Theoretical study of the nonpolar surfaces and their oxygen passivation in 4H- and 6H-SiC

Structure and stability of nonpolar surfaces in 4H- and 6H-SiC have been investigated within the framework of a self-consistent charge density functional based tight binding method. The lowest energy stoichiometric surface is corrugated for (10 (1) over bar0) but atomically smooth for (11 (2) over bar0). The most stable clean surfaces are Si rich. independent of the growth conditions. Unlike the polar surfaces both nonpolar surfaces can completely be passivated by a single SiO2 adlayer

Oral Air Pressure, Nasal Air Flow, and Velopharyngeal Area in the Speech of Young Children

There are limited data regarding velopharyngeal (VP) aerodynamics for children younger than six years of age. Aerodynamic data can serve as evidence in the decision-making process regarding adequacy of VP function. Data available for older populations support the conclusion that VP aerodynamics do differ across the age ranges. Velopharyngeal aerodynamics from 32 children with typically developing speech were assessed. The purposes of the study were to describe VP aerodynamic measures in preschool-aged children, evaluate variables other than age as influential factors on these measures, describe stability over two recordings sessions, and compare preschool-aged to school-aged children. Findings were that preschool-aged children had VP aerodynamic measures similar to those from school-aged children. Body size measurements of height, weight, head circumference, chest circumference and cross-sectional VP area were not strong correlates to the VP measures. Nasal flow measures were stable over two recordings, but oral pressure was higher at the second recording

Shock Wave Behavior of Particulate Composites

Material heterogeneity at some scale is common in present engineering and structural materials as a means of strength improvement, weight reduction, and performance enhancement in a great many applications such as impact and blast protection, construction, and aerospace. While the benefits of transitioning toward composites in practical applications is obvious, the methods of measurement and optimization required to handle spatial heterogeneity and bridge length scale differences across multiple orders of magnitude are not. This is especially true as loading rates transition into the shock regime. Composite materials, such as concrete, have advantages afforded to them by their microstructure that allow them to dissipate and scatter impact energy. The mechanical mismatch between constituent phases in composites (mortar and cement paste in concrete, crystals and binder in polymer bonded explosives, ceramic powder and epoxy in potting materials, etc.) provides the interfaces required for shock wave reflection. The degree to which a shock is disrupted from its accepted form as a propagating discontinuity in stress and particle velocity is highly dependent upon the size, shape, and density of the interfaces present. The experimental and computer aided simulations in this thesis seek to establish a scaling relationship between composite microstructure and shock front disruption in terms of particulate size and density through the use of multi-point heterodyne velocity interferometry. A model particulate composite has been developed to mimic the wave reflection properties of materials such as Ultra High Performace Composite (UHPC) concrete and polymer bonded explosives, while also being simple to source and manufacture repeatably. Polymethyl Methacrylate (PMMA), a thermoplastic polymer, and silica glass spheres satisfy the manufacturing constraints with a shock impedance mismatch of 4.1, when placed in-between the shock impedance of UHPC concretes (~ 10) and polymer bondedexplosives (~ 2). The flexibility afforded by the model composite allows for the use of mono-disperse bead particle diameter distributions centered at 5 discrete diameters centered in the range associated with high scattering effectiveness (5-50 times the shock thickness in the pure matrix material). Shock front disruption is measured at multiple points on the rear surface of a plate impact target to observe shock spreading and spatial heterogeneity in material response due to random particle placement. Shock rise times are reported for composites of 30% and 40% glass spheres by volume, with glass spheres of 100, 300, 500, 700, and 1000 micron diameter. Composites with single mode as well as bi-modal bead diameter distributions are subjected to plate impact loading at an average pressure of 5 GPa. In single mode composites, a linear dependence of shock wave rise time on particle diameter is observed, with a constant of proportionality equal to the bulk shock speed in the material. Bi-modal bead diameter composites were fabricated in order to achieve higher volume fractions without composite degradation. The addition of a second phase to a base 30% glass by volume composite mix results in significant increases in shock wave rise time for base mixes of 500 micron beads, while a point of maximum scattering effectiveness is observed for base mixes of 1000 micron diameter beads. A comprehensive two dimensional series of CTH hydrocode simulations has been completed in tandem with experiments. An evaluation of the discrepancies in simulation and experimental results is presented. Shock disruption mechanisms and matrix/interface damage effects are discussed as possible sources of error and potential avenues for model improvement. The scaling arguments and model deficiency corrections made in this thesis have the potential to drive the development of new approaches of modeling shock waves in heterogeneous materials as well as optimization of microstructure for maximum shock front disruption.</p

Shock Wave Structure in Particulate Composites

An experimental study of shock wave profiles in particulate composites of various compositions is undertaken to determine how shock width and rise times depend on the mean particulate size. The composites under examination serve as a model for concrete or polymer bonded explosives, based upon the impedance mismatch between the relatively stiff particulates and compliant matrix. Polymethyl Methacrylate (PMMA) and glass spheres ranging in size from 100 μm to 1000 μm are used in concentrations of 30% and 40% glass by volume for experiments with a single bead size, and up to 50% glass by volume for multi-mode particle size distributions. A linear change in shock wave rise time is observed as a function of mean particulate diameter

Ab initio study: Investigating the adsorption behaviors of polarized greenhouse gas molecules on pillar[5]arenes

Supramolecular organic frameworks (SOFs) based on Pillar[5]arenes (P[5]A) have shown great potential in capturing and separating greenhouse gases. In this study, we investigate the adsorption behaviors of other environmentally harmful gas molecules, including NO, NH3, CO, and NO2 on pillar[5]arenes (P[5]A) using DFTB and DFT calculations. The P[5]A structures exhibit the capability to adsorb including NO, NH3, CO, and NO2 at both the cavity site and OH group, facilitated by π-π interactions and hydrogen bonding. CO exhibits the lowest binding energy among the studied gases, primarily due to its weak dipole moment. In contrast, the cavity-in positions for NO2 and NH3, characterized by high polarization, exhibit the highest adsorption energies. The adsorption energies at the top-out positions are relatively similar for all gases examined. These findings provide valuable insights for the targeted design and optimization of P[5]A, enabling its potential applications in effectively capturing toxic gases.publishedVersio