abstract: The two-dimensional electron gas (2DEG) at SrTiO3-based oxide interfaces has been extensively studied recently for its high carrier density, high electron mobility, superconducting, ferromagnetic, ferrroelectric and magnetoresistance properties, with possible application for all-oxide devices. Understanding the mechanisms behind the 2DEG formation and factors affecting its properties is the primary objective of this dissertation. Advanced electron microscopy techniques, including aberration-corrected electron microscopy and electron energy-loss spectroscopy (EELS) with energy-loss near-edge structure (ELNES) analysis, were used to characterize the interfaces. Image and spectrum data-processing algorithms, including subpixel atomic position measurement, and novel outlier detection by oversampling, subspace division based EELS background removal and bias-free endmember extraction algorithms for hyperspectral unmixing and mapping were heavily used. Results were compared with density functional theory (DFT) calculations for theoretical explanation. For the γ-Al2O3/SrTiO3 system, negative-Cs imaging confirmed the formation of crystalline γ-Al2O3. ELNES hyperspectral unmixing combined with DFT calculations revealed that oxygen vacancies, rather than polar discontinuity, were the key to the 2DEG formation. The critical thickness can be explained by shift of the Fermi level due to Ti out diffusion from the substrate to the film. At the LaTiO3/SrTiO3 interface, aberration-corrected imaging showed crystallinity deterioration in LaTiO3 films a few unit cells away from the interface. ELNES showed that oxygen annealing did not alter the crystallinity but converted Ti3+ near the interface into Ti4+, which explained disappearance of the conductivity. At the EuO/SrTiO3 interface, both high-resolution imaging and ELNES confirmed EuO formation. ELNES hyperspectral unmixing showed a Ti3+ layer confined to within several unit cells of the interface on the SrTiO3 side, confirming the presence of oxygen vacancies. At the BaTiO3/SrTiO3 interface, spontaneous polarization and local lattice parameters were measured directly in each unit cell column and compared with oxidation state mapping using ELNES with unit-cell resolution. The unusually large polarization near the interface and the polarization gradient were explained by oxygen vacancies and the piezoelectric effect due to epitaxial strain and strain gradient from relaxation. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2018

abstract: As selenium and chromium are toxic even at low levels, it is very necessary to remove them from drinking water with proper ways. In this work, titanium dioxide based photocatalysts were mainly investigated in detail for their photoreduction ability towards selenate and chromate in aqueous environment. Firstly, photoreduction ability of layered double hydroxide (LDH) nanosheets with commercial TiO2 particle hybrid materials was investigated towards selenate or chromate. The results showed that commercial LDH/TiO2 (P90) composite, homemade LDH nanosheets/TiO2 (P90) composite and also in situ LDH/TiO2 (P25) composite all did not indicate significant improvement on photoreduction performance towards selenate or chromate. Secondly, TiO2 nanosheets material was synthesized with TiS2 as precursor via hydrothermal treatment. Morphology of TiO2 nanosheets were characterized by SEM, AFM and TEM. Photodegradation of MB (methylene blue) with TiO2 nanosheets was performed. In the future, first approach is to synthesize visible-light driven LDH photocatalyst NiFe-LDH nanosheets with TiO2 nanosheets hybrid material for selenate removal. Second approach is to use anion intercalation/insertion via electrochemical process to remove anions in drinking water. / Dissertation/Thesis / Masters Thesis Materials Science and Engineering 2018

abstract: The electrochemical behavior of nanoscale solids has become an important topic to applications, such as catalysis, sensing, and nano–electronic devices. The electrochemical behavior of elemental metal and alloy particles was studied in this work both theoretically and experimentally. A systematic thermodynamic derivation for the size–dependent Pourbaix Diagram for elemental metal particles is presented. The stability of Pt particles was studied by in situ electrochemical scanning tunneling microscopy (ECSTM). It is shown that small Pt particles dissolve at a lower potential than the corresponding bulk material. For the alloy particles, two size ranges of AuAg particles, ∼4 nm and ∼45 nm in diameter, were synthesized by co–reduction of the salts of Au and Ag from an aqueous phase. The alloy particles were dealloyed at a series of potential by chronoamperometry in acid, and the resulting morphology and composition were characterized by electron microscopy, energy dispersive X–ray spectroscopy (EDX). In the case of the smaller particles, only surface dealloying occurred yielding a core–shell structure. A porous structure was observed for the larger particles when the potential was larger than a critical value that was within 50 mV of the thermodynamic prediction. / Dissertation/Thesis / Ph.D. Engineering 2012

