Search NASA⌕ Search

SEARCH · Search NASA

Results for “Grain size”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

A new spectrum of Triton near the time of the Voyager encounter

A 5200-10,000 A spectrum of Triton that was telescopically obtained during the summer of 1989, just before the Voyager II encounter with the Neptune system, exhibits a measurable 8900 A CH4 ice absorption band. A combination of these data with those of Voyager indicates that the absorption is caused solely by Triton surface CH4 ice. A Hapke-type model for the Triton spectrum (1) sets a 20-micron lower limit on the CH4 ice's mean grain size (although it is suspected that actual grain size is closer to 100 microns), and (2) indicates that CH4 ice is widely distributed on the southern-hemisphere surface of Triton.

Grundy, William M.↗
NASA NTRS2013

Oxygen Permeability and Grain-Boundary Diffusion Applied to Alumina Scales

High-temperature oxygen permeability measurements had determined grain-boundary diffusivities (deltaD(sub gb)) in bulk polycrystalline alumina (Wada, Matsudaira, and Kitaoka). They predict that oxygen deltaD(sub gb,O) varies with oxygen pressure as P(O2)(sup -1/6) at low pressure whereas aluminum deltaD(sub gb),Al varies with P(O2)(sup +3/16) at high pressure. These relations were used to evaluate alumina scale growth in terms of diffusivity and grain size. A modified Wagner treatment for dominant inward oxygen growth produces the concise solution: П(sub i) = k(sub p,i)×G(sub i) = 12 deltaD(sub gb,O,int), where П(sub i) is a constant and k(sub p,i) and G(sub i) refer to instantaneous values of the scale parabolic growth constant and grain size, respectively. A commercial FeCrAl(Zr) alloy was oxidized at 1100 to 1400 degC to determine k(sub p,i), interfacial grain size, П, and thus deltaD(sub gb,O,int). The deltaD(sub gb,O,int) values predicted from oxidation at (375 kJ/mole) were about 20 times less than those obtained above (at 298 kJ/mole), but closer than extrapolations from high-temperature bulk measurements. The experimental oxidation results agree with similar FeCrAl(X) studies, especially where both k(sub p,i) and G(sub i) were characterized. This complete approach accounts for temperature-sensitive oxidation effects of grain enlargement, equilibrium interface pressure variation, and grain-boundary diffusivity.

Smialek, James L.↗
NASA NTRS1978

Carbon-14 in lunar soil and in meteorites

C-14 was measured in grain-size fractions of lunar soil 10084 and in samples of the Bruderheim chondrite and of several meteorites recently found in Antarctica (Allan Hills no. 5, 6, and 8). Temperature-release patterns were investigated. It was found that C-14 is released at temperatures below melting from small soil grains (less than 74 microns), but not from meteorites or from large soil grains. Below-melting C-14 contents increase with decreasing grain size in a manner similar to solar-wind-implanted rare-gas isotope contents (Eberhardt et al., 1970), whereas the C-14 released above melting temperatures is independent of grain size, suggesting that below-melting C-14 is solar-wind-implanted and above-melting C-14 is the result of cosmic ray spallations. The activity of C-14 in lunar samples is half that measured in the Bruderheim meteorite, which fell on May 4, 1970. No C-14 activity was observed in the Allan Hills chondrites; the C-14 limits suggest that these meteorites fell more than 25,000 years ago.

Fireman, E. L.↗
NASA NTRS1984

Grainsize evolution and differential comminution in an experimental regolith

The comminution of planetary surfaces by exposure to continuous meteorite bombardment was simulated by impacting the same fragmental gabbro target 200 times. The role of comminution and in situ gardening of planetary regoliths was addressed. Mean grain size continuously decreased with increasing shot number. Initially it decreased linearly with accumulated energy, but at some stage comminution efficiency started to decrease gradually. Point counting techniques, aided by the electron microprobe for mineral identification, were performed on a number of comminution products. Bulk chemical analyses of specific grain size fractions were also carried out. The finest sizes ( 10 microns) display generally the strongest enrichment/depletion factors. Similar, if not exactly identical, trends are reported from lunar soils. It is, therefore, not necessarily correct to explain the chemical characteristics of various grain sizes via different admixtures of materials from distant source terrains. Differential comminution of local source rocks may be the dominating factor.

