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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1996

Arabidopsis thaliana sku mutant seedlings show exaggerated surface-dependent alteration in root growth vector

Roots of wild-type Arabidopsis thaliana seedlings in the Wassilewskija (WS) and Landsberg erecta (Ler) ecotypes often grow aslant on vertical agar surfaces. Slanted root growth always occurs to the right of the gravity vector when the root is viewed through the agar surface, and is not observed in the Columbia ecotype. Right-slanted root growth is surface-dependent and does not result directly from directional environmental stimuli or gradients in the plane of skewing. We have isolated two partially dominant mutations in WS (sku1 and sku2) that show an exaggerated right-slanting root-growth phenotype on agar surfaces. The right-slanting root-growth phenotype of wild-type and mutant roots is not the result of diagravitropism or of an alteration in root gravitropism. It is accompanied by a left-handed rotation of the root about its axis within the elongation zone, the rate of which positively correlates with the degree of right-slanted curvature. Our data suggest that the right-slanting root growth phenotype results from an endogenous structural asymmetry that expresses itself by a directional root-tip rotation.

NASA Program Space Biology↗
NASA NTRS2019

Modeling Spectral Degradation of MODIS and VIIRS Solar Diffusers

Solar diffusers (SDs) onboard the Moderate Resolution Imaging Spectroradiometer (MODIS) on Terra and Aqua spacecraft and the Visible Infrared Imaging Radiometer Suite (VIIRS) on SNPP and NOAA-20 spacecraft have been used as the primary onboard radiometric calibrator for reflective solar bands and their spectral reflectance is known to degrade on-orbit. All of the solar diffusers on these instrument show faster degradation of reflectance in the 0.4 to 0.6 µm channels than the longer wavelength channels. The spectral degradation of these SDs is due to the surface roughness growth under the exposure to solar UV and energetic particle radiation in space. In this paper, the origin of SD degradation is modeled by the physics-based Surface Roughness-induced Rayleigh Scattering (SRRS) model. The longterm growths of surface roughness of the SD on these instruments are characterized to show the consistency of SD roughness growth rates between Terra/Aqua MODIS and SNPP/NOAA-20 VIIRS. There was coincidental flattening or reverse in the growth trend of the surface roughness for the Terra/Aqua MODIS and SNPP VIIRS SDs, which can be due to the space weather variation. The applicability of the SRRS model is demonstrated with the long term spectral reflectance data from independent spaceborne SDs.

Calibration↗
NASA NTRS1996

Silicon Carbide Epitaxial Films Studied by Atomic Force Microscopy

Silicon carbide (SiC) holds great potential as an electronic material because of its wide band gap energy, high breakdown electric field, thermal stability, and resistance to radiation damage. Possible aerospace applications of high-temperature, high-power, or high-radiation SiC electronic devices include sensors, control electronics, and power electronics that can operate at temperatures up to 600 C and beyond. Commercially available SiC devices now include blue light-emitting diodes (LED's) and high-voltage diodes for operation up to 350 C, with other devices under development. At present, morphological defects in epitaxially grown SiC films limit their use in device applications. Research geared toward reducing the number of structural inhomogeneities can benefit from an understanding of the type and nature of problems that cause defects. The Atomic Force Microscope (AFM) has proven to be a useful tool in characterizing defects present on the surface of SiC epitaxial films. The in-house High-Temperature Integrated Electronics and Sensors (HTIES) Program at the NASA Lewis Research Center not only extended the dopant concentration range achievable in epitaxial SiC films, but it reduced the concentration of some types of defects. Advanced structural characterization using the AFM was warranted to identify the type and structure of the remaining film defects and morphological inhomogeneities. The AFM can give quantitative information on surface topography down to molecular scales. Acquired, in part, in support of the Advanced High Temperature Engine Materials Technology Program (HITEMP), the AFM had been used previously to detect partial fiber debonding in composite material cross sections. Atomic force microscopy examination of epitaxial SiC film surfaces revealed molecular-scale details of some unwanted surface features. Growth pits propagating from defects in the substrate, and hillocks due, presumably, to existing screw dislocations in the substrates, were imaged. Away from local defects, step bunching was observed to yield step heights of hundreds of angstroms, with possible implications for the uniformity of dopants incorporated in SiC devices during fabrication. The quantitative topographic data from the AFM allow the relevant defect information to be extracted, such as the size and distribution of step bunching and the Burgers vector of screw dislocations. These atomic force microscopy results have furthered the understanding of the dynamic epitaxial SiC growth process. A model describing the observed hillock step bunching has been proposed. This cooperation between researchers involved in crystal growth, electronic device fabrication, and surface structural characterization is likely to continue as atomic force microscopy is used to improve SiC films for high-temperature electronic devices for NASA's advanced turbine engines and space power devices, as well as for future applications in the automotive industry.

