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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 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Partitioning of K, U, and Th between sulfide and silicate liquids - Implications for radioactive heating of planetary cores

Experimental partitioning studies are reported of K, U, and Th between silicate and FeFeS liquids designed to test the proposal that actinide partitioning into sulfide liquids is more important then K partitioning in the radioactive heating of planetary cores. For a basaltic liquid at 1450 C and 1.5 GPa, U partitioning into FeFeS liquids is five times greater than K partitioning. A typical value for the liquid partition coefficient for U from a granitic silicate liquid at one atmosphere at 1150 C and low fO2 is about 0.02; the coefficient for Th is similar. At low fO2 and higher temperature, experiments with basaltic liquids produce strong Ca and U partitioning into the sulfide liquid with U coefficient greater than one. The Th coefficient is less strongly affected.

Murrell, M. T.↗
NASA NTRS1986

The shapes of the circumstellar silicate features

Around oxygen-rich stars the spectra of most long-period variables (LPV) show an excess infrared emission which is attributed to circumstellar silicate dust grains. It is known that the spectral energy distribution of the 10 micron emissions shows variations from star to star. With the availability of many Infrared Astronomy Satellite (IRAS) Low Resolution Spectra (LRS) in the 8 to 22 micron region, the 10 micron feature can be studied to determine its uniformity (or lack thereof). The excess silicate emissions (10 micron emission), divided into three groups characterized by similar spectral shapes, are discussed.

Little-Marenin, I. R.↗
NASA NTRS1987

Reflectance spectra of mafic silicates and phyllosilicates from .6 to 4.6 microns

The results of spectral measurements for mafic silicates are given. The study provided valuable spectral reflectance information about mafic silicates and phyllosilicates in the 2.5 to 4.6 micron wavelength region. It was shown that the reflectance of these materials is strongly affected by the presence of H2O and OH. Therefore, the identification of these absorbing species is greatly enhanced.

Roush, Ted L.↗
NASA NTRS1986

Electrically conductive, black thermal control coatings for spacecraft application. I - Silicate matrix formulation

The formulation of the graphite silicate paints MH-11 and MH-11Z, which will serve as electrically conductive, heat-resistant thermal control coatings for the Galileo spacecraft's 400 Newton engine plume shield, 10 Newton thruster plume shields, and external shunt radiators, is described, and performance results for these paints are reported. The MH-11 is produced by combining a certain grade of graphite powder with a silicate base to produce a black, inorganic, electrically conductive, room temperature cure thermal control paint having high temperature capability. Zinc oxide is added to the MH-11 formulation to produce the blister resistant painta MH-11Z. The mechanical, chemical, thermal, optical, and radiation characteristics of the coatings are reported. The formulation, mixing, application, and surface preparation of the substrates are described, and a method of determining the electrical resistance of the coatings is demonstrated.

Bauer, J. L.↗
NASA NTRS1987

The silicate component of Martian dust

Absorption features in telescopic reflectance spectra of Mars obtained during 1978, and absorption features in Mariner 7 and Mariner 9 spectra, are analyzed which are assigned to structural hydroxyl within a strongly hydrogen-bonded acidic material suggested to be a silicate. Deduced fundamentals indicate that the Si-O(b)-Si bond angle may be approximately linear, and that the estimated Si-O(b) bridge length may be about 1.61 A. Viking compositional data indicate that Mg(2+), rather than Ca(2+) may be the dominant cation in the Martian material. The results suggest that the silicate component of the dust may be an incipient hydrolysis product of olivine-rich ultramafic or mafic material, involving a process that resulted in minimum loss of mobile cations and that maintained the high O/Si ratio of the starting material.

