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At least 1,117 records · Page 62
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

The effects of ion mass variation and domain size on octupolar out-of-plane magnetic field generation in collisionless magnetic reconnection

Graf von der Pahlen and Tsiklauri [Phys. Plasmas 21, 060705 (2014)] established that the generation of octupolar out-of-plane magnetic field structure in a stressed X-point collapse is due to ion currents. The field has a central region, comprising of the well-known quadrupolar field (quadrupolar components), as well as four additional poles of reversed polarity closer to the corners of the domain (octupolar components). In this extended work, the dependence of the octupolar structure on domain size and ion mass variation is investigated. Simulations show that the strength and spatial structure of the generated octupolar magnetic field is independent of ion to electron mass ratio; thus showing that ion currents play a significant role in out-of-plane magnetic structure generation in physically realistic scenarios. Simulations of different system sizes show that the width of the octupolar structure remains the same and has a spacial extent of the order of the ion inertial length. The width of the structure thus appears to be independent on boundary condition effects. The length of the octupolar structure, however, increases for greater domain sizes, prescribed by the external system size. This was found to be a consequence of the structure of the in-plane magnetic field in the outflow region halting the particle flow and thus terminating the in-plane currents that generate the out-of-plane field. The generation of octupolar magnetic field structure is also established in a tearing-mode reconnection scenario. The differences in the generation of the octupolar field and resulting qualitative differences between X-point collapse and tearing-mode are discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Size scaling of negative hydrogen ion sources for fusion

The RF-driven negative hydrogen ion source (H{sup −}, D{sup −}) for the international fusion experiment ITER has a width of 0.9 m and a height of 1.9 m and is based on a ⅛ scale prototype source being in operation at the IPP test facilities BATMAN and MANITU for many years. Among the challenges to meet the required parameters in a caesiated source at a source pressure of 0.3 Pa or less is the challenge in size scaling of a factor of eight. As an intermediate step a ½ scale ITER source went into operation at the IPP test facility ELISE with the first plasma in February 2013. The experience and results gained so far at ELISE allowed a size scaling study from the prototype source towards the ITER relevant size at ELISE, in which operational issues, physical aspects and the source performance is addressed, highlighting differences as well as similarities. The most ITER relevant results are: low pressure operation down to 0.2 Pa is possible without problems; the magnetic filter field created by a current in the plasma grid is sufficient to reduce the electron temperature below the target value of 1 eV and to reduce together with the bias applied between the differently shaped bias plate and the plasma grid the amount of co-extracted electrons. An asymmetry of the co-extracted electron currents in the two grid segments is measured, varying strongly with filter field and bias. Contrary to the prototype source, a dedicated plasma drift in vertical direction is not observed. As in the prototype source, the performance in deuterium is limited by the amount of co-extracted electrons in short as well as in long pulse operation. Caesium conditioning is much harder in deuterium than in hydrogen for which fast and reproducible conditioning is achieved. First estimates reveal a caesium consumption comparable to the one in the prototype source despite the large size.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Effect of dislocation pile-up on size-dependent yield strength in finite single-crystal micro-samples

Recent research has explained that the steeply increasing yield strength in metals depends on decreasing sample size. In this work, we derive a statistical physical model of the yield strength of finite single-crystal micro-pillars that depends on single-ended dislocation pile-up inside the micro-pillars. We show that this size effect can be explained almost completely by considering the stochastic lengths of the dislocation source and the dislocation pile-up length in the single-crystal micro-pillars. The Hall–Petch-type relation holds even in a microscale single-crystal, which is characterized by its dislocation source lengths. Our quantitative conclusions suggest that the number of dislocation sources and pile-ups are significant factors for the size effect. They also indicate that starvation of dislocation sources is another reason for the size effect. Moreover, we investigated the explicit relationship between the stacking fault energy and the dislocation “pile-up” effect inside the sample: materials with low stacking fault energy exhibit an obvious dislocation pile-up effect. Our proposed physical model predicts a sample strength that agrees well with experimental data, and our model can give a more precise prediction than the current single arm source model, especially for materials with low stacking fault energy.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Morphological impact on the reaction kinetics of size-selected cobalt oxide nanoparticles

