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+ Transport on the Cathode Surface within Lithium-Ion Batteries
Published in Ming-Fa Lin, Wen-Dung Hsu, Green Energy Materials Handbook, 2019
Figure 2.1 illustrates the chemical structures for the five different polymer binders focused in this work, including PVDF, PAN, PEO, PSS, and PNVF. The electrolyte was composed of 1 M LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) with 1:1 volume ratio. The all-atom optimized potentials for liquid simulations (OPLS-AA) force field was applied to describe all molecules, including polymers, organic solvents, and lithium salts.42 The OPLS force field has been widely used in MD studies of ionic liquids, polymers, and solid polymer electrolyte systems.43–46 The parameters for PF− were taken from updated OPLS potentials developed by Lopes and Pauda.45 The LiFePO4 (LFP) cathode contained 3 × 8 × 6 unit crystal cell with the [010] surface normal to the z-axis.47 The van der Waals interactions of LFP atoms were described with the OPLS force field, whereas the atomic charges were assigned based on the molecular model by Smith et al.47Chemical structures for all the functional polymer binders, including (a) poly(vinylidene fluoride) PVDF, (b) polyacrylonitrile PAN, (c) poly(ethylene oxide) PEO, (d) poly(styrene sulfonate) PSS, and (e) poly(N-vinylformamide) PNVF.
Molecular Dynamics Method For Microscale Heat Transfer
Published in W.J. Minkowycz, E.M. Sparrow, Advances in Numerical Heat Transfer, 2018
TIP4P potential proposed by Jorgensen et al. [14] employed the structure of water molecule as rOH = 0.09572 nm and ∠HOH = 104.52° based on the experimentally assigned value for the isolated molecule. The positive point charges q were on hydrogen atoms, and the negative charge -2q was set at rOM from the oxygen atom on the bisector of the HOH angle, as in Fig. 2b. The function can be written as Eq. (5) without S(R12) function. The parameters listed in Table 3 were optimized for thermodynamic data such as density, potential energy, specific heat, evaporation energy, self-diffusion coefficient and thermal conductivity, and structure data such as the radial distribution function and neutron diffraction results at 25 °C and 1 atm. This potential is regarded as one of the OPLS (optimized potential for liquid simulations) set covering liquid alcohols and other molecules with hydroxyl groups developed by Jorgensen [15].
Modeling of Mechanical Properties in Nanoparticle Reinforced Polymers Using Atomistic Simulations
Published in Frank Abdi, Mohit Garg, Characterization of Nanocomposites, 2017
Samit Roy, Avinash Reddy Akepati
Work is currently underway to quantify and verify the difference due to the entropic contribution at even higher temperatures. All the simulations till now have been done using optimized potentials for liquid simulations (OPLS) force field. Work is currently underway to use ReaxFF force field (van Duin, 2001) for MD simulations. ReaxFF is a bond order–based force field which can model chemical reactions including formation and breaking of chemical bonds. This force field is useful for fracture simulation because it can model crack propagation by breaking chemical bonds, which is a more accurate representation of the physical phenomenon occurring at atomic scale. This would give a better understanding of the effect of nanographene on fracture properties of EPON 862. As mentioned earlier, our ultimate goal is to be able to use MD simulations to compute parameters such as the partition function, free energy density, bond density function (for bonded as well as nonbonded interactions), Piola–Kirchoff stress, etc., in order to quantify the influence of nanofillers such as graphene nanoplatelets on macroscale phenomenon such as delamination crack propagation in a nanoreinforced laminated polymer composite. As mentioned in an earlier section, the local harmonic (LH) approximation is not valid for an amorphous (disordered) crosslinked polymer network. A methodology for the derivation of a proper partition function (Z) for a flexible polymer crosslinked network through the use of statistical mechanics of phantom networks is described in (Weiner, 1983) and will be adapted for evaluating J-integral for polymers at finite temperatures. In the event, the partition function computation proves to be computationally intractable for a large amorphous polymeric system, an alternate approach using a cohesive zone–based technique will be employed for computation of J-integral at the nanoscale, as outlined by Klein and Gao (1998). Atomistic J-integral evaluation scheme discussed earlier can then be applied to polymeric systems to evaluate performance metrics such as work of separation at the polymer–nanoparticle interface. These data can subsequently be incorporated in a multiscale model for simulating failure at the macroscale. The multiscale analysis will be performed using a concurrent coupling scheme as discussed in by Roy and Nair (2011). A major advantage of the coupled simulation over pure MD simulation of crack propagation is that the spurious effect of stress waves reflected from the boundaries of the MD domain impinging on the moving crack tip is mitigated. The nanoscale fracture toughness data obtained will subsequently be hierarchically incorporated at the microscale to predict fuzzy fiber/matrix debonding or at the macroscale to predict inter-ply delamination in a composite laminate reinforced with interlaced layers of CNT forest.
Nonequilibrium Molecular Dynamics Modeling Of A Fuel Nanojet In Sub/supercritical Environments: Chamber Pressure Effects On Characteristics Of The Gas–liquid Interface
Published in Nanoscale and Microscale Thermophysical Engineering, 2018
Wu Wei, Hongsheng Liu, Lei Deng, Ming Jia, Maozhao Xie
MD methodology is founded upon the basic principles of classical mechanics and can provide insight into the microscopic dynamical behavior of the individual atoms that make up a given system. In order to provide a description of the microscopic behavior of a system from the laws of classical mechanics, MD requires, as an input, information of the interparticle interactions. In the current study, n-heptane is chosen as the real fluid to model the diesel fuel. The classical all-atom force fields Optimized Potentials for Liquid Simulations All Atom Potential Functions (OPLS-AA) [25], including bond stretching and angle bending, Fourier series for torsional energetics, and Coulomb plus Lennard-Jones 12-6 potentials for the nonbonded interactions, can achieve high accuracy for systems of hydrocarbons, organics, and biomolecules. However, because the interaction between all atoms must be considered, the OPLS-AA force fields are complicated and computationally expensive. To reduce computation time, in this work, the united-atom force field of n-heptane suggested by Kalyanasundaram et al. [26] is used and shown in Figure 1, where the effects of hydrogen atoms are not directly computed but added to the nearby carbon atom.