In this work, a constitutive theory is developed, within the context of continuum mechanics, to describe the creep deformation of single crystal superalloys. The con- stitutive model that is developed here is based on the fact that as bodies deform the stress free state that corresponds to the current configuration (referred to as the "natural configuration", i.e., the configuration that the body would attain on the removal of the external stimuli) evolves. It is assumed that the material possesses an infinity of natural (or stress-free) configurations, the underlying natural configuration of the body changing during the deformation process, with the response of the body being elastic from these evolving natural configurations. It is also assumed that the evolution of the natural configurations is determined by the tendency of the body to undergo a process that maximizes the rate of dissipation. Central to the theory is the prescription of the forms for the stored energy and rate of dissipation functions. The stored energy reflects the fact that the elastic response exhibits cubic symmetry. Consistent with experiments, the elastic response from the natural configuration is assumed to be linearly elastic and the model also takes into account the fact that the symmetry of single crystals does not change with inelastic deformation. An ap- propriate form for the inelastic stored energy (the energy that is `trapped' within dislocation networks) is also utilized based on simple ideas of dislocation motion. In lieu of the absence of any experimental data to corroborate with, the form for the inelastic stored energy is assumed to be isotropic. The rate of dissipation function is chosen to be anisotropic, in that it reflects invariance to transformations that belong to the cubic symmetry group. The rate of dissipation is assumed to be proportional to the density of mobile dislocations and another term that takes into account the damage accumulation due to creep. The model developed herein is used to simulate uniaxial creep of <001>, <111> and <011> oriented single crystal nickel based su- peralloys for a range of temperatures. The predictions of the theory match well with the available experimental data for CMSX-4. The constitutive model is also imple- mented as a User Material (UMAT) in commercial finite element software ABAQUS to enable the analysis of more general problems. The UMAT is validated for simple problems and the numerical scheme based on an implicit backward difference formula works well in that the results match closely with those obtained using a semi-inverse approach.

Intermetallic clathrates have long been of interest for materials science research. The promise these materials hold for useful applications ranges from thermoelectrics to photovoltaics and optoelectronics to potentially ultra-hard materials and magnetic cooling applications. Their unique physical properties are intimately related to their intriguing structural properties. Thus a fundamental understanding of the chemistry and physics of inorganic clathrates offers the possibility to assess their potential for use in the various applications mentioned above. The purpose of the current work is to expand the current knowledge of the synthetic routes for obtaining clathrate materials, their structural, chemical, and physical properties, particularly those that from in the type I, II and VIII crystal structures. New synthesis routes are presented and used for preparation of single crystals of Na 8Si46 and Na 24Si136. Single-crystal X-ray analysis, and resistivity, Seebeck coefficient and thermal conductivity measurements are presented. In addition, two "inverse" clathrates with compositions Sn 24P19.3Br8 and Sn17Zn7P22Br8 have been characterized in terms of their transport properties. Since the magnetic refrigeration based on the magnetocaloric effect is a topic of great interest, type VIII Eu 8Ga16Ge30 clathrates are also explored in terms of their application for magnetic cooling.

Three sheet materials, including high purity aluminum, commercial purity aluminum, and an aluminum-magnesium alloy with 3 wt% magnesium, were produced into single-crystals in the solid-state. The method, developed in 1939 by T. Fujiwara at Hiroshima University, involves straining a fully recrystallized material then passing it into a furnace with a high temperature gradient at a specific rate. This method preserves composition and particulate distributions that melt-solidification methods do not. Large single crystals were measured for their orientation preferences and growth rates. The single-crystals were found to preferably orient their growth direction to the <120> to <110> directions, and <100> to <111> directions normal to the specimen surface. The grain boundary mobility of each material was found to be a function of impurity content. The mobility constants observed were similar to those reported in the literature, indicating that this method of crystal growth provides an estimate of grain boundary mobility. This is the first study the effect of impurities and alloying to this single-crystal production process, and to show this method’s applicability in determining grain boundary mobility information. / text

