujourd'hui. La structure de Ca3Co4O9+δ est constituée de l'alternance de couches Ca2CoO3-δ de type NaCl et de couches hexagonales CoO2. L'étude de céramiques texturées a démontré une diffusion facilitée parallèlement aux couches, probablement au sein des couches de type NaCl, lacunaires en oxygène. Par ailleurs, les premières mesures par LEIS ont montré la présence préférentielle de calcium à l'extrême surface du matériau. / The main objective of this work was the characterization of the oxygen transport properties of oxygen in two cobaltite materials, Ba2Co9O14 et Ca3Co4O9+δ, promising as SOFC cathode or SOEC anode. A significant part of this work was devoted to the set-up of oxygen diffusion profiles measurement by combining isotopic exchange and SIMS analysis. It has been demonstrated that these ceramics are mixed ionic–electronic conducting (MIEC) materials. Even though Ba2Co9O14's oxygen transport coefficients are relatively low, Ca3Co4O9+δ and derivatives show surface exchange coefficients close to those encountered in the today's most promising cathode materials.Ca3Co4O9+δ is built upon the stacking of Ca2CoO3-δ rock-salt layers and CoO2 hexagonal layers. The study of textured ceramics showed a preferential diffusion along the layers, probably inside the rock-salt layers which contain oxygen vacancies. In addition, first LEIS measurements showed that the uppermost atomic layer of the structure is mainly made up of calcium atoms.

No description available.

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfillment of the academic requirements for the degree of Master of Science. Johannesburg, March 2016. / Zinc oxide (ZnO) continues to receive widespread attention due to its excellent optical and electronic properties; it demonstrates the combined characteristics of high transmittance and electrical conductivity. Despite the tremendous drive for its application in optoelectronic devices, the full nature of the point defects and defect complexes have not been characterised comprehensively. In this work, luminescence characteristics of the intrinsic defects in ZnO thin films and the rare-earth ions Eu3+ and Tb3+ in ZnO powders are investigated under blue-laser excitation. The thin films used in this study were deposited using the radio-frequency magnetron sputtering method over a 2 hour duration under varied power and substrate bias conditions. The powders were synthesized by the chemical bath deposition method with dopant concentrations of 1.0 mol%. Grazing incidence X-ray diffraction (XRD) was used to determine the lattice properties of the samples. Photoluminescence studies were primarily conducted at room temperature (300 K) with the 457.9 nm, 476.5 nm and 488.0 nm laser lines as excitation sources. For the ZnO thin films, XRD patterns of a hexagonal wurtzite structure with a c/a ratio of about 1.60 and a u-parameter of 0.38 were obtained. Photoluminescence measurements show a broad emission band in the 500.0-900.0 nm range, centred at 656.0 nm. Annealed films yielded relatively more intense photoluminescence spectra than the as-prepared films. The intrinsic point defects and defect level transitions responsible for the broad emission are discussed. For the ZnO powders, the XRD patterns of the annealed samples of pristine ZnO, ZnO:Eu3+ and ZnO:Tb3+ are similarly consistent with a hexagonal wurtzite ZnO phase. Energy dispersive spectroscopy (EDS) confirmed the presence of the Eu3+ and Tb3+ dopants in the respective ZnO host while scanning electron microscopy (SEM) measurements showed the morphology of the sample powders. Photoluminescence spectra of pelletized samples, obtained in the 460.0-900.0 nm range, exhibit relatively intense Eu3+ and Tb3+ emission bands superimposed on a broad emission background. The RE3+ emission bands are attributed to the 5D0 → 7FJ (J = 0, 1, 2, 3, 4) and the 5D4 → 7FJ (J = 0, 1, 2, 3, 4, 5, 6) electronic transitions of Eu3+ and Tb3+, respectively, while the background emission is attributed to intrinsic defects. Crystal-field energy levels for the Eu3+ ion and the Tb3+ ion occupying a C3v symmetry site were deduced from fitting Gaussian curves to the RE3+ emission bands. Possible channels for transfer of energy from the intrinsic defects to Eu3+ and Tb3+ are discussed. / LG2017

A dissertation submitted to the Faculty of Science University of the Witwatersrand, Johannesburg for the degree of Master of Science. Johannesburg, 1994. / Molecular mechanics calculations are used to interpret and predict metal ion discrimination by coordinating ligands. Of particular interest are chelates exhibiting characteristics that Single them out for potential medical application; Selectivity patterns for several series of ligands are investigated with the help of strain energy profiles as a function of metal-donor atom bond distance. Ligands include simple; open-chain oxygen- and nitrogen-donors ana triaza- and tetraazamacrocyeles. Results are compared with X-ray crystallographic and solution data. Factors such as chelate ring size, conformational flexibility and preferred metal coordination geometry are found to influence metal specificity. Addition of pendent donor groups to macrocyoles leads to rigid structures and selectivity predictions according to cavity size. Interpretation of specific. metal ion recognition by polyetner antibiotics is attempted. Structural and steric factors are probed as possible determinants of metal choice. both covalent and ionic bonding models are explored. The covalent approach results in predictions of metal selectivity which correlate with mown selectivity patterns. Unfortunately, inability to optimise force field parameters in the ionic bonding approach forced us to abandon this model. The main force field used is the TRIPOS (1992,1993) force field. It performs well in calculations involving a univariate scanning technique but has to be modified to obtain reasonable structure reproduction with the large antibiotics, Errors in thermodynamic data predictions are obtained, nonbonding parameters have yet to be properly parameterized and the allocation of partial atomic charges warrants closer examination . All of these factors contribute to the poor performance of the force field when ionic interactions between metal and donor atoms of the polyethers are assumed. / AC2017

