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All sources NASA NTRS DOE OSTI

DOE OSTI2016

Epitaxial GaN layers formed on langasite substrates by the plasma-assisted MBE method

In this publication, the results of development of the technology of the epitaxial growth of GaN on single-crystal langasite substrates La{sub 3}Ga{sub 5}SiO{sub 14} (0001) by the plasma-assisted molecular-beam epitaxy (PA MBE) method are reported. An investigation of the effect of the growth temperature at the initial stage of deposition on the crystal quality and morphology of the obtained GaN layer is performed. It is demonstrated that the optimal temperature for deposition of the initial GaN layer onto the langasite substrate is about ~520°C. A decrease in the growth temperature to this value allows the suppression of oxygen diffusion from langasite into the growing layer and a decrease in the dislocation density in the main GaN layer upon its subsequent high-temperature deposition (~700°C). Further lowering of the growth temperature of the nucleation layer leads to sharp degradation of the GaN/LGS layer crystal quality. As a result of the performed research, an epitaxial GaN/LGS layer with a dislocation density of ~10{sup 11} cm{sup –2} and low surface roughness (<2 nm) is obtained.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Investigating the RTA Treatment of Ohmic Contacts to n-Layers of Heterobipolar Nanoheterostructures

The preparation of ohmic contacts to heterobipolar nanostructures has a number of characteristic features. In addition to the basic requirement of minimizing contact resistance, contacts to this type of structures have a transition layer whose depth of penetration must not exceed the emitter layer’s thickness, due to the possibility of short-circuiting the emitter base p–n junction. In this work, the effect the main technological parameters of rapid thermal annealing have on a contact’s characteristics are examined, and the process of obtaining a low-resistance ohmic contact to heterobipolar transistor layers is optimized. Ohmic contacts to the n-layers of heterobipolar nanoheterostructures based on gallium arsenide and produced via layer-by-layer electron-beam deposition of Ge/Au/Ni/Au are considered. The diffusion distribution profiles of doping with Ge impurities are calculated as a function of the time and temperature of rapid thermal annealing, and are examined via scanning electron microscopy. It is found that rapid thermal annealing for 60 s at a temperature of 398°C yields ohmic contacts with low resistance, smooth surface morphology, and the minimum size of the transition layer.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2019

Template Synthesis, Structure, and Magnetic Properties of Layered Nanowires

Nanowires (NWs) consisting of Ni/Cu and Co/Cu alternating layers with a diameter of 100 nm and layer thicknesses varying between 10 and 500 nm are prepared by template synthesis in pores of polymer track-etched membranes. Bath compositions and different regimes for pulsed electrodeposition of NWs are explored. A procedure for electrodeposition of NWs using pulses of equal charge is developed. By diminishing the amount of charge per pulse, initially we manage to lower the layer thickness to 10–15 nm, but further diminishing of charge in pulses leads to the blending of elemental composition of adjacent layers and/or formation of rod–shell nanostructures within the NWs. The coercive force (15–30 mT) and residual magnetization of our layered NWs are determined from magnetization measurements. For NWs with a layer thickness of 50–100 nm, the magnetization curves recorded in the out-of-plane and in-plane geometries are similar in shape and have similar parameters. For NWs with thicker layers (250 and 500 nm), magnetization curves are markedly different due to magnetic anisotropy (an easy magnetization axis emerges longitudinally to NWs) and interference between neighboring NWs. Magnetic force microscopy of isolated NWs identifies that the NWs comprise magnetic regions extending over ~100–150 nm. The NW can be partially remagnetized by applying an external magnetic field (+16 mT) longitudinally.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

DOE OSTI2025

TorchBraid: High-Performance Layer-Parallel Training of Deep Neural Networks with MPI and GPU Acceleration

TorchBraid is a high-performance implementation of layer-parallel training for deep neural networks (DNNs) supporting MPI-based parallelism and GPU acceleration. Layer-parallel training has been developed to overcome the serialization inherent in forward and backward propagation of DNNs that limits utilization of computational resources in the strong scaling limit. To achieve this, TorchBraid integrates the PyTorch neural network framework with the state-of-the-art XBraid time-parallel library. Furthermore, this article presents the use and performance of TorchBraid, in addition to solutions for overcoming the algorithmic challenges inherent in combining automatic differentiation with layer-parallel. Results are presented with and without GPU acceleration for the Tiny ImageNet and MNIST image classification data sets, as well as recurrent neural networks. Overall, TorchBraid enables fast training of DNNs, both in a strong and weak scaling context. In addition to the TorchBraid software, several new advances in applying layer-parallel algorithms are detailed. Integration of layer-parallel with data-parallel algorithms is presented for the first time, showing the computational advantages of the combination. Standard deep learning techniques, like batch-normalization, are developed for layer-parallel training. Finally, a new approach combining layer-parallel with spatial coarsening in order to accelerate training for 3D image classification shows roughly a 10× speedup over serial execution.