Ο σκοπός της παρούσας διατριβής είναι να συμβάλει στην επιλογή του βέλτιστου σπινθηριστή ανάμεσα σε αυτούς που χρησιμοποιούνται στα σύγχρονα απεικονιστικά συστήματα τομογραφίας εκπομπής ποζιτρονίων PET και PET μικρών ζώων, ο οποίος θα μπορούσε να χρησιμοποιηθεί σε ένα ανιχνευτικό σύστημα πολλαπλών εφαρμογών PET/CT, MV CBCT, κλπ.) με έναν κοινό ανιχνευτή. Για το σκοπό αυτό μελετήθηκαν δείγματα από τους ακόλουθους κρυσταλλικούς σπινθηριστές: α) πυριτικού οξειδίου του γαδολινίου (GSO), β) πυριτικού οξειδίου του λουτεσίου (LSO), γ) πυριτικού οξειδίου του λουτεσίου-υτρίου (LΥSO), δ) αλουμινικού οξειδίου του υτρίου (YAP) και ε) αλουμινικού οξειδίου του λουτεσίου-υτρίου (LuYAP). Όλα τα δείγματα των σπινθηριστών ήταν ενεργοποιημένα με τρισθενές δημήτριο (Ce+3). Η μελέτη έγινε σε ευρεία κλίμακα ενεργειών (20kV-18MV). / The aim of this thesis was to select the best scintillator among the ones currently used in PET and animal PET systems, which could be used in a single detector multimodality scanner. To this aim crystal samples of GSO, LSO, LYSO, LuYAP and YAP scintillators, doped with cerium (Ce+3) were examined under a wide energy range (from 20kV-18MV). Measurements concerning determination of absolute efficiency, energy absorption efficiency, intrinsic conversion efficiency, detector optical gain and detector quantum efficiency, giving information on light yield and the intrinsic properties of the scintillators. Information on the compatibility of the light emission spectrum of the scintillators with the currently used optical photon detectors was obtained by calculating the spectral matching factors of each scintillator examined.

Natural and synthetic common-rock forming minerals were examined using single-crystal X-ray diffraction (SXRD) and electron microprobe (EMP) analyses. The influences of common defect features, such as inclusions in spinel and oxygen positional disorder and twinning in kalsilite, were reported on the respective structures. The case studies show that these defect features could lead to a misinterpretation of X-ray intensity data. The structural interpretations obtained from these XSRD analyses could be significantly different when physical properties of the crystals are considered.In the second part of my dissertation, comparative crystal chemical studies on mantle-derived minerals such as spinel, clinopyroxene, and olivine are reported. These studies were carried out to examine temperature, pressure, and compositional effects on the structures of these phases. In particular, packing arrangements of oxygen atoms were examined in detail to investigate how the packing affects element partitioning among upper-mantle minerals. At ambient conditions, oxygen packing is more distorted in the order of spinel < olivine < clinopyroxene. The packing of oxygen atoms in olivine might have a significant control on element substitutions at high pressure. Because elements whose radius is larger than that of Mg distort the packing of mantle olivine (Fo~89), olivine might limit the amount of those elements, such as Fe2+, entering the structure. In contrast, substitutions of smaller cations in C2/c clinopyroxenes increase packing distortion. For clinopyroxenes enclosed in peridotite and eclogite, higher equilibration pressures are associated with more distorted, less efficiently packed structures. Unlike many minerals reported in Thompson and Downs (2001), spinel becomes more packed with rising temperature when intracrystalline cation exchange reactions are possible. Despite wide chemical variations, spinel samples from one geological environment display a constant packing distortion, which might suggest that spinel is capable of achieving an optimal packing configuration at a given P and T.

An experimental technique based on the immobilisation of dislocations by segregation of impurity atoms to the dislocation core (dislocation locking) has been developed and used to investigate the critical conditions for slip occurrence in Czochralski-grown and nitrogen-doped floating-zone-grown silicon crystals. The accumulation of nitrogen and oxygen impurities along a dislocation and the resulting dislocation locking effect has been investigated in silicon samples subjected to different annealing conditions. In particular, the stress needed to unlock the dislocations after their decoration by impurities has been measured as a function of annealing duration and temperature. The approach used in this study has allowed the determination of new diffusivity data for oxygen and nitrogen in silicon in the technologically important range of temperatures 350-850°C. No other data covering such wide temperature range are available in the literature. In addition to transport properties, the binding energy of an impurity atom to a dislocation in silicon has been deduced from the experimental data in the case of oxygen and nitrogen impurities. A discussion in terms of the impurity species responsible for transport (monomers or dimers) and dislocation locking is also presented. The role of oxide precipitates in the generation of glide dislocation loops and the parameters affecting the occurrence of slip have been investigated in silicon samples containing precipitates of different sizes and different morphologies. The fundamental parameters deduced in this work have been used to develop a numerical model to investigate the effect of different heat treatments on the mechanical properties of silicon wafers containing a controlled distribution of impurities. This model has then been used to simulate real wafer processing conditions during device fabrication to show how they may be modified to increase dislocation locking. It is hoped that these results will have relevance to how wafers are processed in order to minimise or eliminate dislocation multiplication and consequent warpage.

