[en] The numerical study of frustrated spin systems using the Monte Carlo simulation method and the analytic study of fluctuation phenomenon of the thermoelectric power near the superconducting transition using Green's function techniques are presented. The first frustrated system considered is the B-site antiferromagnetic (AF) spinel. Based on an Ising model, various thermodynamic and magnetic properties were studied for both the fully frustrated structure and partially frustrated cases of a small tetragonal distortion. When fully frustrated, an interesting short-range order and some unusual scaling behavior were obtained. The other frustrated spin system studied is the magnetic phase of YBa₂Cu₃O_6+x via a classical spin model, with appropriate anisotropic exchange couplings and randomly located spins of distribution probability as a function of x. There is a first order boundary between Type 1 and Type 2 in the Ising case, while there is no real phase boundary in the cases of continuous spin. In the study on the thermopower fluctuation, the thermopower was determined by the linear response of the electric and heat currents to an electric field, and the linear responses were in turn calculated from correlation functions of the current

[en] The numerical study of frustrated spin systems using the Monte Carlo simulation method and the analytic study of fluctuation phenomenon of the thermoelectric power near the superconducting transition using Green's function techniques are presented. The first frustrated system considered was the B-site antiferromagnetic (AF) spinel. Based on an Ising model, various thermodynamic and magnetic properties for both the fully frustrated structure and partially frustrated cases of a small tetragonal distortion were studied. When fully frustrated, an interesting short range order and some unusual scaling behavior were obtained. In the two tetragonally distorted cases, contracting and expanding in the crystallographic c-direction, AF long range orders and some hysteresis behavior were found. A general phase diagram was constructed as a function of the degree of the distortion. The other frustrated spin system that was studied is the magnetic phase of YBa2Cu3O(6+x). A classical spin model, was constructed, and various properties in its Ising, Heisenberg, and x-y versions were studied. The susceptibility was calculated as a function of temperature for various values of x. In the study on the thermopower fluctuation, the thermopower was determined by the linear response of the electric and heat currents to an electric field, and the linear responses were in turn calculated from correlation functions of the current

[en] This thesis describes the experimental studies of nitrogen oxide (NO, NO₂, N₂O) emissions in pressurized fluidized bed combustion (PFBC). In the first part of the thesis the background and the objectives of this study are introduced. The second part summarizes the fundamental knowledge about the formation and destruction of nitrogen oxides in coal combustion, particularly in the conditions of PFBC. The instrumentation of test facilities, measurement and data analysis is described in the third part. Then the most important experimental results follow in the next parts. The forth part describes the results from a PFBC test rig and an empirical modelling for predicting the emissions of NO_x and N₂O. Finally, the fundamental work on coal combustion and fuel nitrogen conversion in a PFBC batch reactor is presented. These studies clearly confirm the potential of PFBC technology in the control nitrogen of oxide emissions. The research in the test rig was concentrated on determining the effects of process parameters on the emissions of nitrogen oxides with different fuels. Another objective was to examine the reduction of nitrogen oxides with the control methods in PFBC conditions, including ammonia injection and air staging combustion for reducing NO, and high temperature operations for reducing N₂0. The results indicate that pressurized operation suppresses the conversion of fuel-N to nitrogen oxides and favors with employing the reduction methods for further nitrogen oxide reduction, for instance the temperature window of NO reduction with ammonia injection has been found to be widened to even lower temperature range. Maximum reductions of 80-85 % with ammonia injection and 75-80 % with air staging combustion were achieved in the conditions examined. Considerably low emissions of N₂O (<7 ppm) were obtained in the tests of N₂O control, and thermal decomposition proved to be the laming pathway of N₂O destruction in PFBC. In the examined pressure range, increasing pressure causes a decrease of NO emission but a slight increase of NO₂ emission; its influence on N2_O emission seems to be minor. The share of NO₂ to the total NO_x has been observed to be small in the trials of PFBC test rig. The work of empirical modelling continues the analysis of available data from the PFBC test rig for various fuels. As a first-generation model empirical correlation is developed for predicting emissions of NO_x and N₂0 as a function of primary operating parameters and properties of fuel and sorbent. Key variables are identified through statistical inference, while a model is selected by maximizing the correlation coefficient using multiple linear regression analysis. Despite some uncertainties in the modelling work, the correlations provide the new aspects of the nitrogen oxides in PFBC, particularly the effects on emissions of the interaction terms between parameters and fuel properties. Detailed fundamental research on pressurized combustion was conducted in a laboratory-scale PFBC batch reactor, which provides the opportunity to control the operations at welldefined conditions and to separate devolatilization and char oxidation during coal combustion. A special focus of the work is to study how the pressure affects the combustion process and fuel nitrogen (fuel-N) conversion. The influence of pressure was divided into two parts: O₂ partial pressure and total pressure. Results indicated that nitrogen chemistry in the batch reactor depends stron