abstract: There has been much interest in photoelectrochemical conversion of solar energy in recent years due to its potential for low-–cost, sustainable and renewable production of fuels. Despite the huge potential, there are still a number of technical barriers due to the many constraints needed in order to drive photoelectrochemical reactions such as overall water splitting and the identification of efficient and effective semiconductor materials. To this end, the search for novel semiconductors that can act as light absorbers is still needed. The copper hydroxyphosphate mineral libethenite (CHP), which has a chemical formula of Cu2(OH)PO4, has been recently shown to be active for photocatalytic degradation of methylene blue under UV-–irradiation, indicating that photo-excited electrons and holes can effectively be generated and separated in this material. However, CHP has not been well studied and many of its fundamental electrochemical and photoelectrochemical properties are still unknown. In this work, the synthesis of different morphologies of CHP using hydrothermal synthesis and precipitation methods were explored. Additionally, a preliminary investigation of the relevant fundamental characteristics such as the bandgap, flatband potential, band diagram, electrochemical and photoelectrochemical properties for CHP was performed. Better understanding of the properties of this material may lead to the development of improved catalysts and photocatalysts from natural sources. / Dissertation/Thesis / M.S. Materials Science and Engineering 2013

<p> The mitigation of greenhouse gas emissions on the environment led to the development of non-polluting hydrogen fuel cell use in automobiles. Syngas produced from coal gasification is converted to H₂ and CO2 gasses by the water shift reaction. Metallic membranes are used to separate H₂ from CO₂ and other gasses obtained from the water shift reaction of coal-derived syngas. Commercial crystalline Pd-Ag membranes are widely used for this purpose; however, Pd is an expensive strategic metal. Thus, inexpensive Ni-Nb-Zr alloys are studied.The permeation property of amorphous membranes are known, however, the mechanism of permeation and the nature of the local atomic order of the amorphous membranes was not fully understood. </p><p> In this study, atom dynamics studied by synchrotron x-ray photon correlation spectroscopy (XPCS) showed the movement of heavier elements such as Ni, Nb, and Zr, at room temperature and 373K. The addition of hydrogen significantly accentuates the motion of atoms as the hydrogen occupies the tetrahedral sites within the icosahedra leading to expansion and short-range diffusion, and no long-range diffusion is observed estimated to be &sim;10^-22 m²/s. Vacuum removal of hydrogen from these membranes showed a contraction of the icosahedra and approached to its original position. This suggests that the process reversible due to the pressure gradient. The XPCS results did not reveal the specific position of hydrogen atoms in the icosahedra; hydrogen goes into the tetrahedral sites of Zr₄ and distorted Nb₄ sites as determined by neutron vibrational spectroscopy. Total neutron scattering and DFT-MD simulation determine the short-range order of up to 1.8 nm and the nearest neighbor bond distances. Determination of cluster formation was first attempted by using small neutron scattering, but it did not have appropriate &ldquo;Q&rdquo; range. Thus atom probe tomography (APT) was attempted. This APT study revealed Nb-rich and Zr-rich clusters embedded in Ni-rich matrix, whose compositions are reported. DFT-MD simulation reveals interconnected icosahedra in the metal matrix. The atom dynamics (NVS and XPCS), atom probe tomography, total neutron scattering studies are discussed which have implication in the mechanisms of hydrogen permeation in amorphous metallic membranes.</p><p>

<p> This study investigated localized corrosion behavior of austenitic stainless steels under stressed and unstressed conditions, as well as corrosion of metallic thin films. While austenitic stainless steels are widely used in corrosive environments, they are vulnerable to pitting and stress corrosion cracking (SCC), particularly in chloride-containing environments. The corrosion resistance of austenitic stainless steels is closely tied to the alloying elements chromium, nickel, and molybdenum. Polarization curves were measured for five commercially available austenitic stainless steels of varying chromium, nickel, and molybdenum content in 3.5 wt.% and 25 wt.% NaCl solutions. The alloys were also tested in tension at slow strain rates in air and in a chloride environment under different polarization conditions to explore the relationship between the extent of pitting corrosion and SCC over a range of alloy content and environment. The influence of alloy composition on corrosion resistance was found to be consistent with the pitting resistance equivalent number (PREN) under some conditions, but there were also conditions under which the model did not hold for certain commercial alloy compositions. Monotonic loading was used to generate SCC in in 300 series stainless steels, and it was possible to control the failure mode through adjusting environmental and polarization conditions. Metallic thin film systems of thickness 10-200 nm are being investigated for use as corrosion sensors and protective coatings, however the corrosion properties of ferrous thin films have not been widely studied. The effects of film thickness and substrate conductivity were examined using potentiodynamic polarization and scanning vibrating electrode technique (SVET) on iron thin films. Thicker films undergo more corrosion than thinner films in the same environment, though the corrosion mechanism is the same. Conductive substrates encourage general corrosion, similar to that of bulk iron, while insulating substrates supported only localized corrosion.</p><p>

