Comparative Studies on Thermal Decompositions of Dinitropyrazole-Based Energetic Materials.

Dinitropyrazole is an important structure for the design and synthesis of energetic materials. In this work, we reported the first comparative thermal studies of two representative dinitropyrazole-based energetic materials, 4-amino-3,5-dinitropyrazole (LLM-116) and its novel trimer derivative (LLM-226). Both the experimental and theoretical results proved the active aromatic N-H moiety would cause incredible variations in the physicochemical characteristics of the obtained energetic materials. Thermal behaviors and kinetic studies of the two related dinitropyrazole-based energetic structures showed that impressive thermal stabilization could be achieved after the trimerization, but also would result in a less concentrated heat-release process. Detailed analysis of condensed-phase systems and the gaseous products during the thermal decomposition processes, and simulation studies based on ReaxFF force field, indicated that the ring opening of LLM-116 was triggered by hydrogen transfer of the active aromatic N-H moiety. In contrast, the initial decomposition of LLM-226 was caused by the rupture of carbon-nitrogen bonds at the diazo moiety.

Design of saccharide based organic binder for low-grade iron ore pelletization using atomistic simulations and machine learning methods.

Inorganic binders like bentonite, used for pelletization of low-grade iron ore, generate iron ore slimes with comparatively high silica and alumina content necessitating extra steps for their removal during iron making process. This demands the usage of organic binders as full or partial replacement of bentonite for iron ore pelletization. In this work, adsorption of organic binders with saccharides skeleton and -H, -OH, -CH2OH and -CH2CH2OH as polar substituents, on goethite surface was studied using density functional theory, molecular dynamics and machine learning. It was observed that adsorption energy of binders on goethite surface had weak dependence on number of hydrogen bonds between them. With this favorable interaction in mind, a library containing 64 organic binders was constructed and adsorption energy of 30 of these binders was computed using molecular dynamics, followed by training of a linear regression model, which was then used to predict the adsorption energy of rest of the binders in the library. It was found that the introduction of -CH2CH2OH at R2 position resulted in statistically significant higher adsorption energy. Binder34 and Binder44 were identified as viable candidates for both goethite and hematite ore pelletization and adsorption of their n-mers on goethite and hematite surfaces was also quantified.

Structural Toughness of Fluorinated-Nitrided Interphase Passivation Layers for Lithium-Ion Batteries: A Reactive Molecular Dynamics Study.

The performance of lithium-ion batteries largely depends on the stability of the solid electrolyte interphase (SEI) layer formed on the anode surface. Strategies to improve SEI robustness often rely on optimizing its composition through electrolytic additives. Recently, the amalgamation of fluorinated cosolvents with nitride sources as additives has been shown to enable the construction of sustainable fluorinated-nitrided SEI layers (FN-SEI). Furthermore, the presence of lithiophilic nitrides embedded in lithium fluoride (LiF) was found to contribute toward stability of a beneficial amorphous phase for interfacial passivation. However, there is a lack of understanding on how key indicators of mechanical longevity, like plasticity and fracture resistance, may evolve in such multiphase SEI building blocks. Herein, in conjunction with first-principles calculations, a reactive force field (ReaxFF) has been developed for deriving new mechanistic insights into the intriguing FN-SEI. Our studies demonstrate that owing to a significant elasticity mismatch, the hard nitride phases have a propensity to affect the native deformation modes when embedded in a soft amorphous LiF-rich matrix. Impact of the volume fraction and distribution of the nitride (Li3N) phases are discussed from the perspective of how they interfere with the propagation of shear bands. Interestingly, brittle-ductile-brittle regimes are recognized along the nitride infusion window, providing a glimpse into the effect of phase distribution on the structural toughness of the LiF-Li3N-enhanced SEI.

Reactive molecular dynamics simulations on thermal decomposition of 3-methyl-2,6-dinitrophenol.

In this paper, we simulated the decomposition mechanism of 3-methyl-2,6-dinitrophenol (MDNP) based on reaction molecular dynamics using ReaxFF force field. In addition, the evolution of some main products over time at different heating rates (10, 15, and 20 K·ps-1) was studied. As indicated by the simulation results, with the elevation at different heating rates, the time required for the system to reach equilibrium was shortened, and more products were obtained. At three heating rates, C7H7O5N2, C7H6O4N2, C7H5O5N2, C7H5O4N2, HON, NO, and NO2 were the main intermediate products, and N2, CO2, H2O, H2, and NH3 were the primary final products. To be specific, C7H5O5N2 was the first produced intermediate product, and H2O was the first produced final product with the largest abundance. The intermediate products first increased and then decreased to zero. Moreover, the primary chemistry reactions in the MDNP pyrolysis were simulated through ReaxFF MD simulations.

Investigation of kerogen thermal decomposition mechanisms and kinetics via ReaxFF molecular dynamics simulations.

This study investigates the mechanisms and kinetics of kerogen thermal decomposition using molecular dynamics simulations with the ReaxFF force field. The cook-off simulation at the constant heating rate shows that the decomposition of kerogen begins with the cracking at terminals and weaker linkages of kerogen molecule, and the final products are formed by radicals recombination, dehydrogenation, and other reactions. The Flynn-Wall-Ozawa kinetic analysis based on the thermal decomposition simulations at various heating rates shows that the activation energy increases with the conversion of decomposition. These results reveal the thermal decomposition mechanisms and the thermal stability of kerogen in different stages during the process of thermal decomposition.

Molecular Simulations of the Microstructure Evolution of Solid Electrolyte Interphase during Cyclic Charging/Discharging.

