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Calculations of theoretical efficiencies for electrochemically-mediated tandem solar water splitting as a function of bandgap energies and redox shuttle potential
Tandem Z-scheme solar water splitting devices composed of two light-absorbers that are connected electrochemically by a soluble redox shuttle constitute a promising technology for cost-effective solar hydrogen production. In this work, efficiency limits of these devices are modeled by combining the detailed-balance model of the light-absorbers with Butler–Volmer electron-transfer kinetics. The impacts of the redox shuttle thermodynamic potential, light-absorber bandgaps, and electrocatalytic parameters on the solar-to-hydrogen conversion (STH) efficiency are modeled. We report that the thermodynamic potential of the redox shuttle with respect to the hydrogen and oxygen evolution potentials has a direct effect on both the STH efficiency and the optimal tandem light-absorber bandgaps needed to achieve the maximum possible STH efficiency. At 1 sun illumination and assuming ideal and optimally selective electrocatalytic parameters, the STH efficiency varies from a minimum of 21%, for a redox shuttle potential of 0 V vs. the reversible hydrogen electrode (RHE), to a maximum of 34%, for a redox shuttle potential of either 0.36 V or 1.06 V vs. RHE. To attain the maximum possible STH efficiency of 34%, the light-absorber bandgaps must be 1.53 eV and 0.75 eV, yet the optimal redox shuttle potential depends on whether the hydrogen-evolving or oxygen-evolving light-absorber has the larger bandgap. Results also underscore the importance of optimizing the absorptance of the top light-absorber, which enables large STH efficiencies to be achieved with a wider range of bandgap combinations. Moreover, given the large overpotentials for the oxygen evolution reaction and reasonably low overpotentials for most redox shuttle reactions, the tandem design is more efficient than a single light-absorber design even when the potential of the redox shuttle exceeds 1.23 V vs. RHE. When the exchange current density of the redox shuttle reactions is as low as 10 -5 mA cm -2 , STH efficiencies as large as 22% are still achievable as long as selective catalysis and optimal redox shuttle potential are assumed, suggesting that even slow redox shuttle reactions may not limit the practicality of these devices.
Neutron-induced fission: properties of prompt neutron and γ rays as a function of incident energy
We have applied the Hauser-Feshbach statistical theory, in a Monte-Carlo implementation, to the de-excitation of fission fragments, obtaining a reasonable description of the characteristics of neutrons and gamma rays emitted before beta decays toward stability. Originally implemented for the spontaneous fission of 252 Cf and the neutroninduced fission of 235 U and 239 Pu at thermal neutron energy, in this contribution we discuss the extension of the formalism to incident neutron energies up to 20 MeV. For the emission of pre-fission neutrons, at incident energies beyond second-chance fission, we take into account both the pre-equilibrium and statistical pre-fission components. Phenomenological parameterizations of mass, charge and TKE yields are used to obtain the initial conditions for the fission fragments that subsequently decay via neutron and emissions. Finally, we illustrate this approach for 239 Pu(n,f).
Nuclear shape transitions in neutron-rich medium-mass nuclei
We study the isotopic evolution of the ground-state nuclear shapes in neutron-rich Kr, Rb, Sr, Y, Zr, Nb, and Mo isotopic chains. Both even-even and odd-A nuclei are included in the analysis. For the latter we also study the systematics of their one-quasiparticle low-lying configurations. The theoretical approach is based on a selfconsistent Hartree-Fock-Bogoliubov formalism with finite range Gogny energy density functionals. Neutron separation energies, charge radii, and the spin-parity of the ground states are calculated and compared with available data. Shape-transition signatures are identified around N= 60 isotones as discontinuities in both charge radii isotopic shifts and spin-parities of the ground states. The nuclear deformation including triaxiality is shown to play a relevant role in the understanding of the bulk and spectroscopic features of the ground and low-lying one-quasiparticle states.