<p> Abrupt changes in grain boundary energy (GBE) and character are associated with a change in GB complexion, GB mobility and exaggerated grain growth. In this thesis, complexion transitions and their effect of relative GBE and GB character distributions (GBCD) are studied for ceramics: europium doped spinel (Eu doped spinel), yttria doped (Y-doped), undoped alumina (Al₂O₃), and strontium titanate (SrTiO₃). </p><p> The population of GB planes (GBPD) of Eu doped spinel was investigated at temperatures before and after a previously identified a complexion transition. The microstructure changed from unimodal (1400 &deg;C) to bimodal (1600 &deg;C) and the relative area of {111} increased. This information was used to understand the GBs involved in the transition. </p><p> Atomic force microscopy was used to measure relative GBE from thermal grooves on the surfaces of Al₂O₃, 100 ppm Y-doped Al₂O₃, and 500 ppm Y-doped Al₂O₃ heated between 1350 &deg;C and 1650 &deg;C. The relative GBE of Al₂O₃ decreased slightly with increased temperature. When the doped samples were heated, there was an overall increase in the relative GBE, interrupted by abrupt reductions (increases) in relative GBE (mobility) between 1450 &deg;C and 1550 &deg;C. When the 100 ppm Y-doped sample was cooled, there was an increase in the relative GBE at the same complexion transition temperature, indicating that the transition is reversible. </p><p> Relative GBE was measured from thermal grooves on (SrTiO₃ in a region of non-Arrhenius grain growth. Between 1350 &deg;C and 1390 &deg;C, an abrupt decrease in relative GBE and exaggerated grain growth indicated that a complexion transition occurred. Solidified liquid eutectic wet the boundaries at 1550 &deg;C so these GB interfaces could not be compared to the lower temperatures. </p><p> 3D microstructures of samples heated below (1350 &deg;C), within (1390 &deg;C), and above (1425 &deg;C) the non-Arrhenius region in (SrTiO₃ were measured. Internal dihedral angles indicated that slowly growing grains have lower GBE than more rapidly growing grains. Low GBE grains increased in fraction with increased temperature until 1425 &deg;C. The GBE distribution and GBCD at 1425 &deg;C indicated a correlated high population and low energy for the {001}, the (111) sigma 3 and (2&macr;21) sigma 9 GBs. </p><p>

<p> Heusler based functional or smart materials are a deep well of solutions to future energy, heat transport and mechanization problems. The half-metallic ferromagnetic nature of these crystalline intermetallic compounds is the source of their extraordinary properties. The loss of this magnetic ordering places limits on the range of application temperatures making knowledge of the Curie point of these novel materials essential for understanding of their limitations. High throughput continuous wavelet transform spectrum analysis of magnetic balance data generated on a custom modified Setaram Setsys Evolution 16/18 Differential Scanning Calorimeter-Differential Thermal Analyzer with simultaneous Thermogravimetric Analyzer was performed on select Fe, Co and Mn based Heusler compounds. The phase stability of Co-Fe-Si compounds is explored in relation to the high-Curie Temperature <i>Co2FeSi</i> and <i>Fe2CoSi </i> compounds via generation of equilibrium ternary isothermal phase diagrams at 1160 &deg;C and 800 &deg;C to enable greater control of the microstructure for future thermomechanically processed bulk smart device fabrication.</p>

