22,524 research outputs found
Determining the gluonic content of isoscalar mesons
No full textWe develop tools to determine the gluonic content of a resonance of known mass, width, and J(PC) from its branching fraction in radiative quarkonium decays and production cross section in gamma gamma collisions. We test the procedures by applying them to known <q(q)over bar> mesons, then analyze four leading glueball candidates. We identify inconsistencies in data for J/psi-->gamma f(0)(1500) and J/psi-->f(J)(1710) whose resolution can quantify their glueball status. When Gamma(f(0)(1500)-->gamma gamma) and Gamma(f(J)(1710)-->gamma gamma) are known, the <n(n)over bar>,<s(s)over bar>,gg mixing angles can be determined. The enigmatic situation in the 1400-1500 MeV region of the isosinglet 0(-+) sector is discussed.Astronomy & AstrophysicsPhysics, Particles & FieldsSCI(E)0ARTICLE95749-57665
Thermoelasticity of Hexagonal Close-Packed Iron from the Phonon Density of States
No full textIron is the main constituent in Earth’s core, along with ~5 to 10 wt% Ni and some light elements (e.g., H, C, O, Si, S). This thesis explores the vibrational thermodynamic and thermoelastic properties of pure hexagonal close-packed iron (ε-Fe), in an effort to improve our understanding of the properties of a significant fraction of this remote region of the deep Earth and in turn, better constrain its composition.
In order to access the vibrational properties of pure ε-Fe, we directly probed its total phonon density of states (DOS) by performing nuclear resonant inelastic x-ray scattering (NRIXS) and in situ x-ray diffraction (XRD) experiments at Sector 3-ID-B of the Advanced Photon Source (APS) at Argonne National Laboratory. NRIXS and in situ XRD were collected over the course of ~14 days at eleven compression points between 30 and 171 GPa, and at 300 K. Our in situ XRD measurements probed the sample volume at each compression point, and our long NRIXS data-collection times and high-energy resolution resulted in the highest statistical quality dataset of this type for ε-Fe to outer core pressures. Hydrostatic conditions were achieved in the sample chamber for our experiments at smaller compressions (P ≤ 69 GPa) via the loading of a neon pressure transmitting medium at the GeoSoilEnviroCARS (GSECARS) sector of the APS. For measurements made at P > 69 GPa, the sample was fully embedded in boron epoxy, which served as the pressure transmitting medium.
From each measured phonon DOS and thermodynamic definitions, we determined a wide range of vibrational thermodynamic and thermoelastic parameters, including the Lamb-Mössbauer factor; vibrational components of the specific heat capacity, free energy, entropy, internal energy, and kinetic energy; and the Debye sound velocity. Together with our in situ measured volumes, the shape of the total phonon DOS and these parameters gave rise to a number of important properties for ε-Fe at Earth’s core conditions.
For example, we determined the Debye sound velocity (vD) at each of our compression points from the low-energy region of the phonon DOS and our in situ measured volumes. In turn, vD is related to the compressional and shear sound velocities via our determined densities and the adiabatic bulk modulus. Our high-statistical quality dataset places a new tight constraint on the density dependence of ε-Fe’s sound velocities to outer core pressures. Via comparison with existing data for iron alloys, we investigate how nickel and candidate light elements for the core affect the thermoelastic properties of iron. In addition, we explore the effects of temperature on ε-Fe’s sound velocities by applying pressure- and temperature-dependent elastic moduli from theoretical calculations to a finite-strain model. Such models allow for direct comparisons with one-dimensional seismic models of Earth’s solid inner core (e.g., the Preliminary Reference Earth Model).
Next, the volume dependence of the vibrational free energy is directly related to the vibrational thermal pressure, which we combine with previously reported theoretical values for the electronic and anharmonic thermal pressures to find the total thermal pressure of ε-Fe. In addition, we found a steady increase in the Lamb-Mössbauer factor with compression, which suggests restricted thermal atomic motions at outer core pressures. This behavior is related to the high-pressure melting behavior of ε-Fe via Gilvarry’s reformulation of Lindemann’s melting criterion, which we used to obtain the shape of ε-Fe’s melting curve up to 171 GPa. By anchoring our melting curve shape with experimentally determined melting points and considering thermal pressure and anharmonic effects, we investigated ε-Fe’s melting temperature at the pressure of the inner–core boundary (ICB, P = 330 GPa), where Earth’s solid inner core and liquid outer core are in contact. Then, combining this temperature constraint with our thermal pressure, we determined the density of ε-Fe under ICB conditions, which offers information about the composition of Earth’s core via the seismically inferred density at the ICB.
