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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 1,711 records · Page 95
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

On the structural and impedance characteristics of Li- doped PEO, using n-butyl lithium in hexane as dopant

Nowadays polymer based solid state electrolytes for applications in rechargeable battery systems are highly sought after materials, pursued extensively by various research groups worldwide. Numerous methods are discussed in literature to improve the fundamental properties like electrical conductivity, mechanical stability and interfacial stability of polymer based electrolytes. The application of these electrolytes in Li-ion cells is still in the amateur state, due to low ionic conductivity, low lithium transport number and the processing difficulties. The present work is an attempt to study the effects of Li doping on the structural and transport properties of the polymer electrolyte, poly-ethelene oxide (PEO) (Molecular weight: 200,000). Li doped PEO was obtained by treating PEO with n-Butyllithium in hexane for different doping concentrations. Structural characterization of the samples was done by XRD and FTIR techniques. Impedance measurements were carried out to estimate the ionic conductivity of Li doped PEO samples. It is seen that, the crystallinity of the doped PEO decreases on increasing the doping concentration. XRD and FTIR studies support this observation. It is inferred that, ionic conductivity of the sample is increasing on increasing the doping concentration since less crystallinity permits more ionic transport. Impedance measurements confirm the results quantitatively.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Unraveling multi-spin effects in rotational resonance nuclear magnetic resonance using effective reduced density matrix theory

A quantum-mechanical model integrating the concepts of reduced density matrix and effective Hamiltonians is proposed to explain the multi-spin effects observed in rotational resonance (R{sup 2}) nuclear magnetic resonance (NMR) experiments. Employing this approach, the spin system of interest is described in a reduced subspace inclusive of its coupling to the surroundings. Through suitable model systems, the utility of our theory is demonstrated and verified with simulations emerging from both analytic and numerical methods. The analytic results presented in this article provide an accurate description/interpretation of R{sup 2} experimental results and could serve as a test-bed for distinguishing coherent/incoherent effects in solid-state NMR.

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

Effects of a static electric field on two-color photoassociation between different atoms

We study non-perturbative effects of a static electric field on two-color photoassociation of different atoms. A static electric field induces anisotropy in scattering between two different atoms and hybridizes field-free rotational states of heteronuclear dimers or polar molecules. In a previous paper [D. Chakraborty et al., J. Phys. B 44, 095201 (2011)], the effects of a static electric field on one-color photoassociation between different atoms has been described through field-modified ground-state scattering states, neglecting electric field effects on heteronuclear diatomic bound states. To study the effects of a static electric field on heteronuclear bound states, and the resulting influence on Raman-type two-color photoassociation between different atoms in the presence of a static electric field, we develop a non-perturbative numerical method to calculate static electric field-dressed heteronuclear bound states. We show that the static electric field induced scattering anisotropy as well as hybridization of rotational states strongly influence two-color photoassociation spectra, leading to significant enhancement in PA rate and large shift. In particular, for static electric field strengths of a few hundred kV/cm, two-color PA rate involving high-lying bound states in electronic ground-state increases by several orders of magnitude even in the weak photoassociative coupling regime.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Non-linear dynamic characteristics and optimal control of giant magnetostrictive film subjected to in-plane stochastic excitation

The non-linear dynamic characteristics and optimal control of a giant magnetostrictive film (GMF) subjected to in-plane stochastic excitation were studied. Non-linear differential items were introduced to interpret the hysteretic phenomena of the GMF, and the non-linear dynamic model of the GMF subjected to in-plane stochastic excitation was developed. The stochastic stability was analysed, and the probability density function was obtained. The condition of stochastic Hopf bifurcation and noise-induced chaotic response were determined, and the fractal boundary of the system's safe basin was provided. The reliability function was solved from the backward Kolmogorov equation, and an optimal control strategy was proposed in the stochastic dynamic programming method. Numerical simulation shows that the system stability varies with the parameters, and stochastic Hopf bifurcation and chaos appear in the process; the area of the safe basin decreases when the noise intensifies, and the boundary of the safe basin becomes fractal; the system reliability improved through stochastic optimal control. Finally, the theoretical and numerical results were proved by experiments. The results are helpful in the engineering applications of GMF.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Stochastic bifurcation and fractal and chaos control of a giant magnetostrictive film-shape memory alloy composite cantilever plate subjected to in-plane harmonic and stochastic excitation