Despite advances in diagnostic procedures and treatments, the overall survival rate from cancer has not improved substantially over the past 30 years. One promising development is the encapsulation of toxic cancer chemotherapeutic reagents within biocompatible nanocomposite materials. The targeted stimuli triggered drug release restrict the toxic drugs to the tumour site, thereby reducing the effects of “free drug” on healthy tissues. One of the most versatile and safe materials used in medicine are iron oxide nanoparticles. This project describes the development of several formulations based on magnetite nanoparticles for drug delivery applications. Utilising magnetic nanoparticles in drug delivery systems allowed for the synergistic effects of hyperthermia and heat triggered drug released. The drug delivery systems developed in this project include magnetoliposomes, magnetic micelles, mesoporous silica-magnetite core-shell nanoparticles, liposome capped mesoporous silica-magnetite core-shell nanoparticles (protocells) and polymer capped mesoporous silica-magnetite core-shell nanoparticles. The drug loading and release profiles of the developed nanomaterials were assessed using two different anticancer drugs; Mitomycin C (MMC) and Doxorubicin (DOX). The drug loading content and drug loading efficiency for different nanocomposites ranged from 0.48 to 10.30% and 16.16 to 85.85%, respectively. Drug release profiles were studied in vitro at 37°C at pH 5.5 and pH 7.4 and at hyperthermia elevated temperature of 43°C to evaluate the effects of pH and temperature on the release profiles. An AC magnetic field with frequency of 406 kHz and variable field of up to 200 G was used to induce magnetic heating and keep the temperature within hyperthermia treatment range. Compared to uncapped mesoporous silica nanoparticles capping the mesopores of the silica nanoparticles with liposome or polymer reduced the drug release by 52.7% and 41.5%, respectively. The efficacy of doxorubicin-containing nanoparticles were evaluated in vitro against breast cancer and glioblastoma cell lines where different formulations demonstrated comparable or increased cytotoxicity compared to free drug. The cells treated with DOX loaded nanoparticles and hyperthermia demonstrated up to 89% lower viability compared to cells treated with free DOX. Silica coated magnetic nanoparticles were also used as enzymes (Pseudomonas Fluorescens Lipase (PFL) and Candida Rugosa Lipase (CRL)) supports in catalysis reactions. The enzymes were immobilised onto nanoparticles through physical adsorption and chemical bonding. The immobilised lipases were used in hydrolysis of pNPP and hydrolysis of cis-3,5-diacetoxy-1-cyclopentene to investigate the catalytic activity of the immobilized enzymes compared to free enzymes. The results indicated that free lipases provided slightly higher conversion than immobilised lipases in the first cycle however, the immobilised lipases were easily recycled and reused in sequential cycles which provides higher total yield per mg of lipase. The chemically immobilised lipase exhibited good reusability without loss of its activity in sequential cycles, however the physically adsorbed lipase showed reduced activity which could be explained by loss of enzyme during recycling between successive reactions. The CRL lipase activity were further assessed in the presence of an AC field where the results showed that exposure to the AC magnetic field resulted in increased lipase activity. The effect of reaction temperature on immobilised lipase activity were studied by performing the hydrolysis of cis-3,5-diacetoxy-1-cyclopentene at two temperatures of 25°C and 37°C where it was observed that both lipases exhibited higher activity at higher temperature which could be due to the fact that for PFL and CRL the optimum temperature is close to 37°C.

I. Crack wake bridges were studied in glassy alumina s-DCB specimens at elevated temperatures. Direct observation of fracture surfaces in unmodified s-DCB specimens revealed the sizes and shapes of the high temperature bridges in the crack wake relative to the position of the crack front. Similar fracture surface observations made on s-DCB specimens modified to include a rear notch wedge, allowing the measurement of the high temperature bridging forces, were used to analyze bridge behavior in terms of viscous and power law creep models. The sizes and positions of the bridges were found to be highly stochastic but there was a clear trend toward decreased area bridged with increasing COD. Simple uniaxial viscous or creeping columns were inadequate to explain the load supported by the bridges. II. R-curve behavior and slow crack growth were studied in 99.5% alumina chevron notched short bar specimens at temperatures up to $1200\sp\circ\rm C.$ Constant loading rate tests measured toughness as a function of crack length at various loading rates. Constant load tests measured subcritical crack growth as a function of time. The intrinsic toughness decreased with increasing temperature while the bridging contribution to the R-curve remained independent of temperature. The effect of loading rate on R-curve tests was masked by experimental scatter at all temperatures except $1200\sp\circ\rm C.$ Subcritical crack growth tests showed high n-values at 700 and $1000\sp\circ\rm C,$ indicative of region III type behavior and an n-value indicative of region I at $1200\sp\circ\rm C.$ Predictions using the obtained subcritical crack growth parameters showed a small decrease in the level of the R-curve with decreasing loading rate at 700 and $1000\sp\circ\rm C$ that was within the range of the experimental scatter. A discrepancy between the R-curve predictions and the data at $1200\sp\circ\rm C$ was attributed to a rate dependence of the bridging behavior that was not considered in the analysis.