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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,009 records · Page 56
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Understanding S-shaped current-voltage characteristics of organic solar cells: Direct measurement of potential distributions by scanning Kelvin probe

We present a comparison of the potential distribution along the cross section of bilayer poly(3-hexylthiophene)/1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]C61 (P3HT/PCBM) solar cells, which show normal and anomalous, S-shaped current-voltage (IV) characteristics. We expose the cross sections of the devices with a focussed ion beam and measure them with scanning Kelvin probe microscopy. We find that in the case of S-shaped IV-characteristics, there is a huge potential drop at the PCBM/Al top contact, which does not occur in solar cells with normal IV-characteristics. This behavior confirms the assumption that S-shaped curves are caused by hindered charge transport at interfaces.

36 MATERIALS SCIENCE↗
DOE OSTI2013

The analysis of ball's motion in the torus-shaped autoequalizer

The reduction of vibrations in rotary systems is necessary condition for the development of constructions with high quality requirements. Among the devices that are able to reduce rotor's vibrations are automatic balancers with balls, liquid, etc. Autoequalizers with torus-shaped body contain one ball-shaped adjustment mass that can freely roll both in the ring direction and in the cross section. This enables reducing to the minimum the force of resistance at the ball's moving, but simultaneously decreases the chance of launching the autoequalizer in the autobalancing regime. It has been stated that the ball in the working state is influenced by forces that are aimed at moving it in the body of the autoequalizer towards the side opposite to the imbalance. The design of an autoequalizer that ensures working state in which at rotation the ball stops relative to the autoequalizer's body opposite to the imbalance has been experimentally worked out. The autoequalizer is made up of the mathematical model with two differential equations. In the result of the calculations it has been proved that the ball of an equalizer with the torus-shaped body has at least two movement conditions one of which is a working state, and another one is a spare state.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Laboratory transferability of optimally shaped laser pulses for quantum control

Optimal control experiments can readily identify effective shaped laser pulses, or “photonic reagents,” that achieve a wide variety of objectives. An important additional practical desire is for photonic reagent prescriptions to produce good, if not optimal, objective yields when transferred to a different system or laboratory. Building on general experience in chemistry, the hope is that transferred photonic reagent prescriptions may remain functional even though all features of a shaped pulse profile at the sample typically cannot be reproduced exactly. As a specific example, we assess the potential for transferring optimal photonic reagents for the objective of optimizing a ratio of photoproduct ions from a family of halomethanes through three related experiments. First, applying the same set of photonic reagents with systematically varying second- and third-order chirp on both laser systems generated similar shapes of the associated control landscape (i.e., relation between the objective yield and the variables describing the photonic reagents). Second, optimal photonic reagents obtained from the first laser system were found to still produce near optimal yields on the second laser system. Third, transferring a collection of photonic reagents optimized on the first laser system to the second laser system reproduced systematic trends in photoproduct yields upon interaction with the homologous chemical family. These three transfers of photonic reagents are demonstrated to be successful upon paying reasonable attention to overall laser system characteristics. The ability to transfer photonic reagents from one laser system to another is analogous to well-established utilitarian operating procedures with traditional chemical reagents. The practical implications of the present results for experimental quantum control are discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Tomographic analysis of neutron and gamma pulse shape distributions from liquid scintillation detectors at Joint European Torus

