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At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Predicting initial dissolution rates using structural features from molecular dynamics simulations

Predicting chemical durability of glass materials is important for various applications from daily life such as drink glass and kitchen ware to advanced technologies such as nuclear waste disposal and biomedicine. In this work, we explored prediction of initial dissolution rate through structural features from molecular dynamics (MD) simulations for a wide range of glass compositions (total 28) including borosilicate and aluminosilicate glasses, ZrO 2 -containing and V 2 O 5 -containing boroaluminosilicate glasses. The initial dissolution rates (r 0 ) measured experimentally at 90 °C with varying solution conditions were correlated with structural features (e.g., polyhedral linkages and non-bridging oxygen species) obtained from MD simulations, either from this study or from literature. Since hydrolysis of the glass network through breaking of the network former linkages (e.g., Si-O-Si, Si-O-Al, etc.) is a critical step of network glass dissolution, the statistics of these linkages obtained from MD were correlated to r0 through linear regression, where the coefficient of determination (R 2 ) and root mean square error are found to be 0.949 and 0.681, respectively. This model was compared and discussed with existing models developed by various approaches including machine learning, the kinetic rate equation, topological constraint theory, and other descriptors from MD simulations. The discussion provides insights on future model improvements to predict glass dissolution. In addition, the impact of V 2 O 5 on the glass dissolution was examined in detail, implicating that the impact is not the same across all glass compositions and test conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2022

An initial investigation of accuracy required for the identification of small molecules in complex samples using quantum chemical calculated NMR chemical shifts

The majority of primary and secondary metabolites in nature have yet to be identified, representing a major challenge for metabolomics studies that currently require reference libraries from analyses of authentic compounds. Using currently available analytical methods, complete chemical characterization of metabolomes is infeasible for both technical and economic reasons. For example, unambiguous identification of metabolites is limited by the availability of authentic chemical standards, which, for the majority of molecules, do not exist. Computationally predicted or calculated data are a viable solution to expand the currently limited metabolite reference libraries, if such methods are shown to be sufficiently accurate. For example, determining nuclear magnetic resonance (NMR) spectroscopy spectra in silico has shown promise in the identification and delineation of metabolite structures. Many researchers have been taking advantage of density functional theory (DFT), a computationally inexpensive yet reputable method for the prediction of carbon and proton NMR spectra of metabolites. However, such methods are expected to have some error in predicted 13 >C and 1 H NMR spectra with respect to experimentally measured values. This leads us to the question–what accuracy is required in predicted 13 C and 1 H NMR chemical shifts for confident metabolite identification? Using the set of 11,716 small molecules found in the Human Metabolome Database (HMDB), we simulated both experimental and theoretical NMR chemical shift databases. We investigated the level of accuracy required for identification of metabolites in simulated pure and impure samples by matching predicted chemical shifts to experimental data. We found 90% or more of molecules in simulated pure samples can be successfully identified when errors of 1 H and 13 C chemical shifts in water are below 0.6 and 7.1 ppm, respectively, and below 0.5 and 4.6 ppm in chloroform solvation, respectively. In simulated complex mixtures, as the complexity of the mixture increased, greater accuracy of the calculated chemical shifts was required, as expected. However, if the number of molecules in the mixture is known, e.g., when NMR is combined with MS and sample complexity is low, the likelihood of confident molecular identification increased by 90%.

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

Two-Electron–Two-Proton Transfer from Colloidal ZnO and TiO 2 Nanoparticles to Molecular Substrates

Transfers of multiple electrons and protons are challenging yet central to many energy-conversion processes and other chemical and biochemical reactions. Semiconducting oxides can hold multiple redox equivalents. This study describes the 2e – /2H + transfer reactivity of photo-reduced ZnO and TiO 2 nanoparticle (NP) colloids with molecular 2e – /2H + acceptors, to form new O-H, N-H, and C-H bonds. The reaction stoichi-ometries were monitored by NMR and optical spectroscopies. Faster 2e – /2H + transfer rates were observed for substrates forming O-H or N-H bonds, presumably due to initial hydrogen bonding at the oxide surface. Chemically reduced ZnO NPs stabilized by Na + or Ca 2+ also engage in 2e – /2H + transfer reactivity, showing that protons transferred in these processes are inherent to the oxide nanoparticles and do not exclusively stem from photoreduction. Furthermore, these results highlight the potential of ZnO and TiO 2 for multiple proton-coupled electron transfer (PCET) reactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1980