Stochastic bifurcation and fractal and chaos control of a giant magnetostrictive film–shape memory alloy (GMF–SMA) composite cantilever plate subjected to in-plane harmonic and stochastic excitation were studied. Van der Pol items were improved to interpret the hysteretic phenomena of both GMF and SMA, and the nonlinear dynamic model of a GMF–SMA composite cantilever plate subjected to in-plane harmonic and stochastic excitation was developed. The probability density function of the dynamic response of the system was obtained, and the conditions of stochastic Hopf bifurcation were analyzed. The conditions of noise-induced chaotic response were obtained in the stochastic Melnikov integral method, and the fractal boundary of the safe basin of the system was provided. Finally, the chaos control strategy was proposed in the stochastic dynamic programming method. Numerical simulation shows that stochastic Hopf bifurcation and chaos appear in the parameter variation process. The boundary of the safe basin of the system has fractal characteristics, and its area decreases when the noise intensifies. The system reliability was improved through stochastic optimal control, and the safe basin area of the system increased.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Measurement of positive direct current corona pulse in coaxial wire-cylinder gap

In this paper, a system is designed and developed to measure the positive corona current in coaxial wire-cylinder gaps. The characteristic parameters of corona current pulses, such as the amplitude, rise time, half-wave time, and repetition frequency, are statistically analyzed and a new set of empirical formulas are derived by numerical fitting. The influence of space charges on corona currents is tested by using three corona cages with different radii. A numerical method is used to solve a simplified ion-flow model to explain the influence of space charges. Based on the statistical results, a stochastic model is developed to simulate the corona pulse trains. And this model is verified by comparing the simulated frequency-domain responses with the measured ones.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Quasi-bound states and continuum absorption background of polar Al{sub 0.5}Ga{sub 0.5}N/GaN quantum dots

A theoretical interpretation of the photoluminescence excitation spectra of self-organized polar GaN/(Al,Ga)N quantum dots is presented. A numerical method assuming a realistic shape of the dots and including the built-in electric field effects is developed to calculate their energy structure and hence their optical absorption. The electron and hole spectra show the existence of a set of quasi-bound states that does not originate from the wetting layer and plays a crucial role in the observed absorption spectrum of the GaN/(Al,Ga)N dots. Transitions involving these quasi-bound states and wetting layer states give a sufficient explanation for the observed continuum absorption background. The properties of this absorption band, especially its extension, depend strongly on the dot's size. Our simulation provides a natural explanation of the experimental luminescence excitation spectra of ensembles of dots of different heights. Our theoretical model can be convenient for future optical studies including systems with more complicated potentials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Limitation of parallel flow in double diffusive convection: Two- and three-dimensional transitions in a horizontal porous domain

Two-dimensional (2D) and three-dimensional (3D) numerical simulations of double diffusion natural convection in an elongated enclosure filled with a binary fluid saturating a porous medium are carried out in the present work. The Boussinesq approximation is made in the formulation of the problem, and Neumann boundary conditions for temperature and concentration are adopted, respectively, on vertical and horizontal walls of the cavity. The used numerical method is based on the control volume approach, with the third order quadratic upstream interpolation scheme in approximating the advection terms. A semi implicit method algorithm is used to handle the velocity-pressure coupling. To avoid the excessively high computer time inherent to the solution of 3D natural convection problems, full approximation storage with full multigrid method is used to solve the problem. A wide range of the controlling parameters (Rayleigh-Darcy number Ra, lateral aspect ratio Ay, Lewis number Le, and the buoyancy ration N) is investigated. We clearly show that increasing the depth of the cavity (i.e., the lateral aspect ratio) has an important effect on the flow patterns. The 2D perfect parallel flows obtained for small lateral aspect ratio are drastically destabilized by increasing the cavity lateral dimension. This yields a 3D fluid motion with a much more complex flow pattern and the usually considered 2D parallel flow model cannot be applied.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Mode composition analysis on experimental results of a Gigawatt-class Ka-band overmoded Cerenkov oscillator

In this paper, a modified numerical method is used to investigate the mode composition of a Gigawatt-class Ka-band overmoded Cerenkov oscillator, which has been proposed and studied in our previous experiment. In the experiment, the measured angular distribution of radiation did not fit a single TM{sub 01} mode. So the particle in cell code calculations and the antenna radiation calculations are carried out, which show a consistent picture: the dominant modes are the TM{sub 01} mode and the TM{sub 03} mode, and their phase relationship is constant with time; therefore, a steady radiation pattern is produced, which matches the experimental data. As a conclusion, the comprehensive analysis shows that the existing modes of the output microwave in our experiment are the first five TM{sub 0n} modes (n = 1–5), with corresponding power ratios of 36.64%, 0.78%, 56.26%, 5.70%, and 0.52%, and relative phase differences of 0°, 146°, 54°, 169°, and 133°.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Asymmetric modes decomposition in an overmoded relativistic backward wave oscillator