[en] Considerable progress has been made on automatic hexahedral mesh generation in recent years. Several automatic meshing algorithms have proven to be very reliable on certain classes of geometry. While it is always worth pursuing general algorithms viable on more general geometry, a combination of the well-established algorithms is ready to take on classes of complicated geometry. By partitioning the entire geometry into meshable pieces matched with appropriate meshing algorithm the original geometry becomes meshable and may achieve better mesh quality. Each meshable portion is recognized as a meshing feature. This paper, which is a part of the feature based meshing methodology, presents the work on shape recognition and volume decomposition to automatically decompose a CAD model into meshable volumes. There are four phases in this approach: (1) Feature Determination to extinct decomposition features, (2) Cutting Surfaces Generation to form the ''tailored'' cutting surfaces, (3) Body Decomposition to get the imprinted volumes; and (4) Meshing Algorithm Assignment to match volumes decomposed with appropriate meshing algorithms. The feature determination procedure is based on the CLoop feature recognition algorithm that is extended to be more general. Results are demonstrated over several parts with complicated topology and geometry

[en] The potential native point defects and diffusion behaviors of helium impurities in zirconium carbide (ZrC) are discussed by first-principles calculations. It is found that C-related defects are easier to form in energy than Zr-related defects, consistent with experimental observations. By comparing incorporation energies at various sites, the energetically favored location for helium in intrinsic ZrC is determined to be the Zr substitution site. The positive but small incorporation energy suggests that helium is at the edge of solubility in ZrC. It is revealed that Zr-vacancy assisted diffusion mechanism for helium plays a dominant role in ZrC system with a small energy barrier of 0.70 eV. Besides, helium is likely trapped in Zr pre-existing vacancies, which may impact on the mechanical properties and dimensional stability of ZrC materials. - Highlights: • C-related defects are easier to form in energy than Zr-related defects in cubic ZrC. • The most favorable location for helium in crystal ZrC is Zr substitution site. • Zr-vacancy assisted diffusion manner for helium plays a dominant role in ZrC system.

[en] The samples of VO₂ thin films are irradiated by excimer laser with a wavelength of 308 nm under different output energy. Precipitation and recrystallization phenomenon on the surface are observed in the irradiated samples with lower and higher irradiation energy of laser respectively. Hence, the obvious variation of morphology is observed in irradiated samples with different irradiation energy. Precipitation process just slightly brings down the intensity of X-ray diffraction (XRD) peaks but recrystallization process can change the intensity and width of XRD peaks of samples. The results of laser Raman spectroscopy (LRS) and scanning electron miscroscopy (SEM) analysis indicate that the size of crystalline grain increases after recrystallization. The other optical properties of samples are also measured before and after irradiation

[en] Purpose: To determine whether the order of treatment plan rankings derived from CT scans taken on different days remains the same, or whether ranking depends on the specific CT data set used. Materials and methods: Five prostate patients each with three CT scans taken on three different days are used in this study. For each CT scan of a given patient, anatomical structures and the target are separately contoured, and three different treatment plans (4-field box, 6-field, and 4-field with wedge) are generated, all with the target receiving 100% of the prescribed dose. A total of 45 three-dimensional dose distributions and 225 dose-volume histograms (DVHs) are computed. The plans based on the same CT scan are compared and ranked according to DVHs of the target, rectum, and bladder. For each patient, plan rankings from three different CT scans are examined for consistency with each other. Results: There are considerable variations in rectum and bladder DVHs from one CT scan to another. However, in four out of five patients, the relative plan ranking based on the DVHs does not change from one CT scan to another. The plans are ranked as (1) 4-field with wedge, (2) 6-field, and (3) 4-field box according to the rectum DVHs, and as (1) 6-field, (2) 4-field with wedge, and (3) 4-field box according to the bladder DVHs. For the remaining patient, the ranking according to the bladder DVHs is the same as for the other patients for all three CT scans. However there is a possible change in the plan ranking according to the rectum DVHs for one CT scan comparing to the other scans. Conclusion: The process of selecting a treatment technique based on DVHs from a single CT scan appears to be generally valid, despite the variation in anatomy and volume irradiated from day to day for the same patient. For most patients, the plan ranking does not seem to depend on which CT data set of a patient is used. Exceptions may occur on a small fraction of patients

[en] Synchrotron radiation phase contrast imaging (PCI) is a new imaging method in recent years, which has high brightness, resolution, and coherence. PCI could detect articular cartilage histological structure noninvasively. The early osteoarthritis appearances, such as surface fibrosis or small cracks, could be detected. PCI has important implications in the early intervention and treatment of disease. The principles of PCI technique, basic research and clinical progress were reviewed in this article. (authors)

[en] The fluctuation contribution to the thermopower above the superconducting transition temperature T_c is calculated including all leading p-h asymmetric corrections. A fluctuation term is obtained which produces a peak in the thermopower at T_c in the dirty limit. The result is compared to some recent experiments on the high-T_c superconductors. (authors)

[en] Using the method of theoretical calculation and finite element model in ANSYS code, the studies of temperature distribution and the heat load of the liquid helium tube for the cryopump and the connecting feed through were performed. The results show that the steady heat load of the liquid helium tube is 5.45 W; particle deposition heat load is 75.68 W, and 5.04 W in the feed through. These results could be the reference for thermal stress analysis, cryopump flow velocity control and cryo system designing later. (authors)