The primary focus of this research is to investigate the non-linear behavior of single crystal and polycrystalline relaxor ferroelectric PMN-xPT and PZN-xPT through experimentation and modeling. Characterization of single crystal and polycrystalline specimens with similar compositions was performed. These data give experimental insight into the differences that may arise in a polycrystal due to local interaction with inhomogeneities. Single crystal specimens were characterized with a novel experimental technique that reduced clamping effects at the boundary and gave repeatable results. The measured experimental data was used in conjunction with electromechanical characterizations of other compositions of single crystal specimens with the same crystallographic orientation to study the compositional effects on material properties and phase transition behavior. Experimental characterization provided the basis for the development of a model of the continuous phase transformation behavior seen in PMN-xPT single crystals. In the modeling it is assumed that a spatial chemical and structural heterogeneity is primarily responsible for the gradual phase transformation behavior observed in relaxor ferroelectric materials. The results are used to simulate the effects of combined electrical and mechanical loading. An improved rate-independent micromechanical constitutive model based on the experimental observations of single crystal and polycrystalline specimens under large field loading is also presented. This model accounts for the non-linear evolution of variant volume fractions. The micromechanical model was calibrated using single crystal data. Simulations of the electromechanical behavior of polycrystalline ferroelectric materials are presented. These results illustrate the effects of non-linear single crystal behavior on the macroscopic constitutive behavior of polycrystals.

The need for miniaturized frequency-selective components in electronic systems is clear. The questions are whether and how micro-electro-mechanical systems (MEMS) can satisfy the need. This dissertation aims to address these questions from a scientific perspective. Silicon is the focus of this work, as it can benefit from scaling of the semiconductor industry. Silicon also offers many technical advantages. The characteristics of silicon resonators from 32 kHz to 1 GHz are described. The temperature stability and phase noise of a 6-MHz temperature-compensated oscillator and a 100-MHz temperature-controlled oscillator are reported. Silicon resonator design and characterization, with a focus on quality factor, linearity, and the electrical equivalent circuit, are included. Electrical tuning, electromechanical coupling, finite element modeling, and unexpected findings of these resonators are also described. A manufacturability technique employing batch process compensation is demonstrated. Results indicate that silicon is an excellent material for micromechanical resonators. The aim of this research is to explore the fundamental limitations, provide a foundation for future work, and also paint a clearer picture on how micromechanical resonators can complement alternative technologies.

The search of materials relevant for thermoelectric and magnetocaloric applications, as well as materials that interact with light, is an important aspect of the materials science. Such materials can be used for solid-state power generation and refrigeration, as light sources, detectors, or controllers. Intermetallic clathrates have long been of interest for the materials science research. The promise these materials hold for useful applications ranges from thermoelectrics to photovoltaics and optoelectronics to potentially ultra-hard materials and magnetic cooling applications. Their unique physical properties are intimately related to their intriguing structural properties. Thus a fundamental understanding of the chemistry and physics of inorganic clathrates offers the possibility to assess their potential for use in the various applications mentioned above. In this work the selective, phase pure, single-crystal growth of AxSi46 and AySi136 (A = Na, K) intermetallic clathrates by the new vapor-phase intercalation method is presented. The approach appears promising for accessing regions of the equilibrium diagrams for Na-Si and K-Si clathrates that can be otherwise difficult to reach due to the greatly differing properties of the constituent elements. Physical properties of these materials were investigated in terms of single-crystal diffraction, electrical and thermal properties measurements. The synthesis and structural properties of single crystals of NaxSi136 are presented. A two-step synthetic approach was employed for the synthesis of NaxSi136 which also allowed for low temperature transport measurements of polycrystalline NaxSi136. The potential of the Eu8Ga16Ge30 type-I and VIII - EuO composites for magnetocaloric applications is discussed. The type-I clathrate - EuO composites hold promise for active magnetic refrigeration around 70 K.

京都大学 / 新制・課程博士 / 博士(工学) / 甲第23912号 / 工博第4999号 / 新制||工||1780(附属図書館) / 京都大学大学院工学研究科物質エネルギー化学専攻 / (主査)教授 陰山 洋, 教授 阿部 竜, 教授 安部 武志 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM