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Register a new userMolecular dynamics computer simulation of amorphous silica under high pressure
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Authors
Juergen Horbach
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The structural and dynamic properties of silica melts under high pressure are studied using molecular dynamics (MD) computer simulation. The interactions between the ions are modeled by a pairwise-additive potential, the so-called CHIK potential, that has been recently proposed by Carre et al. The experimental equation of state is well-reproduced by the CHIK model. With increasing pressure (density), the structure changes from a tetrahedral network to a network containing a high number of five- and six-fold Si-O coordination. In the partial static structure factors, this change of the structure with increasing density is reflected by a shift of the first sharp diffraction peak towards higher wavenumbers q, eventually merging with the main peak at densities around 4.2 g/cm^3. The self-diffusion constants as a function of pressure show the experimentally-known maximum, occurring around a pressure of about 20 GPa.
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Explicit molecular dynamics simulations were applied to a pair of amorphous silica nanoparticles in aqueous solution, of diameter 4.4 nm with four different background electrolyte concentrations, to extract the mean force acting between the pair of silica nanoparticles. Dependences of the interparticle forces with separation and the background electrolyte concentration were demonstrated. The nature of the interaction of the counter-ions with charged silica surface sites (deprotonated silanols) was investigated. A patchy double layer of adsorbed sodium counter-ions. was observed. Dependences of the interparticle potential of mean force with separation and the background electrolyte concentration were demonstrated. Direct evidence of the solvation forces is presented in terms of changes of the water ordering at the surfaces of the isolated and double nanoparticles. The nature of the interaction of the counter-ions with charged silica surface sites (deprotonated silanols) was investigated in terms of quantifying the effects of the number of water molecules separately inside each of the pair of nanoparticles by defining an impermeability measure. A direct correlation was found between impermeability (related to the silica surface hairiness) and the disruption of water ordering. Differences in the impermeability between the two nanoparticles are attributed to differences in the calculated electric dipole moment.
Explicit molecular dynamics simulations were applied to a pair of amorphous silica nanoparticles of diameter 3.2 nm immersed in a background electrolyte. Mean forces acting between the pair of silica nanoparticles were extracted at four different background electrolyte concentrations. Dependence of the inter-particle potential of mean force on the separation and the silicon to sodium ratio, as well as on the background electrolyte concentration, are demonstrated. The pH was indirectly accounted for via the ratio of silicon to sodium used in the simulations. The nature of the interaction of the counter-ions with charged silica surface sites (deprotonated silanols) was also investigated. The effect of the sodium double layer on the water ordering was investigated for three Si:Na+ ratios. The number of water molecules trapped inside the nanoparticles was investigated as the Si:Na+ ratio was varied. Differences in this number between the two nanoparticles in the simulations are attributed to differences in the calculated electric dipole moment. The implications of the form of the potentials for aggregation are also discussed.
A fitting scheme is proposed to obtain effective potentials from Car-Parrinello molecular dynamics (CPMD) simulations. It is used to parameterize a new pair potential for silica. MD simulations with this new potential are done to determine structural and dynamic properties and to compare these properties to those obtained from CPMD and a MD simulation using the so-called BKS potential. The new potential reproduces accurately the liquid structure generated by the CPMD trajectories, the experimental activation energies for the self-diffusion constants and the experimental density of amorphous silica. Also lattice parameters and elastic constants of alpha-quartz are well-reproduced, showing the transferability of the new potential.
The nature of the amorphous state has been notably difficult to ascertain at the microscopic level. In addition to the fundamental importance of understanding the amorphous state, potential changes to amorphous structures as a result of radiation damage have direct implications for the pressing problem of nuclear waste encapsulation. Here, we develop new methods to identify and quantify the damage produced by high-energy collision cascades that are applicable to amorphous structures and perform large-scale molecular dynamics simulations of high-energy collision cascades in a model zircon system. We find that, whereas the averaged probes of order such as pair distribution function do not indicate structural changes, local coordination analysis shows that the amorphous structure substantially evolves due to radiation damage. Our analysis shows a correlation between the local structural changes and enthalpy. Important implications for the long-term storage of nuclear waste follow from our detection of significant local density inhomogeneities. Although we do not reach the point of convergence where the changes of the amorphous structure saturate, our results imply that the nature of this new converged amorphous state will be of substantial interest in future experimental and modelling work.
We report an {it ab initio} simulation study of changes in structural and dynamic properties of liquid Si at 7 pressures ranging from 10.2 GPa to 24.3 GPa along the isothermal line 1150~K, which is above the minimum of the melting line. The increase of pressure from 10.2 GPa to 16 GPa causes strong reduction in the tetrahedral ordering of the most close neighbors. The diffusion coefficient shows a linear decay vs drop in atomic volume, that agrees with theoretical prediction for simple liquid metals, thus not showing any feature at the pressures corresponding to the different crystal phase boundaries. The Fourier-spectra of velocity autocorrelation function shows two-peak structure at pressures 20 GPa and higher. These characteristic frequencies correspond well to the peak frequencies of the transverse current spectral function in the second pseudo-Brillouin zone. Two almost flat branches of short-wavelength transverse modes were observed for all the studied pressures. We discuss the pressure evolution of characteristic frequencies in the longitudinal and transverse branches of collective modes.
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