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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 847 records · Page 47
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

The CRISPR RNA-guided surveillance complex in Escherichia coli accommodates extended RNA spacers

Bacteria and archaea acquire resistance to foreign genetic elements by integrating fragments of foreign DNA into CRISPR (clustered regularly interspaced short palindromic repeats) loci. In Escherichia coli , CRISPR-derived RNAs (crRNAs) assemble with Cas proteins into a multi-subunit surveillance complex called Cascade (CRISPR-associated complex for antiviral defense). Cascade recognizes DNA targets via protein-mediated recognition of a protospacer adjacent motif and complementary base pairing between the crRNA spacer and the DNA target. Previously determined structures of Cascade showed that the crRNA is stretched along an oligomeric protein assembly, leading us to ask how crRNA length impacts the assembly and function of this complex. We found that extending the spacer portion of the crRNA resulted in larger Cascade complexes with altered stoichiometry and preserved in vitro binding affinity for target DNA. Longer spacers also preserved the in vivo ability of Cascade to repress target gene expression and to recruit the Cas3 endonuclease for target degradation. Lastly, longer spacers exhibited enhanced silencing at particular target locations and were sensitive to mismatches within the extended region. These findings demonstrate the flexibility of the Type I-E CRISPR machinery and suggest that spacer length can be modified to fine-tune Cascade activity.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2011

Inelastic neutron scattering study of the hydrogenated (Zr{sub 55}Cu{sub 30}Ni{sub 5}Al{sub 10}){sub 99}Y{sub 1} bulk metallic glass

The lattice dynamics of both as-charged and room-temperature-aged, hydrogenated bulk metallic glasses (BMGs) with the stoichiometry of (Zr{sub 55}Cu{sub 30}Ni{sub 5}Al{sub 10}){sub 99}Y{sub 1} have been investigated by inelastic neutron scattering at 4 K. The vibrational density of states of dissolved hydrogen atoms in the as-charged BMG shows a broad band maximized at {approx}135 meV. Three months of room-temperature aging caused this band to narrow somewhat, losing intensity at both lower and higher energies. The results suggest that hydrogen atoms preferably occupy the interstitial tetrahedral-like t sites comprised of four Zr atoms, with a local atomic arrangement similar to that of {delta}-zirconium hydride. At higher hydrogen loadings, hydrogen atoms can also occupy octahedral-like and/or more Zr-deficient tetrahedral-like o sites. The spectral differences between the as-charged and aged specimens and accompanying Vickers hardness measurements suggested that around two-thirds of the hydrogen atoms in the as-charged specimen were trapped in the strongly binding t sites and relatively immobile at room temperature, while the remaining one-third occupied the more weakly binding o-sites and disappeared from the specimen upon aging. These latter sites most likely play a crucial role in the diffusion of hydrogen in BMGs at room temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Spin-glass Behavior of Semiconducting KxFe2−yS2

We report the discovery of K{sub x}Fe{sub 2-y}S{sub 2} single crystals, isostructural to K{sub x}Fe{sub 2-y}Se{sub 2} superconductors. The sulfide compound is a small gap semiconductor and shows spin-glass behavior below 32 K. Our results indicate that stoichiometry, defects, and the local environment of FeCh (Ch=S,Se) tetrahedra have important effects on the physical properties of isostructural and isoelectronic K{sub x}Fe{sub 2-y}Ch{sub 2} compounds.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Magnetic Structure of Fe-doped CoFe(2)O(4) Probed by X-ray Magnetic Spectroscopies

The magnetic properties of iron-doped cobalt ferrite (Co{sub 1-x}Fe{sub 2+x}O{sub 4}) (001) thin films grown epitaxially on MgO (001) substrates are investigated by superconducting quantum interference device magnetometry and soft x-ray magnetic linear and circular dichroisms. All Co{sub 1-x}Fe{sub 2+x}O{sub 4} (0.01 {le} x {le} 0.63) samples have out-of-plane magnetic easy axes and large coercive fields, unlike Fe{sub 3}O{sub 4}, due to a large Co{sup 2+} orbital moment. The magnetic moments for those samples are significantly reduced from their bulk values; however, as x increases, the magnetic moments tend nearer to their bulk values and increase more rapidly as x approaches 1. This reduction in magnetic moment is attributed to spin canting among the Co{sup 2+} cations, owing to a small in-plane tensile strain in the film and to an increased antiferromagnetic alignment among all the cations caused by a partially inverse spinel cubic structure and the likely presence of antiphase boundaries. Our results show that small changes in stoichiometry can lead to significant changes in the magnetic moment of Co{sub 1-x}Fe{sub 2+x}O{sub 4}, especially at large values of x.

