<p> In this thesis, we study star formation in clustered environment within molecular clouds using Smooth Particle Hydrodynamics (SPH) simulations. Our first approach was to use "sink particles" to replace the dense gas particles where stars are forming. We implemented this type of particle in GASOLINE, and ran a simulation with a similar set of parameters to Bate et al. (2003). We found a good general agreement with this study. However, this work raised increasing concerns about some of the approximations used to follow the fragmentation process over many orders of magnitude in density. Our first issue was with the polytropic equation of state used to simulate gas of high density, that we believe would require some form of radiative transfer to be reliable. We also had concerns about the sink particles themselves, potentially overestimating the accretion rates.</p> <p> This guided our following work, where we choose to avoid both sinks and polytropic assumptions; allowing us to concentrate on the role of turbulence in forming prestellar cores. Supersonic turbulence is known to decay rapidly even when considering magnetic fields and gravity. However these studies are based on grid codes for periodic boxes. Our simulations are not periodic, they have open boundaries. Therefore the gravitational collapse can occur for the whole molecular cloud, not only for small portions of it. Hence the picture we observe in our self-gravitating turbulent molecular clouds is different. We found that under gravitational collapse turbulence is naturally developed and maintained with properties in good agreement with the current observational and theoretical picture.</p> <p> We also compared the cores we formed with observations. We looked at several observable properties of cores: density profiles, velocity dispersion and rotation of the cores, core-core velocity dispersion, core-envelope velocity dispersion, velocity dispersion vs. core size relation and the core mass function. We found a good general agreement between our simulated and observed cores, which indicates that extra physics like magnetic fields, outflows, proper equation of state or radiative transfer would have only secondary effects at this formation stages, or would tend to cancel each other.</p> / Thesis / Doctor of Philosophy (PhD)

<p> Simple theoretical expressions for the zero temperature energy gap and the transition temperature of a weak coupling superconductor are derived and applied to an investigation of several phenomena.</p> <p> The anisotropy of the energy gap in aluminium arising from the anisotropy in the phonon spectrum is calculated. The effect of this energy gap anisotropy on some thermodynamic properties of superconducting aluminium is investigated.</p> / Thesis / Doctor of Philosophy (PhD)

This work aims at improving the previously developed design procedure accounting for anisotropic dimensional changes on sintering. The goal of the project is both enlarging the reference database and investigating in depth the parameters responsible for anisotropic dimensional changes. The effects of green density, geometry, and compaction parameters (compaction speed hold down force and hold down time) on the anisotropy of dimensional changes on sintering were investigated. Ring shaped parts made of eight different iron-based materials were investigated in order to cover a large range of dimensional changes and different sintering mechanisms. The work is divided into two main parts. The first part focuses on investigating the effects of green density and geometry on the anisotropic dimensional changes, also enlarging the database. The application of the design methodology previously developed showed the significant role of anisotropy in the compaction plane for accurate prediction of dimensional changes, which was not highlighted previously. To solve this critical point a new anisotropy parameter is proposed and implemented in the design procedure. With the new anisotropy parameter, prediction of dimensional changes was improved while another critical point was highlighted, which is the scatter in the database. In order to have a reliable design procedure, database must be as accurate as possible, as demonstrated by a careful analysis on data processing. In the second part, the effect of compaction parameters on the anisotropy of dimensional changes on sintering was investigated, with a focus on the anisotropy in the compaction plane highlighted in the first part. No direct correlations were found between anisotropy of dimensional changes and compaction parameters, but compaction settings leading to lower anisotropy in the compaction plane were highlighted. Application of design methodology confirmed that the accuracy of predictions is higher when the sampling is less scattered. Also, a numerical study based on the experimental results was done to evaluate the possibility of neglecting anisotropy in the compaction plane for predicting dimensional changes. It was demonstrated that neglecting anisotropy in the compaction plane is acceptable, for some materials produced using the compaction settings minimizing such anisotropy. Additionally, a correlation between springback during ejection of the parts after compaction and anisotropy in the compaction plane was found.

