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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 1,351 records · Page 75
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

A direct comparison of particle-resolved and point-particle methods in decaying turbulence

We use particle-resolved direct numerical simulation (PR-DNS) as a model-free physics-based numerical approach to validate particle acceleration modelling in gas-solid suspensions. To isolate the effect of the particle acceleration model, we focus on point-particle direct numerical simulation (PP-DNS) of a collision-free dilute suspension with solid-phase volume fraction $\unicode[STIX]{x1D719}=0.001$ in a decaying isotropic turbulent particle-laden flow. The particle diameter $d_{p}$ in the suspension is chosen to be the same as the initial Kolmogorov length scale $\unicode[STIX]{x1D702}_{0}$ ( $d_{p}/\unicode[STIX]{x1D702}_{0}=1$ ) in order to overlap with the regime where PP-DNS is valid. We assess the point-particle acceleration model for two different particle Stokes numbers, $St_{\unicode[STIX]{x1D702}}=1$ and 100. For the high Stokes number case, the Stokes drag model for particle acceleration under-predicts the true particle acceleration. In addition, second moment quantities which play key roles in the physical evolution of the gas–solid suspension are not correctly captured. Considering finite Reynolds number corrections to the acceleration model improves the prediction of the particle acceleration probability density function and second moment statistics of the point-particle model compared with the particle-resolved simulation. We also find that accounting for the undisturbed fluid velocity in the acceleration model can be of greater importance than using the most appropriate acceleration model for a given physical problem.

Mechanics↗
DOE OSTI2019

A semi-Lagrangian direct-interaction closure of the spectra of isotropic variable-density turbulence

A study of variable-density homogeneous stationary isotropic turbulence based on the sparse direct-interaction perturbation (SDIP) and supporting direct numerical simulations (DNS) is presented. The non-solenoidal flow considered here is an example of turbulent mixing of gases with different densities. The spectral statistics of this type of flow are substantially more difficult to understand theoretically than those of the similar solenoidal flows. In the approach described here, the nonlinearly coupled velocity and scalar (which determine the density of the fluid) equations are expanded in terms of a normalised density ratio parameter. A new set of coupled integro-differential SDIP equations are derived and then solved numerically for the first-order correction to the incompressible equations in the variable-density expansion parameter. By adopting a regular expansion approach, one obtains leading-order corrections that are universal and therefore interesting in their own right. The predictions are then compared with DNS of forced variable-density flow with different density contrasts. It is found that the velocity spectrum owing to variable density is indistinguishable from that of constant-density turbulence, as it is supported by a wealth of indirect experimental evidence, but the scalar spectra show significant deviations, and even loss of monotonicity, as a function of the type and strength of the large-scale source of the mixing. Furthermore, the analysis helps clarify what may be the proper approach to interpret the power spectrum of variable-density turbulence.

Mechanics↗
DOE OSTI2020

Roles of Alkali Metals and Ionic Networks in Directing the Formation of Anionic Metal–Organic Frameworks

Integrating different types of fundamental chemical interactions to direct the self-assembly processes and to control the dimension of the network formed by each type of interactions is a promising strategy to develop advanced functional materials. Here, the ionic interaction of alkali metals (Na, K, Cs) towards multi-functional 2,2’-disulfo-4,4’-oxydibenzoic acid is utilized in two solvent systems, aqueous-amide and uro-amide, to synthesize six novel anionic framework materials with dramatically different features and dimensionalities: (Na 3 [Zn 4 O(L 4- ) 3 (e-murea)(H 2 O) 3 ](H 2 N(CH 3 ) 2 +) 3 (CPM-s1), [Zn 4 O(L 4- ) 3 (DMF)]((NH 2 (CH 3 ) 2 +) 6 ) (CPM-s2), Na 6 [(Zn 2 OH 2 ) 3 (L 3- ) 6 ](H 2 O) 3 (CPM-s3), K 3 [Zn 2 (L 4- ) 2 ](H 2 N(CH 3 ) 2 +)(e-murea) 2 (H 2 O) 3 (CPM-s4), K 4 [Zn(L 3- ) 2 ](H 2 O) 2 (CPM-s5), and Cs 6 [Zn(L 4- ) 2 ](H 2 O) 2 (CPM-s6), L 4- = 2,2’-disulfo-4,4’-oxydibenzoate, DMF = N,N-dimethylformamide, e-murea = 1,3-dimethyl-1-2-imidazolidinone). In CPM-s1 to CPM-s6, the alkali-sulfonate ionic bonding networks transformed from 0D clusters with Na + to 1D chains and 2D sheets with K + , and 3D network with Cs + . These vastly differing ionic domains influenced the coordination modes and spatial arrangements of the metal-carboxylate linkers, which consequently, directed the dimensionality of the coordination networks from 3D with Na + to 1D with Cs + . CPM-s1 and CPM-s2, made from tetrameric Zn 4 O clusters are rare examples of anionic MOF-5-type structures. CPM-s3 features an unprecedented hexameric Zn 6 cluster. Even though a large portion of the charge-balancing cations in these materials would hamper hydrogen bonding networks, some of them still exhibit very high ionic conductivity (e.g. 1.25 x 10 -3 S cm -1 in CPM-s3), suggesting the potential of these materials as solid-state electrolytes.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Polymers for Extreme Conditions Designed Using Syntax-Directed Variational Autoencoders

