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NASA NTRS2015

Crystal Growth of Germanium-Silicon Alloys on the ISS

A series of Ge(1-x)Si(x) crystal growth experiments are planned to be conducted in the Low Gradient Furnace (LGF) onboard the International Space Station. The experiments are part of the investigation "Influence of Containment on the Growth of Silicon-Germanium" (ICESAGE). The primary objective of the research is to determine the influence of containment on the processing-induced defects and impurity incorporation in germanium-silicon alloy crystals. A comparison will be made between crystals grown by the normal and "detached" Bridgman methods and the ground-based float zone technique. Crystals grown without being in contact with a container have superior quality to otherwise similar crystals grown in direct contact with a container, especially with respect to impurity incorporation, formation of dislocations, and residual stress in crystals. "Detached" or "dewetted" Bridgman growth is similar to regular Bridgman growth in that most of the melt is in contact with the crucible wall, but the crystal is separated from the wall by a small gap, typically of the order of 10-100 microns. Long duration reduced gravity is essential to test the proposed theory of detached growth. Detached growth requires the establishment of a meniscus between the crystal and the ampoule wall. This meniscus can exist over a much larger range of processing parameters in microgravity and the meniscus is more stable under microgravity conditions. The plans for the flight experiments will be described.

Volz, M. P.↗

NASA NTRS2016

Detached Solidification of Germanium-Silicon Crystals on the ISS

A series of Ge(sub 1-x) Si(sub x) crystal growth experiments are planned to be conducted in the Low Gradient Furnace (LGF) onboard the International Space Station. The primary objective of the research is to determine the influence of containment on the processing-induced defects and impurity incorporation in germanium-silicon alloy crystals. A comparison will be made between crystals grown by the normal and "detached" Bridgman methods and the ground-based float zone technique. Crystals grown without being in contact with a container have superior quality to otherwise similar crystals grown in direct contact with a container, especially with respect to impurity incorporation, formation of dislocations, and residual stress in crystals. "Detached" or "dewetted" Bridgman growth is similar to regular Bridgman growth in that most of the melt is in contact with the crucible wall, but the crystal is separated from the wall by a small gap, typically of the order of 10-100 microns. Long duration reduced gravity is essential to test the proposed theory of detached growth. Detached growth requires the establishment of a meniscus between the crystal and the ampoule wall. The existence of this meniscus depends on the ratio of the strength of gravity to capillary forces. On Earth, this ratio is large and stable detached growth can only be obtained over limited conditions. Crystals grown detached on the ground exhibited superior structural quality as evidenced by measurements of etch pit density, synchrotron white beam X-ray topography and double axis X-ray diffraction.

Volz, M. P.↗

NASA NTRS2016

Detached Solidification of Germanium-Silicon Crystals on the ISS

Volz, M. P.↗

NASA NTRS1985

Nonstoichiometric defects in GaAs and the EL2 bandwagon

In the present paper, an attempt is made to formulate a common framework for a discussion of nonstoichiometric defects, especially EL2 and dislocations. An outline is provided of the most important settled and unsettled issues, taking into account not only fundamental interests, but also urgent needs in advancing IC technology. Attention is given to stoichiometry-controlled compensation, the expected role of melt stoichiometry in electrical conductivity for the basic atomic disorders, defect equilibria-dislocations and EL2, and current issues pertaining to the identification of EL2. It is concluded that nonstoichiometric defects play a critical role in the electronic properties of GaAs and its electronic applications. Very significant progress has been recently made in learning how to adjust melt stoichiometry in order to maximize its beneficial effects and minimize its detrimental ones.

Lagowski, J.↗

NASA NTRS1986

On control of stresses in silicon web growth

It has been observed that the residual stresses and dislocations during the silicon crystal growth for photovoltaic applications are caused by thermal stresses. The temperatures along the boundaries of the silicon crystal ribbon are prescribed to meet the requirements of the crystal growth. It is shown that by allowing the temperatures to satisfy a second-order partial differential equation in the ribbon, all thermal stresses, and others induced by them, may be eliminated for the stress-free growth of the silicon crystal.