Layer-parallel↗

DOE OSTI2019

Seasonal-to-Interannual Response of Southern Ocean Mixed Layer Depth to the Southern Annular Mode from a Global 1/10° Ocean Model

The relationship between the southern annular mode (SAM) and Southern Ocean mixed layer depth (MLD) is investigated herein using a global 0.1° resolution ocean model. The SAM index is defined as the principal component time series of the leading empirical orthogonal function of extratropical sea level pressure from September to December, when the zonally symmetric SAM feature is most prominent. Following positive phases of the SAM, anomalous deep mixed layers occur in the subsequent fall season, starting in May, particularly in the southeast Pacific. Composite analyses reveal that for positive SAM phases enhanced surface cooling caused by anomalously strong westerlies weakens the stratification of the water column, leading to deeper mixed layers during spring when the SAM signal is at its strongest. During the subsequent summer, the surface warms and the mixed layer shoals. However, beneath the warm surface layer, anomalously weak stratification persists throughout the summer and into fall. When the surface cools again during fall, the mixed layer readily deepens due to this weak interior stratification, a legacy from the previous springtime conditions. Therefore, the spring SAM–fall MLD relationship is interpreted here as a manifestation of reemergence of interior water mass anomalies. The opposite occurs after negative phases of the SAM, with anomalously shallow mixed layers resulting. Additional analyses reveal that for the MLD region in the southeast Pacific, the effects of salinity variations and Ekman heat advection are negligible, although Ekman heat transport may play an important role in other regions where mode water is formed, such as south of Australia and in the Indian Ocean.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2019

Cold-Air Outbreaks in the Marine Boundary Layer Experiment (COMBLE) Science Implementation Plan

Boreal regions have experienced warming faster than the rest of the Earth, and the arctic ice cover has declined at a rate faster than predicted by most climate models. Uncertainties in climate projections for this region are large, in part due to the lack of observational guidance to constrain the treatment of aerosol-cloud-precipitation linkages in climate models. To remedy this shortfall, field campaigns have been conducted to collect integrated, comprehensive measurements under specific atmospheric regimes. While considerable efforts have been devoted to subtropical marine boundary-layer clouds, as well as to clouds in the stable arctic environment, very few targeted observations exist of convective boundary-layer clouds that form over open water when cold airmasses are advected off ice-covered regions or boreal continents. There is an urgent need to shed light on the dynamics and microphysical properties of clouds and precipitation in the high-latitude marine boundary layer during cold-air outbreaks. Despite the common occurrence of linear and cellular cloud patterns that typify cold-air outbreaks, little is known about the properties of these clouds; how they vary with surface, environmental, and aerosol conditions; the role of cold-air outbreaks in the global atmospheric and ocean circulation; and the accuracy of the treatment of this atmospheric regime in climate models. Thus, we proposed to conduct the Cold-Air Outbreaks in the Marine Boundary Layer Experiment (COMBLE), to focus on marine boundary-layer clouds during cold-air outbreaks. COMBLE will deploy the first ARM Mobile Facility (AMF1) and an AMF “satellite” station in the far North Atlantic in January–May 2020. COMBLE will take advantage of the synergy with several coincident campaigns, notably MOSAIC, which will characterize the source airmasses of cold-air outbreaks over the arctic ice, and (AC) 3 , which will operate several aircraft between northern Scandinavia and the MOSAIC deployment to document evolution of these airmasses. COMBLE and its sister campaigns will compose a four-member array of supersites between northern Scandinavia and the Arctic, each with profiling and in situ cloud, precipitation, radiation, and aerosol measurements. COMBLE will be guided by six science themes. The first five deal with boundary-layer convection in cold-air outbreaks: (1) the fetch-dependent mesoscale organization of clouds and precipitation, including linear and cellular convection; (2) surface heat and momentum fluxes and vertical profiles of temperature, humidity, wind, and turbulence; (3) vertical structure of clouds and precipitation; (4) the sources and sinks of aerosol, including ice-nucleating particles, and the role of cloud-active aerosol on cloud processes and radiative fluxes; and (5) the influence of these four themes on polar cyclogenesis and polar low vertical structure. The overarching sixth theme is that COMBLE will provide integrated data sets of dynamical, thermodynamic, and cloud microphysical characteristics of marine boundary-layer convection in cold-air outbreaks, including cloud and aerosol properties, that will enable constraining high-resolution numerical simulations, developing process-level understanding, and, subsequently, evaluating and improving representations of shallow convection in weather and climate models.

