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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Improved performance of silicon nitride-based high temperature ceramics

Results are presented regarding experiments intended for improving the strength and toughness of hot-pressed Si3N4 (HPSN), improving the strength and oxidation resistance of reaction-sintered Si3N4 (RSSN), and improving the strength and oxidation resistance of sinterable Si-Al-O-N compositions. It is shown that the use of ZrO2 instead of MgO as a sintering aid improved the room-temperature and high-temperature flexural strength of HPSN, in addition to enhancing the rupture strength and Charpy impact resistance. The use of crushable energy absorbing layers increased the impact resistance of HPSN. Impregnation of RSSN with solutions that oxidize to Al2O3 or ZrO2 resulted in increased bending strength at room temperature. Beta-prime Si-Al-O-N sintered to full density by means of the sintering aids CeO2, Y2O3, and ZrO2 yielded the greatest strength with Y2O3 and the greatest oxidation resistance with ZrO2.

Ashbrook, R. L.↗
NASA NTRS1978

Near-opposition limb darkening of solids of planetary interest

Results are presented for a general laboratory study of the limb darkening at small phase angles of various materials of planetary interest, with particular attention given to the wavelength dependence of limb darkening. Only substances stable at room temperature were measured, including basalt, graphite, mixtures of MgO and charcoal powders, olivine, goethite, bronzite, CaCO3, MgSO4, BaSO4, limonite, and hematite. Previous studies aimed at determining the nature of limb darkening at opposition for different common materials are reviewed. It is found that the scattering properties of most particulate materials near phase zero can be described adequately by Minnaert's law, but that such a representation is totally inadequate for materials that tend to fracture into flakes with mirrorlike surfaces (e.g., bronzite and graphite). The results also indicate that materials such as olivine have scattering properties within deep absorption bands which exhibit definite departures from Minnaert's law at large angles of incidence or emission.

Veverka, J.↗
NASA NTRS1977

Coarse-grained basalt 71597 - A product of partial olivine accumulation

Rock 71597, a coarse-grained plagioclase-poikilitic basalt that contains 15.8% MgO and 19.3% modal olivine, is described. Olivine and ilmenite crystals occur in two size generations, and large subequant olivine crystals, small 'matrix' olivines, and large ilmenite crystals are characterized. Since large olivine and ilmenite crystals with skeletal habits suggestive of crystallization under relatively rapid cooling are found in a rock whose texture is consonant with slower cooling, it is suggested that the large skeletal crystals formed toward the more rapidly cooled margin of a flow and accumulated by gravitational crystal settling toward the flow's interior. In support of this hypothesis, calculations indicate that the observed olivine compositions are approximately 10 mol% too Fe-rich to have crystallized from a melt of composition corresponding to that of 71597.

Warner, R. D.↗
NASA NTRS1978

Thermal barrier coatings: Burner rig hot corrosion test results

A Mach 0.3 burner rig test program was conducted to examine the sensitivity of thermal barrier coatings to Na and V contaminated combustion gases simulating potential utility gas turbine environments. Coating life of the standard ZrO2-12Y2O3/Ni-16.2Cr-5.6Al-0.6Y NASA thermal barrier coating system which was developed for aircraft gas turbines was significantly reduced in such environments. Two thermal barrier coating systems, Ca2SiO4/Ni-16.2Cr-5.6Al-0.6Y and ZrO2-8Y2O3/Ni-16.4Cr-5.1Al-0.15Y and a less insulative cermet coating system, 50 volume percent MgO-50 volume percent Ni-19.6Cr-17.1Al-0.97Y/Ni-16.2Cr-5.6Al-0.6Y, were identified as having much improved corrosion resistance compared to the standard coating.

