Doctor of Philosophy / Department of Mechanical and Nuclear Engineering / Douglas S. McGregor / Strengthening the crystal lattice of lanthanide halides, which are brittle, anisotropic, ionic crystals may increase the availability and ruggedness of these scintillators for room-temperature γ-ray spectroscopy applications. Eight dopants for CeBr[subscript]3, including CaBr[subscript]2, SrBr[subscript]2, BaBr[subscript]2, ZrBr[subscript]4, HfBr[subscript]4, ZnBr[subscript]2, CdBr[subscript]2, and PbBr[subscript]2, were explored at two different doping levels, 500ppm and 1000ppm, in an effort to identify potential aliovalent strengthening agents which do not adversely affect scintillation performance. All dopants and doping levels exhibited improved ingot yields over the undoped case, indicating an improvement in the ease of crystal growth. Scintillation performance was gauged using four key metrics. Scintillation emission spectra or, rather, radioluminescence spectra were recorded using x-ray irradiation. Total light yield was estimated through pulse height comparison with bismuth germanate (BGO) scintillators. Scintillation kinetics were checked by measuring single interaction pulses directly output by a fast response PMT. Finally, light yield proportionality was measured using a Compton coincidence system. Samples from each ingot were harvested to benchmark their performance with the four metrics. Of the eight dopants explored, only BaBr[subscript]2 and PbBr[subscript]2 clearly altered scintillation spectral emission characteristics significantly. The remaining dopants, CaBr[subscript]2, SrBr[subscript]2, ZrBr[subscript]4, HfBr[subscript]4, CdBr[subscript]2 and ZnBr[subscript]2, altered scintillation performance to a lesser degree. No dopant appeared to affect light yield proportionality, nor did any drastically alter the light decay characteristics of CeBr[subscript]3. HfBr[subscript]4 and ZnBr[subscript]2-doped CeBr[subscript]3 exhibited the highest light yields, significantly higher than the undoped CeBr[subscript]3 samples tested. Finally, aliovalent doping appeared to greatly improve CeBr[subscript]3 ingot yields, regardless of the dopant, thus it is a promising method for improving crystal strength while not deleteriously affecting scintillation performance. HfBr[subscript]4 and ZnBr[subscript]2 both demonstrated high performance without any noticeable negative side-effects and are prime candidates for future study.

Master of Science / Department of Mechanical and Nuclear Engineering / Kevin B. Lease / The purpose of this study was to experimentally characterize the compressive and shear behavior of square cell titanium honeycomb cores according to the American Society of Testing and Materials Standards C 365-05 and C 273-06. By varying the honeycomb cell size and height as well as the foil thickness, many configurations of titanium honeycomb were manufactured utilizing a laser welding and expansion method. The test matrix consisted of 1080 compression and 1080 shear specimens. The compression specimens were split evenly into stabilized and unstabilized tests, and the shear specimens were split evenly to test three different shear orientations. At the conclusion of the characterization, a comprehensive statistical analysis was performed on the data. It was determined that both the compressive and shear strengths have a strong dependence on the relative density of the honeycomb. The compressive strength was found to be slightly affected by the presence of a stabilizing face sheet, and largely unaffected by specimen core height. The compressive modulus was affected by both the core height and the presence of a face sheet. Shear strength was found to decrease with increasing core height and was influenced by the shear orientation. Additionally, the rate of increase of shear modulus with respect to relative density was proportional to core height. Although no clear trend was observed, orientation did seem to have an effect on shear modulus. The compression and shear behavior of the honeycomb was compared with experimental results of honeycomb from existing publications and found to be consistent.

Selectively-transparent and conducting photonic crystals (STCPCs) made of alternating layers of sputtered indium-tin oxide (ITO) and spin-coated silica (SiO2) nanoparticle films have potential applications in micromorph solar cells and building integrated photovoltaics (BIPVs). In this work, theoretical calculations have been performed to show performance enhancement of the micromorph solar cell upon integration of the STCPC an intermediate reflector. Thin semi-transparent hydrogenated amorphous silicon (a-Si:H) solar cells with STCPC rear contacts are demonstrated in proof-of-concept devices. A 10% efficiency increase in a 135nm thick a-Si:H cell on an STCPC reflector with Bragg peak at 620nm was observed, while the transmitted solar irradiance and illuminance are determined to be 295W/m2 and 3480 lux, respectively. The STCPC with proper Bragg peak positioning can boost the a-Si:H cell performance while transmitting photons that can be used as heat and lighting sources in building integrated photovoltaic applications.