Apart from large surface areas, low activation energies are essential for efficient reactions, particularly in heterogeneous catalysis. Here, we show that not only the size of nanoparticles but also their detailed morphology can crucially affect reaction kinetics, as demonstrated for mass-selected, soft-landed, and oxidized cobalt clusters in a 6 nm to 18 nm size range. The method of reflection high-energy electron diffraction is extended to the quantitative determination of particle activation energies which is applied for repeated oxidation and reduction cycles at the same particles. We find unexpectedly small activation barriers for the reduction reaction of the largest particles studied, despite generally increasing barriers for growing sizes. We attribute these observations to the interplay of reaction-specific material transport with a size-dependent inner particle morphology.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Effect of size on electronic states in a strained pyramidal InAs-GaAs quantum dot system

The effect of size on electronic states in a strained pyramidal InAs-GaAs quantum dot system was studied. A comparison was made between two InAs quantum pyramids of different sizes embedded inside a cubic GaAs susbtrate material. Strain relaxation was carried out via the Metropolis Monte Carlo method and the calculated local strain tensors were then included to solve the energy values and the wave functions of the electronic states inside the two simulation cube. The 3D finite difference scheme was employed to solve the time independent Schrödinger equation based on the decoupled electron-hole model. Calculated energy values of the four lowest electronic states showed that the transitions between the electron and hole states widen as the size of the dot becomes smaller especially between the ground states. The confinement of electrons and holes become weaker as the size of the dot reduces.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2015

Reduction of effective terahertz focal spot size by means of nested concentric parabolic reflectors

An ongoing limitation of terahertz spectroscopy is that the technique is generally limited to the study of relatively large samples of order 4 mm across due to the generally large size of the focal beam spot. We present a nested concentric parabolic reflector design which can reduce the terahertz focal spot size. This parabolic reflector design takes advantage of the feature that reflected rays experience a relative time delay which is the same for all paths. The increase in effective optical path for reflected light is equivalent to the aperture diameter itself. We have shown that the light throughput of an aperture of 2 mm can be increased by a factor 15 as compared to a regular aperture of the same size at low frequencies. This technique can potentially be used to reduce the focal spot size in terahertz spectroscopy and enable the study of smaller samples.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Investigation of finite-size effects in chemical bonding of AuPd nanoalloys

In this paper, the size-dependent changes in energetic, vibrational, and electronic properties of C–O gas molecule interacting with surface Pd atom of a variety of AuPd nanoalloy structures are investigated by means of first principles calculations. The variation in C–O adsorption energies, C–O vibration frequencies (ν{sub C−O}), and Pd d-bond centers (ε{sub d}) on a series of non-supported Au{sub n−1}–Pd{sub 1} nanoparticles (with n varying from 13 to 147) and on two semi-finite surfaces are inspected with cluster size. We demonstrate for the first time that, with small AuPd bimetallic three-dimensional clusters as TOh{sub 38}, one can reach cluster size convergence even for such a sensitive observable as the adsorption energy on a metal surface. Indeed, the results show that the adsorbate-induced perturbation is extremely local and it only concerns the isolated Pd interacting with the reactive gas molecule. Except for 13 atom clusters, in which molecular behaviour is predominant, no finite-size effects are observed for surface Pd atom substituted in AuPd free nanoclusters above 38 atoms.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Exciton size and binding energy limitations in one-dimensional organic materials

In current organic photovoltaic devices, the loss in energy caused by the charge transfer step necessary for exciton dissociation leads to a low open circuit voltage, being one of the main reasons for rather low power conversion efficiencies. A possible approach to avoid these losses is to tune the exciton binding energy to a value of the order of thermal energy, which would lead to free charges upon absorption of a photon, and therefore increase the power conversion efficiency towards the Shockley-Queisser limit. We determine the size of the excitons for different organic molecules and polymers by time dependent density functional theory calculations. For optically relevant transitions, the exciton size saturates around 0.7 nm for one-dimensional molecules with a size longer than about 4 nm. For the ladder-type polymer poly(benzimidazobenzophenanthroline), we obtain an exciton binding energy of about 0.3 eV, serving as a lower limit of the exciton binding energy for the organic materials investigated. Furthermore, we show that charge transfer transitions increase the exciton size and thus identify possible routes towards a further decrease of the exciton binding energy.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