Most of the investigated overmoded relativistic backward wave oscillators (RBWOs) are azimuthally symmetric; thus, they are designed through two dimensional (2-D) particle-in-cell (PIC) simulations. However, 2-D PIC simulations cannot reveal the effect of asymmetric modes on beam-wave interaction. In order to investigate whether asymmetric mode competition needs to be considered in the design of overmoded RBWOs, a numerical method of determining the composition of both symmetric and asymmetric modes in three dimensional (3-D) PIC simulations is introduced in this paper. The 2-D and 3-D PIC simulation results of an X-band overmoded RBWO are analyzed. Our analysis indicates that the 2-D and 3-D PIC simulation results of our device are quite different due to asymmetric mode competition. In fact, asymmetric surface waves, especially EH{sub 11} mode, can lead to serious mode competition when electron beam propagates near the surface of slow wave structures (SWSs). Therefore, additional method of suppressing asymmetric mode competition, such as adjusting the reflections at both ends of SWSs to decrease the Q-factor of asymmetric modes, needs to be utilized in the design of overmoded RBWOs. Besides, 3-D PIC simulation and modes decomposition are essential for designing overmoded RBWOs.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Study on the thermal resistance in secondary particles chain of silica aerogel by molecular dynamics simulation

In this article, molecular dynamics simulation was performed to study the heat transport in secondary particles chain of silica aerogel. The two adjacent particles as the basic heat transport unit were modelled to characterize the heat transfer through the calculation of thermal resistance and vibrational density of states (VDOS). The total thermal resistance of two contact particles was predicted by non-equilibrium molecular dynamics simulations (NEMD). The defects were formed by deleting atoms in the system randomly first and performing heating and quenching process afterwards to achieve the DLCA (diffusive limited cluster-cluster aggregation) process. This kind of treatment showed a very reasonable prediction of thermal conductivity for the silica aerogels compared with the experimental values. The heat transport was great suppressed as the contact length increased or defect concentration increased. The constrain effect of heat transport was much significant when contact length fraction was in the small range (<0.5) or the defect concentration is in the high range (>0.5). Also, as the contact length increased, the role of joint thermal resistance played in the constraint of heat transport was increasing. However, the defect concentration did not affect the share of joint thermal resistance as the contact length did. VDOS of the system was calculated by numerical method to characterize the heat transport from atomic vibration view. The smaller contact length and greater defect concentration primarily affected the longitudinal acoustic modes, which ultimately influenced the heat transport between the adjacent particles.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Communication: Newton homotopies for sampling stationary points of potential energy landscapes

One of the most challenging and frequently arising problems in many areas of science is to find solutions of a system of multivariate nonlinear equations. There are several numerical methods that can find many (or all if the system is small enough) solutions but they all exhibit characteristic problems. Moreover, traditional methods can break down if the system contains singular solutions. Here, we propose an efficient implementation of Newton homotopies, which can sample a large number of the stationary points of complicated many-body potentials. We demonstrate how the procedure works by applying it to the nearest-neighbor ϕ{sup 4} model and atomic clusters.

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

Numerical matching of the sheath and presheath solutions for a spherical probe in radial-motion theory

The theory of positive-ion collection by a probe immersed in a low-pressure plasma was reviewed and extended by Allen, Boyd, and Reynolds [Proc. Phys. Soc. 70, 297 (1957)]. For a given value of the ion current, the boundary values of the matched “nonneutral” or “sheath” solution V{sup ~}{sub nn}{sup (m)}(r; r{sub m}) were obtained from the “quasineutral” or “presheath” solution V{sup ~}{sub qn}(r) by choosing the small potential and electric-field values corresponding to some large “matching radius” r{sub m}. Here, a straightforward but efficient numerical method is presented for systematically determining an optimal value of the matching radius at which the presheath and sheath solutions are joined to yield the “matched” potential profile. Some suitable initial matching radius r{sub m1} is chosen and the related potential and electric-field values of the quasineutral solution are calculated. Using these as boundary conditions, Poisson's equation is integrated to yield the matched nonneutral solution including the corresponding potential at the probe surface. This procedure is repeated for increasing values r{sub m2}, r{sub m3},…. until the resulting potential at the probe surface becomes practically constant. The corresponding value of r{sub m} is taken as the “optimal” matching radius r{sub mo} at which the presheath and sheath solutions are ultimately joined to yield the “optimal” matched potential profile in the entire plasma-probe transition region. It is also shown that the Bohm criterion is inapplicable in the present problem.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Evolution of chirped laser pulses in a magnetized plasma channel