42 ENGINEERING↗
DOE OSTI2011

Correlation between electrical properties and thermodynamic stability of ACoO{sub 3-{delta}} perovskites (A= La, Pr, Nd, Sm, Gd)

For perovskites with the general formula ACoO{sub 3-{delta}} (A = La, Pr, Nd, Sm, and Gd) the influence of the A-site cation on the electrical conductivity, electronic structure, thermodynamic stability, and oxygen stoichiometry was studied. The perovskite oxide powders were produced by a combined citric acid and ethylenediaminetetraacetic acid complexing method. Ceramic specimens sintered at 1100 deg. C in air were single-phase perovskites. With increasing temperature, the electrical conductivity shows three discrete regimes. All compositions show semiconductivity up to a transition temperature of {approx}300 deg. C-450 deg. C and then behave like metallic conductors. The activation energies for the semiconductivity, as well as the transition temperatures to the metallic-like conduction, decrease monotonically with increasing pseudocubic lattice parameters, i.e., with increasing ionic radii of the A cation. This behavior correlates with decreasing oxygen nonstoichiometry and increased thermodynamic stability. The highest conductivity and the lowest activation energy of 0.66 eV were found for LaCoO{sub 3-{delta},} which also had the lowest semiconductor-metal transition temperature at 269 deg. C, the lowest oxygen nonstoichiometry of {delta}= 0.008, and the highest Gibbs free energy change for the decomposition reaction of 42.37 kJ/mol at 850 deg. C. GdCoO{sub 3-{delta}} had the highest oxygen nonstoichiometry with {delta}0.032, a high activation energy of 1.19 eV for the semiconductivity with a high transition temperature at 452 deg. C, and the lowest Gibbs free energy change of 26.54 kJ/mol at 850 deg. C. X-ray absorption spectroscopy data imply an increasing Co low-spin character with decreasing cation radius from La to Gd, while an increase in temperature increases the number of holes or Co 3d bandwidth. This correlates well with the electrical conductivity data.

36 MATERIALS SCIENCE↗
DOE OSTI2011

First-principles study of misfit strain-stabilized ferroelectric SnTiO{<_3}.

Toxicity of lead and bismuth has motivated an active search for isovalent ferroelectric oxides free of these elements. Using first-principles density-functional calculations, we survey Sn(ii) titanates with SnTiO{sub 3} stoichiometry to evaluate the phase stability of polar and nonpolar polymorphs: we predict a tetragonal perovskite P4mm phase with a large axial ratio (c/a=1.134) and ferroelectric polarization (1.28 C/m{sup 2}) to be the ground-state equilibrium structure. We also show that heteroepitaxial thin films of perovskite SnTiO{sub 3} promote the stereochemical lone-pair activity and simultaneously enable control over the direction of the net electric polarization and magnitude of the electronic band gap. Finally, we examine the consequence of antisite defects on the polar cation displacements by studying the substitution of Sn on Ti sites. We demonstrate that local metallic screening resulting from site substitution diminishes the magnitude of the polar distortions but does not completely quench it. Based on these calculations, we suggest that polar perovskite SnTiO{sub 3} ferroelectrics are viable thin-film alternatives to Pb- and Bi-containing oxides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Quantum paramagnetism in a non-Kramers rare-earth oxide: Monoclinic Pr 2 Ti 2 O 7

Little is so far known about the magnetism of the A 2 B 2 O 7 monoclinic layered perovskites that replace the spin-ice supporting pyrochlore structure for r A /r B > 1.78. We show that high quality monoclinic Pr 2 Ti 2 O 7 single crystals with a three-dimensional network of non-Kramers Pr 3+ ions that interact through edge-sharing superexchange interactions, form a singlet ground-state quantum paramagnet that does not undergo any magnetic phase transitions down to, at least, 1.8 K. The chemical phase stability, structure, and magnetic properties of the layered perovskite Pr 2 Ti 2 O 7 were investigated using x-ray diffraction, transmission electron microscopy, and magnetization measurements. Synthesis of polycrystalline samples with the nominal compositions of Pr 2 Ti 2+x O 7 (–0.16 ≤ x ≤ 0.16 ) showed that deviations from the Pr 2 Ti 2 O 7 stoichiometry lead to secondary phases of related structures including the perovskite phase Pr 2/3 TiO 3 and the orthorhombic phases Pr 4 Ti 9 O 24 and Pr 2 TiO 5 . No indications of site disordering (stuffing and antistuffing) or vacancy defects were observed in the Pr 2 Ti 2 O 7 majority phase. A procedure for growth of high-structural-quality stoichiometric single crystals of Pr 2 Ti 2 O 7 by the traveling solvent floating zone method is reported. Thermomagnetic measurements of single-crystalline Pr 2 Ti 2 O 7 reveal an isolated singlet ground state that we associate with the low-symmetry crystal electric-field environments that split the (2J + 1 = 9)-fold degenerate spin-orbital multiplets of the four differently coordinated Pr 3+ ions into 36 isolated singlets resulting in an anisotropic temperature-independent van Vleck susceptibility at low T. Here, a small isotropic Curie term is associated with 0.96(2)% noninteracting Pr 4+ impurities.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Neural Network Molecular Dynamics at Scale

