<p> The fatigue softening behaviour of copper single crystals was investigated as a function of temperature. Copper crystals, prestrained in tension, were softened by "push-pull" cycling at constant plastic strain amplitude, in the low amplitude range, and the cyclic stress-strain curves determined at various temperatures. Transmission electron microscopy was employed to determine the detailed microstructural changes which occured during softening while X-ray and slip line observations were utilized to indicate the overall structural changes on a macroscopic scale. The results were correlated and a rationale, based on dipole production, proposed for the observed softening behaviour. </p> / Thesis / Master of Science (MSc)

<p> The galvanomagnetic properties and shape of the Fermi surface of copper single crystals containing different density of dislocations have been studied experimentally and theoretically through magnetoresistivity measurement and using effective medium theory respectively. </p> <p> In experiments, two crystallographic orientations of copper single crystal samples with tensile axis parallel to < 100 > and < 541 > have been plastically strained to various stress levels to introduce different density Nd of dislocations. The experimental data of angle and field-dependent magnetoresistivity measured at temperature T = 2K and in the magnetic filed up to 9 Tesla show that dislocations influence substantially the galvanomagnetic properties of copper crystal samples in the open, extended and closed orbit crystallographic orientations. The results reveal that the pure samples with resistivity ratio RR equal to or larger than 151 show quadratic dependence of transverse magnetoresistivity as a function of the magnetic field in the open-orbit orientation, which changes to linear variation of magnetoresistivity with magnetic fields in highly deformed samples with RR smaller than 138. A quadratic dependence of transverse magnetoresistivity as a function of the magnetic field is significantly suppressed as the density of dislocations increases. The magnetoresistivity decrease with the increase of the density of dislocations was also observed in the closed-orbit crystallographic orientation. Such effect is independent upon the type of dislocations introduced to the crystal lattice.</p> <p> Measurements of the de Hass-van Alphen effect in plastically deformed copper single crystals have been carried out with torque magnetometer and AC susceptibility options of Quantum Design PPMS-9 system. The oscillation frequencies for the extremal orbits normal to the principal crystallographic directions are obtained through Fourier transform of torque versus inversed field characteristics. By comparing these frequencies with the analogous frequencies obtained for undeformed copper crystals, the changes in the cross-sectional area of the Fermi surface corresponding to the extremal orbits are obtained and the shape of the dislocation-distorted Fermi surface is postulated based on measurements performed.</p> <p> The effective-medium approximation and Green's function method are applied to model the magnetoresistivity data and to gain insight into the fundamental material properties responsible for the observed magnetoresistivity behavior. The effective magnetoresistivity calculated using a self-consistent method shows a good agreement with the experimental results.</p> / Thesis / Doctor of Philosophy (PhD)

No description available.

In this thesis, the physical properties of certain multi-functional organic crystals were studied in detail. This study involves the growth of single crystals of Glucuronic acidγ-lactone (GAL), Imidazoliumtartarate (IMLT), (Bis)imidazoliumtartarate (BIMLT), and Diisopropylammonium iodide (DPI) and investigations of their optical, dielectric, piezoelectric, pyroelectric, and ferroelectric properties as a function of temperature and dependence on crystal structure in these organic crystals. Piezoelectric resonance was observed at certain frequencies when dielectric constant was monitored along the b-plate of GAL crystals. The electro-mechanical coupling coefficient estimated at the resonance near 1 MHz frequency revealed an exceptionally large value in GAL similar to that in inorganic lead titanate. The dependence of the piezoelectric resonance frequency on temperature was studied in detail. These crystals showed excellent second- and third-order nonlinear optical properties as well as high laser damage threshold. The high values of χ(2) andχ(3), laser damage threshold, and low UV cut-off makes GAL crystals an interesting prospect for NLO and laser applications. Towards this goal, GAL crystals were studied in detail with regard to determination of directions of dielectric axes, optic axes, and collinear phase-matching. Single crystals of another promising NLO organic crystal, IMLT were also grown which showed interesting dielectric, piezoelectric, and NLO properties. The dielectric dispersion with temperature provided an insight to the polarization mechanisms. Like GAL, IMLT also exhibits piezoelectric resonance. The existence of only one easy axis of vibration in IMLT enabled the candidate to identify the first resonance peak as corresponding to the fundamental mode of oscillation in the sample. This also helped to determine many piezoelectric parameters. By angular phase matching, one direction of phase matching in IMLT was identified. The conversion efficiency of IMLT along this direction was determined which was high in comparison to that in a standard KDP crystal. At piezoelectric resonance frequencies, the electro-optic response due to photo-elastic contributions is enhanced. Single crystals of organic ferroelectric BIMLT were grown by mixing two moles of imidazole with one mole of l-tartaric acid. The controversy with regard to the phase transition temperature of BIMLT was clarified by the DSC and structural analysis in this work. Previously, studies on BIMLT were limited to polycrystalline samples and single crystals with inclusions primarily due to the difficulty in growing good quality single crystals from aqueous solution. However, by experimenting the growth process using different solvents, good quality single crystals were achieved without the trapping of mother solution. This remarkable find is a notable result in these crystals for ferroelectric applications. The mechanism of ferroelectricity in BIMLT is mainly attributed to the transfer of protons along N–H---O hydrogen bonds in the direction of b-axis. Interestingly, the values of spontaneous polarization and Curie-temperature in the organic ferroelectric material DPI were significantly high and comparable to several popular inorganic ferroelectrics. The polarization obtained in this material is the highest among reported organic ferroelectrics. In addition to the high Curie temperature and spontaneous polarization, there were unique phase transitions that were revealed in DPI. The mechanism of ferroelectricity is quite complex, mainly being displacive type on account of the change in orientation of dipoles with electric field. Some contribution to ferroelectricity comes from the order-disorder nature of Nitrogen atom.

