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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 1,603 records · Page 89
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Integration of lead-free ferroelectric on HfO2/Si (100) for high performance non-volatile memory applications

We introduce a novel lead-free ferroelectric thin film (1-x)BaTiO3-xBa(Cu1/3Nb2/3)O3 (x 5 0.025) (BT-BCN) integrated on to HfO2 buffered Si for non-volatile memory (NVM) applications. Piezoelectric force microscopy (PFM), x-ray diffraction, and high resolution transmission electron microscopy were employed to establish the ferroelectricity in BT-BCN thin films. PFM study reveals that the domains reversal occurs with 1806 phase change by applying external voltage, demonstrating its effectiveness for NVM device applications. X-ray photoelectron microscopy was used to investigate the band alignments between atomic layer deposited HfO2 and pulsed laser deposited BT-BCN films. Programming and erasing operations were explained on the basis of band-alignments. The structure offers large memory window, low leakage current, and high and low capacitance values that were easily distinguishable even after ,106 s, indicating strong charge storage potential. This study explains a new approach towards the realization of ferroelectric based memory devices integrated on Si platform and also opens up a new possibility to embed the system within current complementary metal-oxide-semiconductor processing technology.

42 ENGINEERING↗
DOE OSTI2015

Allostery through protein-induced DNA bubbles

Allostery through DNA is increasingly recognized as an important modulator of DNA functions. Here, we show that the coalescence of protein-induced DNA bubbles can mediate allosteric interactions that drive protein aggregation. We propose that such allostery may regulate DNA's flexibility and the assembly of the transcription machinery. Mitochondrial transcription factor A (TFAM), a dual-function protein involved in mitochondrial DNA (mtDNA) packaging and transcription initiation, is an ideal candidate to test such a hypothesis owing to its ability to locally unwind the double helix. Numerical simulations demonstrate that the coalescence of TFAM-induced bubbles can explain experimentally observed TFAM oligomerization. The resulting melted DNA segment, approximately 10 base pairs long, around the joints of the oligomers act as flexible hinges, which explains the efficiency of TFAM in compacting DNA. Since mitochondrial polymerase (mitoRNAP) is involved in melting the transcription bubble, TFAM may use the same allosteric interaction to both recruit mitoRNAP and initiate transcription.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2015

Elevated CO 2 shifts the functional structure and metabolic potentials of soil microbial communities in a C 4 agroecosystem

Atmospheric CO 2 concentration is continuously increasing, and previous studies have shown that elevated CO 2 (eCO 2 ) significantly impacts C 3 plants and their soil microbial communities. However, little is known about effects of eCO 2 on the compositional and functional structure, and metabolic potential of soil microbial communities under C 4 plants. Here we showed that a C 4 maize agroecosystem exposed to eCO 2 for eight years shifted the functional and phylogenetic structure of soil microbial communities at both soil depths (0–5 cm and 5–15 cm) using EcoPlate and functional gene array (GeoChip 3.0) analyses. The abundances of key genes involved in carbon (C), nitrogen (N) and phosphorus (P) cycling were significantly stimulated under eCO 2 at both soil depths, although some differences in carbon utilization patterns were observed between the two soil depths. Consistently, CO 2 was found to be the dominant factor explaining 11.9% of the structural variation of functional genes, while depth and the interaction of depth and CO 2 explained 5.2% and 3.8%, respectively. This study implies that eCO 2 has profound effects on the functional structure and metabolic potential/activity of soil microbial communities associated with C 4 plants, possibly leading to changes in ecosystem functioning and feedbacks to global change in C 4 agroecosystems.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2016

The Role of Climate Covariability on Crop Yields in the Conterminous United States

