The physics of the series Pr_yY_1-yBa₂Cu₃O_6+x, and ability of Pr to suppress superconductivity, has been a subject of frequent discussions in the literature for more than a decade. This thesis describes a polarised neutron diffraction (PND) experiment performed on PrBa₂Cu₃O_6.24 designed to find out something about the electron structure. This experiment pushed the limits of what can be done using the PND technique. The problem is one of a limited number of measured Fourier components that need to be inverted to form a real space image. To accomplish this inversion the maximum entropy technique has been employed. In some cases, the maximum entropy technique has the ability to increase the resolution of ‘inverted’ data immensely, but this ability is found to depend critically on the choice of constants used in the method. To investigate this a Bayesian robustness analysis of the maximum entropy method is carried out, resulting in an improvement of the maximum entropy technique for analysing PND data. Some results for nickel in the literature have been re-analysed and a comparison is made with different maximum entropy algorithms. Equipped with an improved data analysis technique and carefully measured PND data for PrBa₂Cu₃O_6.24 a number of new interesting features are observed, putting constraints on existing theoretical models of Pr_yY_1-yBa₂Cu₃O_6+x and leaving room for more questions to be answered.

This thesis reports the synthesis and characterisation of the layered oxypnictides Sr₂MO₃FeAs (M = Sc, V and Cr) and CeOMnAs. In these materials the choice of transition metal cation at the tetrahedral site in the arsenide layer chiefly dictates the physical properties that are observed. The bulk of this work has focussed on the development of a new family of iron arsenide superconductor with the general formula Sr₂MO₃FeAs (M = Sc, V, Cr). This structure is comprised of anti-PbO-type [FeAs]^- layers which alternate with insulating [Sr₂MO₃]⁺ oxide fragments that resemble a portion of the K₂NiF₄ structure. In contrast to other FeAs parent materials, no member of the Sr₂MO₃FeAs family exhibits any strong evidence for long range Fe order or a tetragonal to orthorhombic distortion upon cooling. Attempts to electron and hole dope Sr₂ScO₃FeAs into the superconducting regime have as yet been unsuccessful. Although Sr₂ScO₃FeAs shows no evidence for Fe ordering, a checkerboard arrangement of Cr³⁺ spins in the ab-plane is observed below 40 K (k = (½, ½, 0)) analogous to that seen in Pr₂CuO₄. The partial substitution of Fe²⁺ (d⁶) by Co²⁺ (d⁷) in Sr₂CrO₃Fe_1-xCo_xAs has been shown to be a fruitful strategy for electron-doping this material into the superconducting regime with T_c maximised at 18 K in Sr₂CrO₃Fe_0.92Co_0.08As. It is also established that this substitution influences the ordering on the Cr sub-lattice with a doubling in the size of the magnetic cell along the c axis (k = (½, ½, ½)). Sr₂VO₃FeAs, a rare example of an “undoped” superconductor (T_c = 25 K), is shown to be electron-doped by mixed valence vanadium +3.13(5). Magnetometry measurements also reveal a series of magnetic transitions in Sr₂VO₃FeAs, however μSR and powder neutron diffraction studies suggest that this system is some way from commensurate long range order. In contrast to Sr₂CrO₃FeAs, electron-doping strategies in Sr₂VO₃FeAs have the effect of decreasing T_c and ultimately suppressing superconductivity entirely as Sr₂V_1-xTi_xO₃FeAs and Sr₂VO₃Fe_1-xCo_xAs materials are over electron-doped. Sr₂V_1-xMg_xO₃FeAs samples were also prepared, but rather than this strategy hole-doping the FeAs layer it preferentially oxidises vanadium towards V⁴⁺. This substitution also has a considerable effect on the superconducting critical temperature (T_c) which is raised as high as 31 K in Sr₂V_0.775Mg_0.225O₃FeAs. The isovalent substitution of Sr²⁺ by Ca²⁺ in Sr_2-xCa_xVO₃FeAs has been shown to strongly influence the superconducting properties of this material and a clear correlation between the evolution of T_c and the shape of the FeAs₄ tetrahedron has been established. These results demonstrate that superconductivity in iron-based superconductors is extremely sensitive to both electron count and the crystal structure. Finally, investigations into the manganese oxide arsenide CeOMnAs reveal room temperature ordering of Mn²⁺ spins and a spin reorientation transition of Mn moments at 36 K. This transition is concomitant with Ce ordering and an apparent weak structural distortion, demonstrating that f electrons are able to dictate the orientation of Mn moments.