This thesis investigates the effect of microstructural strengthening mechanisms on the overall mechanical performance of micro-truss periodic cellular metals (PCMs). Prior to the author’s work, the primary design considerations of micro-truss PCMs had been topological issues, i.e. the architectural arrangement of the load-supporting ligaments. Very little attention had been given to investigate the influence of microstructural effects within the cellular ligaments. Of the four broad categories of strengthening mechanisms in metals, only solute and second phase strengthening had previously been used in micro-trusses; the potential for strengthening micro-truss materials by work-hardening or grain size reduction had not been addressed. In order to utilize these strengthening mechanisms in micro-truss PCMs, two issues needed to be addressed. First, the deformation-forming method used to produce the micro-trusses was analyzed in order to map the fabrication-induced (in-situ) strain as well as the range of architectures that could be reached. Second, a new compression testing method was developed to simulate the properties of the micro-truss as part of a common functional form, i.e. as the core of a light-weight sandwich panel, and test the effectiveness of microstructural strengthening mechanisms without the influence of typical high-temperature sandwich panel joining processes, such as brazing. The first strengthening mechanism was achieved by controlling the distribution of plastic strain imparted to the micro-truss struts during fabrication. It was shown that this strain energy can lead to a factor of three increase in compressive strength without an associated weight penalty. An analytical model for the critical inelastic buckling stress of the micro-truss struts during uniaxial compression was developed in terms of the axial flow stress during stretch forming fabrication. The second mechanism was achieved by electrodeposition of a high-strength nanocrystalline metal sleeve around the cellular ligaments, producing new types of hybrid nanocrystalline cellular metals. It was shown that despite the added mass, the nanocrystalline sleeves could increase the weight-specific strength of micro-truss hybrids. An isostrain model was developed based on the theoretical behaviour of a nanocrystalline metal tube network in order to predict the compressive strength of the hybrid materials.

Doctor of Philosophy / Department of Grain Science and Industry / Xiuzhi Susan Sun / Renewable materials are desirable for many applications due to the finite fossil resources and environmental issues. Plant oil is one of the most promising renewable feedstocks. Plant oils and functionalized oleo-chemicals including functionalized soybean oils have become attractive sustainable chemicals for industrial applications. Especially, epoxidized oleo-chemicals such as epoxidized soybean oil (ESO) are one of the most well-known readily available inexpensive functionalized plant oils. In this study, novel polymers and nanocomposites for sustainable materials applications were designed and prepared via ring-opening of epoxide in plant oils, and their chemical and physical properties were characterized. The novel transparent elastomers derived from functionalized plant oils have a great potential as flexible electronic and biological applications with their inherent low toxicity. Especially, their rheological properties showed a potential for pressure sensitive adhesives (PSAs). The dominant thermal stability and transparency were obtained via green processing: one pot, single step, fast reactions in moderate conditions, or solvent-free UV curing conditions. These oleo-based elastomers presented excellent end-use properties for PSAs application comparable to commercial PSA tapes. Based on the principal chemical studies, the roles of the each component have been identified: polymer derived from the ring-opening of epoxides as an elastomer, and dihydroxylated triglycerides as a tackifier. Their interaction was also elucidated with an element label analysis. The mechanical and rheological properties of the oleo-polymer as PSAs were able to be improved with a rosin ester tackifier. In addition, biogreases and bio-thermoplastics were developed via the environmentally benign process, which will contribute to further application on the production of new bio-based materials. Further, this study essays a novel acid functionalized iron/iron oxide nanoparticles catalyst with excellent product yields for epoxide ring opening of oleochemicals for a greener synthetic method of biopolyols, and excellent environmental benefits with life cycle assessment of syntheses. Those functionalized iron/iron oxide core shell nanoparticles catalysts has great potential for biomedical engineering process with the highest magnetization of Fe(0) core among all metals.