Source record↗
NASA NTRS1997

Protein Crystal Growth Apparatus for Microgravity

Apparatus for growing protein crystals under microgravity environment includes a plurality of protein growth assemblies stacked one above the other within a canister. Each of the protein growth assemblies includes a tray having a number of spaced apart growth chambers recessed below an upper surface. the growth chambers each having an upstanding pedestal and an annular reservoir about the pedestal for receiving a wick and precipitating agents. A well is recessed below the top of each pedestal to define a protein crystal growth receptacle. A flexible membrane is positioned on the upper surface of each tray and a sealing plate is positioned above each membrane, each sealing plate having a number of bumpers corresponding in number and alignment to the pedestals for forcing the membrane selectively against the upper end of the respective pedestal to seal the reservoir and the receptacle when the sealing plate is forced down.

Carter, Daniel C.↗
NASA NTRS1991

Growth morphologies of crystal surfaces

A previously proposed Monte Carlo model (Xiao et al., 1988, 1990) is extended to three dimensions, and reevaporation after accommodation and growth on dislocation-induced steps are included. It is found again that, for a given set of growth parameters, the critical size beyond which a crystal cannot retain its macroscopically faceted shape scales linearly with the mean free path in the vapor. However, the three-dimensional systems show increased shape stability as compared with the corresponding two-dimensional cases. The experimental observation that the critical size of a growing crystal depends on the prevailing growth mechanism is confirmed, and detailed insight into the processes leading to the loss of face and facet stability is obtained.

Xiao, Rong-Fu↗
NASA NTRS1999

Polarity Effects of Substrate Surface in Epitaxial ZnO Film Growth

Epitaxial ZnO films were grown on the two polar surfaces (0-face and Zn-face) of (0001) ZnO single crystal substrates using off-axis magnetron sputtering deposition. As a comparison, films are also deposited on the (000 I) Al203 substrates. It is found that the two polar surfaces have different photoluminescence (PL) spectrum, surface structure and morphology, which are strongly inference the epitaxial film growth. The morphology and structure of epitaxial films on the ZnO substrates are different from the film on the Al203 substrates. An interesting result shows that high temperature annealing of ZnO single crystals will improve the surface structure on the O-face surface rather than the opposite Surface. The measurements of PL, low-angle incident x-ray diffraction, and atomic force microscopy of ZnO films indicate that the O-terminated surface is better for ZnO epitaxial film growth using reactive sputtering deposition.

Zhu, Shen↗
NASA NTRS1985

Numerical Estimation of the Curvature of Biological Surfaces

Many biological systems may profitably be studied as surface phenomena. A model consisting of isotropic growth of a curved surface from a flat sheet is assumed. With such a model, the Gaussian curvature of the final surface determines whether growth rate of the surface is subharmonic or superharmonic. These properties correspond to notions of convexity and concavity, and thus to local excess growth and local deficiency of growth. In biological models where the major factors controlling surface growth are intrinsic to the surface, researchers thus gained from geometrical study information on the differential growth undergone by the surface. These ideas were applied to an analysis of the folding of the cerebral cortex, a geometrically rather complex surface growth. A numerical surface curvature technique based on an approximation to the Dupin indicatrix of the surface was developed. A metric for comparing curvature estimates is introduced, and considerable numerical testing indicated the reliability of this technique.

Todd, P. H.↗
NASA NTRS2000

Influence of Contact Angle, Growth Angle and Melt Surface Tension on Detached Solidification of InSb

We extended the previous analysis of detached solidification of InSb based on the moving meniscus model. We found that for steady detached solidification to occur in a sealed ampoule in zero gravity, it is necessary for the growth angle to exceed a critical value, the contact angle for the melt on the ampoule wall to exceed a critical value, and the melt-gas surface tension to be below a critical value. These critical values would depend on the material properties and the growth parameters. For the conditions examined here, the sum of the growth angle and the contact angle must exceed approximately 130, which is significantly less than required if both ends of the ampoule are open.