We report the design/discovery of new materials is highly nontrivial owing to the near-infinite possibilities of material candidates and multiple required property/performance objectives. Thus, machine learning tools are now commonly employed to virtually screen material candidates with desired properties by learning a theoretical mapping from material-to-property space, referred to as the forward problem. However, this approach is inefficient and severely constrained by the candidates that the human imagination can conceive. Thus, in this work on polymers, we tackle the materials discovery challenge by solving the inverse problem: directly generating candidates that satisfy desired property/performance objectives. We utilize syntax-directed variational autoencoders (VAE) in tandem with Gaussian process regression (GPR) models to discover polymers expected to be robust under three extreme conditions: (1) high temperatures, (2) high electric field, and (3) high temperature and high electric field, useful for critical structural, electrical, and energy storage applications. This approach to learn from and augment) human ingenuity is general and can be extended to discover polymers with other targeted properties and performance measures.

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

Direct Synthesis of Six-Monolayer (1.9 nm) Thick Zinc-Blende CdSe Nanoplatelets Emitting at 585 nm

Here, quasi-two-dimensional semiconductor nanoplatelets (NPLs) have garnered widespread interest because of their uniquely narrow emission spectra and favorable characteristics for optical gain and lasing.(1–5) Strongly quantum confined along thickness, NPLs exhibit the electronic structure of quantum wells determined by their thickness, which can be controlled with atomic precision.(6–10) Among all semiconductor NPLs, CdSe NPLs are probably the most studied. So far, four kinds of CdSe NPLs, emitting at around 396 nm, 463 nm, 513m, and 553 nm, have been intensely investigated for almost a decade.(11–18) Despite some early confusions in identifying actual thicknesses of those NPLs, they have been successfully assigned to be 2-monolayer (2 ML) to 5-monolayer (5 ML) thickness by high-resolution TEM.(19) Here, nML thickness corresponds to n chalcogen layers sandwiched between n + 1 metal layers. Another kind of NPLs with 6MLs of CdSe, once briefly mentioned in 2013,(20) however, was not explored, likely because its synthesis proved hard to reproduce. The 6 ML CdSe NPLs can be prepared by the colloidal atomic layer deposition (c-ALD) method,(21) but there have been no reports on direct synthesis of 6 ML CdSe NPLs, and their direct synthesis has been in doubt as being difficult by traditional approaches.(22,23)

36 MATERIALS SCIENCE↗
DOE OSTI2020

Direct Conversion of Biomass Carbohydrates to Platform Chemicals: 5-Hydroxymethylfurfural (HMF) and Furfural

5-hydroxymethylfurfural (HMF) and furfural are promising platform molecules which can be converted to fuel intermediates and hydrocarbon fuels. Herein, one-step, direct conversion of the carbohydrates in never treated biomass to HMF and furfural in a microwave reactor was investigated. The influence of reaction temperature, time, feedstock, addition of Lewis and Bronsted acids, solvent and solvent to aqueous ratio were studied to find the optimum process conditions to maximize the yields of furfurals. The reaction conditions of 200 degrees C for 5 min with 33 mM HCl and 8 mM AlCl 3 using a dioxane/water (4:1) miscible solvent system were found to be the most conducive for direct dehydration of the glucan and hemicelluloses present in untreated poplar wood to furfural and HMF in yields of 91 and 69%, respectively. Furfural could be easily produced in yields greater than 80%, and once formed, it was found to be stable under the various process conditions explored. In contrast, HMF yields varied with both reaction time and temperature, as it readily underwent rehydration to levulinic acid in yields approaching 25-30% at increased reaction severity. The additions of both Bronsted (HCl) and Lewis (AlCl 3 ) acids in catalytic amounts, i.e., 33 and 8 mM, respectively, were required to maximize the yield of furfurals. Reactions were also performed on other feedstocks (corn stover, switchgrass, loblolly pine, and pure cellulose) to investigate the effect of feedstock type on furfurals yields. Results showed that HMF and furfural yields were primarily dependent on hexosan and pentosan loading regardless of the type of feedstock. In addition, biomass physical features, such as crystallinity or the presence of other biomass constituents, such as lignin, had little influence on furfurals yields.