In addition, the shape of the phonon DOS remained similar at all compression points, while the maximum (cutoff) energy increased regularly with decreasing volume. As a result, we were able to describe the volume dependence of ε-Fe’s total phonon DOS with a generalized scaling law and, in turn, constrain the ambient temperature vibrational Grüneisen parameter. We also used the volume dependence of our previously mentioned vD to determine the commonly discussed Debye Grüneisen parameter (γD), which we found to be ~10% smaller than our vibrational Grüneisen parameter at any given volume. Finally, applying our determined vibrational Grüneisen parameter to a Mie-Grüneisen type relationship and an approximate form of the empirical Lindemann melting criterion, we predict the vibrational thermal pressure and estimate the high-pressure melting behavior of ε-Fe at Earth’s core pressures, which can be directly compared with our previous results.
Finally, we use our measured vibrational kinetic energy and entropy to approximate ε-Fe’s vibrational thermodynamic properties to outer core pressures. In particular, the vibrational kinetic energy is related to the pressure- and temperature-dependent reduced isotopic partition function ratios (β-factors) of ε-Fe and in turn, provide information about the partitioning behavior of solid iron in equilibrium processes. In addition, the volume dependence of vibrational entropy is directly related to the product of ε-Fe’s vibrational component of the thermal expansion coefficient and the isothermal bulk modulus, which we find to be independent of pressure (volume) at 300 K. In turn, this product gives rise to the volume-dependent thermal expansion coefficient of ε-Fe at 300 K via established EOS parameters, and the vibrational Grüneisen parameter and temperature dependence of the vibrational thermal pressure via thermodynamic definition.</p
Physiological and Mechanistic Studies of Phototrophic Fe(II) Oxidation in Purple Non-sulfur Bacteria
No full textPhototrophic Fe(II)-oxidizing bacteria use electrons from ferrous iron [Fe(II)] and energy from light to drive reductive CO₂ fixation. This metabolism is thought to be ancient in origin, and plays an important role in environmental iron cycling. It has been implicated in the deposition of Banded Iron Formations, a class of ancient sedimentary iron deposits. Consistent with this hypothesis, we discovered that hydrogen gas, a thermodynamically favorable electron donor to Fe(II), in an Archean atmosphere would not have inhibited phototrophic Fe(II) oxidation. To understand this physiology and the connection to BIF formation at the molecular level, the mechanisms of phototrophic Fe(II) oxidation were examined in two purple non-sulfur bacteria, Rhodopseudomonas palustris TIE-1 and Rhodobacter sp. SW2.
Important advances were made in elucidating genes critical to phototrophic Fe(II) oxidation. In R. palustris TIE-1, the first genetically tractable phototrophic Fe(II) oxidizer isolated, transposon mutagenesis identified a putative integral membrane protein and a potential cobalamin (vitamin B₁₂) biosynthesis protein involved in Fe(II) oxidation.
Increased expression of a putative decaheme c-type cytochrome, encoded by pioA, was observed when cells were grown under Fe(II)-oxidizing conditions. Two genes located immediately downstream of pioA in the same operon, pioB and pioC, encode a putative outer membrane beta-barrel protein and a putative high potential iron-sulfur protein, respectively. Deletion studies demonstrated that all three genes are involved in phototrophic Fe(II) oxidation.
In Rhodobacter sp. SW2, a three-gene operon, foxEYZ, was found to be involved in phototrophic Fe(II) oxidation through heterologous expression in a close relative, Rhodobacter capsulatus SB1003. The first gene, foxE, encodes a novel c-type cytochrome located in the periplasm. Expression of foxE alone confers light-dependent Fe(II) oxidation activity to SB1003, but maximal activity is achieved when foxE is co-expressed with foxY and foxZ. FoxY appears to contain the redox cofactor pyrroloquinoline quinone and FoxZ a cytoplasmic membrane transporter. Recombinant PioC was overexpressed and partially purified from Escherichia coli.
This research presents a detailed study of the physiology and genetics of phototrophic Fe(II) oxidation in two purple non-sulfur bacteria, and provides our first insight into the molecular mechanisms of this metabolism.</p
A study of breakwater gap wave diffraction using close range photogrammetry and finite and infinite elements.
