Search NASA⌕ Search

SEARCH · Search NASA

Results for “SILICATE”

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 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS2003

Lifetime Predictions of a Titanium Silicate Glass with Machined Flaws

A dynamic fatigue study was performed on a Titanium Silicate glass to assess its susceptibility to delayed failure and to compare the results with those of a previous study. Fracture mechanics techniques were used to analyze the results for the purpose of making lifetime predictions. The material strength and lifetime was seen to increase due to the removal of residual stress through grinding and polishing. Influence on time-to-failure is addressed for the case with and without residual stress present. Titanium silicate glass otherwise known as ultra-low expansion (ULE)* glass is a candidate for use in applications requiring low thermal expansion characteristics such as telescope mirrors. The Hubble Space Telescope s primary mirror was manufactured from ULE glass. ULE contains 7.5% titanium dioxide which in combination with silica results in a homogenous glass with a linear expansion coefficient near zero. delayed failure . This previous study was based on a 230/270 grit surface. The grinding and polishing process reduces the surface flaw size and subsurface damage, and relieves residual stress by removing the material with successively smaller grinding media. This results in an increase in strength of the optic during the grinding and polishing sequence. Thus, a second study was undertaken using samples with a surface finish typically achieved for mirror elements, to observe the effects of surface finishing on the time-to-failure predictions. An allowable stress can be calculated for this material based upon modulus of rupture data; however, this does not take into account the problem of delayed failure, most likely due to stress corrosion, which can significantly shorten lifetime. Fortunately, a theory based on fracture mechanics has been developed enabling lifetime predictions to be made for brittle materials susceptible to delayed failure. Knowledge of the factors governing the rate of subcritical flaw growth in a given environment enables the development of relations between lifetime, applied stress and failure probability for the material under study. Dynamic fatigue is one method of obtaining the necessary information to develop these relationships. In this study, the dynamic fatigue method was used to construct a time-to-failure diagram for polished ULE glass.

Tucker, Dennis S.↗
NASA NTRS2003

Silicate Stardust from Comets

Interplanetary dust particles (IDPs) collected in the Earth s stratosphere are 5-50 pm fragments of asteroids and comets. Anhydrous IDPs have escaped significant parent body hydrothermal alteration and are further distinct from meteorites in their greater abundances of volatile elements and C, fine grained (l00 - 500 nm) textures, and unequilibrated mineralogies. Many anhydrous IDPs contain high abundances interstellar organic compounds and grains of silicate stardust. These observations are consistent with properties of comets inferred from remote astronomical observations. Comets have been thought to be pristine aggregates of interstellar materials. However, spectroscopic observations of crystalline silicates in comets has challenged this notion, given their apparent absence in the interstellar medium.

Messenger, S.↗
NASA NTRS2003

Siliceous Shrubs in Yellowstone's Hot Springs: Implications for Exobiological Investigations

Potential relict hot springs have been identified on Mars and, using the Earth as an analog, Martian hot springs are postulated to be an optimal locality for recognizing preserved evidence of extraterrestrial life. Distinctive organic and inorganic biomarkers are necessary to recognize preserved evidence of life in terrestrial and extraterrestrial hot spring accumulations. Hot springs in Yellowstone National Park, Wyoming, U.S.A., contain a wealth of information about primitive microbial life and associated biosignatures that may be useful for future exobiological investigations. Numerous siliceous hot springs in Yellowstone contain abundant, centimeter-scale, spinose precipitates of opaline silica (opal-A). Although areally extensive in siliceous hot spring discharge channel facies, these spinose forms have largely escaped attention. These precipitates referred to as shrubs, consist of porous aggregates of spinose opaline silica that superficially resemble miniature woody plants, i.e., the term shrubs. Shrubs in carbonate precipitating systems have received considerable attention, and represent naturally occurring biotically induced precipitates. As such, shrubs have great potential as hot spring environmental indicators and, more importantly, proxies for pre-existing microbial life.