The Joint European Torus (JET, Culham, UK) is the largest tokamak in the world devoted to nuclear fusion experiments of magnetic confined Deuterium (D)/Deuterium-Tritium (DT) plasmas. Neutrons produced in these plasmas are measured using various types of neutron detectors and spectrometers. Two of these instruments on JET make use of organic liquid scintillator detectors. The neutron emission profile monitor implements 19 liquid scintillation counters to detect the 2.45 MeV neutron emission from D plasmas. A new compact neutron spectrometer is operational at JET since 2010 to measure the neutron energy spectra from both D and DT plasmas. Liquid scintillation detectors are sensitive to both neutron and gamma radiation but give light responses of different decay time such that pulse shape discrimination techniques can be applied to identify the neutron contribution of interest from the data. The most common technique consists of integrating the radiation pulse shapes within different ranges of their rising and/or trailing edges. In this article, a step forward in this type of analysis is presented. The method applies a tomographic analysis of the 3-dimensional neutron and gamma pulse shape and pulse height distribution data obtained from liquid scintillation detectors such that n/γ discrimination can be improved to lower energies and additional information can be gained on neutron contributions to the gamma events and vice versa.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Investigation on edge fringing effect and oxide thickness dependence of inversion current in metal-oxide-semiconductor tunneling diodes with comb-shaped electrodes

A particular edge-dependent inversion current behavior of metal-oxide-semiconductor (MOS) tunneling diodes was investigated utilizing square and comb-shaped electrodes. The inversion tunneling current exhibits the strong dependence on the tooth size of comb-shaped electrodes and oxide thickness. Detailed illustrations of current conduction mechanism are developed by simulation and experimental measurement results. It is found that the electron diffusion current and Schottky barrier height lowering for hole tunneling current both contribute on inversion current conduction. In MOS tunneling photodiode applications, the photoresponse can be improved by decreasing SiO{sub 2} thickness and using comb-shaped electrodes with smaller tooth spacing. Meantime, the high and steady photosensitivity can also be approached by introducing HfO{sub 2} into dielectric stacks.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Charge-signal multiplication mediated by urea wires inside Y-shaped carbon nanotubes

In previous studies, we reported molecular dynamics (MD) simulations showing that single-file water wires confined inside Y-shaped single-walled carbon nanotubes (Y-SWNTs) held strong and robust capability to convert and multiply charge signals [Y. S. Tu, P. Xiu, R. Z. Wan, J. Hu, R. H. Zhou, and H. P. Fang, Proc. Natl. Acad. Sci. U.S.A. 106, 18120 (2009); Y. Tu, H. Lu, Y. Zhang, T. Huynh, and R. Zhou, J. Chem. Phys. 138, 015104 (2013)]. It is fascinating to see whether the signal multiplication can be realized by other kinds of polar molecules with larger dipole moments (which make the experimental realization easier). In this article, we use MD simulations to study the urea-mediated signal conversion and multiplication with Y-SWNTs. We observe that when a Y-SWNT with an external charge of magnitude 1.0 e (the model of a signal at the single-electron level) is solvated in 1 M urea solutions, urea can induce drying of the Y-SWNT and fill its interiors in single-file, forming Y-shaped urea wires. The external charge can effectively control the dipole orientation of the urea wire inside the main channel (i.e., the signal can be readily converted), and this signal can further be multiplied into 2 (or more) output signals by modulating dipole orientations of urea wires in bifurcated branch channels of the Y-SWNT. This remarkable signal transduction capability arises from the strong dipole-induced ordering of urea wires under extreme confinement. We also discuss the advantage of urea as compared with water in the signal multiplication, as well as the robustness and biological implications of our findings. This study provides the possibility for multiplying signals by using urea molecules (or other polar organic molecules) with Y-shaped nanochannels and might also help understand the mechanism behind signal conduction in both physical and biological systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Spectral shapes of Ar-broadened HCl lines in the fundamental band by classical molecular dynamics simulations and comparison with experiments

Spectral shapes of isolated lines of HCl perturbed by Ar are investigated for the first time using classical molecular dynamics simulations (CMDS). Using reliable intermolecular potentials taken from the literature, these CMDS provide the time evolution of the auto-correlation function of the dipole moment, whose Fourier-Laplace transform leads to the absorption spectrum. In order to test these calculations, room temperature spectra of various lines in the fundamental band of HCl diluted in Ar are measured, in a large pressure range, with a difference-frequency laser spectrometer. Comparisons between measured and calculated spectra show that the CMDS are able to predict the large Dicke narrowing effect on the shape of HCl lines and to satisfactorily reproduce the shapes of HCl spectra at different pressures and for various rotational quantum numbers.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Spatial shaping for generating arbitrary optical dipole traps for ultracold degenerate gases

