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NASA NTRS1998

Determining the Molecular Growth Mechanisms of Tetragonal Lysozyme Crystals

Studies of the growth of tetragonal lysozyme crystals employing atomic force microscopy (AFM) have shown the advantages of this technique in investigating the growth mechanisms of protein crystals [1]. The resolution of these studies was in the micron range, which revealed surface features such as the occurrence of dislocations and 2D nucleation islands, similar to those found in inorganic systems. They clearly showed that the crystals grew by these surface growth mechanisms. However, the studies also revealed some surprising features, such as bimolecular growth step heights and pronounced growth anisotropies on the (110) face, which could not be explained. In previous studies we employed Periodic Bond Chain (PBC) theory to tetragonal lysozyme crystal growth and found that the crystals were constructed by strongly bonded molecular chains forming helices about the 43 axes [2,3]. The helices were connected to each other with weaker bonds. The growth process was shown to proceed by the formation of these 43 helices, resulting in bimolecular growth steps on the (110) face. It was also shown to explain many other observations on tetragonal lysozyme crystal growth. Although PBC analysis is not a new technique [4], it has not been widely used as the mechanisms predicted from it could not be experimentally verified. In this study the growth process of these crystals was investigated, particularly for the (110) face, employing some newly developed high resolution AFM techniques. These techniques allowed individual lysozyme molecules on the crystal faces to be resolved and predictions from PBC analyses to be tested. The analyses had shown that of the two possible packing arrangements on (110) faces, only one would actually occur. Employing the first of the newly developed techniques, these faces were scanned by high resolution AFM. The resulting images were then compared with the theoretically constructed images for the two possible packing arrangements on the (110) face. The theoretical images were constructed by convolution of the crystal surface shape obtained from crystallographic data with the AFM tip shape. The comparison confirmed the prediction that the molecular packing arrangement of these faces corresponded to that for complete 43 helices. The second AFM technique that was developed was used to follow the growth process by measuring the dimensions of individual growth units on the (110) face. Linescans across a growth step, performed near the saturation limit of the crystals, allowed the growth unit dimensions to be measured. These measurements showed that growth on the (110) face proceeded by the formation of new 43 helices from the addition of at least tetramer units in the [110] direction. In the [001] direction growth proceeded by the addition of various aggregate units corresponding to the 4(sub 3) helices.

Li, Huayu↗

NASA NTRS1988

Microstructure and growth model for rice-hull-derived SiC whiskers

The microstructure of silicon carbide whiskers grown from rice hulls has been studied using methods of high-resolution analytical electron microscopy. Small, partially crystalline inclusions (about 10 nm) containing calcium, manganese, and oxygen are concentrated in whisker core regions, while peripheral regions are generally inclusion free. The distinct microphase distribution is evidence of a two-stage growth process in which the core region grows first, followed by normal growth toward whisker sides. Partial dislocations extend radially from the core region to the surface and tend to be paired in V-shaped configurations. Whisker surfaces exhibit microroughness due to a tendency to develop small facets on close-packed planes. The microstructural data obtained from TEM observations are used as a basis for discussion of the mechanisms involved in whisker growth, and a model of the growth process is proposed. The model includes a two-dimensional growth mechanism involving vapor, liquid, and solid phases, although it is significantly different from the classical vapor-liquid-solid (VLS) process of whisker growth.

Nutt, Steven R.↗

NASA NTRS1999

Photoluminescence Studies of ZnSe Starting Materials and Vapor Grown Bulk Crystals

Low-temperature photoluminescence (PL) spectra and glow discharge mass spectroscopy (GDMS) were measured on ZnSe starting materials provided by various vendors and on bulk crystals grown from these starting materials by physical vapor transport (PVT) to study the effects of purification and contamination during crystal growth process. The purification effect of pre-growth heat treatments and the PVT process is evidenced from the GDMS results which showed orders of magnitude reduction in the Li and Na concentration and a factor of 3 reduction in the 0 content after growth. The PL spectra showed that the strong emissions associated with Li (or Na) in one of the starting materials disappeared after growth. To evaluate the contamination of the crystal during the high temperature growth process three growth runs were processed using similar growth parameters but with different furnace environments. The GDMS results showed orders of magnitude increase in the Al and Si contents after growth. The PL spectra suggest that the Al contamination was originated from the fused silica ampoule and the Inconel cartridge might have been the cause for the broad Cu green and Cu red bands observed in one of the grown crystal.

