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Time-resolved UV−visible absorption spectroscopy was used to examine chemical decomposition in neat liquid nitromethane (NM) subjected to stepwise shock loading to 19 GPa. Up to a peak pressure (temperature) of 13.8 GPa (854 K), no sign of chemical reaction was observed in the nπ* absorption band centered at 270 nm. For shock compression resulting in peak temperatures above 940 K, extensive reaction was indicated by irreversible red-shifting of the absorption band edge that occurred after peak pressure was reached. This red-shift was followed by a loss of transmission through the sample, which was attributed to the formation of absorbing reaction products. Comparison of these reaction-induced spectral changes with previous absorption results for NM sensitized by ethylenediamine (EDA) suggests that the presence of the amine causes a change in the early stages of shock-induced decomposition. An induction time was observed between the attainment of peak pressure and the onset of reaction-induced spectral changes in neat NM. Significant decreases in the induction time were produced by modest increases of 25−50 K in the initial sample temperature. The induction time data are consistent with the thermal explosion model of shock initiation in energetic materials. The observed induction time correlates well with the final shock temperature; no pressure dependence is observable within the pressure range examined here. Measured induction times for the absorption experiments are consistently shorter than for continuum experiments reaching similar temperatures likely because the absorption technique probes earlier stages of the reaction process. This suggests that induction times measured using different experimental techniques are not necessarily equivalent.

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Time-resolved Raman spectroscopy was used to examine chemical changes in neat liquid nitromethane subjected to stepwise loading to peak pressures of 14−17 GPa. After a peak pressure at 14 GPa was reached, no changes in the CN (917 cm-1), NO2 (1400 cm-1), and CH3 (2968 cm-1) symmetric stretching modes were observed. After a peak pressure at 16 GPa was reached, time-dependent changes were observed during the induction period reported in previous absorption experiments. At this peak pressure, the extent of reaction was small, and the observed changes in the CH3 and CN modes indicated prereaction changes in the sample bulk. After a peak pressure at 17 GPa was reached (980 K peak temperature), all three peaks disappeared, indicating that the extent of reaction was substantial under these conditions. The broadening of the CH3 peak and the softening of the CN mode observed in this work suggest strong intermolecular interactions. These interactions lead to a reaction precursor involving proposed head-to-tail intermolecular associations with decomposition proposed to follow through a bimolecular reaction, put forward by Bardo, which forms nitrosomethane and nitromethanol. Confirmation of these ideas will require different spectroscopic methods, since Raman measurements are primarily useful for probing initial changes in the sample.

A thermodynamically consistent equation of state (EOS) was developed for unreacted liquid nitromethane (NM). The specific heat cv, the coefficient of thermal pressure (∂P/∂T)v, and the isothermal bulk modulus BT, were modeled as functions of temperature and volume using existing experimental data. To test our EOS predictions, temperature measurements using time-resolved Raman spectroscopy were obtained from NM subjected to stepwise loading. In contrast to previous EOS developments, calculations using our EOS show good agreement with the measured temperatures. Comparison with previous EOS models shows that simplifying assumptions, such as holding (∂P/∂T)v or Γ/v constant, lead to significant inaccuracies in temperature predictions for shocked NM. The assumption that the Gruneisen parameter Γ is a function of volume only is not consistent with our EOS.

Second-order elastic constants of pentaerythritol tetranitrate and cyclotrimethylene trinitramine using impulsive stimulated thermal scattering J. Appl. Phys.The second-order elastic constants for cyclotetramethylene tetranitramine ͑␤-HMX͒ single crystals were determined using the impulsive stimulated thermal scattering ͑ISTS͒ method. Despite the low symmetry of these crystals, the complete set of 13 elastic constants were determined accurately from acoustic velocity measurements using samples cut parallel to three different crystal planes. Our acoustic velocities are consistent with the limited sound speed data available from ultrasonic measurements. However, significant differences are observed between the elastic constants determined from our experiments and those obtained previously using Brillouin scattering. Our results demonstrate the usefulness and efficiency of the ISTS method for determining the full set of elastic constants of low-symmetry molecular crystals, including energetic crystals.

Impulsive stimulated thermal scattering (ISTS) was used to determine the complete set of second-order elastic constants for pentaerythritol tetranitrate (PETN) and cyclotrimethylene trinitramine (RDX) single crystals. Despite the weak scattering efficiency of these materials, excellent signal quality was obtained by using an optical heterodyne detection approach. The elastic constants for PETN agree well with previous values obtained from ultrasonic velocity measurements. The elastic constants for RDX are consistent with previous values obtained from ultrasonic velocity measurements and from resonant ultrasound spectroscopy, but show significant differences with values obtained from Brillouin scattering data. The present results demonstrate that the ISTS method, with optical heterodyne detection, provides a useful and accurate approach for determining the elastic constants of energetic crystals.

A nonlinear anisotropic continuum framework for describing the thermoelastic-plastic response of single crystals shocked along arbitrary orientations has been developed. Our modeling approach incorporates nonlinear elasticity, crystal plasticity, and thermodynamic consistency within an incremental tensor formulation. Crystal plasticity was incorporated by considering dislocation motion along specified slip planes. The theoretical developments presented here are sufficiently general to also accommodate other types of inelastic deformation mechanisms. As representative applications of the theoretical developments, numerical simulations of shock wave propagation in lithium fluoride (LiF) and copper single crystals are presented and compared to wave profile data for several crystal orientations. Simulations of shock wave propagation along low-symmetry directions, where data are not available, are also presented to examine the propagation of quasilongitudinal and quasishear waves in crystals undergoing elastic-plastic deformation. Temperature calculations for the shocked single crystals are discussed.