Lithium (Li) metal is regarded as one of the most promising anode materials for use in next-generation high-energy-density rechargeable batteries because of its high volumetric and gravimetric specific capacity, as well as low reduction potential. Unfortunately, uncontrolled dendritic Li growth during cyclic charging/discharging leads to low columbic efficiency and critical safety issues. Hence, comprehensive understanding of the formation mechanism for Li-dendrite growth, particularly at the onset of dendrite formation, is essential for developing Li-metal anode batteries. In this study, reactive molecular dynamics (MD) simulations in combination with the electrochemical dynamics with implicit degrees of freedom (EChemDID) method were performed to investigate the formation and evolution of solid electrolyte interphase (SEI) films for a Li-metal anode under cyclic charging/discharging processes in two distinct dimensions, namely, electrolyte compositions and initial surface morphologies. Our simulations indicated that regardless of the electrolyte compositions and initial anode morphologies, inhomogeneous Li reduction, namely, the formation of Li-reduction "hotspots" during cyclic charging cycles, took place and could serve as the seed for subsequent dendrite growth. The fluorine-containing electrolyte additives could notably mitigate the Li-anode roughening processes by forming dense-SEI-layer products or suppressing electrolyte decomposition. A series of Li-ion-drifting simulations suggest that Li ions navigate through the SEI layer via pathways composed of low-density atoms and become reduced at these reduction hotspots, promoting inhomogeneous deposition and subsequent dendrite growth. The present study reveals atomistic details of the early stage of dendrite growth during cyclic loadings under different electrolyte compositions and anode morphologies, thereby providing insights for designing artificial SEI layers or electrolytes for long-life, high-capacity Li-ion batteries.

Adsorption of ethanol molecules on the Al (1 1 1) surface: a molecular dynamic study.

The adsorption process of ethanol molecules on Al slabs was investigated by molecular dynamic simulations with a ReaxFF force field. The force field used in this paper has been validated by comparing adsorption energy results with quantum mechanical (QM) calculations. All simulations were performed under the canonical (NVT) ensemble. The single-molecule adsorption simulation shows that the hydroxyl group plays a more important role in the whole progress than the ethyl group. Besides, decomposition of hydroxyl groups was also observed during multimolecule adsorption processes. Simulations of adsorption processes of Al slab by ethanol molecules at different temperatures and pressures (controlled by the number of ethanol molecules) was also performed. System energy and radial distribution function (RDF) plots were invoked to describe adsorption processes and centro-symmetry parameter (CSP) analysis was adopted to study the surface properties with coating layers. Our results indicate that the whole adsorption process can be divided into two periods and the greater the pressure, the more ethanol molecules diffuse into the Al slab. How raising the temperature helps the adsorption processes is related to the initial number of molecules. The crystal structure of the Al surface will become amorphous under the constant impact of ethanol molecules.

Structure-Property of Lithium-Sulfur Nanoparticles via Molecular Dynamics Simulation.

Lithium-sulfur (Li-S) batteries offer higher energy densities than most reported lithium-ion batteries. However, our understanding of Li-S battery is still largely unknown at the level of the nanoscale. The structural properties of Li-S materials were investigated via molecular dynamics (MD) simulations using the ReaxFF force field. Several Li-S nanoparticles with different Li/S composition ratios (2:1 and 2:8) and various structures are studied. Our MD simulations show that among the four structures we constructed for Li2S8 nanoparticles, the core-shell structure is the most thermodynamically stable one during the charging (delithiation) process. In contrast to bulk crystal Li2S, we find the presence of mixed lithium sulfide and polysulfide species are common features for these Li-S (Li2S, Li2S8) nanoparticles. The complex distribution of these sulfide and polysulfide speciation are dictated by both stoichiometry and local atomic structures in the nanoparticle. These findings will provide insight into further development of functionalized lithium-sulfur cathodes.

Atomistic Insights into Nucleation and Formation of Hexagonal Boron Nitride on Nickel from First-Principles-Based Reactive Molecular Dynamics Simulations.

Atomistic-scale insights into the growth of a continuous, atomically thin hexagonal boron nitride (hBN) lattice from elemental boron and nitrogen on Ni substrates were obtained from multiscale modeling combining density functional theory (DFT) and reactive molecular dynamics. The quantum mechanical calculations focused on the adsorption and reaction energetics for the hBN building-block species, i.e., atomic B, N, BxNy (x, y = 1, 2), on Ni(111) and Ni(211), and the diffusion pathways of elemental B and N on these slab model surfaces and in the sublayer. B can diffuse competitively on both the surface and in the sublayer, while N diffuses strictly on the substrate surface. The DFT data were then used to generate a classical description of the Ni-B and Ni-N pair interactions within the formulation of the reactive force field, ReaxFF. Using the potential developed from this work, the elementary nucleation and growth process of an hBN monolayer structure from elemental B and N is shown at the atomistic scale. The nucleation initiates from the growth of linear BN chains, which evolve into branched and then hexagonal lattices. Subsequent DFT calculations confirmed the structure evolution energetically and validate the self-consistency of this multiscale modeling framework. On the basis of this framework, the fundamental aspects regarding crystal quality and the role of temperature and substrates used during hBN growth can also be understood.

Direct MD Simulations of Terahertz Absorption and 2D Spectroscopy Applied to Explosive Crystals.

A direct molecular dynamics simulation of the THz spectrum of a molecular crystal is presented. A time-dependent electric field is added to a molecular dynamics simulation of a crystal slab. The absorption spectrum is composed from the energy dissipated calculated from a series of applied pulses characterized by a carrier frequency. The spectrum of crystalline cyclotrimethylenetrinitramine (RDX) and triacetone triperoxide (TATP) were simulated with the ReaxFF force field. The proposed direct method avoids the linear response and harmonic approximations. A multidimensional extension of the spectroscopy is suggested and simulated based on the nonlinear response to a single polarized pulse of radiation in the perpendicular polarization direction.