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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 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Thermal history effects on electrical relaxation and conductivity for potassium silicate glass with low alkali concentrations

Electrical response measurements from 10 Hz to 100 kHz between 120 and 540 C were made on potassium-silicate glasses with alkali oxide contents of 2, 3, 5 and 10 mol percent. Low alkali content glasses were chosen in order to try to reduce the Coulombic interactions between alkali ions to the point that frozen structural effects from the glass could be observed. Conductivity and electrical relaxation responses for both annealed and quenched glasses of the same composition were compared. Lower DC conductivity (sigma(sub DC)) activation energies were measured for the quenched compared to the annealed glasses. The two glasses with the lowest alkali contents exhibited a non-Arrhenius concave up curvature in the log(sigma(sub DC)) against 1/T plots, which decreased upon quenching. A sharp decrease in sigma(sub DC) was observed for glasses containing K2O concentrations of 5 mol percent or less. The log modulus loss peak (M'') maximum frequency plots against 1/T all showed Arrhenius behavior for both annealed and quenched samples. The activation energies for these plots closely agreed with the sigma(sub DC) activation energies. A sharp increase in activation energy was observed for both series as the potassium oxide concentration decreased. Changes in the electrical response are attributed to structural effects due to different alkali concentrations. Differences between the annealed and quenched response are linked to a change in the distribution of activation energies (DAE).

Angel, Paul W.↗
NASA NTRS1993

Partitioning of Nb, Mo, Ba, Ce, Pb, Th and U between immiscible carbonate and silicate liquids: Evaluating the effects of P2O5,F, and carbonate composition

Previously we have reported carbonate liq./silicate liq. partition coefficients (D) for a standard suite of trace elements (Nb, Mo, Ba, Ce, Pb, Th, and U) and Ra and Pa as well. In brief, we have found that immiscible liquid partitioning is a strong function of temperature. As the critical temperature of the carbonate-silicate solvus is approached, all partition coefficients approach unity. Additionally, for the overwhelming majority of the partitioning elements, InD is a linear function of 'ionic field strength,' z/r, where z is the charge of the partitioned cation and r is its ionic radius.

Jones, J. H.↗
NASA NTRS1993

Experimental partitioning of Zr, Ti, and Nb between silicate liquid and a complex noble metal alloy and the partitioning of Ti between perovskite and platinum metal

El Goresy et al.'s observation of Nb, Zr, and Ta in refractory platinum metal nuggets (RPMN's) from Ca-Al-rich inclusions (CAI's) in the Allende meteorite led them to propose that these lithophile elements alloyed in the metallic state with noble metals in the early solar nebula. However, Grossman pointed out that the thermodynamic stability of Zr in the oxide phase is vastly greater than metallic Zr at estimated solar nebula conditions. Jones and Burnett suggested this discrepancy may be explained by the very non-ideal behavior of some lithophile transition elements in noble metal solutions and/or intermetallic compounds. Subsequently, Fegley and Kornacki used thermodynamic data taken from the literature to predict the stability of several of these intermetallic compounds at estimated solar nebula conditions. Palme and Schmitt and Treiman et al. conducted experiments to quantify the partitioning behavior of certain lithophile elements between silicate liquid and Pt-metal. Although their results were somewhat variable, they did suggest that Zr partition coefficients were too small to explain the observed 'percent' levels in some RPMN's. Palme and Schmitt also observed large partition coefficients for Nb and Ta. No intermetallic phases were identified. Following the work of Treiman et al., Jurewicz and Jones performed experiments to examine Zr, Nb, and Ti partitioning near solar nebula conditions. Their results showed that Zr, Nb, and Ti all have an affinity for the platinum metal, with Nb and Ti having a very strong preference for the metal. The intermetallic phases (Zr,Fe)Pt3, (Nb,Fe)Pt3, and (Ti,Fe)Pt3 were identified. Curiously, although both experiments and calculations indicate that Ti should partition strongly into Pt-metal (possibly as TiPt3), no Ti has ever been observed in any RPMN's. Fegley and Kornacki also noticed this discrepancy and hypothesized that the Ti was stabilized in perovskite which is a common phase in Allende CAI's.

Jurewicz, Stephen R.↗
NASA NTRS1993

Complex radiation-thermal history of Kaidun meteorite on data of track study of silicate minerals

The results of track study of approximately 80 individual silicate mineral crystals (ol, px, plag) picked out from Kaidun meteorite are presented. A wide range of observed rho(sub VH) value distributions indicate the complex irradiation history of Kaidun minerals. In one anortite crystal having two track groups with different parameters the pre-accretion irradiation traces were observed in all probability.