54 ENVIRONMENTAL SCIENCES↗

NASA NTRS1951

Discussion of boundary-layer characteristics near the casing of an axial-flow compressor

Boundary-layer velocity profiles on the casing of an axial-flow compressor behind the guide vanes and rotor were measured and resolved into two components: along the streamline of the flow and perpendicular to it. Boundary-layer thickness and the deflection of the boundary layer at the wall were the generalizing parameters. By use of these results and the momentum-integral equations, the characteristics of boundary on the walls of axial-flow compressor are qualitatively discussed. Important parameters concerning secondary flow in the boundary layer appear to be turning of the flow and the product of boundary-layer thickness and streamline curvature outside the boundary layer. Two types of separation are shown to be possible in three dimensional boundary layer.

BOUNDARY LAYER CHARACTERISTICS - INTERNAL AERODYNA↗

NASA NTRS2005

The Closure of the Ocean Mixed Layer Temperature Budget using Level-Coordinate Model Fields

Entrainment is an important element of the mixed layer mass, heat, and temperature budgets. Conventional procedures to estimate entrainment heat advection often do not permit the closure of heat and temperature budgets because of inaccuracies in its formulation. In this study a rigorous approach to evaluate the effect of entrainment using the output of a general circulation model (GCM) that does not have an explicit prognostic mixed layer model is described. The integral elements of the evaluation are 1) the rigorous estimates of the temperature difference between mixed layer water and entrained water at each horizontal grid point, 2) the formulation of the temperature difference such that the budget closes over a volume greater than one horizontal grid point, and 3) the apparent warming of the mixed layer during the mixed layer shoaling to account for the weak vertical temperature gradient within the mixed layer. This evaluation of entrainment heat advection is compared with the estimates by other commonly used ad hoc formulations by applying them in three regions: the north-central Pacific, the Kuroshio Extension, and the Nino-3 areas in the tropical Pacific. In all three areas the imbalance in the mixed layer temperature budget by the ad hoc estimates is significant, reaching a maximum of about 4 K yr(exp -1).

temperature gradient↗

NASA NTRS2017

Approach to Modeling Boundary Layer Ingestion Using a Fully Coupled Propulsion-RANS Model

Airframe-propulsion integration concepts that use boundary layer ingestion have the potential to reduce aircraft fuel burn. One concept that has been recently explored is NASA's Starc-ABL aircraft configuration, which offers the potential for 12% mission fuel burn reduction by using a turbo-electric propulsion system with an aft-mounted electrically driven boundary layer ingestion propulsor. This large potential for improved performance motivates a more detailed study of the boundary layer ingestion propulsor design, but to date, analyses of boundary layer ingestion have used uncoupled methods. These methods account for only aerodynamic effects on the propulsion system or propulsion system effects on the aerodynamics, but not both simultaneously. This work presents a new approach for building fully coupled propulsive-aerodynamic models of boundary layer ingestion propulsion systems. A 1D thermodynamic cycle analysis is coupled to a RANS simulation to model the Starc-ABL aft propulsor at a cruise condition and the effects variation in propulsor design on performance are examined. The results indicates that both propulsion and aerodynamic effects contribute equally toward the overall performance and that the fully coupled model yields substantially different results compared to uncoupled. The most significant finding is that boundary layer ingestion, while offering substantial fuel burn savings, introduces throttle dependent aerodynamics effects that need to be accounted for. This work represents a first step toward the multidisciplinary design optimization of boundary layer ingestion propulsion systems.

Boundary Layer Ingestion↗

NASA NTRSDate not supplied

Effect of Water Droplets Crossing the Boundary Layer in a Stagnation Point Configuration

An experimental study was conducted in the Vertical Icing Studies Tunnel at the Icing Physics Flow Laboratory of NASA Glenn Research Center to study the effect of water droplets crossing the boundary layer in a stagnation point configuration. The objective of the experiment was to determine if water droplets crossing a boundary layer create turbulent spots that accelerate the boundary layer transition from laminar to turbulent. Water droplets that crossed the boundary layer were generated with a nozzle installed in the plenum. The turbulence level in the boundary layer was measured with a hot wire system when the nozzle was off, air on, and air and water on. The results indicate that the presence of the nozzle alone did not affect the boundary layer. The activation of the nozzle to eject air only or to generate water droplets affected the turbulence level in the boundary layer. The continued study of this effect is needed because of its implications in the development of heat transfer models for icing codes.

