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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 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Rayleigh-Taylor instability at spherical interfaces between viscous fluids: Fluid/vacuum interface

For a spherical interface of radius R separating two different homogeneous regions of incompressible viscous fluids under the action of a radially directed acceleration, we perform a linear stability analysis in terms of spherical surface harmonics Y n to derive the dispersion relation. The instability behavior is investigated by computing the growth rates and the most-unstable modes as a function of the spherical harmonic degree n. This general methodology is applicable to the entire parameter space spanned by the Atwood number, the viscosity ratio, and the dimensionless number B = (α R Ρ² 2 /μ² ² )¹ / ³ R (where α R , Ρ 2 and μ 2 are the local radial acceleration at the interface, and the density and viscosity of the denser overlying fluid, respectively). While the mathematical formulation here is general, this paper focuses on instability that arises at a spherical viscous fluid/vacuum interface as there is a great deal to be learned from the effects of one-fluid viscosity and sphericity alone. To quantify and understand the effect that curvature and radial accelerationhave on the Rayleigh-Taylor instability, a comparison of the growth rates, under homologous driving conditions, between the planar and spherical interfaces is performed. The derived dispersion relation for the planar interface accounts for an underlying finite fluid region of thickness L and normal acceleration α R . Under certain conditions, the development of the most-unstable modes at a spherical interface can take place via the superposition of two adjacent spherical harmonics Y n and Y n+1 . This bimodality in the evolution of disturbances in the linear regime does not have a counterpart in the planar configuration where the most-unstable modes are associated with a unique wave number.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Impact of Inverter-Interfaced Renewable Generation on Transient Stability at Varying Levels of Penetration: Preprint

The transient dynamics of conventional power systems are dominated by synchronous generators, and low-inertia inverters serve as the primary interface between power systems and renewable energy resources. Increasing penetration levels of inverter-interfaced generation are impacting the transient dynamics of power systems. In this paper, we attempt to quantify these impacts at different penetration levels. An elementary system including one inverter, one synchronous generator, and one load is studied under different parameter variations to assess the transient dynamics of the system. To quantify transient stability, the post-fault regions of attraction of the system are estimated by the sample-based basin stability approach with parallel computing technology. The simulation results suggest that different inverter penetration levels, inertias of the generator, inverter controller gains, and phase-locked loop controller gains have a clear impact on the regions of attraction as well as on the transient dynamics of proposed systems.

24 POWER TRANSMISSION AND DISTRIBUTION↗
DOE OSTI2014

Thermal Performance and Reliability Characterization of Bonded Interface Materials (BIMs): Preprint

Thermal interface materials are an important enabler for low thermal resistance and reliable electronics packaging for a wide array of applications. There is a trend towards bonded interface materials (BIMs) because of their potential for low thermal resistivity (< 1 mm2K/W). However, BIMs induce thermomechanical stresses in the package and can be prone to failures and integrity risks. Deteriorated interfaces can result in high thermal resistance in the package and degradation and/or failure of the electronics. DARPA's Thermal Management Technologies program has addressed this challenge, supporting the development of mechanically-compliant, low resistivity nano-thermal interface (NTI) materials. In this work, we describe the testing procedure and report the results of NREL's thermal performance and reliability characterization of an initial sample of four different NTI-BIMs.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2020

The roles of yield strength mismatch, interface strength, and plastic strain gradients in fatigue crack growth across interfaces

An irreversible cohesive zone model was used to understand how plastic strain gradients and yield strength mismatches affect fatigue crack growth across bi-material and multilayer interfaces for a range of interface strengths. Parametric studies identified regimes of penetration through the interface, deflection into the interface, and a transition where cracks extend on both paths. Plastic strain gradients strongly influence the transition regime, and a yield strength mismatch is necessary to cause deflection. For crack growth across an interlayer, interactions between the interfaces occur when the crack-tip plastic zone spans the interlayer. Plastic strain gradients promote penetration through the interlayer.