Kashkarov, L. L.↗
NASA NTRS1993

Chromite-rich mafic silicate chondrules in ordinary chondrites: Formation by impact melting

Chromium-rich chondrules constitute less than 0.1 percent of all ordinary chondrite (OC) chondrules and comprise three groups: chromian-spinel chondrules, chromian-spinel inclusions, and chromite-rich mafic silicate (CRMS) chondrules. Chromian-spinel chondrules (typically 100-300 microns in apparent diameter) exhibit granular, porphyritic and unusual textures and occur mainly in H chondrites. Their morphologies are distinct from the irregularly shaped chromian-spinel inclusions of similar mineralogy. Chromian-spinel chondrules and inclusions consist of grains of chromian-spinel embedded in plagioclase (Pl) or mesostasis of Pl composition. Many also contain accessory ilmenite (Ilm), high-Ca pyroxene (Px), merrillite (Mer), and rare olivine (Ol); some exhibit concentric mineral and chemical zoning. CRMS chondrules (300-1100 microns in apparent diameter) are generally larger than chromian-spinel chondrules and occur in all metamorphosed OC groups. Most CRMS chondrules are nearly spherical although a few are ellipsoidal with a/b aspect ratios ranging up to 1.7. Textures include cryptocrystalline, granular, radial, barred, and porphyritic varieties; some contain apparently relict grains. The chondrules consist of chromite (Chr), Ol and Pl, along with accessory Mer, troilite (Tr), metallic Fe-Ni (Met), Px and Ilm. The mesostasis in CRMS chondrules is nearly opaque in transmitted light; thus, they can be easily recognized in the optical microscope. Based on the similarity of mineralogy and chemistry between CRMS chondrules of different textures (opaque chromite-rich mesostasis, skeletal morphology of Ol grains, similar bulk compositions) we suggest that these chondrules form a genetically related population.

Krot, Alexander N.↗
NASA NTRS1993

Mapping the distribution of vesicular textures on silicic lavas using the Thermal Infrared Multispectral Scanner

To investigate the effect of vesicularity on TIMS (Thermal Infrared Multispectral Scanner) imagery independent of chemical variations, we studied a large rhyolitic flow of uniform composition but textural heterogeneity. The imagery was recalibrated so that the digital number values for a lake in the scene matched a calculated ideal spectrum for water. TIMS spectra for the lava show useful differences in coarsely and finely vesicular pumice data, particularly in TIMS bands 3 and 4. Images generated by ratioing these bands accurately map out those areas known from field studies to be coarsely vesicular pumice. These texture-related emissivity variations are probably due to the larger vesicles being relatively deeper and separated by smaller septa leaving less smooth glass available to give the characteristic emission of the lava. In studies of inaccessible lava flows (as on Mars) areas of coarsely vesicular pumice must be identified and avoided before chemical variations can be interpreted. Remotely determined distributions of vesicular and glassy textures can also be related to the volatile contents and potential hazards associated with the emplacement of silicic lava flows on Earth.

Ondrusek, Jaime↗
NASA NTRS1994

Silicate interactions with ammonia-water fluids on early Titan

Plausible models of the early history of Titan suggest that ammonia and water were present in liquid form at the surface. We show here by thermodynamic modeling that such an ocean could have reacted with silicates to put substantial quantities of sodium and potassium into solution. Following the formation of an ice crust by cooling, mantle ammonia-water fluids enriched in potassium would have been brought to the surface through the cryogenic equivalent of volcanism. Later impacts would have released the Ar-40 produced by decay of the K-40 into the atmosphere. The abundance of atmospheric Ar-40, measurable by the Huygens probe gas chromatograph mass spectrometer, may be dominated by this source and hence gives a proxy indication of the volume of ammonia-water resurfacing on Titan over geologic time.