We present two spatial-shaping approaches – phase and amplitude – for creating two-dimensional optical dipole potentials for ultracold neutral atoms. When combined with an attractive or repulsive Gaussian sheet formed by an astigmatically focused beam, atoms are trapped in three dimensions resulting in planar confinement with an arbitrary network of potentials – a free-space atom chip. The first approach utilizes an adaptation of the generalized phase-contrast technique to convert a phase structure embedded in a beam after traversing a phase mask, to an identical intensity profile in the image plane. Phase masks, and a requisite phase-contrast filter, can be chemically etched into optical material (e.g., fused silica) or implemented with spatial light modulators; etching provides the highest quality while spatial light modulators enable prototyping and realtime structure modification. This approach was demonstrated on an ensemble of thermal atoms. Amplitude shaping is possible when the potential structure is made as an opaque mask in the path of a dipole trap beam, followed by imaging the shadow onto the plane of the atoms. While much more lossy, this very simple and inexpensive approach can produce dipole potentials suitable for containing degenerate gases. High-quality amplitude masks can be produced with standard photolithography techniques. Amplitude shaping was demonstrated on a Bose-Einstein condensate.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Calculating electronic tunnel currents in networks of disordered irregularly shaped nanoparticles by mapping networks to arrays of parallel nonlinear resistors

We have shown both theoretically and experimentally that tunnel currents in networks of disordered irregularly shaped nanoparticles (NPs) can be calculated by considering the networks as arrays of parallel nonlinear resistors. Each resistor is described by a one-dimensional or a two-dimensional array of equal size nanoparticles that the tunnel junction gaps between nanoparticles in each resistor is assumed to be equal. The number of tunnel junctions between two contact electrodes and the tunnel junction gaps between nanoparticles are found to be functions of Coulomb blockade energies. In addition, the tunnel barriers between nanoparticles were considered to be tilted at high voltages. Furthermore, the role of thermal expansion coefficient of the tunnel junction gaps on the tunnel current is taken into account. The model calculations fit very well to the experimental data of a network of disordered gold nanoparticles, a forest of multi-wall carbon nanotubes, and a network of few-layer graphene nanoplates over a wide temperature range (5-300 K) at low and high DC bias voltages (0.001 mV–50 V). Our investigations indicate, although electron cotunneling in networks of disordered irregularly shaped NPs may occur, non-Arrhenius behavior at low temperatures cannot be described by the cotunneling model due to size distribution in the networks and irregular shape of nanoparticles. Non-Arrhenius behavior of the samples at zero bias voltage limit was attributed to the disorder in the samples. Unlike the electron cotunneling model, we found that the crossover from Arrhenius to non-Arrhenius behavior occurs at two temperatures, one at a high temperature and the other at a low temperature.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Roles of V-shaped pits on the improvement of quantum efficiency in InGaN/GaN multiple quantum well light-emitting diodes

The roles of V-shaped pits on the improvement of quantum efficiency in InGaN/GaN multiple quantum well (MQW) light-emitting diodes are investigated by numerical simulation. The simulation results show that V-shaped pits cannot only screen dislocations, but also play an important role on promoting hole injection into the MQWs. It is revealed that the injection of holes into the MQW via the sidewalls of the V-shaped pits is easier than via the flat region, due to the lower polarization charge densities in the sidewall structure with lower In concentration and (10–11)-oriented semi-polar facets.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

The equilibrium shape of fluid-fluid interfaces: Derivation and a new numerical method for Young’s and Young-Laplace equations