Su, Ching-Hua↗

NASA NTRS1998

Global Observation of Substorm Growth Phase Processes in the Polar Caps

Global images of the polar cap region during the substorm growth phase by the Polar Ultraviolet Imager reveals evidence of the processes which are not completely explained by current models. In particular, it was found that size of the polar cap region increases during the growth phase even if the interplanetary magnetic field has no southward component. Three phenomena were observed to produce an increase in the size of the polar cap: (1) motion of the auroral oval to lower latitude, (2) thinning of the auroral oval, and (3) reduction of intense aurora[ precipitation in the polar region. Correlation of image intensities with in situ particle measurements from the FAST satellite are being conducted to study the three growth phase phenomena; and to help identify the source regions of the particles, the mechanisms involved in producing the auroral structures and what may be reducing the polar cap precipitation during the substorm growth phase.

Brittnacher, M.↗

DOE OSTI2020

Remote Epitaxial Growth of Compound Semiconductors through Graphene

Epitaxial growth has been widely used for high-quality growth of crystalline materials. However, epitaxial growth techniques usually require lattice match and similar thermal expansion coefficient between growth material and substrate, which blocks this growth technique from wide applicability. Recently, van der Waals (vdW) epitaxy has been demonstrated that allows to relax the limitation of epitaxial growth. vdW epitaxy on layered or two-dimensional (2D) materials is mediated by weak vdW interactions. In addition to these interactions, a substrate below 2D materials also plays a role to intervene in the epitaxial growth process, leaving room to realize a novel epitaxial growth process: remote epitaxy. Potential fluctuation—the difference between potential energy maxima and minima along the surface of the substrate—is influenced by this remote interaction; the fluctuation is predominant over the vdW force, due to the atomic thickness of 2D materials. The remote interactions can be controlled by modulation of polarities of 2D materials and epitaxy materials. Polar-material-based heterostructure, such as GaAs, InP, GaP and GaN with their substrate sandwiching 2D materials, enables the semiconductor film to be exfoliated from the underlying substrates through the 2D materials. This process is expected to open a novel avenue for the field of non-silicon electronics and photonics, where the ability to re-use graphene-coated substrates allows savings on the high cost of non-silicon substrates.

Lee, Kyusang↗

DOE OSTI2023

GaN MOCVD Growth on Native substrates for High Voltage (15-20 KV) Vertical Power Devices

This project aims to develop metalorganic chemical vapor deposition (MOCVD) homoepitaxy of GaN on native substrates with fast growth rate (15-20 μm/hr), low background doping (low-10 15 cm -3 ) and smooth surface morphology via comprehensive understanding of the crystal growth process including high quality GaN substrate development and surface preparation, impurity and native defects control, and their impacts on the breakdown field. The team successfully developed a new growth process by introducing the laser-assisted MOCVD (LA-MOCVD) process to address the limited growth rates of GaN in the traditional MOCVD process. Specifically, the use of the CO2 laser with lasing wavelength of 9.219 μm, the strong coupling between the laser beam with the ammonia (GaN MOCVD precursor) leads to efficient decomposition of NH3 which significantly increases the effective group V/III molar ratio and thus suppresses C impurity incorporation in MOCVD GaN. The reduction in C incorporation in LA-MOCVD GaN is especially prominent when the GaN growth rate is fast (> 10 μm/hr), which allows to develop thick GaN films with high crystalline quality and low controllable doping needed for vertical high power device applications. The results of this project lead to the demonstration of vertical GaN PN diodes with record breakdown voltage of ~ 8kV with high Baliga’s figure of merit.

24 POWER TRANSMISSION AND DISTRIBUTION↗

NASA NTRS1987

An electric current associated with gravity sensing in maize roots

The study of gravisensing would be greatly enhanced if physiological events associated with gravity sensing could be detected separately from subsequent growth processes. This report presents a means to discriminate sensing from the growth processes. By using a vibrating probe, we have found an electric current generated by the gravity sensing region of the root cap of maize (Zea mays cv Merit) in response to gravistimulation. On the upper surface of the root cap, the change from the endogenous current has a density of 0.55 microampere per square centimeter away from gravity. The onset of the current shift has a characteristic of lag of three to four minutes after gravistimulation, which corresponds to the presentation time for gravity sensing in this tissue. A description of the current provides some information about the sensing mechanism, as well as being a valuable means to detect gravity sensing independently of differential growth.