Remote sensing data acquisition often requires a revisit to the same target. Therefore, it is not always possible to have the same illumination and viewing conditions. Bidirectional Reflectance Distribution Function (BRDF) is an attempt to predict the reflectance of an object for any given viewing and illumination geometry by explaining the interaction of the incident energy with the target object, the medium lying between the source and the target, and the interaction of the reflected energy with the medium between the target and the sensor. In this study various factors affecting BRDF were explored. Various factors contribute to this characteristic of the surface to reflect unequally in different directions like its structure, shape, degree of absorption and transmittance. Bidirectional Reflectance Factor, Anisotropic Factor, and Anisotropic Index were used in the research. Radiances were recorded using the Sandmeier Field Goniometer of target areas at the agricultural farms of Mississippi State University.

No description available.

<p> This thesis focuses on the use of time-resolved fluorescence anisotropy (TRF A) for the analysis of peptide-silica and protein-silica interactions. Previous studies from our group have shown that strong ionic binding of the cationic probe rhodamine 6G (R6G) to the anionic surface of silica particles in water provides a convenient labeling procedure to study both particle growth kinetics and surface modification by time-resolved fluorescence anisotropy (TRF A). The decays for R6G dispersed in diluted Ludox silica sols usually fit to a sum of picosecond and nanosecond decay components, along with a significant residual anisotropy component. The first objective of my work was to assess the nature of the R6G:silica interaction to determine the origin of the nanosecond decay component, and ultimately validate the model used to fit the TRFA data and gain further insight into the physical meaning of the anisotropy decay parameters. Our results show the origin of the nanosecond decay component ( ¢2) is due to the presence of a subpopulation of small nanoparticles in the Ludox sol. </p> <p> With the correct physical model in place, we have been able use TRFA ofR6G in aqueous Ludox to monitor peptide adsorption onto the silica particles in situ. Steady-state anisotropy and TRF A of R6G in Ludox sols were measured to characterize the extent of the ionic binding of the probe to silica particles in the presence of varying levels of tripeptides of varying charge, including Lys-Trp-Lys (KWK), N-acetylated Lys-Trp-Lys (Ac-KWK), Glu-Trp-Glu (EWE) and N-acetylated Glu-Trp-Glu (Ac-EWE). R6G showed significant decreases in anisotropy in the presence of cationic peptides, consistent with the addition of cationic peptides blocking the adsorption of the dye to the silica surface. The study shows that the competitive binding method can be used to assess the binding of various biologically relevant compounds onto silica surfaces, and demonstrates the potential of TRF A for probing peptide: silica and protein: silica interactions. </p> <p> We have also extended the application of TRF A to monitor protein adsorption onto plain and modified silica particles using a recently reported cationic long-lifetime quinolinium dye, CG437, which strongly binds to anionic silica particles through electrostatic interactions. In this case, alterations in the rotational correlation time of Ludox particles resulting from increases in the diameter of the rotating body upon binding of protein to the silica surface were monitored. The study shows that TRFA analysis of long-lived cationic probes such as CG437 can provide an effective method to investigate interactions between proteins and modified silica surfaces, extending the utility of the TRF A method. </p> / Thesis / Master of Science (MSc)

No description available.

