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At least 991 records · Page 55
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Low temperature atomic layer deposition of PbO 2 for electrochemical applications

A low temperature atomic layer deposition (ALD) process for PbO 2 was developed using bis(1-dimethylamino-2-methyl-2-propanolate)lead(II), Pb(DMAMP) 2 , and O 3 as the reactants, with a high growth rate of 2.6 Å/cycle. PbO 2 readily reduces under low oxygen partial pressures at moderate temperatures making it challenging to deposit ALD PbO 2 from Pb 2+ precursors. However, thin films deposited with this process showed small crystalline grains of α-PbO 2 and β-PbO 2 , without signs of reduced PbO x phases. The ALD PbO 2 thin films show the high electrical conductivity characteristic of bulk PbO 2 . In situ measurements of ALD PbO 2 film conductivity during growth suggest a reaction mechanism by which sub-surface oxygen mobility contributes to the growth of resistive PbO or PbO x during the Pb(DMAMP) 2 surface reaction step, which is only fully oxidized from Pb 2+ to Pb 4+ during the O 3 reaction step. These films were electrochemically reduced to PbSO 4 in H 2 SO 4 and then reoxidized to PbO 2 , demonstrating their suitability for use as an electrode material for fundamental battery research and other electrochemical applications.

atomic layer deposition↗
DOE OSTI2022

Growth and structural transitions of core-shell nanorods in nanocrystalline Al-Ni-Y

Unique nanorod precipitates with a core-shell structure are found to nucleate from the grain boundaries of a bulk nanocrystalline Al-Ni-Y alloy fabricated via powder consolidation, contributing significantly to stabilization and strengthening. The local structure, chemistry, and evolution of these features during annealing are reported here. In the as-consolidated state, the nanorods can be either structurally ordered or disordered, yet a consistent chemical patterning is found where the core is primarily Al plus C while the shell is enriched with Y. As annealing time increases, more nanorods transform to an ordered structure as they coarsen while the core composition remains unchanged. In contrast, the shell chemistry transitions from Y-rich to Ni-rich with longer annealing treatments, most likely due to the different diffusivities of Y and Ni in Al. Furthermore, a spatial and chemical correlation between the nanorods and amorphous complexions is observed, suggesting that these complexions serve as preferential nucleation sites.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Novel architectures for stabilization of Mn-rich cathodes: a high-valent approach to interfaces

Lithium- and manganese-rich (LMR) layered oxides continue to generate significant interest as promising, earth-abundant cathode materials for next-generation Li-ion batteries. In spite of their attractive capacity and cost advantages, a few long-standing challenges still hamper their widespread adoption, with manganese dissolution being one of the most persistent and vexing issues. In the present study, we explore the incorporation of Sb5+ as a high-valent cation and exploit its ability to form unique lithium-rich surface and grain-boundary structures that can integrate directly with the LMR lattice. When synthesized under appropriate conditions, Sb5+ orders strongly with Li+ to form localized Li+–Sb5+ motifs, which play a key role in restructuring the surface and grain-boundary regions. These restructured regions act as protective, stabilizing entities that substantially suppress electrolyte-driven side reactions, reduce impedance growth, limit manganese dissolution, and help retain cyclable lithium during long-term electrochemical cycling. Further improvements of the electrochemical performance of Sb-treated LMR were achieved using a well-known additive to mitigate Mn dissolution and highlight the synergistic effects of combined strategies. Overall, this work showcases how high-valent elements such as Sb5+ can help tailor the surface and intergranular regions and work in synergy with other modifiers (e.g. electrolyte additives), enhancing the cycle life and practical viability of LMR cathodes for use in graphite-based full cells.

Mallick, Subhadip [Argonne National Laboratory (AN↗
NASA NTRS1979

Some TEM observations of Al2O3 scales formed on NiCrAl alloys

The microstructural development of Al2O3 scales on NiCrAl alloys has been examined by transmission electron microscopy. Voids were observed within grains in scales formed on a pure NiCrAl alloy. Both voids and oxide grains grew measurably with oxidation time at 1100 C. The size and amount of porosity decreased towards the oxide-metal growth interface. The voids resulted from an excess number of oxygen vacancies near the oxidemetal interface. Short-circuit diffusion paths were discussed in reference to current growth stress models for oxide scales. Transient oxidation of pure, Y-doped, and Zr-doped NiCrAl was also examined. Oriented alpha-(Al, Cr)2O3 and Ni(Al, Cr)2O4 scales often coexisted in layered structures on all three alloys. Close-packed oxygen planes and directions in the corundum and spinel layers were parallel. The close relationship between oxide layers provided a gradual transition from initial transient scales to steady state Al2O3 growth.

