Nicolas Desbiens

Molecular simulations of Hugoniots of detonation product mixtures at chemical equilibrium: Microscopic calculation of the Chapman-Jouguet state

In this work, we used simultaneously the reaction ensemble Monte Carlo (ReMC) method and the adaptive Erpenbeck equation of state (AE-EOS) method to directly calculate the thermodynamic and chemical equilibria of mixtures of detonation products on the Hugoniot curve. The ReMC method [W. R. Smith and B. Triska, J. Chem. Phys. 100, 3019 (1994)] allows us to reach the chemical equilibrium of a reacting mixture, and the AE-EOS method [J. J. Erpenbeck, Phys. Rev. A 46, 6406 (1992)] constrains the system to satisfy the Hugoniot relation. Once the Hugoniot curve of the detonation product mixture is established, the Chapman-Jouguet (CJ) state of the explosive can be determined. A NPT simulation at P(CJ) and T(CJ) is then performed in order to calculate direct thermodynamic properties and the following derivative properties of the system using a fluctuation method: calorific capacities, sound velocity, and Gruneisen coefficient. As the chemical composition fluctuates, and the number of particles is not necessarily constant in this ensemble, a fluctuation formula has been developed to take into account the fluctuations of mole number and composition. This type of calculation has been applied to several usual energetic materials: nitromethane, tetranitromethane, hexanitroethane, PETN, and RDX.

Mesoscopic simulations of shock-to-detonation transition in reactive liquid high explosive

An extension of the model described in a previous work (see Maillet J. B. et al., EPL, 78 (2007) 68001) based on Dissipative Particle Dynamics is presented and applied to a liquid high explosive (HE), with thermodynamic properties mimicking those of liquid nitromethane. Large scale nonequilibrium simulations of reacting liquid HE with model kinetic under sustained shock conditions allow a better understanding of the shock-to-detonation transition in homogeneous explosives. Moreover, the propagation of the reactive wave appears discontinuous since ignition points in the shocked material can be activated by the compressive waves emitted from the onset of chemical reactions.

Molecular based equation of state for shocked liquid nitromethane

An approach is proposed to obtain the equation of state of unreactive shocked liquid nitromethane. Unlike previous major works, this equation of state is not based on extended integration schemes [P.C. Lysne, D.R. Hardesty, Fundamental equation of state of liquid nitromethane to 100 kbar, J. Chem. Phys. 59 (1973) 6512]. It does not follow the way proposed by Winey et al. [J.M. Winey, G.E. Duvall, M.D. Knudson, Y.M. Gupta, Equation of state and temperature measurements for shocked nitromethane, J. Chem. Phys. 113 (2000) 7492] where the specific heat C(v), the isothermal bulk modulus B(T) and the coefficient of thermal pressure (deltaP/deltaT)(v) are modeled as functions of temperature and volume using experimental data. In this work, we compute the complete equation of state by microscopic calculations. Indeed, by means of Monte Carlo molecular simulations, we have proposed a new force field for nitromethane that lead to a good description of shock properties [N. Desbiens, E. Bourasseau, J.-B. Maillet, Potential optimization for the calculation of shocked liquid nitromethane properties, Mol. Sim. 33 (2007) 1061; A. Hervouët, N. Desbiens, E. Bourasseau, J.-B. Maillet, Microscopic approaches to liquid nitromethane detonation properties, J. Phys. Chem. B 112 (2008) 5070]. Particularly, it has been shown that shock temperatures and second shock temperatures are accurately reproduced which is significative of the quality of the potential. Here, thermodynamic derivative properties are computed: specific heats, Grüneisen parameter, sound velocity among others, along the Hugoniot curve. This work constitutes to our knowledge the first determination of the equation of state of an unreactive shocked explosive by molecular simulations.

Potential optimization for the calculation of shocked liquid nitromethane properties

We present the results of the optimization of a classical molecular force field used to calculate the properties of shocked nitromethane by Monte Carlo simulations. The optimization technique allows a good transferability of the potential parameters on a broad range of thermodynamic conditions (temperature and pressure) since a large variety of reference data can be used in the optimization procedure, including densities, vaporization enthalpies or pressures along the Hugoniot curve. Results of calculated properties of shocked nitromethane are in good agreement with experimental shock Hugoniot data, including temperature measurements of second shock Hugoniot.