Horz, F.↗
NASA NTRS1981

Spectral albedos of midlatitude snowpacks

Spectral albedos of impure-nonhomogeneous snowpacks, typical of midlatitudes, from 400 to 2200 nm were modeled through a numerical solution of the radiative transfer equation in the two-stream approximation. Discrete depth-dependent values of density, grain size and impurity concentration were used to characterize the snowpacks. The model is for diffuse incident radiation, and the numerical method is based on doubling and invariant imbedding. The effect of soot impurities on snowpack albedos is illustrated when a snowpack is several centimeters deep and soot reduces the albedos at visible wavelengths, however, when a snowpack is only a few centimeters deep, soot may increase the albedos at visible wavelengths. By adjusting soot content and snow grain size, good quantitative agreement with some observations at the Cascade Mountains (Washington) and at Point Barrow (Alaska) are obtained; however, the model grain sizes are found to be fifty to four hundred percent larger than the measured values. For satellite snowcover observations, a model for effective albedo of partially snow-covered areas was developed and compared with some NOAA-2 observations of the southeastern United States.

Choudhury, B.↗
NASA NTRS1991

The absorption coefficient of the liquid N2 2.15-micron band and application to Triton

The present measurements of the temperature dependence exhibited by the liquid N2 2.15-micron 2-0 collision-induced band's absorption coefficient and integrated absorption show the latter to be smaller than that of the N2 gas, and to decrease with decreasing temperature. Extrapolating this behavior to Triton's nominal surface temperature yields a new estimate of the N2-ice grain size on the Triton south polar cap; a mean N2 grain size of 0.7-3.0 cm is consistent with grain growth rate calculation results.

Grundy, William M.↗
NASA NTRS1995

Thermal emission from particulate surfaces: A comparison of scattering models with measured spectra

Emissivity spectra of particulate mineral samples are highly dependent on particle size when that size is comparable to the wavelength of light emitted (5-50 micrometers for the midinfrared). Proper geologic interpretation of data from planetary infrared spectrometers will require that these particle size effects be well understood. To address this issue, samples of quartz powders were produced with narrow, well-characterized particle size distributions. Mean particle diameters in these samples ranged from 15 to 227 micrometers. Emission spectra of these powders allow the first detailed comparison of the complex spectral variations with particle size observed in laboratory data with the predictions of radiative transfer models. Four such models are considered here. Hapke's relectance theory (converted to emissivity via Kirchoff's law) is the first model tested. Hapke's more recently published emission theory is also employed. The third model, the 'Mie/Conel' model, uses Mie single scattering with a two-stream approximation for multiple scattering. This model, like the first, is a converted reflec- tance model. Mie scattering assumes particles are both spherical and well separated, which is not true for the quartz powders, but includes diffraction effects. The fourth model uses the Mie solution for single scattering by spheres and inputs those results into the multiple scattering formalism of Hapke's emission theory. The results of the four models are considered in relation to the values of the optical constants n and k. We have grouped these as class 1 (k large), class 2 (k moderate, n is approximately 2), class 3 (k small, n is approximately 2), and class 4 (k small, n is approximately 1). In general, the Mie/Hapke hybrid model does best at predicting variations with grain size. In particular, it predicts changes of the correct pattern, although incorrect magnitude, for class 1 bands, where large increases in emissivity with decreasing grain size are observed. This model also does an excellent job on moderate (class 2) and very weak and intraband (class 3) regions, and correctly predicts the emission maximum and its invariance with grain size near the Christiansen frequency (class 4). The Mie/Hapke hybrid model also has the fewest free parameters of the four models examined, while maintaining the most physical treatment of the radiative transfer. The Mie/Conel model performs as well as the Mie/Hapke hybrid model in strong bands (class 1) but does not accurately model the behavior of moderate (class 2) and very weak (class 3) bands.

