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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 703 records · Page 39
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Comments on the BLAG model - The carbonate-silicate geochemical cycle and its effect on atmospheric carbon dioxide over the past 100 million years

A self-consistent method of determining initial conditions for the model presented by Berner, Lasaga, and Garrels (1983) (henceforth, the BLAG model) is derived, based on the assumption that the CO2 geochemical cycle was in steady state at t = -100 m.y. (million years). This initialization procedure leads to a dissolved magnesium concentration higher than that calculated by Berner, Lasaga, and Garrels and to a low ratio of dissolved calcium to bicarbonate prior to 60 m.y. ago. The latter prediction conflicts with the geologic record of evaporite deposits, which requires that this ratio remain greater than 0.5. The contradiction is probably caused by oversimplifications in the BLAG model, such as the neglect of the cycles of organic carbon and sulfur.

Kasting, J. F.↗
NASA NTRS1986

Cooling rate calculations for silicate glasses

The effects of thermal properties, sample size, and quench medium on cooling rates achieved during quenching are evaluated for a variety of samples with different thermal properties. It is found that, depending on the composition and the property being observed, quench rate may or may not be fast enough to preserve a high-temperature property. If the cooling rate dependence of a property in a given system is unknown, then the cooling rate must be specified so that others may duplicate specific conditions. The effect of cooling rate can be experimentally tested by manipulation of sample size and quench medium. These results should add to understanding of glass-forming phenomena on the moon.

Birnie, D. P., III↗
NASA NTRS1986

Carbon in oxides and silicates - Dissolution versus exsolution

A theory of CO2 dissolution in the solid state is developed, using the idea proposed by Freund (1983) concerning dissolution of CO/CO2 in MgO on the basis of their experimental results obtained with an MgO-containing carbon impurity. It is shown that the dissolution mechanism may be linked to an internal redox reaction by which a certain number of lattice oxygens change their formal oxidation state from -2 to -1, while the carbon becomes reduced. The similarities between the mechanisms of CO and/or CO2 dissolution and that of H2O dissolution are pointed out. A hypothesis is proposed concerning the exsolution of reduced carbon from supersaturated solid solutions under conditions which permit C-C bond formation.

Freund, F.↗
NASA NTRS1987

The geology of Picacho Butte, a silicic volcanic dome in northwest Arizona

The purpose of this investigation was to determine the geologic history of Picacho Butte and vicinity through careful mapping of a 38 square kilometer area surrounding the peak. A detailed analysis of the geochemistry and petrology will aid in the development of a petrogenetic model for the area. The relationship of Picacho Butte to regional volcanism in Arizona, and more specifically to nearby volcanic centers can thus be established. Furthermore, in conjunction with this study a search will be made for possible planetary analogs exhibiting photogeologic characteristics similar to those in northern Arizona.

Kisiel, Andrew P.↗
NASA NTRS1986

Abundances of cobalt, nickel, and volatiles in the silicate portion of the moon

The trace abundances of Ni have been determined in 20 high-Mg magmas produced by partial melting of the lunar mantle. Associated with the eruption of the high-Mg magmas was the release of indigenous volatile elements. Data suggest that these volatiles were derived from primordial blocks of debris from the outer solar system that became entrained in the circumterrestrial accretion disk. The survival of these reservoirs within the lunar interior places constraints on the moon's thermal history.

Delano, John W.↗
NASA NTRS1986

An analysis of refractory silicate nucleation via the theory of the physical spinodal

An analysis of experimental data has been conducted using the theory of the spinodal, as a test of its applicability to refractory vapors. A promising aspect of this approach is its prediction of a temperature and supersaturation at which there no longer exists a barrier to nucleation; such a phenomenon has been observed in both the SiO-H2 and MgSiO-H2 systems, where a small change in ambient temperature yielded 5-10 orders of magnitude increases in the nucleation rate within a few seconds.

