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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Study of Microphysical Signatures Based on Spectral Polarimetry during the RELAMPAGO Field Experiment in Argentina

Weather radars with dual-polarization capabilities enable the study of various characteristics of hydrometeors, including their size, shape, and orientation. Radar polarimetric measurements, coupled with Doppler information, allow for analysis in the spectral domain. This analysis can be leveraged to reveal valuable insight into the microphysics and kinematics of hydrometeors in precipitation systems. This paper uses spectral polarimetry to investigate precipitation microphysics and kinematics in storm environments observed during the Remote Sensing of Electrification, Lightning, and Mesoscale/Microscale Processes with Adaptive Ground Observations (RELAMPAGO) field experiment in Argentina. This study uses range–height indicator scan measurements from a C-band polarimetric Doppler weather radar deployed during the field campaign. Here, in this work, the impact of storm dynamics on hydrometeors is studied, including the size sorting of hydrometeors due to vertical wind shear. In addition, particle microphysical processes because of aggregation and growth of ice crystals in anvil clouds, as well as graupel formation resulting from the riming of ice crystals and dendrites, are also analyzed here. The presence of different particle size distributions because of the mixing of hydrometeors in a sheared environment and resulting size sorting has been reported using spectral differential reflectivity (sZ dr ) slope. Spectral reflectivity sZ h and sZ dr have also been used to understand the signature of ice crystal aggregation in an anvil cloud. The regions of pristine ice crystals are identified from vertical profiles of spectral polarimetric variables in anvil cloud because of sZ h < 0 dB and sZ dr values around 2 dB. It is also found that the growth process of these ice crystals causes a skewed bimodal sZ h spectrum due to the presence of both pristine ice crystals and dry snow. Next, graupel formation due to riming has been studied, and it is found that the riming process produces sZ h values of about 10 dB and corresponding sZ dr values of 1 dB. This positive sZ dr indicates the presence of needle/columnar secondary ice particles formed by ice multiplication processes in the riming zones. Last, the temporal evolution of a storm is investigated by analyzing changes in hydrometeor types with time and their influence on the spectral polarimetric variables.

54 ENVIRONMENTAL SCIENCES↗
NASA NTRS1980

Cyclic plasticity and failure of structural components

An analytical approach for low-cycle fatigue prediction is presented. The approach combines a cyclic plasticity model with the finite element method and a damage accumulation criterion for ductile metals. The cyclic plasticity model is based on the concept of the combination of several yield surfaces. The surfaces are related to the material uniaxial stress-strain curve idealized by piecewise linear segments. The damage criterion is based on the Coffin-Manson formulae modified for the mean stress variation effect. It is extended to the multiaxial varying stress-strain field and applied for both the crack initiation and the crack growth processes. The stable slow crack growth rate is approximated by the damage accumulation gradient computed from the cracked finite element models. This procedure requires fatigue testing data of only smooth specimens under constant strain amplitudes. The present approach is illustrated by numerical examples of an aircraft wing stiffened panel subjected to compression, which causes material yielding and residual tension.

Kalev, I.↗
NASA NTRS2014

Structural Characterization of Lateral-grown 6H-SiC am-plane Seed Crystals by Hot Wall CVD Epitaxy