Many physical problems require explicit knowledge of the equilibrium shape of the interface between two fluid phases. Here, we present a new numerical method which is simply implementable and easily adaptable for a wide range of problems involving capillary deformations of fluid-fluid interfaces. We apply a simulated annealing algorithm to find the interface shape that minimizes the thermodynamic potential of the system. First, for completeness, we provide an analytical proof that minimizing this potential is equivalent to solving the Young-Laplace equation and the Young law. Then, we illustrate our numerical method showing two-dimensional results for fluid-fluid menisci between vertical or inclined walls and curved surfaces, capillary interactions between vertical walls, equilibrium shapes of sessile heavy droplets on a flat horizontal solid surface, and of droplets pending from flat or curved solid surfaces. Finally, we show illustrative three-dimensional results to point out the applicability of the method to micro- or nano-particles adsorbed at a fluid-fluid interface.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Shape transition in nano-pits after solid-phase etching of SiO{sub 2} by Si islands

We study the nano-pits formed during the etching of a SiO{sub 2} film by reactive Si islands at T≈1000 °C. Combining low energy electron microscopy, atomic force microscopy, kinetic Monte Carlo simulations, and an analytic model based on reaction and diffusion at the solid interface, we show that the shape of the nanopits depend on the ratio R/x{sub s} with R the Si island radius and x{sub s} the oxygen diffusion-length at the Si/SiO{sub 2} interface. For small R/x{sub s}, nanopits exhibit a single-well V-shape, while a double-well W-shape is found for larger R/x{sub s}. The analysis of the transition reveals that x{sub s}∼60 nm at T≈1000 °C.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Neutron yield enhancement in laser-induced deuterium-deuterium fusion using a novel shaped target

Neutron yields have direct correlation with the energy of incident deuterons in experiments of laser deuterated target interaction [Roth et al., Phys. Rev. Lett. 110, 044802 (2013) and Higginson et al., Phys. Plasmas 18, 100703 (2011)], while deuterated plasma density is also an important parameter. Experiments at the Shenguang II laser facility have produced neutrons with energy of 2.45 MeV using d (d, n) He reaction. Deuterated foil target and K-shaped target were employed to study the influence of plasma density on neutron yields. Neutron yield generated by K-shaped target (nearly 10{sup 6}) was two times higher than by foil target because the K-shaped target results in higher density plasma. Interferometry and multi hydro-dynamics simulation confirmed the importance of plasma density for enhancement of neutron yields.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Spin splitting generated in a Y-shaped semiconductor nanostructure with a quantum point contact

We have studied the spin splitting of the current in the Y-shaped semiconductor nanostructure with a quantum point contact (QPC) in a perpendicular magnetic field. Our calculations show that the appropriate tuning of the QPC potential and the external magnetic field leads to an almost perfect separation of the spin-polarized currents: electrons with opposite spins flow out through different output branches. The spin splitting results from the joint effect of the QPC, the spin Zeeman splitting, and the electron transport through the edge states formed in the nanowire at the sufficiently high magnetic field. The Y-shaped nanostructure can be used to split the unpolarized current into two spin currents with opposite spins as well as to detect the flow of the spin current. We have found that the separation of the spin currents is only slightly affected by the Rashba spin-orbit coupling. The spin-splitter device is an analogue of the optical device—the birefractive crystal that splits the unpolarized light into two beams with perpendicular polarizations. In the magnetic-field range, in which the current is carried through the edges states, the spin splitting is robust against the spin-independent scattering. This feature opens up a possibility of the application of the Y-shaped nanostructure as a non-ballistic spin-splitter device in spintronics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Dependence of the ferroelectric domain shape on the electric field of the microscope tip

A theory of an equilibrium shape of the domain formed in an electric field of a scanning force microscope (SFM) tip is proposed. We do not assume a priori that the domain has a fixed form. The shape of the domain is defined by the minimum of the free energy of the ferroelectric. This energy includes the energy of the depolarization field, the energy of the domain wall, and the energy of the interaction between the domain and the electric field of the SFM tip. The contributions of the apex and conical part of the tip are examined. Moreover, in the proposed approach, any narrow tip can be considered. The surface energy is determined on the basis of the Ginzburg-Landau-Devonshire theory and takes into account the curvature of the domain wall. The variation of the free energy with respect to the domain shape leads to an integro-differential equation, which must be solved numerically. Model results are illustrated for lithium tantalate ceramics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Contact resonances of U-shaped atomic force microscope probes