Master of Science / Department of Civil Engineering / Kyle A. Riding / When transverse cracks meander there is a high possibility for transverse cracks to meet at a point and connect to another transverse crack, creating a Y-crack. Y-cracks have been blamed for being the origin of punchouts and spallings in CRCPs. When the direction of maximum principal stress changes, it could cause a change in the crack direction, potentially forming a Y-crack. Finite Element Models (FEMs) were run to model the change in principal stress direction based on design and construction conditions. The finite element model of CRCP using typical Oklahoma CRCP pavement conditions and design was assembled. The model included the concrete pavement, asphalt concrete subbase, and soil subgrade. The effect of areas of changed friction on the direction of principal stress was simulated by considering a patch at the pavement-subbase interaction. Investigated factors related to this patch were location of patch, friction between patch and subbase, and patch size. Patches were placed at two different locations in the pavement: a patch at the corner of the pavement and a patch at the longitudinal edge between pavement ends. A change in the friction at the corner had a large effect on the stress magnitude and direction of principal stress, while a patch in the middle did not significantly change the stress state. Also, patch size had a noticeable effect on stress magnitude when the patch was at the corner. Another model was developed to understand the effect of jointed shoulder on direction of maximum principal stress. Analysis of this model showed that the stresses were not symmetric and changed along the width of the pavement. This meandering pattern shows a high potential for Y-cracking. Also, several finite element models were run to understand the effects of different shrinkage between mainline and shoulder. In order to simulate the effects of the differential drying shrinkage between the hardened mainline concrete and the newly cast shoulder, different temperature changes were applied on the mainline and shoulder. For these models, the orientation of the maximum principal stress was not significantly changed from different amounts of temperature decreases between mainline and shoulder. Also, effect of different longitudinal steel percentages was investigated by comparing two finite element models with different steel percentage. The model with higher steel percentage (0.7%) indicated more variation in stress, potentially leading to more crack direction diverging.

Doctor of Philosophy / Department of Chemical Engineering / James H. Edgar / The exceptional radiation resistance, high melting point, and wide energy bandgap (3.2 eV) of icosahedral boron arsenide, B[subscript]12As[subscript]2, make it an attractive candidate for applications in radiation intense environments, for example, in radioisotope batteries. These devices have potential lifetimes of decades rather than days or weeks that are typical of conventional chemical power cells. Solid state neutron detectors are another potential application of this semiconductor, as the boron-10 isotope has a high thermal neutron capture cross-section, orders of magnitude higher than most elements. To produce high quality crystalline B[subscript]12As[subscript]2 for these applications, this research focused on the epitaxy and electrical properties of B[subscript]12As[subscript]2 thin films. The major findings include the following. Twin-free heteroepitaxial B[subscript]12As[subscript]2 layers were obtained on m-plane 15R-SiC and c-plane 4H-SiC inclined 4° and 7° off-axis in the [1-100] direction. These substrates exposed asymmetric step-terrace surface structures that force B[subscript]12As[subscript]2 layers to adopt a single orientation, thus, twins were eliminated. Consequently, the crystal quality was greatly improved over films on on-axis c-plane 6H-SiC, yielding a maximum hole mobility of 80 cm[superscript]2V[superscript]-1s[superscript]-1, nearly 100 times higher than previously reported values. B[subscript]12As[subscript]2 epilayers grown at 1300°C had the lowest defect densities, smallest residual strains, highest mobility and highest deposition rate. Excess AsH[subscript]3 concentration was advantageous to prevent the loss of arsenic from the epilayer. Undoped B[subscript]12As[subscript]2 exhibited a variable-range-hopping conduction, indicating it was a highly disordered system. All films were p-type with a room temperature hole concentration on the order of 10[superscript]12~10[superscript]15cm[superscript]-3. The thermal activation energy of acceptors varied from 0.15 eV to 0.33 eV. The Hall mobility was dominated by impurity scattering at low temperatures and by polar phonon scattering at high temperatures. H, C, O and Si were the major impurities present in the undoped B[subscript]12As[subscript]2 films with concentrations on the order of 10[superscript]18~10[superscript]19 cm[superscript]-3. Si doping and annealing decreased the resistivity by up to two orders of magnitude. The density of localized states was small in the undoped B[subscript]12As[subscript]2 as the intrinsic acceptor levels (IALs) were compensated by the boron interstitials. However, in Si-doped B[subscript]12As[subscript]2, Si may prevent the interstitial boron atoms from compensating the IALs, yielding a decreased density of localized states. The Hall mobility of B[subscript]12As[subscript]2 epilayer was significantly reduced with increasing silicon concentration.