No full textIn this thesis the diffraction of water waves passing through a gap in a breakwater is investigated experimentally, using close range photogrammetry, and numerically, using finite and infinite elements. The author was particularly interested in validating specific breakwater gap diffraction diagrams given in popular coastal engineering design manuals. Breakwater gap configurations with the following gap width to wave length ratios (B/L ratios) were analysed, both experimentally and numerically, namely: B/L = 1,64; 1,41; 1,2; 1; 0,75; 0,5. These configurations are symmetrical, i.e. both breakwater arms lie on the same straight line. An asymmetrical B/L = 1,64 breakwater gap configuration was also analysed. Previous experimental breakwater gap diffraction investigations are reviewed leading to the conclusion that the reported results are inconclusive due to (1) the relatively poor accuracy with which the wave heights were measured and (2) secondary basin effects which were superimposed upon and thus distorted the pure diffraction phenomena. In the experimental breakwater gap configurations investigated by the author, splitter plates were used to eliminate the reflection problems on the seaward side of the breakwaters, whilst a novel photogrammetric wave height measurement technique was used to measure accurately the wave heights in the entire basin, before they could be distorted by reflecting waves, basin resonance effects, etc. This "infinite basin technique" was used to simulate experimentally and measure the diffraction of a continuous wave train entering an infinite basin via a gap in an approximate totally absorbing breakwater. A number of different photogrammetric wave height measurement techniques based on analogue procedures, the theory of projective transformations, and the theory of the deformed reference plane, are investigated and developed. It was found that the technique based on the projective transformation theory, and in which the plates are analysed using a stereo-comparator linked to a microcomputer, is the most accurate. Using this technique it was found that, with the cameras situated approximately 5 m above the water surface, the wave heights in the basin can be measured with an accuracy of better than 2 mm. The above method, in conjunction with the infinite basin technique, was used to analyse the experimental breakwater gap configurations. The basic linear wave theory is described leading to the derivation of the Helmholtz diffraction equation. The classical diffraction theories for the semi-infinite breakwater and breakwater gap configurations are reviewed and compared. The Better-off refraction - diffraction equation is then briefly derived. A review of previous numerical refraction - diffraction investigations, and also of modern numerical methods for water wave diffraction and refraction-diffraction, is given. This review led to the adoption of the finite and infinite element program "WAVE", developed at the University College of Swansea, to model numerically the experimental breakwater gap configurations. The use of the "WAVE" program to model breakwater gap wave diffraction is novel and certain conceptual problems had to be overcome. Finally, the experimental and numerical diffraction diagrams obtained were compared to analytical diagrams where these were available. The correlation between the finite element and analytical results is excellent. When comparing the experimental and finite element results the general conclusions are : 1) in regions outside the shadow zones the linear diffraction theory is conservative except close to small gaps (B/L ≤ 1); and 2) within the shadow zones the linear theory is not conservative and one will have to allow for non-linear effects such as radiating second-order waves generated at the breakwater tips, and increased wave orthogonal spreading near the gap centre line and subsequent orthogonal bunching in the shadow zones caused by wave steepness differences along the crests. Other conclusions drawn are : 1) the photogrammetric techniques described are the best available for the experimental simulation and analysis of infinite domain diffraction and refraction - diffraction problems; and 2) the finite and infinite element program "WAVE" is a very useful tool for the prediction of wave heights in large harbour basins
The dynamic diffusion behaviors of 2D small Fe clusters on a Fe(110) surface
No full textIn this paper, the diffusion behaviors of Fe clusters on a Fe( 110) surface have been investigated using molecular dynamics simulations based on a modified analytic embedded-atom method. The stable configurations of Fe clusters are predicted to be close-packed islands configuration for Fe clusters up to nine atoms or even larger in size. The activation energy of surface diffusion exhibits an interesting, oscillatory behavior as a function of cluster size. As compared to the structures with extra atoms at the periphery, compact geometric configurations of Fe clusters (four-and seven-atom clusters) have an obviously higher activation energy. The reason is that for clusters of more than two atoms the diffusion mechanisms of 2D small clusters are achieved by the migration of extra atoms at the periphery