Recent approaches used to characterize the elastic or viscoelastic properties of materials with nanoscale resolution have focused on the contact resonances of atomic force microscope (CR-AFM) probes. The experiments for these CR-AFM methods involve measurement of several contact resonances from which the resonant frequency and peak width are found. The contact resonance values are then compared with the noncontact values in order for the sample properties to be evaluated. The data analysis requires vibration models associated with the probe during contact in order for the beam response to be deconvolved from the measured spectra. To date, the majority of CR-AFM research has used rectangular probes that have a relatively simple vibration response. Recently, U-shaped AFM probes have created much interest because they allow local sample heating. However, the vibration response of these probes is much more complex such that CR-AFM is still in its infancy. In this article, a simplified analytical model of U-shaped probes is evaluated for contact resonance applications relative to a more complex finite element (FE) computational model. The tip-sample contact is modeled using three orthogonal Kelvin-Voigt elements such that the resonant frequency and peak width of each mode are functions of the contact conditions. For the purely elastic case, the frequency results of the simple model are within 8% of the FE model for the lowest six modes over a wide range of contact stiffness values. Results for the viscoelastic contact problem for which the quality factor of the lowest six modes is compared show agreement to within 13%. These results suggest that this simple model can be used effectively to evaluate CR-AFM experimental results during AFM scanning such that quantitative mapping of viscoelastic properties may be possible using U-shaped probes.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Circularly polarized triple band glass shaped monopole patch antenna with metallic reflector for bluetooth & wireless applications

This paper presents the design and performance of strip line fed glass shaped monopole patch antenna having with overall size 30mm × 30 mm × 1.59 mm. In the patch; an eight shaped slot and in the ground plane an eight shaped ring are introduced. A metallic ground plane is also introduced at appropriate location beneath the ground plane. The proposed antenna is simulated by applying CST Microwave Studio simulator. Antenna provides circularly polarized radiations, triple broad impedance bandwidth of 203MHz (2.306GHz to 2.510GHz), 42MHz (2.685GHz to 2.757GHz) & GHz (3.63 GHz to 6.05 GHz), high flat gain (close to 5dBi) and good radiation properties in the desired frequency range. This antenna may be a very useful tool for 2.45GHz Bluetooth communication band as well as for 2.4GHz/5.2 GHz /5.8 GHz WLAN bands & 3.7GHz/5.5 GHz Wi-Max bands.

42 ENGINEERING↗
DOE OSTI2016

Linear gyrokinetic simulations of microinstabilities within the pedestal region of H-mode NSTX discharges in a highly shaped geometry

Linear (local) gyrokinetic predictions of edge microinstabilities in highly shaped, lithiated and non-lithiated NSTX discharges are reported using the gyrokinetic code GS2. Microtearing modes dominate the non-lithiated pedestal top. The stabilization of these modes at the lithiated pedestal top enables the electron temperature pedestal to extend further inwards, as observed experimentally. Kinetic ballooning modes are found to be unstable mainly at the mid-pedestal of both types of discharges, with un- stable trapped electron modes nearer the separatrix region. At electron wavelengths, ETG modes are found to be unstable from mid-pedestal outwards for η e, exp ~2.2 with higher growth rates for the lithiated discharge. Near the separatrix, the critical temperature gradient for driving ETG modes is reduced in the presence of lithium, re ecting the reduction of the lithiated density gradients observed experimentally. A preliminary linear study in the edge of non-lithiated discharges shows that the equilibrium shaping alters the electrostatic modes stability, found more unstable at high plasma shaping.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