Este trabalho tem por objetivo principal a caracterização dos microcristais de niobato de lítio (LiNbO3) dopados com óxido de cromo (CrO3) e com óxido de ferro (Fe2O3), crescidos no Departamento de Física e Ciências dos Materiais do Instituto de Física e Química de São Carlos - USP. Para isso, utilizamos as seguintes técnicas: Ataque Químico Preferncial, Ressonância Paragnética Eletrônica (RPE), Absorção Ótica no Visível e Infravermelho Próximo e a Técnica Holográfica. Através da técnica de ataque químico preferencial os domínios ferroelétricos destes cristais foram observados. Os possíveis sítios que os íons Cr3+ e os íons Fe3+ ocupam na rede do LiNbO3 foram estudados por RPE, e os resultados obtidos indicam que o íon Cr3+ ocupa preferencialmente as vacâncias estruturais, enquanto os íons Fe3+ ocupam o sítio do nióbio ou do lítio. O efeito de tratamentos térmicos em atmosferas redutoras e oxidantes sobre a concentração dos íons Fe3+, Fe2+ e dos centros OH-, foram estudados através das técnicas de RPE, absorção ótica no visível e no infravermelho. Através da técnica holográfica, foram observadas altas eficiências de difração holográficas em cristais dopados com ferro. Foram feitas também medidas da variação temporal desta eficiência em cristais tratados e não tratados termicamente, e em alguns casos o efeito de acoplamento de ondas foi observado. / The present work shows the characterization of the lithium niobate (LiNbO3) doped with chromium oxide and iron oxide. Those crystals were grown in the Departamento de Física e Ciência dos Materiais - IFQSC - USP São Carlos. Several experimental techniques were applied like Preferential Chemical Etching, Electron Paramagnetic Resonance (EPR), Near Infrared and Visible Optical Absorption and the Holagraphic Technique. Through the preferential chemical etching we observed the ferroelectric domains. The probable Cr3+ and Fe3+ ion site were studied with EPR, it shows the Cr3+ ions fill possibly structural vacancies, while the Fe3+ ions possibly take place on Li1+ or Nb5+ substitucionally. The thermal treatment effects at reducing and oxidizing atmospheres under several Fe3+, Fe2+ and OH- concentration were studied through EPR, visible and near infrared optical absorption. By using the holographic technique we observed holographic diffraction highly efficient in iron doped crystals. Also were made time dependence efficiency at thermally treated and non treated samples, and in some cases we observed wave coupling effect.

