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  • 1
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    Optimization of an exchange-correlation density functional for water (2016)
    American Institute of Physics (AIP)
    Details
    Publikationsdatum: 2016-06-10
    Beschreibung: We describe a method, that we call data projection onto parameter space (DPPS), to optimize an energy functional of the electron density, so that it reproduces a dataset of experimental magnitudes. Our scheme, based on Bayes theorem, constrains the optimized functional not to depart unphysically from existing ab initio functionals. The resulting functional maximizes the probability of being the “correct” parameterization of a given functional form, in the sense of Bayes theory. The application of DPPS to water sheds new light on why density functional theory has performed rather poorly for liquid water, on what improvements are needed, and on the intrinsic limitations of the generalized gradient approximation to electron exchange and correlation. Finally, we present tests of our water-optimized functional, that we call vdW-DF-w, showing that it performs very well for a variety of condensed water systems.
    Print ISSN: 0021-9606
    Digitale ISSN: 1089-7690
    Thema: Chemie und Pharmazie , Physik
    Publiziert von American Institute of Physics (AIP)
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  • 2
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    Characterizing solid-state ignition of runaway chemical reactions in Ni-Al nanoscale multilayers under uniform heating (2015)
    American Institute of Physics (AIP)
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    Publikationsdatum: 2015-10-08
    Beschreibung: Nanoscale layers of nickel and aluminum can mix rapidly to produce runaway reactions. While self-propagating high temperature synthesis reactions have been observed for decades, the solid-state ignition of these reactions has been challenging to study. Particularly elusive is characterization of the low-temperature chemical mixing that occurs just prior to the ignition of the runaway reaction. Characterization can be challenging due to inhomogeneous microstructures, uncontrollable heat losses, and the nonuniform distribution of heat throughout the material prior to ignition. To reduce the impact of these variables, we heat multilayered Ni/Al foils in a highly uniform manner and report ignition temperatures as low as 245 °C for heating rates ranging from 2000 °C/s to 50 000 °C/s. Igniting in this way reveals that there are four stages before the reaction is complete: heating to an ignition temperature, low temperature solid-state mixing, a separate high temperature solid-state mixing, and liquid-state mixing. Multiple bilayer spacings, heating rates, and heating times are compared to show that the ignition temperature is a function of the bilayer spacing. A symmetric numerical diffusion model is used to show that there is very little chemical mixing in the first 10 ms of heating but significant mixing after 50 ms. These predictions suggest that ignition temperatures should increase for the slowest heating rates but this trend could not be identified clearly. The modeling was also used to examine the kinetic parameters governing the early stages of solid-state diffusion and suggest that grain boundary diffusion is dominant.
    Print ISSN: 0021-8979
    Digitale ISSN: 1089-7550
    Thema: Physik
    Publiziert von American Institute of Physics (AIP)
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  • 3
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    Highly textured oxypnictide superconducting thin films on metal substrates (2014)
    American Institute of Physics (AIP)
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    Publikationsdatum: 2014-10-31
    Beschreibung: Highly textured NdFeAs(O,F) thin films have been grown on ion beam assisted deposition-MgO/Y 2 O 3 /Hastelloy substrates by molecular beam epitaxy. The oxypnictide coated conductors showed a superconducting transition temperature ( T c ) of 43 K with a self-field critical current density ( J c ) of 7.0 × 10 4   A / cm 2 at 5 K, more than 20 times higher than powder-in-tube processed SmFeAs(O,F) wires. Albeit higher T c as well as better crystalline quality than Co-doped BaFe 2 As 2 coated conductors, in-field J c of NdFeAs(O,F) was lower than that of Co-doped BaFe 2 As 2 . These results suggest that grain boundaries in oxypnictides reduce J c significantly compared to that in Co-doped BaFe 2 As 2 and, hence biaxial texture is necessary for high J c.
