Disertacija yra skirta relaksacijos procesų, vykstančių optoelektronikos medžiagose ir prietaisuose, tyrimui dažninės skyros metodu. Įprastas fluorescencijos gesimo trukmės tyrimo metodas, veikiantis harmoniškai moduliuoto žadinimo režimu, buvo adaptuotas žadinimui puslaidininkiniais šviestukais ir signalų registravimui radijo dažnių faziniu detektoriumi. Neorganiniai vandeniniu zolių-gelių metodu susintetinti fosforai, skirti baltų puslaidininkinių šviestukų gamybai, ir modernios organinės optoelektronikos medžiagos buvo ištirtos fotoliuminescencijos gesimo dėsnio ir kvantinės išeigos matavimo metodikomis, siekiant nustatyti krūvininkų rekombinacijos savybes ir optimizuoti sintezės parametrus. Realizuotas GaN epitaksinių sluoksnių liuminescencijos kinetikos tyrimas labai žemo kvazitolydinio sužadinimo atveju patvirtino rekombinacijos per priemaišas svarbą šio tipo medžiagose. Buvo sukurtos naujos prekinių šviestukų šiluminių savybių in-situ tyrimo metodikos, veikiančios dažninės skyros režimu. Bangos ilgio keitiklių temperatūra baltuose šviestukuose ir šilumos relaksacijos konstantos įvairaus tipo šviestukų konstrukciniuose elementuose buvo išmatuotos esant vardiniam šviestukų veikimo režimui. / The thesis is devoted to the frequency-resolved investigation of the relaxation processes in optoelectronic materials and devices. Conventional fluorescence decay time measurement technique in the frequency domain was adapted for the use with light-emitting diode (LED) excitation and signal registration by a lock-in amplifier. Inorganic phosphors synthesized by aqueous sol-gel combustion method for using as wavelength converters in white LEDs as well as advanced organic semiconducting materials were investigated by photoluminescence decay time and quantum yield measurement techniques. The photoluminescence decay time measurement technique with extremely low quasi-continuous UV LED excitation was applied for the carrier dynamics research in GaN epitaxial layers. The investigation under such a low excitation conditions revealed the contribution of donor-acceptor recombination in the yellow luminescence of GaN. The techniques for in-situ thermal characterization of encapsulated LEDs, including the measurements of phosphors converter temperature and heat relaxation time constants inside a LED, were developed and demonstrated for the investigation of commercial low- and high- power LEDs.

In this study, it was shown that improvements in the quality of continuously cast steel billets, similar to those which can be achieved by electromagnetic stirring (EMS) of the liquid pool during solidification, can be obtained if casting superheats can be consistently controlled at low levels. A lack of casting temperature control is not only detrimental to the quality of the cast product, but also to a shop's overall productivity. The important variables for temperature control in the ladle, tundish and mould were quantified and possible methods of controlling these variables were proposed. / The thermal state of the ladle lining is one of the major contributors to the variability in casting temperature. It was shown that by reducing energy losses from the refractory ladle lining, by way of incorporating an insulating refractory tile between the ladle shell and the safety lining and by using a ladle lid throughout the cycle of the ladle, temperature losses from the liquid steel can be substantially decreased. (Abstract shortened by UMI.)

The aim of this study was to investigate rubber toughening of resol type phenol-formaldehyde resin. For this purpose, phenolic resin was first modified by only acrylonitrile butadiene rubber, and then by using nitrile rubber together with 3-aminopropyltriethoxysilane. Test specimens were prepared by mixing and casting of liquid phenolic resin in three groups. In the first one, neat phenolic resin specimens were produced. In the second group, phenolic resin was modified with 0.5, 1, 2, and 3 wt.% nitrile rubber, while in the last group modification was carried out by using 0.5 wt.% nitrile rubber together with 1, 2, and 4wt.% amino silane (with respect to nitrile rubber). All specimens were heat cured in the oven. In order to observe behaviors of the specimens, Three-Point Bending, Charpy Impact, Plane-Strain Fracture Toughness, and Dynamic Mechanical Analysis tests were conducted according to the related ISO standards for all specimens groups. Scanning Electron Microscopy was also used for the fractographic analysis of some samples. It can be concluded that, although there were problems in mixing and casting of liquid resol type phenolic resin, its toughness could be improved by using nitrile rubber and amino silane. Modification by using nitrile rubber and amino silane together was much more effective than by using only nitrile rubber. In this synergistic case for instance, Charpy impact strength and fracture toughness values of the neat phenolic specimens were increased 63% and 50%, respectively. SEM studies indicated that the main rubber toughening mechanism was shear yielding observed as deformation lines especially initiated at the domains of nitrile rubber and amino silane.