Neural network molecular dynamics (NNMD) simulations could revolutionize atomistic modeling of materials with quantum-mechanical accuracy at a fraction of computational cost. However, popular NNMD frameworks are generally implemented for a single computing node, and conventional energy-based NN models still suffer from large time-to-solution (T2S), prohibiting the application of NNMD to challenging materials simulations encompassing large spatiotemporal scales. Consequently, no leadership-scale NNMD simulation has thus far been reported. Here, we present a scalable parallel NNMD software (RXMD-NN) based on our scalable reactive molecular dynamics simulation engine named RXMD. RXMD-NN has achieved high scalability up to 786,432 IBM BlueGene/Q cores involving 1.7 billion atoms. Furthermore, we have achieved 4.6-fold reduction of T2S by using a novel network that directly predicts atomic forces from feature vectors. Reduced T2S has for the first time allowed the study of large-scale off-stoichiometry effect in a widely used phase change material, Ge 2 Se 2 Te 5 , thereby resolving its "firstsharp diffraction peak mystery".

fist sharp diffraction peak↗
DOE OSTI2011

Nanoscale Measurements of the Surface Photovoltage in Cu(In,Ga)Se2, Cu2ZnSnS4, and Cu2ZnSnSe4 Thin Films: The Role of the Surface Electronics on the Efficiency of Solar Cells

We report on recent advances in the development of nanoscale measurements of the surface photovoltage (SPV) based on scanning tunneling microscopy (STM) and its application to the kesterites Cu{sub 2}ZnSnS{sub 4} (CZTS) and Cu{sub 2}ZnSnSe{sub 4} (CZTSe). One critical aspect of the electronic structure of Cu(In, Ga)Se{sub 2} (CIGS) that has yet to be determined in their related kesterite compounds is the character of the surface electronics. In CIGS, spontaneous deviations in the stoichiometry of the surface cause a depletion (or even a type inversion) region that reinforces the CIGS homojunction. First-principle calculations predict that this inversion region will be more difficult to form in CZTS. In this contribution, the characteristics of the surface space charge region for both CIGS and CZTS(e) are investigated by STM. The implications of the results of these measurements on the future development of CZTS solar cells will be discussed.

14 SOLAR ENERGY↗
DOE OSTI2022

Revisiting the growth rate hypothesis: Towards a holistic stoichiometric understanding of growth

The growth rate hypothesis (GRH) posits that variation in organismal stoichiometry (C:P and N:P ratios) is driven by growth-dependent allocation of P to ribosomal RNA. The GRH has found broad but not uniform support in studies across diverse biota and habitats. We synthesise information on how and why the tripartite growth-RNA-P relationship predicted by the GRH may be uncoupled and outline paths for both theoretical and empirical work needed to broaden the working domain of the GRH. We found strong support for growth to RNA (r 2 = 0.59) and RNA-P to P (r 2 = 0.63) relationships across taxa, but growth to P relationships were relatively weaker (r 2 = 0.09). Together, the GRH was supported in ~50% of studies. Mechanisms behind GRH uncoupling were diverse but could generally be attributed to physiological (P accumulation in non-RNA pools, inactive ribosomes, translation elongation rates and protein turnover rates), ecological (limitation by resources other than P), and evolutionary (adaptation to different nutrient supply regimes) causes. These factors should be accounted for in empirical tests of the GRH and formalised mathematically to facilitate a predictive understanding of growth.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2018

Understanding variation in salamander ionomes: A nutrient balance approach

1. Ecological stoichiometry uses information on a few key biological elements (C, N and P) to explain complex ecological patterns. Although factors driving variation in these elements are well established, expanding stoichiometric principles to explore dynamics of the many other essential elements comprising biological tissues (i.e. the ionome) is needed to determine their metabolic relationships and better understand biological control of elemental flows through ecosystems. 2. Here in this paper, we report observations of ionomic variation in two species of salamander (Ambystoma opacum and A. talpoideum) across ontogenic stages using specimens from biological collections of two wetlands sampled over a 30-year period. This unique data set allowed us to explore the extent of ionomic variation between species, among ontogenic stages, between sites and through time. 3. We found species- and, to a lesser extent, site-specific differences in C, N and P along with 13 other elements forming salamander ionomes but saw no evidence of temporal changes. Salamander ionomic composition was most strongly related to ontogeny with relatively higher concentrations of many elements in adult males (i.e. Ca, P, S, Mg, Zn and Cu) compared to metamorphic juveniles, which had greater amounts of C, Fe and Mn. 4. In addition to patterns of individual elements, covariance among elements was used to construct multi-elemental nutrient balances, which revealed differences in salamander elemental composition between species and sites and changes in elemental proportions across ontogenic development. These multi-elemental balances distinguished among species-site-ontogenic stage groups better than using only C, N and P. 5. Overall, this study highlights the responsiveness of consumer ionomes to life-history and environmental variation while reflecting underlying relationships among elements tied to biological function. As such, ionomic studies can provide important insights into factors shaping consumer elemental composition and for predicting how these changes might affect higher-order ecological processes.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2017