Well-logging and aerospace applications require electronics capable of withstanding elevated temperature operation. A key element of high temperature packaging technology is the Si die attach material, and a comparative study of two die attach systems for use in harsh environment has been performed. Die bond sample packages, using commercial adhesives and an Au-Si eutectic solder, have been manufactured and were subsequently thermally exposed for various times at 250 and 300°C respectively. The adhesive die bond packages comprised a high temperature co-fired ceramic (HTCC) substrate with W, Ni and Au metallisations whereas the Au-Si die bond packages used thick film Au metallised on a Al₂O₃ substrate. Optimisation of the eutectic die bonding parameters was successfully performed for the Au-Si system by an experimental design method, which improved mean and spread of maximum bonded areas and consequently, the shear load to failure. Bonded area was systematically assessed by scanning acoustic microscopy (SAM) followed by digital image analysis (DIA). Accelerated testing comprised thermal cycling and thermal shock and although showing some degradation, Au-2wt%Si die bonds were surprisingly robust, showing excellent subsequent stability during industrial device testing investigations.

This thesis describes neutron scattering experiments on strongly correlated systems exhibiting a range of emergent phenomena: antiferromagnetism, charge order, superconductivity and multiferroicity. I have examined the La_{2}CoO_{4} compound which is a Mott insulator and orders antiferromagnetically near room temperature. The La_{2}CoO_{4} sample was studied using spherical neutron polarimetry and I present magnetic structure models to describe the two antiferromagnetic phases of the compound. Furthermore, the magnetic fluctuations have been investigated using neutron time-of-flight technique. This has allowed us to extract the dominant exchange interactions in the system. More interestingly, the work on La_{2}CoO_{4} presented in this thesis provides a basis for the experimental evidence of an hourglass dispersion in La_{5/3}Sr_{1/3}CoO_{4}, previously only observed in the copper oxide based superconductors. This dispersion has been understood in terms of a stripe ordered magnetic phase and was found to be well described by a linear spin-wave model. Neutron scattering experiments were also carried out on the new iron-based high-temperature superconductors, FeSe_{x}Te_{1−x}. A range of compositions were studied, including both antiferromagnetically ordered and superconducting. Below the superconducting phase transition temperature, a spin resonance mode was found centred on the antiferromagnetic wavevector. This is an important feature shared by many unconventional superconductors. The spin resonance intensity was found to reflect the order parameter of the superconducting state. Polarised inelastic neutron scattering experiments have revealed a small anisotropy between the in-plane and out-of-plane magnetic fluctuations at the resonance. This anisotropy cannot be readily explained by the usual anisotropic terms in the Hamiltonian. This could be evidence of new physics in the FeSe_{x}Te_{1−x} superconductors. Finally, I have studied CuO – a high-temperature multiferroic. Analysis of polarised neutron diffraction experiments shows that the magnetic domain population can be varied using an externally applied electric field. This unambiguously demonstrates coupling between the magnetic and ferroelectric degrees of freedom. Using representation analysis I derive the incommensurate magnetic structure in the multiferroic phase. The origin of the magnetoelectric coupling is consistent with models based on the inverse Dzyaloshinskii-Moriya interaction.