The covariability of temperature (T), precipitation (P) and radiation (R) is an important aspect in understanding the climate influence on crop yields. Here in this paper, we analyze county-level corn and soybean yields and observed climate for the period 1983–2012 to understand how growing-season (June, July and August) mean T, P and R influence crop yields jointly and in isolation across the CONterminous United States (CONUS). Results show that nationally averaged corn and soybean yields exhibit large interannual variability of 21% and 22%, of which 35% and 32% can be significantly explained by T and P, respectively. By including R, an additional of 5% in variability can be explained for both crops. Using partial regression analyses, we find that studies that ignore the covariability among T, P, and R can substantially overestimate the sensitivity of crop yields to a single climate factor at the county scale. Further analyses indicate large spatial variation in the relative contributions of different climate variables to the variability of historical corn and soybean yields. Finally, the structure of the dominant climate factors did not change substantially over 1983–2012, confirming the robustness of the findings, which have important implications for crop yield prediction and crop model validations.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2014

Ion transport and softening in a polymerized ionic liquid

Polymerized ionic liquids (PolyILs) are promising materials for various solid state electronic applications such as dye-sensitized solar cells, lithium batteries, actuators, field-effect transistors, light emitting electrochemical cells, and electrochromic devices. However, fundamental understanding of interconnection between ionic transport and mechanical properties in PolyILs is far from complete. In this paper, local charge transport and structural changes in films of a PolyIL are studied using an integrated experiment-theory based approach. Experimental data for the kinetics of charging and steady state current–voltage relations can be explained by taking into account the dissociation of ions under an applied electric field (known as the Wien effect). Onsager's theory of the Wien effect coupled with the Poisson–Nernst–Planck formalism for the charge transport is found to be in excellent agreement with the experimental results. The agreement between the theory and experiments allows us to predict structural properties of the PolyIL films. We have observed significant softening of the PolyIL films beyond certain threshold voltages and formation of holes under a scanning probe microscopy (SPM) tip, through which an electric field was applied. Finally, the observed softening is explained by the theory of depression in glass transition temperature resulting from enhanced dissociation of ions with an increase in applied electric field.

36 MATERIALS SCIENCE↗
DOE OSTI2016

A mechanism of Cu work function reduction in CsBr/Cu photocathodes

Thin films of CsBr deposited on Cu(100) have been proposed as next-generation photocathode materials for applications in particle accelerators and free-electron lasers. However, the mechanisms underlying an improved photocathode performance remain poorly understood. We present density Functional Theory (DFT) calculations of the work function reduction following the application of CsBr thin film coatings to Cu photocathodes. The effects of structure and van der Waals forces are examined. Calculations suggest that CsBr films can reduce the work function by around 1.5 eV, which would explain the exponential increase in quantum efficiency (QE) of coated vs. uncoated photocathodes. In conclusion, a model explaining experimentally observed laser activation of photocathode is provided whereby the photo-induced creation of di-vacancies at the surface, and their subsequent diffusion throughout the lattice and segregation at the interface leads to a further increase in QE after a period of laser irradiation.

36 MATERIALS SCIENCE↗
DOE OSTI2015

The atomistic origin of the extraordinary oxygen reduction activity of Pt 3 Ni 7 fuel cell catalysts

Recently Debe et al. reported that Pt 3 Ni 7 leads to extraordinary Oxygen Reduction Reaction (ORR) activity. However, several reports show that hardly any Ni remains in the layers of the catalysts close to the surface (“Pt-skin effect”). This paradox that Ni is essential to the high catalytic activity with the peak ORR activity at Pt 3 Ni 7 while little or no Ni remains close to the surface is explained here using large-scale first-principles-based simulations. We make the radical assumption that processing Pt–Ni catalysts under ORR conditions would leach out all Ni accessible to the solvent. To simulate this process we use the ReaxFF reactive force field, starting with random alloy particles ranging from 50% Ni to 90% Ni and containing up to ~300 000 atoms, deleting the Ni atoms, and equilibrating the resulting structures. We find that the Pt 3 Ni 7 case and a final particle radius around 7.5 nm lead to internal voids in communication with the exterior, doubling the external surface footprint, in fair agreement with experiment. Then we examine the surface character of these nanoporous systems and find that a prominent feature in the surface of the de-alloyed particles is a rhombic structure involving 4 surface atoms which is crystalline-like but under-coordinated. Using density-functional theory, we calculate the energy barriers of ORR steps on Pt nanoporous catalysts, focusing on the O ad -hydration reaction (O ad + H 2 O ad → OH ad + OH ad ) but including the barriers of O 2 dissociation (O 2ad → O ad + O ad ) and water formation (OH ad + H ad → H 2 O ad ). We find that the reaction barrier for the O ad -hydration rate-determining-step is reduced significantly on the de-alloyed surface sites compared to Pt(111). Moreover we find that these active sites are prevalent on the surface of particles de-alloyed from a Pt–Ni 30 : 70 initial composition. These simulations explain the peak in surface reactivity at Pt 3 Ni 7 , and provide a rational guide to use for further optimization of improved catalytic and nanoporous materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Structure-dependent vibrational dynamics of Mg(BH 4 ) 2 polymorphs probed with neutron vibrational spectroscopy and first-principles calculations