The performance of commercially available silicon carbide (SiC) power devices is limited due to inherently high density of screw dislocations (SD), which are necessary for maintaining polytype during boule growth and commercially viable growth rates. The NASA Glenn Research Center (GRC) has recently proposed a new bulk growth process based on axial fiber growth (parallel to the c-axis) followed by lateral expansion (perpendicular to the c-axis) for producing multi-faceted m-plane SiC boules that can potentially produce wafers with as few as one SD per wafer. In order to implement this novel growth technique, the lateral homoepitaxial growth expansion of a SiC fiber without introducing a significant number of additional defects is critical. Lateral expansion is being investigated by hot wall chemical vapor deposition (HWCVD) growth of 6H-SiC am-plane seed crystals (0.8mm x 0.5mm x 15mm) designed to replicate axially grown SiC single crystal fibers. The post-growth crystals exhibit hexagonal morphology with approximately 1500 m (1.5 mm) of total lateral expansion. Preliminary analysis by synchrotron white beam x-ray topography (SWBXT) confirms that the growth was homoepitaxial, matching the polytype of the respective underlying region of the seed crystal. Axial and transverse sections from the as grown crystal samples were characterized in detail by a combination of SWBXT, transmission electron microscopy (TEM) and Raman spectroscopy to map defect types and distribution. X-ray diffraction analysis indicates the seed crystal contained stacking disorders and this appears to have been reproduced in the lateral growth sections. Analysis of the relative intensity for folded transverse acoustic (FTA) and optical (FTO) modes on the Raman spectra indicate the existence of stacking faults. Further, the density of stacking faults is higher in the seed than in the grown crystal. Bundles of dislocations are observed propagating from the seed in m-axis lateral directions. Contrast extinction analysis of these dislocation lines reveals they are edge type basal plane dislocations that track the growth direction. Polytype phase transition and stacking faults were observed by high-resolution TEM (HRTEM), in agreement with SWBXT and Raman scattering.

Single Crystal↗
NASA NTRS1977

Laser-zone growth in a Ribbon-To-Ribbon (RTR) process. Silicon sheet growth development for the large area silicon sheet task of the low cost silicon solar array project

The Ribbon-to-Ribbon (RTR) approach to silicon ribbon growth is investigated. An existing RTR apparatus is to be upgraded to its full capabilities and operated routinely to investigate and optimize the effects of various growth parameters on growth results. A new RTR apparatus was constructed to incorporate increased capabilities and improvements over the first apparatus and to be capable of continuous growth. New high power lasers were implemented and this led to major improvements in growth velocity -- 4 inch/min. growth has been demonstrated. A major step in demonstration of the full feasibility of the RTR process is reported in the demonstration of RTR growth from CVD polyribbon rather than sliced polyribbon ingots. Average solar cell efficiencies of greater than 9% and a best cell efficiency of 11.7% are reported. Processing was shown to provide a substantial improvement in material minority carrier diffusion length. An economic analysis is reported which treats both the polyribbon fabrication and RTR processes.

Gurtler, R. W.↗
NASA NTRS1977

Laser-zone growth in a Ribbon-To-Ribbon (RTR) process silicon sheet growth development for the large area silicon sheet task of the low cost silicon solar array project

The Ribbon-to-Ribbon (RTR) approach to silicon ribbon growth was investigated. An existing RTR apparatus, RTR#1, was upgraded to allow for 5 cm wide ribbon growth with a finite stroke length of at least 15 cm. A second RTR apparatus, RTR#2, was designed, built, and operated which utilizes continuous feed mechanisms and allows continuous growth of 7.5 cm wide ribbons. RTR#2 includes development and utilization of advanced beam scanning (or shaping), high power lasers, and thermal profile modification elements to attain maximum growth velocities (with a design goal of 18 cm/min). Materials studies, process development, and thermal analyses are also described. Residual stresses and dislocation densities were minimized through theoretical and experimental efforts towards optimization of thermal profiles. Growth runs were performed on RTR#2 and solar cells were fabricated which demonstrated efficiencies greater than 10%.

Gurtler, R. W.↗
DOE OSTI2025

Model form and sensitivity analysis of CALPHAD-based nucleation models in b-stabilized Ti alloys

