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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

O(-) identified at high temperatures in CaO-based catalysts for oxidative methane dimerization

A technique called charge-distribution analysis (CDA) is employed to study mobile charge carriers in the oxidation catalysts CaO, CaO with 11 percent Na2O, and CaO with 10 percent La2O3. A threshold temperature of about 550-600 C is identified at which highly mobile charge carriers are present, and the CDA studies show that they are O(-) states. The present investigation indicates the usefulness of CDA in catalysis research with pressed powder samples and gas/solid reactions.

Freund, F.↗
NASA NTRS1988

The effect of Al2O3, CaO, Cr2O3 and MgO on devitrification of silica

The effect of doping on devitrification of vitreous silica was studied at 1100, 1200, and 1300 C. Dispersion of dopants on a molecular scale was accomplished via a sol-gel technique. All dopants accelerated the devitrification of silica but to different degrees. The most active was CaO followed by MgO, Al2O3, and Cr2O3. Pure silica and silica containing Cr2O3 and Al2O3 devitrified to alpha-cristobalite only, whereas silica doped with CaO and MgO produced alpha-quartz and alpha-cristobalite. It appears that prolonged heat treatment would cause alpha-quartz to transform to alpha-cristobalite.

Zaplatynsky, Isidor↗
NASA NTRS1990

Energy storage for a lunar base by the reversible chemical reaction: CaO+H2O reversible reaction Ca(OH)2

A thermochemical solar energy storage concept involving the reversible reaction CaO + H2O yields Ca(OH)2 is proposed as a power system element for a lunar base. The operation and components of such a system are described. The CaO/H2O system is capable of generating electric power during both the day and night. The specific energy (energy to mass ratio) of the system was estimated to be 155 W-hr/kg. Mass of the required amount of CaO is neglected since it is obtained from lunar soil. Potential technical problems, such as reactor design and lunar soil processing, are reviewed.

Perez-Davis, Marla E.↗
NASA NTRS2021

Melting and Crystallization Behavior of CaO-MgO-Al2O3-SiO2 Silicates Relevant to Turbine Engine Applications

The melting and crystallization behavior of four quaternary CaO-MgO-Al2O3-SiO2(CMAS) silicates were investigated. The CaO:SiO2 ratios of these systems were based on various terrestrial sources of ingested particles relevant to gas turbine engine operating environments. Melting behavior was characterized using differential scanning calorimetry, and high temperature intrinsic crystallization products were determined by furnace heat treatments of the glasses at 1200°C, 1300°C, and 1400°C. The silicates exhibited a wide range of melting temperatures from ~1240°C up to ~1500°C, with most of the compositions exhibiting incongruent melting behavior. High temperature crystallization products included CaSiO3,CaAl2Si2O8, Ca2MgSi2O7, and Ca(Mg,Al)Si2O6, although SiO2 was the only crystalline phase observed at 1400°C.

Jamesa L. Stokes↗
NASA NTRSDate not supplied

Tribological performance of Al2O3-B2O3 composites: Role of CuOand CaO constituents

Al2O3-B2O3 based composites for wear resistant applications (e.g., seal rings and bearings) were manufactured using powder metallurgy route. Four different compositions were considered to control amount of in situ phases, mainly aluminum borate, formed during sintering. The fabricated composites were characterized using X-ray diffraction to identify the phases. Hardness was measured in terms of micro-hardness. The tribological performance of the composites were evaluated by conducting dry sliding tests against alumina balls at different humidity levels (10 – 95% relative humidity). The X-ray spectrum revealed that increase in B2O3 content in the composites increased the amount of aluminum borate phase that enhanced the micro-hardness. Results revealed that aluminum borate enhanced the wear resistance of the composites, whereas the humidity-sensitive alumina phase reduced friction at higher humidity levels. Furthermore, CuO and CaO (2-5 wt.%) were added separately to the composite, resulting in increased density by 22-48 %. Athough the addition of CuO and CaO led to increase in hardness and wear resistance of the composites, the coefficient of friction was negatively affected. The friction and wear performance along with wear mechanism are discussed with respect to observed phases in the composites.

Ashish K. Kasar↗
NASA NTRS2016

Thermochemistry of CaO-MgO-Al2O3-SiO2 (CMAS) and Advanced Thermal and Environmental Barrier Coating Systems

CaO-MgO-Al2O3-SiO2 (CMAS) oxides are constituents in a broad number of materials and minerals which have recently inferred to discussions in materials science, planetary science, geochemistry and cosmochemistry communities. In materials science, there is increasing interest in the degradation studies of thermal (TBC) and environmental (EBC) barrier coatings of gas turbines by molten CMAS. These coatings have been explored to be applied on silicon-based ceramics and composites which are lighter and more temperature capable hot-section materials of gas turbines than the current Ni-based superalloys. The degradation of the coatings occurs when CMAS minerals carried by the intake air into gas turbines, e.g. in aircraft engines, reacts at high temperatures (1000C) with the coating materials. This causes premature failure of the static and rotating components of the turbine engines. We discuss some preliminary results of the reactions between CMAS and Rare-Earth (RE Y, Yb and Gd) oxide stabilized ZrO2 systems, and stability of the resulting oxides and silicates.