Correlation and transport properties for mixtures at constant pressure and temperature

Transport properties of mixtures of elements in the dense plasma regime play an important role in natural astrophysical and experimental systems, e.g., inertial confinement fusion. We present a series of orbital-free molecular dynamics simulations on dense plasma mixtures with comparison to a global pseudo ion in jellium model. Hydrogen is mixed with elements of increasingly high atomic number (lithium, carbon, aluminum, copper, and silver) at a fixed temperature of 100 eV and constant pressure set by pure hydrogen at 2g/cm^{3}, namely, 370 Mbars. We compute ionic transport coefficients, such as self-diffusion, mutual diffusion, and viscosity for various concentrations. Small concentrations of the heavy atoms significantly change the density of the plasma and decrease the transport coefficients. The structure of the mixture evidences a strong Coulomb coupling between heavy ions and the appearance of a broad correlation peak at short distances between hydrogen atoms. The concept of an effective one component plasma is used to quantify the overcorrelation of the light element induced by the admixture of a heavy element.

Parametrization of pair correlation function and static structure factor of the one component plasma across coupling regimes

We present a parametrization of the pair correlation function and the static structure factor of the Coulomb one component plasma (OCP) from the weakly coupled regime to the strongly coupled regime. Recent experiments strongly suggest that the OCP model can play the role of a reference system for warm dense matter. It can provide the ionic static structure factor that is necessary to interpret the x-ray Thomson scattering measurements, for instance. We illustrate this with the interpretation of an x-ray diffraction spectrum recently measured, using a Bayesian method that requires many evaluations of the static structure factor to automatically calibrate the parameters. For strongly coupled dusty plasmas, the proposed parametrization of the Coulomb OCP pair correlation function can be related to the Yukawa one, including screening. Further prospects to parametrize the static structure of Yukawa systems are also discussed.

Theoretical study of the influence of the equation of state mixture properties on the velocitycurvature relationship for heterogeneous solid explosives

DSD is probably the most popular engineering tool used to model the dynamics of detonation. In this model the normal shock velocity (Dn) depends only on the local curvature (k) of the front. One way to reproduce this behavior is to construct a model for the explosive which obeys the 1D quasi-steady weakly curved detonation theory. In its simplest form, such a model is composed of a biphasic reactive equation of state coupled with a single step burning law. To complete the description of EOS different authors proposed various closure hypotheses (isobaric isothermal, isobaric adiabatic, isodensity isothermal,…). Given this form for the EOS, the reaction rate law is generally calibrated to match the experimental detonation velocity-curvature curve. In this paper we theoretically examine the influence of the closure hypotheses on the (Dn, k) curve with a fixed rate law. As the 1D curved theory of detonation is extremely sensitive to the calculation of the sonic surface downstream the reaction zone, we show th...

Bayesian inference of x-ray diffraction spectra from warm dense matter with the one-component-plasma model

We show that the Bayesian inference of recently measured x-ray diffraction spectra from laser-shocked aluminum [L. B. Fletcher et al., Nat. Photon. 9, 274 (2015)10.1038/nphoton.2015.41] with the one-component-plasma (OCP) model performs remarkably well at estimating the ionic density and temperature. This statistical approach requires many evaluations of the OCP static structure factor, which were done using a recently derived analytic fit. The atomic form factor is approximated by an exponential function in the diffraction window of the first peak. The electronic temperature is then estimated from a comparison of this approximated form factor with the electronic structure of an average atom model. Out-of-equilibrium states, with electrons hotter than ions, are diagnosed for the spectra obtained early after the pump, whereas at a late time delay the plasma is at thermal equilibrium. Apart from the present findings, this OCP-based modeling of warm dense matter has an important role to play in the interpretation of x-ray Thomson scattering measurements currently performed at large laser facilities.

Seeking an accurate generalized-gradient approximation functional for high pressure molecular fluids

We propose to assess the performance of density functional theory calculations to predict the properties of CO2, H2O, and N2 fluids under high pressure (up to 40u2009GPa), which are representatives of not only detonation products but also giant planet interiors. Twenty-two generalized-gradient approximation functionals, presently in the ABINIT code, have been compared to molecular data and experimental equations of state of supercritical fluids. We found that the Perdew, Burke, and Ernzerhof (PBE) functional with Grimmes dispersion correction (D3) gives the best results. The residual error of PBE-D3 on pressure is estimated around 15%.