Smialek, J.↗
NASA NTRS1979

Some TEM observations of Al2O3 scales formed on NiCrAl alloys

The microstructural development of Al2O3 scales on NiCrAl alloys has been examined by transmission electron microscopy. Voids have been observed within grains in scales formed on a pure NiCrAl alloy. Both voids and oxide grains grew measurably with oxidation time at 1100 C. The size and amount of porosity decreased towards the oxide-metal growth interface. It was postulated that the voids resulted from an excess number of oxygen vacancies near the oxide-metal interface. Short-circuit diffusion paths were discussed in reference to current growth stress models for oxide scales. Transient oxidations of pure, Y-doped, and Zr-doped NiCrAl was also examined. Oriented alpha-(Al,Cr)2O3 and Ni(Al,Cr)2O4 scales often coexisted in layered structures on all three alloys. Close-packed oxygen planes and directions in the corundum and spinel layers were parallel. The close relationships between oxide layers provided a gradual transition from initial transient scales to steady state Al2O3 growth.

Smialek, J.↗
NASA NTRS1984

Analysis techniques for tracer studies of oxidation

Analysis techniques to obtain quantitative diffusion data from tracer concentration profiles were developed. Mass balance ideas were applied to determine the mechanism of oxide growth and to separate the fraction of inward and outward growth of oxide scales. The process of inward oxygen diffusion with exchange was theoretically modelled and the effect of lattice diffusivity, grain boundary diffusivity and grain size on the tracer concentration profile was studied. The development of the tracer concentration profile in a growing oxide scale was simulated. The double oxidation technique was applied to a FeCrAl-Zr alloy using 0-18 as a tracer. SIMS was used to obtain the tracer concentration profile. The formation of lacey oxide on the alloy was discussed. Careful consideration was given to the quality of data required to obtain quantitative information.

Basu, S. N.↗
NASA NTRS2002

Superhard Nanocrystalline Homometallic Stainless Steel on Steel for Seamless Coatings

The objective of this work is to deposit nanocrystalline stainless steel onto steel substrates (homometallic) for enhanced wear and corrosion resistance. Homometallic coatings provide superior adhesion, and it has been shown that ultrafine-grained materials exhibit the increased hardness and decreased permeability desired for protective coatings. Nanocrystals will be produced by controlling nucleation and growth and use of an ion beam during deposition by e-beam evaporation or sputtering. Phase I is depositing 31 6L nanocrystalline stainless steel onto 31 6L stainless steel substrates. These coatings exhibit hardnesses comparable to those normally obtained for ceramic coatings such ZrO2, and possess the superior adhesion of seamless, homometallic coatings. Hardening the surface with a similar material also enhances adhesion, by avoiding problems associated with thermal and lattice mismatch. So far we have deposited nanocrystalline homometallic 316L stainless steel coatings by varying the ions and the current density of the ion beams. For all deposition conditions we have produced smooth, uniform, superhard coatings. All coatings exhibit hardness of at least 200% harder than that of bulk materials. Our measurements indicate that there is a direct relationship between nanohardness and the current density of the ion beam. Stress measurements indicate that stress in the films is increasingly proportional to current density of the ion beam. TEM, XPS, and XRD results indicate that the coated layers consist of FCC structure nanocrystallites with a dimension of about 10 to 20 nm. The Ni and Mo concentration of these coating are lower than those of bulk 316L but the concentration of Cr is higher.