The linearized Vlasov-Maxwell equations describing anisotropic plasma oscillations and waves are studied using an operator theoretic approach. The model considered is one dimensional so that after velocity averages perpendicular to this direction. have been taken, the equations can be naturally grouped into one set of equations for longitudinal modes and another set of equations for transverse modes. The problems of longitudinal and transverse plasma oscillations are studied by Fourier transforming the equations in the space variable and analyzing the resulting operator equations using the theory of semigroups. Existence and uniqueness theorems are proved, and solutions are constructed by the resolvent integration technique. The solutions are put into the form of a generalized eigenfunction expansion with eigenmodes corresponding to zeros of the appropriate plasma dispersion function. The expansion coefficients for eigenmodes corresponding to simple and second order real zeros of the plasma dispersion function are also presented, and constitute some of the new results obtained by our analysis. Existence and uniqueness of the solution to the longitudinal plasma wave boundary value problem is proved by writing the longitudinal equations in operator form and again using the theory of semigroups. The solution to the plasma wave boundary value problem is arrived at by a Fourier time transformation of the Vlasov equation coupled to Ampere's Law rather than Gauss‘ Law, and analyzing a scalar operator as opposed to the more complicated matrix operator that has previously been studied. Special care is used in constructing the half range transport operator whose resolution of the identity yields the solution in the form of a half range generalized eigenfunction expansion where again, new results are presented for the expansion coefficients for eigenfunctions corresponding to simple and second order real zeros of the fixed frequency longitudinal plasma dispersion function. Since this study is concerned with anisotropic plasmas, a non-even plasma equilibrium distribution function is assumed with the direct result that more stable and unstable plasma modes corresponding to real and complex zeros of the plasma dispersion function are possible that has previously been considered. Also, for the longitudinal plasma wave problem, the Wiener-Hopf factorization of the fixed frequency longitudinal plasma dispersion function is presented and the coupled nonlinear integral equations for the Wiener-Hopf factors are studied. These Wiener-Hopf factors are required in the construction of the half range transport operator. / Ph. D.

This study considers an infinite orthotropic plate on a frictionless elastic foundation subjected to a transverse point load. We are concerned with the stress distribution in the vicinity of the load application. Based on the assumption of a uniform stress distribution under the load, a Fourier Transform technique is utilized to formulate and solve the problem in the transform domain. The inverse transformation is carried out numerically, via the Gaussian Quadrature scheme, to obtain the real response. Symmetry of the transform response due to the material orthotropy has been used to reduce the effort involved in performing the integration. Results indicated a similar symmetric behavior for the real stress distribution, and satisfied the boundary conditions of the problem. The transverse equilibrium is also verified by summing up the reaction forces exerted by the foundation. / M.S.