Weighted-density functionals for cavity formation and dispersion energies in continuum solvation models
Continuum solvation models enable efficient first principles calculations of chemical reactions in solution, but require extensive parametrization and fitting for each solvent and class of solute systems. Here, we examine the assumptions of continuum solvation models in detail and replace empirical terms with physical models in order to construct a minimally-empirical solvation model. Specifically, we derive solvent radii from the nonlocal dielectric response of the solvent from ab initio calculations, construct a closed-form and parameter-free weighted-density approximation for the free energy of the cavity formation, and employ a pair-potential approximation for the dispersion energy. We show that the resulting model with a single solvent-independent parameter: the electron density threshold (n{sub c}), and a single solvent-dependent parameter: the dispersion scale factor (s{sub 6}), reproduces solvation energies of organic molecules in water, chloroform, and carbon tetrachloride with RMS errors of 1.1, 0.6 and 0.5 kcal/mol, respectively. We additionally show that fitting the solvent-dependent s{sub 6} parameter to the solvation energy of a single non-polar molecule does not substantially increase these errors. Parametrization of this model for other solvents, therefore, requires minimal effort and is possible without extensive databases of experimental solvation free energies.
Soft X-ray irradiation of methanol ice: Formation of products as a function of photon energy
Pure methanol ices have been irradiated with monochromatic soft X-rays of 300 and 550 eV close to the 1s resonance edges of C and O, respectively, and with a broadband spectrum (250-1200 eV). The infrared (IR) spectra of the irradiated ices show several new products of astrophysical interest such as CH{sub 2}OH, H{sub 2}CO, CH{sub 4}, HCOOH, HCOCH{sub 2}OH, CH{sub 3}COOH, CH{sub 3}OCH{sub 3}, HCOOCH{sub 3}, and (CH{sub 2}OH){sub 2}, as well as HCO, CO, and CO{sub 2}. The effect of X-rays is the result of the combined interactions of photons and electrons with the ice. A significant contribution to the formation and growth of new species in the CH{sub 3}OH ice irradiated with X-rays is given by secondary electrons, whose energy distribution depends on the energy of X-ray photons. Within a single experiment, the abundances of the new products increase with the absorbed energy. Monochromatic experiments show that product abundances also increase with the photon energy. However, the abundances per unit energy of newly formed species show a marked decrease in the broadband experiment as compared to irradiations with monochromatic photons, suggesting a possible regulatory role of the energy deposition rate. The number of new molecules produced per absorbed eV in the X-ray experiments has been compared to those obtained with electron and ultraviolet (UV) irradiation experiments.
Density of States for a Specified Correlation Function and the Energy Landscape
Not Available
Investigation of Materials for Radio Frequency Antenna Plasma Facing Components
Here, the interaction of radio frequency (RF) sheaths with fusion reactor relevant materials (e.g., tungsten and titanium diboride) is being studied on the RF Plasma Interaction Experiment (RF PIE). The RF PIE consists of an electron cyclotron resonance (ECR) plasma source (2.45 GHz, 5 kW) with a biased and heated RF electrode that is used to simulate antenna surfaces in contact with the edge plasma. Helium plasmas (density of ~1e18/m 3 , electron temperature of 4–5 eV) are being used to explore sheath formation on material surfaces with biases up to 500 V. The erosion of a tungsten surface is being studied spectroscopically using a mirror-linked 1 m Czerny-Turner UV imaging spectrometer with a spectral resolution of 0.012 nm for measuring plasma emission in and near the sheath. Tungsten line emission intensity is higher for RF versus dc biasing for similar plasma conditions and average ion energy. RF biasing causes a broadening of the ion energy distribution function (IEDF) due to the RF sheath, as determined from the hPIC2 code, and results in enhanced sputtering. Calculations of the expected sputtering yield for dc and RF biasing are consistent with experimental observations of changes in the 400.9 nm tungsten line emission intensity as a function of ion energy.
