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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 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Quantum Confinement and Decoherence Effect on Excited Electron Transfer at the Semiconductor–Molecule Interface: A First-Principles Dynamics Study

Excited electron dynamics at the semiconductor–molecule interface play a vital role in many solar energy conversion applications and beyond. First-principles dynamics simulation is used to study how excited electron dynamics in a type-II semiconductor–molecule interface is affected when the surface of a bulk semiconductor is replaced with that of a quantum dot. In particular, the interface between the hydrogen-terminated silicon(111) surface and a simple organic molecule is investigated. Quantum confinement was found to significantly slow down the excited electron relaxation and the interfacial transfer of the excited electron. Additionally, the role of decoherence was examined. The decoherence affects the dynamics much more significantly for the interface with the quantum dot, while the bulk surface case is affected only marginally by it. Finally, compared to the excited electron relaxation within the quantum dot, the decoherence effect was found to slow down the interfacial transfer from the quantum dot to the molecule by more than an order of magnitude, exhibiting a significant quantum Zeno effect.

charge transfer↗
DOE OSTI2021

New insights into the mechanism governing the elasticity of calcium silicate hydrate gels exposed to high temperature: A molecular dynamics study

When exposed to fire, the integrity of cement-based materials is governed by thermally-induced changes in the mechanical properties of their binding phase, i.e., the calcium–silicate–hydrate (C–S–H) gel. However, the effect of temperature on the structure, density, and mechanical properties of C–S–H remains only partially known. Here, based on reactive molecular dynamics simulations, we reveal the nature of thermally-induced damage in C–S–H gels. In general, we show that, at the atomic scale, exposure to high temperature results in partial dehydration, volumetric shrinkage, disordering, and stiffening in the C–S–H grains. However, we show that the thermal response of C–S–H strongly depends on its chemical composition, wherein C–S–H systems associated with lower Ca/Si molar ratios are able to undergo higher temperatures before amorphization. Based on these results, we demonstrate that the stiffness of C–S–H gels (i.e., including porosity—as probed by nanoindentation) is governed by a competition between the stiffening of the grains and the decrease in packing density—wherein the latter eventually become predominant.

36 MATERIALS SCIENCE↗
NASA NTRS1993

Structure and energetics of model amphiphilic molecules at the water liquid-vapor interface - A molecular dynamics study

A molecular dynamics study of adsorption of p-n-pentylphenol at infinite dilution at the water liquid-vapor interface is reported. The calculated free energy of adsorption is -8.8 +/- 0.7 kcal/mol, in good agreement with the experimental value of -7.3 kcal/mol. The transition between the interfacial region and the bulk solution is sharp and well-defined by energetic, conformational, and orientational criteria. At the water surface, the phenol head group is mostly immersed in aqueous solvent. The most frequent orientation of the hydrocarbon tail is parallel to the interface, due to dispersion interactions with the water surface. This arrangement of the phenol ring and the alkyl chain requires that the chain exhibits a kink. As the polar head group is being moved into the solvent, the chain length increases and the tail becomes increasingly aligned toward the surface normal, such that the nonpolar part of the molecule exposed to water is minimized. The same effect was achieved when phenol was replaced by a more polar head group, phenolate.

Pohorille, Andrew↗
DOE OSTI2026

Dataset_for_Molecular_Motion_Below_the_Glass_Transition_A_Solid-State_NMR_Study_of_Siloxane_Polymer_Dynamics Study

This dataset contains solid-state 1H and 13C NMR relaxometry data, differential scanning calorimetry (DSC) data, and size exclusion chromatography (SEC/GPC) data supporting the study of sub-glass-transition (sub-Tg) molecular dynamics in a composition- and sequence-controlled series of diphenyl-substituted polysiloxanes (PDMS, 14Ph, 33Ph, 50Ph, 67Ph, and 100Ph; 0–100% diphenylsiloxane content by mole).All solid-state NMR data were acquired on a 200 MHz Bruker Avance III HD spectrometer using a static 7 mm HX probe or a 4 mm HX probe under 4 kHz magic-angle spinning. Raw Bruker TopSpin experiment folders are included for: (1) variable-temperature 1H lineshape measurements used to determine linewidth (FWHM) as a function of temperature across the glass transition; (2) 1H T1 (saturation recovery with solid-echo detection), probing nanosecond-scale dynamics near the 1H Larmor frequency; (3) 1H T1rho (direct spin-lock, 62.5 kHz), probing microsecond-scale segmental dynamics; (4) 13C-detected Lee–Goldburg cross-polarization 1H T1rho (LGCPH T1rho) for 33Ph and 50Ph, resolving aromatic and aliphatic proton environments; and (5) 13C T1 relaxation for 33Ph and 50Ph. Differential scanning calorimetry data (TA Instruments DSC 25, −150 to +120 °C, up to +300 °C for 100Ph, 10 °C/min) are included for all six compositions and support the glass-transition temperatures in Table 1 and Figure 1. Size exclusion chromatography data (Agilent 1200 Series, PL-Gel 300 mixed-C column, THF mobile phase, polystyrene calibration standards) are included for the three synthesized copolymers (33Ph, 50Ph, 67Ph) and support the number-average molecular weights in Table 1. Processed data include per-composition relaxation-time summaries (Excel), curve-fitting and Bloembergen-Purcell-Pound (BPP) model analysis notebooks (Jupyter/Python), and Igor Pro (.pxp) master files used to generate the manuscript's figures.

