The lead-free tin telluride (SnTe) is considered as a potential candidate to substitute lead telluride (PbTe) for thermoelectric power generation based on their similar crystal and electronic structures. However, the relatively high lattice thermal conductivity and low Seebeck coefficient of pristine SnTe are detrimental for real-life applications. This dissertation explored elements-doping/substituting of SnTe to overcome those shortcomings and improve SnTe thermoelectric performance. A series of the Sn1-xGexTe phases were synthesized and studied. When the Ge amount reaches 50% or higher, Sn1-xGexTe undergoes a phase transition from the rock-salt structure (Fm3̅m) to the rhombohedral one (R3m). The Sn0.5Ge0.5Te phase was explored in more details because it delivers the best thermoelectric performance with the Sn1-xGexTe series. The electron-richer Sb and Bi were substituted on the Sn/Ge site to optimize the charge transport properties, and Cu2Te was added into the matrix to improve the thermoelectric performance further. The In/Sb and In/Bi co-doping on the Sn/Ge sites was employed for Seebeck coefficient optimization. A comparative study of the electronic structure of the Sn0.5Ge0.5Te-based samples was performed. The calculations indicated a band convergence and changes in the valence band, thus providing insight into the co-doping effects. Suppression of the lattice thermal conductivity of SnTe was performed via alloying with AgSnSe2 and PbTe, which introduced strong atomic disorder. Additionally, AgSnSe2 showed a hole donor behavior in SnTe, and the increased carrier concentration compensated for the reduction in the carrier mobility, thus rendering a decent electrical conductivity in alloyed samples. As a result, the alloying effectively improved the samples' thermoelectric performance. / Thesis / Doctor of Philosophy (PhD) / In recent decades, renewable energy has attracted a lot of attention due to an increase in the global energy use and depletion of fossil fuel reserves. Thermoelectric materials are expected to play a vital role as green energy generators to overcome the upcoming energy crisis as they can directly convert waste heat into electricity through the Seebeck effect. In this dissertation, the main goal is optimizing the thermoelectric performance of SnTe for the above room temperature applications. Different doping/ substituting/alloying strategies were applied to improve the performance. The obtained thermoelectric properties of the SnTe-based materials were rationalized in terms of the charge carrier behavior, changes in the electronic structure, and phonon propagation.

Scattering effects from rough surfaces are non-paraxial diffraction phenomena resulting from random phase variations in the reflected wavefront. The ability to predict these effects is important in a variety of applications including x-ray and EUV imaging, the design of stray light rejection systems, and reflection modeling for rendering realistic scenes and animations of physical objects in computer graphics. Rayleigh-Rice (small perturbation method) and Beckmann-Kirchoff (Kirchhoff approximation) theories are commonly used to predict surface scatter effects. In addition, Harvey and Shack developed a linear systems formulation of surface scatter phenomena in which the scattering behavior is characterized by a surface transfer function. This treatment provided insight and understanding not readily gleaned from the two previous theories, and has been incorporated into a variety of computer software packages (ASAP, Zemax, Tracepro). However, smooth surface and paraxial approximations have severely limited the range of applicability of each of the above theoretical treatments. In this dissertation, a linear systems formulation of non-paraxial scalar diffraction theory is first developed and then applied to sinusoidal phase gratings, resulting in diffraction efficiency predictions far more accurate than those provided by classical scalar theories. The application of the theory to these gratings was motivated by the fact that rough surfaces are frequently modeled as a superposition of sinusoidal surfaces of different amplitudes, periods, and orientations. The application of the non-paraxial scalar diffraction theory to surface scatter phenomena resulted first in a modified Beckmann-Kirchhoff surface scattering model, then a generalized Harvey-Shack theory, both of which produce accurate results for rougher surfaces than the Rayleigh-Rice theory and for larger incident and scattering angles than the classical Beckmann-Kirchhoff theory. These new developments enable the analysis and simplify the understanding of wide-angle scattering behavior from rough surfaces illuminated at large incident angles. In addition, they provide an improved BRDF (Bidirectional Reflectance Distribution Function) model, particularly for the smooth surface inverse scattering problem of determining surface power spectral density (PSD) curves from BRDF measurements.