We investigated the structure-dependent vibrational properties of different Mg(BH 4 ) 2 polymorphs (α, β, γ, and δ phases) with a combination of neutron vibrational spectroscopy (NVS) measurements and density functional theory (DFT) calculations, with emphasis placed on the effects of the local structure and orientation of the BH 4 - anions. DFT simulations closely match the neutron vibrational spectra. The main bands in the low-energy region (20–80 meV) are associated with the BH4 - librational modes. The features in the intermediate energy region (80–120 meV) are attributed to overtones and combination bands arising from the lower-energy modes. The features in the high-energy region (120–200 meV) correspond to the BH 4 - symmetric and asymmetric bending vibrations, of which four peaks located at 140, 142, 160, and 172 meV are especially intense. There are noticeable intensity distribution variations in the vibrational bands for different polymorphs. We can explain these differences using the spatial distribution of BH 4 - anions within various structures. An example of the possible identification of products after the hydrogenation of MgB 2 , using NVS measurements, is presented. Our results provide fundamental insights of benefit to researchers currently studying these promising hydrogen-storage materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Structure and thermodynamic stability of UTa 3 O 10 , a U( v )-bearing compound

Heating a mixture of uranyl(VI) nitrate and tantalum(V) oxide in the molar ratio of 2 : 3 to 1400 °C resulted in the formation of a new compound, UTa 3 O 10 . The honey colored to yellow brown crystals of UTa 3 O 10 crystallize in an orthorhombic structure with the space group Fddd (no. 70), lattice parameters a = 7.3947(1), b = 12.7599(2), c = 15.8156(2) Å, and Z = 8. Vertex sharing [TaO 6 ] 7- octahedra of two crystallographically distinct Ta cations form a three dimensional tantalate framework. Within this framework, six membered rings of [TaO 6 ] 7- octahedra are formed within the (001) plane. The center of these rings is occupied by the uranyl cations [UO 2 ] + , with an oxidation state of +5 for uranium. The pentavalence of U and Ta was confirmed by X-ray photoelectron spectroscopy and X-ray adsorption spectroscopy. The enthalpy of formation of UTa 3 O 10 from Ta 2 O 5 , β-U 3 O 7 , and U 3 O 8 has been determined to be 13.1 ± 18.1 kJ mol -1 using high temperature oxide melt solution calorimetry with sodium molybdate as the solvent at 700 °C. The close to zero enthalpy of formation of UTa 3 O 10 can be explained by closely balanced structural stabilizing and destabilizing factors, which may also apply to other UM 3 O 10 compounds.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Magnetic iron oxide and manganese-doped iron oxide nanoparticles for the collection of alpha-emitting radionuclides from aqueous solutions