Wang, Yazhen↗
NASA NTRS1985

Monte Carlo study of reversible growth of clusters on a surface

The effect of binding energy on the growth of clusters on a two-dimensional square lattice was studied by a means of a Monte Carlo simulation. The results indicate that for binding energies higher than 0.5kT, clusters grow relative to the initial random distribution, while for binding energies lower than 0.5 kT, clusters decompose. It was also found that at low binding energies the equilibrium percentage of atoms that are bound in clusters is linearly dependent on the binding energy. The cluster density behaved in a similar manner. This similarity reflects the fact that at low binding energies the growth proceeds mainly by the formation of new clusters rather than by an increase in the size of existing ones.

Salik, J.↗
NASA NTRS1973

VLS /vapor-liquid-solid/ - Newly discovered growth mechanism on the lunar surface.

A probable vapor-liquid-solid (VLS) type of growth has been discovered for the first time in nature on the surface of lunar rock 15015. Scanning electron microprobe and energy dispersive x-ray data indicate that the growth occurs as metallic iron stalks from about 0.015 to 0.15 micrometer in diameter, with bulbous tips consisting of a mixture of iron and sulfur and measuring from about 0.03 to 0.2 micrometer in diameter. The stalk length is two to ten times the bulb diameter.

Carter, J. L.↗
NASA NTRS2009

Survey Analysis of Materials Processing Experiments Aboard STS-47: Spacelab J

This Technical Memorandum (TM) is a survey outline of materials processing experiments aboard Space Shuttle Mission STS-47: Spacelab J, a joint venture between NASA and the National Space Development Agency of Japan. The mission explored materials processing experiments including electronics and crystal growth materials, metals and alloys, glasses and ceramics, and fluids. Experiments covered include Growth of Silicone Spherical Crystals and Surface Oxidation, Growth Experiment of Narrow Band-Gap Semiconductor Lead-Tin-Tellurium Crystals in Space, Study on Solidification of Immiscible Alloys, Fabrication of Very-Low-Density, High-Stiffness Carbon Fiber/Aluminum Hybridized Composites, High Temperature Behavior of Glass, and Study of Bubble Behavior. The TM underscores the historical significance of these experiments in the context of materials processing in space.

Sharpe, R. J.↗
NASA NTRS2002

Surface Processes of Faceted Growth

Atomic force microscopy and high precision optical interferometry allow to analyze the processes that are in the core of our present understanding of faceted crystal growth. Some of these processes will be reviewed. Recent experiments suggest that the Gibbs-Thomson Law (GTL) may not be applicable to the weakly fluctuating strongly polygonized steps, with low kink density. Propagation rate of a straight short step segment changes with the segment length much steeper than predicted by GTL. If the step fluctuations are still well developed, the typical transition length may be determined just by the radius of rounded comers of the polygonized step. If fluctuations do not have enough time to develop, the kinetic effects may be essential. Indeed, 'communications' between comers of a short straight step segment may occur and, thus, the GTL may be implemented only via kink exchange. E.g., dissolutions of this short segment at macroscopic equilibrium occurs as follows: The kinks are split from one comer as a result of fluctuations, and annihilate with kinks of the opposite sign generated by another comer. Similarly, an exchange via chain of kinks on a rough step is the mechanism to implement the GT Law. If there is a supersaturation with respect to a large crystal, the kinks not only diffuse along the step, but move back the generating comers. This 'wind' prevents communication between the comers and places upper limit supersaturation only below which GTL is applicable. Steps replace kinks in the 3D case, though, of course, another dimensionality brings about another physics. Another specific problem to think about is if fluctuations are always fast enough to provide enough kinks for linear dependence of step rate on supersaturation. Increasing azimuthal polygonization of growth hillocks is a sign of insufficient fluctuation rate. Morphological stability of vicinal faces is usually considered on the basis of mutually parallel steps. Azimuthal anisotropy and interlacing of step bunches inducing the well known bunch splitting is another challenge. Among other challenging problems are: existence of surface diffusion on the crystal/so1ution interface, step interaction in solutions, achievement of step flow modes from liquids, impurity effects. New phenomena essential in biomacromolecular crystallization is another interesting area.

