Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session Q20: Matter at Extreme Conditions: Theory for Explosives and ChemistryFocus
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Sponsoring Units: GSCCM Chair: Malcolm McMahon, The University of Edinburgh Room: Room 212 |
Wednesday, March 8, 2023 3:00PM - 3:36PM |
Q20.00001: Supervised and Unsupervised Learning in Atomistic Simulations of Materials Under Reactive Conditions Invited Speaker: Nir Goldman Knowledge of the equation of state and chemical kinetics of materials under reactive conditions is needed for a wide number of research areas, including studies of planetary interiors, astrobiology, and candidate materials for hydrogen storage. The characterization of these systems under dynamic conditions is often nontrivial, especially for disordered or amorphous phases in which the underlying symmetry of the atomic geometries can be difficult to determine. Here, we focus specifically on two of our efforts to create computational tools to address these problems, namely: (1) the Chebyshev Interaction Model for Efficient Simulation (ChIMES), which is a machine-learned molecular dynamics potential based on linear combinations of many-body Chebyshev polynomials, and (2) the Scalar Graph Order Parameter (SGOP), which characterizes atomic structures based on their graph topologies. These methods will be discussed in the context of molecular dynamics simulations of chemical decomposition under high pressures, Δ-learning with semi-empirical quantum models to yield hybrid functional and coupled cluster accuracy with orders of magnitude improvement in computational cost, and the characterization of multi-element materials over a broad range of pressures and temperatures. Our methods provide a way to conduct computationally efficient and highly accurate simulations over varying conditions, where physical and chemical properties can be difficult to interrogate directly and there is historically a significant reliance on theoretical approaches for interpretation and validation of experimental results. |
Wednesday, March 8, 2023 3:36PM - 3:48PM |
Q20.00002: Multiscale Strategy for Predicting Radiation Chemistry in Polymers Matthew P Kroonblawd, Anthony Yoshimura, Nir Goldman, Amitesh Maiti, James P Lewicki, Andrew P Saab Polymers are routinely subjected to ionizing radiation for sterilization, as part of planned usage, and as a driver for curing or to accelerate aging. A primary mode for radiation chemistry arises from ballistic electrons that induce electronic excitations, but subsequent chemical mechanisms are poorly understood. We develop a multiscale modeling strategy to predict this chemistry starting from quantum electrodynamics scattering calculations. Ensembles of nonadiabatic molecular dynamics simulations based on time-dependent density functional theory are used to sample initial bond-breaking events following the most likely excitations. These excited state configurations in turn feed into semiempirical quantum-based simulations of the approach towards chemical equilibrium. Application to polyethylene shows that local backbone conformation plays a significant role in the initial steps of radiolysis, providing an explanation for experimental observations of a morphology dependence in network crosslinking. |
Wednesday, March 8, 2023 3:48PM - 4:00PM |
Q20.00003: Ab Initio Study of the Origin of Life in the Upper Mantle of Earth Tao Li, Ding Pan How life started on Earth is a long-time unsolved mystery. There are a few hypotheses ranging from outer space to deep Earth. In this work, we studied the possibility of the origin of life at extreme pressure (P) and temperature (T) conditions as found in Earth's upper mantle. We applied extensive ab initio molecular dynamics (AIMD) simulations to study chemical reactions in C-H-O-N fluids at high P and high T. We found many important precursors, such as formaldehyde, formic acid, isocyanic acid, and formamide, which have great potential to form building blocks of life. Particularly, glycine, the smallest amino acid, was detected in the chemical products. We applied free energy calculations to estimate the suitable P-T conditions for those important organic molecules. Our study helps to advance our understanding of biomolecular reactions under extreme conditions with important implications for the deep Earth hypothesis of the origin of life. |
Wednesday, March 8, 2023 4:00PM - 4:12PM |
Q20.00004: High explosive shock initiation model with shear band reactivity and carbon condensation effects Joel G Christenson, Matthew P Kroonblawd, Laurence E Fried Hot spot formation in shocked high explosive materials, usually due to local heat generation around collapsed pores and other defects, has commonly been used to explain the onset of chemical decomposition in the shock initiation regime. Many ignition and growth reactive flow models employ this concept, by assuming that local reaction at hot spots, and the subsequent thermally driven consumption, or growth, into the surrounding cooler explosive, is responsible for the build up to detonation. Past computational investigations have shown, however, that in addition to localized heating, pore collapse may also yield non-local effects, in particular shear band formation in the vicinity of collapsed pores and in the bulk. Recent molecular dynamics simulations of shocked TATB have shown that such shear bands lose much of their crystallinity, and have significantly lower reaction energy barriers compared to the bulk crystal. We therefore hypothesize that the increased reactivity of shear bands may play a crucial role in the shock to detonation transition. We present a reactive flow model that incorporates the enhanced reactivity of shear bands, in addition to other reaction pathways common to ignition and growth, such as the aforementioned hot spot reaction and growth, and carbon condensation kinetics. The model is applicable to both shock initiation and detonation experiments. |
