Morphological and Phylogenetic Characterization of New Gephyrocapsa Isolates Suggests Introgressive Hybridization in the Emiliania/Gephyrocapsa Complex (Haptophyta)

The coccolithophore genus Gephyrocapsa contains a cosmopolitan assemblage of pelagic species, including the bloom-forming Gephyrocapsa oceanica, and is closely related to the emblematic coccolithophore Emiliania huxleyi within the Noëlaerhabdaceae. These two species have been extensively studied and are well represented in culture collections, whereas cultures of other species of this family are lacking. We report on three new strains of Gephyrocapsa isolated into culture from samples from the Chilean coastal upwelling zone using a novel flow cytometric single-cell sorting technique. The strains were characterized by morphological analysis using scanning electron microscopy and phylogenetic analysis of 6 genes (nuclear 18S and 28S rDNA, plastidial 16S and tufA, and mitochondrial cox1 and cox3 genes). Morphometric features of the coccoliths indicate that these isolates are distinct from G. oceanica and best correspond to G. muellerae. Surprisingly, both plastidial and mitochondrial gene phylogenies placed these strains within the E. huxleyi clade and well separated from G. oceanica isolates, making Emiliania appear polyphyletic. The only nuclear sequence difference, 1 bp in the 28S rDNA region, also grouped E. huxleyi with the new Gephyrocapsa isolates and apart from G. oceanica. Specifically, the G. muellerae morphotype strains clustered with the mitochondrial β clade of E. huxleyi, which, like G. muellerae, has been associated with cold (temperate and sub-polar) waters. Among putative evolutionary scenarios that could explain these results we discuss the possibility that E. huxleyi is not a valid taxonomic unit, or, alternatively the possibility of past hybridization and introgression between each E. huxleyi clade and older Gephyrocapsa clades. In either case, the results support the transfer of Emiliania to Gephyrocapsa. These results have important implications for relating morphological species concepts to ecological and evolutionary units of diversity.

Flash-Sintering and Characterization of La0.8Sr0.2Ga0.8Mg0.2O3-δ Electrolytes for Solid Oxide Fuel Cells

La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-δ (LSGM), a promising electrolyte material for intermediate temperature solid oxide fuel cells, can be sintered to a fully dense state by a flash-sintering technique. In this work, LSGM is sintered by the current-limiting flash-sintering process at 690°C under an electric field of 100 V cm^-1, in comparison with up to 1400°C or even higher temperature in conventional furnace sintering. The resultant LSGM samples are investigated by scanning electron microscopy, X-ray diffraction, and electrochemical impedance spectroscopy. The SEM images exhibit well-densified microstructures while XRD results show that the perovskite structure after flash-sintering does not changed. EIS results show that the conductivity of LSGM sintered by the current-limiting flash-sintering process increases with sintering current density value. The conductivity of samples sintered at 120 mA mm^-2 reaches 0.049 σ cm^-1 at 800°C, which is approximate to the value of conventional sintered LSGM samples at 1400°C. Additionally, the flash-sintering process is interpreted by Joule heating theory. Therefore, the current-limiting flash-sintering technique is proved to be an energy-efficient and eligible approach for the densification of LSGM and other materials requiring high sintering temperature.

Fabrication and characterization of SSZ tape cast electrolyte-supported solid oxide fuel cells

A 10 mol%Sc₂O₃, 1 mol%CeO₂ stabilized-ZrO₂ (SSZ) powder was successfully prepared using the sol-gel method. Subsequent SSZ electrolyte pellets were prepared by tape casting technique and sintered at 1400 °C, 1450 °C, 1500 °C, 1550 °C and 1600 °C. These were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). SSZ showed a pure cubic phase after sintering, the grain size of SSZ increased with the increase of sintering temperature. The SSZ sintered at 1550 °C showed the highest ion conductivity. The maximum power densities of Ni-SSZ/SSZ/La_0.8Sr_0.2MnO_3-δ (LSM)-SSZ single cells sintered at 1550 °C were 0.18, 0.36, 0.51 and 0.72 W cm^-2 at 650, 700, 750 and 800 °C, respectively. The polarization resistance (R_p) of the single cell attained 0.201 Ω cm² at 800 °C.

Microstructure characterization of 316L deformed at high strain rates using EBSD

Specimens from split Hopkinson pressure bar experiments, at strain rates between ~ 1000–9000 s− 1 at room temperature and 500 °C, have been studied using electron backscatter diffraction. No significant differences in the microstructures were observed at different strain rates, but were observed for different strains and temperatures. Size distribution for subgrains with boundary misorientations > 2° can be described as a bimodal lognormal area distribution. The distributions were found to change due to deformation. Part of the distribution describing the large subgrains decreased while the distribution for the small subgrains increased. This is in accordance with deformation being heterogeneous and successively spreading into the undeformed part of individual grains. The variation of the average size for the small subgrain distribution varies with strain but not with strain rate in the tested interval. The mean free distance for dislocation slip, interpreted here as the average size of the distribution of small subgrains, displays a variation with plastic strain which is in accordance with the different stages in the stress-strain curves. The rate of deformation hardening in the linear hardening range is accurately calculated using the variation of the small subgrain size with strain.

