No description available.

No description available.

No description available.

Bimolecular rate constants for the quenching of up to 15 excited triplet aromatic hydrocarbons by eight S-diketonate complexes of Cr(III) and four of Co(III) have been obtained using nanosecond laser flash photolysis. The bimolecular rate constants have been plotted against donor energy and for those quenchers whose rate constants follow the available spectroscopic energy levels in the quencher, electronic energy transfer is established to be the mechanism of quenching. As the electrochemical reduction potential of the quencher is reduced by the introduction of trifluoromethyl groups into the S-diketonate complexes, an increase in quenching efficiency is observed which may be explained by modifying the mechanism of quenching to include electron transfer. Theoretical models have been developed in order to predict quenching constants for the different mechanistic pathways. For the quenching of excited triplet states by y-substituted chromium (III) complexes, rate constants greater than those predicted by the Debye Equation, once spin statistics have been introduced, were obtained. Diffusion theories were examined and a Spernol-Wirtz type treatment was employed,since this takes into account microfiction between solute and solvent molecules of differing molecular radii. Using this diffusion theory allowed all of the results presented to be interpreted within the framework of the theoretical models. The effect of geometrical isomerism on the quenching efficency of a chromium(III) complex was investigated and it was found that the aisisomers is a more efficient electronic energy transfer quencher than the trans form, especially when energy is being accepted by the quartet state. The general nature of the approach developed in the thesis was examined by measuring the quenching rate constants for four Co(III) a-diketonate complexes.

No description available.

Asphaltenes correspond to the fraction of oil that is insoluble in heptane but is soluble in toluene. The aggregates of asphaltene are of interest because they cause serious problems in the production of oil. Asphaltenes contain fluorescent moieties, and as such they can be studied by fluorescence techniques. The first objective of this work was to develop methodologies to study the fluorescence of asphaltenes, and to investigate the fluorescence of asphaltenes at various concentrations. Time-resolved fluorescence studies indicate that asphaltenes have different chromophores with different lifetimes. The average lifetime of the asphaltene emission decreased when the asphaltene concentration was increased because of quenching processes occurring within the aggregates. The measurement of lifetimes at different excitation and emission wavelengths demonstrated that different components of asphaltene aggregate at different concentrations. The second objective of this work was to investigate how accessible the asphaltene aggregate is to small molecules by fluorescence quenching experiments. Nitromethane was the quencher used in the fluorescence of asphaltenes. The quenching efficiencies were found to be independent of the concentration of asphaltenes. However, the quenching efficiencies differed for different chromophores, suggesting a selective quenching for nitromethane of the excited states for the different chromophores of asphaltenes. The third objective of this thesis was to investigate the fluorescence of externally added probes that might be incorporated in asphaltene aggregates through π−π stacking. Pyrene was chosen as the probe because its fluorescence properties are strongly affected by its surroundings. The pyrene emission was quenched by nitromethane. The quenching efficiencies determined for pyrene in the absence or the presence of asphaltene aggregates were the same. This suggests that pyrene is located in an open environment, where the asphaltene aggregates do not offer any protection for pyrene from nitromethane. / Graduate

The basic principles of spray control during casting speed changes in the continuous casting of steel have been studied. The normal spray control practice in which water flow in the sprays is made proportional to the casting speed has been found to be completely inadequate from the standpoint of controlling the surface temperature of the strand. A computer model based upon the principle of controlling the sprays as a function of the age of the metal passing through the machine has been developed. The model predicts both the average local residence time and water flux in a spray zone for any casting speed change. An accompanying heat flow model has also been developed for characterization of cooling rates and simulation of the effects of spray control practices on the surface temperature of the strand. The control model has been used for the development of a new spray control practice in an industrial slab caster. Tests have been performed both using the new practice and the normal spray control practice. Although, a complete verification has not been possible at this time, the model appears to realistically predict the thermal requirements of the strand during casting speed change. The necessity for automatic control of the sprays has become evident during the development of the present work. The principles of automatic control of the sprays have been studied and the control model adapted for this purpose. It has been shown that a true automatic control of spray cooling can be attained using the mathematical model for spray control. / Applied Science, Faculty of / Materials Engineering, Department of / Unknown

In this thesis photoluminescence measurements were made on ZnS:Mn thin-films with different concentration of Mn in order to observe the phenomenon of concentration quenching. A mathematical model, based on migration of energy among Mn centres and subsequent transfer to traps, is proposed to explain it. Probability of energy transfer to traps is considered independent of Mn concentration. It is consistent with experimental evidence reported in literature. Using this model we can give physical interpretation to the empirical decay-law for luminescence of Mn centres n(t) =n(0)exp[ - (t/r)a ] , where j and a change with Mn concentration. We are able to quantify the probability of energy loss to traps using Dexter's equation for dipole-dipole transfer. It is suggested that energy loss to the traps can be influenced by growth technique. / Thesis / Master of Engineering (ME)