S A Guidry↗
NASA NTRS2003

H2O-Silicate Microphysics in Ascending Volcanic Plumes on Mars

We begin this study with a simple investigation of the role of the particle size frequency distribution on adsorptive scavenging in the ascending cloud. We define a parcel of silicates and H2O, and allow that parcel to cool, monitoring the total H2O remaining in the vapor phase. In this case, we have defined 10(exp 7) kg of silicates in the test volume, with a log-normal size-frequency distribution, a mean grain size of 10(exp -5) m, and a minimum of 10(exp -6) square meters per particle. This is based on observations of terrestrial ash particles that reveal enormous internal surface area. We assume 10(exp 5) kg of H2O (approximately equal to 1 wt%). The results of this study show significant dependence of the H2O sequestered in the adsorbed phase.

Zent, A. P.↗
NASA NTRS2004

Radiogenic Xenon-129 in Silicate Inclusions in the Campo Del Cielo Iron Meteorite

Iron meteorites present a challenge for the I-Xe dating technique because it is usually the inclusions, not metal, that contain radiogenic xenon and iodine. Silicate inclusions are frequent in only types IAB and IIE, and earlier studies of irons have demonstrated that I-Xe system can survive intact in these inclusions preserving valuable age information. Our previous studies of the I-Xe record in pyroxene grains from Toluca iron suggested an intriguing relationship between apparent I-Xe ages and (Mg+Fe)/Fe ratios. The I-Xe system in K-feldspar inclusions from Colomera (IIE) had the fingerprint of slow cooling, with an indicated cooling rate of 2-4 C/Ma. Here we present studies of the iodine-xenon system in a silicate-graphite-metal (SiGrMet) inclusion of the IA Campo del Cielo iron meteorite from the collection of the Museum of Natural History in Vienna.

Meshik, A.↗
NASA NTRS2004

Clear Evidence for Fe-60 in Silicate from a Semarkona Chondrule

Fe-60 (t(sub 1/2) = 1.5 Ma) is key to understanding the sources of short-lived radionuclides in the early solar system because it is the only one among those known from meteoritic material that is produced only in stars [1]. Within the last year, it has become clear that Fe-60 was present in sulfides from primitive ordinary and enstatite chondrites in amounts sufficient to require a recent stellar input [2-5]. The sulfide data indicate an initial Fe-60/Fe-56 ratio for the early solar system of between approx. 3 10(exp -7) and approx. 1.6 10(exp -6) [2-4]. However, iron (and nickel?) in sulfides is easily mobilized by very mild heating [e.g., 6], so there is considerable uncertainty over the true initial ratio. To resolve this uncertainty, we have begun a search for evidence of Fe-60 in silicates from primitive chondrites. In olivine from type 3.0-3.1 ordinary chondrites, diffusive exchange of iron and magnesium has not occurred to any significant degree, and diffusive exchange in pyroxene is slower [7]. However, the relatively small elemental fractionation of iron from nickel in silicates, coupled with the fact that the daughter nuclide, Ni-60, makes up approx. 26 % of normal nickel, make detection of excesses of radiogenic Ni-60 very difficult. Fortunately, we have found a fine-grained radiating-pyroxene chondrule in Semarkona (LL3.0) with a very high Fe/Ni ratio that gives clear evidence of Fe-60.

Huss, G. R.↗
NASA NTRS2005

Pyrolytic Synthesis of Carbon Nanotubes from Sucrose on a Mesoporous Silicate

Multiwall carbon nanotubes were synthesized from sucrose by a pyrolytic technique using mesoporous MCM-41 silicate templates without transition metal catalysts. The Nanotubes were examined in the carbon/silicate composite and after dissolution of the silicate. High resolution transmission electron microscopy study of the multiwall nanotubes showed them to be 15 nm in diameter, 200 nm in length and close-ended. There was variation in crystallinity with some nanotubes showing disordered wall structures.

Abdel-Fattah, Tarek↗
NASA NTRS2004

The unique radar properties of silicic lava domes

Silicic lava domes exhibit distinct morphologic characteristics at scales of centimeters to kilometers. Multiparameter radar observations capture the unique geometric signatures of silicic domes in a set of radar scattering properties that are unlike any other natural geologic surfaces.

radar remote sensing volcanology↗
NASA NTRS2006

Thermal Expansion and Thermal Conductivity of Rare Earth Silicates

Rare earth silicates are considered promising candidate materials for environmental barrier coatings applications at elevated temperature for ceramic matrix composites. High temperature thermophysical properties are of great importance for coating system design and development. In this study, the thermal expansion and thermal conductivity of hot-pressed rare earth silicate materials were characterized at temperatures up to 1400 C. The effects of specimen porosity, composition and microstructure on the properties were also investigated. The materials processing and testing issues affecting the measurements will also be discussed.