The chemical abundances of the Cassiopeia A fast-moving knots - Explosive nucleosynthesis on a minicomputer

A simplified nuclear reaction network for explosive nucleosynthesis calculations is described in which only the most abundant nuclear species and the most important reactions linking these species are considered. This scheme permits the exploration of many cases without excessive computational effort. Good agreement with previous calculations employing more complex reaction networks is obtained. This scheme is applied to the observed chemical abundances of the fast-moving knots in the supernova remnant Cassiopeia A and it is found that a wide range of initial conditions could yield the observed abundances. The abundances of four of the knots with significant and different amounts of elements heavier than oxygen are consistent with an origin in material of the same initial composition but processed at different peak temperatures and densities. Despite the observed high oxygen abundances and low abundances of light elements in the knots, they did not necessarily undergo incomplete oxygen burning; in fact, it is not even necessary that oxygen have been present in the initial composition. The agreement between the calculated and observed chemical abundances in Cas A and similar supernova remnants depends primarily upon the relevant nuclear physics and does not provide strong evidence in favor of any particular model of the supernova event.

Johnston, M. D.↗
DOE OSTI2025

Chemical timescale effects on detonation convergence

Numerical simulations of detonation-containing flows have emerged as crucial tools for designing next- generation power and propulsion devices. As these tools mature, it is important for the combustion community to properly understand and isolate grid resolution effects when simulating detonations. To this end, the objective of this work is to provide a comprehensive analysis of the numerical convergence of unsteady detonation simulations, with focus on isolating the impacts of chemical timescale modifications on convergence characteristics in the context of operator splitting. With the aid of an AMReX-based adaptive mesh refinement flow solver-which enables resolutions up to ($\mathcal{O}$ (1000) cells-per-induction length-the convergence analysis is conducted using two kinetics configurations: (1) the simplified three-step Arrhenius-based model mechanism of Short and Quirk (1997), where chemical timescales in the detonation are modified by adjusting activation energies in the initiation and branching reactions, and (2) a detailed hydrogen- air mechanism, where the chemical timescales are adjusted by varying the ambient pressure. The convergence of unsteady self-sustained detonations in one-dimensional channels is then analyzed with reference to steady-state theoretical baseline solutions using these mechanisms. The goal of the analysis is to provide a detailed comparison of the effects of grid resolution on both macroscopic (peak pressures and wave speeds) and microscopic (wave structure) quantities of interest, drawing connections between the deviations from steady-state baselines and minimum chemical timescales. In particular, chemical timescale reductions were found to have minimal impact on the convergence of macroscopic properties. Furthermore, analyses of microscopic convergence trends, particularly in the reaction front location, revealed a key insight: maintaining the induction time while eliminating prohibitive chemical timescales through mechanism simplifications and combustion modeling can significantly enhance detonation convergence properties. Ultimately, this work uncovers resolution-dependent unsteady detonation convergence regimes and highlights the important role played by not only the chemical timescales, but also the ratio between the chemical timescale and induction time on the numerical convergence of the detonation wave structure.

Adaptive mesh refinement↗
DOE OSTI2024

Properties of molecular clumps and cores in colliding magnetized flows

ABSTRACT We simulate the formation of molecular clouds in colliding flows of warm neutral medium with the adaptive mesh refinement code flash in eight simulations with varying initial magnetic field strength, between 0.01–5 μG. We include a chemical network to treat heating and cooling and to follow the formation of molecular gas. The initial magnetic field strength influences the fragmentation of the forming cloud because it prohibits motions perpendicular to the field direction and hence impacts the formation of large-scale filamentary structures. Molecular clump and core formation occurs anyhow. We identify 3D clumps and 3D cores, which are defined as connected, CO-rich regions. Additionally, 3D cores are heavily shielded. While we do not claim those 3D objects to be directly comparable to observations, this enables us to analyse their full virial state. With increasing field strength, we find more fragments with a smaller average mass; yet the dynamics of the forming clumps and cores only weakly depends on the initial magnetic field strength. The molecular clumps are mostly unbound, probably transient objects, which are weakly confined by ram pressure or thermal pressure, indicating that they are swept up by the turbulent flow. They experience significant fluctuations in the mass flux through their surface, such that the Eulerian reference frame shows a dominant time-dependent term due to their indistinct nature. We define the cores to encompass highly shielded molecular gas. Most cores are in gravitational-kinetic equipartition and are well described by the common virial parameter $\alpha _\mathrm{vir}$, while some undergo minor dispersion by kinetic surface effects.