Nuth, Joseph A., III↗
NASA NTRS1987

Rb-Sr and Sm-Nd systematics of cherts and other siliceous deposits

An analysis of marine and lacustrine cherts demonstrates that Rb-Sr and Sm-Nd systems may be used in elucidating the origin and evolution of cherts, and that the Rb-Sr system may be used for dating. All samples have high Rb/Sr ratios, and though the site of these elements in cherts was not established, they appear to be correlated with Al content. Sm/Nd ratios were typical of continental sources, and the results show the Rb-Sr system in cherts to be isolated from the associated carbonate, with little Sr exchange. Data suggest that in many cases cherts are formed by dissolved SiO2 nucleating on clay-like material which preserves the isotropic characteristics of the detrital source, while in some cases it is possible that cherts were formed from silica-saturated waters from a nonmarine source.

Weis, D.↗
NASA NTRS1987

Mutual interactions of redox couples via electron exchange in silicate melts - Models for geochemical melt systems

The mutual interactions via electron exchange of redox couples in glass-forming melts were investigated both theoretically and experimentally. A thermodynamic approach for considering the mutual interactions leads to conclusion that the degree of mutual interaction in the melt should be proportional in part to the difference in relative reduction potentials of the interacting redox couples. Experimental studies verify this conclusion for numerous redox couples in several composition/temperature/oxygen fugacity regimes. Geochemical systems simultaneously possess many potentially multivalent elements; the stabilized redox states in the resulting magmas can be explained in part by mutual interactions and by redox buffering through the central Fe(III)- Fe(II) couples in the melts. The significance of these results for basaltic magmas of the earth, moon, and meteorites is addressed.

Schreiber, Henry D.↗
NASA NTRS1986

Dependence of the critical cooling rate for lithium-silicate glass on nucleating agents

The critical cooling rate, Rc, for glass formation of a glass containing 40 mol pct Li2O and 60 mol pct SiO2, doped with small amounts of Pt, Au, P2O5, and TiO2 nucleating agents, has been measured. Rc increases with increasing Pt and Au additions, but Pt has a larger effect than Au. Additions of P2O5 tend to decrease Rc, whereas TiO2 has practically no effect on Rc. The devitrified glass nucleated with Pt contains only crystalline Li2O-SiO2, but all the other devitrified glasses, including the undoped glass, contain both Li2O-SiO2 and Li2O-2SiO2. Analysis shows that Rc increases as the concentration of the Li2O-SiO2 phase in the devitrified glass increases.

Huang, W.↗
NASA NTRS1987

Experimental correlation of melt structures, nucleation rates, and thermal histories of silicate melts

The theory and measurement of the structure of liquids is an important aspect of modern metallurgy and igneous petrology. Liquid structure exerts strong controls on both the types of crystals that may precipitate from melts and on the chemical composition of those crystals. An interesting aspect of melt structure studies is the problem of melt memories; that is, a melt can retain a memory of previous thermal history. This memory can influence both nucleation behavior and crystal composition. This melt memory may be characterized quantitatively with techniques such as Raman, infrared and NMR spectroscopy to provide information on short-range structure. Melt structure studies at high temperature will take advantage of the microgravity conditions of the Space Station to perform containerless experiments. Melt structure determinations at high temperature (experiments that are greatly facilitated by containerless technology) will provide invaluable information for materials science, glass technology, and geochemistry. In conjunction with studies of nucleation behavior and nucleation rates, information relevant to nucleation in magma chambers in terrestrial planets will be acquired.

Boynton, W. V.↗
NASA NTRS1990

Silicates and variability in comets Okazaki-Levy-Rudenko (1989r) and Austin (1989c1)

Comet Okazaki-Levy-Rudenko (1989r) was studied over a wide range of wavelengths from the UV to the submillimeter. Using visual magnitude estimates from the IAU Circulars and reported submillimeter fluxes in conjunction with the thermal infrared spectroscopy and filter photometry, the nature of the grains and the behavior of the comet can be assessed and compared to other comets studied under this program. In addition, preliminary results from a collaborative infrared study of Comet Austin are presented which address the nature of the dust in that comet. Progress on a model for cometary properties and brightness behavior is discussed.