Microionization chamber air-kerma calibration coefficients as a function of photon energy for x-ray spectra in the range of 20-250 kVp relative to {sup 60}Co
Purpose: To investigate the applicability of a wide range of microionization chambers for reference dosimetry measurements in low- and medium-energy x-ray beams. Methods: Measurements were performed with six cylindrical microchamber models, as well as one scanning chamber and two Farmer-type chambers for comparison purposes. Air-kerma calibration coefficients were determined at the University of Wisconsin Accredited Dosimetry Calibration Laboratory for each chamber for a range of low- and medium-energy x-ray beams (20-250 kVp), with effective energies ranging from 11.5 keV to 145 keV, and a {sup 60}Co beam. A low-Z proof-of-concept microchamber was developed and calibrated with and without a high-Z silver epoxy on the collecting electrode. Results: All chambers composed of low-Z materials (Z{<=} 13), including the Farmer-type chambers, the scanning chamber, and the PTW TN31014 and the proof-of-concept microchambers, exhibited air-kerma calibration coefficients with little dependence on the quality of the beam. These chambers typically exhibited variations in calibration coefficients of less than 3% with the beam quality, for medium energy beams. However, variations in air-kerma calibration coefficients of greater than 50% were measured over the range of medium-energy x-ray beams for each of the microchambers containing high-Z collecting electrodes (Z > 13). For these high-Z chambers, which include the Exradin A14SL and A16 chambers, the PTW TN31006 chamber, the IBA CC01 chamber, and the proof-of-concept chamber containing silver, the average variation in air-kerma calibration coefficients between any two calibration beams was nearly 25% over the entire range of beam qualities investigated. Conclusions: Due to the strong energy dependence observed with microchambers containing high-Z components, these chambers may not be suitable dosimeters for kilovoltage x-ray applications, as they do not meet the TG-61 requirements. It is recommended that only microchambers containing low-Z materials (Z{<=} 13) be considered for air-kerma calibrations for reference dosimetry in low- and medium-energy x-ray beams.
The synthesis of one-dimensional functional nanostructures for energy storage applications [Slides]
Abstract not provided.
Using effective models to “translate” ab initio energies and wave functions into physics and chemistry [Slides]
Abstract not provided.
Fixed-Energy Response-Function Analysis with Multi-Efficiency (FRAM) Software, Version 6
Researchers at Los Alamos National Laboratory have developed FixedEnergy Response- Function Analysis with Multiple Efficiency (FRAM), a powerful software tool for gamma-ray spectrometry isotopic analysis. This tool leverages Los Alamos expertise in nuclear safeguards, radiation detection systems, and related analysis techniques. A household name in the non-destructive assay community for over two decades, FRAM determines a special nuclear material source's isotopic composition by analyzing gamma-ray spectra. Los Alamos researchers have released the latest version of FRAM that incorporates several updates to the 2013 major release of FRAM, Version 5.
Coloration of MgO single crystals by low energy protons - A function of selected defects
MgO single crystals dominant coloration in solar spectral region by electron hole pair diffusion, trapping and recombination
Grid-generated isotropic homogeneous turbulence at high Reynolds numbers
Consideration is given to an empirical formula for the longitudinal correlation function for grid-generated incompressible fluid turbulence at Reynolds numbers above 12,800. The formula, which relates the longitudinal correlation function to the inverse cube of a dimensionless geometrical ratio, is shown to minimize the global correlation integrals into which the two-point velocity correlation tensor has been substituted subject to a global constraint on the Sobolev concomitent of the longitudinal correlation function. Furthermore, the energy spectrum function associated with the empirical formula is shown to satisfy a tertiary Helmholtz-type linear condition throughout the initial period of decay.
The muon content of EAS as a function of primary energy
The muon content of extensive air showers (EAS) was measured over the wide primary energy range 10 to the 16th power to 10 to the 20th power eV. It is reported that the relative muon content of EAS decreases smoothly over the energy range 10 to the 17th power to 10 to the 19th power eV and concluded that the primary cosmic ray flux has a constant mass composition over this range. It is also reported that an apparent significant change in the power index occurs below 10 to the 17th power eV rho sub c (250 m) sup 0.78. Such a change indicates a significant change in primary mass composition in this range. The earlier conclusions concerning EAS of energy 10 to the 17th power eV are confirmed. Analysis of data in the 10 to the 16th power - 10 to the 17th power eV range revealed a previously overlooked selection bias in the data set. The full analysis of the complete data set in the energy range 10 to the 16th power - 10 to the 17th power ev with the selection bias eliminated is presented.
First Order Variation of the Dispersion Function with Particle Energy Deviation
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A Note on the Relationship Between the Emittance, the Beta Function, and the Energy in a Linear Collider
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