A theoretical investigation of the influence of gold nanosphere size on the decay and energy transfer rates and efficiencies of quantum emitters

We present in this contribution a comprehensive investigation of the effect of the size of gold nanospheres on the decay and energy transfer rates of quantum systems placed close to these nanospheres. These phenomena have been investigated before, theoretically and experimentally, but no comprehensive study of the influence of the nanoparticle size on important dependences of the decay and energy transfer rates, such as the dependence on the donor-acceptor spectral overlap and the relative positions of the donor, acceptor, and nanoparticle, exists. As such, different accounts of the energy transfer mechanism have been presented in the literature. We perform an investigation of the energy transfer mechanisms between emitters and gold nanospheres and between donor-acceptor pairs in the presence of the gold nanospheres using a Green’s tensor formalism, experimentally verified in our lab. We find that the energy transfer rate to small nanospheres is greatly enhanced, leading to a strong quenching of the emission of the emitter. When the nanosphere size is increased, it acts as an antenna, increasing the emission of the emitter. We also investigate the emission wavelength and intrinsic quantum yield dependence of the energy transfer to the nanosphere. As evidenced from the literature, the energy transfer process between the quantum system and the nanosphere can have a complicated distance dependence, with a r{sup −6} regime, characteristic of the Förster energy transfer mechanism, but also exhibiting other distance dependences. In the case of a donor-acceptor pair of quantum systems in the presence of a gold nanosphere, when the donor couples strongly to the nanosphere, acting as an enhanced dipole; the donor-acceptor energy transfer rate then follows a Förster trend, with an increased Förster radius. The coupling of the acceptor to the nanosphere has a different distance dependence. The angular dependence of the energy transfer efficiency between donor and acceptor exhibits a strong focusing effect and the same enhanced donor-dipole character in different angular arrangements. The spectral overlap of the donor emission and acceptor absorption spectra shows that the energy transfer follows the near-field scattering efficiency, with a red-shift from the localized surface plasmon peak for small sphere sizes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

All-in-all-out magnetic domain size in pyrochlore iridate thin films as probed by local magnetotransport

Pyrochlore iridates have attracted growing attention because of a theoretical prediction of a possible topological semimetal phase originating from all-in-all-out spin ordering. Related to the topological band structure, recent findings of the magnetic domain wall conduction have stimulated investigations of magnetic domain distribution in this system. Here, we investigate the size of magnetic domains in Eu{sub 2}Ir{sub 2}O{sub 7} single crystalline thin films by magnetoresistance (MR) using microscale Hall bars. Two distinct magnetic domains of the all-in-all-out spin structure are known to exhibit linear MR but with opposite signs, which enables us to estimate the ratio of the two domains in the patterned channel. The linear MR for 80 × 60 μm{sup 2} channel is nearly zero after zero-field cooling, suggesting random distribution of domains smaller than the channel size. In contrast, the wide distribution of the value of the linear MR is detected in 2 × 2 μm{sup 2} channel, reflecting the detectable domain size depending on each cooling-cycle. Compared to simulation results, we estimate the average size of a single all-in-all-out magnetic domain as 1–2 μm.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Polyethylene glycol (PEG) assisted size-controlled SnO{sub 2} nanoparticles by sol-gel process