CaO-MgO-Al2O3-SiO2 (CMAS)↗
NASA NTRS1969

Exciton spectra of CaO and MgO.

CaO and MgO reflectance spectra measurements at high resolution and at low temperature including exciton spectra at 25 K, obtaining fine structure

Walker, W. C.↗
NASA NTRS1978

Electronic structure of CaO. I

Electronic wavefunctions for 15 states of CaO arising (nominally) from six electron occupancies are calculated at the single configuration (sc) SCF level and analyzed qualitatively in terms of Mulliken populations and an approximate overlap criterion. The accuracy of the sc approximation is assessed and shown to be highly inadequate in several cases. Approximate Te's are extracted from these results when warranted by the quality of the sc wavefunctions. An improved two configuration wavefunction for the lowest 1 Sigma + state is also determined and discussed in detail.

Bauschlicher, C. W., Jr.↗
NASA NTRS1985

Modelling Equilibrium and Fractional Crystallization in the System MgO-FeO-CaO-Al2O3-SiO2

A mathematical modelling technique for use in petrogenesis calculations in the system MgO-FeO-CaO-Al2O3-SiO2 is reported. Semiempirical phase boundary and elemental distribution information was combined with mass balance to compute approximate equilibrium crystallization paths for arbitrary system compositions. The calculation is applicable to a range of system compositions and fractionation calculations are possible. The goal of the calculation is the computation of the composition and quantity of each phase present as a function of the degree of solidification. The degree of solidification is parameterized by the heat released by the solidifying phases. The mathematical requirement for the solution of this problem is: (1) An equation constraining the composition of the magma for each solid phase in equilibrium with the liquidus phase, and (2) an equation for each solid phase and each component giving the distribution of that element between that phase and the magma.

Herbert, F.↗
NASA NTRS2017

Thermochemistry of CaO-MgO-Al2O3-SiO2 (CMAS) and Advanced Thermal and Environmental Barrier Coating Systems

CaO-MgO-Al2O3-SiO2 (CMAS) oxides are constituents in a broad number of materials and minerals which have recently inferred to discussions in materials science, planetary science, geochemistry and cosmochemistry communities. In materials science, there is increasing interest in the degradation studies of thermal (TBC) and environmental (EBC) barrier coatings of gas turbines by molten CMAS. CMAS minerals usually are carried by the intake air into gas turbines, e.g. in aircraft engines, and their deposits react at high temperatures (1000C) with the coating materials. This causes degradation and accelerated failure of the static and rotating components of the turbine engines. We discuss some preliminary results of the reactions between CMAS and Rare-Earth (RE Y, Yb, Dy, Gd, Nd and Sm) oxide stabilized ZrO2 or HfO2 systems, and the stability of the resulting oxides and silicates. Plasma sprayed hollow tube samples ( 2.2 mm and 26 mm height) were half filled with CMAS powder, wrapped and sealed with platinum foil, and heat treated at 1310 C for 5h. Samples were characterized by differential scanning calorimetry, X-ray diffraction and cross section electron microscopy analysis.

degradation↗
NASA NTRS2017

High Temperature Degradation of Advanced Thermal and Environmental Barrier Coatings (TEBCs) by CaO-MgO-Al2O3-SiO2 (CMAS)

There is increasing interest in the degradation studies of thermal and environmental barrier coatings (TEBCs) of gas turbines by molten CaO-MgO-Al2O3-SiO2 (CMAS). CMAS minerals are usually referred as silica-containing sand dust and volcano ash materials that are carried by the intake air into gas turbines, e.g. aircraft engines. The low-melting deposits react at high temperatures (1000C) with the coating materials. This causes degradation and accelerated coating failure of the static and rotating components of the turbine engines. We discuss some preliminary results of the reactions between CMAS and Rare-Earth (RE Y, Yb, Dy, Gd, Nd and Sm) oxide stabilized ZrO2 or HfO2 systems, and the stability of the resulting oxides and silicates. Plasma sprayed hollow tube samples (outer diameter 4.7 mm, wall thickness 0.76 mm and 26 mm height) were half filled with CMAS powder, wrapped and sealed with platinum foil, and heat- treated at 1310 C for 5h. Samples were characterized by differential scanning calorimetry, X-ray diffraction and cross section electron microscopy analysis.