Weis, M. (ORCID:0000000256838860)↗
DOE OSTI2022

Cell design for laser-induced breakdown spectroscopy measurements on reactive gas samples

Uranium hexafluoride (UF 6 ) is the primary material used for the enrichment of uranium in the production of light water nuclear reactor fuels worldwide. Accurate, rapid quantification of uranium isotopic composition in nuclear materials is required for safeguards programs and nonproliferation purposes. One potential technique for isotopic measurements in uranium species in the field is laser induced breakdown spectroscopy (LIBS). Safe and effective application of LIBS to UF 6 for enrichment measurements is uniquely challenging due to the chemical and physical properties of UF 6 , which necessitate specific handling procedures. The objective of this work is to design a cell for isotopic analysis of UF 6 that is (1) compatible with chemical and physical properties of UF 6 , (2) compatible with LIBS laser, and (3) portable-sized for nuclear safeguards applications. Along with cell design, initial testing of the cell for basic performance and chemical compatibility is performed. As designed and constructed, the portable gas cell was gas-tight, chemically compatible with UF 6 , and withstood long-duration laser exposure. In this work, the cell has proven capability for handling reactive gasses, such as UF 6 , with specification application to isotopic analysis.

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

ChemoGraph: Interactive Visual Exploration of the Chemical Space

Exploratory analysis of the chemical space is an important task in the field of cheminformatics. For example, in drug discovery research, chemists investigate sets of thousands of chemical compounds in order to identify novel yet structurally similar synthetic compounds to replace natural products. Manually exploring the chemical space inhabited by all possible molecules and chemical compounds is impractical, and therefore presents a challenge. To fill this gap, we present ChemoGraph, a novel visual analytics technique for interactively exploring related chemicals. In ChemoGraph, we formalize a chemical space as a hypergraph and apply novel machine learning models to compute related chemical compounds. It uses a database to find related compounds from a known space and a machine learning model to generate new ones, which helps enlarge the known space. Moreover, ChemoGraph highlights interactive features that support users in viewing, comparing, and organizing computationally identified related chemicals. With a drug discovery usage scenario and initial expert feedback from a case study, we demonstrate the usefulness of ChemoGraph.

chemical space exploration↗
NASA NTRS1996

Parallelization and Visual Analysis of Multidimensional Fields: Application to Ozone Production, Destruction, and Transport in Three Dimensions

The three-dimensional, spectral transport model used in the current project was first successfully integrated over climatological time scales by Dr. Guang Ping Lou for the simulation of atmospheric N2O using the United Kingdom Meteorological Office (UKMO) 4-dimensional, assimilated wind and temperature data set. A non-parallel, FORTRAN version of this integration using a fairly simple N2O chemistry package containing only photo-chemical reactions was used to verify our initial parallel model results. The integrations reproduced the gross features of the observed stratospheric climatological N2O distributions but also simulated the structure of the stratospheric Antarctic vortex and its evolution. Subsequently, Dr. Thomas Kindler, who produced much of the parallel version of our model, enlarged the N2O model chemistry package to include N2O reactions involving O(D-1) and also introduced assimilated wind data from NASA as well as UKMO. Initially, transport calculations without chemistry were run using Carbon-14 as a non-reactive tracer gas with the result that large differences in the transport properties of the two assimilated wind data sets were apparent from the resultant Carbon-14 distributions. Subsequent calculations for N2O, including its chemistry, with the two input winds data sets with verification from UARS satellite observations have refined the transport differences between the two such that the model's steering capabilities could be used to infer the correct climatological vertical velocity fields required to support the N2O observations. During this process, it was also discovered that both the NASA and the UKMO data contained spurious values in some of the higher frequency wave components, leading to incorrect local transport calculations and ultimately affecting the large scale properties of the model's N2O distributions, particularly at tropical latitudes. Subsequent model runs with wind data that had been filtered to remove some of the high frequency components produced much more realistic N2O distributions. During the past few months, the UKMO wind data base for a complete two-year period was processed into spectral form for model use. This new version of the input transport data base now includes complete temperature fields as well as the necessary wind data. This was done to facilitate advanced chemical calculations in the parallel model which often depend upon temperature. Additional UKMO data is being added as it becomes available.