De nombreuses pièces sont sujettes à des sollicitations de frottement à haute température dans le domaine de l’aéronautique. En effet, dans ce domaine, l’augmentation constante des puissances des réacteurs implique une augmentation de la température de fonctionnement des systèmes mécaniques et en particulier des contacts frottants. Cette augmentation de température a pour conséquence la diminution de la durabilité des matériaux lubrifiants solides actuellement utilisés pour ce type de contact. C’est pourquoi il est indispensable de sélectionner de nouveaux revêtements aux propriétés tribologiques à chaud améliorées. Dans cette optique, une thèse ayant pour sujet la durabilité de rotules aéronautiques d’attache mât-moteur de l’A380 soumises à des conditions extrêmes de fonctionnement en termes de température et de conditions glissement, a été effectuée au LTDS. Dans un premier temps, les endommagements de surface (abrasion, adhésion) des pièces réelles ont été analysés, à travers des analyses MEB/EDX du revêtement d’argenture déposé dans le contact entre les bagues intérieure et extérieure de la rotule. Des essais tribologiques en configuration classiquement utilisée, cylindre/plan (sur Cameron-Plint), ont ensuite été réalisés sur le revêtement d’argenture afin de comparer les endommagements observés in situ et sur banc. Dans le but de sélectionner un revêtement ou un couple de revêtements susceptibles de remplacer l’argenture dans le contact étudié, une dizaine de revêtements différents (couches accommodantes et couches dures) ont ensuite été testés individuellement en configuration cylindre/plan dans les mêmes conditions d’essai que dans le cas de l’argenture. Cette campagne d’essai a permis de dégager les plus performants/endurants tribologiquement. Parallèlement, un tribomètre original a été conçu au laboratoire afin de pouvoir simuler au mieux les conditions de contact réelles (sollicitations mécaniques, température de contact…). Ce tribomètre rotatif à haute température (TRHT), utilisable en configuration couronne/plan, permet d’appliquer simultanément au contact une force normale pouvant aller jusqu’à 50 kN, un mouvement rotatif alternatif, et une température de contact pouvant aller jusqu’à 1000°C grâce à un système de chauffage par induction. Les revêtements ayant démontré le meilleur comportement en termes de résistance à l’usure et au frottement au cours des essais en configuration cylindre/plan ont également été testés individuellement sur ce tribomètre en configuration couronne/plan. Les essais finaux ont été réalisés en interface complète, c'est-à-dire avec une couche accommodante déposée sur la pièce représentant la bague intérieure de la rotule, une couche dure déposée sur la pièce représentant la bague extérieure, et une couche à bas frottement (graisse ou vernis). Deux solutions de lubrification de ce type ont été déterminées pour le contact rotulé : une pour les rotules d’attache mât-moteur avant qui fonctionnent jusqu’à 160°C (CuNiIn + vernis Everlub / WC-C) et une pour les rotules d’attache mât-moteur arrière qui fonctionnent jusqu’à 570°C (stellite + vernisEverlub / TiHfCN). / In aeronautics tribology, mechanical parts are required to operate with increasing temperature. The increased functioning temperature of the contacts prone to friction and wear (such as bearings and other structural parts) is a direct consequence of the increasing power of jet engines. In the case of ball bearings, the substrate materials as well as the coating durability are affected by temperature. There is then a pressing need to introduce new coatings demonstrating effective tribological behaviour at high temperature. A PhD research subject was defined in partnership with Airbus Aerospace and SKF Aerospace and conducted at LTDS. The main goal of this research is to study the durability of ball bearings functioning under extreme conditions in terms of temperature and sliding friction. First, SEM/EDX analyses were done on the silver coating deposited in the contact between the ball bearings rings, in order to acknowledge the actual bearings surface damage. Tribological testing of the silver coating was then performed in a standard cylinder-on-flat configuration in order to compare in situ and experimental damages. More than ten different coatings (both soft and hard coatings, with solid lubrication properties and/or wear resistance properties) were then tested in the same configuration and conditions as the silver coating. These tests allowed us to identify the best suitable coatings for the application. An original test rig was also designed in the lab, in order to better simulate the bearings functioning conditions. This tribometer (TRHT) makes it possible to perform tests in ring-on-flat configuration (close contact) and to simultaneously apply a normal force up to 50 kN and a reciprocating rotating motion. Using this tribometer, the final tests were conducted on the best candidates among the different coatings, individually at first to assess their tribological behavior, and then in a configuration as close to reality as possible, with a hard coating covered ring and a soft coating covered flat, the contact between the two being greased or varnished. Finally, several possible solutions were determined, for two different functioning temperatures:160°C (CuNiIn + varnish Everlub / WC-C) and 570°C (stellite + varnish Everlub / TiHfCN).