    Print ISSN: 0003-6951
    Digitale ISSN: 1077-3118
    Thema: Physik
    Publiziert von American Institute of Physics (AIP)
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  • 4
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    Spectral encoding method for measuring the relative arrival time between x-ray/optical pulses (2014)
    American Institute of Physics (AIP)
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    Publikationsdatum: 2014-08-29
    Beschreibung: The advent of few femtosecond x-ray light sources brings promise of x-ray/optical pump-probe experiments that can measure chemical and structural changes in the 10–100 fs time regime. Widely distributed timing systems used at x-ray Free-Electron Laser facilities are typically limited to above 50 fs fwhm jitter in active x-ray/optical synchronization. The approach of single-shot timing measurements is used to sort results in the event processing stage. This has seen wide use to accommodate the insufficient precision of active stabilization schemes. In this article, we review the current technique for “measure-and-sort” at the Linac Coherent Light Source at the SLAC National Accelerator Laboratory. The relative arrival time between an x-ray pulse and an optical pulse is measured near the experimental interaction region as a spectrally encoded cross-correlation signal. The cross-correlation provides a time-stamp for filter-and-sort algorithms used for real-time sorting. Sub-10 fs rms resolution is common in this technique, placing timing precision at the same scale as the duration of the shortest achievable x-ray pulses.
    Print ISSN: 0034-6748
    Digitale ISSN: 1089-7623
    Thema: Elektrotechnik, Elektronik, Nachrichtentechnik , Physik
    Publiziert von American Institute of Physics (AIP)
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  • 5
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    The creation of large-volume, gradient-free warm dense matter with an x-ray free-electron laser (2015)
    American Institute of Physics (AIP)
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    Publikationsdatum: 2015-03-25
    Beschreibung: The efficiency and uniformity of heating induced by hard x-ray free-electron laser pulse is investigated for 0.5  μ m silver foils using the X-ray Pump Probe instrument at the Linac Coherent Light Source facility. Intense 8.9 keV x-ray pulses of 60 fs duration deposit energy predominantly via inner-shell ionization to create a non-equilibrium Ag solid density plasma. The x-ray pulses are focused to 14 × 17  μ m 2 by means of beryllium lenses and by varying the total beam energy, the energy deposition is varied over a range of irradiances from 4.4 to 6.5 × 10 15  W/cm 2 . Two time-and-space resolved interferometers simultaneously probed the expansion of the front and rear sample surfaces and find evidence of a nearly symmetric expansion pointing to the uniformity of energy deposition over the full target thickness. The experimental results are compared with two different hydrodynamic simulations of the sample expansion. The agreement between experimental and theoretical results yields an estimate of the temperature evolution as a function of x-ray irradiance that varies from 8 to 10 eV for the x-ray irradiances studied.
    Print ISSN: 1070-664X
    Digitale ISSN: 1089-7674
    Thema: Physik
    Publiziert von American Institute of Physics (AIP)
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  • 6
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    Performance of a beam-multiplexing diamond crystal monochromator at the Linac Coherent Light Source (2014)
    American Institute of Physics (AIP)
    Details
    Publikationsdatum: 2014-06-07
    Beschreibung: A double-crystal diamond monochromator was recently implemented at the Linac Coherent Light Source. It enables splitting pulses generated by the free electron laser in the hard x-ray regime and thus allows the simultaneous operations of two instruments. Both monochromator crystals are High-Pressure High-Temperature grown type-IIa diamond crystal plates with the (111) orientation. The first crystal has a thickness of ∼100 μm to allow high reflectivity within the Bragg bandwidth and good transmission for the other wavelengths for downstream use. The second crystal is about 300 μm thick and makes the exit beam of the monochromator parallel to the incoming beam with an offset of 600 mm. Here we present details on the monochromator design and its performance.
    Print ISSN: 0034-6748
    Digitale ISSN: 1089-7623
    Thema: Elektrotechnik, Elektronik, Nachrichtentechnik , Physik
    Publiziert von American Institute of Physics (AIP)
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  • 7
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    p+-doping analysis of laser fired contacts for silicon solar cells by Kelvin probe force microscopy (2016)
    American Institute of Physics (AIP)
    Details
    Publikationsdatum: 2016-03-15
    Beschreibung: Local rear contacts for silicon passivated emitter and rear contact solar cells can be established by point-wise treating an Al layer with laser radiation and thereby establishing an electrical contact between Al and Si bulk through the dielectric passivation layer. In this laser fired contacts (LFC) process, Al can establish a few μ m thick p + -doped Si region below the metal/Si interface and forms in this way a local back surface field which reduces carrier recombination at the contacts. In this work, the applicability of Kelvin probe force microscopy (KPFM) to the investigation of LFCs considering the p + -doping distribution is demonstrated. The method is based on atomic force microscopy and enables the evaluation of the lateral 2D Fermi-level characteristics at sub-micrometer resolution. The distribution of the electrical potential and therefore the local hole concentration in and around the laser fired region can be measured. KPFM is performed on mechanically polished cross-sections of p + -doped Si regions formed by the LFC process. The sample preparation is of great importance because the KPFM signal is very surface sensitive. Furthermore, the measurement is responsive to sample illumination and the height of the applied voltage between tip and sample. With other measurement techniques like micro-Raman spectroscopy, electrochemical capacitance-voltage, and energy dispersive X-ray analysis, a high local hole concentration in the range of 10 19  cm −3 is demonstrated in the laser fired region. This provides, in combination with the high spatial resolution of the doping distribution measured by KPFM, a promising approach for microscopic understanding and further optimization of the LFC process.