42 ENGINEERING↗
DOE OSTI2011

In situ imaging of orthoclase-aqueous solution interfaces with x-ray reflection interface microscopy

The use of x-ray reflection interface microscopy (XRIM) to image molecular-scale topography at the aqueous-solid interface, in situ, is described. Specifically, we image interfacial topography of the orthoclase-(001)-aqueous solution interface at room temperature and describe the challenges associated with in situ XRIM imaging. The measurements show that the reflectivity signal for in situ XRIM measurements is substantially smaller than that for ex situ measurements, because of both intrinsic and extrinsic factors. There is also a systematic temporal reduction in the image intensity with increasing x-ray dose, revealing that interaction of the focused x-ray beam with the orthoclase interfaces leads to interfacial perturbations, presumably in the form of surface roughening. This image fading is localized to the x-ray beam footprint, suggesting that the primary damage mechanism is initiated by photoelectrons produced by x-ray beam absorption near the substrate-electrolyte interface. Finally, the role of aqueous solution composition in controlling the sensitivity of the orthoclase surface to x-ray beam-induced effects is explored. A substantial increase in the orthoclase (001) surface stability was observed in solutions having elevated ionic strength, apparently as a result of the reduced lifetime of radiation chemistry products at these conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Reduction in interface state density of Al{sub 2}O{sub 3}/InGaAs metal-oxide-semiconductor interfaces by InGaAs surface nitridation

We report the decrease in interface trap density (D{sub it}) in Al{sub 2}O{sub 3}/InGaAs metal-oxide-semiconductor (MOS) capacitors by using electron cyclotron resonance plasma nitridation of the InGaAs surfaces. The impact of the nitridation process on the MOS interface properties is quantitatively examined. The plasma nitridation process is observed to form a nitrided layer at the InGaAs surface. The nitridation using microwave power (P{sub microwave}) of 250 W and nitridation time (t{sub nitridation}) of 420 s can form Al{sub 2}O{sub 3}/InGaAs MOS interfaces with a minimum D{sub it} value of 2.0 Multiplication-Sign 10{sup 11} cm{sup -2} eV{sup -1}. On the other hand, the nitridation process parameters such as P{sub microwave} and t{sub nitridation} are found to strongly alter D{sub it} (both decrease and increase are observed) and capacitance equivalent thickness (CET). It is found that the nitridation with higher P{sub microwave} and shorter t{sub nitridation} can reduce D{sub it} with less CET increase. Also, it is observed that as t{sub nitridation} increases, D{sub it} decreases first and increases later. It is revealed from XPS analyses that minimum D{sub it} can be determined by the balance between the saturation of nitridation and the progress of oxidation. As a result, it is found that the superior MOS interface formed by the nitridation is attributable to the existence of oxide-less InGaN/InGaAs interfaces.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Effect of MgO/Co interface and Co/MgO interface on the spin dependent transport in perpendicular Co/Pt multilayers

Effect of the metal/oxide interface on spin-dependent transport properties in perpendicular [Co/Pt]{sub 3} multilayers was investigated. The saturation Hall resistivity (ρ{sub xy}) is significantly increased by 45% with 1.4 nm thick CoO layer inserted at the top Co/MgO interface; whereas it is increased only 25% with 1 nm thick CoO layer at the bottom MgO/Co interface. The interfacial structures characterized by X-ray photoelectron spectroscopy show that the MgO/Co interface and Co/MgO interface including chemical states play a dominant role on spin-dependent transport, leading to different anomalous Hall behavior.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Investigations of interface spin asymmetry and interfacial resistance in Fe{sub x}Co{sub 100−x}/Ag interface