NASA Discipline Number 40-10↗

NASA NTRS1986

Thermal-capillary analysis of Czochralski and liquid encapsulated Czochralski crystal growth. II - Processing strategies

The pseudosteady-state heat transfer model developed in a previous paper is augmented with constraints for constant crystal radius and melt/solid interface deflection. Combinations of growth rate, and crucible and bottom-heater temperatures are tested as processing parameters for satisfying the constrained thermal-capillary problem over a range of melt volumes corresponding to the sequence occuring during the batchwise Czochralski growth of a small-diameter silicon crystal. The applicability of each processing strategy is judged by the range of existence of the solution, in terms of melt volume and the values of the axial and radial temperature gradients in the crystal.

Derby, J. J.↗

DOE OSTI2022

Growth mechanism of glucose-based hydrochar under the effects of acid and temperature regulation

Improving the tailorability of hydrochar synthesis is an effective way to enhance its performance and utilization efficiency. In this study, the growth rate, morphology, and molecular structure of hydrochar were controlled by regulating the pH and temperature of the hydrothermal carbonization process. Growth process analysis indicates that hydrochar has three growth periods: induction, rapid, and stable growth periods. It is mainly controlled by 5-hydroxymethylfurfural (HMF), which is formed by converting glucose and its transformation products. Further, the regulation of acid can significantly shorten the induction period of hydrochar, even under low-temperature conditions (<180 °C), and increase the growth rate of hydrochar. However, the degree of hydrochar adhesion varies: the lower the temperature, the greater the degree of its adhesion. Molecular structural analysis demonstrates that hydrochar mainly consists of furan structural domains and aromatic clusters, and its surface is rich in oxygen-containing functional groups. The degree at which hydrochar was aromatized was improved by increasing the reaction temperature (160–220 °C); whereas the regulation of acid reduced it and increased the content of oxygen-containing functional groups on the hydrochar surface. Based on these results, it is proposed that hydrochar formation has five stages and three growth periods with or without acid regulation.

59 BASIC BIOLOGICAL SCIENCES↗

NASA NTRS1968

Compound equation developed for postnatal growth of birds and mammals

Compound growth equation was developed in which the rate of this linear growth process is regarded as proportional to the mass already attained at any instant by an underlying Gompertz process. This compound growth model was fitted to the growth data of a variety of birds and mammals of both sexes.

Laird, A. K.↗

NASA NTRS1999

Melt Stabilization of PbSnTe in a Magnetic Field

Both the experimental observation and numerical simulation indicate that the Bridgman growth of PbSnTe under the microgravity environment in space is still greatly influenced by buoyancy-induced convection. The application of a magnetic field during the semiconductor growth can dampen the convective flow in the metal-like melt. However, for Bridgman growth of PbSnTe on earth (with either vertical or horizontal configuration), both experimental observation and numerical modeling suggest that even with a strong magnetic furnace (5-Tesla constant axial magnetic field), the convective flow in the melt still cannot be sufficiently suppressed to reach the diffusion-controlled level. In order to completely dampen the buoyancy-induced convection on earth, estimates based on scaling analysis indicate that for common experimental conditions, an extremely high magnetic field is required, far beyond the capacity of the experimental apparatus currently available. Therefore, it is proposed that only the combination of microgravity environment and magnetic damping will produce the desired diffusion-controlled growth state for this particular material. The primary objectives of this study are to provide a quantitative understanding of the complex transport phenomena during solidification of non-dilute binarys, to furnish a numerical tool for furnace design and growth condition optimization, to provide estimates of the required magnetic field strength for low gravity growth, and to assess the role of magnetic damping for space and earth control of the double-diffusive convection. As an integral part of a NASA research program, our numerical simulation supports both the flight and ground-based experiments in an effort to bring together a complete picture of the complex physical phenomena involved in the crystal growth process. For Bridgman growth of PbSnTe under microgravity (with both vertical and horizontal configurations), the simulations suggest that a moderate axial magnetic field of only a few kilo-Gauss in strength could effectively eliminate buoyancy-induced convection in the melt and control solute segregation. Therefore, this work confirms the idea that the combination of microgravity environment and the magnetic damping will indeed be sufficient to produce the desired diffusion-controlled growth state for PbSnTe.