Schwan, Karsten↗
NASA NTRS2003

Mid-Infrared Spectroscopy of Polycyclic Aromatic Nitrogen Heterocycles (PANHS) and their Ions

In recent years, polycyclic aromatic nitrogen heterocycles (PANHs) have attracted a good deal of attention because of their potent carcinogenic and mutagenic properties, and their prevalence in our environment. Such species also play a prominent role in the chemistry of life up to and including the very nucleobases from which our DNA is constructed. Surprisingly, these compounds may even be common outside of our terrestrial environment. To wit, it is now widely accepted that polycyclic aromatic materials are abundant in space and represent a major reservoir of organic carbon in the interstellar medium and developing planetary systems. Given that nitrogen is the fourth most abundant chemically reactive element in space (surpassed only by hydrogen, carbon, and oxygen), it is entirely reasonable to suspect that PANHs may represent an important component of that organic reservoir. Motivated by their intrinsic merit and with special attention toward evaluating their exobiological significance, we have initiated a program to study the spectroscopic and chemical properties of P A " s under conditions relevant to extraterrestrial environments. Here we present the first results of that program-infrared spectroscopic measurements on a series of PANH"s in neutral and cationic forms, isolated in inert matrices at cryogenic temperatures.temperatures. The species studied include: 1 -, and 2-azabenz[a]anthracene, 1-, 2-, and 4- azachrysene, dibenz[a,h]acridine, and dibenz[a,J)acridine. The experimental measurements are also compared with theoretical spectra calculated using density functional theory. General spectroscopic trends observed in this series of compounds are discussed and the implications of these results for Astrophysics and Exobiology are considered.

Mattioda, Andrew L.↗
DOE OSTI2022

Nanoscale trace-element zoning in pyrite framboids and implications for paleoproxy applications

Pyrite framboids (spherical masses of nanoscale pyrite) are among the earliest textures of pyrite to form in sediments. It has been proposed that their trace-element (TE) contents can be used to track the TE composition of the water column in which they formed. However, it is not clear how these TEs are associated with the framboidal pyrite grains. For instance, it is important to know whether they are incorporated uniformly or are enriched in different regions of the framboid. Here, we used high-resolution scanning transmission electron microscopy to identify chemical zoning within pyrite framboids. We found that initial, nanoscale pyrite euhedral crystals, which make up the volumetric majority of the framboids, are covered/infilled by later pyrite that templates on the earlier pyrite. Further, this later pyrite is enriched in TEs, suggesting that many TEs are incorporated in pyrite relatively late (during early diagenesis; not in the water column). This observation suggests that although chemical analyses of pyrite framboids may provide ocean-water chemistry trends through time, the details are complex. Specifically, the TEs found in pyrite may be linked to adsorption onto organic matter, detrital material, and authigenic minerals such as Fe- and Mn-oxide phases followed by desorption in the sediments or release via dissolution and incorporation into pyrite as overgrowths on the initial nanoscale euhedral crystals that make up framboids. While the use of pyrite chemistry to understand past ocean conditions remains promising, and even diagenetic additions may not preclude the utility of pyrite for reconstructing ancient ocean conditions, care must be taken in interpretations because the end concentration may be influenced by diagenesis.

