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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 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Factors governing Yb magnetism in Yb{sub 0.95}PtIn{sub 2} and other MgCuAl{sub 2}-type structures

The ternary compound 'YbPtIn{sub 2}' has been synthesized and its crystal structure determined using single crystal X-ray diffraction techniques. The compound crystallizes in the MgCuAl{sub 2} structure, a ternary structure type often formed when rare earth metals, Group 10 transition metals, and indium are present in a 1:1:2 ratio. The unit cell is C-centered orthorhombic, space group Cmcm (no. 63) with lattice parameters of a=4.3410(1) A, b=10.3230(2) A, and c=7.8510(2) A. Evidence is found for a substantial non-stoichiometry on the Yb site, in contrast to the Pt and In sites which behave as expected, and it is believed that the stoichiometry of this compound is more properly described as Yb{sub 0.95}PtIn{sub 2}. Magnetic and resistivity measurements reveal that 'YbPtIn{sub 2}' is a weakly interacting, non-magnetic metal exclusively containing Yb{sup 2+}, and with a resistivity ({rho}) varying from 34 {mu}{Omega} cm at room temperature to a residual resistivity {rho}{sub 0} of 23 {mu}{Omega} cm at 1.8 K. Density functional theory calculations suggest that the observed diamagnetic Yb behavior is expected for this compound, but that certain substitutions on the Pt or In site could help stabilize magnetic Yb{sup 3+} and potentially induce heavy fermion behavior. - Graphical abstract: The crystal structure, magnetic behavior, and temperature-dependent resistivity of the new ternary compound Yb{sub 0.95}PtIn{sub 2.} Highlights: Black-Right-Pointing-Pointer Flux growth method used to produce Yb{sub 0.95}PtIn{sub 2} single crystals. Black-Right-Pointing-Pointer Found rare example of non-stoichiometry in a MgCuAl{sub 2}-type structure. Black-Right-Pointing-Pointer The resistivity and diamagnetic response of Yb{sub 0.95}PtIn{sub 2} were modeled. Black-Right-Pointing-Pointer DFT correctly predicts presence of Yb{sup 2+} in Yb{sub 0.95}PtIn{sub 2}. Black-Right-Pointing-Pointer COHP results suggest other MgCuAl{sub 2}-type indides that may contain paramagnetic Yb{sup 3+}.

36 MATERIALS SCIENCE↗
DOE OSTI2011

The role of boron segregation in enhanced thermoelectric power factor of CoSi{sub 1-x}B{sub x} alloys

We report on the influence of boron segregation on the thermoelectric properties of CoSi. Contrary to previous suggestions, and in stark contrast to aluminum substitution, boron does not enter the lattice on the Si site, but rather segregates to the grain boundaries in these alloys. Through a combination of x-ray diffraction, scanning electron microscope, and scanning Auger techniques, we present clear evidence of the formation of a CoB phase at the grain boundaries. Consistent with the failure of B to substitute for Si, we observe no changes in the electron concentration or the Seebeck coefficient under boron substitution. The electrical resistivity, on the other hand, displays a non-monotonic behavior with increasing boron concentration, first decreasing for small amounts of boron, before increasing at higher levels of substitution. We attribute this behavior to a combination of an initial healing effect of boron on microcracks, followed by the eventual increase in electron scattering by the secondary CoB phase at higher concentrations.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Seven years of aerosol scattering hygroscopic growth measurements from SGP: Factors influencing water uptake: Aerosol Scattering Hygroscopic Growth

Long-term measurements of changes in the aerosol scattering coefficient hygroscopic growth at the U.S. Department of Energy Southern Great Plains site provide information on the seasonal as well as size and chemical dependence of aerosol hygroscopic growth. Annual average sub 10 um fRH values (the ratio of aerosol scattering at 85%/40% RH) were 1.75 and 1.87 for the gamma and kappa fit algorithms, respectively. The study found higher growth rates in the winter and spring seasons that correlated with high aerosol nitrate mass fraction. FRH, exhibited strong, but differing correlations with the scattering Ångström exponent and backscatter fraction, two optical size-dependent parameters. The aerosol organic fraction had a strong influence, with fRH decreasing with increases in the organic mass fraction and absorption Ångström exponent and increasing with the aerosol single scatter albedo. Uncertainty analysis if the fit algorithms revealed high uncertainty at low scattering coefficients and slight increases in uncertainty at high RH and fit parameters values.

Jefferson, A.↗
DOE OSTI2018

The role of environmental driving factors in historical and projected carbon dynamics of wetland ecosystems in Alaska

Abstract Wetlands are critical terrestrial ecosystems in Alaska, covering ~177,000 km 2 , an area greater than all the wetlands in the remainder of the United States. To assess the relative influence of changing climate, atmospheric carbon dioxide (CO 2 ) concentration, and fire regime on carbon balance in wetland ecosystems of Alaska, a modeling framework that incorporates a fire disturbance model and two biogeochemical models was used. Spatially explicit simulations were conducted at 1‐km resolution for the historical period (1950–2009) and future projection period (2010–2099). Simulations estimated that wetland ecosystems of Alaska lost 175 Tg carbon (C) in the historical period. Ecosystem C storage in 2009 was 5,556 Tg, with 89% of the C stored in soils. The estimated loss of C as CO 2 and biogenic methane (CH 4 ) emissions resulted in wetlands of Alaska increasing the greenhouse gas forcing of climate warming. Simulations for the projection period were conducted for six climate change scenarios constructed from two climate models forced under three CO 2 emission scenarios. Ecosystem C storage averaged among climate scenarios increased 3.94 Tg C/yr by 2099, with variability among the simulations ranging from 2.02 to 4.42 Tg C/yr. These increases were driven primarily by increases in net primary production (NPP) that were greater than losses from increased decomposition and fire. The NPP increase was driven by CO 2 fertilization (~5% per 100 parts per million by volume increase) and by increases in air temperature (~1% per °C increase). Increases in air temperature were estimated to be the primary cause for a projected 47.7% mean increase in biogenic CH 4 emissions among the simulations (~15% per °C increase). Ecosystem CO 2 sequestration offset the increase in CH 4 emissions during the 21st century to decrease the greenhouse gas forcing of climate warming. However, beyond 2100, we expect that this forcing will ultimately increase as wetland ecosystems transition from being a sink to a source of atmospheric CO 2 because of (1) decreasing sensitivity of NPP to increasing atmospheric CO 2 , (2) increasing availability of soil C for decomposition as permafrost thaws, and (3) continued positive sensitivity of biogenic CH 4 emissions to increases in soil temperature.

Lyu, Zhou↗
DOE OSTI2012

Factors affecting the microstructural stability and durability of thermal barrier coatings fabricated by air plasma spraying

The high-temperature behavior of high-purity, low-density (HP-LD) air plasma sprayed (APS) thermal barrier coatings (TBCs) with NiCoCrAlY bond coats deposited by argon-shrouded plasma spraying is described. The high purity yttria-stabilized zirconia resulted in top coats which are highly resistant to sintering and transformation from the metastable tetragonal phase to the equilibrium mixture of monoclinic and cubic phases. The thermal conductivity of the as-processed TBC is low but increases during high temperature exposure even before densification occurs. The porous topcoat microstructure also resulted in good spallation resistance during thermal cycling. The actual failure mechanisms of the APS coatings were found to depend on topcoat thickness, topcoat density, and the thermal cycle frequency. The failure mechanisms are described and the durability of the HP-LD coatings is compared with that of state-of-the-art electron beam physical vapor deposition TBCs.

36 MATERIALS SCIENCE↗