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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 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
NASA NTRSDate not supplied

Effect of Pressure and Oxidation State on Pnictogen Metal-Silicate Partitioning

The partitioning of trace elements between metal and silicate melts serves as the foundation for understanding the differentiation of a planetary body into a metallic core and silicate mantle. Element activity influences metal-silicate partitioning behavior. Activity coefficients are directly dependent on composition and temperature, and can be indirectly dependent on oxygen fugacity and pressure. Distinguishing the effect of pressure from other variables on the activity coefficients and partitioning is important for understanding the chemical evolution of different planetary bodies during differentiation. In this study, we investigated the influence of pressure on the activity coefficients of Cu, Mo, Pd, Pt, As, Sb, and Bi in Fe-Si metallic liquids. All of these elements exhibit moderate to high activity coefficients in Fe-Si liquids at low pressure, which significantly controls their metal-silicate partitioning behavior. Identifying whether this strong dependence persists at higher pressures is critical to modeling and understanding the chemical consequences of core formation. New experiments at 10 GPa were used to derive activity coefficients for these metals which can be compared to activity coefficients determined at 1 GPa. Experiments were conducted at 10 GPa and 2373K using a 10/5 assembly in the 880-ton multi anvil press at NASA Johnson Space Center. The standard 10/5 COMPRES assembly was slightly modified to accommodate a sample capsule machined from single-crystal MgO that minimizes melt percolation out of the sample volume during the experiment. Experiment starting materials were comprised of 70 wt.% Knippa basalt and 30 wt.% metal. The metal mixture (~85 wt.% Fe) was created by adding the elements of interest (Cu, Mo, Pd, Pt, As, Sb, Bi) to Fe metal powder. Varied amounts of Si metal (0-10 wt.% Si) were added to the metal-silicate mixtures to generate a systematic series of starting materials. For each element, an epsilon interaction parameter in Fe-Si liquid was derived from the results of our 0-10 wt.% Si metal series. To investigate whether pressure influences the trace element activity coefficients in the 1-10 GPa pressure range, we compared our results at 10 GPa to those at 1-4 GPa [1-3]. Our results can also be directly compared to interaction parameters for Au, P, V, Mn, Ga, Zn, Cd, Sn, W, Pb, and Nb previously determined at 10 GPa and 2373K following the same methods [4]. Combined, this suite of interaction parameters will directly inform metal- silicate partitioning between 1 and 10 GPa, and assess whether these values can be extrapolated to modeling differentiation processes at pressures >10 GPa. [1] Righter et al. (2018) GCA 232, 101-123 [2] Righter et al. (2019) MaPS 54, 1379-1394 [3] Steenstra et al. (2020) Icarus, 335, 113391 [4] Righter et al. (2020) Geochem. Persp. Let. 15, 44-49

Kelsey Prissel↗
DOE OSTI2023

Palladium-Percolated Networks Enabled by Low Loadings of Branched Nanorods for Enhanced H 2 Separations

Nanoparticles (NPs) at high loadings are often used in mixed matrix membranes (MMMs) to improve gas separation properties, but they can lead to defects and poor processability that impede membrane fabrication. Herein, it is demonstrated that branched nanorods (NRs) with controlled aspect ratios can significantly reduce the required loading to achieve superior gas separation properties while maintaining excellent processability, as demonstrated by the dispersion of palladium (Pd) NRs in polybenzimidazole for H 2 /CO 2 separation. Increasing the aspect ratio from 1 for NPs to 40 for NRs decreases the percolation threshold volume fraction by a factor of 30, from 0.35 to 0.011. An MMM with percolated networks formed by Pd NRs at a volume fraction of 0.039 exhibits H 2 permeability of 110 Barrer and H 2 /CO 2 selectivity of 31 when challenged with simulated syngas at 200 °C, surpassing Robeson's upper bound. In conclusion, this work highlights the advantage of NRs over NPs and nanowires and shows that right-sizing nanofillers in MMMs is critical to construct highly sieving pathways at minimal loadings. In conclusion, this work paves the way for this general feature to be applied across materials systems for a variety of chemical separations.