Due to the extreme operating conditions present in the combustion sections of gas turbines, designers have relied heavily on specialized engineering materials. For blades, which must retain substantial strength and resistance to fatigue, creep, and corrosion at high temperatures, directionally-solidified (DS) nickel-base superalloys have been used extensively. Complex thermomechanical loading histories makes life prediction for such components difficult and subjective. Costly product inspection and refurbishment, as well as capital expense required in turbine forced outage situations, are significant drains on the resources of turbine producers. This places a premium on accurate endurance prediction as the foundation of viable long-term service contracts with customers. In working towards that end, this work characterizes the behavior of the blade material CM247LC DS subjected to a variety of in-phase (IP) and out-of phase (OP) loading cycles in the presence of notch stress concentrations. The material response to multiaxial notch effects, highly anisotropic material behavior, time-dependent deformation, and waveform and temperature cycle characteristics is presented. The active damage mechanisms influencing crack initiation are identified through extensive microscopy as a function of these parameters. This study consisted of an experimental phase as well as a numerical modeling phase. The first involved conducting high temperature thermomechanical fatigue (TMF) tests on both smooth and notched round-bar specimens to compile experimental results. Tests were conducted on longitudinal and transverse material grain orientations. Damage is characterized and conclusions drawn in light of fractography and microscopy. The influences of microstructure morphology and environmental effects on crack initiation are discussed. The modeling phase utilized various finite element (FE) simulations. These included an anisotropic-elastic model to capture the purely elastic notch response, and a continuum-based crystal visco-plastic model developed specifically to compute the material response of a DS Ni-base superalloy based on microstructure and orientation dependencies. These FE simulations were performed to predict and validate experimental results, as well as identify the manifestation of damage mechanisms resulting from thermomechanical fatigue. Finally, life predictions using simple and complex analytical modeling methods are discussed for predicting component life at various stages of the design process.

Thesis (M.S.)--Air Force Institute of Technology, 2008. / Title from title page of PDF document (viewed on: Dec 10, 2009).

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / No intuito de melhorar a eficiência de dispositivos ópticos, pesquisadores têm investigado os efeitos da incorporação de nanopartículas metálicas em materiais contendo terras-raras. Neste trabalho, foram estudados os efeitos da variação de AgNO3 em vidros teluritos (72,5TeO2-23WO3-3Na2CO3-1,5Nb2O5 (% em mol)) dopados com 0,1% em mol de Pr3+ produzidos pelo método de fusão seguido de tratamento térmico. As amostas foram caracterizadas por Difratometria de Raios X (DRX), Espectroscopia de Infravermelho com Transformada de Fourier (FTIR), Calorimetria Exploratória Diferencial (DSC), espectroscopia de absorção e luminescência, tempo de vida e índice refração. As curvas de DRX evidenciaram a natureza não-cristalina das amostras enquanto que os espectros de FTIR mostraram que os grupos vibracionais não sofrem alterações após a adição de prata. Por meio das curvas de DSC foi verificado um aumento de aproximadamente 70°C na estabilidade térmica dos vidros TWNN com a adição de prata. A partir dos espectros de absorção na região do visível foi possível verificar a banda de ressonância de plasmon de superfície (SPR) centrada em 500 nm. As medidas de luminescência, por sua vez, mostraram um aumento e quenching nas intensidades das bandas provenientes das transições eletrônicas do Pr3+. Foi observado que a banda entre 480-520 nm apresenta quenching, isso se deve possivelmente pela proximidade da banda SPR. As bandas entre 575-675 nm, por sua vez, tiveram uma melhora na intensidade das luminescências devido à incorporação das AgNPs evidenciando que a adição de AgNO3 proporciona melhorias na intensidade de algumas transições eletrônicas do íon Pr3+. Trabalhos futuros envolvendo medidas complementares serão realizados bem como o uso de outros íons terras-raras na matriz TWNN. / In order to improve the efficiency of optical devices, researchers have investigated the effects of the incorporation of metallic nanoparticles into materials containing rare earth. In this work, the effects of AgNO3 variation on doped telurite glass with 0.1 mol% of Pr3+ produced by the melting-quenching method were studied. The samples were characterized by X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Differential scanning calorimetry (DSC), absorption and luminescence spectroscopy, lifetime and index refraction. The XRD curves demonstrated the non-crystalline nature of the samples whereas the FTIR spectra showed that the vibrational groups did not change after silver addition. By means of the DSC curves an increase of approximately 70 °C in the thermal stability of the TWNN glasses with the addition of silver was verified. From the absorption spectra in the visible region it was possible to verify the surface plasmon resonance band (SPR) centered at 500 nm. The luminescence measurements, in the other hand, showed an increase and quenching in the intensities of the bands coming from the electronic transitions of Pr3+. It was observed that the band between 480-520 nm presents quenching, this is possibly due to the proximity of the SPR band. The bands between 575-675 nm, in turn, had an improvement in the intensity of the luminescences due to the incorporation of AgNPs evidencing that the addition of AgNO3 provides improvements in the intensity of some electronic transitions of the Pr3+ ion. Future works involving complementary measures will be performed as well as the use of other rare earth ions in the TWNN matrix.

CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / CNPq - Conselho Nacional de Desenvolvimento Científico e Tecnológico / FAPEMIG - Fundação de Amparo a Pesquisa do Estado de Minas Gerais / Este trabalho apresenta um novo método experimental desenvolvido para a obtenção simult ânea da condutividade e difusividade térmica em materiais sólidos. O método se baseia na utilização de um novo aparato experimental na superfície que possui uma resistência elétrica para aquecimento, dois termopares e um sensor de medição de uxo de calor. A amostra é parcialmente aquecida em uma superfície frontal. Duas regiões da superfície frontal, aquecida e não aquecida, são usadas para estimar as propriedades térmicas. A técnica se baseia na minimização de duas funções distintas. Ambos são de nidos a partir das mesmas medições de temperatura na superfície frontal, mas usando diferentes conceitos térmicos. A primeira usa uma razão de ganho entre duas temperaturas medidas na superfície para a identi cação da difusividade térmica. A segunda é de nido pela conhecida função de erro quadrático calculada entre os valores experimentais e teóricos para as mesmas temperaturas super ciais. Este segundo procedimento permite a estimativa da condutividade térmica. Em adição, apresentase um estudo sobre o comportamento de alguns parâmetros que in uenciam o conceito de razão de ganho de temperatura. As propriedades foram mensuradas em duas amostras diferentes, PVC e cobre, materiais não condutor e condutor, com o desvio inferior a 2% dos valores encontrados na literatura. Após as estimativas experimentais, os modelos térmicos originais foram reduzidos a outros modelos simpli cados. Uma nova função para estimar a difusividade térmica foi proposta usando o modelo térmico simpli cado com a mesma ideia da razão de ganho entre duas temperaturas medidas na superfície. A condutividade térmica é determinada com o mesmo conceito quando se usa o modelo original, mas é necessária uma nova posição de medição de temperatura experimental sob o centro do uxo de calor. Esses modelos reduzidos usam apenas uma superfície para estimar as propriedades térmicas. Os resultados da estimativa de propriedades são os mesmos usando o modelo original ou os modelos reduzidos, mas tem uma diminuição signi cativa no tempo computacional usando os modelos reduzidos devido ao tempo para minimizar as funções. Dessa forma, esta técnica tem todos os requisitos para ser aplicada em medições "in situ". / This work presents a new experimental method developed for obtaining, simultaneously, the thermal conductivity and thermal di usivity of solid materials. The method is based on the use of a new surface probe that has one resistance heater, two termocouples, and one heat ux sensor. The sample is partially heated in a frontal surface. Two regions, heated and non-heated front surface are used to estimate the thermal properties. The technique relies on the minimization of two distinct functions. Both are de ned from the same frontal surface temperature measurements, but using di erent thermal concepts. The rst uses a gain ratio between two surface temperatures for the identi cation of the thermal di usivity. The second is de ned by the well-known square quadratic error function calculated between the experimental and theoretical values of the same surface temperatures. This second procedure allows the estimation of thermal conductivity. In addition, has a study about the behaviour of some parameters that in uence the a gain ratio temperatura concept. The properties were mensure in two di erent sample, PVC and coopper, non-condutor and conductor materials with the deviation less than 1% of the values found in literature. After mensurements, the original thermal models were reduced to others more simple. A new function to mensure the thermal di usivity was proposed using the simple thermal model wiht the same ideia of the gain ratio between two surface temperature measurements. The thermal conductivy is determinated with the same concept when use the original model, but a new position of mensurement of experimental temperatura under the center of the heat ux is necessary. Those reduced models use only one surface to estimate the thermal properties. The results of the properties estimation are the same using the original or the reduce models, but has a signi cant reduction in computacional estimation time using the reduce models because of the time to minimize the functions. So, this thecnique has all requirements to be applied in in situ measurements. / Tese (Doutorado)