Magnetic nanoparticles are well known to possess chemically active surfaces and high surface areas that can be employed to extract a range of ions from aqueous solutions. Additionally, their paramagnetic property provides a convenient means for bulk collection of the material from solution after the targeted ions have been adsorbed. Herein, two nanoscale amphoteric metal oxides, each possessing useful magnetic attributes, were evaluated for their ability to collect both naturally occurring radioactive isotopes (polonium (Po), radium (Ra), and uranium (U)) as well as the transuranic element americium (Am) from a suite of naturally occurring aqueous matrices. The nanomaterials include commercially available paramagnetic magnetite (Fe3O4) and magnetite that was modified to incorporate manganese (Mn) into the crystal structure. The chemical stability of these nanomaterials was evaluated in Hanford Site, WA ground water between the natural pH (~8) and pH 1 (acidified with HCl). Whereas the magnetite was observed to have good stability over the pH range, the Mn-doped material was observed to leach Mn at low pH. The materials were evaluated in parallel to characterize their uptake performance of the aforementioned alpha-emitting radionuclide spikes from Hanford Site ground water across a range of pH (from ~8 down to 2). In addition, radiotracer uptake experiments were performed on Columbia River water, seawater, and human urine at their natural pH and at pH 2. Despite the observed leaching of Mn from the Mn-doped nanomaterial in the lower pH range, it exhibited generally superior analyte extraction performance compared to the magnetite, and analyte uptake was observed across a broader pH range. The uptake behavior of the various radiotracers on these two materials at different pH levels can generally be explained by the amphoteric nature of the nanoparticle surfaces. Finally, the rate of sorption of the radiotracers on the two materials in unacidified groundwater was evaluated. The uptake curves generally indicate that equilibrium is obtained within a few minutes, which is attributed to the high surface areas of the nanomaterials and the high level of dispersion in the liquids. Overall, the results indicate that these nanomaterials may have the potential to be employed for a range of applications to extract radionuclides from aqueous solutions. These applications may include analytical chemistry, waste water treatment and remediation, mining, and in vitro radiobioassay.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Electrochemical properties and structural evolution of O3-type layered sodium mixed transition metal oxides with trivalent nickel

The electrochemical properties of NaNi 0.5 Co 0.5 O 2 and NaNi 0.5 Fe 0.5 O 2 and their structural transitions as a function of Na extraction associated with redox reactions are investigated. Synthesized in the O3-type layered structure, both materials show reasonable electrochemical activities at room temperature, delivering approximately 0.5 Na per formula unit at C/10 discharge. More Na can be reversibly cycled in NaNi 0.5 Co 0.5 O 2 at elevated temperature and/or in an extended voltage window, while NaNi 0.5 Fe 0.5 O 2 shows significant capacity fading at a high voltage cutoff which is likely due to Fe 4+ migration. In situ X-ray diffraction shows that the structural changes in the two materials upon desodiation are very different. NaNi 0.5 Co 0.5 O 2 goes through many different two-phase reactions including three different O3-type and three different P3-type structures during cycling, producing a voltage profile with multiple plateau-like features. In contrast, NaNi 0.5 Fe 0.5 O 2 has a smooth voltage profile and shows the typical O3–P3 phase transition without lattice distortion seen in other materials. Finally, this different structural evolution upon desodiation and re-sodiation can be explained by the electronic structure of the mixed transition metals and how it perturbs the ordering between Na ions differently.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Two mechanisms of nanoparticle generation in picosecond laser ablation in liquids: the origin of the bimodal size distribution

The synthesis of chemically clean and environmentally friendly nanoparticles through pulsed laser ablation in liquids has shown a number of advantages over conventional chemical synthesis methods and has evolved into a thriving research field attracting laboratory and industrial applications. The fundamental understanding of processes leading to the nanoparticle generation, however, still remains elusive. In particular, the origin of bimodal nanoparticle size distributions in femto- and picosecond laser ablation in liquids, where small nanoparticles (several nanometers) with narrow size distribution are commonly observed to coexist with larger (tens to hundreds of nanometers) ones, has not been explained so far. In this paper, joint computational and experimental efforts are applied to understand the mechanisms of nanoparticle formation in picosecond laser ablation in liquids and to explain the bimodal nanoparticle size distributions. The results of a large-scale atomistic simulation reveal the critical role of the dynamic interaction between the ablation plume and the liquid environment, leading to the generation of large nanoparticles through a sequence of hydrodynamic instabilities at the plume-liquid interface and a concurrent nucleation and growth of small nanoparticles in an expanding metal-liquid mixing region. The computational predictions are supported by a series of stroboscopic videography experiments showing the emergence of small satellite bubbles surrounding the main cavitation bubble generated in single pulse experiments. Lastly, carefully timed double pulse irradiation triggers expansion of secondary cavitation bubbles indicating, in accord with the simulation results, the presence of localized sites of laser energy deposition (possibly large nanoparticles) injected into the liquid at the early stage of the bubble formation.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Reaction mechanism of the selective reduction of CO 2 to CO by a tetraaza [Co II N 4 H] 2+ complex in the presence of protons