Huguenin, R. L.↗
NASA NTRS1989

Effects of silicate-graphite dust in H II regions

Interstellar dust that is present in H II regions will most likely absorb H-ionizing photons more efficiently than He-ionizing photons. Assuming a silicate-graphite dust composition, the effects of such selective absorption on the ionization structure of H, He, O, and N are investigated with emphasis on a blister geometry. Scattering by the dust grains is included in the generalized on-the-spot approximation. For a relatively soft radiation field of effective temperature about 35,000 K, the radiation hardening may cause the He(+) zone to be nearly coextensive with the H(+) zone, and the O(++) volume is strongly increased relative to the N(++) volume. The smaller grains (radii about 0.005 micron) contribute most to the selective absorption. We show that because of a cancellation of effects a simple analytical formula for the fraction of ionizing photons absorbed by the dust given by Petrosian, Silk, and Field is quite accurate in the case of silicate-graphite dust, even for blister regions.

Aannestad, Per A.↗
NASA NTRS1988

Interactions of silicate glasses with aqueous environments under conditions of prolonged contact and flow

This paper discusses mechanisms involving saturation and reactions that lead to the formation of altered phases in silicate glasses considered for use in geologic repositories for nuclear waste. It is shown that the rate of dissolution of silicate glasses exposed to a broad range of contact times, leachant compositions, and surface-to-volume ratios is strongly affected by the presence of reactive species such as Al, Mg, and Fe. The reactive materials may originate in the leachant or, under conditions of high surface-to-volume ratio, in the glass itself. The effects of glass composition on the course of the corrosion process can be viewed in terms of the formation of a surface layer on the leached glass; the type, composition, and structure of this layer control the dissolution behavior of the glass.

Barkatt, Aaron↗
NASA NTRS1989

Proton irradiation of SiH4-Fe(CO)5-H2O ices - Production of refractory silicates and implications for the solar nebula

The results of a number of experiments are reported in which SiH4-Fe(CO)5-H2O ice mixtures were irradiated by 1 MeV protons at 15 K and then warmed to room temperature in approximately 50 K steps. The properties of a sample were monitored by observing its infrared absorption spectrum as a function of its processing history. In each experiment in which ice samples containing SiH4 were irradiated, a refractory amorphous silicate residue remained after warming to room temperature. In addition to the usual broad silicate features at 10 and 20 microns, features due to water or hydroxyl at 3 and 6.2 microns, and a feature due to CO at 5 microns, a surprisingly stable infrared feature at 4.6 microns, attributed to the SiH stretching fundamental, was observed.

Nuth, J. A.↗
NASA NTRS1989

Processes affecting the composition and structure of silicate grains in cometary nuclei

Characteristics which might be observed in silicate grains from a cometary nucleus are examined on the basis of theoretical studies of the life cycle of refractory grains and laboratory experiments using simple analogs to the complex natural system. The life-cycle of a typical silicate grain is reviewed from condensation in a circumstellar outflow to incorporation into a cometary parent body. The possibility for grain metamorphism in the cometary environment is discussed and results are presented from experiments on the properties of interstellar grains. Also, consideration is given to the information about presolar refractory grains that might be gained from studying samples returned from a cometary nucleus.

Nuth, Joseph A., III↗
NASA NTRS1990

Signatures of aging silicate dust

A self-consistent explanation for the width, strength, and position of spectral features which may be diagnostic of the structure and composition of circumstellar and interstellar silicate grains is presented based on a long series of laboratory experiments using simple oxide analogs. The ratio of the integrated absorption strength of the grains near 10 mu to that near 20 mu is shown to decrease monotonically with increased processing. This ratio may be diagnostic of the relative age of the silicate. The oxidation state of iron in the grains is directly related to both the near-IR grain opacity and enhanced absorption in the UV. The oxidation state of both iron and silicon is directly related to the rate of nucleation, growth, and processing in a circumstellar outflow, as well as to the processing history of the grain after it reaches the interstellar medium. Other diagnostic indicators of the average silicon oxidation state, degree of structural polymerization, and dispersion in the average degree of structural polymerization are discussed.