Accurate prediction of α-phase nucleation and growth in β-stabilized titanium alloys is crucial for designing heat treatments to optimize mechanical properties in additively manufactured lightweight components. Ideally, predictions of nucleation and growth would incorporate both top-down observations of past experimental heat treatments and bottom-up modeling of phase transformations; however, the appropriate method of combining these information sources is not self-evident. Combining top-down and bottom-up information requires a unified form of model that can connect between spatiotemporal scales, as well as sets of fitting parameters that can be identified by each data source. The selection of which parameters to fit to which data source can be made based on expert opinion, or by performing a sensitivity analysis. In solid-solid nucleation, direct observation of the nucleation and growth process is challenging. Most data on the heat treatment-controlled phase transformations are not in-situ. To predict the process and outcome of the nucleation, growth and coarsening of precipitates, theoretical models of the nucleation pathway are used to bridge the gap. Many sources of uncertainty affect the modeling of this nucleation process. It can be influenced by small variations in the thermomechanical processing history, chemical composition, and initial microstructure. If molecular dynamics (MD) simulations are used to determine thermodynamic quantities and inform CALPHAD modeling, additional uncertainty can be introduced and accounted for using Bayesian methods. Top-down uncertainties require additional steps to quantify. The influence of nucleation model form on the sensitivity of predictions to input parameters and physical conditions is the focus of this study. Classical nucleation theory (CNT) allows modeling to formulate the nucleation as homogeneous or, more commonly, heterogeneous. Non-classical nucleation models are also increasingly explored as a means of reconciling top-down and bottom-up data. In this study, the sensitivity of the intragranular nucleation of α in a β-annealed, slow-cooled aging (BASCA) heat treatment of β-stabilized Ti5553 alloy is explored using CNT and both heterogeneous and homogeneous assumptions. The Kampmann-Wagner Numerical model of precipitate nucleation and growth is employed. Using open-source tools (pyCalphad and thermodynamic modeling of TiMo as a surrogate system, a sensitivity analysis is performed to measure variations in key parameters, including chemical driving force, interfacial energy, and diffusivity, as they relate to predictions of precipitate number density. The inclusion of top-down and bottom-up data in selection of nucleation model form is discussed.

Rodriguez Negron, A. M.↗
NASA NTRS1984

Overview of materials processing in space activity at Marshall Space Flight Center

An overview of activities involving the Space Transportation System (STS), now in the operational phase, and results of some of the current space experiments, as well as future research opportunities in microgravity environment, are presented. The experiments of the Materials Processing in Space Program flown on the STS, such as bioseparation processes, isoelectric focusing, solidification and crystal growth processes, containerless processes, and the Materials Experiment Assembly experiments are discussed. Special consideration is given to the experiments to be flown aboard the Spacelab 3 module, the Fluids Experiments System, and the Vapor Crystal Growth System. Ground-based test facilities and planned space research facilities, as well as the nature of the commercialization activities, are briefly explained.

Williams, J. R.↗
DOE OSTI2024

On the role of geometrically necessary dislocations in void formation and growth in response to shock loading conditions in wrought and additively manufactured Ta

This study investigates the role of geometrically necessary dislocations (GNDs) and microstructure on void nucleation and growth in wrought and additively manufactured (AM) tantalum subjected to high-strain rate loading. Multi-modal 3D data was collected using TriBeam tomography to calculate GND densities and their spatial relationship to voids. A microstructural comparison between the wrought and AM samples identified distinct void shapes and locations, with intragranular voids and more spherical voids frequently observed in the AM dataset. Results indicate that voids preferentially form at both high-angle grain boundaries and low-angle subgrain boundaries, the latter of which are frequently observed in the AM material. Through a radial distribution analysis of all voids in the datasets, significant GND localization to near-void-surface regions was observed in both samples. 3D crystal plasticity simulations were employed to extend the experimental observations, revealing higher void growth rates in [111] oriented grains when compared to [001] grains. The simulations also suggest that GNDs can be generated as part of the void growth process, with more GND accumulation for growth in a [111] grain than a [001] grain. These findings provide valuable insights into the links between nanoscale void nucleation, mesoscale void growth, and microstructural effects in dynamically loaded tantalum.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Indirect Measurements of the Composition of Ultrafine Particles in the Arctic Late-Winter