The tetraaza [Co II N 4 H] 2+ complex ( 1 ) is remarkable for its ability to selectively reduce CO 2 to CO with 45% Faradaic efficiency and a CO to H 2 ratio of 3 : 2. We employ density functional theory (DFT) to determine the reasons behind the unusual catalytic properties of 1 and the most likely mechanism for CO 2 reduction. The selectivity for CO 2 over proton reduction is explained by analyzing the catalyst's affinity for the possible ligands present under typical reaction conditions: acetonitrile, water, CO 2 , and bicarbonate. After reduction of the catalyst by two electrons, formation of [Co I N 4 H] + –-CO 2 - is strongly favored. Based on thermodynamic and kinetic data, we establish that the only likely route for producing CO from here consists of a protonation step to yield [Co I N 4 H] + –CO 2 H, followed by reaction with CO 2 to form [Co II N 4 H] 2+ –CO and bicarbonate. This conclusion corroborates the idea of a direct role of CO 2 as a Lewis acid to assist in C–O bond dissociation, a conjecture put forward by other authors to explain recent experimental observations. The pathway to formic acid is predicted to be forbidden by high activation barriers, in accordance with the products that are known to be generated by 1 . Calculated physical observables such as standard reduction potentials and the turnover frequency for our proposed catalytic cycle are in agreement with available experimental data reported in the literature. The mechanism also makes a prediction that may be experimentally verified: that the rate of CO formation should increase linearly with the partial pressure of CO 2 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Unexpected compositional and structural modification of CoPt 3 nanoparticles by extensive surface purification

We combined synchrotron small angle X-ray scattering, X-ray fluorescence and extended X-ray absorption fine structure spectroscopy to probe the structure of chemically synthesized CoPt 3 nanoparticles (NPs) after ligand removal via the commonly accepted solvent/nonsolvent approach. We showed that the improved catalytic activity of extensively purified NPs could not be explained only in terms of a “cleaner” surface. We found that extensive surface purification results in the substantial leaching of the Co atoms from the chemically synthesized CoPt 3 NPs transforming them into CoPt 3 /Pt core/shell structures with an unexpectedly thick (~0.5 nm) Pt shell. We indicated that the improved catalytic activity of extensively purified NPs in octyne hydrogenation reaction can be explained by the formation of CoPt 3 /Pt core/shell structures. Also, we demonstrated that drastic compositional and structural transformation of water transferred CoPt 3 NPs was rather a result of extensive removal of native ligands via a solvent/nonsolvent approach than leaching of cobalt atoms in aqueous media. We expect that these findings can be relevant to other transition metal based multicomponent NPs.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Tutorial review on structure – dendrite growth relations in metal battery anode supports