09 BIOMASS FUELS↗
DOE OSTI2025

Seeding Advanced Treated Wastewater for Purposes of Direct Potable Reuse

Direct potable reuse (DPR) is a promising solution to address water scarcity. However, a better understanding of how introducing advanced treated water (ATW) affects microbial communities present in distribution systems is needed. Here, in this study, we measured changes to the microbial water quality in simulated distribution systems that were conditioned using treated, unimpaired surface water (SW) and then transitioned to ATW. In addition, we investigated whether adding a biological filtration step would seed the microbial community of the ATW and whether the influence would persist in the simulated distribution systems. We found that the bulk water in the ATW-fed distribution systems had lower cell counts and ATP concentrations and a distinct microbial community (based on 16S amplicon sequencing) compared to the SW-fed or the seeded ATW-fed systems. However, biofilm community composition and biomass remained consistent regardless of the feedwater. Increased microbial biomass and diversity were present in the seeded ATW, with several amplicon sequence variants identified as being introduced by the biological filter. Our results suggest that directly introducing ATW to distribution systems could disturb the existing microbial community. Preparing ATW for distribution via biological filtration may deliver more predictable and stable microbial water quality than introducing unseeded ATW.

16S↗
DOE OSTI2021

Multiscale Modeling of a Direct Nonoxidative Methane Dehydroaromatization Reactor with a Validated Model for Catalyst Deactivation

Due to the recent boom in shale gas production, aromatics production using direct nonoxidative methane dehydroaromatization (DHA) is being investigated extensively. However, due to rapid coke formation, catalysts in the nonoxidative methane DHA reactors get deactivated, which is one of the critical issues for the commercial success of the methane DHA process. In this paper, a model for catalyst deactivation is developed. Rate models for other DHA reactions are developed by considering the decrease in the catalyst activity with time. Due to the very fast coke formation rate on the fresh catalyst, there is coke formation immediately upon the introduction of the feed. Therefore, an algorithm is developed for estimation of the initial state of the reactor and the kinetic parameters by coupling an iterative direct substitution approach with an optimization approach. Transient experimental data from an in-house reactor are first reconciled and then used for developing the kinetic model including the coke formation model. Using the rate model, a dynamic, heterogeneous, multiscale reactor model with embedded heating is developed. Here, the model couples the catalyst pellet level model with a reactor level model. Impacts of temperature, L/D ratio, and scheduling of reactors on variability in conversion and yield with time are studied.

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

Direct Measurement of the Selective Microwave-Induced Heating of Agglomerates of Dipolar Molecules: The Origin of and Parameters Controlling a Microwave Specific Superheating Effect

Agglomerates of polar molecules in nonpolar solvents are selectively heated by microwave radiation. The magnitude of the selective heating was directly measured by using the temperature dependence of the intensities of the Stokes and anti-Stokes bands in the Raman spectra of p-nitroanisole (pNA) and mesitylene. Under dynamic heating conditions, a large apparent temperature difference (ΔT) of over 100 °C was observed between the polar pNA solute and the nonpolar mesitylene solvent. This represents the first direct measurement of the selective microwave heating process. The magnitude of the selective microwave heating was affected by the properties of the agglomerated pNA. As the concentration of the pNA increases, the magnitude of the selective heating of the pNA was observed to decrease. This is explained by the tendency of the pNA dipoles to orient in an antiparallel fashion in the aggregates as measured by the Kirkwood g value, which decreased with increasing concentration. This effect reduces the net dipole moment of the agglomerates, which decreases the microwave absorption. After the radiation was terminated, the effective temperature of the dipolar molecules returned slowly to that of the medium. The slow heat transfer was modeled successfully by treating the solutions as a biphasic solvent/solute system. Based on modeling and the fact that the agglomerate can be heated above the boiling temperature of the solvent, an insulating layer of solvent vapor is suggested to form around the heated agglomerate, slowing convective heat transfer out of the agglomerate. Furthermore, this is an effect unique to microwave heating.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Complex-Valued Intermolecular Coupling Enables Directional Exciton Transport in Excitonic Circuits