Hodge, P. E.↗
NASA NTRS1978

Development and evaluation of die materials for use in the growth of silicon ribbons by the inverted ribbon growth process, task 2. LSSA project

The results of emission spectroscopic analysis indicate that molten silicon can remain in contact with hot-pressed Si3N4 (99.2 percent theoretical density) for prolonged periods without attaining the impurity content level of the nitride. Although MgO was used as binder, Mg was not found present in the silicon sessile drop in quantities much above the level initially present in the silicon source material. Preliminary experiments with EFG-type dies coated with CVD Si3N4 or CVD SiOxNy indicate that capillary rise does not occur readily in these dies. The same was found to be true of hot-pressed and reaction-sintered Si3N4 obtained commercially. However, when dies were formed by depositing CVD layers on shaped silicon slabs, a column of molten silicon was maintained in each CVD die while being heated in contact with a crucible of molten silicon. Preliminary wetting of dies appears necessary for EFG growth. Several ribbon growth experiments were performed from V-shaped dies.

Duffy, M. T.↗
NASA NTRS1979

Refractory-element-rich inclusions in CM meteorites

Refractory-element-rich inclusions of a variety of types occur in CM meteorites. Based on detailed study of Murchison and more limited investigations of Murray, Nogoya, Cold Bokkeveld and Mighei, the most common of these inclusions are small spinel-hibonite bodies rimmed with diopside. Bulk chemical compositions are estimated to be approximately 67% Al2O3, 21% MgO, 5.5% CaO, 5.5% TiO2, and 1% SiO2. Although this chemical composition does not agree closely with current theoretically predicted compositions of early condensate assemblages, the inclusions have many features which suggest that they are indeed early condensates. These include texture, mineralogical composition, sequence of minerals and magnesium isotopic composition of hibonite. The CM refractory inclusions exhibit several differences from those observed in Allende and other CV meteorites. These probably record variations in time, place or physical conditions of origin for the two cases.

Macdougall, J. D.↗
NASA NTRS1978

Chemical alterations of the Holbrook chondrite resulting from terrestrial weathering

Major, minor and trace element analysis as well as noble gas measurements were performed on three Holbrook chondrite samples collected fresh in 1912, in 1931, and in 1968. Decreases in metallic iron and MgO and an increase in total iron were detected. Elemental concentrations of Ti, Ca, Al, P, Mn, Ni, and Cr did not change significantly during weathering. While sodium might have been lost, and K slightly enriched, the trace elements C, Rb, and Sr increased at least two-fold during weathering. Gas abundances decreased, and the concentration ratios of noble gas nuclides varied unpredictably.

Gibson, E. K., Jr.↗
NASA NTRS1978

Grain size and the evolution of Luna 24 soils

The grain size distribution of six Luna 24 samples has been determined. These samples are characterized by a bimodal distribution which is indicative of soils formed primarily by mixing rather than by reworking. Although agglutinate content decreases with depth, it is not likely that the Luna 24 soils have undergone appreciable in situ reworking. Particle types and abundances in each of four size fractions have been determined petrographically. Mineral fragments are very abundant in all analyzed size fractions. Pyroxene and plagioclase increase slightly in abundance at finer grain sizes, but olivine decreases significantly. Compared with typical mare soils, the Luna 24 trends are anomalous. They are compatible with the hypothesis than many, if not most, of the mineral grains in the 20-250-micron fractions come from coarse-grain rocks having average mineral grain sizes greater than 250 microns. The mineralogy and chemistry of the coarse-grained rocks has not been well characterized, but there is evidence that at least some of them are higher in MgO than the analyzed finer-grained basalts.

Mckay, D. S.↗
NASA NTRS1978

Luna 24 - Lateral heterogeneity in the lunar crust

Results of petrological investigations of Luna 24 soil samples are compared with data from Apollo and other Luna missions in order to determine the large scale composition of the lunar surface and crust. Modal analysis of the 90 to 150 micron soil fraction and electron microprobe analysis of glass spheres and fragments of the Luna 24 core sample are presented. High and low MgO mare basalt compositional clusters are found, suggesting the derivation of southern Mare Crisium basalts from two magmas. A model of Luna 24 soils as being composed of a mixture of glass-poor mare material and glass-rich highland material is found to be consistent with glass and agglutinate modal abundances. Comparisons show that potassium, and by inference all lithophile trace elements, are depleted on the eastern limb of the moon relative to the central nearside. The observed depletion is interpreted to be characteristic of the underlying crust and mantle as well as the surface.