Moersch, J. E.↗
NASA NTRS2024

A Physics-Based Antarctic Melt Detection Technique: Combining Advanced Microwave Scanning Radiometer 2, Radiative-Transfer Modeling, and Firn Modeling

Surface melt on ice shelves has been linked to hydrofracture and subsequent ice shelf breakup. Since the 1990s, scientists have been using microwave radiometers to detect melt on ice shelves and ice sheets by applying various statistical thresholding techniques to identify significant increases in brightness temperature that are associated with melt. In this study, instead of using a fixed threshold, we force the Snow Radiative Transfer model (SMRT) with outputs from the Community Firn Model (CFM) to create a dynamic, physics-based threshold for melt. In the process, we also combine our method with statistical thresholding techniques and produce microwave grain size information in the process. We run this “Hybrid Method” across the Larsen C ice shelf as well as thirteen sites on the Antarctic Ice Sheet. Melt and non-melt days from the Hybrid Method and three statistical thresholding techniques match with surface energy balance within 94±1%; the effect of melt on the passive microwave is mostly binary and thus largely detectable by statistical thresholding techniques as well as physics-based techniques. Rather than always replacing statistical thresholding techniques with the Hybrid Method, we recommend using the Hybrid Method in studies where melt volume or grain size is of interest. In this study, we show that the Hybrid Method can be used to (a) model dry snow brightness temperatures of Antarctic snow and (b) derive a measure of grain size; therefore, it is an important step forwards towards using firn and radiative transfer modeling to quantify melt rather than to simply detect melt days.

Marissa E Dattler↗
NASA NTRS1989

Estimation of properties of alpine snow from Landsat Thematic Mapper

On the basis of a simulation of a range of snow types and various atmospheric profiles in a variety of possible illumination conditions, it has proven possible to develop typical spectral signatures above the atmosphere over mountainous terrain. Landsat TM data of the southern Sierra Nevada are presently analyzed to distinguish several classes of snow from other surface covers. It is found that snow can be reliably mapped at all sun angles encountered in the midlatitudes, and that large surface grain sizes can be distinguished from areas where the grain size is finer at the snow surface.

Dozier, Jeff↗
NASA NTRS1993

Remote sensing studies of immature soils on the Moon (Reiner-gamma formation)

On the base of laboratory results and telescopic data it is shown that the spectropolarization ratio P(sub max(sup B))/P(sub max(sup R)) for blue and red spectral regions is a remote sensing parameter of lunar soil maturity. It correlates with value of maturity index derived from morphological or ferromagnetic methods of exposure age determination. This parameter is equal to 0.315 for Reiner-gamma formation. So Reiner-gamma area is covered by immature soil. An extensive spectral mapping of the Reiner-gamma formation with high spatial resolution (0.2 km/pixel) was produced. This result was obtained at the 2-meter aperture telescope of Pic-du-Midi (France). The data sets consist in repeated runs comprising 10 selected narrow-band images (from 0.4 to 1.05 micron). The analysis of these data suggests that such a type of immature material includes not more than 28% of agglutinattes. We find the model grain size of fine fraction to be 40 micrometers grain size, of more immature soil 400-500 micrometers, and of the formation soil 120-150 micrometers. The exposure age of the Reiner-gamma immature soil is equal about 10 x 10(exp 6) years.

Shevchenko, V. V.↗
NASA NTRS1980

Long-time creep behavior of the niobium alloy C-103

The creep behavior of C-103 was studied as a function of stress, temperature, and grain size for test times to 19000 hr. Over the temperature range 827 to 1204 C and the stress range 6.89 to 138 MPa, only tertiary (accelerating) creep was observed. The creep strain epsilon can be related to time t by an exponential relation epsilon = epsilon(0) + K e raised to power (st) - 1), where epsilon (0) is initial creep strain, K is the tertiary creep strain parameter, and s is the tertiary creep rate parameter. The observed stress exponent 2.87 is similar to the three power law generally observed for secondary (linear) creep of Class I solid solutions. The apparent activation energy 374 kj/g mol is close to that observed for self diffusion of pure niobium. The initial tertiary creep rate was slightly faster for fine grained than for coarse-grained material. The strain parameter K can be expressed as a combination of power functions of stress and grain size and an exponential function of temperature. Strain time curves generated by using calculated values for K and s showed reasonable agreement with observed curves to strains of at least 4 percent. The time to 1 percent strain was related to stress, temperature, and grain size in a similar manner as the initial tertiary creep rate.

