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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
411

None

Ling, Ming-Hui 04 July 2000 (has links)
None
412

Thermal Etching of Single Crystal Quartz and Willemite: Effects of Boron Oxide, Defects, Lattice Anisotropy and Capillary Force

Chao, Pei-Tong 03 August 2000 (has links)
This thesis is about thermal etching of quartz single crystals with boron oxide melt and thermal etchings on inorganic polymeric single crystals of orthosilicates, willemite (Zn2SiO4) and phenakite (Be2SiO4), where isolated [SiO4] groups are polymerized by corner-sharing with other tetrahedral groups, such as [ZnO4] and [BeO4]. On the thermal etching of quartz, experiments were performed on quartz (10 0), (0001), (10 1) and (11 1) from 500¢J to 700¢J. Three types of etch figures were recognized by scanning electron microscopy: isolated dislocation etch pit, aligned etch pits and flat etch pits. The effects of defect specification and £\-£] displacive phase transformation of quartz on its development of thermal etch figures were evaluated. By doing so, boron oxide melt was proved to be a useful etchant on the studies of defect types and dynamics of quartz. As for the thermal etching of phenakite type silicate, we conducted thermal-cycle etching of willemite at 1250¢J, hydrochloric and hydrofluoric acid etchings of willemite and phenakite at room temperature, and boron oxide melt etching of willemite and phenakite at 700¢J. Surface premelting, anisotropic lattice etching and defect etching were found to play important roles on the thermal etching of willemite. Impurity segregation at dislocation outcrops on willemite (0001) should occur in the first thermal cycling in order to nucleate hillocks at the centers of the hexagonal dislocation etch pits. Reflection IR spectroscopic analysis indicated the surface premelt has the same structural units as willemite, although the subsequent crystallization follows a silica rich path. A silica-rich surface coverage impedes the etching of crystal plane underneath. There is significant polygonization and cleaving-healing of willemite single crystal upon thermal cycling according to transmission electron microscopy observation. Phenakite has remarkable chemical and thermal etching resistance in comparison to its isostructure willemite due to site energy difference of Be and Zn in coordination number 4.
413

Structural Characterization of TiO2 and BaTiO3 Thin Films by MOCVD

Hung, Yi-Min 06 July 2001 (has links)
In recent years, there has been increasing demands for high dielectric materials to replace SiO2 for high-density dynamic random access memories with ultra large scale integration (ULSI). As the dimensions of the charge storage node decrease in high-density dynamic random access memories (DRAMs), TiO2 and BaTiO3 are very promising candidates for applications with exhibiting higher dielectric constant, high refractive index and high chemical stability. The growth of TiO2 and BaTiO3 thin films on various substrates i.e. (100) silicon¡B(100) GaAs¡B(100) InP and (100) MgO are studied by MOCVD using Ti(i-OC3H7)4, Ba(DPM)2, N2O and O2 as precursors. The growth was performed in a cold wall horizontal system in the temperature range of 280~750¢J. The growth rates and structure of TiO2 and BaTiO3 films are affected by the substrate temperature and reactor pressure, etc. The phase transition properties of TiO2 were studied via X-ray diffraction measurements. X-ray diffraction examination shows that phase transition of TiO2 films are at the same temperature of 450 oC on different substrates. Phase-pure rutile is obtained down to 450¢J on InP (100) and GaAs (100), while phase-pure anatase is obtained up to 450¢Jon MgO (100). The optical and electrical properties are associated with the film structures. TiO2 single phase films with rutile (110) orientation were successfully grown on InP (100) at 500¢J. In-plane epitaxial relationship of anatase TiO2 (100) // MgO (100) is present between 300¢J and 375¢J. In addition, the influences of substrate temperature and oxidizer on the structural and electrical properties of BaTiO3 films will be also studied. However, TiO2 and BaTiO3 films have columnar structures acted the paths of leakage current resulting low dielectric constant. We use thermal annealing to improve the quality of TiO2 with respect to leakage current density and dielectric constant. Dielectric constants of annealed TiO2 films were as high as 110.08. Leakage current density reduced to 5 ¡Ñ 10-5 A/cm2. In the future, to improve the crystal structure of the films is the goal in our study.
414

A new mineralogical approach to predict coefficient of thermal expansion of aggregate and concrete