Zhu, Dongming↗
NASA NTRS2007

The Oxidation State of Tungsten in Iron Bearing and Iron Free Silicate Glasses: Results from W L-Edge Xanes Measurements

Tungsten is important in constraining core formation of the Earth because this element is a moderately siderophile element (depleted approx. 10 relative to chondrites) and, as a member of the Hf-W isotopic system, it is useful in constraining the timing of core formation. A number of previous experimental studies have been carried out to determine the silicate solubility and metal-silicate partitioning behavior of W, including its concomitant oxidation state. However, results of previous studies (figure 1) are inconsistent on whether W occurs as W(4+) or W(6+).

Danielson, Lisa R.↗
NASA NTRS2007

Polymer-Layered Silicate Nanocomposites for Cryotank Applications

Previous composite cryotank designs have relied on the use of conventional composite materials to reduce microcracking and permeability. However, revolutionary advances in nanotechnology derived materials may enable the production of ultra-lightweight cryotanks with significantly enhanced durability and damage tolerance, as well as reduced propellant permeability. Layered silicate nanocomposites are especially attractive in cryogenic storage tanks based on results that have been reported for epoxy nanocomposite systems. These materials often exhibit an order of magnitude reduction in gas permeability when compared to the base resin. In addition, polymer-silicate nanocomposites have been shown to yield improved dimensional stability, strength, and toughness. The enhancement in material performance of these systems occurs without property trade-offs which are often observed in conventionally filled polymer composites. Research efforts at NASA Glenn Research Center have led to the development of epoxy-clay nanocomposites with 70% lower hydrogen permeability than the base epoxy resin. Filament wound carbon fiber reinforced tanks made with this nanocomposite had a five-fold lower helium leak rate than the corresponding tanks made without clay. The pronounced reduction observed with the tank may be due to flow induced alignment of the clay layers during processing. Additionally, the nanocomposites showed CTE reductions of up to 30%, as well as a 100% increase in toughness.

Miller, Sandi G.↗
NASA NTRS2000

Float processing of high-temperature complex silicate glasses and float baths used for same

A float glass process for production of high melting temperature glasses utilizes a binary metal alloy bath having the combined properties of a low melting point, low reactivity with oxygen, low vapor pressure, and minimal reactivity with the silicate glasses being formed. The metal alloy of the float medium is exothermic with a solvent metal that does not readily form an oxide. The vapor pressure of both components in the alloy is low enough to prevent deleterious vapor deposition, and there is minimal chemical and interdiffusive interaction of either component with silicate glasses under the float processing conditions. Alloys having the desired combination of properties include compositions in which gold, silver or copper is the solvent metal and silicon, germanium or tin is the solute, preferably in eutectic or near-eutectic compositions.

Cooper, Reid Franklin↗
NASA NTRS2006

Depletion of Vandium in Planetary Mantles: Controlled by Metal, Oxide, or Silicate?

Vanadium concentrations in planetary mantles can provide information about the conditions during early accretion and differentiation. Because V is a slightly siderophile element, it is usually assumed that any depletion would be due to core formation and metal-silicate equilibrium. However, V is typically more compatible in phases such as spinel, magnesiowuestite and garnet. Fractionation of all of these phases would cause depletions more marked than those from metal. In this paper consideration of depletions due to metal, oxide and silicate are critically evaluated.

Righter, Kevin↗
NASA NTRS2009

Selective Clay Placement within a Silicate Clay-Epoxy Blend Nanocomposite and the Effect on Physical Properties

Many epoxy systems under consideration for composite pressure vessels are composed of toughened epoxy resins. In this work, epoxy blends containing both rigid aromatic and flexible aliphatic components were prepared, to model toughened systems, and determine the optimum route of silicate addition. Compositions were chosen such that both glassy and rubbery resins were obtained at room temperature. The physical properties of the nanocomposites varied with T(g) and silicate placement, however, nanocomposite T(g)s were observed which exceeded that of the base resin by greater than 10 C. The tensile strength of the glassy resin remained constant or decreased on the dispersion of clay while that of the rubbery material doubled. Selectively placing the clay in the aliphatic component of the rubbery blend resulted in a greater than 100% increase in material toughness.