Engel, Steffi↗
NASA NTRS1994

Silicates and aromatic hydrocarbons in the 10 micron spectrum of the Taurus dark cloud source Elias 1

We have obtained 10.4-12 micrometer spectra at spectral resolution R approximately equal to 190 of the Herbig Ae star Elias 1 in the Taurus dark cloud with the United Kingdom Infrared Telescope (UKIRT) CGS3 spectrometer, along with an 8-13 micrometer spectrum at R approximately equal to 55. The 11.2 micrometer emission feature matches the wavelength, shape, and full width at half maximum (FWHM) of the 11.22 micrometer aromatic hydrocarbon band, consistent with the presence of other aromatic features. A strong 11.06 micrometer feature is present, and there are two possible new emission features at 11.6 and 11.76 micrometer. The strong silicate emission feature is broader and peaks at a longer wavelength than can be reproduced with the Trapezium emissivity. Larger grains, with mean radius a approximately 1.5 micrometers appear to be required, a possible indication of grain growth around a young star.

Hanner, Martha S.↗
NASA NTRS1994

Aubrite basalt vitrophyres: High sulfur silicate melts and a snapshot of aubrite formation

Two aubrite basalt vitrophyre clasts have been found within AMNH thin sections from the Parsa EH3 chondrite and the Khor Temiki aubrite. Polished sections of the Parsa Aubrite Inclusion (PAI) and the Khor Temiki Inclusion (KTI) were studied by optical, electron probe microanalysis (EPMA), and scanning electron microscopy (SEM) techniques with broad-beam and low absorbed EPMA currents used to minimize glass volatile loss. Some data have previously been reported for PAI and KTI may possibly correlate to a previously reported inclusion in Khor Tiimiki. In polished sections, PAI and KTI are approximately equal 4 mm in diameter and contain a large volume of glass. The clasts have similar textural characteristics and are akin to lunar vitrophyre textures. The glasses have high alkali rhyodacitic compositions Al-though PAI is peraluminous, KTI is significantly peralkaline. Additionally, the glasses have elevated sulfur concentrations that are extremely high by geochemical standards. SEM examination for beam overlap of microscopic CaS, FeS, and (Mg, Mn, Fe) S inclusions showed no such contamination. Furthermore, homogeneity of glass S content and low FeO contents help rule out contamination. Materials research data show that under reducing conditions alumino-silicate melts can dissolve up to several weight percent sulfur in the absence of Fe. The high S and alkali contents, the lack of associated high shock features, and the rationalized phase equilibria suggest that PAI and KTI are igneous melting products of an E-chondrite-like source material. Although large-scale impact melting cannot totally be ruled out, the above observations eliminate the possibility of in-situ shock melting.

Fogel, R. A.↗
NASA NTRS1994

The stability of hibonite, melilite and other aluminous phases in silicate melts: Implications for the origin of hibonite-bearing inclusions from carbonaceous chondrites

Phase fields in which hibonite and silicate melt coexist with spinel CaAl4O7, gehlenitic melilite, anorthite or corundum at 1 bar in the system CaO-MgO-Al2O3-SiO2-TiO2 were determined. The hibonites contain up to 1.7 wt% SiO2. For TiO2, the experimentally determined partition coefficients between hibonite and coexisting melt D(sub i)(sup Hib/L), vary from 0.8 to 2.1 and generally decrease with increasing TiO2 in the liquid. Based on Ti partitioning between hibonite and melt, bulk inclusion compositions and hibonite-saturated liquidus phase diagrams, the hibonite in hibonite-poor fluffy Type A inclusions from Allende and at least some hibonite from hibonite-rich inclusions is relict, although much of the hibonite from hibonite-glass spherules probably crystallized metasably from a melt. Bulk compositions for all of these CAIs are consistent with an origin as melite + hibonite + spinel + perovskite phase assembalges that were partially altered and in some cases partially or completely melted. The duration of the melting event was sufficient to remove any Na introduced by the alteration process but frequently insufficient to dissolve all of the original hibonite. Simple thermochemical models developed for meteoritic melilite and hibonite solid solutions were used to obtain equilibration temperatures of hibonite-bearing phase assemblages with vapor. Referenced to 10(exp -3) atm, hibonite + corundum + vapor equilibrated at approximately 1260 C and hibonite + spinel +/- melilite + vapor at 1215 +/- 10 C. If these temperatures reflect condensation in a cooling gas of solar composition, then hibonite +/- corundum condensed first, followed by spinel and then melilite. The position of perovskite within this sequence is uncertain, but it probably began to condense before spinel. This sequence of phase appearances and relative temperatures is generally consistent with observed textures but differs from expectations based on classical condensation calculations in that equilibration temperatures are generally lower than predicted and melilite initially condenses with or even after spinel. Simple thermochemical modes for the substitution of trace elements into the Ca site of meteoritic hibonites suggest that virtually all Eu is divalent in early condensate hibonites but that Eu(2+)/Eu(#+) decreases by a factor of 20 or more during the course of condensation primarily because the ratio is proportional to the partial pressure of Al, which decreases dramatically as aluminous phase condense. The relative sizes of Eu and Yb anomalies in meteoritic hibonites and inclusions may be partly due to this effect.

