Session C2: ERM: Phase, Melt, and Diffusion

In Situ Investigation of Phase Transformation in Cyclotrimethylene Trinitramine (RDX) During Shock Loading Using X-ray Diffraction

Particularly important for explosives are the discrete changes in stress-strain rate relationships (e.g. phase boundaries) that have a profound effect on the temperature distribution, on the mechanisms for flow, and hence, on the chemical reactions leading to detonation initiation. The Hugoniot path through phase space for cyclotrimethylene trinitramine (RDX) has been investigated. The equilibrium phase diagrams reported in literature for RDX have a 90 K discrepancy in the triple point between the $\alpha $, $\gamma $, and $\varepsilon $ phases. The Hugoniot calculated from volumetric equation of state contributions transits through the region in dispute, and multiple two-wave features have been observed in single crystal velocimetry profiles shocked from 1.5 to 18.5 GPa. In situ x-ray diffraction has been performed to resolve the phase boundaries and identify the phases in single and polycrystalline samples during gas gun and laser-driven shock loading at the Dynamic Compression Sector at the Advanced Photon Source and the Materials in Extreme Conditions instrument at the Linac Coherent Light Source. Direct numerical simulations of the experiments have been performed to assess the effects of transformation kinetics.   

Effect of nonhydrostatic compression on the structural and chemical stability of FOX-7 crystals.

Nonhydrostatic compression conditions were imposed in diamond anvil cell (DAC) experiments on an insensitive energetic crystal -- 1,1-diamino-2,2-dinitroethene (FOX-7) -- to gain insight into the role of nonhydrostatic stresses on its structural and chemical stability. Samples of oriented single-crystals were subjected to compression with controlled amount of nonhydrostaticity. These conditions were achieved by \textit{in-situ} monitoring of the nonhydrostaticity through the splitting of R1 and R2 fluorescence lines of oriented ruby single-crystals and by selecting appropriate pressure transmitting media (PTM). By changing PTM from hydrostatic to strongly nonhydrostatic compression, structural and chemical changes in FOX-7 crystals were examined using Raman spectra measurements for pressures up to 30 GPa. The results revealed that the transformation of wave-shaped layers to planar layers, the $\alpha $'- $\varepsilon $ transition observed under hydrostatic pressure at 4.5 GPa, was significantly affected by the imposed nonhydrostaticity. Both the onset and completion pressures for this transformation depended on the extent of the nonhydrostaticity and orientation of FOX-7 crystal with respect to the cell axis. Unlike hydrostatic compression, changes in the Raman spectra were not reversible upon release from nonhydrostatic compression. This finding shows that the high-pressure, $\varepsilon $-phase can be recovered as a metastable phase under ambient conditions.   

Pressure-induced phase transitions in energetic materials revealed by single-crystal diffraction studies

Understanding the high-pressure structure of energetic materials is essential for the realistic modeling of shock initiation and improvement of code. Pressure-induced structural phase transitions where the symmetry and structure are altered can be exceptionally difficult or impossible to unravel using conventional high-pressure powder X-ray diffraction (XRD). To address this problem, we performed single crystal (SC) XRD studies. In the case of TATB, an insensitive energetic material, our SCXRD results reveal a structural phase transition, reported for the first time, towards a monoclinic structure above 4-5GPa. These experimental results are further supported by calculations that suggest alteration of the stacking of the layers of the TATB molecules. We also investigate Al(IO3)3(HIO3)2(H2O)6, a promising energetic salt, and find through SCXRD a hexagonal to monoclinic phase change at \textasciitilde 6 GPa. These systems highlight the use of SCXRD in characterizing complex, high pressure phase changes.   

Melt-Curve and Liquid-State Transport Properties of TATB: A Molecular Dynamics Study

Phase boundaries and pressure/temperature-dependent properties of reactants are critical inputs for predictive modeling of high-explosives at the meso/macro scales. The pressure-dependent melting of TATB (1,3,5-Triamino-2,4,6-trinitrobenzene) and pressure/temperature-dependent transport properties of liquid TATB are predicted using classical, non-reactive molecular dynamics simulations for pressures up to P$=$ 20 kbar. The melt-curve, obtained using solid-liquid coexistence simulations, is well represented by the Simon-Glatzel equation. The shear viscosity and self-diffusion coefficient of liquid TATB are predicted to have an Arrhenius temperature dependence at all pressures. A linear temperature dependence (P \textless 15 kbar) and a linear density dependence (\textgreater 1200 kgm-3) is predicted for thermal conductivity. At similar densities: (1) the shear viscosity of liquid TATB is close to liquid Nitromethane (NM) but lower than liquid HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) and RDX (hexahydro-1,3,5-trinitro-s-triazine) and (2) the self-diffusion coefficient is higher than that of liquid nitromethane, HMX, and RDX. These differences could be attributed to the lower conformational flexibility of TATB and NM molecules compared to HMX and RDX. The thermal conductivity of TATB is predicted to be 20{\%} greater than the conductivity of liquid HMX at a given density.   

Search for an elastic-plastic transition in single-crystal TATB using a laser drive

While shock initiation and sensitivity of TATB-based explosives is believed to be controlled by their heterogeneous microstructure, the development of comprehensive physical models understanding these processes requires knowledge of the mechanical response of the crystalline material itself. Elastic-plastic transitions are helpful in guiding such development, but two-wave structure measurements have not yet been reported in single-crystal TATB due to a lack of samples large enough for experimentation. In this work we attempt to overcome these limitations by investigating 50-150 micrometer thick single crystals of TATB using the Janus platform at the Jupiter Laser Facilities. We present measured wave profiles at different experimental conditions and our efforts to model these observations using a crystal-mechanics framework.   

Diffusion Effects near Discontinuities in Explosions

We study the problem of diffusion effects near contact surfaces CS in TNT explosions. The flow is modeled by the compressible Navier-Stokes equations in 1-D spherical coordinates. The hyperbolic terms of the conservation laws are integrated with a 2-order Godunov scheme, while the viscous terms are advanced by a 2-order Runge-Kutta method. A tabular EOS, based on equilibrium thermodynamics, is used. Two numerically-converged solutions were found: the inviscid and viscous solutions. The blast wave solution scaled gas-dynamically i.e., with the cube-root of the charge mass . However, species concentrations and peak temperatures near the DP-air contact surface: CS were smeared by molecular diffusion effects. Similarity solutions in a Lagrangian frame of the CS were derived and found to agree with the numerical solution. Species diffusion near CS scales according to an erfc function which depends on the mass Péclet function and diffusivity D. Thermal diffusion near CS scales according to the exponential function which depends on the heat Péclet functions. Thermal diffusion drops the peak air temperature from 10,500 K (inviscid) to 4,000 K (viscous).