Miller, Sandi G.↗
NASA NTRS2009

Lightning and Mass Independent Oxygen Isotopic Fractionation in Nebular Silicates

Lightning has long been postulated as the agent of Chondru|e formation in the solar nebula, but it may have an additional role to play as well. Lightning bolts of almost any scale will both vaporize dust and liberate oxygen atoms that will then interact with both nebular gases as well as the refractory silicate vapor as it re-condenses. Such processes should result in the addition of the heavy oxygen isotopes to the growing silicate grains while the light oxygen-16 becomes part of the gas phase water. This process will proceed to some extent throughout the history of any turbulent nebula and will result in the gradual increase of O-16 in the gas phase and in a much larger relative increase in the O-17 and O-18 content of the nebular dust. Laboratory experiments have demonstrated the production of such "heavy oxygen enriched", non-mass-dependently-fractionated dust grains in a high voltage discharge in a hydrogen rich gas containing small quantities of silane, pentacarbonyl iron and oxygen.

Nuth, Joseph A.↗
NASA NTRS2011

Mechanical Properties and Durability of Advanced Environmental Barrier Coatings in Calcium-Magnesium-Alumino-Silicate Environments

Environmental barrier coatings are being developed and tested for use with SiC/SiC ceramic matrix composite (CMC) gas turbine engine components. Several oxide and silicate based compositons are being studied for use as top-coat and intermediate layers in a three or more layer environmental barrier coating system. Specifically, the room temperature Vickers-indentation-fracture-toughness testing and high-temperature stability reaction studies with Calcium Magnesium Alumino-Silicate (CMAS or "sand") are being conducted using advanced testing techniques such as high pressure burner rig tests as well as high heat flux laser tests.

Miladinovich, Daniel S.↗
NASA NTRS2011

Crystalline Silicates in Comets: Modeling Irregularly-Shaped Forsterite Crystals and Its Implications on Condensation Conditions

Crystalline silicates in comets are a product of the condensation in the hot inner regions (T > or approx. equals 1400 K [1]) of our proto-planetary disk or annealing at somewhat lower temperatures (T > or approx. equals 1000-1200 K) [2, 3, 4] in shocks coupled with disk evolutionary processes that include radial transport of crystals from their formation locations out to the cold outer regions where comet nuclei formed. The grain shape of forsterite (crystals) could be indicative of their formation pathways at high temperatures through vapor-solid condensation or at lower temperatures through vapor-liquid-solid formation and growth [5, 6, 7]. Experiments demonstrate that crystals that formed from a rapidly cooled highly supersaturated silicate vapor are characterized by bulky, platy, columnar/needle and droplet shapes for values of temperature and supersaturation, T and sigma, of 1000-1450 C and < 97, 700-1000 C and 97-161, 580-820 C and 131-230, and <500 C and > 230, respectively [7]. The experimental columnar/needle shapes, which form by vapor-liquid-solid at lower temperatures (<820 C), are extended stacks of plates, where the extension is not correlated with an axial direction: columnar/needles may be extended in the c-axis or a-axis direction, can change directions, and/or are off-kilter or a bit askew extending in a combination of the a- and c-axis direction.

Wooden, Diane H.↗
NASA NTRS2013

Metal-Silicate Partitioning of Bi, In, and Cd as a Function of Temperature and Melt Composition

The origin of volatile elements in the Earth, Moon and Mars is not known; however, several theories have been proposed based on volatile elements such as In, As, Se, Te and Zn which are in lower concentration in the Earth, Moon, and Mars than in chondrites. Explanations for these low concentrations are based on two contrasting theories for the origin of Earth: equilibrium core formation versus late accretion. One idea is that the volatiles were added during growth of the planets and Moon, and some mobilized into the metallic core while others stayed in the mantle (e.g., [1]). The competing idea is that they were added to the mantles after core formation had completed (e.g., [2]). Testing these ideas involves quantitative modeling which can only be performed after data is obtained on the systematic metal-silicate partitioning behavior of volatile elements with temperature, pressure and melt composition. Until now, such data for Bi, In, and Cd has been lacking. After conducting a series of high pressure, high temperature experiments, the metal-silicate partition coefficients of Bi, In, and Cd as a function of temperature and melt composition can be used to evaluate potential conditions under which terrestrial planets differentiated into core and mantle, and how they acquired volatiles.

Marin, Nicole↗