36 MATERIALS SCIENCE↗
NASA NTRS2010

A Spectral Lyapunov Function for Exponentially Stable LTV Systems

This paper presents the formulation of a Lyapunov function for an exponentially stable linear timevarying (LTV) system using a well-defined PD-spectrum and the associated PD-eigenvectors. It provides a bridge between the first and second methods of Lyapunov for stability assessment, and will find significant applications in the analysis and control law design for LTV systems and linearizable nonlinear time-varying systems.

Zhu, J. Jim↗
NASA NTRSDate not supplied

Effect of Pressure on Trace Element Activity Coefficients in Metal-Silicate Systems

The partitioning of trace elements between metal and silicate melts serves as the foundation for understanding the differentiation of a planetary body into a metallic core and silicate mantle. Element activity influences metal-silicate partitioning behavior. Activity coefficients are directly dependent on composition and temperature, and can be indirectly dependent on oxygen fugacity and pressure. Distinguishing the effect of pressure from other variables on the activity coefficients and partitioning is important for understanding the chemical evolution of different planetary bodies during differentiation. In this study, we investigated the influence of pressure on the activity coefficients of Cu, Mo, Pd, Pt, As, Sb, and Bi in Fe-Si metallic liquids. All of these elements exhibit moderate to high activity coefficients in Fe-Si liquids at low pressure, which significantly controls their metal-silicate partitioning behavior. Identifying whether this strong dependence persists at higher pressures is critical to modeling and understanding the chemical consequences of core formation. New experiments at 10 GPa were used to derive activity coefficients for these metals which can be compared to activity coefficients determined at 1 GPa. Experiments were conducted at 10 GPa and 2373K using a 10/5 assembly in the 880-ton multi-anvil press at NASA Johnson Space Center. The standard 10/5 COMPRES assembly was slightly modified to accommodate a sample capsule machined from single-crystal MgO that minimizes melt percolation out of the sample volume during the experiment. Experiment starting materials were comprised of 70 wt.% Knippa basalt and 30 wt.% metal. The metal mixture (~85 wt.% Fe) was created by adding the elements of interest (Cu, Mo, Pd, Pt, As, Sb, Bi) to Fe metal powder. Varied amounts of Si metal (0-10 wt.% Si) were added to the metal-silicate mixtures to generate a systematic series of starting materials. For each element, an epsilon interaction parameter in Fe-Si liquid was derived from the results of our 0-10 wt.% Si metal series. To investigate whether pressure influences the trace element activity coefficients in the 1-10 GPa pressure range, we compared our results at 10 GPa to those at 1-4 GPa [1-3]. Our results can also be directly compared to interaction parameters for Au, P, V, Mn, Ga, Zn, Cd, Sn, W, Pb, and Nb previously determined at 10 GPa and 2373K following the same methods [4]. Combined, this suite of interaction parameters will directly inform metal-silicate partitioning between 1 and 10 GPa, and assess whether these values can be extrapolated to modeling differentiation processes at pressures >10 GPa.

Kelsey Prissel↗
NASA NTRS1986

Dendrite characteristics in directionally solidified Pb-8 pct Au and Pb-3 pct Pd alloys

An investigation of Pb-8 pct Au and Pb-3 pct Pd alloy specimens which have been partially directionally solidified and then quenched is performed in order to characterize their dendritic microstructural details and solute composition profiles. A controlled sectioning technique is employed to measure dendrite tip radii. It is found that most of the observed behavior is predicted quite well by a modification of the Burden and Hunt (1974) model proposed by Laxmanan (1984), in addition to the models based on the marginal stability approach. Results indicate that quantitative comparison of the primary arm spacing measurements can not form the basis of distinguishing among the various dendrite growth models in a positive temperature gradient.

Tewari, S. N.↗
DOE OSTI2016

The catalytic behavior of precisely synthesized Pt–Pd bimetallic catalysts for use as diesel oxidation catalysts