Fe-Cluster Pushing Electrons to N‑Doped Graphitic Layers with Fe<sub>3</sub>C(Fe) Hybrid Nanostructure to Enhance O<sub>2</sub> Reduction Catalysis of Zn-Air Batteries
No full textNon-noble metal catalysts with catalytic
activity toward oxygen reduction reaction (ORR) comparable or even
superior to that of Pt/C are extremely important for the wide application
of metal–air batteries and fuel cells. Here, we develop a simple
and controllable strategy to synthesize Fe-cluster embedded in Fe3C nanoparticles (designated as Fe3C(Fe)) encased
in nitrogen-doped graphitic layers (NDGLs) with graphitic shells as
a novel hybrid nanostructure as an effective ORR catalyst by directly
pyrolyzing a mixture of Prussian blue (PB) and glucose. The pyrolysis
temperature was found to be the key parameter for obtaining a stable
Fe3C(Fe)@NDGL core–shell nanostructure with an optimized
content of nitrogen. The optimized Fe3C(Fe)@NDGL catalyst
showed high catalytic performance of ORR comparable to that of the
Pt/C (20 wt %) catalyst and better stability than that of the Pt/C
catalyst in alkaline electrolyte. According to the experimental results
and first principle calculation, the high activity of the Fe3C(Fe)@NDGL catalyst can be ascribed to the synergistic effect of
an adequate content of nitrogen doping in graphitic carbon shells
and Fe-cluster pushing electrons to NDGL. A zinc–air battery
utilizing the Fe3C(Fe)@NDGL catalyst demonstrated a maximum
power density of 186 mW cm–2, which is slightly
higher than that of a zinc–air battery utilizing the commercial
Pt/C catalyst (167 mW cm–2), mostly because of the
large surface area of the N-doped graphitic carbon shells. Theoretical
calculation verified that O2 molecules can spontaneously
adsorb on both pristine and nitrogen doped graphene surfaces and then
quickly diffuse to the catalytically active nitrogen sites. Our catalyst
can potentially become a promising replacement for Pt catalysts in
metal-air batteries and fuel cells
Precipitation of supersaturated solute in H ion irradiated Fe-Au and Fe-Au-W alloys studied by positron annihilation spectroscopy
No full textThe effect of thermal aging of homogenized Fe-Au and Fe-Au-W alloys, irradiated at room temperature with hydrogen ions, was studied for an aging treatment at 300 °C for aging times up to 100 h. The aging behavior of the Fe-based alloys is compared to the results for pure Fe. The precipitation behavior of Au-rich and W-rich precipitates and its correlation to the H+ irradiation-induced defects is investigated by variable energy positron annihilation spectroscopy (VEPAS). The formation of open-volume defects after irradiation is monitored by an increase in the S parameter, while the recovery of the vacancy-like defects and the formation of precipitates are signalled by an increase in the W parameter. Au-rich precipitation continuously develops during long-term aging, as indicated by the increase in the W parameter. The change of the W parameter in the Fe-Au-W alloy is not only due to the effect of solute W on the Au precipitates, but also because of the interface of W-rich Laves phase with matrix.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Novel Aerospace MaterialsRST/Fundamental Aspects of Materials and Energ
A Pyridinic Fe-N4 Macrocycle Effectively Models the Active Sites in Fe/N-Doped Carbon Electrocatalysts
No full textIron- and nitrogen-doped carbon (Fe-N-C) materials are leading candidates to replace platinum in fuel cells, but their active site structures are poorly understood. A leading postulate is that iron active sites in this class of materials exist in an Fe-N4 pyridinic ligation environment. Yet, molecular Fe-based catalysts for the oxygen reduction reaction (ORR) generally feature pyrrolic coordination and pyridinic Fe-N4 catalysts are, to the best of our knowledge, non-existent. We report the synthesis and characterization of a molecular pyridinic hexaazacyclophane macrocycle, (phen2N2)Fe, and compare its spectroscopic, electrochemical, and catalytic properties for oxygen reduction to a prototypical Fe-N-C material, as well as iron phthalocyanine, (Pc)Fe, and iron octaethylporphyrin, (OEP)Fe, prototypical pyrrolic iron macrocycles. N 1s XPS signatures for coordinated N atoms in (phen2N2)Fe are positively shifted relative to (Pc)Fe and (OEP)Fe, and overlay with those of Fe-N-C. Likewise, spectroscopic XAS signatures of (phen2N2)Fe are distinct from those of both (Pc)Fe and (OEP)Fe, and are remarkably similar to those of Fe-N-C with compressed Fe–N bond lengths of 1.97 Å in (phen2N2)Fe that are close to the