Beckett, J. R.↗
NASA NTRS1994

Silicate emission feature in the spectrum of comet Mueller 1993a

An 8- to 13-micron spectrum of comet Mueller 1993a, a dynamically new comet, was acquired when the comet was at R = 2 AU. Strong, structured silicate emission is present, closely resembling that seen in Comet P/Halley at smaller R. For the first time in a new comet, the 11.2-micron peak of crystalline olivine was detected, demonstrating that crystalline olivine particles were widespread in the solar nebula. Crystalline olivine particles could have formed in the inner protosolar nebula at temperatures greater than 1200 K; extensive radial mixing would have been required to transport these grains to the region of comet formation. Either there was more radial mixing in the solar nebula than some current theories predict or the olivine grains have a presolar origin.

Hanner, Martha S.↗
NASA NTRS1995

Study of Luminescence Characteristics of Trivalent Terbium in Silicate Glass

An important use of silicate glasses doped with terbium oxide (Tb2O3) is their use as fiber optic sensors for high-resolution imaging applications requiring the detection of x-rays (e.g. tomography and radiography). The x-ray radiation is absorbed by the glass, producing electron-hole pairs (excitons). The excitons migrate through the glass matrix and then recombine, emitting characteristic Tb(3+) luminescence in the optical wavelength region. This emission is due to forbidden transitions of 4f electrons and therefore has a long decay time. Long decay time is undesirable when imaging transient events since it results in blurring in time of the images. It has been reported elsewhere that in crystals Tb(3+) ions can act both as luminescence centers and as fluorescence traps. These traps can capture excitons and delay their recombination. This delayed fluorescence is seen as a long lived, secondary component to the luminescence decay curve, or afterglow. Such a secondary decay component to the luminescence decay of Tb(3+) has been observed before in soda glass following pulsed optical excitation. In order to determine the conditions under which afterglow occurs, an understanding of the material's luminescent properties is required.

West, Mike S.↗
NASA NTRS1995

Silicate and related dust emission in stars on the asymptotic giant branch

The Low-Resolution Spectrometer (LRS) on the Infrared Astronomical Satellite (IRAS) produced a rich set of spectra from oxygen-rich circumstellar dust shells. Little-Marenin and Little found that in addition to the classic 10 micron emission feature due to silicate dust, many oxygen-rich shells also produce components at 11 and 13 microns. Some shells exhibit only a broad, low-contrast feature which peaks longward of 11 microns and has been attributed to alumina dust. We have modified the classification method of Little-Marenin and Little, applied it to a large sample of bright oxygen-rich variables on the asymptotic giant branch, and undertaken a study of the 13 micron emission feature and the sources which produce it. We present some of the results of these studies.

Sloan, G. C.↗
NASA NTRS1999

Development of Refractory Silicate-YSZ Dual Layer TBCs

Development of advanced thermal barrier coatings (TBCs) is the most promising approach for increasing the efficiency and performance of gas turbine engines by enhancing the temperature capability of hot section metallic components. Spallation of the yttria-stabilized zirconia (YSZ) top coat, induced by the oxidation of the bond coat coupled with the thermal expansion mismatch strain, is considered to be the ultimate failure mode for current state-of-the-art TBCS. Enhanced oxidation resistance of TBCs can be achieved by reducing the oxygen conductance of TBCs below that of thermally grown oxide (TGO) alumina scale. One approach is incorporating an oxygen barrier having an oxygen conductance lower than that of alumina scale. Mullite, rare earth silicates and glass ceramics have been selected as potential candidates for the oxygen barrier. This paper presents the results of cyclic oxidation studies of oxygen barrier/YSZ dual layer TBCs.

He, Yirong↗
NASA NTRS2000

Comparative FeNi and Silicate Chronology in Portales Valley

Re-Os and U-Pb data on Portales Valley suggest an early formation for the metal and silicates. These two chronometers and Rb-Sr and Sm-Nd require a young disturbance. This is inconsistent with the 39 Ar-40 Ar age and in need of clarification.

Chen, J. H.↗