Tetragonal phase tin oxide (SnO{sub 2}) nanoparticles have been synthesized by sol–gel method using SnCl{sub 4}.5H{sub 2}O and polyethylene glycol (PEG) of different concentration. The phase, size and purity of the final products are characterized by X-ray diffraction (XRD). The morphology is confirmed by scanning electron microscopy (SEM) analysis. There exists relationship between the concentration of PEG and particle size of SnO{sub 2} nanoparticles. Increase in concentration of PEG caused the reduction of particle size of tin oxide nanoparticles. The results suggest that the concentration of PEG plays a significant role in determining the size of SnO{sub 2} nanoparticles synthesized via this method. The optical property of the product has been explored by Ultraviolet (UV-visible) and Fourier Transform Infrared (FTIR) spectroscopic techniques.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Insights on finite size effects in ab initio study of CO adsorption and dissociation on Fe 110 surface

Adsorption and dissociation of hydrocarbons on metallic surfaces represent crucial steps on the path to carburization, eventually leading to dusting corrosion. While adsorption of CO molecules on Fe surface is a barrier-less exothermic process, this is not the case for the dissociation of CO into C and O adatoms and the diffusion of C beneath the surface that are found to be associated with large energy barriers. In practice, these barriers can be affected by numerous factors that combine to favour the CO-Fe reaction such as the abundance of CO and other hydrocarbons as well as the presence of structural defects. From a numerical point of view, studying these factors is challenging and a step-by-step approach is necessary to assess, in particular, the influence of the finite box size on the reaction parameters for adsorption and dissociation of CO on metal surfaces. Here, we use density functional theory (DFT) total energy calculations with the climbing-image nudged elastic band method to estimate the adsorption energies and dissociation barriers for different CO coverages with surface supercells of different sizes. We further compute the effect of periodic boundary condition for DFT calculations and find that the contribution from van der Waals interaction in the computation of adsorption parameters is important as they contribute to correcting the finite-size error in small systems. The dissociation process involves carbon insertion into the Fe surface causing a lattice deformation that requires a larger surface system for unrestricted relaxation. We show that, in the larger surface systems associated with dilute CO-coverages, C-insertion is energetically more favourable, leading to a significant decrease in the dissociation barrier. This observation suggests that a large surface system with dilute coverage is necessary for all similar metal-hydrocarbon reactions in order to study their fundamental electronic mechanisms, as an isolated phenomenon, free from finite-size effects.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Observation of propane cluster size distributions during nucleation and growth in a Laval expansion

We report on molecular-level studies of the condensation of propane gas and propane/ethane gas mixtures in the uniform (constant pressure and temperature) postnozzle flow of Laval expansions using soft single-photon ionization by vacuum ultraviolet light and mass spectrometric detection. The whole process, from the nucleation to the growth to molecular aggregates of sizes of several nanometers (∼5 nm), can be monitored at the molecular level with high time-resolution (∼3 μs) for a broad range of pressures and temperatures. For each time, pressure, and temperature, a whole mass spectrum is recorded, which allows one to determine the critical cluster size range for nucleation as well as the kinetics and mechanisms of cluster-size specific growth. The detailed information about the size, composition, and population of individual molecular clusters upon condensation provides unique experimental data for comparison with future molecular-level simulations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

X-ray intensity and source size characterizations for the 25 kV upgraded Manson source at Sandia National Laboratories

At Sandia National Laboratories, the x-ray generator Manson source model 5 was upgraded from 10 to 25 kV. The purpose of the upgrade is to drive higher characteristics photon energies with higher throughput. In this work we present characterization studies for the source size and the x-ray intensity when varying the source voltage for a series of K-, L-, and M-shell lines emitted from Al, Y, and Au elements composing the anode. We used a 2-pinhole camera to measure the source size and an energy dispersive detector to monitor the spectral content and intensity of the x-ray source. As the voltage increases, the source size is significantly reduced and line intensity is increased for the three materials. We can take advantage of the smaller source size and higher source throughput to effectively calibrate the suite of Z Pulsed Power Facility crystal spectrometers.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2018

Quantifying Spot Size Reduction of a 1.8 kA Electron Beam for Flash Radiography

The spot size of Axis-I at the Dual Axis Radiographic Hydrodynamic Test facility was reduced by 15.5% by including a small diameter drift tube that acts to aperture the outer diameter of the electron beam. Comparing the measured values to both analytic calculations and results from a particle-in-cell model shows that one-third to one-half of the spot size reduction is due to a drop in beam emittance. We infer that one-half to two-thirds of the spot-size reduction is due to a reduction in beam-target interactions. Sources of emittance growth and the scaling of the final focal spot size with emittance and solenoid aberrations are also presented.