In this work, we present indirect measurements of size-resolved ultrafine particle composition conducted during the Ocean-Atmosphere-Sea Ice-Snowpack (OASIS) Campaign in Utqiagvik, Alaska, during March 2009. This study focuses on measurements of size-resolved particle hygroscopicity and volatility measured over two periods of the campaign. During a period that represents background conditions in this location, particle hygroscopic growth factors (HGF) at 90% relative humidity ranged from 1.45 to 1.51, which combined with volatility measurements suggest a mixture of ~30% ammoniated sulfates and ~70% oxidized organics. Two separate regional ultrafine particle growth events were also observed during this campaign. Event 1 coincided with elevated levels of H2SO4 and solar radiation. These particles were highly hygroscopic (HGF = 2.1 for 35 nm particles), but were almost fully volatilized at 160 °C. The air masses associated with both events originated over the Arctic Ocean. Event 1 was influenced by the upper marine boundary layer (200–350 m AGL), while Event 2 spent more time closer to the surface (50–150 m AGL) and over open ocean leads, suggesting marine influence in growth processes. Event 2 particles were slightly less hygroscopic (HGF = 1.94 for 35 nm and 1.67 for 15 nm particles), and similarly volatile. We hypothesize that particles formed during both events contained 60–70% hygroscopic salts by volume, with the balance for Event 1 being sulfates and oxidized organics for Event 2. These observations suggest that primary sea spray may be an important initiator of ultrafine particle formation events in the Arctic late-winter, but a variety of processes may be responsible for condensational growth.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2023

Large-area epitaxial growth of InAs nanowires and thin films on hexagonal boron nitride by metal organic chemical vapor deposition

Large-area epitaxial growth of III–V nanowires and thin films on van der Waals substrates is key to developing flexible optoelectronic devices. In our study, large-area InAs nanowires and planar structures are grown on hexagonal boron nitride templates using metal organic chemical vapor deposition method without any catalyst or pre-treatments. The effect of basic growth parameters on nanowire yield and thin film morphology is investigated. Under optimised growth conditions, a high nanowire density of 2.1 x 10 9 cm -2 is achieved. A novel growth strategy to achieve uniform InAs thin film on h-BN/SiO 2 /Si substrate is introduced. The approach involves controlling the growth process to suppress the nucleation and growth of InAs nanowires, while promoting the radial growth of nano-islands formed on the h-BN surface. A uniform polycrystalline InAs thin film is thus obtained over a large area with a dominant zinc-blende phase. The film exhibits near-band-edge emission at room temperature and a relatively high Hall mobility of 399 cm -2 /(Vs). This work suggests a promising path for the direct growth of large-area, low-temperature III–V thin films on van der Waals substrates.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
NASA NTRS1982

Electromigration process for the purification of molten silicon during crystal growth

A process for the purification of molten materials during crystal growth by electromigration of impurities to localized dirty zones. In the Czochralski crystal growing process, the impurities are electromigrated away from the crystallization interface by applying a direct electrical current to the molten silicon for electromigrating the charged impurities away from the crystal growth interface. The edge-defined film-fed crystal growth process, a direct electrical current is applied between the two faces which are used in forming the molten silicon into a ribbon. The impurities, migrated to one side only of the crystal ribbon, may be removed or left in place. If left in place, they will not adversely affect the ribbon when used in solar collectors. The migration of the impurity to one side only of the silicon ribbon is especially suitable for use with asymmetric dies which preferentially crystallize uncharged impurities along one side or face of the ribbon.

Shlichta, P. J.↗
NASA NTRS1979

Laser-zone Growth in a Ribbon-to-ribbon (RTR) Process Silicon Sheet Growth Development for the Large Area Silicon Sheet Task of the Low Cost Solar Array Project

A technique for growing limited-length ribbons continually was demonstrated. This Rigid Edge technique can be used to recrystallize about 95% of the polyribbon feedstock. A major advantage of this method is that only a single, constant length silicon ribbon is handled throughout the entire process sequence; this may be accomplished using cassettes similar to those presently in use for processing Czochralski waters. Thus a transition from Cz to ribbon technology can be smoothly affected. The maximum size being considered, 3 inches x 24 inches, is half a square foot, and will generate 6 watts for 12% efficiency at 1 sun. Silicon dioxide has been demonstrated as an effective, practical diffusion barrier for use during the polyribbon formation.