Nuth, Joseph A., III↗
NASA NTRS1990

Metal silicate mixtures - Spectral properties and applications to asteroid taxonomy

The reflectance spectra of combinations of olivine, orthopyroxene, and iron meteorite metal are experimentally studied, and the obtained variations in spectral properties are used to constrain the physical and chemical properties of the assemblages. The presence of metal most noticeably affects band area ratios, peak-to-peak and peak-to-minimum reflectance ratios, and band widths. Band width and band areas are useful for determining metal abundance in olivine and metal and orthopyroxene and metal assemblages, respectively. Mafic silicate grain size variations are best determined using band depth criteria. Band centers are most useful for determining mafic silicate composition. An application of these parameters to the S-class asteroid Flora is presented.

Cloutis, Edward A.↗
NASA NTRS1989

Type of silicate feature in oxygen rich stellar envelopes

In many astronomical studies, the emissivity function derived from the Trapezium emission feature has been used. A statistical study of a large sample of objects is described that deals with the applicability of this function (hereafter TR). For comparison, another emissivity function derived for lunar silicate (LS) sample 14321 was used which has a maximum at 10.2 microns instead of at 9.7 microns as for TR. The IRAS low resolution spectra sources classified as 7n was used which has a silicate absorption feature without any atomic line emission. Most of these sources are likely to be oxygen rich stellar envelopes or hotspots in molecular clouds. The central source as assumed to emit a Plankian spectrum characterized by a temperature (t) and an absorbing envelope with an emissivity dependence of type TR or LS. Values of t and tau, the absorption optical depth were obtained by minimizing chi(exp 2) between the observed and fitted spectra in the 7 to 13 micron range. Values of reduced chi(exp 2) were obtained by taking the noise listed in the LRS Catalog as standard deviation. The tau values obtained correspond to 9.7 microns for TR and 10.2 for LS.

Iyengar, K. V. K.↗
NASA NTRS1991

IRAS low-resolution spectral observations of the 10 and 18 micron silicate emission feature

The IRAS LRS Atlas contains 8-23 micron spectra of 1816 stars with silicate emission features. In order to study the shapes of the silicate dust features, the 117 stars whose emission features appear to be optically thin and which have the best signal-to-noise ratio at the longest wavelengths were analyzed. Simple spherical dust shell models were calculated in both the optically thin and the slightly optical thick approximations. From the comparison of the predicted spectra of the dust and stellar continuum, the emissivity function Kappa(lambda) was derived. In the different stars, the shape of the 10-micron feature is either narrow or broad and it is peaked either at about 9.7 microns or at 10 microns. Either particle size effects (particles above 0.75 microns in radius) or optical depth effects (central optical depth about 1) could broaden the 10-micron feature. Chemical composition differences no doubt are also important, particularly as regards the position of the peak of the 10-micron feature. The stars with the peak at 10 microns are more closely confined to the Galactic plane than the stars with the peak at 9.7 microns. The shape of the 18-micron feature is essentially the same in all stars, and can be used to extend the interstellar extinction curve past 13 to 22 microns.

Simpson, Janet P.↗
NASA NTRS1991

Degradation of Fe-Mg silicates in hot CO2 atmospheres: Applications to Venus

Experiments demonstrated that oxidation of ferromagnesian silicates and magnetite occurs when these minerals are heated at 800 C in 1 atmosphere of CO2, under which conditions hematite is thermodynamically stable. The 30 ppm oxygen impurity in CO2 presumably facilitates the oxidation of some of the ferrous iron initially present in the crystal structures of the minerals. Mossbauer spectral measurements reveal, however, that only CO2 degraded olivine and pigeonite is hematite formed as a magnetically ordered phase at ambient temperatures. In orthopyroxene, some of the ferric iron produced by oxidation is present as nanophase hematite which, because it remains superparamagnetic until 4.2 K, must exist as particles less than or equal to 4 nm in diameter. In the calcic pyroxenes much of the oxidized ferrous iron may still remain as structural Fe3(+) in the host silicates. Some ferric iron may also be present as unit cell sized Fe2O3 inclusions in the pyroxenes, or be segregated along cleavage planes, or be coating mineral grains. In these states of aggregation, the Fe2O3 is unidentifiable by x ray diffraction and in low temperature Mossbauer spectra. Applications of this research to the surface of Venus are discussed.