Wednesday, March 8, 2023 4:12PM - 4:24PM Author not Attending |
Q20.00005: Microscopic mechanism of nanoscale shear bands in an energetic molecular crystal (α-RDX): a first-order structural phase transition Sergei Izvekov, Betsy M Rice Nanoscale shear bands formed in many energetic molecular crystals upon shock compression [including 1,3,5-trinitro-s-triazine (RDX)] are considered as a defect-free mechanism for formation and growth of hot-spots which control detonation initiation. We predict the formation of similar nanoscale shear bands in RDX subjected to quasistatic isothermal uniaxial compression indicating a common mechanism of shear strain localization under both shock and quasistatic conditions. In the framework of the Ginzburg-Landau phenomenology coupled with the coarse-grained (CG) Helmholtz free energy of the crystal from first principles, we explore the thermodynamics of stress-induced lattice transformations under quasistatic uniaxial load. We show that the shear banding exhibits a critical behavior associated with a first-order structural phase transition with bands of localized twinning strain as transient microstructure. Analysis of the CG Helmholtz free energy suggests that the stress-induced core-softening of effective intermolecular interaction is a fundamental mechanism for a structural phase transition leading to the nanoscale shear bands. |
Wednesday, March 8, 2023 4:24PM - 4:36PM Author not Attending |
Q20.00006: Large scale atomistic simulations on polymer-bonded energetic materials Chunyu Li Polymer-bonded explosives (PBXs) potentially have both improved safety and high performance if the combination and composition are well tuned. Considering the costly experiments, computational simulations can provide valuable insight to this tuning. In recent years, there have been some continuum modeling or mesoscale modeling PBXs. But there have been little research on PBXs on the atomistic scale. Recently, we have developed a PBX builder that can generate three dimensional all-atom PBX systems. Based on this builder, we created multiple PBX systems, each with about 10 million atoms. We conducted large-scale non-reactive atomistic simulations on these PBX systems. The size distribution of high energetic material particles follows a bimodal distribution based on knowledge of experimental observations. The microstructures, such as void distribution etc., were computationally characterized and their relationship with PBX mechanical properties and hotspot distributions are established. |
Wednesday, March 8, 2023 4:36PM - 4:48PM |
Q20.00007: Modeling Carbon Condensation in Detonation of High Explosives: A Tale of Two Approximations Kirill A Velizhanin, Erik B Watkins Detonation of carbon-rich high explosives produces significant amounts of carbon soot. This soot consists of nanometer-size carbon particles (e.g., nanodiamonds), often aggregated into large fractal structures. The mechanism of formation of this soot is believed to consist of two steps where, first, molecules of high explosive decompose into gaseous molecules (H2O, CO2, N2 etc.) and the so-called excess carbon in the form of small (few atoms) carbon-rich fragments. During the second step - the so-called carbon condensation or clustering - those small fragments undergo the slow (compared to the first step) diffusion-limited coagulation into successively larger carbon clusters and particles. Accordingly, modeling of the condensation kinetics has traditionally been done within the paradigm of the Smoluchowski coagulation with the two standard approximations: (i) the volume fraction of the excess carbon in detonation products is taken to be low, and (ii) the time scale of coalescence of carbon particles upon collision is assumed very short, rendering the overall process diffusion-limited. In this work, we consider the ramifications of lifting these two approximations on modeling of the kinetics of carbon condensation and compare modeling results with recent experimental observations. |
Wednesday, March 8, 2023 4:48PM - 5:00PM |
Q20.00008: Ab-initio Dynamics of the sp2/sp3 Structural Evolution in Amorphous Carbon (a-C) and (a-CN) Brad Steele, Sorin Bastea, I-Feng Kuo Amorphous carbon (a-C) has attracted considerable interest due to its desirable properties and to understand its complex structure that varies with density and number of impurities. Here we show using ab initio molecular dynamics simulations that the sp2 and sp3 content of amorphous carbon, which defines its local structure, evolves over 10s of ps at 3000 K. At lower densities of 2.7 and 2.3 gcc a-C transforms into a layered graphitic structure with a large amount of sp2 bonding >90 %. At higher densities of 3.7 and 3.3 gcc the sp3 content steadily increases over time. The simulations show that the structure of a-C is dependent not only on pressure and temperature but also time, i.e. the kinetics associated with its structural evolution. Amorphous carbon with 20.3 % nitrogen were found to increase the specific volume compared to pure carbon and to slightly hinder the kinetics for the transformation to a layered graphitic structure. Overall, the simulations provide insight into the structural dynamics of a-C and phase transformation processes of carbon. |