Titran, R. H.↗
NASA NTRS1996

The Spectral Energy Distribution of HH 100 IRS

Recent progress in the modeling of the radiative transfer in star forming regions has lead to improved dusty envelope models. Such models can now explain in great detail the observed infrared spectrum. The success of such models suggests that input parameters correspond to the true physical situation of the environment of the young stellar object. However, so far only minor attention has been given to models which include the spectroscopic signature of ice bands. Such models are applied to the Herbig-Haro energy source HH100 IRS. Calculations have been performed to interpret the spectral energy distribution as a function of dust parameters such as the grain size, the ice volume fraction, and the 'fluffiness' of the particles. The infrared spectrum together with the strength of the water ice band of HH 100 IRS is successfully reproduced if an upper limit of the grain size below 1 micron is used. Comet-like grains, with sizes above 1 micron, result in a poor fit of the observations.

Siebenmorgen, Ralf↗
NASA NTRS2000

Percussive Force Magnitude in Permafrost

An in-depth look at percussive drilling shows that the transmission efficiency is very important; however, data for percussive drilling in hard rock or permafrost is rarely available or the existing data are very old. Transmission efficiency can be used as a measurement of the transmission of the energy in the piston to the drill steel or bit and from the bit to the rock. Having a plane and centralized impact of the piston on the drill steel can optimize the transmission efficiency from the piston to the drill steel. A transmission efficiency of near 100% between piston and drill steel is possible. The transmission efficiency between bit and rock is dependent upon the interaction within the entire system. The main factors influencing this transmission efficiency are the contact area between cutting structure and surrounding rock (energy loss due to friction heat), damping characteristics of the surrounding rock (energy dampening), and cuttings transport. Some of these parameters are not controllable. To solve the existing void regarding available drilling data, an experiment for gathering energy data in permafrost for percussive drilling was designed. Fifteen artificial permafrost samples were prepared. The samples differed in the grain size distribution to observe a possible influence of the grain size distribution on the drilling performance. The samples were then manually penetrated (with a sledge-hammer) with two different spikes.

Eustes, A. W., III↗
NASA NTRS1999

An X-Ray Study of Grain Evolution behind Non-Radiative Shock

We model the dynamics, the destruction by sputtering, and the infrared (IR) emission from collisionally heated dust grains in fast (>/= 400 km s(exp -1)) astrophysical shocks in order to develop IR diagnostics for the destruction of grains in these environments. The calculations take into account the feedback from sputtering and IR emission on the gas-phase abundances, the cooling, and the ionization and thermal structure of the shock. Sputtering changes the initial grain size distribution, creating a deficiency of small (radius <50 A) grains compared to their preshock abundances. The altered grain size distribution depends on shock velocity and the density of the interstellar medium. Dust particles with sizes below approximately equal 300 A are stochastically heated, undergo temperature fluctuations, and radiate an excess of near-infrared emission (i </= 40 (micron)meter) over that expected for grains in thermal equilibrium. This near-infrared excess is a measure of the abundance of small grains and therefore a powerful diagnostic for the amount of destruction the grains were subjected to in the shock. We present here IR spectra from collisionally heated dust for a variety of shocks, and depict the changes in the spectra as a function of postshock column density. Our studies compliment those of Vancura et at. that examined the effects of the release of the sputtered refractory elements on the ultraviolet and X-ray emission. Multiwavelength observations at X-ray, Ultraviolet (UV), and IR wavelengths are therefore essential in piecing together a comprehensive picture of the physics of grain destruction in fast astrophysical shocks.