Thermal and environmental barrier coatings↗
NASA NTRS1984

A thermodynamic model for subsolidus equilibria in the system CaO-MgO-Al2O3-SiO2

It is shown that the high temperature enthalpy of solution data for pure phases and solid solutions in the CMAS system are, for 'gabbroic' and 'peridotitic' compositions, consistent with available phase equilibrium data for the MAS, CAS and CMAS systems. A refined set of values of thermodynamic properties for these phases and solid solutions is tabulated. The small differences between the new data set and that of Helgeson et al. (1978) arise from new data on heat capacity and enthalpy being incorporated. The important constraints applied and the major difficulties in fitting which arose are summarized.

Wood, B. J.↗
NASA NTRS1990

Low temperature synthesis of CaO-SiO2 glasses having stable liquid-liquid immiscibility by sol-gel process

Calcium silicate glass compositions lying within the liquid-liquid immiscibility dome of the phase diagram, which could not have been prepared by the conventional melting method, were synthesized by the sol-gel process. Hydrolysis and polycondensation of tetraethyl orthosilicate (TEOS) solutions containing up to 20 mol percent calcium nitrate resulted in the formation of clear and transparent gels. The gel formation time decreased with increase in water:TEOS mole ratio, calcium content, and the reaction temperature. Smaller values of gel times in the presence of calcium nitrate are probably caused by lowering of the ionic charge on the sol particles by the salt present. The gelation activation energy, E(sub gel), was evaluated from temperature dependence of the gel time. Presence of Ca(2+) ions or the water:TEOS mole ratio did not have an appreciable effect on the value of E(sub gel). Presence of glycerol in the solution helped in the formation of crack-free monolithic gel specimens. Chemical and structural changes occurring in the gels, as a function of the heat treatments, have been monitored using DTA, TGA, IR-spectroscopy, x ray diffraction, surface area and pore size distribution measurements.

Bansal, Narottam P.↗
NASA NTRS1992

Low temperature synthesis of CaO-SiO2 glasses having stable liquid-liquid immiscibility by the sol-gel process

Calcium silicate glass compositions lying within the liquid-liquid immiscibility dome of the phase diagram, which could not have been prepared by the conventional melting method, were synthesized by the sol-gel process. Hydrolysis and polycondensation of tetraethyl orthosilicate (TEOS) solutions containing up to 20 mol percent calcium nitrate resulted in the formation of clear and transparent gels. The gel formation time decreased with increase in water: TEOS mole ratio, calcium content, and the reaction temperature. Smaller values of gel times in the presence of calcium nitrate are probably caused by lowering of the ionic charge on the sol particles by the salt present. The gelation activation energy, E(sub gel), was evaluated from temperature dependence of the gel time. Presence of Ca(2+) ions or the water:TEOS mole ratio did not have an appreciable effect on the value of E(sub gel). Presence of glycerol in the solution helped in the formation of crack-free monolithic gel specimens. Chemical and structural changes occurring in the gels, as a function of the heat treatments, have been monitored using DTA, TGA, IR-spectroscopy, X-ray diffraction, surface area and pore size distribution measurements.

Bansal, N. P.↗
NASA NTRS2024

Aerosol Size Distribution Properties Associated With Cold-Air Outbreaks in the Norwegian Arctic

The aerosol particles serving as cloud condensation and ice nuclei contribute to key cloud processes associated with cold-air outbreak (CAO) events but are poorly constrained in climate models due to sparse observations. Here we retrieve aerosol number size distribution modes from measurements at Andenes, Norway, during the Cold-Air Outbreaks in the Marine Boundary Layer Experiment (COMBLE) and at Zeppelin Observatory, approximately 1000 km upwind from Andenes at Svalbard. During CAO events at Andenes, the sea-spray-mode number concentration is correlated with strong over-ocean winds with a mean of 8±4 cm−3 that is 71 % higher than during non-CAO conditions. Additionally, during CAO events at Andenes, the mean Hoppel minimum diameter is 6 nm smaller than during non-CAO conditions, though the estimated supersaturation is lower, and the mean number concentration of particles that likely activated in-cloud is 109±61 cm−3 with no statistically significant difference from the non-CAO mean of 99±66 cm−3. For CAO trajectories between Zeppelin Observatory and Andenes, the upwind-to-downwind change in number concentration is the largest for the accumulation mode with a mean decrease of 93±95 cm−3, likely attributable primarily to precipitation scavenging. These characteristic properties of aerosol number size distributions during CAO events provide guidance for evaluating CAO aerosol–cloud interaction processes in models.

accumulation mode particle scavenging↗