The propagation of intense, short, sinusoidal laser pulses in a magnetized plasma channel has been studied. The wave equation governing the evolution of the radiation field is set up and a variational technique is used to obtain the equations describing the evolution of the laser spot size, pulse length and chirp parameter. Numerical methods are used to analyze the simultaneous evolution of these parameters. The effect of the external magnetic field on initially chirped as well as unchirped laser pulses on the spot size, pulse length and chirping has been analyzed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Stabilization of solitons under competing nonlinearities by external potentials

We report results of the analysis for families of one-dimensional (1D) trapped solitons, created by competing self-focusing (SF) quintic and self-defocusing (SDF) cubic nonlinear terms. Two trapping potentials are considered, the harmonic-oscillator (HO) and delta-functional ones. The models apply to optical solitons in colloidal waveguides and other photonic media, and to matter-wave solitons in Bose-Einstein condensates loaded into a quasi-1D trap. For the HO potential, the results are obtained in an approximate form, using the variational and Thomas-Fermi approximations, and in a full numerical form, including the ground state and the first antisymmetric excited one. For the delta-functional attractive potential, the results are produced in a fully analytical form, and verified by means of numerical methods. Both exponentially localized solitons and weakly localized trapped modes are found for the delta-functional potential. The most essential conclusions concern the applicability of competing Vakhitov-Kolokolov (VK) and anti-VK criteria to the identification of the stability of solitons created under the action of the competing SF and SDF terms.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Clamping characteristics study on different types of clamping unit

Plastic products are becoming more and more widely used in aerospace, IT, digital electronics and many other fields. With the development of technology, the requirement of product precision is getting higher and higher. However, type and working performance of clamping unit play a decisive role in product precision. Clamping characteristics of different types of clamping unit are discussed in this article, which use finite element numerical analysis method through the software ABAQUS to study the clamping uniformity, and detect the clamping force repeatability precision. The result shows that compared with toggled three-platen clamping unit, clamping characteristics of internal circulation two-platen clamping unit are better, for instance, its mold cavity deformation and force that bars and mold parting surface suffered are more uniform, and its clamping uniformity and repeatability precision is also better.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Spin-crossover molecule based thermoelectric junction

Using ab-initio numerical methods, we explore the spin-dependent transport and thermoelectric properties of a spin-crossover molecule (i.e., iron complex of 2-(1H-pyrazol-1-yl)-6-(1H-tetrazole-5-yl)pyridine) based nano-junction. We demonstrate a large magnetoresistance, efficient conductance-switching, and spin-filter activity in this molecule-based two-terminal device. The spin-crossover process also modulates the thermoelectric entities. It can efficiently switch the magnitude as well as spin-polarization of the thermocurrent. We find that thermocurrent is changed by ∼4 orders of magnitude upon spin-crossover. Moreover, it also substantially affects the thermopower and consequently, the device shows extremely efficient spin-crossover magnetothermopower generation. Furthermore, by tuning the chemical potential of electrodes into a certain range, a pure spin-thermopower can be achieved for the high-spin state. Finally, the reasonably large values of figure-of-merit in the presence and absence of phonon demonstrate a large heat-to-voltage conversion efficiency of the device. We believe that our study will pave an alternative way of tuning the transport and thermoelectric properties through the spin-crossover process and can have potential applications in generation of spin-dependent current, information storage, and processing.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

A matching approach to communicate through the plasma sheath surrounding a hypersonic vehicle

In order to overcome the communication blackout problem suffered by hypersonic vehicles, a matching approach has been proposed for the first time in this paper. It utilizes a double-positive (DPS) material layer surrounding a hypersonic vehicle antenna to match with the plasma sheath enclosing the vehicle. Analytical analysis and numerical results indicate a resonance between the matched layer and the plasma sheath will be formed to mitigate the blackout problem in some conditions. The calculated results present a perfect radiated performance of the antenna, when the match is exactly built between these two layers. The effects of the parameters of the plasma sheath have been researched by numerical methods. Based on these results, the proposed approach is easier to realize and more flexible to the varying radiated conditions in hypersonic flight comparing with other methods.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