Neekhra, Siddharth 17 February 2005 (has links)
A new mineralogical approach is introduced to predict aggregate and concrete coefficient of thermal expansion (CoTE). Basically, a modeling approach is suggested based on the assumption that the CoTE of aggregate and concrete can be predicted from the CoTE of their constituent components. Volume percentage, CoTE and elastic modulus of each constituent mineral phase are considered as input for the aggregate CoTE model, whereas the same properties for coarse aggregate and mortar are considered for the concrete CoTE model. Methods have been formulated to calculate the mineral volume percentage from bulk chemical analysis for different type of rocks commonly used as aggregates in Texas. The dilatometer testing method has been established to measure the CoTE of aggregate, pure minerals, and concrete. Calculated aggregate CoTE, based on the determined CoTE of pure minerals and their respective calculated volume percentages, shows a good resemblance with the measured aggregate CoTE by dilatometer. Similarly, predicted concrete CoTE, based on the calculated CoTE of aggregate and mortar and their respective volume percentages compares well with the measured concrete CoTE by dilatometer. Such a favorable comparison between predicted and measured CoTE provided a basis to establish the composite model to predict aggregate and concrete CoTE. Composite modeling will be useful to serve as a check of aggregate source variability in terms of quality control measures and improved design and quality control measures of concrete.
415

Thermal alteration of collagenous tissue subjected to biaxial isometric constraints

Wells, Paul B. 29 August 2005 (has links)
Clinical thermal therapies are widespread and gaining in appeal due to improved technology of heating devices and promising results. Outcomes of thermal treatment are often unpredictable and suboptimal, however, due in part to a lack of appreciation of the underlying biothermomechanics. There is a pressing need, therefore, to understand better the role of clinically-controllable parameters on the thermal damage processes of tissue. Heretofore, researchers have primarily sought to understand this process through various uniaxial experiments on tissues containing collagen as their primary constituent. Most biological tissues experience multiaxial loading, however, with complex boundary constraints inclusive of both isotonic and isometric conditions. The primary focus of this work is on the isothermal denaturation of fibrillar collagen subjected to a biaxial isometric constraint. Results from our tests reveal a complicated process, the kinetics of which are not easily measured. Evolving isometric contraction forces during heating do not correlate with resultant mechanical behaviors, as thermal shrinkage does in biaxial isotonic tests. Furthermore, resultant mechanical behaviors at variousdurations of heating reveal a two phase process with a rate dependent on the amount of isometric stretch. For tissues heated at 75oC for 15 minutes, at which point the first phase of mechanical alteration dominates for all constraints herein, resultant mechanical behaviors correlate well with the amount of isometric stretch. The correlation is similar to that between isotonic loads and resultant mechanical behaviors from previous studies. In light of the need for a better measure of thermal damage in isometric tests, we performed a histological analysis of tissues heated under varying constraints. Results show a good correlation between the level of isometric constraint and thermally-induced histological aberrations. Finally, we demonstrate that our seemingly limited and qualitative knowledge can be applied well to a specific clinical application: namely, the use of glycerol as a clearing agent for laser therapies. Our results suggest that glycerol is safe to use for such therapies because it increases the thermal stability of fibrillar collagen, and its hyperosmotic effects on mechanical behavior are fully reversed upon rehydration.
416

Thermal decomposition study of hydroxylamine nitrate during storage and handling

Zhang, Chuanji 17 September 2007 (has links)
Hydroxylamine nitrate (HAN), an important agent for the nuclear industry and the U.S. Army, has been involved in several costly incidents. To prevent similar incidents, the study of HAN safe storage and handling boundary has become extremely important for industries. However, HAN decomposition involves complicated reaction pathways due to its autocatalytic behavior and therefore presents a challenge for definition of safe boundaries of HAN storage and handling. This research focused on HAN decomposition behavior under various conditions and proposed isothermal aging testing and kinetic-based simulation to determine safety boundaries for HAN storage and handling. Specifically, HAN decomposition in the presence of glass, titanium, stainless steel with titanium, or stainless steel was examined in an Automatic Pressure Tracking Adiabatic Calorimeter (APTAC). n-th order kinetics was used for initial reaction rate estimation. Because stainless steel is a commonly used material for HAN containers, isothermal aging tests were conducted in a stainless steel cell to determine the maximum safe storage time of HAN. Moreover, by changing thermal inertia, data for HAN decomposition in the stainless steel cell were examined and the experimental results were simulated by the Thermal Safety Software package. This work offers useful guidance for industries that manufacture, handle, and store HAN. The experimental data acquired not only can help with aspects of process safety design, including emergency relief systems, process control, and process equipment selection, but also is a useful reference for the associated theoretical study of autocatalytic decomposition behavior.
417

Numerical Analysis of Temperature and Thermal Stress in Cr4+:YAG fiber manufacturing process