Bloembergen-Purcell-Pound theory↗
DOE OSTI2019

Surface, Interface, and Temperature Effects on the Phase Separation and Nanoparticle Self Assembly of Bi-Metallic Ni0.5Ag0.5: A Molecular Dynamics Study

Classical molecular dynamics (MD) simulations were used to investigate how free surfaces, as well as supporting substrates, affect phase separation in a NiAg alloy. Bulk samples, droplets, and droplets deposited on a graphene substrate were investigated at temperatures that spanned regions of interest in the bulk NiAg phase diagram, i.e., miscible and immiscible liquid, liquid-crystal, and crystal-crystal regions. Using MD simulations to cool down a bulk sample from 3000 K to 800 K, it was found that phase separation below 2400 K takes place in agreement with the phase diagram. When free surface effects were introduced, phase separation was accompanied by a core-shell transformation: spherical droplets created from the bulk samples became core-shell nanoparticles with a shell made mostly of Ag atoms and a core made of Ni atoms. When such droplets were deposited on a graphene substrate, the phase separation was accompanied by Ni layering at the graphene interface and Ag at the vacuum interface. Thus, it should be possible to create NiAg core-shell and layer-like nanostructures by quenching liquid NiAg samples on tailored substrates. Furthermore, interesting bimetallic nanoparticle morphologies might be tuned via control of the surface and interface energies and chemical instabilities of the system.

core-shell nanoparticles↗
DOE OSTI2022

Phosphorothioate-Based Site-Specific Labeling of Large RNAs for Structural and Dynamic Studies

Pulsed electron-electron double resonance (PELDOR) spectroscopy, X-ray scattering interferometry (XSI), and single-molecule Forster resonance energy transfer (smFRET) are molecular rulers that provide inter- or intramolecular pair-wise distance distributions in the nanometer range, thus being ideally suitable for structural and dynamic studies of biomolecules including RNAs. The prerequisite for such applications requires site-specific labeling of biomolecules with spin labels, gold nanoparticles, and fluorescent tags, respectively. Recently, site-specific labeling of large RNAs has been achieved by a combination of transcription of an expanded genetic alphabet containing A-T/G-C base pairs and NaM-TPT3 unnatural base pair (UBP) with posttranscriptional modifications at UBP bases by click chemistry or amine-NHS ester reactions. However, due to the bulky sizes of functional groups or labeling probes used, such strategies might cause structural perturbation and decrease the accuracy of distance measurements. Here, we synthesize an a-thiophosphorylated variant of rTPT3TP (rTPT3aS), which allows for post-transcriptional site-specific labeling of large RNAs at the internal a-phosphate backbone via maleimide-modified probes. Subsequent PELDOR, XSI, and smFRET measurements result in narrower distance distributions than labeling at the TPT3 base. The presented strategy provides a new route to empower the molecular rulers for structural and dynamic studies of large RNA and its complex.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2022

Nucleation of Frank Dislocation during the Squeeze-Out Process in Boundary Lubrication: A Molecular Dynamics Study

Liquid–vapor molecular dynamics (LVMD) simulations are performed to reinvestigate the phase transition and solvation force oscillation behavior of a simple argon liquid film confined between two solid surfaces. Our simulations present a novel scenario in which the n → n – 1 layering transitions are accompanied by the formation, climb, and annihilation of Frank partial dislocations during the squeeze-out process under compression. This is indicated by the splitting of the repulsive peaks in the solvation force profile. The detailed analysis reveals that the formation–climb–annihilation mechanism of Frank dislocation occurs during approach and disappears during receding, which would result in force hysteresis. In combination with our recent works, this study provides new insights into the physical property of nanoconfined lubricant films in boundary lubrication.