We investigated current-perpendicular-to-plane (CPP) magnetotransport parameters of Fe{sub x}Co{sub 100−x}/Ag interfaces: interface specific resistance (AR{sub F/N}), extended interface resistance (AR{sup *}{sub F/N}), and spin scattering asymmetry (γ). We also investigated the dependence of interfacial parameters on the giant magnetoresistance effect with CPP geometry. For measuring these parameters, we prepared magnetic multilayers and pseudo spin valves (PSVs), combining the ferromagnetic (F) alloys Fe, Co, Fe{sub 50}Co{sub 50}, and Fe{sub 30}Co{sub 70} with the nonmagnetic (N) metal Ag. In all cases, the largest AR{sub F/N} value was found to be ∼0.68 mΩ μm{sup 2} with an enhanced AR{sup *}{sub F/N} value of ∼1.9 mΩ μm{sup 2} for a (001)-oriented Fe/Ag interface, which suggests that spin-up and spin-down electrons on the Fermi surface have very different transmission probabilities. Such an interface can act as a spin filter through which only one type of electrons can pass.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Band offsets for mismatched interfaces: The special case of ZnO on CdTe (001)

High-quality planar interfaces between ZnO and CdTe would be useful in optoelectronic applications. Although CdTe is zinc blende with cubic lattice constant a = 6.482 Å while ZnO is hexagonal wurtzite with a = 3.253 Å and c = 5.213 Å, (001)-oriented cubic zinc blende ZnO films could be stabilized epitaxially on a CdTe (001) surface in an √2 × √2 R45° configuration with a lattice mismatch of <0.5%. Modeling such a configuration allows density-functional total-energy electronic-structure calculations to be performed on several interface arrangements (varying terminations and in-plane fractional translations) to identify the most likely form of the interface, and to predict valence-band offsets between CdTe and ZnO in each case. Growth of ZnO on Te-terminated CdTe(001) is predicted to produce small or even negative (CdTe below ZnO) valence band offsets, resulting in a Type I band alignment. Growth on Cd-terminated CdTe is predicted to produce large positive offsets for a Type II alignment as needed, for example, in solar cells. To corroborate some of these predictions, thin layers of ZnO were deposited on CdTe(001) by pulsed laser deposition, and the band alignments of the resulting heterojunctions were determined from x-ray photoelectron spectroscopy measurements. Although zinc blende ZnO could not be confirmed, the measured valence band offset (2.0–2.2 eV) matched well with the predicted value.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Adaptive Interface-PINNs (AdaI-PINNs): An Efficient Physics-Informed Neural Networks Framework for Interface Problems

Here, we present an efficient physics-informed neural networks (PINNs) framework, termed Adaptive Interface-PINNs (AdaI-PINNs), to improve the modeling of interface problems with discontinuous coefficients and/or interfacial jumps. This framework is an enhanced version of its predecessor, Interface PINNs or I-PINNs (Sarma et al.; https://doi.org/10.1016/j.cma.2024.117135), which involves domain decomposition and assignment of different predefined activation functions to the neural networks in each subdomain across a sharp interface, while keeping all other parameters of the neural networks identical. In AdaI-PINNs, the activation functions vary solely in their slopes, which are trained along with the other parameters of the neural networks. This makes the AdaI-PINNs framework fully automated without requiring preset activation functions. Comparative studies on one-dimensional, two-dimensional, and three-dimensional benchmark elliptic interface problems reveal that AdaI-PINNs outperform I-PINNs, reducing computational costs by 2-6 times while producing similar or better accuracy.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2019

Interface-Induced and Interface-Enhanced Superconductivity

Among the various theoretical mechanisms proposed to reach a higher Tc, the interface mechanism in different forms has been the one most explored, and it provides continual inspiration and hope for realizing the ultimate goal for researchers in the superconductivity field—room-temperature superconductivity. Difficulties do exist, as most of the materials proposed to exhibit the interfacial mechanism are artificially formed heterostructures and are by nature delicate and easily disturbed by strain and change in the stoichiometry at the interface. The discoveries of superconductivity in naturally assembled single crystals of rare earth (R)-doped CaFe2As2 (Ca122) with a Tc up to 49 K and undoped CaFe2As2 with a Tc up to 25 K have opened an alternate route to tackle the problem. The experimental observation has provided the most direct evidence for interface-induced superconductivity in Ca122 to date and possibly in R-doped Ca122. Given the fact that neither chemical doping nor the application of physical pressure has ever induced a Tc higher than 40 K in bulk FeSe, the reports of Tc up to 45–109 K in FeSe/STO thin films demonstrate that FeSe/STO is an ideal system in which to explore interface-enhanced superconductivity. Here we review the above three specific examples of our effort toward superconductors of higher Tc to demonstrate that the interface mechanism may be a promising paradigm to achieve higher Tc.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Electrolyte layering at the calcite(104)-water interface indicated by Rb + - and Se(VI) K-edge resonant interface diffraction

