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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 1,045 records · Page 58
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Resolving experimental biases in the interpretation of diffusion experiments with a user-friendly numerical reactive transport approach

The reactive transport code CrunchClay was used to derive effective diffusion coefficients (D e ), clay porosities (ε), and adsorption distribution coefficients (K D ) from through-diffusion data while considering accurately the influence of unavoidable experimental biases on the estimation of these diffusion parameters. These effects include the presence of filters holding the solid sample in place, the variations in concentration gradients across the diffusion cell due to sampling events, the impact of tubing/dead volumes on the estimation of diffusive fluxes and sample porosity, and the effects of O-ring-filter setups on the delivery of solutions to the clay packing. Doing so, the direct modeling of the measurements of (radio)tracer concentrations in reservoirs is more accurate than that of data converted directly into diffusive fluxes. While the above-mentioned effects have already been described individually in the literature, a consistent modeling approach addressing all these issues at the same time has never been described nor made easily available to the community. A graphical user interface, CrunchEase, was created, which supports the user by automating the creation of input files, the running of simulations, and the extraction and comparison of data and simulation results. While a classical model considering an effective diffusion coefficient, a porosity and a solid/solution distribution coefficient (D e –ε–K D ) may be implemented in any reactive transport code, the development of CrunchEase makes it easy to apply by experimentalists without a background in reactive transport modeling. CrunchEase makes it also possible to transition more easily from a D e –ε–K D modeling approach to a state-of-the-art process-based understanding modeling approach using the full capabilities of CrunchClay, which include surface complexation modeling and a multi-porosity description of the clay packing with charged diffuse layers.

58 GEOSCIENCES↗
DOE OSTI2024

Probing ferroelectric domain structures and their switching dynamics in SrBi2Ta2O9 by in-situ electric biasing in transmission electron microscopy

Abstract Lead-free SrBi 2 Ta 2 O 9 (SBT) has been a promising ferroelectric material for various applications such as electronics and data storage due to its outstanding ferroelectric properties including high fatigue endurance and low leakage current. However, the atomic-scale domain structure and switching dynamics of ferroelectric SBT remain elusive. This study reveals that spontaneous polarization arises from canted bismuth-cation displacements, forming 90° and Ising-type 180° domain walls. Interestingly, topological pairs of ferroelectric vortex and antivortex connect ferroelectric boundaries where three domain walls converge. In situ electrical biasing transmission electron microscopy (TEM) reveals the dominance of 180° switching over 90°, where oxygen octahedral connectivity is protected by ferroelastic energy in the 90° domain wall. Consequently, all 180° domain walls and (anti)vortices are removed, leaving only 90° domain walls in the electrically poled states. Chemical deterioration along domain walls highlights vulnerability of SBT to ferroelectric fatigue. This study provides insight into crucial aspects for practical applications of SBT.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Alternating-bias assisted annealing of amorphous oxide tunnel junctions

Superconducting quantum bits (qubits) rely on ultra-thin, amorphous oxide tunneling barriers that can have significant inhomogeneities and defects as grown. This can result in relatively large uncertainties and deleterious effects in the circuits, limiting the scalability. Finding a robust solution to the junction reproducibility problem has been a long-standing goal in the field. Here, we demonstrate a transformational technique for controllably tuning the electrical properties of aluminum-oxide tunnel junctions. This is accomplished using a low-voltage, alternating-bias applied individually to the tunnel junctions, with which resistance tuning by more than 70% can be achieved. The data indicates an improvement of coherence and reduction of two-level system defects. Transmission electron microscopy shows that the treated junctions are predominantly amorphous, albeit with a more uniform distribution of alumina coordination across the barrier. This technique is expected to be useful for other devices based on ionic amorphous materials.

36 MATERIALS SCIENCE↗
DOE OSTI2016

The effect of a Ta oxygen scavenger layer on HfO 2 -based resistive switching behavior: Thermodynamic stability, electronic structure, and low-bias transport

Reversible resistive switching between high-resistance and low-resistance states in metal-oxide-metal heterostructures makes them very interesting for applications in random access memories. While recent experimental work has shown that inserting a metallic "oxygen scavenger layer'' between the positive electrode and oxide improves device performance, the fundamental understanding of how the scavenger layer modifies the heterostructure properties is lacking. We use density functional theory to calculate thermodynamic properties and conductance of TiN/HfO 2 /TiN heterostructures with and without a Ta scavenger layer. First, we show that Ta insertion lowers the formation energy of low-resistance states. Second, while the Ta scavenger layer reduces the Schottky barrier height in the high-resistance state by modifying the interface charge at the oxide-electrode interface, the heterostructure maintains a high resistance ratio between high-and low-resistance states. Lastly, we show that the low-bias conductance of device on-states becomes much less sensitive to the spatial distribution of oxygen removed from the HfO 2 in the presence of the Ta layer. By providing a fundamental understanding of the observed improvements with scavenger layers, we open a path to engineer interfaces with oxygen scavenger layers to control and enhance device performance. In turn, this may enable the realization of a non-volatile low-power memory technology with concomitant reduction in energy consumption by consumer electronics and offering significant benefits to society.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Dipole-induced exchange bias

The discovery of dipole-induced exchange bias (EB), switching from negative to positive sign, is reported in systems where the antiferromagnet and the ferromagnet are separated by a paramagnetic spacer (AFM–PM–FM).