Icing↗

NASA NTRSDate not supplied

Hypersonic Boundary-Layer Transition over a Blunt Circular Cone in a Mach 8 Digital Wind Tunnel

To understand the effects of freestream acoustic disturbances on transition reversal over a blunt body, three-dimensional direct numerical simulations (DNS) were conducted to investigate the interaction of a tunnel-like acoustic disturbance field with a Mach 8 laminar boundary layer over a 7 deg half-angle blunt cone with a nose radius of 𝑅 𝑛 = 5.2 mm. The flow conditions and the axisymmetric cone geometry matched those measured in the Sandia Hypersonic Wind Tunnel at Mach 8 (Sandia HWT-8), and the incident freestream acoustic disturbances in the DNS were generated by acoustic radiation from the nozzle-wall turbulent boundary layer in the same tunnel. Consistent with the predictions of the nonmodal instability analysis, the study shows the emergence of a second spectral peak for moderately oblique waves between 150 kHz and 200 kHz in the aft part of the cone. The spectral energy of temperature and streamwise velocity fluctuations is initially concentrated within the entropy layer. These fluctiations are eventually “swallowed” by the boundary layer as the entropy-layer thickness 𝛿 𝑆 becomes similar to the boundary-layer thickness 𝛿h𝑡 at downstream locations. The numerical schlieren contours show the same inclined structures commonly observed in blunt cone experiments, and these structures are found to be three dimensional and advect with a constant speed approximately equal to the mean flow speed near the edge of the boundary layer.

Hypersonic Flow↗

NASA NTRSDate not supplied

Hypersonic Boundary-Layer Transition over a Blunt Circular Cone in a Mach 8 Digital Wind Tunnel

Hypersonic Flow↗

DOE OSTI2018

Removal of the Magnetic Dead Layer by Geometric Design

The proximity effect is used to engineer interface effects such as magnetoelectric coupling, exchange bias, and emergent interfacial magnetism. However, the presence of a magnetic “dead layer” adversely affects the functionality of a heterostructure. Here in this paper, it is shown that by utilizing (111) polar planes, the magnetization of a manganite ultrathin layer can be maintained throughout its thickness. Combining structural characterization, magnetometry measurements, and magnetization depth profiling with polarized neutron reflectometry, it is found that the magnetic dead layer is absent in the (111)-oriented manganite layers, however, it occurs in the films with other orientations. Quantitative analysis of local structural and elemental spatial evolutions using scanning transmission electron microscopy and electron energy loss spectroscopy reveals that atomically sharp interfaces with minimal chemical intermixing in the (111)-oriented superlattices. The polar discontinuity across the (111) interfaces inducing charge redistribution within the SrTiO 3 layers is suggested, which promotes ferromagnetism throughout the (111)-oriented ultrathin manganite layers. The approach of eliminating problematic magnetic dead layers by changing the crystallographic orientation suggests a conceptually useful recipe to engineer the intriguing physical properties of oxide interfaces, especially in low dimensionality.

36 MATERIALS SCIENCE↗

DOE OSTI2020

An In Situ Formed Surface Coating Layer Enabling LiCoO 2 with Stable 4.6 V High‐Voltage Cycle Performances

Abstract The development of high‐voltage LiCoO 2 is essential for achieving lithium‐ion batteries with high volumetric energy density, however, it faces a great deal of challenges owing to the materials, structure and interfacial instability issues. In this work, a strategy is developed, through heat annealing a precoated surface layer to in situ form a high‐voltage‐stable surface coating layer, which is demonstrated to be highly effective to improve the high‐voltage performance of LiCoO 2 . It is discovered that LiCoO 2 reacts with Li 1.5 Al 0.5 Ti 1.5 (PO 4 ) 3 (LATP) at 700 °C to form exclusively spinel phases in addition to Li 3 PO 4 , which are structurally coherent to the layered lattice of LiCoO 2 . The heat annealing of the precoated thin layer of LATP enables the formation of a high‐quality surface layer. Spinel phases possess high‐voltage‐stable structures with much weaker oxidizing ability of lattice oxygen than layered structure. In addition, the Li 3 PO 4 is a good lithium‐ion conductor with excellent chemical stability at high voltages. All these benefits contribute to the construction of a uniform and conformal high‐voltage‐stable surface layer with favorable lithium conducting kinetics at the LiCoO 2 surface. The modified LiCoO 2 shows excellent 4.6 V high‐voltage cycle performance at both room temperature and 45 °C. The thermal stability is greatly enhanced as well.