47 OTHER INSTRUMENTATION↗
DOE OSTI2019

The grain-size effect on thermal conductivity of uranium dioxide

We have investigated the grain-boundary scattering effect on the thermal transport behavior of uranium dioxide (UO 2 ). The polycrystalline samples having different grain-sizes (0.125, 1.8, and 7:2 µm) have been prepared by a spark plasma sintering technique and characterized by x-ray powder diffraction, scanning electron microscope, and Raman spectroscopy. The thermal transport properties (the thermal conductivity and thermoelectric power) have been measured in the temperature range of 2–300 K, and the results were analyzed in terms of various physical parameters contributing to thermal conductivity in these materials in relation to grain-size. We show that thermal conductivity decreases systematically with lowering grain-size in the temperatures below 30 K, where the boundary scattering dominates the thermal transport. At higher temperatures, more scattering processes are involved in the heat transport in these materials, making the analysis difficult. We determined the grain-boundary Kapitza resistance that would result in the observed increase in thermal conductivity with grain-size and compared the value with Kapitza resistances calculated for UO 2 using molecular dynamics from the literature.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Palladium oxidation leads to methane combustion activity: Effects of particle size and alloying with platinum

Pd- and Pt-based catalysts are highly studied materials due to their widespread use in emissions control catalysis. However, claims continue to vary regarding the active phase and oxidation state of the metals. Different conclusions have likely been reached due to the heterogeneous nature of such materials containing various metal nanoparticle sizes and compositions, which may each possess unique redox features. In this work, using uniform nanocrystal catalysts, we study the effect of particle size and alloying on redox properties of Pd-based catalysts and show their contribution to methane combustion activity using operando quick extended x-ray absorption fine structure measurements. Results demonstrate that for all studied Pd sizes (3 nm–16 nm), Pd oxidation directly precedes CH 4 combustion to CO 2 , suggesting Pd oxidation as a prerequisite step to methane combustion, and an oxidation pretreatment shows equal or better catalysis than a reduction pretreatment. Results are then extended herein to uniform alloyed Pt x Pd 1–x nanoparticles, where oxidative pretreatments are shown to enhance low-temperature combustion. In these uniform alloys, we observe a composition-dependent effect with Pt-rich alloys showing the maximum difference between oxidative and reductive pretreatments. In Pt-rich alloys, we initially observe that the presence of Pt maintains Pd in a lower-activity reduced state. However, with time on stream, PdO eventually segregates under oxidizing combustion conditions, leading to a slowly increasing activity. Overall, across particle sizes and alloy compositions, we relate increased catalytic activity to Pd oxidation, thus shedding light on previous contrasting results related to the methane combustion activity of these catalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

The influence of particle size on the fluid dynamics of a laser-induced plasma

The interaction of a laser-induced shock wave with nanoparticles and microparticles of aluminum oxide is investigated through experiments and modeling. The chemistry and physics of the interaction between the particles and plasma generated from laser ablation shows similarities and discrete differences for the two particle sizes. For both particle sizes, early stage (< 10 μs) ionization was dominant and evidenced by higher concentrations of Al II. While both sizes exhibit ionization over the same duration, the intensity of emission was greater for nanoparticles indicating greater concentrations of ionized species. Moreover, the dispersion of species was notably more elongated for microparticles while radial dispersion was more pronounced for nanoparticles with elevated drag forces. At later stages (i.e., > 10 μs), oxidation reactions were dominant for both particle sizes, but the same distinctions in flow field were observed and attributed to particle drag. In all stages of interaction, microparticles expand axially with less drag that suppresses their radial expansion. As a result, the dispersion of reactive species was mapped over an up to 80% larger area for nanoparticles relative to microparticles. Results shown here can be applied toward advancing experimental diagnostics and particle-shock wave modeling and simulation efforts for energetic materials.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