High temperature thermal energy harvesting has attracted much attention recently. In order to achieve stable operation at high temperatures there is emerging need to develop efficient and oxidation-resistant materials. Most of the well-known materials with high dimensionless figure of merit (ZT) values such as Bi2Te3, PbTe, skutterudites, and half-Heusler alloys, are not thermally stable at temperatures approaching 500°C or higher, due to the presence of volatile elements. Oxide thermoelectric materials are considered to be potential candidates for high temperature applications due to their robust thermal and chemical stability in oxidizing atmosphere along with the reduced toxicity, relatively simpler fabrication, and cost. In this dissertation, nanoscale texturing and interface engineering were utilized for enhancing the thermoelectric performance of oxide polycrystalline Ca3Co4O9 materials, which were synthesized using conventional sintering and spark plasma sintering (SPS) techniques. In order to tailor the electrical and thermal properties, Lu and Ga co-doping was investigated in Ca3Co4O9 system. The effect of co-doping at Ca and Co sites on the thermoelectric properties was quantified and the anisotropic behavior was investigated. Because of the effective scattering of phonons by doping-induced defects, lower thermal conductivity and higher ZT were achieved. The layered structure of Ca3Co4O9 has strong anisotropy in the transport properties. For this reason, the thermoelectric measurements were conducted for the samples along both vertical and horizontal directions. The ZT value along the vertical direction was found to be 3 to 4 times higher than that along the horizontal direction. Metallic inclusions along with ionic doping were also utilized in order to enhance the ZT of Ca3Co4O9. The texturing occurring in the nanostructured Ca3Co4O9 through ion doping and Ag inclusions was studied using microscopy and diffraction analysis. Multi-length scale inclusions and heavier ion doping in Ca3Co4O9 resulted in higher electrical conductivity and reduced thermal conductivity. The maximum ZT of 0.25 at 670°C was found in the spark plasma sintered Ca2.95Ag0.05Co4O9 sample. In literature, limited number of studies have been conducted on understanding the anisotropic thermoelectric performance of Ca3Co4O9, which often results in erroneous estimation of ZT. This study addresses this limitation and provides systematic evaluation of the anisotropic response with respect to platelet microstructure. Textured Ca3Co4O9/Ag nanocomposites were fabricated using spark plasma sintering (SPS) technique and utilized for understanding the role of microstructure towards anisotropic thermoelectric properties. The thermoelectric response was measured along both vertical and horizontal direction with respect to the SPS pressure axis. In order to achieve enhanced degree of texturing and increase electrical conductivity along ab planes, a two-step SPS method was developed. Ag nanoinclusions was found to increase the overall electrical conductivity and the thermoelectric power factor because of improved electrical connections among the grains. Through two-step SPS method, 28% improvement in the average ZT values below 400°C and 10% improvement above 400°C in Ca3Co4O9/Ag nanocomposites was achieved. Lastly, this dissertation provides significant progress towards understanding the effect of synthesis method on thermoelectric properties and evolution of textured microstructure. The anisotropy resulting from the crystal structure and microstructural features is systematically quantified. Results reported in this study will assist the continued progress in developing Ca3Co4O9 materials for practical thermoelectric applications. / PHD / Among the wide range of renewable energy sources, wasted thermal energy has attracted worldwide interest as it is freely available from most of the industrial and natural processes. Among various choices for converting thermal energy into electricity, thermoelectric devices are attractive as they are solid state, noiseless, no moving parts, and can be easily integrated with most of the heat sources. Thus, there has been significant efforts to develop high efficiency thermoelectric energy harvesting devices. However, currently available thermoelectric materials are not thermally stable in oxidizing environments because of heavy metals’ evaporation and reactivity. In order to overcome this limitation of thermoelectric materials, in this dissertation, the focus is on developing calcium cobalt oxide (Ca₃Co₄O₉) materials through innovation in the processing, composition design, and modulation of the thermal transport mechanism by exploiting the anisotropy. Ca₃Co₄O₉ is promising candidate for high temperature thermoelectric applications due to its thermal and chemical stability in oxidizing atmosphere, reduced toxicity, easy fabrication, and low cost. Its main disadvantages are the high thermal conductivity and low electrical conductivity. In order to tailor the electrical and thermal properties, Lu and Ga co-doped Ca₃Co₄O₉ were synthesized and characterized. The thermoelectric measurements were conducted along both vertical and horizontal directions with respect to pressure axis during spark plasma sintering. Layered structure of Ca₃Co₄O₉ induces strong anisotropy in the transport properties which indicates that textured microstructure will result in better properties. Texturing and interface engineering were employed to control the grain orientation and thereby improve the electrical and thermal properties. In textured and nanostructured Ca₃Co₄O₉, Ag inclusions along with ionic doping was utilized to enhance the thermoelectric performance. In literature, the importance of the anisotropy in Ca₃Co₄O₉ has been less emphasized, which has restricted accurate thermoelectric evaluation of this material for practical application. In order to address this issue, first textured Ca₃Co₄O₉/Ag nanocomposites were fabricated using spark plasma sintering (SPS) techniques and next detailed investigation was conducted on correlation between microstructure and anisotropic thermoelectric properties. The power factor of the Ca₃Co₄O₉/Ag nanocomposites at high temperatures was almost 50% enhanced, as compared to the pure Ca₃Co₄O₉, which resulted in 50% improvement in ZT both horizontal and vertical directions. The samples with texturing along the vertical direction were used to perform the long-term durability test and almost same value of resistivity was maintained after a long-term heating. Two-step SPS method was developed to improve the in-plane electrical conductivity. Through this newly proposed synthesis process, 28% improvement in the average ZT values below 400°C and 10% improvement above 400°C was obtained in Ca₃Co₄O₉/Ag nanocomposites. Using a wide range of composition and synthesis process, the anisotropy and microstructural effects clarified in this study provides promising pathway towards enhance the thermoelectric performance of Ca₃Co₄O₉ materials.