Russell, Ray W.↗
NASA NTRS1990

Small-scale disequilibrium in a magmatic inclusion and its more silicic host

An investigation of small-scale isotopic, compositional, and mineralogical variation across the interface of a basaltic-andesite inclusion and its dacitic host from Cerro-Chascon, a Holocene dome in northern Chile, is discussed. Serial sectioning across the interface of the inclusion and its host dacite, complemented by microdrill sampling and detailed microprobe work, has enabled an examination of the scale of mixing and chemical disequilibrium. The composition of the inclusion is found to be relatively homogeneous; the dacite host is heterogeneous on a small scale; the isotopic composition in the marginal zone shows the highest Sr-87/Sr-86 and lowest Nd-143/Nd-144; the large plagioclase crystals in the inclusions and host are xenocrystic. These differences are reconciled with a model of magma evolution in a crustal magma chamber.

Davidson, Jon P.↗
NASA NTRS1991

Ablation of silicate particles in high-speed continuum and transition flow with application to the collection of interplanetary dust particles

The ablative deceleration of spheres in the continuum and slip regimes is studied using spherical 7.1-micron-diam soda-lime glass particles launched from vacuum at about 4500 m/sec speed through a 13-micron-thick plastic film into a capture chamber containing Xe at 0.1 or 0.2 atm pressure and 295 K temperature. The results of SEM examinations of the collected ablated particles showed that the ratio of the ablated-particle radius (Rf) to the initial radius (R0) increased with gas pressure (from Rf/R0 about 0.67 at 0.1 atm, to about 0.88 at 0.2 atm). A model was developed to describe the ablation and deceleration of spheres in high-speed continuum and slip flow. The pressure dependence predicted by the model agreed with experimental results.

Rulison, Aaron J.↗
NASA NTRS1992

Mechanical properties of silicate glasses exposed to a low-Earth orbit

The effects of a 5.8 year exposure to low earth orbit environment upon the mechanical properties of commercial optical fused silica, low iron soda-lime-silica, Pyrex 7740, Vycor 7913, BK-7, and the glass ceramic Zerodur were examined. Mechanical testing employed the ASTM-F-394 piston on 3-ball method in a liquid nitrogen environment. Samples were exposed on the Long Duration Exposure Facility (LDEF) in two locations. Impacts were observed on all specimens except Vycor. Weibull analysis as well as a standard statistical evaluation were conducted. The Weibull analysis revealed no differences between control samples and the two exposed samples. We thus concluded that radiation components of the Earth orbital environment did not degrade the mechanical strength of the samples examined within the limits of experimental error. The upper bound of strength degradation for meteorite impacted samples based upon statistical analysis and observation was 50 percent.

Wiedlocher, David E.↗
NASA NTRS1992

Self-diffusion of magnesium in spinel and in equilibrium melts - Constraints on flash heating of silicates

An isotopic tracer is used to measure Mg self-diffusion in spinel and coexisting melt at bulk chemical equilibrium. The diffusion coefficients were calculated from the measured isotope profiles using a model that includes the complementary diffusion of Mg-24, Mg-25, and Mg-26 in both phases with the constraint that the Mg content of each phase is constant. The activation energy and preexponential factor for Mg self-diffusion in spinel are, respectively, 384 +/- 7 kJ and 74.6 +/- 1.1 sq cm/s. These data indicate Mg diffusion in spinel is much slower than previous estimates. The activation energy for Mg self-diffusion in coexisting melt is 343 +/- 25 kJ and the preexponential factor is 7791.9 +/- 1.3 sq cm/s. These results are used to evaluate cooling rates of plagioclase-olivine inclusions (POIs) in the Allende meteorite. Given a maximum melting temperature for POIs of about 1500 C, these results show that a 1-micron radius spinel would equilibrate isotopically with a melt within about 60 min.

Sheng, Y. J.↗