Wang, Yi↗

DOE OSTI2020

An anisotropic viscoplasticity model for shale based on layered microstructure homogenization

Viscoplastic deformation of shale is frequently observed in many subsurface applications. Many studies have suggested that this viscoplastic behavior is anisotropic—specifically, transversely isotropic—and closely linked to the layered composite structure at the microscale. In this work, we develop a two-scale constitutive model for shale in which anisotropic viscoplastic behavior naturally emerges from semianalytical homogenization of a bilayer microstructure. The microstructure is modeled as a composite of soft layers, representing a ductile matrix formed by clay and organics, and hard layers, corresponding to a brittle matrix composed of stiff minerals. This layered microstructure renders the macroscopic behavior anisotropic, even when the individual layers are modeled with isotropic constitutive laws. Using a common correlation between clay and organic content and magnitude of creep, we apply a viscoplastic modified Cam-Clay plasticity model to the soft layers, while treating the hard layers as a linear elastic material to minimize the number of calibration parameters. We then describe the implementation of the proposed model in a standard material update subroutine. The model is validated with laboratory creep data on samples from three gas shale formations. We also demonstrate the computational behavior of the proposed model through simulation of time-dependent borehole closure in a shale formation with different bedding plane directions.

58 GEOSCIENCES↗

DOE OSTI2019

Evolution of photoresist layer structure and surface morphology under fluorocarbon‐based plasma exposure

Abstract Ion bombardment of photoresist materials during plasma etching results in the formation of a surface dense amorphous carbon (DAC) layer that contributes to both etch resistance and the development of surface roughness. Real‐time ellipsometric measurements/analysis reveals that a C 4 F 8 ‐containing plasma interacts with an Ar‐plasma‐formed DAC layer to produce a modified DAC/fluorocarbon (FC) layer by FC deposition/diffusion of fluorine into the surface. The depletion of the DAC layer via modification and ion bombardment causes the etch rate of the bulk layer to increase. As the modified surface layer is formed, a noticeable decrease in surface roughness decrease is observed. These findings provide an understanding of the mechanisms of atomic layer etching processes in photoresist materials.

Pranda, Adam↗

DOE OSTI2018

Highly Enhanced Luminescence Performance of LEDs via Controllable Layer‐Structured 3D Photonic Crystals and Photonic Crystal Beads

Abstract Controllable layer‐structured 3D photonic crystals (PCs) are prepared using the combination of the Langmuir–Blodgett (LB) technique and layer‐by‐layer stacking method. They are used for research into the influence of the number of the layers on the enhancement of the efficiency of light emission from light‐emitting diodes (LEDs) and quantum dots (QDs). The luminescence properties of blue, green, and white LEDs are improved dramatically while applying such controllable layer‐structured 3D PCs as reflectors and the enhancement efficiencies are shown to increase with PC thickness. In addition, the fluorescence intensity increases 7.8‐fold and 5.4‐fold for red and green QDs, respectively, and the fluorescence intensity can be accurately controlled by using PCs with different layers. In addition, PC beads with yellow and red photonic bandgaps are used to improve the luminescence of a white LED with angle‐independent enhancement. These beads are simply doped into the phosphor without any other treatment. Interestingly, the light intensity of the PC bead based LED is remarkably enhanced. These quantized enhanced effect of light intensity on the thickness of PC layers, along with new generation of PC bead based LED, might offer a strategy for designing high brightness LEDs with energy‐saving performance.

Tian, Yu↗

DOE OSTI2018

Tuning SPT-3G Transition-Edge-Sensor Electrical Properties with a Four-Layer Ti–Au–Ti–Au Thin-Film Stack

We have developed superconducting Ti transition-edge sensors with Au protection layers on the top and bottom for the South Pole Telescope’s third-generation receiver (a cosmic microwave background polarimeter, due to be upgraded this austral summer of 2017/2018). The base Au layer (deposited on a thin Ti glue layer) isolates the Ti from any substrate effects; the top Au layer protects the Ti from oxidation during processing and subsequent use of the sensors. We control the transition temperature and normal resistance of the sensors by varying the sensor width and the relative thicknesses of the Ti and Au layers. The transition temperature is roughly six times more sensitive to the thickness of the base Au layer than to that of the top Au layer. The normal resistance is inversely proportional to sensor width for any given film configuration. For widths greater than five micrometers, the critical temperature is independent of width.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