Chernov, A.↗
NASA NTRS2001

Homoepitaxial "Web Growth" of SiC to Terminate C-Axis Screw Dislocations and Enlarge Step-Free Surfaces

Homoepitaxial CVD growth of thin lateral cantilevers emanating from the edges of mesa patterns dry-etched into on-axis commercial 4H-SiC substrates prior to growth is reported. Cantilevers on the order of a micrometer thick extending tens of micrometers from the edge of a mesa have been grown. The termination of vertically propagating screw dislocations, including a micropipe, that are overgrown by the cantilevers has been demonstrated, in large part because the crystal structure of the cantilevers is established laterally from the mesa sidewalls. This technique could help reduce performance-degrading dislocations in SiC electrical devices.

Neudeck, Philip G.↗
NASA NTRS1992

Heat transfer and fluid flow in floating-zone crystal growth with a mostly covered melt surface

Computer simulation was conducted to study a modified floating-zone crystal growth process, in which the melt surface is mostly covered with a heated ring. The growth of 6-mm diameter single crystals of NaNO3 was considered, and the effects of the following parameters were studied: (1) temperature of the ring, (2) growth rate, (3) surface tension-temperature coefficient of the melt, (4) thermal expansion coefficient of the melt, and (5) gravity. It was demonstrated that thermocapillary convection in the melt zone is reduced significantly in this modified process. The model was checked against the measured lengths of the meniscus near the growth front and axial temperature distribution in a growing crystal, and the agreement was good.

Lan, C. W.↗
NASA NTRS2002

Enlargement of Step-Free SiC Surfaces by Homoepitaxial Web-Growth of Thin SiC Cantilevers

Lateral homoepitaxial growth of thin cantilevers emanating from mesa patterns that were reactive ion etched into on-axis commercial SiC substrates prior to growth is reported. The thin cantilevers form after pure stepflow growth removes almost all atomic steps from the top surface of a mesa, after which additional adatoms collected by the large step-free surface migrate to the mesa sidewall where they rapidly incorporate into the crystal near the top of the mesa sidewall. The lateral propagation of the step-free cantilevered surface is significantly affected by pregrowth mesa shape and orientation, with the highest lateral expansion rates observed at the inside concave comers of V-shaped pregrowth mesas with arms lengthwise oriented along the {1100} direction. Complete spanning of the interiors of V's and other mesa shapes with concave comers by webbed cantilevers was accomplished. Optical microscopy, synchrotron white beam x-ray topography and atomic force microscopy analysis of webbed regions formed over a micropipe and closed-core screw dislocations show that c-axis propagation of these defects is terminated by the webbing. Despite the nonoptimized process employed in this initial study, webbed surfaces as large as 1.4 x 10(exp -3) square centimeters, more than four times the pregrowth mesa area, were grown. However, the largest webbed surfaces were not completely free of bilayer steps, due to unintentional growth of 3C-SiC that occurred in the nonoptimized process. Further process optimization should enable larger step-free webs to be realized.

Neudeck, Philip G.↗
NASA NTRS2011

Frost Growth and Densification in Laminar Flow Over Flat Surfaces

One-dimensional frost growth and densification in laminar flow over flat surfaces has been theoretically investigated. Improved representations of frost density and effective thermal conductivity applicable to a wide range of frost circumstances have been incorporated. The validity of the proposed model considering heat and mass diffusion in the frost layer is tested by a comparison of the predictions with data from various investigators for frost parameters including frost thickness, frost surface temperature, frost density and heat flux. The test conditions cover a range of wall temperature, air humidity ratio, air velocity, and air temperature, and the effect of these variables on the frost parameters has been exemplified. Satisfactory agreement is achieved between the model predictions and the various test data considered. The prevailing uncertainties concerning the role air velocity and air temperature on frost development have been elucidated. It is concluded that that for flat surfaces increases in air velocity have no appreciable effect on frost thickness but contribute to significant frost densification, while increase in air temperatures results in a slight increase the frost thickness and appreciable frost densification.

Kandula, Max↗