Doctor of Philosophy / Department of Chemical Engineering / James H. Edgar / Gallium nitride is a promising semiconductor for fabricating field effect transistors for power electronics because of its unique physical properties of wide energy band gap, high electron saturation velocity, high breakdown field and high thermal conductivity. However, these devices are extremely sensitive to the gate leakage current which reduces the breakdown voltage and the power-added efficiency and increases the noise figures. To solve this problem, employing a gate dielectric is crucial to the fabrication of metal insulator semiconductor high electron mobility transistors (MISHEMTs), to reduce the leakage current and increase the magnitude of voltage swings possible. For this device to be successful, imperfections at the oxide-semiconductor interface must be suppressed to maintain the high electron mobility of the device. This research explored multiple high dielectric constant gate oxides (Al[subscript]2O[subscript]3, TiO[subscript]2, and Ga[subscript]2O[subscript]3), deposited on different crystalline orientations and polarities of GaN by atomic layer deposition (ALD) to form metal oxide semiconductor capacitors, including effects of pretreatment on N-polar GaN, ALD TiO[subscript]2/Al[subscript]2O[subscript]3 nano-laminate on thermal oxidized Ga-polar GaN and ALD Al[subscript]2O[subscript]3 on [Italic]c- and [Italic]m-plane GaN Surface pretreatments were shown to greatly alter the morphology of reactive N-polar GaN which is detrimental to the electrical properties. 14 nm thick ALD Al[subscript]2O[subscript]3 films were directly deposited on N-polar GaN without thermal or chemical pretreatments which yield a smooth surface (RMS=0.23 nm), low leakage current (2.09 x 10[superscript]-[superscript]8 A/cm[superscript]2) and good Al[subscript]2O[subscript]3/GaN interface quality, as indicated by the low electron trap density (2.47 x 10[superscript]10 cm[superscript]-[superscript]2eV[superscript]-[superscript]1). In the nano-laminate study, a high dielectric constant of 12.5 was achieved by integrating a TiO[subscript]2/Al[subscript]2O[subscript]3/Ga[subscript]2O[subscript]3 oxide stack layer, while maintaining a low interface trap density and low leakage current. There was a strong correlation between the surface morphology and electrical properties of the device discovered from comparing the ALD Al[subscript]2O[subscript]3 on [Italic]c- and [Italic]m-plane GaN, namely smooth surface lead to small hysteresis. These results indicate the promising potential of incorporation gate dielectric for future GaN devices.

Master of Science / Department of Mechanical and Nuclear Engineering / Xiao J. Xin / Energy consumption due to dairy production constitutes 10% of all energy usage in the U.S. Food Industry. Improving energy efficiency in food refrigeration and freezing plays an important role in meeting the energy challenges of today. Freezing and hardening are important but energy-intensive steps in ice cream manufacturing. This thesis presents a series of models to address these issues. The first step taken to model energy consumption was to create a temperature-dependent ice cream material using empirical properties available in the literature. The homogeneous ice cream material is validated using finite element analysis (FEA) and previously published experimental findings. The validated model is then used to study the efficiency of various package configurations in the ice cream hardening process. The next step taken is to consider product quality by modeling the ice crystal size distribution (CSD) throughout the hardening process. This is achieved through the use of population balance equations (PBE). Crystal size and corresponding hardened ice cream coarseness can be predicted through the PBE model presented in this thesis. The crystallization results are validated through previous experimental study. After the hardening studies are presented, the topic of continuous freezing is discussed. The actual ice cream continuous freezing process is inherently complex, and therefore simplifying assumptions are utilized in this work. Simulation is achieved through combined computational fluid dynamics (CFD) and PBE modeling of a sucrose solution. By assuming constant fluid viscosity, a two-dimensional cross section is able to be employed by the model. The results from this thesis provide a practical advancement of previous ice cream simulations and lay the groundwork for future studies.