Chemistry↗
DOE OSTI2024

The SRG/eROSITA All-Sky Survey: Exploring halo assembly bias with X-ray-selected superclusters

Numerical simulations indicate that the clustering of dark matter halos is not only dependent on the halo masses but has a secondary dependence on other properties, such as the assembly history of the halo. This phenomenon, known as the halo assembly bias (HAB), has been found mostly on galaxy scales; observational evidence on larger scales is scarce. In this work, we propose a novel method for exploring HAB on cluster scales using large samples of superclusters. Leveraging the largest-ever X-ray galaxy cluster and supercluster samples obtained from the first SRG/eROSITA all-sky survey, we constructed two subsamples of galaxy clusters that consist of supercluster members and isolated clusters, respectively. After correcting for the selection effects on redshift, mass, and survey depth, we computed the excess in the concentration of the intracluster gas of isolated clusters with respect to supercluster members, defined as δc gas ≡ c gas , ISO /c gas , SC –1 , to investigate the environmental effect on the concentration of clusters, a sign of HAB on cluster scales. We find that the average gas mass concentration of isolated clusters is a few percent higher than that of supercluster members, with a maximum significance of 2.8σ. The result for δc gas varies with the overdensity ratio, f, in supercluster identification, cluster mass proxies, and mass and redshift ranges but remains positive in almost all the measurements. We measure slightly larger δc gas when adopting a higher f for supercluster identification. The δc gas is also higher for low-mass and low-redshift clusters. We performed weak lensing analyses to compare the total mass concentration of the two classes and find a similar trend in total mass concentration as obtained from the gas mass concentration. Our results are consistent with the prediction of HAB on cluster scales, where halos located in denser environments are less concentrated; this trend is stronger for halos with lower masses and at lower redshifts. These phenomena can be explained by the fact that clusters in denser environments, such as superclusters, have experienced more mergers than isolated clusters in their assembling history. This work paves the way to explore HAB with X-ray superclusters and demonstrates that large samples of superclusters with X-ray and weak-lensing data can advance our understanding of the evolution of the large-scale structure.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Improving electrical performance and bias stability of HfInZnO-TFT with optimizing the channel thickness

RF magnetron sputtered HfInZnO film and atomic layer deposition (ALD) Al{sub 2}O{sub 3} film were employed for thin film transistors (TFTs) as channel layer and gate insulator, respectively. To achieve HfInZnO-TFT with high performance and good bias stability, the thickness of HfInZnO active layer was optimized. The performance of HfInZnO-TFTs was found to be thickness dependent. As the HfInZnO active layer got thicker, the leakage current greatly increased from 1.73 × 10{sup −12} to 2.54 × 10{sup −8} A, the threshold voltage decreased from 7.4 to −4.7 V, while the subthreshold swing varied from 0.41 to 1.07 V/decade. Overall, the HfInZnO film showed superior performance, such as saturation mobility of 6.4 cm{sup 2}/V s, threshold voltage of 4.2 V, subthreshold swing of 0.43 V/decade, on/off current ratio of 3 × 10{sup 7} and V{sub th} shift of 3.6 V under V{sub GS}= 10 V for 7200 s. The results demonstrate the possibility of fabricating TFTs using HfInZnO film as active layer and using ALD Al{sub 2}O{sub 3} as gate insulator.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Recovery from ultraviolet-induced threshold voltage shift in indium gallium zinc oxide thin film transistors by positive gate bias

The effect of short-duration ultraviolet (UV) exposure on the threshold voltage (V{sub th}) of amorphous indium gallium zinc oxide thin film transistors (TFTs) and its recovery characteristics were investigated. The V{sub th} exhibited a significant negative shift after UV exposure. The V{sub th} instability caused by UV illumination is attributed to the positive charge trapping in the dielectric layer and/or at the channel/dielectric interface. The illuminated devices showed a slow recovery in threshold voltage without external bias. However, an instant recovery can be achieved by the application of positive gate pulses, which is due to the elimination of the positive trapped charges as a result of the presence of a large amount of field-induced electrons in the interface region.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Modeling ultrashort laser-induced emission from a negatively biased metal