Utku, S.↗

NASA NTRS2001

Effects of dislocations in Stranski-Krastanow quantum dots

The interrelations between structural and optical properties in Strasnki-Krastanow (S-K) quantum dots are examined trough different experiments that explore III-V quantum dots (QD) formation.

nanotechnology ordered structure photoiluminescenc↗

NASA NTRS1988

Elevated temperature slow plastic deformation of NiAl/TiB2 particulate composites

The 'XD' process for production of discontinuously-reinforced metal-matrix composites has been used to enhance the high-temperature strength of NiAl-TiB2 composites with particulate densities of up to 30 vol pct. SEM, TEM, and optical characterizations of the resulting microstructures showed the average TiB2 particle size to be about 1 micron, while the average grain of the NiAl matrix was of the order of 10 microns. Elevated temperature compression tests conducted at 1200 and 1300 K indicated flow strengths to increase with TiB2 content, so that the 20 vol pct TiB2-reinforced composite was three times stronger than the unreinforced NiAl; this is ascribed to the very high density of microstructural tangled dislocations, loops, and subgrain boundaries connecting the particles.

Whittenberger, J. Daniel↗

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 NTRS1986

Process-induced defects in solar cell silicon

The microstructure of as-grown and processed, edge-defined film-fed grown, silicon ribbons was studied in order to identify the basic mechanisms responsible for the change of the as-grown defect structure during processing at elevated temperatures. The short heat treatment required to diffuse the p-n junction was sufficient to cause heterogeneous precipitation of impurity atoms, particularly transition metals. In addition, an elastic recovery of the ribbon occurred by dislocation glide, resulting in the formation of a polygonizationlike structure of subboundaries in the base. Mechanisms are suggested by which twin boundaries, acting as dislocation obstacles, can concentrate the relatively low average dislocation densities typical of edge-defined film-fed grown ribbons (10 to the 4th to 10 to the 8th/sq cm) to values which are sufficient to induce the formation of subgrainlike boundary structures.

Gleichmann, R.↗

NASA NTRS1993

Rheology of water and ammonia-water ices

Creep experiments on fine-grained water and ammonia-water ices have been performed at one atmosphere and high confining pressure in order to develop constitutive relationships necessary to model tectonic processes and interpret surface features of icy moons of the outer solar system. The present series of experiments explores the effects of temperature, strain rate, grain size, and melt fraction on creep strength. In general, creep strength decreases with increasing temperature, decreasing strain rate, and increasing melt fraction. A transition from dislocation creep to diffusion creep occurs at finer grain sizes, higher temperatures, and lower strain rates.

Goldsby, D. L.↗

NASA NTRS1989

Effect of particle size on fracture toughness of SiC/Al composite material

Discontinuous SiC/Al composites with SiC particles of different sizes were fabricated in order to study the role of particle size on the fracture process. The fracture process is confined to a very narrow band and takes place within the matrix in composites containing small SiC particle sizes. In the composite reinforced with SiC particles of 20 microns and above fracture of SiC begins to dominate. The matrix is influenced by the high density of dislocations generated at SiC/Al interfaces due to the difference in coefficient of thermal expansion between SiC and the Al matrix. Crack initiation fracture toughness does not depend on SiC particle size. Crack growth fracture toughness increases as the size of the SiC particle increase.

Flom, Y.↗

NASA NTRS1993

High purity isotopically enriched Ge-70 and Ge-74 single crystals - Isotope separation, growth, and properties

Ge-70 and Ge-74 isotopes were successfully separated from natural Ge, and were zone purified. Several highly enriched, high purity Ge-70 and Ge-74 single crystals were grown by the vertical Bridgman method. The growth system was designed for reliable growth of low dislocation density, high purity Ge single crystals of about 4 g weight. A Ge-70 and a Ge-74 crystal were selected for complete characterization. In spite of the large surface-to-volume ratio of these ingots, both Ge-70 and Ge-74 crystals contain low electrically active chemical net-impurity concentrations of about 2 x 10 exp 12/cu cm, which is 2 orders of magnitude better than that of Ge-74 crystals previously grown by two different groups. Isotopic enrichment of the Ge-70 and the Ge-74 crystals is 96.3 percent and 96.8 percent, respectively. The residual donors and acceptors present in both crystals were identified as phosphorus and copper, respectively. In addition, less than 10 exp 11/cu cm Ga, Al, and In were found in the Ge-70 crystal.

Itoh, Kohei↗

NASA NTRSDate not supplied

TPSAS-NF1676L-13792-DND

Extreme structural requirements of future aerospace vehicles motivate the development of new, ultra-durable materials and game-changing methodologies for material certification and sustainment. Computational simulations spanning many orders of magnitude in length and time scales—from the nanoscale of underlying damage processes to the larger scales of continuum cracks—are being developed to support these requirements. Results of these simulations are used to deduce key aspects of material response to loads and environments, including: - The microstructural mechanics that govern metal fatigue crack initiation and growth - The variety and complexity of the dislocation-precipitate interac- tions that underpin plastic behavior and damage evolution - The energetic principles that govern the interaction of water with crack surfaces within a structural component - The mechanics of grain boundary separation Modeling the physics behind these metal fatigue behaviors is extremely complex, and requires intensive, high-fidelity simulations. The knowledge gained through these simulations, however, is forming the keystone for advanced material design and will enable development of more capable, reliable structures for aerospace vehicles.