Master of Science / Department of Chemical Engineering / J. H. Edgar / Boron phosphide, BP, is a III-V compound semiconductor with a wide band gap of 2.0 eV that is potentially useful in solid state neutron detectors because of the large thermal neutron capture cross-section of the boron-10 isotope (3840 barns). In this study, cubic BP crystals were grown by crystallizing dissolved boron and phosphorus from a nickel solvent in a sealed (previously evacuated) quartz tube. The boron - nickel solution was located at one end of the tube and held at 1150°C. Phosphorus, initially at the opposite end of the tube at a temperature of 430°C, vaporized, filling the tube to a pressure of 1–5 atmospheres. The phosphorus then dissolved into solution, producing BP. Transparent red crystals up to 4 mm in the largest dimension with mostly hexagonal shape were obtained with a cooling rate of 3°C per hour. The crystal size decreased as the cooling rate increased, and also as growth time decreased. The characterization with x-ray diffraction (XRD) and Raman spectroscopy established that the BP produced through this method were highly crystalline. The lattice constant of the crystals was 4.534 Ǻ, as measured by x-ray diffraction. Intense, sharp Raman phonon peaks were located at 800 cm[superscript]-1 and 830 cm[superscript]-1, in agreement with the values reported in the literature. The FWHM for XRD and Raman spectra were 0.275° and 4 cm[superscript]-1 which are the narrowest ever reported and demonstrates the high quality of the produced crystals. Energy dispersive x-ray spectroscopy (EDS) and scanning electron microscope (SEM) also confirmed the synthesized crystals were cubic BP crystals, with a boron to phosphorus atomic ratio of 1:1. Defect selective etching of BP at 300ºC for two minutes with molten KOH/NaOH revealed triangular and striated etch pits with low densities of defects of ~4 x 10[superscript]7 cm[superscript]-2 and 9.2 x 10[superscript]7 cm[superscript]-2 respectively. The BP crystals were n-type, and an electron mobility of ~39.8 cm[superscript]2/V*s was measured. This is favorable for application in neutron detection. Scaling to larger sizes is the next step through gradient freezing and employing a larger crucible.

Doctor of Philosophy / Department of Physics / Hongxing Jiang / The optical and structural properties of Er-doped GaN/InGaN materials and devices synthesized by metal organic chemical vapor deposition (MOCVD) were investigated. Er-doped GaN via MOCVD emits a strong photoluminescence (PL) emission at 1.54 um using both above and below-bandgap excitation. In contrast to other growth methods, MOCVD-grown Er-doped GaN epilayers exhibit virtually no visible emission lines. A small thermal quenching effect, with only a 20% decrease in the integrated intensity of the 1.54 um PL emission, occurred between 10 and 300 K. The dominant bandedge emission of Er-doped GaN at 3.23 eV was observed at room temperature, which is red-shifted by 0.19 eV from the bandedge emission of undoped GaN. An activation energy of 191 meV was obtained from the thermal quenching of the integrated intensity of the 1.54 um emission line. It was observed that surface morphology and 1.54 um PL emission intensity was strongly dependent upon the Er/NH3 flow rate ratio and the growth temperature. XRD measurements showed that the crystalline ordering of the (002) plane was relatively unperturbed for the changing growth environment. Least-squares fitting of 1.54 um PL measurements from Er-doped GaN of different growth temperatures was utilized to determine a formation energy of 1.82 ± 0.1 eV for the Er-emitting centers. The crystalline quality and surface morphology of Er-doped InGaN (5% In fraction) was nearly identical to that of Er-doped GaN, yet the PL intensity of the 1.54 um emission from Er-doped InGaN (5% In fraction) was 16 x smaller than that of Er-doped GaN. The drop in PL intensity is attributed to the much lower growth temperature in conjunction with the high formation energy of the Er- emitting centers. Er-doped InGaN grown at fixed growth temperature with different growth pressures, NH3 flow rates, and Ga flow rates was also investigated, and showed that increased In fractions also resulted in a smaller 1.54 um PL intensity. Er-doped InGaN p-i-n diodes were synthesized and tested. The electroluminescence (EL) spectra under forward bias shows strong Er based emission in the infrared and visible region. The different emission lines from EL spectra in contrast to PL spectra implies different excitation methods for the Er based emission in the p-i-n diode than in the PL excited epilayer.