Master of Science / Department of Chemistry / Kenneth Klabunde / Remediation of the indoor environment led to the development of novel catalysts which can absorb light in the visible range. These catalysts were prepared using the wet chemistry method known as sol-gel chemistry because preparation via sol-gel provides a homogeneous gel formation, which can be treated via supercritical drying to produce an aerogel. These aerogels have been found to have high surface areas when a combination of titania/silica is used. The increase in surface area has been shown to enhance the activity of the catalysts. Mixed metal oxide systems were prepared using titanium isopropoxide and tetraethyl orthosilicate to yield a 1:1 system of titania/silica (TiO2/SiO2). These systems were doped during the initial synthesis with transition metals (Mn or Co) to create mixed metal oxide systems which absorb light in the visible light range. These materials were assessed for potential as heterogeneous catalysts via gas-solid phase reactions with acetaldehyde. Degradation of acetaldehyde as well as the formation of CO2 was monitored via gas chromatography-mass spectrometery. To increase the activity, visible light was introduced to the system. Experiments have shown that a 10 mol % manganese doped titania/silica system, in the presence of light, can degrade acetaldehyde. The cobalt doped counterpart showed dark activity in the presence of acetaldehyde resulting in the formation of CO2 without the addition of visible light. In the hope of increasing surface area a mixed solvent (toluene/methanol) synthesis procedure was applied to the manganese doped catalyst. The resulting materials were of a low surface area but showed a significant increase in degradation of acetaldehyde. Examination of the interactions between mixed metal oxide systems and semivolatile organic compounds (SVOCs) was studied. The pollutant, triphenyl phosphate, was dissolved in n-pentane and exposed to 10 mg of a given catalyst. These reactions were monitored using UVVis. All systems but the manganese doped titania/silica system resulted in the observation of no activity with triphenyl phosphate. The manganese doped catalyst shown a peculiar activity, the increase in absorbance of the triphenyl phosphate peaks as well as the formation of a new peak.

Master of Science / Department of Chemical Engineering / James H. Edgar / Boron nitride is a purely synthetic material that has been known for over 150 years but only recently has sparked interest as a semiconductor material due to its potential in ultraviolet lasing and neutron detection. Thin-layer hexagonal boron nitride (hBN) is probably most attractive as a complementary material to graphene during its intense research endeavors. But for hBN to be successful in the realm of semiconductor technology, methods for growing large single crystals are critical, and its properties need to be accurately determined. In this study, hBN crystals were grown from metal solvents. The effects of soak temperature, soak time, source materials and their proportions on hBN crystal size and properties were investigated. The largest crystals of hBN measured five millimeters across and about 30 micrometers thick by precipitation from BN powder dissolved in a nickel-chromium solvent at 1700°C. High temperatures promoted outward growth of the crystal along the a-axis, whereas low temperatures promoted growth along the c-axis. Crystal growth at high temperatures also caused bulk hBN to adopt a triangular habit rather than a hexagonal one. A previously unreported method of synthesizing hBN was proven successful by substituting BN powder with elemental boron and a nitrogen ambient. XRD and Raman spectroscopy confirmed hBN from solution growth to be highly crystalline, with an 8.0 cm[superscript]-1 FWHM of the Raman peak being the narrowest reported. Photoluminescence spectra exhibited peaks mid-gap and near the band edge, suggesting impurities and defects in the hBN samples. However, high-purity reactants and post-growth annealing showed promise for synthesizing semiconductor-grade hBN. Several etchants were explored for defect-selective etching of hBN. A molten eutectic mixture of KOH/NaOH was the most effective defect-selective etchant of hBN at temperatures of 430-450°C for about one minute. The two prevalent hexagonal etch pit morphologies observed were deep, pointed-bottom pits and shallow, flat-bottom pits. TEM and SAED confirmed basal plane twists and dislocations in hBN crystals, but due to the highly anisotropic nature of hBN, their existence may be inevitable no matter the growth technique.

Doctor of Philosophy / Department of Mechanical and Nuclear Engineering / Douglas McGregor / The effect of polyethylene addition on the growth of mercuric iodide (HgI[supscript]2) tetragonal prismatic crystals is examined. Three types of polyethylene powder are utilized: low molecular weight (¯Mw ~ 4 x 103), ultra high molecular weight (¯Mw ~ 3 x 6 106), and spectrophotometric grade polyethylenes. Among these types of polyethylene, the low molecular weight polyethylene produces the most significant change in HgI[supscript]2 morphology, with {110} being the most prominent crystal faces. Thermal desorption - gas chromatography/mass spectroscopy (TD-GC/MS) studies show that thermal desorption of the low molecular weight polyethylene at 100°C and 150°C produce isomers of alkynes, odd nalkanes, and methyl (even-n) alkyl ketones. HgI[supscript]2 growth runs with n-alkanes, with either neicosane, n-tetracosane, or n-hexatriacontane, cannot replicate the crystal shapes produced during growth with the low molecular weight polyethylene, whereas HgI[supscript]2 growth runs with ketones, with either 3-hexadecanone or 14-heptacosanone, produce HgI[supscript]2 tetragonal prismatic crystals, similar to the crystals grown with the low molecular weight polyethylene. C-O double bond contained in any ketone is a polar bond and this polar bond may be attracted to the mercury atoms on the top-most layer of the {110} faces through dipoledipole interaction. As a result, the growth of the {110} faces is impeded, with the crystals elongated in the [001] direction and bounded by the {001} faces along with large, prismatic {110} faces.