Molecular systems capable of directing the flow of excitons are key to the development and optimization of optoelectronic materials. The transport of excitons across multiple molecules is governed by an intermolecular electronic coupling network. In this article, we consider the effects of complex-valued intermolecular electronic coupling on exciton transport. Here, we present a molecular motif capable of generating complex-valued coupling under excitation with circularly polarized light. We use theoretical modeling and simulation to illustrate how complex coupling can be leveraged to drive the rotational flux of excitons in cyclic molecular networks and direct the exciton population in branched molecular networks.

chromophores↗
DOE OSTI2018

Insights on the Alumina–Water Interface Structure by Direct Comparison of Density Functional Simulations with X-ray Reflectivity

Density functional theory molecular dynamics (DFT-MD) simulations are frequently used to predict the interfacial structures and dynamical processes at solid-water interfaces in efforts to gain a deeper understanding of these systems. However, the accuracy of these predictions of interfacial structure has not been rigorously quantified. Here, direct comparisons between large-scale DFT-MD simulations and high-resolution X-ray reflectivity (XR) measurements of the well-defined Al2O3(001)-water interface reveal the relative accuracy of these two methods to describe interfacial structure, a comparison that is enabled by XR’s high sensitivity to atomic-scale displacements. The DFT-MD simulated and XR model-fit structures are qualitatively similar, but XR signals calculated directly from the DFT-MD predictions deviate significantly from the experimental data, revealing discrepancies in these two approaches. Differences in the derived interfacial Al2O3 relaxation profiles of ~0.02 A within the top 5 layers are significant to XR but at the limit of the accuracy of DFT. Further differences are found in the surface hydration layer with a simulated average water layer height ~0.2 A higher than that observed experimentally. This is outside the accuracy of both XR and DFT and is not improved by the inclusion of a phenomenological correction for hydrogen bonding (e.g., Grimme).

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2022

Discrete Orientations of Interfacial Waters Direct Crystallization of Mica-Binding Proteins

Understanding the basis of templated molecular assembly on a solid surface requires a fundamental comprehension of both short- and long-range aqueous response to the surface under a variety of solution conditions. Herein we provide a detailed picture of how the molecular-scale response to different mica surfaces yields distinct solvent orientations that produce quasi-static directional potentials onto which macromolecules can adsorb. We connect this directionality to observed (a)symmetric epitaxial alignment of designed proteins onto these surfaces, corroborate our findings with 3D atomic force microscopy experiments, and identify slight differences in surface structure as the origin of this effect. Furthermore, our work provides a detailed picture of the intrinsic electrolyte response in the vicinity of mineral interfaces, with clear predictions for experiment, and highlights the role of solvent on the predictive assembly of hierarchical materials on mineral surfaces.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Theoretical Perspective of Promoting Direct Methane-to-Methanol Conversion at Complex Metal Oxide–Metal Interfaces

Direct methane conversion to methanol has been considered as an effective and economic way to release greenhouse effects and current high demand on methanol in industry. However, the process has long been challenging due to lack of viable catalysts to compromise the activation of methane that typically occurs high temperatures and retaining of produced methanol that requires mild conditions. Here, this perspective demonstrates an effective strategy to promote direct methane to methanol conversion by engineering the active sites and chemical environments at complex metal oxide - copper oxide - copper interfaces. Such effort strongly depends on the extensive theoretical studies by combining density functional theory (DFT) calculations and kinetic Monte Carlo (KMC) simulations to provide in-depth understanding of reaction mechanism and active sites, which built the strong basis to enable the identification of design principles and advance the catalyst optimization for selective CH 4 -to-CH 3 OH conversion.

03 NATURAL GAS↗
DOE OSTI2019

Direct Visualization of Counter-Propagating Surface Plasmons in Real Space-Time

We deploy two-dimensional nano-hole arrays as resonant surface plasmon polariton (SPP) couplers that enable counter-propagation and interference-free imaging of SPP wave-packets. We monitor the spatiotemporal evolution of the resulting SPPs using two-color photoemission electron microscopy (PEEM). The measurements track the electric field envelope of the SPP in real space and time and enable direct characterization of their spatiotemporal properties in a regime where the SPP wave-packet is the principle observable. We provide an analysis of the observables for both the co- and counter-propagating directions via SPP trajectories that are recorded in tandem. Our results highlight the advantages of isolating SPPs through counter-propagation, where field-SPP interactions are suppressed.