This tutorial review explains the emerging understanding of the surface and bulk chemistry – electrochemical performance relations in anode supports (aka secondary current collectors, substrates, templates, hosts) for lithium, sodium and potassium metal batteries (LMBs, SMBs or NMBs, and KMBs or PMBs). In relation to each section, the possible future research directions that may yield both new insight and improved cycling behavior are explored. Representative case studies from Li, Na and K metal anode literature are discussed. The tutorial starts with an overview of the solid electrolyte interphase (SEI), covering both the “classic” understanding of the SEI structure and the “modern” insights obtained by site-specific cryogenic stage TEM analysis. Next, the multiple roles of supports in promoting cycling stability are detailed. Without an optimized support architecture, the metal–electrolyte interface becomes geometrically unstable at a lower current density and cycle number. Taking into consideration the available literature on LMBs, SMBs and KMBs, it is concluded that effective architectures are geometrically complex and electrochemically lithiophilic, sodiophilic or potassiophilic, so as to promote conformal electrochemical wetting of the metal during plating/stripping. One way that philicity is achieved is through support oxygen surface chemistry, which yields a reversibly reactive metal–support interface. Examples of this include the well-known oxygen–carbon moieties in reduced graphene oxide (rGO), as well as classic ion battery reversible conversion reaction oxides such as SnO 2 . Unreactive surfaces lead to dewetted island growth of the metal, which is a precursor to dendrites, and possibly to non-uniform dissolution. Surveying the literature on various Li, Na and K metal supports, it is concluded that the key bulk thermodynamic property that will predict electrochemical wetting behavior is the enthalpy of infinite solution (Δ sol H ∞ ) of the metal (solute) into the support (solvent). Large and negative Δ sol H ∞ promotes uniform metal wetting on the support surface, corresponding to relatively low plating overpotential. Positive Δ sol H ∞ promotes dewetted islands and a relatively high overpotential. This simple rule explains a broad range of studies on Li, Na and K metal – support interactions, including the previously reported correlation between mutual solubility and wetting.

25 ENERGY STORAGE↗
DOE OSTI2022

Development and application of ReaxFF methodology for understanding the chemical dynamics of metal carbonates in aqueous solutions

A new ReaxFF reactive force field has been developed for metal carbonate systems including Na + , Ca 2+ , and Mg 2+ cations and the CO 3 2– anion. This force field is fully transferable with previous ReaxFF water and water/electrolyte descriptions. The Me–O–C (Me = metal) three-body valence angle parameters and Me–C non-reactive parameters of the force field have been optimized against quantum mechanical calculations including equations of state, heats of formation, heats of reaction, angle distortions and vibrational frequencies. The new metal carbonate force field has been validated using molecular dynamics simulations to study the solvation and reactivity of metal and carbonate ions in water at 300 K and 700 K. The coordination radius and self-diffusion coefficient show good consistency with existing experimental and simulation results. The angular distribution analysis explains the structural preference of carbonate ions to form carbonates and bicarbonates, where Na + predominantly forms carbonates due to weaker angular strain, while Ca 2+ and Mg 2+ prefer to form bicarbonate monodentate in nature. Residence time distribution analyses on different systems reveal the role of ions in accelerating and decelerating the dynamics of water and carbonate ions under different thermodynamic conditions. The formation and dissolution of bicarbonates and carbonates in solution were explored on the basis of the protonation capability in different systems. Here, the nucleation phenomenon of metal carbonates at ambient and supercritical conditions is explained from the perspective of cluster formation over time: Ca 2+ ions can form prenucleation clusters at ambient temperature but show saturation with increasing temperature, whereas Na + and Mg 2+ ions show a rapid increase in cluster size and amount upon increasing time and temperature.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Design nanoporous metal thin films via solid state interfacial dealloying

Thin-film solid-state interfacial dealloying (thin-film SSID) is an emerging technique to design nanoarchitecture thin films. The resulting controllable 3D bicontinuous nanostructure is promising for a range of applications including catalysis, sensing, and energy storage. Using a multiscale microscopy approach, we combine X-ray and electron nano-tomography to demonstrate that besides dense bicontinuous nanocomposites, thin-film SSID can create a very fine (5–15 nm) nanoporous structure. Not only is such a fine feature among one of the finest fabrications by metal-agent dealloying, but a multilayer thin-film design enables creating nanoporous films on a wider range of substrates for functional applications. Through multimodal synchrotron diffraction and spectroscopy analysis with which the materials’ chemical and structural evolution in this novel approach is characterized in details, we further deduce that the contribution of change in entropy should be considered to explain the phase evolution in metal-agent dealloying, in addition to the commonly used enthalpy term in prior studies. Here, the discussion is an important step leading towards better explaining the underlying design principles for controllable 3D nanoarchitecture, as well as exploring a wider range of elemental and substrate selections for new applications.

36 MATERIALS SCIENCE↗