58 GEOSCIENCES↗
DOE OSTI2024

Effect of supplementary cementitious materials on the degradation of cement-based barriers in radioactive waste repository: A case study in Korea

This study focuses on investigating the chemical degradation characteristics of cementitious barriers used in low- and intermediate-level radioactive waste repository by reactive transport modeling. The impact of the blending with supplementary cementitious materials (SCMs) in the barriers on the chemical degradation was evaluated to find the optimum barrier design. A number of different barrier designs were examined by replacing ordinary Portland cement (OPC) by SCMs (i.e., fly ash, silica fume, and blast-furnace slag). The simulation results showed that silica fume blended barrier has better durability against chemical degradation by rainwater compared to fly ash or blast-furnace slag blended barriers. In addition, the chemical durability of silica fume-based barrier increased with increasing replacement level up to about 20 %. It seems that the amount of formed calcium silicate hydrate (CSH) in the initial cement-based barrier highly affects the overall chemical durability. The newly developed reactive transport model demonstrated its capability for understanding the barrier performance and investigating the optimal design of the barrier system.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2020

Intrinsic chemical reactivity of solid-electrolyte interphase components in silicon–lithium alloy anode batteries probed by FTIR spectroscopy

Herein we report the solid reaction products from the chemical reaction of aprotic battery electrolyte and three purported components of the Si-based anode SEI : SiO 2 nanoparticles (NPs), lithium silicate (Li x SiO y ) powders, and Si NPs. We use FTIR and classical molecular dynamics/density functional perturbation theory to assess the solid products remaining with these model materials after exposure to electrolyte. The absence of electrochemical bias provides a view of the chemical speciation resulting from early-stage chemical reactivity during battery assembly as well as under open circuit storage conditions. We believe these species represent the initial stages of SEI growth and predict they likely drive subsequent chemical and electrochemical reactions by controlling molecular interactons at the Si active material interface. We find that nominally equivalent materials react differently even before any electrochemistry is performed (e.g., acidic SiO 2 dissolves whereas alkaline SiO 2 is relatively robust), and derive new understanding of the chemical species that could and could not form stable SEI components in Si-based anodes. These results can be used to inform how to passivate Si anode surfaces and potentially generate an artificially engineered SEI that would be stable and enable next-generation battery anodes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1989

Performance comparisons of nuclear thermal rocket and chemical propulsion systems for piloted missions to Phobos/Mars

Performance capability of nuclear thermal rocket (NTR) and chemical propulsion systems, operating with and without aerobraking, are compared for a selected set of Mars mission opportunities in the 2000 to 2020 timeframe. Both high- and low-energy mission opportunities are investigated. Results are presented as the required initial mass in low earth orbit (IMLEO) to perform the missions. Missions exclusively using chemical propulsion systems have the greatest initial masses. Significant mass reductions are realized by utilizing either aerobrake or NTR technology or both. As mission energy requirements increase, the benefit of implementing aerobrake or NTR technology increases, resulting in IMLEO mass reductions on the order of 60 to 75 percent when compared with all-propulsive chemical missions. By combining both advanced technologies, still greater mass reductions are possible.

Borowski, S. K.↗
DOE OSTI2022

Insights into the Effect of Heat Treatment and Carbon Coating on the Electrochemical Behaviors of SiO Anodes for Li-Ion Batteries

The use of SiO as an anode material has attracted significant interest due to its high capacity and long cycling life. Many promising approaches, including structural design and carbon coating at high temperatures, effectively improve its intrinsic low electrical conductivity and poor Coulombic efficiency. However, the “heat treatment process-composition and microstructure-electrochemical properties” relationship of the SiO anode is not fundamentally understood. Here the structure and composition evolution in amorphous SiO and graphene-coated SiO is investigated using different heat-treatment conditions. X-ray absorption near-edge structure techniques are also employed to analyze the surface and bulk composition change during the initial lithiation process, supplemented by physical or chemical characterization and electrochemical testing. Here results reveal the structural transition of SiO during heat treatment, from amorphous to disproportionated hierarchical structure, where the as-formed dielectric exterior SiO 2 shell and interior SiO 2 matrix severely polarizes electrodes, hindering the lithiation process. Carbon coating on SiO effectively restricts the growth of the SiO 2 shell and facilitates charge transfer, leading to improved electrochemical performance. A schematic model is proposed to reveal the relationship between the treatments, the resultant structural evolutions, and corresponding electrochemical behaviors.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Experimental and numerical investigation of ester droplet combustion: Application to butyl acetate