Jacob Hochhalter↗

NASA NTRS1983

TEM and SEM (EBIC) investigations of silicon bicrystals

The electrical and structural properties of low and medium angle tilt grain boundaries in silicon bicrystals were studied in order to obtain insight into the mechanisms determining the recombination activity. The electrical behavior of these grain boundaries was studied with the EBIC technique. Schottky barriers rather than p-n junctions were used to avoid annealing induced changes of the structure and impurity content of the as-grown crystals. Transmission electron spectroscopy reveals that the 20 deg boundary is straight, homogeneous, and free of extrinsic dislocations. It is concluded that, in the samples studied, the electrical effect of grain boundaries appears to be independent of the boundary misorientation. The dominant influence appears to be impurity segregation effects to the boundary. Cleaner bicrystals are required to study intrinsic differences in the electrical activity of the two boundaries.

Gleichmann, R.↗

NASA NTRS1998

Carboxylic and Dicarboxylic Acids Extracted from Crushed Magnesium Oxide Single Crystals

Carboxylic and dicarboxylic acids (glycolic, oxalic, malonic and succinic) have been extracted with tetrahydrofuran (THE) and H2O from large synthetic MgO crystals, crushed to a medium fine powder. The extracts were characterized by infrared spectroscopy and (sup 1)H-NMR (Nuclear Magnetic Resonance). The THF extracts were derivatized with tert-butyldimethylsilyl (t-BDMS) for GC-MS (Gas Chromatography - Mass Spectroscopy) analysis. A single crystal separated from the extract was used for an x-ray structure analysis, giving the monoclinic unit cell, space group P2(sub 1)/c with a(sub o) = 5.543 A, b(sub o) = 8.845 A, c(sub o) = 5.086 A, and beta = 91.9 degrees, consistent with beta-succinic acid, HOOC(CH2)COOH. The amount of extracted acids is estimated to be of the order of 0.1 to 0.5 mg/g MgO. The MgO crystals from which these organic acids were extracted grew from the 2360 C hot melt, saturated with CO/CO2 and H2O, thereby incorporating small amounts of the gaseous components to form a solid solution (ss) with MgO. Upon cooling, the ss becomes supersaturated, causing solute carbon and other solute species to segregate not only to the surface but also internally, to dislocations and subgrain boundaries. The organic acids extracted from the MgO crystals after crushing appear to derive from these segregated solutes that formed C-C, C-H, and C-O bonds along dislocations and other defects in the MgO structure, leading to entities that can generically be described as (HxCyOz)(sup n-). The processes underlying the formation of these precursors are fundamental in nature and expected to be operational in any minerals, preferentially those with dense structures, that crystallized in H2O-CO2-laden environments. This opens the possibility that common magmatic and metamorphic rocks when weathering at the surface of a tectonically active planet like Earth may be an important source of abiogenically formed complex organic compounds.

Freund, Friedemann↗

NASA NTRS2003

Creep Resistance of ZrO2 Ceramic Improved by the Addition of a Small Amount of Er2O3