Oxidação de CO dissolvido em Pt(111) e Pt(110) foi realizada em soluções de ácido sulfúrico e perclórico em eletrodo rotatório em configuração de menisco (HMRDE). O uso de diferentes soluções eletrolíticas revelou a influência do anion na cinética de oxidação. Uma vez oxidada a camada de CO adsorvido, ânions podem adsorver na superfície do eletrodo provocando mudanças na corrente estacionária em potenciais anódicos. Na varredura negativa, os ânions adsorvidos são deslocados da superfície quando a velocidade de oxidação de CO é menor que a velocidade de adsorção de CO. A carga associada a esse deslocamento foi obtida para altas velocidades de varredura, resultado semelhante aos experimentos de deslocamento espécies adsorvidas só que a baixos potenciais. A oxidação de CO dissolvido também foi realizada em superfícies escalonadas pertencente à série Pt(s)[n(111)×(111)] usando a mesma configuração dos experimentos com eletrodos de baixo índice de Miller em solução de ácido sulfúrico. O perfil voltamétrico não foi significantemente modificado pela presença de defeitos (degraus). No entanto, as curvas são deslocadas para potencias negativos com o aumento da densidade de degraus. Assim, na varredura positiva, foi observada uma relação linear do potencial de inicio de oxidação (pico de ignição) e a densidade de degraus para os eletrodos com terraços de largura superior a 5 átomos de Pt. Na varredura negativa, o mesmo comportamento foi observado no potencial onde a reação não é mais favorecida. Neste caso, o eletrodo de Pt(111) mostrou um comportamento distinto das superfícies escalonadas, devido a formação e uma ordenada camada de (bi)sulfato adsorvido O processo de adsorção de ânion foi também observada na varredura negativa sob altas velocidades de varredura, com cargas relacionadas a dessorção do ânion similares a Pt(111). Apenas para o eletrodo com terraços de largura de 3 átomos mostrou valores similares a Pt(110). Todos os resultados foram analisados em função do mecanismo de reação e o possível efeito dos degraus na formação da camada de adsorção de CO. Para finalizar, um modelo para a oxidação de CO dissolvido foi proposto usando para isso equações diferenciais que descrevem o comportamento dos processos envolvidos na reação de oxidação. Através das voltametrias cíclicas obtidas pelas simulações foi possível investigar o papel da camada de adsorção de CO, a variação das constantes de velocidade das reações envolvidas e as condições do transporte de massa na resposta eletroquímica. Os resultados mostraram que a oxidação de CO dissolvido é um processo complexo com um delicado balanço das velocidades de reações envolvidas, e o deslocamento de ânions contribui para a velocidade da reação global. / Bulk CO oxidation experiments have been carried out in sulphuric and perchloric acid solutions on Pt(111) and Pt(110) electrodes under hanging meniscus rotating disk electrode (HMRDE) configuration. The comparison between the two different electrolytic media reveals an important influence of the anion in the oxidation kinetics. Once the adsorbed CO layer has been oxidized after the ignition peak, anions are re-adsorbed on the electrode surface and the presence of these anions affects the stationary currents measured at positive potentials. In the negative-going sweep, adsorbed anions are displaced from the surface when the CO oxidation rate is lower than the corresponding CO adsorption rate. The charge associated to this displacement has been measured at high scan rates and is in agreement with that expected from the CO displacement experiments performed at low potentials. Bulk CO oxidation has been studied on platinum stepped surfaces belonging to the series Pt(s)[n(111)×(111)] using the same configuration for low indice Miller electrodes. The general shape of the voltammograms is not significantly affected by the presence of the steps. However, the curves shift towards negative values as the step density increases. Thus, in the positive going scan, a linear relationship is observed for the dependence of the potential for the ignition peak with the step density for surfaces with terraces wider than 5 atoms. In the negative going scan a similar situation is observed for the potential where the current drops to zero. In this case, Pt(111) electrode deviates from the expected behavior, because the formation of the ordered bisulfate adlayer on the electrode. The anion adsorption process is also observed by recordings the voltammograms at high scan rate. All these results have been analyzed in light of the mechanism, discussing the possible effects of the steps and the defects in the CO adlayer. A simple model for the electrocatalytical carbon monoxide bulk oxidation on a platinum electrode was studied by Mean Field Approximation simulation. Under potentiodynamic conditions, the model enabled the investigation of the role of CO adlayer, rate constants and different mass transport conditions on the macroscopic electrochemical response. The results showed that the CO bulk oxidation is a complex process with a delicate balance between the three reaction rates and the displacement of anions by CO adsorption contributes to the overall reaction rate.