Durante os anos desde 1910, diversos trabalhos científicos foram desenvolvidos, tornando o processo de têmpera bem estabelecido nas plantas industriais. Atualmente, existem diferentes tipos de têmpera e mais técnicas estão sendo desenvolvidas para aumentar as propriedades de componentes de aço. Um deles é o de têmpera intensiva, que pode ser considerado como um processo relativamente novo. O método de têmpera intensiva visa otimizar o processo produtivo, ao mesmo tempo em que diminui consideravelmente o custo da etapa de tratamento térmico. Além disso, atualmente o uso de soluções ambientalmente amigáveis torna este processo bem menos agressivo ao planeta. Neste trabalho realizou-se a têmpera intensiva e a têmpera convencional em corpos de prova preparados para teste de tenacidade à fratura. Trata-se, porém, de um método alternativo de teste, relativamente recente, no qual são considerados parâmetros de fratura dúctil e fratura frágil para o cálculo de K1C. Estes ensaios de tenacidade foram feitos a partir de ensaios de tração em corpos de prova com pré-trinca. Embora as tensões residuais compressivas tenham atingido os valores mais altos na têmpera intensiva, os resultados de tenacidade à fratura foram mais positivos para as amostras com têmpera convencional. A junção e aplicação desses métodos pode trazer um novo parâmetro de fabricação e análise de materiais metálicos, em especial aços-mola, que foi o objeto de estudo deste trabalho. / Since the beginning of 20th Century, several scientific works were developed and conventional quenching process became well established into the industrial area. Nowadays new quenching process were introduced increasing mechanical properties of the heat treated components. Intensive quenching is one of these process which optimize the heat treatment process using also quenchants considered nontoxic to the environment. In this work it was made comparative studies in the SAE 5160 samples which were submitted to conventional quenching and intensive quenching. Toughness fracture were evaluated using an alternative test where are analyzed ductile fracture and brittle fracture parameters to calculate KIC. In this method tensile test are performed in pre-cracked samples. Residual stresses were also measured and although intensive quenching promoted highest compressive stresses in the surface, KIC obtained in such samples presented low values compared with conventional quenching. The presented method of analysis will bring a new parameter for production and analysis for metallic materials, particularly spring steel, where compressive stress and toughness are important as properties for suspension components.

Durante os anos desde 1910, diversos trabalhos científicos foram desenvolvidos, tornando o processo de têmpera bem estabelecido nas plantas industriais. Atualmente, existem diferentes tipos de têmpera e mais técnicas estão sendo desenvolvidas para aumentar as propriedades de componentes de aço. Um deles é o de têmpera intensiva, que pode ser considerado como um processo relativamente novo. O método de têmpera intensiva visa otimizar o processo produtivo, ao mesmo tempo em que diminui consideravelmente o custo da etapa de tratamento térmico. Além disso, atualmente o uso de soluções ambientalmente amigáveis torna este processo bem menos agressivo ao planeta. Neste trabalho realizou-se a têmpera intensiva e a têmpera convencional em corpos de prova preparados para teste de tenacidade à fratura. Trata-se, porém, de um método alternativo de teste, relativamente recente, no qual são considerados parâmetros de fratura dúctil e fratura frágil para o cálculo de K1C. Estes ensaios de tenacidade foram feitos a partir de ensaios de tração em corpos de prova com pré-trinca. Embora as tensões residuais compressivas tenham atingido os valores mais altos na têmpera intensiva, os resultados de tenacidade à fratura foram mais positivos para as amostras com têmpera convencional. A junção e aplicação desses métodos pode trazer um novo parâmetro de fabricação e análise de materiais metálicos, em especial aços-mola, que foi o objeto de estudo deste trabalho. / Since the beginning of 20th Century, several scientific works were developed and conventional quenching process became well established into the industrial area. Nowadays new quenching process were introduced increasing mechanical properties of the heat treated components. Intensive quenching is one of these process which optimize the heat treatment process using also quenchants considered nontoxic to the environment. In this work it was made comparative studies in the SAE 5160 samples which were submitted to conventional quenching and intensive quenching. Toughness fracture were evaluated using an alternative test where are analyzed ductile fracture and brittle fracture parameters to calculate KIC. In this method tensile test are performed in pre-cracked samples. Residual stresses were also measured and although intensive quenching promoted highest compressive stresses in the surface, KIC obtained in such samples presented low values compared with conventional quenching. The presented method of analysis will bring a new parameter for production and analysis for metallic materials, particularly spring steel, where compressive stress and toughness are important as properties for suspension components.