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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 1,081 records · Page 60
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Squeezing a Prism into a Surface: Emulating Bulk Optics with Achromatic Metasurfaces

Metasurfaces promise to replace bulky prisms and lenses with 2D surfaces, revolutionizing wavefront control with technologically significant advantages in size, weight, and planar fabrication. However, conventional implementations suffer from large chromatic aberrations and cannot sustain performance over practical bandwidths of real-world signals because of the limited phase modulation margin available in a surface. How can an infinitely thin surface generate the arbitrarily large, broadband phase delay that bulk phase accumulation can provide? Here, equivalence between bulk optics and certain multiresonant metasurfaces is demonstrated, where phase delay arises from trains of multiple resonances in the effective sheet conductivities of the surface itself instead of accumulation of propagation phase. The fundamentally required electromagnetic surface conductivities of a purely achromatic metasurface are derived and general design rules for arbitrarily broadband beam steering and lensing are obtained. Both operation in transmission and reflection can be achieved in a unified way by proper alignment of the resonances, enabling 360° directionality. The arbitrary spectral bandwidth, dual reflection/transmission operation, and complete dispensability of propagation phase constitute a major advance in the state of the art of achromatic metasurfaces for wavefront manipulation.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Ti‐Gradient Doping to Stabilize Layered Surface Structure for High Performance High‐Ni Oxide Cathode of Li‐Ion Battery

Abstract High‐Ni layered oxide cathodes are considered to be one of the most promising cathodes for high‐energy‐density lithium‐ion batteries due to their high capacity and low cost. However, surfice residues, such as NiO‐type rock‐salt phase and Li 2 CO 3 , are often formed at the particle surface due to the high reactivity of Ni 3+ , and inevitably result in an inferior electrochemical performance, hindering the practical application. Herein, unprecedentedly clean surfaces without any surfice residues are obtained in a representative LiNi 0.8 Co 0.2 O 2 cathode by Ti‐gradient doping. High‐resolution transmission electron microscopy (TEM) reveals that the particle surface is composed of a disordered layered phase (≈6 nm in thickness) with the same rhombohedra structure as its interior. The formation of this disordered layered phase at the particle surface is electrochemically favored. It leads to the highest rate capacity ever reported and a superior cycling stability. First‐principles calculations further confirm that the excellent electrochemical performance has roots in the excellent chemical/structural stability of such a disordered layered structure, mainly arising from the improved robustness of the oxygen framework by Ti doping. This strategy of constructing the disordered layered phase at the particle surface could be extended to other high‐Ni layered transition metal oxides, which will contribute to the enhancement of their electrochemical performance.

Kong, Defei↗
DOE OSTI2020

An In Situ Formed Surface Coating Layer Enabling LiCoO 2 with Stable 4.6 V High‐Voltage Cycle Performances

Abstract The development of high‐voltage LiCoO 2 is essential for achieving lithium‐ion batteries with high volumetric energy density, however, it faces a great deal of challenges owing to the materials, structure and interfacial instability issues. In this work, a strategy is developed, through heat annealing a precoated surface layer to in situ form a high‐voltage‐stable surface coating layer, which is demonstrated to be highly effective to improve the high‐voltage performance of LiCoO 2 . It is discovered that LiCoO 2 reacts with Li 1.5 Al 0.5 Ti 1.5 (PO 4 ) 3 (LATP) at 700 °C to form exclusively spinel phases in addition to Li 3 PO 4 , which are structurally coherent to the layered lattice of LiCoO 2 . The heat annealing of the precoated thin layer of LATP enables the formation of a high‐quality surface layer. Spinel phases possess high‐voltage‐stable structures with much weaker oxidizing ability of lattice oxygen than layered structure. In addition, the Li 3 PO 4 is a good lithium‐ion conductor with excellent chemical stability at high voltages. All these benefits contribute to the construction of a uniform and conformal high‐voltage‐stable surface layer with favorable lithium conducting kinetics at the LiCoO 2 surface. The modified LiCoO 2 shows excellent 4.6 V high‐voltage cycle performance at both room temperature and 45 °C. The thermal stability is greatly enhanced as well.