average 1.94 Å length in Fe-N-C. Electrochemical studies establish that both (Pc)Fe and (phen2N2)Fe have relatively high Fe(III/II) potentials at ~0.6 V, ~300 mV positive of (OEP)Fe. The ORR onset potential is found to directly correlate with the Fe(III/II) potential leading to a ~300 mV positive shift in the onset of ORR for (Pc)Fe and (phen2N2)Fe relative to (OEP)Fe. Consequently, the ORR onset for (phen2N2)Fe and (Pc)Fe is within 150 mV of Fe-N-C. Unlike (OEP)Fe and (Pc)Fe, (phen2N2)Fe displays excellent selectivity for 4-electron ORR with 2O2 production, comparable to Fe-N-C materials. The aggregate spectroscopic and electrochemical data establish (phen2N2)Fe as a pyridinic iron macrocycle that effectively models Fe-N-C active sites, thereby providing a rich molecular platform for understanding this important class of catalytic materials.</p
Investigation of the unusual magnetic properties of Fe and Co on MgO with high spatial, energy and temporal resolution
No full textNanometer-sized magnets are used as magnetic bits in data storage devices to hold information. As such devices are further miniaturized, the control of magnetism at the atomic scale becomes essential. One of the critical parameters to realize nanoscopic magnets is a large magnetic anisotropy. Magnetic anisotropy gives its magnetization a preferred axis and thus its directionality. At the atomic scale, magnetic anisotropy originates from anisotropy in the orbital angular momentum and the spin-orbit coupling that connects the spin moment of a magnetic atom to the spatial symmetry of its ligand field environment. Thus far, the magnetic anisotropy energy per atom in single-molecule magnets and ferromagnetic films remains typically one to two orders of magnitude below the theoretical limit imposed by the atomic spin-orbit interaction. Here we investigate the magnetic properties of individual magnetic atoms on thin magnesium oxide (MgO) films. We find highly unusual magnetic behavior for Fe and Co on the oxygen binding site of MgO. By coordinating a single Co atom to this binding site we can even realized the maximum magnetic anisotropy for a 3d transition metal atom.
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At the heart of this work we combine scanning tunneling microscopy and X-ray absorption spectroscopy experiments and we find striking agreement between these experimental techniques. Scanning tunneling spectroscopy reveals a record-high zero-field splitting of 58 millielectron volts for Co as well as 14 millielectron volts for Fe on the oxygen binding site. This behavior originates from the dominating axial ligand field of this adsorption site, which leads to out-of-plane uniaxial anisotropy combined with large orbital moment, as observed by X-ray magnetic circular dichroism. The bonding geometry and electronic configuration are calculated by density functional theory, a multiplet analysis and a model developed here, that uses a point-charge calculation combined with Stevens operator equivalents. Scanning tunneling microscopy also allows the tuning of the magnetic anisotropy and spin-polarized measurements that confirm the applied model by revealing further transitions and by allowing the measurement of magnetic moments on single atoms.
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A further critical parameter for obtaining miniaturized magnets, for applications in data storage devices, is the magnetic stability, the ability of magnets to retain their magnetic orientation despite external influences. The magnetic stability of larger magnets is determined by a thermal barrier, which scales with the magnetic anisotropy. At the atomic scale, magnetization reversal is often dominated by quantum tunneling of the magnetization. Quantum tunneling allows transitions between states without having to overcome the anisotropy barrier. This is for example caused by mixing between different states, induced by the ligand symmetry. Here we use an all-electrical pump-probe scheme to determine the lifetimes of Co and Fe on MgO and we show how such tunneling can be sufficiently suppressed by careful design of the bonding geometry and by reducing the atom’s interaction with the environment. With this approach, we show the longest lifetimes seen so far for 3d transition metal atoms: a lifetime of 200 microseconds for Co and of 10 milliseconds for Fe.
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The research described in this thesis demonstrates how the complementary use of several experimental and theoretical techniques allows a detailed determination of the character and properties of the magnetic states at the atomic level. These results offer a strategy, based on symmetry arguments and careful tailoring of the interaction with the environment, for the rational design of nanoscopic permanent magnets and single atom magnets
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