This thesis presents the results of a 28 GHz continuous-wave (CW) diffraction measurement campaign in the Washington, DC area. It describes the measurement approach including information on equipment and testing methods. Also described are the various parameters that affected the diffraction loss. Observed diffraction losses showed little dependence on polarization and building material. For diffraction angles greater than 5 degrees, a simple linear equation was fit to the data and accurately describes the diffraction loss. A logarithmic equation describes the dependence at smaller angles. The model developed shows very good agreement with theory and other measurements. Also included are an overview of the fixed wireless industry, a discussion of system design issues, and a review of the historical and mathematical development of diffraction theory. / Master of Science

The structural and functional development of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) is essential to understand in order to enable pharmaceutical testing, disease modeling, and ultimately therapeutic use. Recent developments in the field of bioengineering have led to improvements in the efficiency and efficacy of growth methods that allow hiPSC- CMs to be studied in greater detail. However, engineered cardiac tissue still has not achieved a level of maturation necessary for the majority of biomedical applications. Thus, new technologies and methods are necessary to realize the long-term benefits of engineered cardiac tissue. To better understand the development of the tissue, further characterization of the structure and function of these cardiomyocytes is required. In this work, we describe advances using a method not commonly applied to these materials, Small Angle X-ray Scattering (SAXS). SAXS was used to characterize the structural development of hiPSC- CMs on a 3D multicellular platform in their early stages of maturation. The myofilament lattice spacing was found to monotonically decrease as the tissue matured from its initial state post-seeding at a rate between 0.75 and 1 nm per day between days 3 and 10 of maturation. With 49 total samples across three different batches of tissue, the p value for correlation between the lattice plane spacing and maturation time was p<0.05, indicating a statistically significant correlation. In tests of the tissue response to fixation with varying doses of KCl relaxation buffer, results showed a general trend of decreased myofilament spacing with increasing KCl concentration. However, in the concentrations between 60mM and 120mM, a characteristic increase in spacing is observed. Beat force was also measured prior to measuring myofilament spacing and this resulted in a graphically suggestive correlation. However, ANOVA analysis results in a p value of 0.35 which is statistically insignificant. Finally, methods were tested to monitor the myofilament lattice spacing in contracting tissue and found no evidence of contraction-based changes in the myofilament lattice. / 2024-01-15T00:00:00Z

A series of new germanates was prepared and their structures were characterized with X-ray diffraction (XRD). We employed solid-state synthesis, flux growth, and crystallization from melts to obtain crystals of the new materials. The crystals were studied by means of single crystal XRD, providing the information on the structure and composition of the new materials. Germanates suitable to accommodate Mn4+ – a well-known activator ion for the preparation of the rare-earth-free red phosphors – were of a particular interest. The Ge4+ substitution for Mn4+ is possible if the crystal structure features octahedrally coordinated germanium atoms, and we indeed were able to prepare such germanates. The crystal structures of the following phases were characterized: Mg3Ge1-O4(1-)F2(1+2) ( ≈ 0.1), Mg14Ge4O20F4, Mg2Pb2Ge2O7F2, Sr3GeO4Cl2, Ba3GeO4Br2, Sr6Ge2O7Cl6, Ba5GeO4Br6, Na2BaGe8O18, Rb2BaGe8O18, Na0.36Sr0.82Ge4O9, Na2SrGe6O14, and K2SrGe8O18. Two phases, Mg3Ge1-O4(1-)F2(1+2) and Na0.36Sr0.82Ge4O9 demonstrate deficiency on certain crystallographic sites. We analyzed the connectivity of the GeO4 and GeO6 units in the new and reported tetra- and octagermanates. Despite the similar stoichiometry, the connectivity of GeOn polyhedra is different in RI2Ge4O9, RIIGe4O9 and RIRIIGe8O18 germanates (RI = alkali, RII = alkaline earth metal). The Ge4+ substitution for Mn4+ was successfully done for the Na2BaGe8O18, Rb2BaGe8O18, Na2SrGe6O14, and K2SrGe8O18 phases yielding new red phosphors. Based on the powder XRD data, the optimal synthetic strategies were developed yielding high purity (≈ 99 wt. %) samples. The photoluminescent excitation and emission spectra were collected for the new phosphors. Strong absorption of the UV light and emission in the far-red region of the visible spectra were confirmed, which is in a good agreement with the literature. The Mn4+ doping level was optimized to achieve the highest luminescence in the studied phases. Temperature-dependent luminescence spectra were collected for the Na2SrGe6O14 : Mn4+ and K2SrGe8O18: Mn4+, and the K2SrGe8O18: Mn4+ showed the highest resistance to temperature quenching. / Thesis / Doctor of Philosophy (PhD)