Tobin, Eric J.↗
NASA NTRS2000

Influence of microgravity on ultrastructure and storage reserves in seeds of Brassica rapa L

Successful plant reproduction under spaceflight conditions has been problematic in the past. During a 122 d opportunity on the Mir space station, full life cycles of Brassica rapa L. were completed in microgravity in a series of three experiments in the Svet greenhouse. Ultrastructural and cytochemical analyses of storage reserves in mature dry seeds produced in these experiments were compared with those of seeds produced during a high-fidelity ground control. Additional analyses were performed on developing Brassica embryos, 15 d post pollination, which were produced during a separate experiment on the Shuttle (STS-87). Seeds produced on Mir had less than 20% of the cotyledon cell number found in seeds harvested from the ground control. Cytochemical localization of storage reserves in mature cotyledons showed that starch was retained in the spaceflight material, whereas protein and lipid were the primary storage reserves in ground control seeds. Protein bodies in mature cotyledons produced in space were 44% smaller than those in the ground control seeds. Fifteen days after pollination, cotyledon cells from mature embryos formed in space had large numbers of starch grains, and protein bodies were absent, while in developing ground control seeds at the same stage, protein bodies had already formed and fewer starch grains were evident. These data suggest that both the late stage of seed development and maturation are changed in Brassica by growth in a microgravity environment. While gravity is not absolutely required for any step in the plant life cycle, seed quality in Brassica is compromised by development in microgravity.

NASA Experiment Number 9600004↗
NASA NTRS2014

Microstructure and Oxidation of a MAX Phase/Superalloy Hybrid Interface

Corrosion resistant, strain tolerant MAX phase coatings are of interest for turbine applications. Thin Cr2AlC MAX phase wafers were vacuum diffusion bonded to an advanced turbine disk alloy, LSHR, at 1100 C. The interface, examined by optical and scanning electron microscopy, revealed a primary diffusion zone consisting of ~ 10 micrometers of beta-Ni(Co)Al, decorated with various NiCoCrAl, MC and M3B2 precipitates. On the Cr2AlC side, an additional ~40 micrometers Al-depletion zone of Cr7C3 formed in an interconnected network with the beta-Ni(Co)Al. Oxidation of an exposed edge at 800 C for 100 h produced a fine-grained lenticular alumina scale over Cr2AlC and beta-Ni(Co)Al, with coarser chromia granules over the Cr7C3 regions. Subsequent growth of the diffusion layers was only ~ 5 micrometers in total. A residual stress of ~ 500 MPa was estimated for the MAX phase layer, but no interfacial damage was observed. Subsequent tests for 1000 h reveal similar results.

heat resistant alloys↗
DOE OSTI2024

Design Solutes to Achieve Columnar-to-Equiaxed Transition and Grain Refinement in Cast Multi-principal-element Alloys

The multi-principal-element alloys (MPEAs), also referred to as high-entropy alloys (HEAs), have attracted extensive attention during the last decade and a half due to their unique and excellent properties. However, many MPEAs show coarse and anisotropic columnar grains in the as-cast state. While constitutional supercooling (CS)-driven parameters have been widely used to evaluate and predict the effect of solutes on columnar-to-equiaxed transition (CET) and grain refinement of diluted binary alloys, similar studies are lacking on MPEAs. Due to the multiple solute elements (solutes) and their high concentrations, the CS-driven parameters for MPEAs are different from those proposed for diluted binary alloys. Here, we derived the CS-driven parameters, including undercooling parameter and growth restriction factor, for MPEAs based on their physical significances, with the help of calculated phase diagrams. The calculated CS-driven parameters were then used to predict the effect of a solute on CET and grain refinement in NiCoFeCr MPEAs. Additional alloying solutes Nb, Ti, and V in the NiCoFeCr MPEA were also evaluated for their different CS-driven characteristics. In this study, the grain size of the as-solidified microstructures of NiCoFeCr with and without Nb, Ti, and V were compared and interpreted with the predicted tendency of the CS-driven parameters.

36 MATERIALS SCIENCE↗
NASA NTRS1978

Remote sensing of crop type and maturity

A red-shift in the chlorophyll absorption edge of heading wheat and grain sorghum is clearly visible in high-spectral-resolution measurements made from a low-flying aircraft over Imperial Valley, California. The position of the absorption edge shifts progressively toward the longer wavelengths during the crop growth cycle. The red-shift of 7 to 10 nm can be measured by using 10 nm-wide spectral bands, centered at 745 nm and 785 nm. These two bands, plus a band in the pigment absorption region at 670 nm, contain enough information to identify wheat and grain sorghum in the heading stage, to indicate the degree of heading, and to indicate canopy density during the heading stage

Collins, W.↗
DOE OSTI2023

Assessing the quality of large-area monolayer graphene grown on liquid copper for size-selective ionic/molecular membrane separations