CHARACTERIZATION OF RADIATION DAMAGE TO A NOVEL PHOTONIC CRYSTAL SENSOR

New methods of nuclear fuel and cladding characterization must be developed and implemented to enhance the safety and reliability of nuclear power plants. One class of such advanced methods is aimed at the characterization of fuel performance by performing minimally intrusive in-core, real time measurements on nuclear fuel on the nanometer scale. Nuclear power plants depend on instrumentation and control systems for monitoring, control and protection. Traditionally, methods for fuel characterization under irradiation are performed using a “cook and look” method. These methods are very expensive and labor-intensive since they require removal, inspection and return of irradiated samples for each measurement. Such fuel cladding inspection methods investigate oxide layer thickness, wear, dimensional changes, ovality, nuclear fuel growth and nuclear fuel defect identification. These methods are also not suitable for all commercial nuclear power applications as they are not always available to the operator when needed. Additionally, such techniques often provide limited data and may exacerbate the phenomena being investigated. This thesis investigates a novel, nanostructured sensor based on a photonic crystal design that is implemented in a nuclear reactor environment. The aim of this work is to produce an in-situ radiation-tolerant sensor capable of measuring the deformation of a nuclear material during nuclear reactor operations. The sensor was fabricated on the surface of nuclear reactor materials (specifically, steel and zirconium based alloys). Charged-particle and mixed-field irradiations were both performed on a newly-developed “pelletron” beamline at Idaho State University's Research and Innovation in Science and Engineering (RISE) complex and at the University of Maryland's 250 kW Training Reactor (MUTR). The sensors were irradiated to 6 different fluences (ranging from 1 to 100 dpa), followed by intensive characterization using focused ion beam (FIB), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) to investigate the physical deformation and microstructural changes between different fluence levels, to provide high-resolution information regarding the material performance. Computer modeling (SRIM/TRIM) was employed to simulate damage to the sensor as well as to provide significant information concerning the penetration depth of the ions into the material.

Controlling Nanostructures for in-situ TEM Characterization

Low dimensional nanostructures, such as nanotubes and 2D sheets, have unique and promising material properties both from a fundamental science and an application standpoint. Theoretical modelling and calculations predict previously unobserved phenomena that experimental scientists often struggle to reproduce because of the difficulty in controlling and characterizing the small structures under real-world constraints. The goal of this dissertation is to controlling these structures so that nanostructures can be characterized in-situ in transmission electron microscopes (TEM) allowing for direct observation of the actual physical responses of the materials to different stimuli. Of most interest to this work are the thermal and electrical properties of carbon nanotubes, boron nitride nanotubes, and graphene. The first topic of the dissertation is using surfactants for aqueous processing to fabricate, store, and deposit the nanostructures. More specifically, thorough characterization of a new surfactant, ammonium laurate (AL), is provided and shows that this new surfactant outperforms the standard surfactant for these materials, sodium dodecyl sulfate (SDS), in almost all tested metrics. New experimental set-ups have been developed by combining specialized in-situ TEM holders with innovative device fabrication. For example, electrical characterization of graphene was performed by using an STM-TEM holder and depositing graphene from aqueous solutions onto lithographically patterned, electron transparent silicon nitride membranes. These experiments produce exciting information about the interaction between graphene and metal probes and the substrate that it rests on. Then, by adding indium to the backside of the membrane and employing the electron thermal microscopy (EThM) technique, the same type of graphene samples could be characterized for thermal transport with high spatial resolution. It is found that reduced graphene oxide sheets deposited onto a silicon nitride membrane and displaying high levels of wrinkling have higher than expected electrical and thermal conduction properties. We are clearly able to visualize the ability of graphene to spread heat away from an electronic hot spot and into the substrate.