Numerical models will be developed for the prediction of silencer transmission loss under the operating conditions present in gas turbine exhausts. In these systems the large diameter ducts and high operating temperatures produce a challenging acoustic environment due to the unverified behaviour of fibrous materials at high temperatures and the existence of complex sound fields. To understand the behaviour of fibrous materials at high temperatures their bulk acoustic properties are measured using a modified impedance tube which can heat material samples up to a temperature of 500 C. It will be demonstrated that the high temperature material properties can be extrapolated from room temperature measurements given knowledge of the temperature dependant flow resistivity. Finite element numerical models using point collocation and mode matching techniques to predict the transmission loss of silencers are developed and successfully validated. Dissipative silencer designs with various cross-sectional designs are explored numerically and experimentally according to common industry standards. It is demonstrated that transmission loss may be optimised by the arrangement of the fibrous material across the cross-section. The accurate numerical models allow for effe cient silencers to be designed reducing silencer size and cost. A new hybrid silencer is presented combining dissipative and reactive elements with the aim of increasing the low frequency attenuation of large silencers while maintaining an effective broadband spectrum. Measurements and predictions show this innovative design to be successfull. Application of the hybrid silencer allows for more flexible noise control solutions when design is limited by low frequency noise.

Doctor of Philosophy / Department of Agronomy / P.V. Vara Prasad / Productivity of grain crops is highly sensitive to changing climates and crop management practices. Response of finger millet [Eleusine coracana (L.) Gaertn.] to high temperature stress, and intensive management practices such as increased seeding rates and fertilizer application are not clearly understood. The objectives of this research were to determine the effects of (a) season-long, and short episodes of high temperature stress on growth and yield traits of finger millet, (b) seeding rates and nitrogen fertilizer application rates on grain and biomass yield, and (c) to evaluate the finger millet minicore collection for high grain and biomass yield. Controlled environment studies were conducted to determine the effects of high temperature stress on physiological, growth and yield traits. Field studies were conducted in Manhattan and Hays (Kansas) and Alupe (Kenya) to determine the effects of seeding and nitrogen fertilizer rates on growth and yield traits. Finger millet minicore collection was evaluated under field conditions in India, for phenology, growth and yield traits. Season long high temperature stress of 36/26 or 38/28°C compared to 32/22°C decreased panicle emergence, number of seeds per panicle, grain yield and harvest index. Finger millet was most sensitive to short episodes (10 d) of high temperature (40/30°C) during booting, panicle emergence and flowering stages, resulting in lower number of seeds, and grain yield. Finger millet responded to the interaction between environmental (locations) and temporal (years) factors. In general, locations with higher rainfall had greater grain and biomass yield than those with low rainfall. There was no influence of seeding rates (3.2 or 6.0 kg ha[superscript]-1) at Hays and Alupe. However, in one of the two years in Manhattan, higher seeding rate of 6.0 kg ha[superscript]-1 increased grain yield compared to 3.2 kg ha[superscript]-1. There was no influence of nitrogen rates (0, 30, 60 or 90 kg ha[superscript]-1) on grain or biomass yield at all three locations. However, higher fertilizer rates had greater percentage lodging. The finger millet minicore collection displayed large ranges for most quantitative traits including days to flowering, plant height, number of fingers panicle[superscript]-1, grain yield, biomass yield, and lodging; and had >60% heritability. Some of the genotypes from the minicore collection have the potential to increase grain and biomass yield and abiotic stress tolerance of finger millet.