Doctor of Philosophy / Department of Chemistry / Kenneth J. Klabunde / For solar environmental remediation, a new generation of nanosized (< 10 nm) titanium dioxide photocatalysts codoped with metals and nonmetals, or metals only were prepared by the xero-gel and aero-gel methods. For silver or cobalt-based xero-gel titanium dioxide photocatalysts, photoactivities tests revealed that codoping of titanium dioxide with a metal (1% Ag or 2% Co) and nonmetals (carbon and sulfur) is necessary to achieve high-activities for acetaldehyde degradation under visible light (wavelength > 420 nm). It was concluded that high visible-light-activities for acetaldehyde degradation over codoped titanium dioxide were attributed to an interplay of anatase crystallinity, high-surface area, reduced band-gap (< 3.0 eV), uniform dispersion of doped metal ions, and suppressed recombination rate of photogenerated electron-hole pairs. Moreover, the nature and amount of codoped metals play a significant role in visible-light-induced photocatalysis. Metals (Al, Ga, and In) doped/codoped titanium dioxide photocatalysts were prepared by the aero-gel method. The photocatalytic studies showed that activities of metal doped/codoped photocatalysts under UV light (wavelength < 400 nm) were found to be dependent on pollutants. Indium demonstrated beneficial effects in both textural and photocatalytic properties. Gallium and indium codoped titanium dioxide photocatalysts displayed even better performance in the CO oxidation reaction under UV light. Notably, titanium dioxide codoped with Ga, In, and Pt, exhibited unique photoactivities for the CO oxidation under both UV and visible light irradiation, indicating that this system could have promise for the water-gas shift reaction for hydrogen production. Silver-based nanostructured titanium dioxide samples were developed for killing human pathogens (Escherichia coli cells and Bacillus subtilis spores). Biocidal tests revealed that silver, carbon, and sulfur codoped titanium dioxide nanoparticles (< 10 nm) possess very strong antimicrobial actions on both E. coli (logarithmic kill > 8) and B. subtilis spores (logarithmic kill > 5) for 30 minute exposures in dark conditions compared with Degussa P25. It was believed that the carbon and sulfur codoped titanium dioxide support and Ag species acted synergistically during deactivation of both E. coli and B. subtilis spores. Thus, titanium dioxide codoped with silver, carbon, sulfur can serve as a multifunctional generic biocide and a visible- light-active photocatalyst.

Master of Science / Department of Mechanical and Nuclear Engineering / William L. Dunn / A prototype system for detecting explosives at standoff distances, using a signature based radiation scanning approach, is being developed at Kansas State University. The prototype will incorporate both a machine x-ray source and a machine neutron source to generate signatures from unknown samples of material. These signatures can be compared to templates measured or calculated from known explosive samples using a figure-of-merit. The machine neutron source uses the fusion of deuterium and tritium to create 14.1 MeV neutrons. Due to its radioactivity, the tritium must be sealed within the system. A new method of controlling the gas pressure with the DT generator was developed using a Zr-V-Fe getter supplied by a commercial firm. The shielding and collimation of the 14.1 MeV neutron source is accomplished using layers of steel, high-density polyethylene and borated high-density polyethylene. This thesis describes the development of the gas control method for the sealed neutron source, design studies for the shielding and collimation of the neutron source and modifications made to the building in which the prototype is being housed.