A Dissertation Submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg for the degree of Master of Science. Johannesburg March 1975

M.Sc., Faculty of Science, University of the Witwatersrand, 2011 / In this dissertation, the synthesis of nitrogen doped carbon nanotubes (N-CNTs) was performed successfully, using a floating catalyst chemical vapour deposition (CVD) method. Fe(CO)5 was utilized as the catalyst and acetonitrile and toluene as nitrogen and carbon sources respectively. Two different procedures were used to add reagents to the reactor: an injection method and a bubbling method. The effect of nitrogen concentration and physical parameters such as reaction temperature, gas flow rate on the morphology, crystallinity and thermal stability of the tubes was studied. The synthesized materials were characterized by means of Raman spectroscopy, TGA and TEM analyses. The presence of nitrogen was confirmed by the presence of the bamboo formations in the tubes by TEM. A comparison of the data from the numerous reactions revealed that N-CNTs can be made from Fe(CO)5 and acetonitrile. Further the main conclusions achieved using the injection method were: i) the maximum number of tubes with bamboo structure were made using on acetonitrile concentration of 15%, ii) The best growth temperature to make N-CNTs was 850 oC, iii) An increase in acetonitrile concentration decreased the yield of NCNTs and iv) Tubes with the narrowest outer diameters were made using an acetonitrile concentration of 15%.

In this project the link between species in solution and the solid state was considered. This is relevant due to the many applications in life where there is this interchange between solid and solution state, for example drug design, environmental metal speciation and the manufacture of materials that are in contact with solution (such as outdoor surface coatings, containers and so on). Complexation of two metal ions, namely cadmium(II) and bismuth(III), was studied. With bismuth showing so much promise in medicinal applications it was pertinent to investigate this interchange since the intake of medication is generally in the solid form which then converts to solution species as it dissolves in the body where it becomes active. For cadmium it is mainly the environmental concerns which we are faced with that call for the examination of speciation of complexes in solution, as well as their disposition upon precipitation or crystallization. A correlation was found between solution species and the complex that was isolated in the crystalline form with regards to pH for a number of metal-ligand species. We show how the results from solution experiments (achieved using direct current polarography) and those of crystalline complexes can complement each other when using species distribution diagrams as the intermediary. The distribution of species can be varied by changing the concentration and ligand-to-metal ratio at which the species distribution diagram is plotted. It is this characteristic which allows the solution and crystalline complexes – which are achieved using differing experimental conditions – to be correlated. The speciation diagram for a metal-ligand system, calculated using formation constants derived from solutions studies, was used in most instances to target specific species for their growth in the solid state. In some cases the solid state structure was used to confirm a suspected solution species, and in others the result was used to identify minor solution species which cannot be detected by the techniques used in determining formation constants. Further, we show that doing solution experiments at a range of temperatures can also aid in elucidating these minor species. The growth of crystalline species at low pH was important for this work because the pH titrations used for solution experiments were conducted from below pH 2 where the diffusion junction potential is large and changing. An in-situ witness ion was incorporated into the experiment to monitor the shifts due to the diffusion junction potential so that they could be compensated for. Additionally, for bismuth-ligand systems, hydrolysis and complexation with nitrates occurs in this same pH region. The formation constants and the species identified below pH 2 therefore do carry some uncertainty, so obtaining crystalline complexes of these species provides further confidence in their prediction in solution.