Zirconia (ZrO2) has great technological importance in structural, electrical, and chemical applications. It is the crucial component for state-of-the art thermal barrier coatings and an enabling component as a solid electrolyte for solid-oxide fuel cell systems. Pure ZrO2 is of limited use for industrial applications because of the phase transformations that occur. Upon the addition of stabilizers, cubic (c-ZrO2) and tetragonal (t-ZrO2) forms can be preserved. It is the stabilized and partially stabilized forms of zirconia that function as thermal barrier coatings, solid electrolytes, and oxygen sensors and that have numerous applications in the electrochemical industry. The cubic form of ZrO2 is typically stabilized through Y2O3 additions. However, Y2O3-stabilized zirconia is susceptible to deformation at high temperatures (greater than 900 C) because of the large number of slip systems and the high oxygen diffusion rates, which result in high creep rates at high temperatures. Successful use of ZrO2 at high temperatures requires that new dopant additives be found that will retain or enhance the desirable properties of cubic ZrO2 and yet produce a material with lower creep rates. At the NASA Glenn Research Center, erbium oxide (Er2O3) was identified as a promising dopant for improving the creep resistance of. ZrO2. The selection of Er2O3 was based on the strong interactions of point defects and dislocations. Single crystals of 5 mol% Er2O3- doped ZrO2 rods (4 mm in diameter) and monofilaments (200 to 300 mm in diameter and 30 cm long) were grown using the laser-heated float zone technique, and their creep behavior was measured as a function of temperature. The addition of 5 mol% Er2O3 to single-crystal ZrO2 improved its creep resistance at high temperatures by 2 to 3 orders of magnitude over state-of-the-art Y2O3-doped crystals. Detailed microstructural characterization of ZrO2-Er2O3 single crystals has identified new mechanisms for improving the creep resistance of this class of materials. Adding Er2O3 to ZrO2 results in microstructure of stable and metastable tetragonal precipitates that with thermal treatment evolve to a tweed structure of nanosize tetragonal lamellae. The superior high-temperature creep resistance of Er2O3-doped ZrO2 is attributed to nanoscale precipitation hardening. Doping with Er2O3 will significantly increase the upper-use temperature limit of ZrO2. Potential applications include using Er2O3-doped ZrO2 as a high-temperature fiber for structural applications and adding Er2O3 to reduce the sintering rates of ZrO2 thermal barrier coatings. This work was conducted at Dpto. de F sica de la Materia Condensada, Universidad de Sevilla, Spain, and at NASA Glenn.

Martinez-Fernandez, Julian↗

NASA NTRS1999

Carboxylic and dicarboxylic acids extracted from crushed magnesium oxide single crystals

Carboxylic and dicarboxylic acids (glycolic, oxalic, malonic and succinic) have been extracted with tetrahydrofuran (THF) and H2O from large synthetic MgO crystals, crushed to a medium fine powder. The extracts were characterized by infrared spectroscopy and 1H-NMR. The THF extracts were derivatized with tert-butyldimethylsilyl (t-BDMS) for GC-MS analysis. A single crystal separated from the extract was used for an x-ray structure analysis, giving the monoclinic unit cell, space group P21/c with ao = 5.543 A, bo = 8.845 A, co = 5.086 A, and beta = 91.9 degrees, consistent with beta-succinic acid, HOOC(CH2)COOH. The amount of extracted acids is estimated to be of the order of 0.1 to 0.5 mg g-1 MgO. The MgO crystals from which these organic acids were extracted grew from the 2860 degrees C hot melt, saturated with CO/CO2 and H2O, thereby incorporating small amounts of the gaseous components to form a solid solution (ss) with MgO. Upon cooling, the ss becomes supersaturated, causing solute carbon and other solute species to segregate not only to the surface but also internally, to dislocations and subgrain boundaries. The organic acids extracted from the MgO crystals after crushing appear to derive from these segregated solutes that formed C-C, C-H and C-O bonds along dislocations and other defects in the MgO structure, leading to entities that can generically be described as (HxCyOz)n-. The processes underlying the formation of these precursors are fundamental in nature and expected to be operational in any minerals, preferentially those with dense structures, that crystallized in H2O-CO2-laden environments. This opens the possibility that common magmatic and metamorphic rocks when weathering at the surface of a tectonically active planet like Earth may be an important source of abiogenically formed complex organic compounds.

NASA Discipline Exobiology↗

NASA NTRS2014

Multiscale Analysis of Structurally-Graded Microstructures Using Molecular Dynamics, Discrete Dislocation Dynamics and Continuum Crystal Plasticity

A multiscale modeling methodology is developed for structurally-graded material microstructures. Molecular dynamic (MD) simulations are performed at the nanoscale to determine fundamental failure mechanisms and quantify material constitutive parameters. These parameters are used to calibrate material processes at the mesoscale using discrete dislocation dynamics (DD). Different grain boundary interactions with dislocations are analyzed using DD to predict grain-size dependent stress-strain behavior. These relationships are mapped into crystal plasticity (CP) parameters to develop a computationally efficient finite element-based DD/CP model for continuum-level simulations and complete the multiscale analysis by predicting the behavior of macroscopic physical specimens. The present analysis is focused on simulating the behavior of a graded microstructure in which grain sizes are on the order of nanometers in the exterior region and transition to larger, multi-micron size in the interior domain. This microstructural configuration has been shown to offer improved mechanical properties over homogeneous coarse-grained materials by increasing yield stress while maintaining ductility. Various mesoscopic polycrystal models of structurally-graded microstructures are generated, analyzed and used as a benchmark for comparison between multiscale DD/CP model and DD predictions. A final series of simulations utilize the DD/CP analysis method exclusively to study macroscopic models that cannot be analyzed by MD or DD methods alone due to the model size.

Saether, Erik↗