Wang, Yi↗
DOE OSTI2021

Effects of surface species and homogeneous reactions on rates and selectivity in ethane oxidation on oxide catalysts

Selective alkane oxidations on metal oxide catalysts involve complex mechanisms with multiple reactions in series and parallel, different types of reduced and oxidized surface species, and potential contributions from gas-phase reactions. We report kinetics and thermodynamics of elementary steps involved in C 2 H 6 -O 2 reactions on SiO 2 -supported small vanadium oxide domains are determined using density functional theory. These surface reactions together with gas-phase mechanisms are incorporated in kinetic simulations to determine how surface and gaseous reactions interact and contribute to rates and selectivity. The results show that gas-phase reactions within pore volumes in contact with the catalyst contribute significantly to C 2 H 6 activation rates, even at conditions where gas-phase reactions in empty volumes without catalyst are negligible. The majority of C 2 H 6 activations occur on the surface, via H abstraction by vanadium oxo species present at terminal lattice oxygens. The gas-phase activations via H-abstraction by OH radicals also exhibit significant contributions. The reduced centers formed by reactions at vanadium oxo species are re-oxidized rapidly and, therefore, are present in very small concentrations at reaction conditions. The re-oxidation steps lead to the formation of HO 2 radicals and surface peroxo species that are also rapidly consumed and are present in small concentrations. The peroxo species preferentially convert C 2 H 4 to its epoxide product and influence selectivity even at low concentrations. The gas-phase reactions decrease the concentrations of peroxo species and improve selectivity slightly. The effects of reaction conditions and catalyst site density provide further insights into how factors beyond conversions at lattice oxygens influence rates and selectivity in alkane oxidation reactions of significant industrial importance.

42 ENGINEERING↗
DOE OSTI2020

Real-time Atomic-scale Visualization of Reversible Copper Surface Activation during the CO Oxidation Reaction

Dynamic structural and chemical information of catalyst surface during a catalytic reaction is critical for deciphering the mechanisms governing the properties of catalysts. Transition-metal based catalysts have emerged as an important alternative to noble metal catalysts, but their catalytic mechanism remains largely elusive, complicated by dynamic structure and valence evolution. Herein, by using in situ aberration-corrected environmental transmission electron microscopy, we capture, for the first time at atomic level, the dynamic evolution of Cu surface during CO oxidation reaction. We discovered that, under reaction condition, the Cu surface is activated, typically involving 2-3 atomic layers with the formation of a reversible meta-stable phase that only exists during catalytic reactions. Further, we reveal the distinctive role of CO and O2 in the surface activation, featuring CO exposure to lead to surface roughening and consequently formation of low-coordinated Cu atoms, while O2 exposure to induce quasi-crystalline CuOx phase. Corrugated by DFT calculations, we rationalize crystalline CuOx reversibly transforms into the amorphous phase, acting as an active species to facilitate the interaction of gas reactants and catalyzing CO oxidation.

Luo, Langli↗
DOE OSTI2023

Resolving the Heat Generated from ZrO 2 Atomic Layer Deposition Surface Reactions

Abstract In situ pyroelectric calorimetry and spectroscopic ellipsometry were used to investigate surface reactions in atomic layer deposition (ALD) of zirconium oxide (ZrO 2 ). Calibrated and time‐resolved in situ ALD calorimetry provides new insights into the thermodynamics and kinetics of saturating surface reactions for tetrakis(dimethylamino)zirconium(IV) (TDMAZr) and water. The net ALD reaction heat ranged from 0.197 mJ cm −2 at 76 °C to 0.155 mJ cm −2 at 158 °C, corresponding to an average of 4.0 eV/Zr at all temperatures. A temperature dependence for reaction kinetics was not resolved over the range investigated. The temperature dependence of net reaction heat and distribution among metalorganic and oxygen source exposure is attributed to factors including growth rate, equilibrium surface hydroxylation, and the extent of the reaction. ZrO 2 ‐forming surface reactions were investigated computationally using DFT methods to better understand the influence of surface hydration on reaction thermodynamics.

Bielinski, Ashley R.↗
DOE OSTI2022

Unveiling the High-valence Oxygen Degradation Across the Delithiated Cathode Surface

Charge compensation on anionic redox reaction (ARR) has been promising to realize extra capacity beyond transition metal redox in battery cathodes. The practical development of ARR capacity has been hindered by high-valence oxygen instability, particularly at cathode surfaces. However, the direct probe of surface oxygen behavior has been challenging. Here, the electronic states of surface oxygen are investigated by combining mapping of resonant Auger electronic spectroscopy (mRAS) and ambient pressure X-ray photoelectron spectroscopy (APXPS) on a model LiCoO 2 cathode. The mRAS verified that no high-valence oxygen can sustain at cathode surfaces, while APXPS proves that cathode electrolyte interphase (CEI) layer evolves and oxidizes upon oxygen gas contact. In conclusion, this work provides valuable insights into the high-valence oxygen degradation mode across the interface. Oxygen stabilization from surface architecture is proven a prerequisite to the practical development of ARR active cathodes.