The demands of stricter diesel engine emission regulations have created challenges for current exhaust systems. With advances in low-temperature internal combustion engines and their operations, advances must also be made in vehicle exhaust catalysts. Most current diesel oxidation catalysts use heavy amounts of precious group metals (PGMs) for hydrocarbon (HC), CO, and NO oxidation. These catalysts are expensive and are most often synthesized with poor bimetallic interaction and dispersion. In this paper, the goal was to study the effect of aging on diesel emission abatement of Pt–Pd bimetallic nanoparticles precisely prepared with different morphologies: well dispersed core–shell vs. well dispersed homogeneously alloyed vs. poorly dispersed, poorly alloyed particles. Alumina and silica supports were studied. Particle morphology and dispersion were analyzed before and after hydrothermal treatments by XRD, EDX, and STEM. Reactivity as a function of aging was measured in simulated diesel engine exhaust. While carefully controlled bimetallic catalyst nanoparticle structure has a profound influence on initial or low temperature catalytic activity, the differences in behavior disappear with higher temperature aging as thermodynamic equilibrium is achieved. The metallic character of Pt-rich alumina-supported catalysts is such that behavior rather closely follows the Pt–Pd metal phase diagram. Nanoparticles disparately composed as well-dispersed core–shell (via seq-SEA), well-dispersed homogeneously alloyed (via co-SEA), and poorly dispersed, poorly alloyed (via co-DI) end up as well alloyed, large particles of almost the same size and activity. With Pd-rich systems, the oxidation of Pd also figures into the equilibrium, such that Pd-rich oxide phases appear in the high temperature forms along with alloyed metal cores. Finally, the small differences in activity after high temperature aging can be attributed to the synthesis methods, sequential SEA and co-DI which give rise, after aging, to a bimetallic surface enriched in Pd.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Strain‐Enhanced Metallic Intermixing in Shape‐Controlled Multilayered Core–Shell Nanostructures: Toward Shaped Intermetallics

Abstract Controlling the surface composition of shaped bimetallic nanoparticles could offer precise tunability of geometric and electronic surface structure for new nanocatalysts. To achieve this goal, a platform for studying the intermixing process in a shaped nanoparticle was designed, using multilayered Pd‐Ni‐Pt core–shell nanocubes as precursors. Under mild conditions, the intermixing between Ni and Pt could be tuned by changing layer thickness and number, triggering intermixing while preserving nanoparticle shape. Intermixing of the two metals is monitored using transmission electron microscopy. The surface structure evolution is characterized using electrochemical methanol oxidation. DFT calculations suggest that the low‐temperature mixing is enhanced by shorter diffusion lengths and strain introduced by the layered structure. The platform and insights presented are an advance toward the realization of shape‐controlled multimetallic nanoparticles tailored to each potential application.

Williams, Benjamin P.↗
DOE OSTI2015

Thickness effect on magnetocrystalline anisotropy of Co/Pd(111) films: A density functional study

In this study, we carried out first-principles calculations on magnetocrystalline anisotropy (MCA) of Co/Pd thin films by adopting two different systems of (i) n-Co/3-Pd and (ii) n-Pd/3-Co. In one system, we vary the thickness of Co layer, fixing the thickness of the Pd layer to 3-monolayers, and in the other system vice versa. MCA is mainly governed by the surface and interface Co atoms, while contributions from other Co atoms are smaller. MCA energy (E{sub MCA}) of the Co/Pd thin film shows oscillatory behavior with the thickness of the Co layer, but is insensitive to the thickness of the Pd layer. In particular, the n-Co/3-Pd films of n = 2, 4, and 6 exhibit strong perpendicular MCA of about 1 meV. Our results suggest that controlling the thickness of the Co layer in Co/Pd (111) is crucial in achieving strong perpendicular MCA.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Enhancing the activity of Pd ensembles on graphene by manipulating coordination environment

Atomic dispersion of metal catalysts on a substrate accounts for the increased atomic efficiency of single-atom catalysts (SACs) in various catalytic schemes compared to the nanoparticle counterparts. However, lacking neighboring metal sites has been shown to deteriorate the catalytic performance of SACs in a few industrially important reactions, such as dehalogenation, CO oxidation, and hydrogenation. Metal ensemble catalysts (M n ), an extended concept to SACs, have emerged as a promising alternative to overcome such limitation. Inspired by the fact that the performance of fully isolated SACs can be enhanced by tailoring their coordination environment (CE), we here evaluate whether the CE of M n can also be manipulated in order to enhance their catalytic activity. We synthesized a set of Pd ensembles (Pd n ) on doped graphene supports (Pd n /X-graphene where X = O, S, B, and N). We found that introducing S and N onto oxidized graphene modifies the first shell of Pd n converting Pd–O to Pd–S and Pd–N, respectively. Also, we further found that the B dopant significantly affected the electronic structure of Pd n by serving as an electron donor in the second shell. We examined the performance of Pd n /X-graphene toward selective reductive catalysis, such as bromate reduction, brominated organic hydrogenation, and aqueous-phase CO 2 reduction. We observed that Pd n /N-graphene exhibited superior performance by lowering the activation energy of the rate-limiting step, i.e., H 2 dissociation into atomic hydrogen. The results collectively suggest controlling the CE of SACs in an ensemble configuration is a viable strategy to optimize and enhance their catalytic performance.