Ce travail de thèse porte sur le développement de films diamantés pour des applications de dissipation de chaleur dans les circuits électroniques de puissance. Dans le cadre d’un accord de cotutelle franco-américain, une approche duale a été adoptée. En France, la fabrication de films composites à matrice cuivre et renforts diamant par métallurgie des poudres a été privilégiée. Aux Etats-Unis, les efforts se sont concentrés sur la croissance de films de diamant par une méthode de combustion de flamme assistée laser. Dans chacune des approches, les corrélations entre microstructure et interfaces (microscopies à balayage et à transmission), composition chimique (spectroscopies Auger, Raman, XPS), et propriétés thermiques (radiométrie flash laser, radiométrie photothermique infrarouge, dilatométrie, cyclage thermique) ont été établies. Enfin, la simulation du comportement des matériaux en situation de fonctionnement opératoire à été abordée. / This PhD work deals with the development of diamond-based heat-spreading films for power electronic packaging applications. In the frame of a French-American dual PhD agreement, a double research approach was adopted. In France, the fabrication of copper-matrix diamond-reinforced composite films through tape casting and hot pressing has been endeavoured. In the US, the efforts were focused on the growth of diamond films through laser-assisted combustion synthesis. In both cases, relationships between the microstructure and the interfaces (scanning and transmission electron microscopy), the chemical composition (Auger, Raman, and XPS spectroscopy), and the thermal properties (flash laser radiometry, infrared photothermal radiometry, dilatometry, thermal cycling) of the final materials were established. Finally, the behaviour of the materials in operating environment was simulated.