Monolayer graphene growth on liquid copper (Cu) has attracted attention due to advantages of a flat/smooth catalytic growth surface, high synthesis temperature (>1080 °C) as well as the possibility of forming graphene domains that are mobile on the liquid Cu with potential to minimize grain boundary defects and self-assemble into a continuous monolayer film. However, the quality of monolayer graphene grown on liquid copper and its suitability for size-selective ionic/molecular membrane separations has not been evaluated/studied. Here, we probe the quality of monolayer graphene grown on liquid Cu (via a metallurgical process, HSMG ® ) using Scanning Electron Microscope (SEM), High-resolution transmission electron microscope (HR-TEM), Raman spectroscopy and report on a facile approach to assess intrinsic sub-nanometer to nanometer-scale defects over centimeter-scale areas. We demonstrate high transfer yields of monolayer graphene (>93% coverage) from the growth substrate to polyimide track etched membrane (PITEM, pore diameter ~200 nm) supports to form centimeter-scale atomically thin membranes. Next, we use pressure-driven transport of ethanol to probe defects > 60 nm and diffusion-driven transport of analytes (KCl ~0.66 nm, L-Tryptophan ~0.7–0.9 nm, Vitamin B12 ~1–1.5 nm and Lysozyme ~3.8–4 nm) to probe nanoscale and sub-nanometer scale defects. Diffusive transport confirms the presence of intrinsic sub-nanometer to nanometer scale defects in monolayer graphene grown on liquid Cu are no less than that in high-quality graphene synthesized via chemical vapor deposition (CVD) on solid Cu. Our work not only benchmarks quality of graphene grown on liquid copper for membrane applications but also provides fundamental insights into the origin of intrinsic defects in large-area graphene synthesized via bottom-up processes for membrane applications.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Texture Memory in Hexagonal Metals and Its Mechanism

Texture memory is a phenomenon in which retention of initial textures occurs after a complete cycle of forward and backward transformations, and it occurs in various phase-transforming materials including cubic and hexagonal metals such as steels and Ti and Zr alloys. Texture memory is known to be caused by the phenomena called variant selection, in which some of the allowed child orientations in an orientation relationship between the parent and child phases are preferentially selected. Without such variant selection, the phase transformations would randomize preferred orientations. In this article, the methods of prediction of texture memory and mechanisms of variant selections in hexagonal metals are explored. The prediction method using harmonic expansion of orientation distribution functions with the variant selection in which the Burgers orientation relationship, {110}β//{0001} α-hex <1$\bar{1}$1>β//$\langle2\bar{11}0\rangle$ α-hex , is held with two or more adjacent parent grains at the same time, called “double Burgers orientation relation (DBOR)”, is introduced. This method is shown to be a powerful tool by which to analyze texture memory and ultimately provide predictive capabilities for texture changes during phase transformations. Variation in nucleation and growth rates on special boundaries and an extensive growth of selected variants are also described. Analysis of textures of commercially pure Ti observed in situ by pulsed neutron diffraction reveals that the texture memory in CP-Ti is indeed quite well predicted by consideration of the mechanism of DBOR. The analysis also suggests that the nucleation and growth rates on the special boundary of 90° rotation about $\langle2\bar{11}0\rangle$ α-hex should be about three times larger than those of the other special boundaries, and the selected variants should grow extensively into not only one parent grain but also other grains in α-hex(hexagonal)→β(bcc) transformation. The model calculations of texture development during two consecutive cycles of α-hex→β→α-hex transformation in CP-Ti and Zr are also shown.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Dendritic deformation modes in additive manufacturing revealed by operando x-ray diffraction

Abstract Dynamic solidification behavior during metal additive manufacturing directly influences the as-built microstructure, defects, and mechanical properties of printed parts. How the formation of these features is driven by temperature variation (e.g., thermal gradient magnitude and solidification front velocity) has been studied extensively in metal additive manufacturing, with synchrotron x-ray imaging becoming a critical tool to monitor these processes. Here, we extend these efforts to monitoring full thermomechanical deformation during solidification through the use of operando x-ray diffraction during laser melting. With operando diffraction, we analyze thermomechanical deformation modes such as torsion, bending, fragmentation, assimilation, oscillation, and interdendritic growth. Understanding such phenomena can aid the optimization of printing strategies to obtain specific microstructural features, including localized misorientations, dislocation substructure, and grain boundary character. The interpretation of operando diffraction results is supported by post-mortem electron backscatter diffraction analyses.