36 MATERIALS SCIENCE↗
NASA NTRS1983

In-situ TEM investigations of graphic-epitaxy and small particles

Palladium was deposited inside a controlled-vacuum specimen chamber of a transmission electron microscope (TEM) onto MgO and alpha-alumina substrate surfaces. Annealing and various effects of gas exposure of the particulate Pd deposits were studied in-situ by high resolution TEM and electron diffraction. Whereas substrate temperatures of 500 C or annealing of room temperature (RT) deposits to 500 C were needed to obtain epitaxy on sapphire, RT deposits on MgO were perfectly epitaxial. For Pd/MgO a lattice expansion of 2 to 4% was noted; the highest values of expansion were found for the smallest particles. The lattice expansion of small Pd particles on alumina substrates was less than 1%. Long-time RT exposure of Pd/MgO in a vacuum yielded some moblity and coalescence events, but notably fewer than for Pd on sapphire. Exposure to air or oxygen greatly enhanced the particle mobility and coalescence and also resulted in the flattening of Pd particles on MgO substrates. Electron-beam irradiation further enhanced this effect. Exposure to air for several tens of hours of Pd/MgO led to strong coalescence.

Heinemann, K.↗
DOE OSTI2020

Strain-Enhanced Metallic Intermixing in Shape-Controlled Multilayered Core–Shell Nanostructures: Toward Shaped Intermetallics

Controlling the surface composition of shaped bimetallic nanoparticles could offer precise tunability of geometric and electronic surface structure for new nanocatalysts. To achieve this goal, a platform for studying the intermixing process in a shaped nanoparticle was designed, using multilayered Pd-Ni-Pt core–shell nanocubes as precursors. Under mild conditions, the intermixing between Ni and Pt could be tuned by changing layer thickness and number, triggering intermixing while preserving nanoparticle shape. Intermixing of the two metals is monitored using transmission electron microscopy. The surface structure evolution is characterized using electrochemical methanol oxidation. DFT calculations suggest that the low-temperature mixing is enhanced by shorter diffusion lengths and strain introduced by the layered structure. The platform and insights presented are an advance toward the realization of shape-controlled multimetallic nanoparticles tailored to each potential application.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Direct strain correlations at the single-atom level in three-dimensional core-shell interface structures

Abstract Nanomaterials with core-shell architectures are prominent examples of strain-engineered materials. The lattice mismatch between the core and shell materials can cause strong interface strain, which affects the surface structures. Therefore, surface functional properties such as catalytic activities can be designed by fine-tuning the misfit strain at the interface. To precisely control the core-shell effect, it is essential to understand how the surface and interface strains are related at the atomic scale. Here, we elucidate the surface-interface strain relations by determining the full 3D atomic structure of Pd@Pt core-shell nanoparticles at the single-atom level via atomic electron tomography. Full 3D displacement fields and strain profiles of core-shell nanoparticles were obtained, which revealed a direct correlation between the surface and interface strain. The strain distributions show a strong shape-dependent anisotropy, whose nature was further corroborated by molecular statics simulations. From the observed surface strains, the surface oxygen reduction reaction activities were predicted. These findings give a deep understanding of structure-property relationships in strain-engineerable core-shell systems, which can lead to direct control over the resulting catalytic properties.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Cooperative mechanisms behind nanoscale smoothing on metal surfaces: From adatom diffusion to step nucleation

Many technologies require stable or metastable surface morphology. Here, in this paper we study the factors that control the metastability of a common feature of rough surfaces: “hillocks.” We use low energy electron microscopy to follow the evolution of the individual atomic steps in hillocks on Pd(111). We show that the uppermost island in the stack often adopts a static, metastable configuration. Modeling this result shows that the degree of the metastability depends on the configuration of steps dozens of atomic layers lower. Our model allows us to link surface metastability to the atomic processes of surface evolution.