Synthesis, characterization and chemical vapor deposition of transition metal di(tert-butyl)amido compounds

Although the transition metal chemistry of many dialkylamido ligands has been well studied, the chemistry of the bulky di(tert-butyl)amido ligand has been largely overlooked. The di(tert-butyl)amido ligand is well suited for synthesizing transition metal compounds with low coordination numbers; such compounds may exhibit interesting structural, physical, and chemical properties. Di(tert-butyl)amido complexes of transition metals are expected to exhibit high volatilities and low decomposition temperatures, thus making them well suited for the chemical vapor deposition of metals and metal nitrides. Treatment of MnBr₂(THF)₂, FeI₂, CoBr₂(DME), or NiBr₂(DME) with two equivalents of LiN(t-Bu)2 in benzene affords the two-coordinate complex M[N(t-Bu)₂]₂, where M is Mn, Fe, Co, or Ni. Crystallographic studies show that the M-N distances decrease across the series: 1.9365 (Mn), 1.8790 (Fe), 1.845 (Co), 1.798 Å (Ni). The N-M- N angles are very close to linear for Mn and Fe (179.30 and 179.45°, respectively), but bent for Co and Ni (159.2 and 160.90°, respectively). As expected, the d⁵ Mn complex has a magnetic moment of 5.53 μΒ that is very close to the spin only value. The EPR spectrum is nearly axial with a low E/D ratio of 0.014. The d⁶ Fe compound has a room temperature magnetic moment of 5.55 μΒ indicative of a large orbital angular momentum contribution. It does not exhibit a Jahn-Teller distortion despite the expected doubly degenerate ground state. Applied field Mössbauer spectroscopy shows that the effective internal hyperfine field is unusually large, Hint = 105 T. The magnetic moments of Co[N(t-Bu)₂]₂ and Ni[N(t-Bu)₂]₂ are 5.24 and 3.02 μΒ respectively. Both are EPR silent at 4.2 K. Treatment of TiCl₄ with three equivalents of LiN(t-Bu)2 in pentane affords the briding imido compound Ti₂[μ-N(t-Bu)]₂Cl₂[N(t-Bu)₂]₂ via a dealkylation reaction. Rotation around the bis(tert-butyl)amido groups is hindered, with activation parameters of ΔH‡ = 12.8 ± 0.6 kcal mol-1 and ΔS‡ = -8 ± 2 cal K-1 ·mol-1, as evidenced by variable temperature 1H NMR spectroscopy. Treatment of TiCl₄ with two equivalents of HN(t-Bu)₂ affords Ti₂Cl₆[N(t-Bu)₂]₂. This complex shows a close-contact of 2.634(3) Å between Ti and the carbon atom of one of the CH₃ substituents on the tert-butyl groups. Theoretical considerations and detailed structural comparisons suggest this interaction is not agostic in nature, but rather is a consequence of interligand repulsions. Treatment of NiI₂(PPh3)₂ and PdCl₂(PPh₃)₂ with LiN(t-Bu)₂in benzene affords Ni[N(t-Bu)₂](PPh₃)I and Pd₃(μ₂-NBut₂)2(μ₂-PPh₂)Ph(PPh₃) respectively. The compound Ni[N(t-Bu)₂](PPh₃)I has distorted T-shape in geometry, whereas Pd₃(μ₂-NBut₂)₂(μ₂-PPh₂)Ph(PPh₃) contains a triangular palladium core. Manganese nitride films were grown from Mn[N(t-Bu)₂]₂ in the presence of anhydrous ammonia. The growth rate was several nanometers per minute even at the remarkably low temperature of 80⁰C. As grown, the films are carbon- and oxygen-free, and have a columnar morphology. The spacings between the columns become smaller and the films become smoother as the growth temperature is increased. The composition of the films is consistent with a stoichiometry of Mn₅N₂.

Low temperature magneto-photoluminescence characterization of high purity gallium arsenide and indium phosphide

Low-temperature magneto-photoluminescence is a very powerful technique to characterize high purity GaAs and InP grown by various epitaxial techniques. These III-V compound semiconductor materials are used in a wide variety of electronic, optoelectronic and microwave devices. The large binding energy differences of acceptors in GaAs and InP make possible the identification of those impurities by low-temperature photoluminescence without the use of any magnetic field. However, the sensitivity and resolution provided by this technique rema1ns inadequate to resolve the minute binding energy differences of donors in GaAs and InP. To achieve higher sensitivity and resolution needed for the identification of donors, a magneto-photoluminescence system 1s installed along with a tunable dye laser, which provides resonant excitation. Donors 1n high purity GaAs are identified from the magnetic splittings of "two-electron" satellites of donor bound exciton transitions 1n a high magnetic field and at liquid helium temperature. This technique 1s successfully used to identify donors 1n n-type GaAs as well as 1n p-type GaAs in which donors cannot be identified by any other technique. The technique is also employed to identify donors in high purity InP. The amphoteric incorporation of Si and Ge impurities as donors and acceptors in (100), (311)A and (3ll)B GaAs grown by molecular beam epitaxy is studied spectroscopically. The hydrogen passivation of C acceptors in high purity GaAs grown by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) 1s investigated using photoluminescence. Si acceptors ~n MBE GaAs are also found to be passivated by hydrogenation. The instabilities in the passivation of acceptor impurities are observed for the exposure of those samples to light. Very high purity MOCVD InP samples with extremely high mobility are characterized by both electrical and optical techniques. It is determined that C is not typically incorporated as a residual acceptor ~n high purity MOCVD InP. Finally, GaAs on Si, single quantum well, and multiple quantum well heterostructures, which are fabricated from III-V semiconductors, are also measured by low-temperature photoluminescence.