O estudo da eletrorredução de nitrato teve início na década de 70 e o catalisador mais estudado para esta reação é a platina, embora, para fins ambientais, o paládio seja mais indicado por apresentar maior seletividade para a formação de N2. Neste trabalho a eletrorredução de nitrato foi estudada em eletrodos de Pd, Pt e Pt com camadas de Pd. Primeiramente foi feito um estudo com nanopartículas de Pt, Pd e Pt com uma monocamada de Pd dispersas em carbono, mas a dificuldade em sugerir um mecanismo nesse tipo de eletrodo levou à utilização de eletrodos monocristalinos, que tem a superfície conhecida. Foram feitos estudos eletroquímicos em eletrodos de Pd(hkl) que não apresentaram grande atividade frente a redução de nitrato, uma vez que o potencial inferior não foi menor que 0,2 V, para evitar excessiva absorção de hidrogênio. Em Pt(hkl) os resultados obtidos estão de acordo com os publicados na literatura e comprovam a sugestão de que o produto final é NH4 +. Para observar a atividade do paládio frente à reação, foram utilizados eletrodos monocristalinos de Pt (hkl) com multicamadas de paládio, que se comportam como monocristais de paládio, mas sem absorver hidrogênio. Nestes materiais foram feitos estudos eletroquímicos e de FTIR. Verificou-se que a atividade dos eletrodos depende da estrutura e do eletrólito, mas a definição de um mecanismo fica comprometida pelo fato do solvente (H2O ou D2O) favorecer a formação de um produto ou outro, embora tenha sido comprovada a formação de NO(ads) em todos os casos. / The study of nitrate electro-reduction started in the 70s and the most studied catalyst for this reaction is platinum, however because of environmental reasons, palladium is more indicated since it has higher selectivity for the formation of N2. In this work the nitrate electro-reduction was studied on Pd, Pt and Pt modified with Pd layers. First, a study involving carbon-supported nanoparticles of Pt, Pd and Pt with a Pd single layer was carried out, but the difficulty suggesting a reaction mechanism for this kind of electrode has driven to the use of single crystals, because of their defined surface structure. So, electrochemical studies were made on Pd(hkl) electrodes, but these materials did not present great activity for nitrate reduction, because the electrode potential was not decreased below 0,2 V, to avoid large hydrogen absorption. For Pt(hkl) the obtained results are in agreement with those published previously, confirming the suggestion that NH4 + is the final product. To characterize the palladium activity for this reaction, Pt(hkl) single crystals with palladium multilayer were employed, because these materials have a behavior similar to palladium single crystals, but without hydrogen absorption. Electrochemical and FTIR experiments were done to characterize the i vs E responses and to follow the reaction intermediates and products. It was seen that the activity depends on the electrode structure and the electrolyte, but the mechanism determination pointed to a uncertain situation where the solvent (H2O or D2O) drives to the formation of one product or another, although the production of NO(ads) has been observed in all cases.

A ênfase de todo o trabalho é relacionada com a preparação de soluções sólidas, concentrando-se na matriz de niobato de lítio, LiNbO3, puro e suas soluções sólidas óxidas, onde obtemos monocristais de LiNbO3 com Cr3+, Fe3+ e Mg2+. Os monocristais de LiNbO3são obtidos pelo método de Czochralski (fusão) e a caracterização de suas propriedades está voltada para a análise da perfeição estrutural, da distribuição de dopantes na matriz cristalina, da orientação dos domínios de dipolos ferroelétricos e o estudo do comportamento do coeficiente de segregação efetivo com os parâmetros de crescimento, com a cinética de adsorção e com o efeito de facetamento / The emphasis of all this work is related to the preparation of solid solutions, concentrating on lithium niobate single crystals, LiNbO3, pure and its oxide solid solutions, where we can obtain LiNbO3 single crystals doped with Cr3+, Fe3+ and Mg2+. The LiNbO3 single crystals are obtained by the Czochralski method (fusion) and the characterization of its properties is on the analysis of the structural perfection, of the distribution of the doping in the crystal matrix, of the orientation of the domains ferroelectrics dipoles and the study of the effective segregation coefficient behavior, with the growing parameters, with the adsorption kinetic and the facet effect