A theoretical study of ultrashort laser-induced electron emission from a negatively biased metallic cathode has been performed. Classical as well as tunneling electron emission mechanisms are considered. It was found that electron emission is governed by an interplay of processes inside as well as above the cathode. A hybrid model is proposed, where the electron distribution within the target is retrieved from Boltzmann scattering integrals, while the charge distribution above it is studied by a Particle-In-Cell simulation. The results indicate that non-equilibrium effects determine the initial emission process, whereas the space charge above the target suppresses the effectively emitted charge.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Two-path transport measurements with bias dependence on a triple quantum dot

We present transport measurements on a lateral triple quantum dot with a star-like geometry and one lead attached to each dot. The system is studied in a regime close to established quadruple points, where all three dots are in resonance. The specific sample structure allows us to apply two different bias voltages to the two source leads and thus to study the influence between the paths with serial double dots.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2013

Bias voltage dependence of the electron spin depolarization in quantum wires in the quantum Hall regime detected by the resistively detected NMR

We performed the resistively-detected nuclear magnetic resonance (RDNMR) to study the electron spin polarization in the non-equilibrium quantum Hall regime. By measuring the Knight shift, we derive source-drain bias voltage dependence of the electron spin polarization in quantum wires. The electron spin polarization shows minimum value around the threshold voltage of the dynamic nuclear polarization.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Bias-dependent interface roughening and its effect on electric bistability of organic devices

Atomic force microscopy (AFM), field-emission scanning electron microscopy, and energy dispersive X-Ray spectroscopy are used to study morphological and compositional variations of metal-organic interfaces in organic bistable devices. The results show that bias voltage causes rougher interfaces with new protrusions, and the switching phenomena origins from the evolution of these protrusions under external electric field. In order to exclude other possible factors, three types of bistable devices are designed and examined. In addition, metal-coated AFM probes are utilized to simulate the switching process, which yields similar results and corroborates our conclusion.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Exchange bias and magnetic properties induced by intrinsic structural distortion in CaMn{sub 3}O{sub 6} nanoribbons

Single-crystalline CaMn{sub 3}O{sub 6} nanoribbons have been synthesized by a molten-salt method. To explore the origin of the magnetism of nanosized antiferromagnetic (AFM) manganites, a comparative study has been conducted for CaMn{sub 3}O{sub 6} (CMO-1) and post-growth vacuum annealed (CMO-2) nanoribbons. A lattice expansion resulting from oxygen release during vacuum annealing is observed. Correspondingly, AFM ordering in CMO-2 is further suppressed, and ferromagnetism and spin-glass (SG)-like behavior are significantly enhanced, which are presumed attributable to the intrinsic structural distortions induced by oxygen vacancies. In this case, side and surface effects are not decisive factors. In addition, this study provided observations of the exchange bias effect in manganite nanoribbons with an AFM-SG-like-ferromagnetic (FM) structure, as compared with the typical AFM-core-FM-shell.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Exchange bias and crystal structure of epitaxial (111) FePt/BiFeO{sub 3} sputtered thin films

Crystallographic structure and magnetic properties of the epitaxial FePt (10 nm)/BiFeO{sub 3} (BFO) (10 nm) bilayer films grown on (111) SrTiO{sub 3} (STO) substrates with different deposition temperatures of FePt layers (T{sub d}) have been investigated using magnetron sputtering. Out-of-plane radial scan along (111) direction and off-normal (002) azimuthal scan, determined by synchrotron radiation x-ray diffractometry, evidence that the FePt layers were well epitaxially grown on the (111) epitaxial BFO layers for the samples with T{sub d} = 300 and 700 °C. On the contrary, for the bilayer films with T{sub d} = 500 °C, the FePt and BFO layers exhibit low epitaxial quality. Large in-plane exchange bias field (H{sub eb}) values of 45–412 Oe are obtained for the L1{sub 0}-FePt/BFO bilayer films measured with applied field of 12 kOe at room temperature. The change of effective interfacial area, observed by scanning electron microscopy, between FePt island-like particles and BFO continuous layers, and epitaxiality of the bilayer were correlated with the evolution of H{sub eb}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Exchange bias and antiferromagnetic interfacial exchange coupling in the mixed-spin oxide Li{sub 2/7}Ni{sub 5/7}O

We present a detailed study on magnetic properties of mixed spin oxide Li{sub 2/7}Ni{sub 5/7}O, where small ferromagnetic (FM) clusters (with averaged diameter of ∼18 Å) immersed with an antiferromagnetic (AFM) host. The exchange bias (EB) with shifts of the hysteresis loop along both the field and magnetization axes is found, which is due to the interfacial interaction between the FM clusters and the AFM host. An AFM interfacial exchange coupling is deduced from the exchange interactions between Ni ions. The type and strength of this interfacial exchange interaction are discussed in terms of the EB at low temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