25 ENERGY STORAGE↗
DOE OSTI2023

Atomic force microscopy and scanning electron microscopy characterization of the controlling of surface morphology of epoxy‐amine‐cured spin‐coated films

Abstract Atomic force microscopy (AFM) was successfully used to study spin‐coated, amine‐cured epoxy film microstructure and morphology. The air‐epoxy and epoxy‐substrate interfaces were examined using tapping‐mode height and phase imaging AFM. The impact of relative humidity on the morphology and microstructure of the surfaces was determined. AFM was able to elucidate the changes on the surface as relative humidity during processing increased. It was observed that large nodular formations formed on the epoxy surface expose to the air but not epoxy surface formed on the substrate in addition to varying regions of more or less compliant structures, which was attributed to carbamate formation caused by the amine curing agent reaction with atmospheric CO 2 . Scanning electron microscopy (SEM) was used to further elucidate interface and interphase morphology of spin‐coated epoxies. Experimentation also demonstrated that post‐curing above the glass transition did not change the morphology structure, suggesting surface structures are “locked‐in.” SEM was used to further elucidate how the interface and interphase change with changing environmental conditions at both the air‐epoxy and epoxy‐substrate interface/interphases, including the impact of atmospheric CO 2 on Marangoni cell formation.

Shenk, Timothy M.↗
DOE OSTI2019

Flexible NO 2 ‐Functionalized N‐Heterocyclic Carbene Monolayers on Au (111) Surface

Abstract The formation of flexible self‐assembled monolayers (SAMs) in which an external trigger modifies the geometry of surface‐anchored molecules is essential for the development of functional materials with tunable properties. In this work, it is demonstrated that NO 2 ‐functionalized N‐heterocyclic carbene molecules (NHCs), which were anchored on Au (111) surface, change their orientation from tilted into flat‐lying position following trigger‐induced reduction of their nitro groups. DFT calculations identified that the energetic driving force for reorientation was the lower steric hindrance and stronger interactions between the chemically reduced NHCs and the Au surface. The trigger‐induced changes in the NHCs′ anchoring geometry and chemical functionality modified the work function and the hydrophobicity of the NHC‐decorated Au surface, demonstrating the impact of a chemically tunable NHC‐based SAM on the properties of the metal surface.

Dery, Shahar↗
DOE OSTI2022

On the Counter‐intuitive Heterogeneous Electron Transfer Barrier Properties of Alkanethiolate Monolayers on Gold: Smooth versus Rough Surfaces

Abstract Alkanethiolate monolayers formed on rough gold surfaces can, somewhat surprisingly, act as stronger barriers to heterogeneous electron transfer than those on smooth gold surfaces. This paper presents a possible explanation for this observation by constructing simple geometric models of a “rough” and “smooth” gold surface to examine how microscopic roughness differences can affect the nucleation/growth of the adlayer and size/density of structural defects. Expectedly, the number of defects predicted for adlayers formed on smooth gold is lower than any of those for rough gold. The counter‐intuitive result is that the sizes of a small portion of the defects in the adlayer on the smooth surface are larger than any of those found on the rough surface. The potential implications of these results are discussed.

Chemistry↗
DOE OSTI2025

Glycerol Adsorption on TiO 2 Surfaces: A Systematic Periodic DFT Study

Abstract Conversion of glycerol to added‐value products is desirable due to its surplus during biodiesel synthesis. TiO 2 has been the most explored catalyst. We performed a systematic study of glycerol adsorption on anatase (101), anatase (001), and rutile (110) TiO 2 at the Density Functional Theory level. We found several adsorption modes on these surfaces, with anatase (101) being the less reactive one, leading to adsorption energies between −0.8 and −0.4 eV, with all adsorptions molecular in nature. On the contrary, anatase (001) is the most reactive surface, leading to both molecular and dissociative adsorption modes, with energies ranging from −4 to −1 eV and undergoing severe surface reconstructions in some cases. Rutile (110) also shows both molecular and dissociative adsorptions, but it is less reactive than anatase (001). Surfaces with oxygen vacancies affects the adsorbed states and energies. The electronic structure analysis reveals that glycerol adsorption mainly affects the band gap of the material and not the individual contributions to the valence and conduction band. Bader charge analysis shows that strong adsorption modes on anatase (001) and rutile (110) are associated with large charge transfer from glycerol to the surface, while weak and molecular adsorption modes involve low charge transfer.