Fripp, Archibald L.↗

NASA NTRS1997

Interface Shape and Growth Rate Analysis of Se/GaAs Bulk Crystals Grown in the NASA Crystal Growth Furnace (CGF)

Selenium-doped gallium arsenide, Se/GaAs, bulk crystals have been grown on earth using NASA's crystal growth furnace (CGF) in preparation for microgravity experimentation on the USML-2 spacelab mission. Peltier cooling pulses of 50 ms duration, 2040 A magnitude, and 0.0033 Hz frequency were used to successfully demark the melt-solid interface at known times during the crystal growth process. Post-growth characterization included interface shape measurement, growth rate calculation, and growth rate transient determinations. It was found that the interface shapes were always slightly concave into the solid. The curvature of the seeding interfaces was typically 1.5 mm for the 15 mm diameter samples. This was in agreement with the predicted interface shapes and positions relative to the furnace determined using a numerical model of the sample/ampoule/cartridge assembly (SACA).

Bly, J. M.↗

DOE OSTI2023

Improving parameterization of ice microphysical processes in Arctic clouds using a synergistic modeling and observational approach (Final Report)

The primary goals of the project were to: 1) Improve the fundamental quantitative models of ice growth processes using ARM radar observations. 2) Better understand the growth of ice particle populations through vapor deposition, riming, and aggregation in Arctic clouds using a new particle-property microphysical model. 3) Robustly estimate and constrain key unknown model parameters using ARM radar observations combined with the particle-property model to develop an improved parameterization of ice growth processes for traditional bulk parameterizations used in cloud and climate models.

58 GEOSCIENCES↗

NASA NTRS2007

Development of Hairpin Vortices in Turbulent Spots and End-Wall Transition

The end-stage phase of boundary layer transition is characterized by the development of hairpin-like vortices which evolve rapidly into patches of turbulent behavior. In general, the characteristics of the evolution form this hairpin stage to the turbulent stage is poorly understood, which has prompted the present experimental examination of hairpin vortex development and growth processes. Two topics of particular relevance to the workshop focus will be covered: 1) the growth of turbulent spots through the generatio and amalgamation of hairpin-like vortices, and 2) the development of hairpin vortices during transition in an end-wall junction flow. Brief summaries of these studies are described below. Using controlled generation of hairpin vortices by surface injection in a critical laminar boundary layer, detailed flow visualization studies have been done of the phases of growth of single hairpin vortices, from the initial hairgin generation, through the systematic generation of secondary hairpin-like flow structures, culminating in the evolution to a turbulent spot. The key to the growth process is strong vortex-surface interactions, which give rise to strong eruptive events adjacent to the surface, which results in the generation of subsequent hairpin vortex structures due to inviscid-viscuous interactions between the eruptive events and the free steam fluid. The general process of vortex-surface fluid interaction, coupled with subsequent interactions and amalgamation of the generated multiple hairpin-type vortices, is demonstrated as a physical mechanism for the growth and development of turbulent spots. When a boundary layer flow along a surface encounters a bluff body obstruction extending from the surface (such as cylinder or wing), the strong adverse pressure gradients generated by these types of flows result in the concentration of the impinging vorticity into a system of discrete vortices near the end-wall juncture of the obstruction, with the extensions of the vortices engirdling the obstruction to form "necklace" or "horseshoe" vortices. Recent hydrogen bubble and particle image visualization have shown that as Reynolds number is increased for a laminar approach flow, the flow will become critical, and a destabilization of the necklace vortices results in the development of an azimuthal waviness, or "kinks", in the vortices. These vortex kinks are accentuated by Biot-Savart effects, causing portions of a distorted necklace vortex to make a rapid approach to the surface, precipitating processes of localized, three-dimensional surface interactions. These interactions result in the rapid generation, focussing, and ejection of thin tongues of surface fluid, which rapidly roll-over and appear as hairpin vortices in the junction region. Subsequent amalgamation of these hairpin vortices with the necklace vortices produces a complex transitional-type flow. A presentation of key results from both these studies will be done, emphasizing both the ubiquity of such hairpin-type flow structures in manifold transitional-type flows, and the importance of vortex-surface interactions n the development of hairpin vortices.