Dass, Adrita (ORCID:000000019100635X)↗
DOE OSTI2023

Exploring and Embracing Heterogeneity in Atomically Thin Energy Materials

Atomically thin semiconductors offer extraordinary opportunities for the manipulation of charge carriers, many-body optical excitations, quantum light emitters, and non-charge based quantum numbers. Confinement and reduced dielectric screening in these two-dimensional (2D) materials give rise to large characteristic energies so that many-body and quantum effects are important even at room temperature. Optical excitations in extended homogeneous areas have been investigated intensely, albeit mostly focusing on a limited set of materials, particularly transition metal dichalcogenides. Much less understood are light-matter interactions for other classes of 2D semiconductors, as well as effects that arise in heterogeneous materials, either near naturally occurring defects, impurities, edges and grain boundaries, or as a result of intentional interface formation in heterostructures. Addressing such systems experimentally involves significant challenges: Understanding the atomistic growth mechanisms of 2D semiconductors, so that novel systems with designed properties, specific ‘imperfections’, or controlled interfaces can be realized; and probing of local excitations at scales that match the relevant (micrometer to nanometer) length scales in heterogeneous materials. In this research project, we addressed these challenges by harnessing quantitative in-situ microscopy to study the growth of 2D and layered semiconductors and heterostructures, combined with local spectroscopic measurements of quasiparticles excited at the nanometer scale. An integral part of the research has been the development of novel experimental approaches, both for in-situ microscopy of synthesis and for nanometer-scale spectroscopy. In particular, advanced techniques were developed for cathodoluminescence in scanning transmission electron microscopy (STEM-CL) where a nanometer-focused electron beam is used to locally excite electron-hole pairs, excitons, as well as propagating hybrid light-matter modes such as exciton-polaritons. Experiments were guided and analyzed via computations of structure, chemistry, and excitation spectra. The particular materials focus has been on group IV chalcogenides, a family of less explored 2D/layered semiconductors whose diversity in crystal structure and properties promises access to novel materials architectures and the discovery of phenomena that can support emerging technology needs.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Dependence of Dust Formation on the Supernova Explosion

We investigate the properties, composition, and dynamics of dust formation and growth for a diverse set of core-collapse supernovae (CCSNe), with 15, 20, and 25 M ⊙ progenitor masses, explosion energies ranging from 0.5 to 120 foe, and varied engine type. These explosions are evolved with a 1D Lagrangian hydrodynamics code out to a minimum of 1157 days to model the ejecta as it expands and cools. A multigrain dust nucleation and growth model is applied to these results. We find that higher explosion energies lead to an earlier onset of dust formation, smaller grain sizes, and larger silicate abundances. Further, we see that nuclear burning during the explosion leads to enhanced formation of silicate dust. Finally, we build composite models from our suite to predict the efficiency of CCSN dust production as a function of metallicity.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1979

Variations in pore structure of reaction-bonded silicon nitride /RBSN/

A discussion is presented relating the observed pore structures (sizes) to the reaction mechanisms in reaction-bonded silicon nitride (alpha- and beta-Si3N4) on the basis of information available from the literature. While the techniques for reducing the residual macroporosity are quite well-developed for reaction-bonded Si3N4 (RBSN), it is important to be aware of three other orders of magnitude for porosity present in RBSN as a result of the nitriding process itself, and how these types of nitridation-induced porosity can be controlled. For ease of description, these types of nitridation-induced porosity are called micropores, nanopores, and picopores in order of their decreasing size. A scanning electron micrograph is presented, showing nanopores isolated in the unreacted Si and picopores in the alpha-matte Si3N4. The assumption that an alpha-matte growth mechanism is active explains the occurrence of nanopores and their partial filling with alpha-Si3N4, leaving behind very fine-grained alpha-matte and picopores.

Danforth, S. C.↗
NASA NTRS1988

Turbulent diffusivity

A method for determining the turbulent diffusivity tensor is proposed which has application to problems such as the mixing in the interior of the sun for the solar neutrino problem, the chemical evolution of the galaxy, and grain sedimentation on the primitive solar nebula. A model for determining the turbulent energy spectral function, the eddy decorrelation rate, and the growth rate of the underlying instability is discussed. Good agreement is obtained between the present values for the coefficient of turbulent diffusivity and those obtained from both numerical simulation studies and laboratory data on shear turbulence.

Canuto, V. M.↗