Neste trabalho foram estudados os efeitos do tratamento térmico e da adição de zircônio, vanádio, titânio e manganês nas propriedades mecânicas em alta temperatura de uma liga AlSi12CuMgNi, comumente utilizada em pistões automotivos. Os resultados de tração a 300°C mostram que a adição de elementos de liga reduz o alongamento sem aumentar o limite de escoamento. Os ensaios de fadiga a 300°C mostram que a aplicação de um tratamento de solubilização a 525°C por 15 minutos é duas vezes melhor que a aplicação de um tratamento de solubilização a 500°C por uma hora. Estes resultados foram atribuídos ao arredondamento das partículas de silício que, por serem partículas frágeis, quando arredondadas ocupam menor área por volume, reduzindo a velocidade de propagação de trincas. Os ensaios de fadiga a 300°C mostram também que a adição de elementos de liga combinados a um tratamento de solubilização a 525°C por 15 minutos é cerca de 30% pior que o mesmo tratamento aplicado a uma liga sem a adição destes elementos de liga. Este fenômeno foi atribuído à fragilização dos intermetálicos que, por consequência, aumentam a velocidade de propagação das trincas de fadiga. / The effects of heat treatment and the addition of zirconium, vanadium, titanium and manganese on the high temperature properties of an AlSi12CuMgSi alloy, usually applied to cast automotive pistons, were studied. The tensile test results at 300°C show that the addition of alloying elements decreases the elongation without significantly increasing the yield strength. The fatigue tests at 300°C show that a solution heat treatment at 525°C for 15 minutes is two times better than a solution heat treatment at 500°C for one hour. These results are due to the spheroidization of the silicone particles that are brittle and, when spheroidized, have a smaller area/volume ratio, slowing the crack propagation speed. The fatigue tests at 300°C also show that the addition of alloying elements combined with a solutionizing treatment at 525°C for 15 minutes is 30% worse than the same treatment applied to an alloy without these alloying elements. This behavior is due to the embrittlement of the intermetallic particles containing these alloying elements, increasing the fatigue crack propagation speed.

Neste trabalho é apresentado um estudo do ponto ZTC (Zero Temperature Coefficient) em dispositivos SOI MOSFETs, funcionando em modo parcialmente (PD-SOI) e totalmente (FD-SOI) depletados. O estudo é realizado a partir de um modelo analítico simples, proposto para determinação da tensão de polarização da porta do transistor no ponto ZTC (VZTC), através dos modelos de primeira ordem das características da corrente de dreno (IDS) em função da tensão aplicada a porta (VGF) do transistor, considerando as regiões de operação linear e de saturação. Para a validação do modelo, os resultados obtidos são confrontados com dados experimentais, e foi obtido um bom ajuste dos valores, apesar das simplificações adotadas para o modelo proposto. Foi realizada uma análise para estudar o impacto no valor de VZTC com a variação no valor de parâmetros de referência, como a concentração de portadores (Naf) e a espessura do óxido de porta (toxf). O erro máximo observado em VZTC, impondo a variação nos parâmetros Naf e toxf, para os dispositivos PD é de 3,1% e 4,6% na região linear, respectivamente; e 3,5% e 7,2% na região de saturação, respectivamente. Para os dispositivos FD o erro máximo observado, devido a variação nos parâmetros Naf e toxf, foi de 11% e 10% operando no regime linear, respectivamente e 5,3% e 8,4% no regime de saturação, respectivamente. Através do modelo proposto foi realizado o estudo da estabilidade do ponto ZTC em função da variação da degradação da mobilidade com a temperatura (fator c), comprimento de canal (L) e a tensão de dreno (VDS) para os dispositivos supracitados. A analise da influência do fator c em VZTC mostrou-se mais importante nos dispositivos parcialmente depletados (PD). A tensão VZTC, para os dispositivos nMOS, apresentou um menor valor operando na região de saturação, e torna-se mais pronunciada essa diferença para dispositivos com menor comprimento de canal, para ambos os tipos de dispositivos. Observando a variação de VZTC com VDS, nota-se uma diminuição no valor de VZTC para altos valores de VDS, para os dois tipos de dispositivos estudados, n e pMOS. Os resultados do modelo proposto foram avaliados com dados experimentais de outras tecnologias SOI MOSFET. Também foi obtido um bom ajuste com os valores para as tecnologias GC-SOI e GC-GAA SOI, operando em regime linear e saturação. / This paper presents a study of ZTC point (\"Zero Temperature Coefficient) in SOI MOSFETs devices, partially (PD-SOI) and fully (FD-SOI) depleted mode. The study is performed from a simple analytical model proposed for the determination of the gate bias voltage at ZTC point (VZTC) using the first-order models of the drain current (IDS) characteristics as a function of the gate voltage (VGF), operating in the linear and saturation regimes. To validate the model proposed results were compared with experimental data, and the analytical predictions are in very close agreement with experimental results in spite of the simplification used for the VZTC model proposed. Analysis was performed to study the impact on the VZTC value with the change in the parameters used as reference, such as Naf and toxf. The maximum error observed for the PD devices is 3.1% and 4.6% in the linear region and 3.5% and 7.2% in the saturation region, respectively. For FD devices the maximum error observed was 11% and 10% operating in the linear and 5.3% and 8.4% in the saturation regime. In order to verify the stability of the ZTC point as a function of the mobility degradation (c), channel length (L) and drain voltage (VDS), the proposed model was applied to the devices mentioned above. The VZTC changes in the temperature range investigated showed a temperature mobility degradation dependence and are more pronounced in PD devices. The VZTC voltage for nMOS devices presented a lower value operating in the saturation region than in the linear region, and this difference becomes more pronounced to devices with smaller channel length for both devices, n and pMOS. Analyzing the VZTC variation with drain voltage (VDS), showed a decrease in VZTC value for higher VDS, for both studied devices. The model proposed results were evaluated using experimental data from other SOI MOSFET technologies. And also we have obtained for the GC SOI and GC-GAA-SOI technologies a very close agreement, operating in both regions, linear and saturation.