    Print ISSN: 0021-8979
    Digitale ISSN: 1089-7550
    Thema: Physik
    Publiziert von American Institute of Physics (AIP)
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  • 8
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    PEDOT:PSS emitters on multicrystalline silicon thin-film absorbers for hybrid solar cells (2015)
    American Institute of Physics (AIP)
    Details
    Publikationsdatum: 2015-02-26
    Beschreibung: We fabricated an efficient hybrid solar cell by spin coating poly(3,4-ethylene-dioxythiophene):polystyrenesulfonate (PEDOT:PSS) on planar multicrystalline Si (mc-Si) thin films. The only 5  μ m thin Si absorber layers were prepared by diode laser crystallization of amorphous Si deposited by electron beam evaporation on glass. On these absorber layers, we studied the effect of SiO x and Al 2 O 3 terminated Si surfaces. The short circuit density and power conversion efficiency (PCE) of the mc-Si/Al 2 O 3 /PEDOT:PSS solar cell increase from 20.6 to 25.4 mA/cm 2 and from 7.3% to 10.3%, respectively, as compared to the mc-Si/SiO x /PEDOT:PSS cell. Al 2 O 3 lowers the interface recombination and improves the adhesion of the polymer film on the hydrophobic mc-Si thin film. Open circuit voltages up to 604 mV were reached. This study demonstrates the highest PCE so far of a hybrid solar cell with a planar thin film Si absorber.
    Print ISSN: 0003-6951
    Digitale ISSN: 1077-3118
    Thema: Physik
    Publiziert von American Institute of Physics (AIP)
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  • 9
    Digitale Medien
    Digitale Medien
    Shock compression of tungsten and molybdenum (1992)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 71 (1992), S. 1721-1728 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: New high-pressure shock-wave data have been obtained for W and Mo. These data have been combined with previous data sets for these materials to extend the range of linear us–up fits for the Hugoniot to 480 GPa for Mo and 680 GPa for W. The shock-wave data, supplemented by the necessary thermodynamic information, have been used to generate several isotherms (100, 200,...1000 K). Tables of pressure versus relative volume up to 380 GPa suitable for comparison with statically obtained data are given.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.351203
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  • 10
    Digitale Medien
    Digitale Medien
    As-cast magnets based on Fe-Nd-C : The 5th Joint MMM−Intermag Conference (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 6447-6449 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Fe14Nd2C (Φ) shows structural and intrinsic magnetic properties comparable to Fe14Nd2B. The phase Fe14Nd2C is found in long-time-annealed samples only ((approximately-greater-than)20 d at 850 °C) because of nucleation difficulties and slow growth kinetics. Compared to the Fe-Nd-B system the phase relations in the Fe-Nd-C system are unfavorable to produce sintered or even as-cast magnet materials, as manifested in the phase diagram presented. Fe14Nd2C is in equilibrium with either ferromagnetic and/or very corrosive phases, therefore the pure Fe-Nd-C material is useless. This situation can be changed by the addition of several elements to the Fe-Nd-C material: boron (even in small amounts of ≈0.05 mass %) accelerates the formation of Φ and reduces annealing times from weeks to hours; Cu added in the correct amount leads to new phase relations between Φ and NdxCuy intermetallic compounds. These intermetallics are nonmagnetic and low melting. Starting from as-cast material using the knowledge about the influences of substituants on phase relations and kinetics, a heat treatment can be chosen that leads directly to an isotropic, magnetically hard material with a rather small grain size ((approximately-less-than)20 μm) and coercivities up to 1 T. The magnetization is comparable to isotropic Fe-Nd-B materials.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.349926
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