Straub, Darcy W.↗
NASA NTRS1992

Estimation of high temperature metal-silicate partition coefficients

It has been known for some time that abundances of siderophile elements in the upper mantle of the Earth are far in excess of those expected from equilibrium between metal and silicate at low pressures and temperatures. Murthy (1991) has re-examined this excess of siderophile element problem by estimating liquid metal/liquid silicate partition coefficients reduces from their measured values at a lower temperature, implying that siderophile elements become much less siderophilic at high temperatures. Murthy then draws the important conclusion that metal/silicate equilibrium at high temperatures can account for the abundances of siderophile elements in the Earth's mantle. Of course, his conclusion is critically dependent on the small values of the partition coefficients he calculates. Because the numerical values of most experimentally-determined partition coefficients increase with increasing temperature at both constant oxygen fugacity and at constant redox buffer, we think it is important to try an alternative extrapolation for comparison. We have computed high temperature metal/silicate partition coefficients under a different set of assumptions and show that such long temperature extrapolations yield values which are critically dependent upon the presumed chemical behavior of the siderophile elements in the system.

Jones, John H.↗
NASA NTRS1992

A carbonate-silicate aqueous geochemical cycle model for Mars

A model for the carbonate-silicate geochemical cycle of an early, wet Mars is under development. The results of this study will be used to constrain models of the geochemical history of Mars and the likely mineralogy of its present surface. Although Mars today is a cold, dry planet, it may once have been much warmer and wetter. Values of total outgassed CO2 from several to about 10 bars are consistent with present knowledge (Pollack et al. 1987), and this amount of CO2 implies an amount of water outgassed at least equal to an equivalent depth of 500-1000 meters (Carr 1986). Pollack et al. (1987), in addition, estimate that a thick CO2 atmosphere may have existed for an extended period of time, perhaps as long as a billion years. The greenhouse effect of such an atmosphere would permit the presence of liquid water on the surface, most likely in the form of a shallow sea in the lowest regions of the planet, such as the northern plains (Schaefer 1990). The treatment of geochemical cycles as complex kinetic chemical reactions has been undertaken for terrestrial systems in recent years with much success (Lasaga 1980, 1981; Berner et al. 1983; Lasaga et al. 1985). Although the Martian system is vastly less well understood, and hence less well-constrained, it is also a much simpler system, due to the lack of biogenic reactions that make the terrestrial system so complex. It should be possible, therefore, to use the same techniques to model the Martian system as have been used for terrestrial systems, and to produce useful results. A diagram of the carbonate-silicate cycle for Mars (simplified from the terrestrial system) is given.

Schaefer, M. W.↗
NASA NTRS1991

Reinterpretation of reduction potential measurements done by linear sweep voltammetry in silicate melts

The equilibrium concentrations of Ni between silicate melt and Pt were determined experimentally as a function of oxygen fugacity. The results demonstrate that metallic species derived in linear sweep voltammetry experiments in silicate melts are diffusing into Pt electrodes and not into the melt, as was concluded by previoius studies. This requires reinterpretation of previous linear sweep voltammetry results and recalculation and correction of reported reduction potentials. This paper reports these corrected reduction potentials. Also reported are the activity coefficients for Ni in synthetic basalt and diopsidic melts and for Co in diopsidic melt.

Colson, R. O.↗
NASA NTRS1992

Silicate dust in a Vega-excess system

The 10-micron spectrum of the K5V star SAO 179815 (= HD 98800) is presented, and conclusively demonstrates the presence of small silicate dust grains around this star. The 9.7-micron silicate dust feature is unusually broad and shallow in this system. This, together with the slow fall-off of flux at longer wavelengths, constrains the size and density distributions of dust grains in models of the disk. It is found that there must be a significant population of small grains, as well as a population of large grains in order to explain all the observed properties of the disk.

Skinner, C. J.↗