High resolution fluorescence spectra are presented of the sideband of the 5D0+ 7F0 transition of Sm++ in KBr and KCl. Several Van Hove singularities of the phonon spectrum of the host material are directly observed. They occur at slightly different frequenciesfromthose predicted by density of states calculations based on neutron diffraction measurements. Numerical calculations of both sidebands are given and compared with experiment, with quite good agreement. Sidebands observed for Eu++ in KBr and KCl are also presented and discussed. / Thesis / Doctor of Philosophy (PhD)

<p> A coarse grained amphibolite from the metamorphosed rim of the Whitestone Anorthosite was prepared in several ways for the purpose of determining the modal abundance of the constituent minerals by calibrated X-ray diffraction. A simple two component amphibolite consisting of plagioclase and amphibole was chosen and five major methods of mounting the specimens for X-ray diffraction were employed.</p> <p> It was hoped that a method could be found which would produce randomly oriented, homogeneous samples. A series of such samples each of a different component ratio would then provide a calibration curve from which the mode of a whole rock specimen could be estimated.</p> <p> The calibrated X-ray charts were produced on Philips scanning X-ray diffractometers.</p> <p> The inconsistencies in the results indicate that better technical procedures are required.</p> / Thesis / Bachelor of Science (BSc)

<p> The crystal structures of three complexes of rhenium have been determined by single crystal x-ray diffraction methods. The structure and the probable position of the hydrogen atom in the complex tetracarbonyl-rhenium(I)-μ-oxo-μ-hydroxotetracarbonylrhenate(I) are discussed.</p> <p> The structural results for the other two complexes show the presence of carboxylato-bridged dinuclear rhenium core. The very short rhenium to rhenium distances and the eclipsed rotomeric configurations are similar to those found in octachlorodirhenate(III) anion. A reaction scheme for the formation of these complexes from rhenium(III) chloride is postulated and correlated with the structural results. The nature of bonding in the carboxylato complexes of rhenium is discussed.</p> / Thesis / Doctor of Philosophy (PhD)

<p> The crystal structure of dioxotetrapyridine-rhenium(V) chloride dihydrate has been determined by single crystal x-ray diffraction methods. The structure has been found to belong to the trans-dioxo- group of compounds. The Re=O bond in the structure has been shown to have a bond-order of about 2, with the average Re=O bond length of 1.76(J) A. The probable structures of the monohydrate and anhydrous dioxotetrapyridine-rhenium(V) chloride have been discussed on the basis of the observed pseudo-symmetric structure for the dihydrate with a non-centrosymmetric space group. The nature and strength of the hydrogen bonds in the structure have also been discussed.</p> / Thesis / Master of Science (MSc)

no abstract provided. / Thesis / Master of Science (MSc)