Crampton, Kevin T.↗
DOE OSTI2019

Unconventional Complex Coacervation between Neutral Polymer and Inorganic Polyoxometalate in Aqueous Solution via Direct Water Mediation

Water, specifically in a hydration shell, is critical for many biological supramolecular aggregations in nature, where water can directly mediate intermolecular association via hydrogen bonding and is regarded as “structured water”. Conversely, little has been reported on the biomimetic water-mediated supramolecular assembly with adequately high water content to date, because of the competing thermodynamic processes of water hydration and water as a building block to participate in self-assembly. In this work, we explore water-mediated complexation based on entropy-driven biphasic coacervate formation using highly hydrophilic neutral polymer and inorganic mineral-analogous nanoclusters. For the first time (to the best of our knowledge), nonelectrostatic liquid–liquid separating coacervate formation is demonstrated between polyethylene glycol (PEG) and polyoxometalate (POM) nanoclusters in aqueous solutions of varied PEG and POM concentrations, POM types, and aqueous medium conditions. Comprehensive characterization using fluorescence microscopy, small-angle X-ray scattering, calorimetry, and other techniques has confirmed that the compositions, microstructure, and thermodynamics of PEG–POM complex coacervation are highly similar to entropy-driven complex coacervation between oppositely charged polyelectrolytes in aqueous solution. However, the effect of heavy water on critical POM concentration for the onset of coacervate formation suggests that water, instead of the counter ions as commonly debated for polyelectrolyte complex coacervation, is responsible for PEG–POM coacervate formation. Specifically, structured water works as a hydrogen bond donor for both highly hydrated PEG and POM to directly mediate the PEG–water–POM association, resulting in the release of excess hydrated water for entropy-driven PEG–POM complex coacervation. Furthermore, water-mediated complex coacervation could be developed as a general and simple strategy to build biomimetic hybrid nanomaterials with high water content for various applications from energy-related functional nanomaterials to biomedical ramification.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Direct Correlation of Charge Carrier Transport to Local Crystal Quality in Lead Halide Perovskites

Although solution processing methods provide an attractive route toward development of low-cost functional materials, these accessible fabrication approaches can engender high concentrations of microscopic structural defects that are detrimental to performance. In lead halide perovskites, structural disorder derived from solution processing has been implicated as an important determiner of photophysical properties. However, a direct correlation between the functional properties of these materials and the local crystal structure in which non-equilibrium states evolve has remained elusive, in part because structural heterogeneities occur on length scales that defy conventional characterization techniques. To address this knowledge gap, in this work we have combined ultrafast pump–probe microscopy and electron backscattering diffraction to directly correlate charge carrier transport with the local diffraction pattern contrast, an indicator of crystal quality. Spatial correlation of these measurements strongly suggests that even on individual single crystal CsPbBr3 domains, microscopic variability in the crystal quality profoundly impacts the efficiency of charge carrier transport.

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

Direct Observation of Large Flexoelectric Bending at the Nanoscale in Lanthanide Scandates

There is a growing interest in the flexoelectric effect, since at the nanoscale it is predicted to be very large. However, there have been no direct observations of flexoelectric bending consistent with current theoretical work that implies strains comparable to or exceeding the yield strains of typical materials. In this work, we show a direct observation of extraordinarily large, two-dimensional reversible bending at the nanoscale in dysprosium scandate due to the converse flexoelectric effect, with similar results for terbium and gadolinium scandate. Within a transmission electron microscope, thin features bend up to 90° with radii of curvature of about 1 μm, corresponding to very large nominal strains. Analysis including independent experimental determination of the flexoelectric coefficient is semiquantitatively consistent with interpreting the results as due to flexoelectricity. These results experimentally demonstrate large flexoelectric bending at the nanoscale.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Direct Laser Writing Crystal Polymorphs of Organic Semiconductors for Phase Change Electronics

The many diverse polymorphic behaviors observed in organic electronic materials offer opportunities to modulate electronic properties through reversibly switching crystal structures. Here, we access the prolific polymorphism observed in two-dimensional quinoidal terthiophene via laser writing to locally heat and direct the phase transitions. We access a metastable polymorph IV through rapid cooling and observe distinct symmetry as well as packing through grazing incidence X-ray diffraction (GIXD). Using our open-source PolyChemPrint patterning platform, we direct laser heating to initiate the IV-I transition, switching the conductance by >2 orders of magnitude. Here, this is confirmed via a combination of GIXD and Raman spectroscopy. Finally, we demonstrate switching of transistor devices as well as discrete tuning of conductance via laser writing.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