Grain growth and pore coarsening in dense nano-crystalline UO 2+ x fuel pellets

Dense nano-sized UO 2+x pellets are synthesized by spark plasma sintering with controlled stoichiometries (UO 2.03 and UO 2.11 ) and grain sizes (~100 nm), and subsequently isothermally annealed to study their effects on grain growth kinetics and microstructure stability. The grain growth kinetics is determined and analyzed focusing on the interaction between grain boundary migration, pore growth and coalescence. Grains grow much bigger in nano-sized UO 2.11 than UO 2.03 upon thermal annealing, consistent with the fact that hyper-stoichiometric UO 2+x is beneficial for sintering due to enhanced U ion diffusion from excessive O ion interstitials. The activation energies of the grain growth for UO 2.03 and UO 2.11 are determined as ~1.0 and 1.3~2.0 eV, respectively. As compared with the micron-sized UO 2 in which volumetric diffusion dominates the grain coarsening with an activation energy of ~3.0 eV, the enhanced grain growth kinetics in nano-sized UO 2+x suggests that grain boundary diffusion controls grain growth. Lastly, the higher activation energy of more hyper-stoichiometric nano-sized UO 2.11 may be attributed to the excessive O interstitials pinning grain boundary migration.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Influence of temperature on the hydrogenated amorphous carbon films prepared by plasma-enhanced chemical vapor deposition

Hydrogenated amorphous carbon (a-C:H) films were deposited in a plasma-enhanced chemical vapor deposition (PECVD) system. The substrate temperature at deposition was found to have significant effects on the film stoichiometry, sp{sup 2} phase, and optical properties. Raman spectroscopy reveals an increase in sp{sup 2}-bonded carbon and a continual structure ordering of the sp{sup 2} phase with increasing substrate temperature at deposition. Thermal desorption spectroscopy analysis revealed that the onset temperature of CH{sub 4} effusion of PECVD a-C:H films increase with increasing substrate temperatures, implicating enhanced structural stability via elevating the substrate temperature at deposition. The extinction coefficient k measured from spectroscopic ellipsometry gradually increases with increasing substrate temperature at deposition, due possibly to the graphitization effect which decreases the optical gap resulting in higher k.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2010

Structural and optical properties of yttrium oxide thin films for planar waveguiding applications

Thin films of yttrium oxide, Y{sub 2}O{sub 3}, were deposited by reactive sputtering and reactive evaporation to determine their suitability as a host for a rare earth doped planar waveguide upconversion laser. The optical properties, structure, and crystalline phase of the films were found to be dependent on the deposition method and process parameters. X-ray diffraction analysis on the ''as-deposited'' thin films revealed that the films vary from amorphous to highly crystalline with a small broad peak at 29 deg. corresponding to the <222> reflections of Y{sub 2}O{sub 3}. The samples with the polycrystalline structure had a stoichiometry close to bulk cubic Y{sub 2}O{sub 3}. Scanning electron microscopy imaging revealed a regular column structure confirming their crystalline nature. The thin film layers which allowed guiding in both visible and infrared regions had lower refractive indices, higher oxygen content, and a more amorphous structure. Higher oxygen pressures during the deposition lead to a more amorphous layer.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Epitaxial growth of CoxMnySiz (111) thin films in the compositional range around the Heusler alloy Co2MnSi

Epitaxial growth and structural properties of Co{sub x}Mn{sub y}Si{sub z} thin films on Ge (111) substrates, including the Heusler alloy Co{sub 2}MnSi (111), have been studied using combinatorial molecular beam epitaxy (MBE) techniques. In situ reflection high energy electron diffraction and ex situ x-ray diffraction experiments show that high quality coherent MBE growth with fcc (111) stacking can be achieved over a relatively large composition space that includes Co{sub 2}MnSi. The highest structural and chemical ordering is observed near the composition of Co{sub 0.63}Mn{sub 0.14}Si{sub 0.23} rather than that at the Heusler stoichiometry of Co{sub 2}MnSi. The in-plane crystallographic axis of the fcc film exhibits a 60{sup o} rotation with respect to that of the Ge substrate. The rotation appears to be originated at the film-substrate interface, as a result of the symmetry and stacking of the Ge (111) surface reconstruction.

36 MATERIALS SCIENCE↗