Warner, J. L.↗
NASA NTRS1978

On the origin of Luna 24 basalts and soils

Analyses of fine-grained very low titanium (VLT) basalt from the Luna 24 drill core suggest that a single homogeneous magma is represented by the sample. In particular, the small variation in MgO contents of the fine-grained basalt, together with the tight clustering of the compositions of brown glasses (which may be pyroclastic equivalents of the VLT basalt), provides evidence for the single-magma hypothesis. The high-Mg component in the soil samples, though not obviously explainable in petrographic terms, may be derived from material similar to olivine vitrophyre and its degraded products, or from some other high-Mg VLT basalt.

Ryder, G.↗
NASA NTRS1978

VLT mare basalts - Impact mixing, parent magma types, and petrogenesis

Available major-element compositions of very low-Ti (VLT) mare basalt lithic fragments and glasses are examined. Many of the lithic fragments contain relic mineral and sometimes lithic clasts, which suggests that many samples of VLT mare basalt glasses and lithic fragments were formed by impact melting. Therefore, some may have compositions that represent mixtures of VLT mare basalts and other types of rocks, not authentic igneous rock compositions. In spite of this problem, a number of possible parent magma compositions are identified. These hypothetical magmas vary in CaO/Al2O3(0.9-1.1), FeO (17 to 24 wt. %), Fe/(Fe + Mg) (0.34-0.46), and K2O (0-0.2 wt. %). Each may have differentiated to produce a wide variety of derivative magmas. Calculations involving olivine fractionation from the proposed parent magmas indicate the Luna 24 ferrobasalts might have formed from a magma whose composition is similar to that of a group of high-MgO (about 13-14 wt. %) Apollo 17 VLT mare basalt glasses.

Taylor, G. J.↗
NASA NTRS1978

Major and trace element chemistry of Luna 24 samples from Mare Crisium

Atomic absorption spectrometry and instrumental neutron activation analysis were employed to analyze six Luna 24 soils for major and trace elements. The analysis revealed well-mixed soils, though size fractions of each of the soils showed quite dissimilar compositions. Thus the regolith apparently has not been extensively reworked. Noritic breccia admixed preferentially to the finest size fractions and differential comminution of one or more other soil components accounted for the observed elemental distributions as a function of grain size. The ferrobasalt composition and one or more components with higher MgO contents have been identified in the samples.

Blanchard, D. P.↗
NASA NTRS1979

Convection, composition, and the thermal state of the lower mantle

A thermal model for the lower mantle is presented, which is constructed from the petrologically derived estimates of the temperature in the transition zone and from an adiabat based on the thermal properties of MgO and SiO2 measured at high pressures. Superadiabatic contributions to the geotherm through the lower mantle are negligibly small. A thermal boundary layer is required at the base of the mantle in order to satisfy an estimate of the lowest possible temperature in the core. It is suggested that a thermal boundary layer is associated with a chemical discontinuity either at the top of the lower mantle or near its base.

Jeanloz, R.↗
NASA NTRS1979

Development of silicon nitride of improved toughness

The application of reaction sintered Si2N4 energy absorbing surface layers to hot-pressed Si3N4 was investigated. The surface layer was formed by in-place nitridation of silicon powder. It was found that reaction sintered Si3N4 layers of 1 mm thickness, fabricated from either -100, +200, -200, or -325 mesh Si powder and nitrided in 96% N2/4% H2 so that approximately 20-25 vol % unnitrided Si remained in the layer, resulted in a sevenfold increase in ballistic impact resistance of a 0.64 cm thick hot-pressed SI3N4 substrate from RT 1370 C. Both NC-132 SI3N4, with MgO additive, and NCX-34 Si3N4, with Y2O3 additive, were evaluated as substrate material. The finer grain size -200 and -325 mesh nitrided Si layers were for their smoothness and relatively high density. It was found that nitriding in N2/H2 mixtures, rather than pure N2, resulted in a microstructure that did not substantially degrade the strength of the hot-pressed Si3N4 substrate. Thermal cycling tests on the RSSN/HPSN combinations from 200 C to 1370 C for 75 cycles in air did not degrade the impact resistance nor the interfacial bonding, although a large amount of internal silica formation occurred within the RSSN layer. Mach 0.8, 5 hr, hot gas erosion tests showed no surface recession of RSSN layers at 1200 C and slight surface recession at 1370 C.

Brennan, J. J.↗
NASA NTRS1979

Reactions of calcium orthosilicate and barium zirconate with oxides and sulfates of various elements

Calcium orthosilicate and barium zirconate were evaluated as the insulation layer of thermal barrier coatings for air cooled gas turbine components. Their reactions with various oxides and sulfates were studied at 1100 C and 1300 C for times ranging up to 400 and 200 hours, respectively. These oxides and sulfates represent potential impurities or additives in gas turbine fuels and in turbine combustion air, as well as elements of potential bond coat alloys. The phase compositions of the reaction products were determined by X-ray diffraction analysis. BaZrO3 and 2CaO-SiO2 both reacted with P2O5, V2O5, Cr2O3, Al2O3, and SiO2. In addition, 2CaO-SiO2 reacted with Na2O, BaO, MgO, and CoO and BaZrO3 reacted with Fe2O3.

Zaplatynsky, I.↗
NASA NTRS1980

Effects of oxide additions and temperature on sinterability of milled silicon nitride

Specimens of milled alpha-Si3N4 with 0 to 5.07 equivalent percent of oxide additions were pressureless sintered at 1650 to 1820 C for 4 hours in nitrogen while covered with powdered Si3N4 + SiO2. Densities of less than or equal to 97.5 percent resulted with approximately 2.5 equivalent percent of MgO, CeO2, Y2O3, and three mixtures involving these oxides. Densities of greater than or equal to 94 percent were obtained with approximately 0.62 equivalent percent of the same additives. At most temperatures, best sinterability (density maxima) was obtained with 1.2 to 2.5 equivalent percent additive.

Arias, A.↗
NASA NTRS1980

MCSCF wave functions for excited states of polar molecules - Application to BeO

A previously reported multi-configuration self-consistent field (MCSCF) algorithm based on the generalized Brillouin theorem is extended in order to treat the excited states of polar molecules. In particular, the algorithm takes into account the proper treatment of nonorthogonality in the space of single excitations and invokes, when necessary, a constrained optimization procedure to prevent the variational collapse of excited states. In addition, a configuration selection scheme (suitable for use in conjunction with extended configuration interaction methods) is proposed for the MCSCF procedure. The algorithm is used to study the low-lying singlet states of BeO, a system which has not previously been studied using an MCSCF procedure. MCSCF wave functions are obtained for three 1 Sigma + and two 1 Pi states. The 1 Sigma + results are juxtaposed with comparable results for MgO in order to assess the generality of the description presented here.

Bauschlicher, C. W., Jr.↗
NASA NTRS1979

Apollo 17 high-Ti mare basalts - New bulk compositional data, magma types, and petrogenesis

Bulk compositional and mineral chemical data for 28 previously unanalyzed samples support the classification of Apollo-17 high-Ti mare basalts into three-types (A, B, and C), defined on the basis of analyses of fine-grained basalts. The most MgO- and TiO2-rich fine-grained basalts of these types appear to be the best choices for representing the compositions of the parent magmas.

Warner, R. D.↗