Smith, Charles R.↗

DOE OSTI2021

Revealing the effect of local stresses on twin growth mechanisms in titanium using synchrotron X-ray diffraction

Deformation twinning has a significant impact on the evolution of microstructure and mechanical response in hexagonal close packed (hcp) metals. Understanding the physical mechanisms associated with twin nucleation and growth processes is important to enable the broad use of hcp metals. While both nucleation and growth are conditioned by the local stress state, few studies have quantified it in bulk deformed samples. In this study, we reveal the effects of local stresses on twin thickening using in-situ synchrotron experiments with differential aperture X-ray microscopy. High purity Ti is deformed under four-point bending to activate {1012} tensile twins. Further, 3D stress fields with a spatial resolution of 0.5μm are mapped in the vicinity of low and high macroscopic Schmid factor (MSF) twins growing inside a grain. Reconciling this experimental analysis with recent twin growth models reveals that the growth of low MSF twins is limited by the nucleation of twin growth defects in the high-purity Ti sample. While growth-inducing defect nucleation may occur in many places on the interface of the high resolved shear stress (RSS)/MSF twin, the nucleation rate of twinning disconnections for the low MSF twin is high only at the end of the twin near a stress concentration. Once nucleated, these defects easily propagate into lower RSS regions of the grain resulting in twin growth. This work highlights the importance of stress concentrations not only for twin (embryo) nucleation, but also to the growth process of twins, particularly in low MSF twins.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Low-bandgap mixed tin–lead iodide perovskites with reduced methylammonium for simultaneous enhancement of solar cell efficiency and stability

High-performance perovskite/perovskite tandem solar cells require high-efficiency and stable low-bandgap perovskite subcells. State-of-the-art low-bandgap mixed tin–lead iodide perovskite solar cells exhibit either a high power-conversion efficiency or improved stability, but not both. In this article we report a two-step bilayer interdiffusion growth process to simultaneously meet both requirements for formamidinium-based low-bandgap mixed tin–lead iodide perovskite solar cells. The bilayer interdiffusion growth process allows for the formation of high-quality and large-grained perovskite films with only 10 mol% volatile methylammonium. Additionally, one-dimensional pyrrolidinium perovskite was applied to passivate the perovskite film and improve the junction quality, which resulted in a carrier lifetime of 1.1 µs and an open circuit voltage of 0.865 V for our perovskite film and device with a bandgap of 1.28 eV. Our strategies enabled a power-conversion efficiency of 20.4% for low-bandgap perovskite solar cells under AM 1.5G illumination. More importantly, an encapsulated device can retain 92% of its initial efficiency after 450 h of continuous 1 sun illumination.

14 SOLAR ENERGY↗

DOE OSTI2020

In situ temperature profile measurements with high-energy X-rays as a probe of optical floating zone crystal growth environment

The ability of optical floating zone (OFZ) furnaces to rapidly produce large single crystals of complex emerging materials has had a transformative effect on many scientific fields that require samples of this type. However, the crystal growth process within the OFZ furnace is not well understood owing to the challenges involved in monitoring the high-temperature crystal growth process. Novel beamline-compatible optical furnaces that approximate the inhomogeneous growth environment within an OFZ furnace have been fabricated and tested in high-energy synchrotron beamlines. In this paper, it is demonstrated that temperature profiles can be effectively extracted from powder diffraction data collected on polycrystalline ceramic rods heated at their tip. Furthermore, these measured temperature profiles can be accurately reproduced using a heat-transfer model that accounts for solid-state thermal conduction, partial sample lamp power absorption, convective air cooling and radiative cooling, allowing key thermal parameters such as thermal conductivity to be extracted from experimental data.

36 MATERIALS SCIENCE↗

NASA NTRS1988

A review of melt and vapor growth techniques for polydiacetylene thin films for nonlinear optical applications

Methods for the growth of polydiacetylene thin films by melt and vapor growth and their subsequent polymerization are summarized. Films with random orientations were obtained when glass or quartz were used as substrates in the vapor growth process. Oriented polydiacetylene films were fabricated by the vapor deposition of diacetylene monomer onto oriented polydiacetylene on a glass substrate and its subsequent polymerization by UV light. A method for the growth of oriented thin films by a melt-shear growth process as well as a method of film growth by seeded recrstallization from the melt between glass plates, that may be applied to the growth of polydiacetylene films, are described. Moreover, a method is presented for the fabrication of single crystal thin films of polyacetylenes by irradiation of the surface of diacetylene single crystals to a depth between 100 and 2000 angstroms.

Penn, B. G.↗