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

Beam Dynamics Studies at the PIP-II Injector Test Facility

A series of beam dynamic studies were performed in 2020-2021 at the PIP-II Injector Test Facility (PIP2IT) that has been built to validate the concept of the front-end of the PIP-II linac being constructed at Fermilab. PIP2IT is comprised of a 30-keV H- ion source, a 2 m-long Low Energy Beam Transport (LEBT), a 2.1- MeV CW RFQ, followed by a 10-m Medium Energy Beam Transport (MEBT), 2 cryomodules accelerating the beam to 16 MeV and a High-Energy Beam Transport (HEBT) bringing the beam to a dump. This paper presents beam dynamics - related measurements performed at PIP2IT such as the Twiss parameters with Allison scanners, beam envelopes along the injector, and transverse and longitudinal rms emittance reconstruction. These measurements are compared with predictions from the beam dynamics code Tracewin.

43 PARTICLE ACCELERATORS↗
NASA NTRS1991

Interaction of monovalent ions with the water liquid-vapor interface - A molecular dynamics study

Results of molecular dynamics calculations are presented for a series of ions at infinite dilution near the water liquid-vapor interface. The free energies of ion transfer from the bulk to the interface are discussed, as are the accompanying changes of water structure at the surface and ion mobilities as a function of their proximity to the interface. It is shown that simple dielectric models do not provide an accurate description of ions at the water surface. The results of the study should be useful in the development of better models incorporating the shape and molecular structure of the interface.

Wilson, Michael A.↗
DOE OSTI2023

Computational Fluid Dynamics Study of a Cross-Flow Marine Hydrokinetic Turbine and the Combined Influence of Struts and Helical Blades

A computational fluid dynamics study was performed for a cross-flow marine hydrokinetic turbine. The analysis was done in three dimensions and used the unsteady Reynolds-averaged Navier-Stokes solver in the commercial code STAR-CCM+. The base turbine configuration is the RivGen(R) Turbine, designed by the Ocean Renewable Power Company. A convergence and uncertainty analysis was performed for both the spatial and temporal discretization; this was done using the base configuration, which features support struts and helical foils. Both struts and helical blades introduce three-dimensional flow effects, influencing the complex flow phenomenon of dynamic stall. The study compares the relative impact of struts on power performance and blade loading for both helical and straight blades, and found that for this turbine the relative loss in power from struts was lower with helical blades.

CFD↗
NASA NTRS1992

Computational fluid dynamics study of the variable-pitch split-blade fan concept

A computational fluid dynamics study was conducted to evaluate the feasibility of the variable-pitch split-blade supersonic fan concept. This fan configuration was conceived as a means to enable a supersonic fan to switch from the supersonic through-flow type of operation at high speeds to a conventional fan with subsonic inflow and outflow at low speeds. During this off-design, low-speed mode of operation, the fan would operate with a substantial static pressure rise across the blade row like a conventional transonic fan; the front (variable-pitch) blade would be aligned with the incoming flow, and the aft blade would remain fixed in the position set by the supersonic design conditions. Because of these geometrical features, this low speed configuration would inherently have a large amount of turning and, thereby, would have the potential for a large total pressure increase in a single stage. Such a high-turning blade configuration is prone to flow separation; it was hoped that the channeling of the flow between the blades would act like a slotted wing and help alleviate this problem. A total of 20 blade configurations representing various supersonic and transonic configurations were evaluated using a Navier Stokes CFD program called ADAPTNS because of its adaptive grid features. The flow fields generated by this computational procedure were processed by another data reduction program which calculated average flow properties and simulated fan performance. These results were employed to make quantitative comparisons and evaluations of blade performance. The supersonic split-blade configurations generated performance comparable to a single-blade supersonic, through-flow fan configuration. Simulated rotor total pressure ratios of the order of 2.5 or better were achieved for Mach 2.0 inflow conditions. The corresponding fan efficiencies were approximately 75 percent or better. The transonic split-blade configurations having large amounts of turning were able to generate large amounts of total turning and achieve simulated total pressure ratios of 3.0 or better with subsonic inflow conditions. These configurations had large losses and low fan efficiencies in the 70's percent. They had large separated regions and low velocity wakes. Additional turning and diffusion of this flow in a subsequent stator row would probably be very inefficient. The high total pressure ratios indicated by the rotor performance would be substantially reduced by the stators, and the stage efficiency would be substantially lower. Such performance leaves this dual-mode fan concept less attractive than originally postulated.

Kepler, C. E.↗