Wednesday, March 8, 2023 5:00PM - 5:12PM |
Q20.00009: Efficiency of Energy Localization in Hotspot Formation for Complex Pore Collapse Mechanisms Brenden W Hamilton, Timothy C Germann Previous works in both atomistic and continuum modeling of shock induced pore collapse have shown that the collapse mechanisms at lower shock speeds, dominated by plastic deformation, are more efficient at localizing temperature than the hydrodynamic collapse invoked at stronger shocks. Yet, these provide significantly lower absolute temperatures due to the low shock pressures and velocities involved. Highly anisotropic crystal structures are known to leads to a variety of complex collapse mechanisms at different shock strengths and pore sizes. However, these mechanisms are not well characterized with respect to one another. Therefore, we run all-atom molecular dynamics simulations of shock induced pore collapse in an anisotropic molecular crystal with a strong hydrogen bonding network, using four different impact velocities and pore sizes that range across an order of magnitude. We find that a complex, lateral collapse mechanism resembling atomic scale extrusion leads to extremely efficient hotspot formation mechanism as low shock pressure. However, the strength of the hydrogen bonding network is shown to greatly limit the ability to produce high temperature hotspots at high shock strengths. |
Wednesday, March 8, 2023 5:12PM - 5:24PM |
Q20.00010: The speciation of Platinum and Palladium in aqueous carbonates Thi Lien Le, Giulia Galli The elements of the Platinum Group (PGEs) are considered essential commodities for the electronic and automotive industries, due to their high melting points, corrosion resistance and catalytic properties. Unfortunately, natural geochemical and petrological processes have concentrated PGE metals in rare ore deposits. It is thus important to understand how these deposits formed to eventually help discover new ores and/or improve processes for the metals’ extraction. At the fundamental level, despite recent experimental advances, the question of how hot, aqueous fluid in the Earth crust may facilitate transport of PGEs remains open. Here we investigate the speciation trend of platinum and palladium in aqueous carbonates using first principles molecular dynamics and the Qbox code (http://qboxcode.org/). We aim at obtaining a fundamental understanding of the ion complexes at extreme conditions, including those relevant to hydrothermal and magmatic environments and we compare our results with EXAFS measurements. Specifically, we study Pt-Pd complexes in carbonate solutions under high temperature between 600K and 1000K and high pressure in range from 0.1 GPa to 10 GPa and we compute Raman spectra to identify vibrational signatures of the ions present in solution. In addition, we aim at using our calculations to provide input for the development of a self-consistent thermodynamic solubility model as a function of temperature and pressure. |
Wednesday, March 8, 2023 5:24PM - 5:36PM |
Q20.00011: Effects of Chain Length on the Dynamics and Structure of a Model Polyimide Under Uniaxial Loading Nicholas T Liesen, Matthew P Kroonblawd, Amitesh Maiti, Christy Fox, Graham Kosiba, Richard H Gee Aromatic polyimides are known for their chemical resistance and thermal stability, and have long been used in high-temperature applications such as the manufacturing and soldering of electronic components. The mechanical properties of these materials can depend sensitively on the molecular composition and length of the polyimide chains, but specific underlying mechanisms are poorly understood. In this work we validate an all-atom molecular dynamics model for a pyromellitic dianhydride (PMDA)-based polyimide with 4,4′-oxydianiline (ODA) separators that include flexible ether linkages. This model is used to predict the chain-level and segmental dynamics, and the molecular structure of amorphous polyimides across a range of temperatures and chain lengths. We find relatively slow dynamics outside of high temperature regimes, even for low molar masses, which is consistent with the high glass transition temperatures characteristic of PMDA-based polyimides. Simulations measuring the stress-strain response of polyimides under ultrafast uniaxial loading are used to assess factors governing material strength. |
Wednesday, March 8, 2023 5:36PM - 5:48PM |
Q20.00012: Pressure dependence of sound velocity, equation of state, and structure of vitons Charles M Zoller, Jonathan K Simon, Rostislav Hrubiak, Curtis Kenney-Benson, Muhtar Ahart, Russell J Hemley Viton A500 and carbon-filled composite Viton are important materials as binders for high explosives and as seals over a wide temperature range. These applications require a thorough understanding of the elastic, thermodynamic, and structural properties over a broad pressure range. Using a Paris-Edinburgh press, the sound velocity measurements were measured by ultrasonic pulse-echo method and x-ray radiography up to a pressure of 7 GPa. Pressure effects on the short and medium range order in the amorphous component and the residual a crystalline component were determined using in-situ x-ray diffraction. The results were compared with measurements on pure polyvinylidene fluoride (PVDF), Sylgard, and polychlorotrifluoroethylene (Kel-F). SEM compositional analysis of Viton A500, its carbon-filled composite, and pure PVDF sample was performed to determine sample homogeneity and the origin of measured residual crystallinity. |
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