This paper presents an experimental and numerical study of the combustion of isolated n–butyl acetate droplets in the standard atmosphere. Numerical simulations are reported using a model that incorporates unsteady gas and liquid transport, variable properties, and radiation. Three skeletal mechanisms of n-butyl acetate, derived from a large detailed mechanism comprised of 819 species and 52,698 reactions, were used in the numerical simulations to evaluate the influence of the kinetic mechanism on burning. The reduced mechanisms comprised 212 species and 5413 reactions, 157 species and 3089 reactions, and 105 species and 1035 reactions. The numerical model did not include soot formation, though qualitatively mild sooting was noted only for droplets larger than 0.7 mm. The numerical predictions were in good agreement with experimental measurements of droplet and flame diameters. Flame extinction was numerically predicted which was attributed to a decrease of the characteristic diffusion time relative to the chemical time as droplet burned. Effects of initial droplet diameter on the evolution of maximum gas temperature (T max ) and peak mole fractions of CO 2 and CO are also examined numerically.

09 BIOMASS FUELS↗
DOE OSTI2020

A Quantum-Based Approach to Predict Primary Radiation Damage in Polymeric Networks

Initial atomistic-level radiation damage in chemically reactive materials is thought to induce reaction cascades that can result in undesirable degradation of macroscale properties. Ensembles of quantum-based molecular dynamics (QMD) simulations can accurately predict these cascades, but extracting chemical insights from the many underlying trajectories is a labor-intensive process that can require substantial a priori intuition. We develop here a general and automated graph-based approach to extract all chemically distinct structures sampled in QMD simulations and apply our approach to predict primary radiation damage of polydimethylsiloxane (PDMS), the main constituent of silicones. A postprocessing protocol is developed to identify underlying polymer backbone structures as connected components in QMD trajectories. Furthermore, these backbones form a repository of radiation-damaged structures. A scheme for extracting and updating a library of isomorphically distinct structures is proposed to identify the spanning set and aid chemical interpretation of the repository. The analyses are applied to ensembles of cascade QMD simulations in which the four element types in PDMS are selectively excited in primary knock-on atom events. Our approach reveals a much higher degree of combinatorial complexity in this system than was inferred through radiolysis experiments. Probabilities are extracted for radiation-induced network changes including formation of branch points, carbon linkages, cycles, bond scissions, and carbon uptake into the Si–O siloxane backbone network. The general analysis framework presented here is readily extendable to modeling chemical degradation of other polymers and molecular materials and provides a basis for future quantum-informed multiscale modeling of radiation damage.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Influence of Water Transport Across Microscale Bipolar Interfaces on the Performance of Direct Borohydride Fuel Cells

Direct borohydride fuel cells (DBFCs) can operate at double the voltage of proton exchange membrane fuel cells (PEMFCs) by employing an alkaline NaBH 4 fuel feed and an acidic H 2 O 2 oxidant feed. The pH-gradient-enabled microscale bipolar interface (PMBI) facilitates the creation and maintenance of an alkaline environment at the anode and an acidic environment at the cathode for the borohydride oxidation and peroxide reduction reactions. However, given the need to dissociate water at the interface to ensure ionic conduction, PMBI can be efficient only when anion exchange ionomer (AEI) moieties enable fast water transport for autoprotolysis. Herein, a series of polynorbornene-based AEIs with a range of water uptake values are examined to unravel the optimum water uptake required to enable high performance DBFCs. The DBFC with PMBI configuration containing the optimal AEI composition delivers a current density of 302 mA cm –2 at 1.5 V and a peak power density of 580 mW cm –2 at 1 V. This AEI composition exhibits high hydroxide ionic conductivity of 90.7 mS cm –1 at 80 °C with an IEC of 2.01 mequiv g –1 and demonstrates impressive chemical stability by retaining 98.75% of its initial ionic conductivity after immersion into anolyte (3 M KOH and 1.5 M NaBH 4 ) at 70 °C for 536 h.

25 ENERGY STORAGE↗