Baghdadi, A.↗
NASA NTRS1978

Laser-zone growth in a Ribbon-To-Ribbon (RTR) process. Silicon sheet growth development for the large area sheet task of the low-cost solar array project

A new calculation of the effects of thermal stresses during growth on silicon ribbon quality is reported. Thermal stress distributions are computed for ribbon growth under a variety of temperature profiles. A growth rate of 55 cu cm/min with a single ribbon was achieved. The growth of RTR ribbon with a fairly uniform parallel dendritic structure was demonstrated. Results with two approaches were obtained for reducing the Mo impurity level in polycrystalline feedstock. Coating the Mo substrate with Si3N4 does not effect thermal shear separation of the polyribbon; this process shows promise of improving cell efficiency and also increasing the useful life of the molybdenum substrate. A number of solar cells were fabricated on RTR silicon grown from CVD feedstock.

Baghdadi, A.↗
NASA NTRS1976

Laser-zone growth in a Ribbon-To-Ribbon (RTR) process, silicon sheet growth development for the large area silicon sheet task of the low cost silicon solar array project

The objective of this research is to fully investigate the Ribbon-To-Ribbon (R-T-R) approach to silicon ribbon growth. Initial work has concentrated on modification and characterization of an existing R-T-R apparatus. In addition, equipment for auxiliary heating of the melt is being evaluated and acquired. Modification of the remote viewing system and mechanical staging are nearly complete. Characterization of the laser and other components is in progress and several auxiliary heating techniques are being investigated.

Gurtler, R. W.↗
NASA NTRS1992

Materials processing in a centrifuge - Numerical modeling of macrogravity effects

The fluid mechanics associated with crystal growth processes on a centrifuge is investigated. A simple scaling analysis is used to examine the relative magnitudes of the forces acting on the system and good agreement is obtained with previous studies. A two-dimensional model of crystal growth on a centrifuge is proposed and calculations are undertaken to help in understanding the fundamental transport processes within the crystal growth cell. Results from three-dimensional calculations of actual centrifuge-based crystal growth systems are presented both for the thermodynamically stable and unstable configurations. The calculations show the existence of flow bifurcations in certain configurations but not in all instances. The numerical simulations also show that the centrifugal force is the dominant stabilizing force on fluid convection in the stable configuration. The stabilizing influence of the Coriolis force is found to be only secondary in nature. No significant impact of gravity gradient is found in the calculations. Simulations of unstable configurations show that the Coriolis force has a stabilizing influence on fluid motion by delaying the onset of unsteady convection. Detailed flow and thermal field characteristics are presented for all the different cases that are simulated.

Ramachandran, N.↗
NASA NTRS1974

Analytics of crystal growth in space

Two crystal growth processes considered for spacelab experiments were studied to anticipate and understand phenomena not ordinarily encountered on earth. Computer calculations were performed on transport processes in floating zone melting and on growth of a crystal from solution in a spacecraft environment. Experiments intended to simulate solution growth at micro accelerations were performed.

Wilcox, W. R.↗
NASA NTRS1979

A numerical cloud model for the support of laboratory experimentation

A numerical cloud model is presented which can describe the evolution of a cloud starting from moist aerosol-laden air through the diffusional growth regime. The model is designed for the direct support of cloud chamber laboratory experimentation, i.e., experiment preparation, real-time control and data analysis. In the model the thermodynamics is uncoupled from the droplet growth processes. Analytic solutions for the cloud droplet growth equations are developed which can be applied in most laboratory situations. The model is applied to a variety of representative experiments.

Hagen, D. E.↗
NASA NTRS1990

Ground-based experiments on thermal and thermosolutal convection in inclined low-aspect-ratio enclosures

Thermal and thermosolutal convection driven by buoyancy forces are investigated experimentally in inclined low-aspect-ratio enclosures. Thermal and thermosolutal convection are important in crystal growth processes. The transport process in the fluid phase during the growth of a crystal has a profound influence on the structure and quality of the solid phase. The present work is concerned with natural convection in a shallow rectangular enclosure with imposed inclined temperature and/or concentration gradients. Various complex flow patterns are observed with different experimental conditions. A flow instability under certain conditions is reported. The flow instability is found to depend not only on the thermal and solutal Grashof numbers, the aspect ratios, but also the inclined angles.

Wang, L. W.↗