Doctor of Philosophy / Department of Civil Engineering / Mustaque Hossain / The engineers from the Kansas Department of Transportation (KDOT) often have to decide whether or not to accept non-conforming Superpave mixtures during construction. The first part of this study focused on estimating lives of deficient Superpave pavements incorporating nonconforming Superpave mixtures. These criteria were based on the Hamburg Wheel-Tracking Device (HWTD) test results and analysis. The second part of this study focused on developing accelerated mix testing models to considerably reduce test duration. To accomplish the first objective, nine fine-graded Superpave mixes of 12.5-mm nominal maximum aggregate size (NMAS) with asphalt grade PG 64-22 from six administrative districts of KDOT were selected. Specimens were prepared at three different target air void levels @ N[subscript]design gyrations and four target simulated in-place density levels with the Superpave gyratory compactor. Average number of wheel passes to 20-mm rut depth, creep slope, stripping slope, and stripping inflection point in HWTD tests were recorded and then used in the statistical analysis. Results showed that, in general, higher simulated in-place density up to a certain limit of 91% to 93%, results in a higher number of wheel passes until 20-mm rut depth in HWTD tests. A Superpave mixture with very low air voids @ N[subscript]design (2%) level performed very poorly in the HWTD test. HWTD tests were also performed on six 12.5-mm NMAS mixtures with air voids @ N[subscript]design of 4% for six projects, simulated in-place density of 93%, two temperature levels and five load levels with binder grades of PG 64-22, PG 64-28, and PG 70-22. Field cores of 150-mm in diameter from three projects in three KDOT districts with 12.5-mm NMAS and asphalt grade of PG 64-22 were also obtained and tested in HWTD for model evaluation. HWTD test results indicated as expected. Statistical analysis was performed and accelerated mix testing models were developed to determine the effect of increased temperature and load on the duration of the HWTD test. Good consistency between predicted and observed test results was obtained when higher temperature and standard load level were used. Test duration of the HWTD can thus be reduced to two hours or less using accelerated mix testing (statistical) models.

Doctor of Philosophy / Department of Civil Engineering / Mustaque Hossain / A Superpave asphalt mixture with 4.75-mm nominal maximum aggregate size (NMAS) is a promising, low-cost pavement preservation treatment for agencies such as the Kansas Department of Transportation (KDOT). The objective of this research study is to develop an optimized 4.75-mm NMAS Superpave mixture in Kansas. In addition, the study evaluated the residual tack coat application rate for the 4.75-mm NMAS mix overlay. Two, hot-in-place recycling (HIPR) projects in Kansas, on US-160 and K-25, were overlaid with a 15- to 19-mm thick layer of 4.75-mm NMAS Superpave mixture in 2007. The field tack coat application rate was measured during construction. Cores were collected from each test section for Hamburg wheel tracking device (HWTD) and laboratory bond tests performed after construction and after one year in service. Test results showed no significant effect of the tack coat application rate on the rutting performance of rehabilitated pavements. The number of wheel passes to rutting failure observed during the HWTD test was dependent on the aggregate source as well as on in-place density of the cores. Laboratory pull-off tests showed that most cores were fully bonded at the interface of the 4.75-mm NMAS overlay and the HIPR layer, regardless of the tack application rate. The failure mode during pull-off tests at the HMA interface was highly dependent on the aggregate source and mix design of the existing layer material. This study also confirmed that overlay construction with a high tack coat application rate may result in bond failure at the HMA interface. Twelve different 4.75-mm NMAS mix designs were developed using materials from the aforementioned but two binder grades and three different percentages of natural (river) sand. Laboratory performance tests were conducted to assess mixture performance. Results show that rutting and moisture damage potential in the laboratory depend on aggregate type irrespective of binder grade. Anti-stripping agent affects moisture sensitivity test results. Fatigue performance is significantly influenced by river sand content and binder grade. Finally, an optimized 4.75-mm NMAS mixture design was developed and verified based on statistical analysis of performance data.