Lai, Sheng-shin 18 August 2009 (has links)
Factors in fiber manufacturing procedures affect fiber¡¦s production, and the fiber quality will be affected accordingly. The residual stresses, in particular, have a significant influence on fiber quality, due to the mechanical strength and the refraction rate that has been changed. Mechanically, residual stresses may cause ruptures in the preform, reducing the intrinsic strength of the fiber and its durability; optically, it may also cause anisotropic distortions of the refractive index profiles. In the process of cooling under high temperature, fiber core and cladding will be compressed owing to the material difference and the residual stress will be in the fiber. Thermal conductance and thermal expansion coefficient contribute to the cracks on the interface and thus affect the refraction rate. Experiments have shown that quartz and Cr4+¡GYAG will rupture on the interface as a result of the huge thermal expansion coefficient. According to researches, stress in different directions will bring about fiber cracks or changes of the refraction rate. This paper mainly investigates the influence of material properties on the temperature field and thermal stress distribution in the cooling process of fiber and preform manufacturing by numerical simulations. The results show the preform temperature profile and the stress distribution in different directions. Also, through the stress distribution, the stress is known to be centered on the interface between core and cladding.
418

Novel cycles using carbon dioxide as working fluid : new ways to utilize energy from low-grade heat sources

Yang, Chen January 2006 (has links)
<p>This licentiate thesis proposes and analyzes three carbon dioxide novel cycles, namely: the carbon dioxide transcritical power cycle, the carbon dioxide Brayton cycle and the carbon dioxide cooling and power combined cycle. Due to the different characteristics of each cycle, the three cycles are suitable for different applications. The CO<sub>2</sub> transcritical power cycle is suitable for harvesting energy from low-grade heat sources, near which a low temperature heat sink is accessible. The CO<sub>2 </sub>Brayton cycle is suitable for harvesting the energy from relatively high-grade heat sources when there is no low temperature heat sink available. The CO<sub>2 </sub>cooling and power combined cycle is suitable for applications, where both power and cooling are needed (e.g. automobile applications, in which the cycle can utilize the energy in the engine exhaust gasses to produce power and provide cooling/heating to the mobile compartment room at the same time).</p><p>Several models have been developed using the software known as Engineering Equation Solver (EES)<sup>1 </sup>for both cycle analysis and computer aided heat exchanger design. Different cycle working conditions have been simulated and different working parameters’ influence on the cycle performance has been explained. In addition, Refprop 7.0<sup>2</sup> is used for calculating the working fluid properties and the CFD tool Femlab has been employed to investigate the particular phenomena influencing the heat exchanger performance.</p>
419

Enhanced boiling heat transfer from a novel nanodendritic micro-porous copper structure

Furberg, Richard January 2006 (has links)
<p>Following licentiate thesis is a summary of the advances made within the research project - Micro- and nano structured surfaces for enhanced boiling heat transfer – which is a collaboration effort between the Divi-sion of Applied Thermodynamics and Refrigeration and the Division of Materials Chemistry at the Royal Institute of Technology (KTH).</p><p>The main objectives with this research project has been to: <i>develop</i> <i>methods for producing highly efficient boiling surfaces with well defined</i> <i>micro- and nano-structured porous surfaces by the use of micro- a</i>nd <i>nano-manufacturing techniques</i>. This objective has been achieved and the result is a novel micro-porous surface structure comprising dendritically ordered nano-particles of cop-per. The structure was fabricated by a high-current-density electrode-position process, in which the evolution of hydrogen bubbles serve as a dynamic masking template to the growth of the dendritic copper struc-ture. Important variables were identified that affect the production of the structure and its features, such as surface orientation during electrode-position, pressure and temperature of electrolyte, and a final heat treat-ment of the surface under reduced atmosphere, all of which have previ-ously not been reported on.</p><p>Experimental tests have been conducted in a widely used refrigerant, R134a, where the micro-porous structure was shown to enhance the boiling performance of a copper surface over 15 times compared to a regular copper surface. The boiling characteristics of the structure were found to be dependent on controllable surface characteristics. The re-markably good boiling performance of the novel micro-porous en-hancement structure has been attributed to its high porosity ( ~94%), a dendritically formed and exceptionally large surface area, and to a high density of well suited vapor escape channels (>50 per mm2).</p><p>A patent application, intended to protect the enhancement structure and its fabrication method, was submitted to the Swedish patent authorities (PRV) on March 1st, 2006.</p>
420

Anisotropic lattice thermal diffusivity in olivines and pyroxenes to high temperatures /

Harrell, Michael D., January 2002 (has links)
Thesis (Ph. D.)--University of Washington, 2002. / Vita. Includes bibliographical references (leaves 121-129).

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