Fernandes Cauduro, Andre Luis↗
DOE OSTI2024

Four-Terminal Electrochemistry: A Back-Gate Controls the Electrochemical Potential of a 2D Working Electrode

We demonstrate that ultrathin semiconductor working electrodes integrated into metal–insulator–semiconductor (MIS) stacks are an enabling platform for understanding non-Faradaic semiconductor electrochemistry. Furthermore, 5 nm thick ZnO electrodes were deposited on 30 nm HfO 2 dielectric on a Pd “gate” electrode. Application of a bias V G between the Pd gate and the ZnO electrode causes electrons to accumulate in the ZnO layer as measured by recording the in-plane sheet conductance. By contacting the top surface of the ZnO layer with the electrolyte in a conventional three-electrode electrochemical cell, we show that the gate voltage V G modulates the electrochemical potential V ZnO of the ZnO film with respect to a reference electrode. Electrochemical potential changes ΔV ZnO up to –1 V vs Ag/Ag + are achieved for V G = +7 V. Furthermore, by measuring V ZnO vs V G , we extract the quantum capacitance CQ of the ZnO film as a function of the Fermi-level position, which provides a direct measure of the ZnO electronic density of states (DOS). Finally, we demonstrate that the gated ZnO working electrodes can disentangle the two principal components of electrochemical potential, namely, the Fermi-level shift Δδ and the double-layer charging energy eΔΦ EDL . This disentanglement hinges on a fundamental difference between back-gating and normal electrochemical control, namely, that electrochemical control requires double-layer charging, while back-gate control does not. Collectively, the results show that the backside gate electrode is an effective fourth terminal that enables measurements that are difficult to achieve in conventional three-terminal electrochemical setups.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Effects of CO and H2O Co-Feed on the Adsorption and Oxidation Properties of a Pd/BEA Hydrocarbon Trap

Hydrocarbon traps for exhaust emissions control adsorb hydrocarbons in low temperature exhaust and release them as the exhaust warms up. In this work, a Pd/BEA hydrocarbon trap was tested under lean exhaust conditions using ethylene and dodecane as model hydrocarbons. Ethylene uptake was partially inhibited by CO and H2O when fed separately. When both were added, the loss in ethylene uptake was 90% relative to the condition with no H2O or CO. Dodecane uptake was unchanged under all conditions tested. During a temperature ramp, ethylene desorbed and was combusted to CO2 and H2O over active Pd centers. Further, oxidation light-off of dodecane generated an exotherm which caused rapid desorption of the remaining hydrocarbon species from the zeolite. For both hydrocarbons, CO co-feed led to a decreased oxidation light-off temperature, and therefore lower desorption temperature. By pretreating the catalyst in CO and H2O at 80 °C, and even after removing CO from the feed, the enhanced oxidation light-off behavior was observed. DRIFTS characterization shows that some form of oxidized Pd was reducible to Pd0 by CO at 80 °C only in the presence of H2O. Further, this reduction appears reversible by high temperature oxygen treatment. We speculate that this reduced Pd phase serves as the active site for low temperature hydrocarbon oxidation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Modulation of contact resistance between metal and graphene by controlling the graphene edge, contact area, and point defects: An ab initio study

A systematic first-principles non-equilibrium Green's function study is conducted on the contact resistance between a series of metals (Au, Ag, Pt, Cu, Ni, and Pd) and graphene in the side contact geometry. Different factors such as the termination of the graphene edge, contact area, and point defect in contacted graphene are investigated. Notable differences are observed in structural configurations and electronic transport characteristics of these metal-graphene contacts, depending on the metal species and aforementioned influencing factors. It is found that the enhanced chemical reactivity of the graphene due to dangling bonds from either the unsaturated graphene edge or point defects strengthens the metal-graphene bonding, leading to a considerable contact resistance reduction for weakly interacting metals Au and Ag. For stronger interacting metals Pt and Cu, a slightly reduced contact resistance is found due to such influencing factors. However, the wetting metals Ni and Pd most strongly hybridize with graphene, exhibiting negligible dependence on the above influencing factors. This study provides guidance for the optimization of metal-graphene contacts at an atomic scale.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