O estudo da extractibilidade em solventes orgânicos de soluções contendo íons de cobre, azoteto, excesso de 2,2\'-dipiridila e perrenato foi aproveitado como reação analítica para o referido ânion e utilizada para fins quantitativos. Elaboraram-se dois métodos de determinação. O primeiro baseia-se na extração com metil, isobutilcetona e a subsequente medida indireta de íons de cobre, no próprio extrato orgânico, pela técnica de absorção atômica, que permite determinar perrenato em concentrações de 3 a 16 µg/ml. O segundo método é espectrofotométrico e os valores de absorção do extrato orgânico possibilitaram a determinação de perrenato em teores de 16 a 40 µg/ml . O procedimento analítico apresenta seletividade apreciável, sendo aplicável na presença de elevados teores de molibdato e grande número de íons estranhos. As condições de trabalho que tornassem possível a extração quantitativa dos íons perrenato do meio aquoso foram elaboradas mediante um estudo preliminar da influência de vários fatores sobre o decurso da reação. Isolou-se uma substância de composição CuN3ReO4(dipy)2 que se provou corresponder a espécie extraída no processo analítico. Realizou-se estudo visando-se a melhor caracterização do composto isolado que levou a admitir ser a espécie contendo perrenato o composto: ‌CuN3(dipy)2‌ReO4. A execução de uma prova de toque para a identificação de íons periodato foi elaborada mediante o aparecimento de coloração amarela no extrato orgânico a partir de soluções aquosas contendo íons de manganês(II), azoteto, 2,2\'-dipiridila e íons periodato extraídas com metil, isotubilcetona. Foi possível identificar 0,45 µg de 10-4 na presença de 5.000 µg de 10-4. Outros ânions estranhos são também tolerados. / A study on the extractability with ·organic solvents of aqueous solutions containing cooper(II), azide and perrhenate ions in the presence of excess of 2,2\'-dipyridyl led to a new analytical reaction for perrhenate. On the basis of this study two methods for the determination of perrhenate were developed, both through the extraction of the aqueous system referred above with methyl isobutyl Ketone, followed by the indirect determination of cupric ions in the organic extract. In the first one atomic absorption is employed al lowing the determination of perrhenate in the range of 3 to 16 µg/ml. In the second method, through the spectrophotometric measure of the extracted copper(II), 16 to 40 µg/ml can be determined. The analytical procedure is highly selective being applicable in the presence of a large concentration of molybdate and a considerable number of foreign ions. The working conditions were established as a result of a previous systematic study fo the influence of various factors on the extraction efficiency. A product corresponding to the composition CuN3ReO4(dipy)2 was isolated and proved to correspond to the extracted species in the analytical procedure. A careful experimental study showed that this compound should correspond to the formula ‌CuN3(dipy)2‌ReO4. A spot test for the identification of periodate was based on t he yellow color of the organic phase when an aqueous developed solution containing manganese(II), azide, 2,2\'-dipyridyl and periodate ions is extracted with methyl isobutyl Ketone. 0,45 µg of 10-4 can be detected in the presence of 5,000 &#181g of 10-3 as well as of a number of foreign ions which do not interfere in the conditions of the test.

Orientador: Marcelo de Freitas Lima / Coorientador: Mario Henrique Gonzalez / Banca: Elma Neide Vasconcelos Martins Carrilho / Banca: Gustavo Orlando Bonila Rodriguez / Resumo: Os íons metálicos estão cada vez mais presentes nos efluentes industriais e sua destinação incorreta pode promover impactos ambientais desastrosos. O desenvolvimento de processos e metodologias para remoção destes diferentes metais em níveis tóxicos deve apresentar baixo custo e serem sustentáveis de forma geral. Nos últimos anos tem havido um considerável interesse nos estudos de materiais sorventes obtidos a partir de biomassa, como as pectinas, que são provenientes principalmente de resíduos de frutas. A utilização da pectina traz vantagens por possuir uma estrutura potencialmente propícia a modificações químicas, além de poder ter sua seletividade modulada devido aos seus diferentes graus de esterificação. Tendo isso em vista, foi sintetizado e caracterizado um derivado ácido hidroxâmico da pectina de frutas cítricas visando seu emprego em estudos de sorção para chumbo. As caracterizações foram realizadas através da determinação do grau de esterificação, espectrofotometria na região do infravermelho, titulação potenciométrica, estudo da afinidade por vapor d‟água, análises térmicas e análise elementar. A caracterização da pectina quanto ao seu grau de esterificação, pelas técnicas de infravermelho e titulação revelaram resultados coerentes, com graus de esterificação de 49,9% e de 51,5% respectivamente. A caracterização do biopolímero modificado foi realizada por espectrofotometria na região do infravermelho e possibilitou a identificação de duas novas bandas em... / Abstract: The metal ions are increasingly present in industrial effluents and their improper disposal can result in environmental impacts. The development of processes and methodologies for removing these toxic metals must be low cost and sustainable. In the last years, there has been a considerable interest in studies of sorbent materials obtained from biomass, such as pectin, which are extracted from fruit residue. The use of pectin brings advantages for having a structure potentially favorable to chemical modifications, besides having its selectivity modulated due to different degrees of esterification. Keeping this in view, a pectin hydroxamic acid derivative was synthesized, which was characterized with commercial citrus fruit pectin for the purpose of both being used in metal ion sorption tests. The characterizations include determination the degree of esterification, spectroscophotometry in the mid-infrared region, potentiometric titration, the water uptake, thermal analysis and CHN. The characterization of pectin as to its degree of esterification, by spectroscophotometry in the mid-infrared region and titration, techniques revealed consistent results, with degrees of esterification of 49.9% and 51.5% respectively. The characterization of the modified biopolymer was performed by infrared radiation absorption and allowed the identification of two new bands at 1548 cm-1 and 1646 cm-1 compared to the starting material, showing its modification. Through of potentiometric titration technique, the pKa value of the commercial pectin was 4.4. The experiments regarding affinity to water, showed a similarity hydrophilic character of commercial pectin compared to its hydroxamic acid derivative. DSC analysis showing the modification of commercial pectin, in which the derivative showed a higher thermal stability compared to the starting material. The CHN results determined the degree of ... / Mestre