Calcite–water interface reactions are of major importance in various environmental settings as well as in industrial applications. Here we present resonant interface diffraction results on the calcite(104)–aqueous solution interface, measured in solutions containing either 10 mmol L -1 RbCl or 0.5 mmol L -1 Se(VI). Results indicate that Rb + ions enter the surface adsorbed water layers and adsorb at the calcite(104)–water interface in an inner-sphere fashion. A detailed analysis based on specular and off-specular resonant interface diffraction data reveals three distinct Rb + adsorption species: one 1.2 Å above the surface, the second associated with surface adsorbed water molecules 3.2 Å above the surface, and the third adsorbed in an outer-sphere fashion 5.6 Å above the surface. A peak in resonant amplitude between L = 1.5 and L = 3.0 is interpreted as signal from a layered electrolyte structure. The presence of a layered electrolyte structure seems to be confirmed by data measured in the presence of Se(VI).

Heberling, F.↗
DOE OSTI2017

Patterned surface anchoring of nematic droplets at miscible liquid–liquid interfaces

We report on the internal configurations of droplets of nematic liquid crystals (LCs; 10–50 μm-in-diameter; comprised of 4-cyano-4'-pentylbiphenyl and 4-(3-acryloyloxypropyloxy)benzoic acid 2-methyl-1,4-phenylene ester) sedimented from aqueous solutions of sodium dodecyl sulfate (SDS) onto interfaces formed with pure glycerol. We observed a family of internal LC droplet configurations and topological defects consistent with a remarkably abrupt transition from homeotropic (perpendicular) to tangential anchoring on the surface of the LC droplets in the interfacial environment. Calculations of the interdiffusion of water and glycerol at the aqueous–glycerol interface revealed the thickness of the diffuse interfacial region of the two miscible liquids to be small (0.2–0.5 μm) compared to the diameters of the LC droplets on the experimental time-scale (15–120 minutes), leading us to hypothesize that the patterned surface anchoring was induced by gradients in concentration of SDS and glycerol across the diameter of the LC droplets in the interfacial region. This hypothesis received additional support from experiments in which the time of sedimentation of the LC droplets onto the interface was systematically increased and the droplets were photo-polymerized to preserve their configurations: the configurations of the LC droplets were consistent with a time-dependent decrease in the fraction of the surface area of each droplet exhibiting homeotropic anchoring. Specifically, LC droplets with <10% surface area with tangential anchoring exhibited a bulk point defect within the LC droplet, whereas droplets with >10% surface area with tangential anchoring exhibited a boojum defect within the tangential region and a disclination loop separated the regions with tangential and homeotropic anchoring. The topological charge of these LC droplet configurations was found to be consistent with the geometrical theorems of Poincaré and Gauss and also well-described by computer simulations performed by minimization of a Landau–de Gennes free energy. Additional experimental observations (e.g., formation of “Janus-like” particles with one hemisphere exhibiting tangential anchoring and the other perpendicular anchoring) and simulations (e.g., a size-dependent set of LC droplet configurations with <10% surface area exhibiting tangential anchoring) support our general conclusion that placement of LC droplets into miscible liquid–liquid interfacial environments with compositional gradients can lead to a rich set of LC droplet configurations with symmetries and optical characteristics that are not encountered in LC droplet systems in homogeneous, bulk environments. Finally, our results also reveal that translocation of LC droplets across liquid–liquid interfaces can define new transition pathways that connect distinct configurations of LC droplets.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