Characterization of nanoparticle mediated laser transfection by femtosecond laser pulses for applications in molecular medicine

Background: In molecular medicine, the manipulation of cells is prerequisite to evaluate genes as therapeutic targets or to transfect cells to develop cell therapeutic strategies. To achieve these purposes it is essential that given transfection techniques are capable of handling high cell numbers in reasonable time spans. To fulfill this demand, an alternative nanoparticle mediated laser transfection method is presented herein. The fs-laser excitation of cell-adhered gold nanoparticles evokes localized membrane permeabilization and enables an inflow of extracellular molecules into cells. Results: The parameters for an efficient and gentle cell manipulation are evaluated in detail. Efficiencies of 90% with a cell viability of 93% were achieved for siRNA transfection. The proof for a molecular medical approach is demonstrated by highly efficient knock down of the oncogene HMGA2 in a rapidly proliferating prostate carcinoma in vitro model using siRNA. Additionally, investigations concerning the initial perforation mechanism are conducted. Next to theoretical simulations, the laser induced effects are experimentally investigated by spectrometric and microscopic analysis. The results indicate that near field effects are the initial mechanism of membrane permeabilization. Conclusion: This methodical approach combined with an automated setup, allows a high throughput targeting of several 100,000 cells within seconds, providing an excellent tool for in vitro applications in molecular medicine. NIR fs lasers are characterized by specific advantages when compared to lasers employing longer (ps/ns) pulses in the visible regime. The NIR fs pulses generate low thermal impact while allowing high penetration depths into tissue. Therefore fs lasers could be used for prospective in vivo applications.

Characterization of Copper Covetic Bulk and Films: Copper with High Carbon Content

Incorporation of carbon nanostructures in metals is desirable to combine the strongly bonded electrons in the metal and the free electrons in carbon nanostructures that give rise to high ampacity and high conductivity, respectively. Carbon in copper has the potential to impact industries such as: building construction, power generation and transmission, and microelectronics. This thesis focuses on the structure and properties of bulk and thin films of a new material, Cu covetic, that contains carbon in concentrations up to 16 at.%. X-ray photoelectron spectroscopy (XPS) shows C 1s peak with both sp2 and sp3 bonded C measuring up to 3.5 wt.% (16 at.%). High resolution transmission electron microscopy and electron diffraction of bulk covetic samples show a modulated structure of ≈ 1.6 nm along several crystallographic directions in regions that have high C content suggesting that the carbon incorporates into the copper lattice forming a network. Electron energy loss spectra (EELS) from covetics reveal that the level of graphitization from the source material, activated carbon, is maintained in the covetic structure. Bulk Cu covetics have a slight increase in the lattice constant, as well as <111> texturing, or possibly a different structure, compared to pure Cu. Density functional theory calculations predict bonding between C and Cu at the edges and defects of graphene sheets. The electrical resistivity of bulk covetics first increases and then decreases with increasing C content. Cu covetic films were deposited using e-beam and pulsed laser deposition (PLD) at different temperatures. No copper oxide or any allotropes of carbon are present in the films. The e-beam films show enhanced electrical and optical properties when compared to pure Cu films of the same thickness even though no carbon was detected by XPS or EELS. They also have slightly higher ampacity than Cu metal films. EELS analysis of the C-K-edge in the PLD films indicate that graphitic carbon is transferred from the bulk into the films with uniform carbon distribution. PLD films exhibit flatter and higher transmittance curves and sheet resistance two orders of magnitude lower than e-beam films leading to a high figure of merit as transparent conductors.

Preparation, Characterization, and Photocatalytic Applications of MWCNTs/TiO2 Composite

The multiwall carbon nanotubes (MWCNTs)/titanium dioxide (P25) composite in different ratios was prepared using simple evaporation and drying process. The composite was characterized by Raman spectroscopy, X-ray diffraction (XRD), UV-visible diffuse reflectance spectroscopy, and scanning electron microscopy (SEM). The photocatalytic activity of this composite was investigated using degradation of the Bismarck brown R dye (BBR). An optimal MWCNTs/TiO2 ratio of 0.5% (w/w) was found to achieve the maximum rate of BBR degradation. It was observed that the composite exhibits enhanced photocatalytic activity compared with TiO2. The enhancement in photocatalytic activity performance of the MWCNTs/P25 composite is explained in terms of recombination of photogenerated electron-hole pairs. In addition, MWCNTs act as a dispersing support to control the morphology of TiO2 particles in the MWCNTs/TiO2 composite.