Doctor of Philosophy / Mechanical and Nuclear Engineering / Douglas S. McGregor / There is a need for compact, rugged neutron detectors for a variety of applications including national security and oil well logging. A solid form neutron detector would have a higher efficiency than present day gas filled ³He and ¹⁰BF ₃ detectors, which are standards currently used in the industry today. A sub-branch of the III-V semiconductors is the filled tetrahedral compounds, known as Nowotny-Juza compounds (A[superscript I]B[superscript II]C[superscript V]). These materials are desirable for their cubic crystal structure and semiconducting electrical properties. Originally studied for photonic applications, Nowotny-Juza compounds have not been fully developed and characterized. Nowotny-Juza compounds are being studied as neutron detection materials here, and the following work is a study of LiZnP and LiZnAs material development and device characterization. Precursor binaries and ternary materials of LiZnAs and LiZnP were synthesized in-house in vacuum sealed quartz ampoules with a crucible lining. Synthesized powders were characterized by x-ray diffraction, where lattice constants of 5.751 ± .001 Å and 5.939 ± .002 Å for LiZnP and LiZnAs, respectively, were determined. A static vacuum sublimation in quartz was performed to help purify the synthesized ternary material. The resulting material from the sublimation process showed characteristics of a higher purity ternary compound. Bulk crystalline samples were grown from the purified material. Ingots up to 9.0 mm in diameter and 13.0 mm in length were harvested. Individual samples were characterized for crystallinity on a Bruker AXS Inc. D2 CRYSO, energy dispersive x-ray diffractometer, and a Bruker AXS D8 DISCOVER, high-resolution x-ray diffractometer with a 0.004° beam divergence. High-resolution XRD measurements indicated reasonable out-of-plane and in-plane ordering of LiZnP and LiZnAs crystals. Devices were fabricated from the LiZnP and LiZnAs crystals. Resistivity of devices were determined within the range of 10⁶ – 10¹¹ Ω cm. Charge carrier mobility and mean free drift time products were characterized for electrons at 8.0 x 10⁻⁴ cm² V⁻¹ ± 4.8% and 9.1 x 10⁻⁴ cm² V⁻¹ ± 4.4% for LiZnP and LiZnAs respectively. Sensitivity to 337 nm laser light (3.68 eV photons) was observed, where an absorption coefficient of 0.147 mm⁻¹ was determined for LiZnAs devices. Thermal neutron sensitivity was evaluated with unpurified and purified LiZnP and LiZnAs devices. Sensitivity was observed, however material quality and crystalline quality significantly hindered device performance.

Doctor of Philosophy / Chemistry / Christer B. Aakeroy / A series of co-crystallizations between four biimidazole based compounds with nine symmetric aliphatic di-acids and fifteen perfluorinated halogen-bond donors were carried out to determine if a MEPS based ranking can be used to effectively assign selectivity in hydrogen- and halogen-bond interactions. The results suggested that a simple electrostatic view provides a reliable tool for successfully implementing the practical co-crystal synthesis with desired connectivity. MEPS based selectivity guidelines for halogen-bond interactions were explored in co-crystallizations between twelve asymmetric ditopic acceptors and nine halogen-bond donors. If the difference between the two acceptor sites is below 35 kJ/mol, no selectivity was observed; above 65 kJ/mol halogen bond selectivity dominates and mid ΔE range was recognized as the grey area where predictions cannot be made. To examine competition between hydrogen and halogen bonds, five heteroaryl-2-imidazoles were co-crystallized with fifteen halogen-bond donors. It was found that halogen bonds prefer best the acceptor site, demonstrating that a suitably activated halogen-bond donor can compete with a strong hydrogen-bond donor. The benefits of ‘double activation’ for promoting halogen bond effectiveness was explored with nine haloethynylnitrobenzenes. The positive potential on halogen atoms was enhanced through a combination of an sp-hybridized carbon and electron-withdrawing nitro group(s). Iodoethynylnitrobenzenes were identified as the most effective halogen-bond donors reported to date and the compounds were exploited for the interaction preferences of nitro group and nitro⋯X-Csp interactions were identified as synthetic tools for energetic co-crystal assembly. A synthetic strategy for the deliberate assembly of molecular polygons was developed utilizing bifurcated halogen bonds constructed from N-oxides and complementary halogen-bond donors via co-crystallization. A convenient, effective, and scalable protocol for stabilizing volatile liquid chemicals with co-crystallization was achieved. Through the use of halogen-bonding, liquid iodoperfluoroalkanes were transformed into crystalline materials with low-vapor pressure, considerable thermal stability and moisture resistance. To stabilize the energetic compound ethylenedinitramine, a co-crystallization approach targeting the acidic protons was employed. Eight co-crystals were obtained and the acceptors were identified as supramolecular protecting groups leading to diminished reactivity and enhanced stability while retaining the desirable energetic properties.