Neste trabalho crescemos monocristais de Bi12SiO20 (BSO) e de Bi12TiO20 (BTO). Os cristais de BSO foram crescidos pela técnica de Czochralski a partir da fase líquida com composição molar 6Bi2O3:1SiO2. Os melhores resultados foram obtidos para taxas de puxamento entre 1 e 2,5mm/h, a velocidade de rotação foi mantida constante em 20rpm. Os cristais de BTO foram crescidos pelo método top-seeded solution Growth (TSSG) a partir da composição molar 10Bi2O3:1SiO2, com taxas de puxamento menores que 0,3mm/h e velocidade de rotação entre 16 e 30 rpm. A qualidade dos cristais foi avaliada utilizando microscopia óptica e eletrônica, corrosão seletiva e raios-x. Por microscopia óptica identificamos os defeitos macroscópicos e discutimos sua natureza e possíveis causas. Utilizando a técnica de ataque químico seletivo, analisamos a morfologia das figuras de ataque e estimamos a densidade de deslocações. Visando identificar defeitos de estequiometria, fizemos medidas de composição por microanálise eletrônica. Para a caracterização cristalográfica, calculamos o parâmetro de rede por difração de raios-x usando o método do pó e confirmamos a estrutura cristalográfica através do método de Rietveld. E, ainda, medimos a atividade óptica que é uma constante característica dos cristais. / In this work we grew Bi12SiO20 (BSO) and Bi12TiO20 single crystals. The BSO crystals have been grown from the melt composition of 6Bi2O3:1SiO2 by the Czochralski method. The best results were obtained at pulling rates from 1 to 2.5m/h, the rotation rate of 20rpm was constant. The BTO crystals have been grown by the top-seeded solution growth technique from a 10Bi2O3:1SiO2 solution with pulling rates less than 0.3mm/h and rotation rates from 16 to 30rpm. The crystal quality was examined by optical and scanning electron microscopy, selective etching, and x-ray diffraction. We identified the macroscopic defects by optical microscopy and discussed their nature and probable origin. Using selective etching, we analyzed the etching pits morphology and evaluated the dislocation density. To identify stoichiometric defects, composition measurements by electron probe microanalysis were made. To obtain crystallographic characterization, we calculated the cell parameter by powder method x-ray diffraction and used the Rietveld method to verify the crystallographic structure. And, also, we measured the optical activity, a constant of the crystals.

A eletro-oxidação de etanol é certamente um dos principais temas de pesquisa no campo de eletrocatálise devido ao sua potencial utilização em sistemas conversores de energia química em energia elétrica, pois é considerado um combustível renovável e de baixa toxicidade, além de possuir alta densidade energética. Quando afastado do equilíbrio termodinâmico, esse sistema apresenta uma dinâmica complexa devido a continua ativação e desativação do catalisador e na presença de instabilidades, é possível alcançar um maior valor de eficiência termodinâmica quando comparado com o mesmo sistema operando em regime convencional. Além disso, a facilidade de emergência de instabilidades e a simplicidade com que os parâmetros experimentais podem ser controlados fazem com que o estudo deste tipo de sistemas se torne atrativo para a comunidade científica. Nesta tese de doutorado são apresentados os resultados obtidos durante a dinâmica convencional e não-linear da eletro-oxidação de etanol em meio alcalino, combinando os dados obtidas em ambos os processos cinéticos para entender plenamente o efeito dos cátions durante a reação catalítica. Após a análise do impacto de diferentes íons na cinética convencional e complexa, foram estimados os valores máximos das frequências de turnover na presença de diferentes ânions e cátions, utilizando os dados obtidos por métodos eletroanalíticos e, assim, discutir a atividade em uma base mais fundamental. Ainda, a partir dos dados da dinâmica oscilatória, foi possível realizar uma avaliação indireta da dissipação química dos sistemas estudados empregando as curvas pseudo-estacionárias. Por fim foi realizada a caracterização do comportamento oscilatório da eletro-oxidação de etanol em meio alcalino em superfícies monocristalinas de platina, relacionando as variações no perfil oscilatório com os defeitos presentes nos planos cristalográficos. / The electro-oxidation of ethanol is certainly one of the main research topics in the field of electrocatalysis due to its potential use in converting chemical energy into electrical energy, since it is considered a renewable fuel with low toxicity and high energy density. When it is far from the thermodynamic equilibrium, this system presents a complex dynamic due to the continuous activation and deactivation of the catalyst and with the emergence of instabilities it is possible to achieve a higher thermodynamic efficiency when compared to the same system operating in conventional regime. In addition, the facility in which instabilities emerge and the simplicity which the experimental parameters can be controlled make the study of such systems attractive to the scientific community. In this thesis the results obtained during the conventional and non-linear dynamics of the electro-oxidation of ethanol in alkaline media are presented, combining the data obtained in both kinetic processes to fully understand the effect of the cations during the catalytic reaction. Once the impact of different ions on conventional and complex kinetics was analyzed, the maximum values of the turnover frequencies in the presence of different anions and cations were estimated using the data obtained by electroanalytical methods and, with the results in hand it was possible to discuss the activity on a more fundamental basis . Also, from the data of the oscillatory dynamics, it was possible to perform an indirect evaluation of the chemical dissipation of the systems studied using the pseudo-stationary curves. Finally, the characterization of the oscillatory behavior of the electro-oxidation of ethanol in alkaline medium on platinum single-crystalline surfaces was performed, connecting the disparities in the oscillatory profile with the defects presented in each crystallographic planes.