Chemistry↗
DOE OSTI2020

Surface modification of bamboo fiber with sodium hydroxide and graphene oxide in epoxy composites

Bamboo fibers (BFs) have high mechanical properties and are candidate reinforcement for epoxy matrix composites. However, to improve performance, good fiber-matrix interaction is required. In this work, unidirectional long BF reinforced epoxy composites at fiber volume content of 22%, 40%, and 50% were made by compression molding. The 40 v% untreated BF reinforced composites exhibited 107% and 439% increase for flexural strength and modulus, respectively, compared to neat epoxy. Sodium hydroxide (NaOH) treatment was used to modify the surface of the BFs, and then the NaOH modified BFs were coated with graphene oxide (GO). The 40 v% NaOH modified BF composites showed an improvement from 259.9 to 327.5 MPa for flexural strength and from 16.7 to 21.5 GPa for flexural modulus, compared to 40 v% untreated BF composites. Slight improvement in properties up to 334.6 MPa for flexural strength and up to 23.8 GPa for flexural modulus was achieved for composites made of 40 v% NaOH/GO modified BF. Surface modification of BF after the NaOH and NaOH/GO treatment was confirmed by X-ray photoelectron spectroscopy and by scanning electron microscopy, which showed differences on the fiber surface morphology and on the composite fracture surface. This BF surface modification approach with GO has potential to impart other properties beyond mechanical to produce multifunctional composite and lead to the use of sustainable plant fibers as alternatives to synthetic fibers.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Holography abhors visible trapped surfaces

We prove that consistency of the holographic dictionary implies a hallmark prediction of the weak cosmic censorship conjecture: that in classical gravity, trapped surfaces lie behind event horizons. In particular, the existence of a trapped surface implies the existence of an event horizon, and that furthermore this event horizon must be outside of the trapped surface. More precisely, we show that the formation of event horizons outside of a strong gravity region is a direct consequence of causal wedge inclusion, which is required by entanglement wedge reconstruction. We make few assumptions beyond the absence of evaporating singularities in strictly classical gravity. We comment on the implication that spacetimes with naked trapped surfaces do not admit a holographic dual, note a possible application to holographic complexity, and speculate on the dual CFT interpretation of a trapped surface.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Spontaneous rearrangement of acetylated xylan on hydrophilic cellulose surfaces

The interaction of xylan, an abundant plant polysaccharide, with cellulose microfibrils is essential for secondary cell wall strength. A deeper understanding of these interactions is crucial both to improve our understanding of plant cell wall architecture and to design alternate strategies to overcome cellulose recalcitrance for the production of biofuels and sustainable biomaterials. Naturally occurring acetate or glucuronic acid substitutions on xylan have been shown to influence xylan-cellulose interactions. In this work, we use unrestrained molecular dynamics simulations to determine the interactions with the (110) hydrophilic face of cellulose fibers of four different xylans. In the absence of cellulose, all xylans, independent of the substitution pattern, adopt a highly flexible threefold helical screw conformation. However, when xylan is spatially close to a cellulose surface 1,2 linked acetyl xylans (2AcX) adopt rigid twofold helical screw conformations. The 2AcX conformations are primarily stabilized by interactions between the acetylated oxygen and the glycosidic linkage with C-O6 of cellulose. In contrast, the glycosidic oxygens and acetyl decorations for 1,3 linked acetyl groups (3AcX) are oriented away from the cellulose surface and the 3AcX xylans maintain threefold helical screw conformations on the cellulose surface. Our results show that evenly spaced chemical functionalization (with acetyl groups) and the position of substitution (1,2) on xylan backbone play key roles in tuning the xylan-cellulose interactions to stabilize the twofold helical screw conformations of xylan on the cellulose surface. A comparison with previous experimental findings further suggests that 1,2 substitutions induce twofold helical screw conformations of xylan on the cellulose surface irrespective of the chemical nature of the substituent, while 1,3 substitutions primarily bind lignin in threefold helical screw conformations rather than cellulose in plant cell walls.

1,2 Ac↗
DOE OSTI2023

Patterns in interactions of variably acetylated xylans with hydrophobic cellulose surfaces

The recalcitrance of plant cell wall lignocellulosic biomass to deconstruction is a major hurdle to sustainable biofuel/bioproduct economy. A multitude of interactions stabilize lignocellulosic biomass structure. Among these, tight packing of hemicellulose-cellulose is partly responsible for biomass recalcitrance. Here, unrestrained molecular dynamics simulations are employed to understand the influence of the nature and pattern of naturally-occuring acetyl decorations of the xylan backbone on interactions with the (100) hydrophobic cellulose surface. Periodically O2-acetylated xylan (2AcX) assume twofold helical screw conformations that are stabilized by a combination of multiple hydrophobic contacts and hydrogen bonds with the hydrophobic cellulose surface. In contrast, acetylation at the O3 position in xylan obstructs interactions, thereby adopting threefold helical screw conformations that potentially preferably interact with lignin rather than cellulose. Fully acetylated xylan desorbs from the surface implying a minimum number of unsubstituted residues on the xylan backbone is required for interaction with the surface. Here, the substituted residues must form ~ 20% fewer contacts than the unsubstituted residues to sustain stable twofold helical screw xylan conformations on the cellulose surface. Thus, specific roles of macromolecular conformations of cellulose and hemicellulose in influencing the supramolecular interactions and function of plant cell walls have been determined.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2025