Neste trabalho de mestrado são descritos o projeto, a montagem e os testes de funcionamento de uma câmara porta amostra para medidas elétricas de quatro pontas de prova dc. A estrutura da câmara porta amostra é em material cerâmico, que garante a estabilidade física e química do sistema, prolongando sua vida útil e melhorando a qualidade das análises realizadas. Este sistema permite a realização de medidas elétricas de diferentes materiais desde a temperatura ambiente até ~1500 °C em ampla faixa de pressões parciais de oxigênio. A funcionalidade da câmara porta amostras foi avaliada por meio da comparação de medidas da dependência da resistividade elétrica com a temperatura de amostras de zircônia estabilizada com ítria. Visando à aferição e a aplicação deste sistema de medidas elétricas, foram fabricados e caracterizados compósitos à base de zircônia estabilizada com ítria e nitreto de titânio, obtidos pela técnica de sinterização por plasma pulsado (\"spark plasma sintering\", SPS). As propriedades gerais destes compósitos foram investigadas por meio de análises térmicas, difratometria de raios X (DRX) e medidas elétricas de quatro pontas de prova dc, usando o sistema construído. As análises das amostras dos compósitos à base de zircônia e TiN mostraram que a técnica SPS produz amostras densas, sem reação entre as fases ou degradação do TiN por oxidação. As amostras com adição de TiN apresentaram comportamento metálico da resistividade elétrica, evidenciando a percolação do nitreto na matriz de zircônia para frações volumétricas ≤ 27 vol.%. Medidas de resistividade elétrica combinadas com análises térmicas e de DRX foram usadas para monitorar a oxidação do TiN nos compósitos em altas temperaturas. As amostras produzidas apresentam propriedades promissoras para aplicações de alta temperatura que requeiram elevada condutividade elétrica. / This study describes the design, construction, and tests of a sample holder for four-probe dc electrical measurements. The structural parts of the sample holder are built using ceramic material (alumina) that ensures physical and chemical stability, prolonging its use and the quality of experiments. The sample holder allows electrical measurements from room temperature up to ~1500 °C in a wide range of oxygen partial pressures. The functionality of the constructed sample holder was assessed by comparing measurements of the temperature dependence of the electrical resistivity of yttria-stabilized zirconia samples. To further explore the capabilities of the measuring apparatus, samples of high-temperature composites based on yttria-stabilized zirconia and titanium nitride were prepared by spark plasma sintering (SPS). The general properties of these composites were investigated by thermal analysis, X-rays diffraction (XRD), and four-probe dc electrical measurements. The study of the composites showed that SPS resulted in dense samples with no detected reaction between phases and free from TiN oxidation. Samples with TiN addition displayed metallic behavior of the electrical resistivity, evidencing that the nitride attained the percolation threshold in the oxide matrix at volume fractions ≤ 27 vol.%. Electrical measurements combined with thermal analysis and XRD were used to monitor the oxidation of TiN at high temperature. The studied composites show good properties indicating that it is a promising material for high temperature applications that require high electrical conductivity.