De nos jours, l'industrie microélectronique utilise des fréquences de fonctionnement plus élevées dans les composants commercialisés. Ces fréquences entraînent des températures de fonctionnement plus élevées et limitent donc l'intégrité et la durée de vie des composants électroniques. Cependant, les besoins actuels nécessitent des dispositifs miniaturisés et de haute densité de puissance. De ce fait, la dissipation thermique dans les composants microélectroniques s’avère capitale. Ainsi, des drains thermiques sont utilisés pour évacuer la chaleur produite par le fonctionnement du composant. Les drains thermiques actuels sont composés de métaux, tels que le cuivre et l’aluminium, présentant des conductivités et des coefficients de dilatation thermiques élevés. Néanmoins, les coefficients de dilatation thermique des différents matériaux présents dans un circuit peuvent induire des contraintes thermo-mécaniques aux interfaces et engendrer une défaillance des composants après plusieurs cycles de fonctionnement. Dans ce contexte, nous proposons de remplacer ces drains métalliques par un système composite à matrice cuivre renforcée par du carbone, sur lequel est déposé un diffuseur thermique sous forme de diamant. Ces composites Cu/C présentent des propriétés thermo-mécaniques adaptatives pouvant palier aux contraintes induites durant l’utilisation des composants. Le transfert optimal des propriétés dans les MMC est souvent compromis par l'absence de liaison chimique interfaciale, en particulier dans les systèmes non réactifs telsque Cu/C. Cependant, pour un assemblage thermiquement efficace, l'interface devrait permettre un bon transfert de charges thermo-mécaniques entre les matériaux. L'objectif de cette étude est de combiner les propriétés exceptionnelles du diamant et les propriétés thermo-mécaniques adaptatives des MMC. Les composites à matrice de cuivre renforcés au carbone sont synthétisés à l'aide d'un processus dit semi-liquide pour obtenir des gradients de composition et des propriétés optimisées d'interface matrice - renfort. Par conséquent, des éléments d'alliage sont insérés dans le matériau pour former des interphases de carbure à l'interface Cu/C. Le film mince de diamant est obtenu par dépôt chimique en phase vapeur assisté par laser. Cette méthode de dépôt permet d’agir sur la qualité du film ainsi que sur l’adhésion avec le substrat composite. Finalement, une importance particulière est portée à l’influence des interfaces sur les propriétés thermiques tant au sein du matériau composite (interface matrice – renfort), qu’au sein de l’assemblage film diamant – MMC.Ces travaux ont été menés dans le cadre d’un accord franco-américain de cotutelle de thèse entre l’Institut de Chimie de la Matière Condensée de l’Université de Bordeaux, en France, et le département d’Ingénierie Electrique de l’Université du Nebraska-Lincoln, aux Etats-Unis. Ils ont été financés, en France, par la Direction Générale de l’Armement (DGA), et par l’équivalent Américain aux Etats-Unis. / Today, the microelectronics industry uses higher functioning frequencies in commercialized components. These frequencies result in higher functioning temperatures and, therefore, limit a component’s integrity and lifetime. Until now, heat-sink materials were composed of metals which exhibit high thermal conductivities (TC). However, these metals often induce large coefficient of thermal expansion (CTE) mismatches between the heat sink and the nonmetallic components of the device. Such differences in CTEs cause thermomechanical stresses at the interfaces and result in component failure after several on/off cycles.To overcome this issue, we suggest replacing the metallic heat sink materials with a heat-spreader (diamond film) deposited on metal matrix composites (MMCs), specifically, carbon-reinforced copper matrices (Cu/C) which exhibit optimized thermomechanical properties. However, proper transfer of properties in MMCs is often compromised by the absence of effective interfaces, especially in nonreactive systems such as Cu/C. Therefore, the creation of a chemical bond is ever more relevant. The goal of this research was to combine the exceptional properties of diamond by means of a thin film and the adaptive thermomechanical properties of MMCs. Carbon-reinforced copper matrix composites were synthesized using an innovative solid-liquid coexistent phase process to achieve designed composition gradients and optimized matrix/reinforcement interface properties. In addition, the lack of chemical affinitybetween Cu and C results in poor thermal efficiency of the composites. Therefore, alloying elements were inserted into the material to form carbide interphases at the Cu/C interface. Their addition enabled the composite’s integrity to be optimized in order to obtain thermally efficient assemblies. The diamond, in the form of a thin layer, was obtained by laser-assisted chemical vapor deposition. This process allowed action on the film’s phase purity and adhesion to the substrate material. Of particular importance was the influence of the interfaces on thermal properties both within the composite material (matrix-reinforcement interface) and within the diamond film-MMC assembly. This work was carried out within the framework of a Franco-American agreement between the Institute of Condensed Matter Chemistry of the University of Bordeaux in France and the Department of Electrical Engineering at the University of Nebraska-Lincoln, in the United States. Funding, in France, was provided by the Direction Générale de l’Armement (DGA), and by the American equivalent in the United States.

This research studies the performance of a salinity gradient solar pond driving an absorption cooling system, as an alternative to a conventional electrically powered cooling system, to provide cool air for a modern single family house in the hot dry climate of Baghdad, Iraq. The system comprises a salinity gradient solar pond, a hot-water-fired absorption water chiller, a chilled-water cooling coil which cools the air in the house, and a cooling tower which rejects heat to the ambient air. Hot brine from the pond circulates through a heat exchanger, where it heats water that is then pumped to the chiller. This arrangement protects the chiller from the corrosive brine. The system is controlled on-off by a room thermostat in the house. The system performance is modelled by dynamic thermal simulation using TMY2 hourly typical weather data. TRNSYS software is used for the main simulation, coupled to a MATLAB model of heat and mass transfer in the pond and the ground beneath it. The model of the pond and the ground is one-dimensional (only vertical transfers are considered). Radiation, convection, conduction, evaporation and diffusion are considered; the ground water at some depth below the pond is treated as being at a fixed temperature. All input data and parameter values in the simulation are based on published, standard or manufacturer's data. Temperature profiles in the pond were calculated and found to be in good agreement with published experimental results. It was found that a pond area of approximately 400 m2 was required to provide satisfactory cooling for a non-insulated house of approximately 125 m2 floor area. It was found that varying the pond area, ground conditions and pond layer thicknesses affected the system performance. The optimum site is one that has soil with low thermal conductivity, low moisture content and a deep water table. It is concluded that Iraq's climate has a potential for solar-pond-powered thermal cooling systems. It is feasible to use a solar-pond-powered cooling system to meet the space cooling load for a single family house in the summer season. Improving the thermal performance of the house by insulation could reduce the required solar pond area.