SUDA: A SUrface Dust Analyser for Compositional Mapping of the Galilean Moon Europa

Abstract The Surface Dust Analyser (SUDA) is a mass spectrometer onboard the Europa Clipper mission for investigating the surface composition of the Galilean moon Europa. Atmosphereless planetary moons such as the Galilean satellites are wrapped into a ballistic dust exosphere populated by tiny samples from the moon’s surface produced by impacts of fast micrometeoroids. SUDA will measure the composition of such surface ejecta during close flybys of Europa to obtain key chemical signatures for revealing the satellite’s composition such as organic molecules and salts, history, and geological evolution. Because of their ballistic orbits, detected ejecta can be traced back to the surface with a spatial resolution roughly equal to the instantaneous altitude of the spacecraft. SUDA is a Time-Of-Flight (TOF), reflectron-type impact mass spectrometer, optimized for a high mass resolution which only weakly depends on the impact location. The instrument will measure the mass, speed, charge, elemental, molecular, and isotopic composition of impacting grains. The instrument’s small size of$268 ~\mathrm {mm} \times 250 ~\mathrm {mm} \times 171$ 268 mm × 250 mm × 171 $~\mathrm {mm}$ mm , radiation-hard design, and rather large sensitive area of 220 cm 2 matches well the challenging demands of the Clipper mission.

Astronomy & Astrophysics↗
DOE OSTI2019

Few-Layers Graphene Film and Copper Surface Morphology for Improved Corrosion Protection of Copper

Graphene has shown excellent corrosion protection of copper (Cu). The corrosion protection of Cu is governed by the characteristics of the deposited graphene and Cu surface morphology underneath. In this work, graphene films (1-2, 4-5 layers) were deposited on Cu using a chemical vapor deposition by changing hydrogen (H{sub 2}) concentration during the annealing stage. The chemical structure and surface microstructural features of graphene/Cu were inspected by Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. The electrochemical corrosion performance of graphene/Cu was studied in 0.5 M sodium chloride solution using potentiodynamic polarization and electrochemical impedance spectroscopic measurements. The 1-2 layers of graphene and rough Cu surface were obtained for low H{sub 2} concentrations (0 and 2.5%), whereas high H{sub 2} concentrations, 20 and 50%, resulted in a smooth Cu surface and 4-5 layers of graphene. Our results showed that Cu with a smooth surface and multilayer graphene film exhibited the best corrosion resistance against electrochemical degradation. Tafel analysis revealed that Cu with 4-5 layers of graphene coating corroded nearly three orders of magnitude slower than the annealed Cu, without graphene coating. The results of this study can be useful for several applications where Cu is in close contact with salt species.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Helium effects on the surface and subsurface evolutions in single-crystalline tungsten

Tungsten (W) has been perceived as one of the most promising plasma facing materials (PFMs) for future fusion reactors. In the past decade, its behavior under irradiation and helium (He) plasma interaction has been extensively studied. However, some key knowledge gaps still exist, such as the influence of crystallographic orientation on the surface and subsurface evolutions. In this work, we focus on the He ion-beam irradiation damage effects in mirror-polished single-crystalline W samples with three different surface planes of {100}, {110} and {111}. Irradiation was performed at room temperature using 40 keV He+ to a fluence of 1 × 10 16 /cm 2 , followed by thermal desorption spectroscopy (TDS) up to ~1920 K. The microstructures of He-irradiated W before and after TDS heat treatment were characterized by scanning and transmission electron microscopy. Subsurface He bubbles were imaged in all irradiated samples, but newly formed <111>-oriented surface grains and surface blisters were only observed in W {100} and {110} starting orientations. These results reveal that radiation damage, He thermal desorption, and surface/subsurface evolution are all strongly dependent on crystallographic orientation. Underlying physical mechanisms are discussed based on ion channeling effects, He-vacancy interactions, and surface diffusion. In conclusion, these findings provide new insights into He effects in W.

36 MATERIALS SCIENCE↗