Vancura, Olaf↗
NASA NTRS2005

Space Weathering of Intermediate-Size Soil Grains in Immature Apollo 17 Soil 71061

Understanding space weathering, which is caused by micrometeorite impacts, implantation of solar wind gases, radiation damage, chemical effects from solar particles and cosmic rays, interactions with the lunar atmosphere, and sputter erosion and deposition, continues to be a primary objective of lunar sample research. Electron beam studies of space weathering have focused on space weathering effects on individual glasses and minerals from the finest size fractions of lunar soils [1] and patinas on lunar rocks [2]. We are beginning a new study of space weathering of intermediate-size individual mineral grains from lunar soils. For this initial work, we chose an immature soil (see below) in order to maximize the probability that some individual grains are relatively unweathered. The likelihood of identifying a range of relatively unweathered grains in a mature soil is low, and we plan to study grains ranging from pristine to highly weathered in order to determine the progression of space weathering. Future studies will include grains from mature soils. We are currently in the process of documenting splash glass, glass pancakes, craters, and accretionary particles (glass and mineral grains) on plagioclase from our chosen soil using high-resolution field emission scanning electron microscopy (FESEM). These studies are being done concurrently with our studies of patinas on larger lunar rocks [e.g., 3]. One of our major goals is to correlate the evidence for space weathering observed in studies of the surfaces of samples with the evidence demonstrated at higher resolution (TEM) using cross-sections of samples. For example, TEM studies verified the existence of vapor deposits on soil grains [1]; we do not yet know if they can be readily distinguished by surfaces studies of samples. A wide range of textures of rims on soil grains is also clear in TEM [1]; might it be possible to correlate them with specific characteristics of weathering features seen in SEM?

Wentworth, S. J.↗
NASA NTRS1982

A search for diffuse band profile variations in the rho Ophiuchi cloud

High signal-to-noise profiles of the broad diffuse interstellar band at 4430 A were obtained on the 2.2-m telescope at the Mauna Kea Observatory, using the newly-developed pulse-counting multi-anode microchannel array detector system in an effort to determine whether the band profile varies with mean grain size as expected if the band is produced by absorbers embedded in grain lattices. The lack of profile variability over several lines of sight where independent evidence indicates that the mean grain size varies shows that lambda 4430 is probably not formed by the same grains that are responsible for interstellar extinction at visible wavelengths. The possibility that this band is created by a population of very small ( approximately 100 A) grains is still viable, as is the hypothesis that it has a molecular origin.

Snow, T. P.↗
NASA NTRS1984

Effects of Non-equilibrium Solidification on the Material Properties of Brick Silicon for Photovoltaics

Silicon ingot growth technologies like the Ubiquitous Crystallization Process (UCP) are solidified within a shaping crucible. The rate at which heat can be lost from this crucible minus the rate at which heat is input from an external source determines the rate at which crystallization will occur. Occasionally, when the process parameters for solidification are exceeded, the normally large multi-centimeter grain size material assocated with the UCP will break down into regions containing extremely small, millimeter or less, grain size material. Accompanying this breakdown in grain growth is the development of so called sinuous grain boundaries. The breakdown in grain growth which results in this type of small grain structure with sinuous boundaries is usually associated with the rapid crystallization that would accompany a system failure. This suggests that there are limits to the growth velocity that one can obtain and still expect to produce material that would possess good photovoltaic properties. It is the purpose to determine the causes behind the breakdown of this material and what parameters will determine the best rates of solidification.

Regnault, W. F.↗
NASA NTRS1971

Generation of long time creep data on refractory alloys at elevated temperatures

Ultrahigh vacuum creep tests were conducted on tantalum, tungsten, and molybdenum alloys to develop creep data and to evaluate the influence of liquid lithium exposure on the creep behavior of the tantalum alloy T-111. Test conditions were generally selected to provide 1% creep in 1000 to 10,000 hours, with test temperatures ranging from 1600 to 2912 F (1144 to 1873 K). Tests on T-111 specimens exposed to vacuum and to liquid lithium for various times and temperatures showed large reductions in creep strength for exposures in the 1800 to 1900 F (1255 to 1310 K) range. Tests on the ASTAR 811c showed a significant influence of grain size on the creep strength of this material, with larger grain size specimens being significantly stronger above the equicohesive temperature of 2000 F (1366 K).

Sheffler, K. D.↗