Behavior of Explosives Under Pressure in a Diamond Anvil Cell

Diamond anvil cell (DAC) studies can yield information about the pressure dependence of materials and reactions under conditions comparable to shock loading. The pressure gradient across the face of the diamonds is often deliberately minimized to create uniform pressure over much of the sample and a simplified data set. To reach very high pressures (30-40 GPa), however, it may be necessary to use ''softer'', high nitrogen content diamonds that are more susceptible to bending under pressure. The resulting enhanced pressure gradient then provides a view of high-pressure behavior under anisotropic conditions similar to those found at the burn front in a bulk sample. We discuss visual observations of pressure-induced changes relative to variations in burn rate of several explosives (Triaminotrinitrobenzene, Nitromethane, CL-20) in the DAC. The burn rate behavior of both Nitromethane (NM) and Triaminotrinitrobenzene (TATB) were previously reported for pressures up to {approx}40 GPa. Nitromethane showed a near monotonic increase in burn rate to a maximum at {approx}30 GPa after which the burn rate decreased, all without color change. At higher pressures, the TATB samples had shiny (metallic) polycrystalline zones or inclusions where the pressure was highest in the sample. Around the shiny zones was a gradation of color (red to yellow) that appeared to follow the pressure gradient. The color changes are believed related to disturbances in the resonance structure of this explosive as the intermolecular separations decrease with pressure. The color and type of residue found in unvented gaskets after the burn was complete also varied with pressure. The four polymorphs of CL-20 ({alpha}, {beta}, {gamma}, {var_epsilon}-Hexanitrohexaazaisowurtzitane, HNIW) did not change color up to the highest pressure applied ({approx}30 GPa), and each polymorph demonstrated a distinctly different burn rate signature. One polymorph {beta} was so sensitive to laser ignition over a narrow pressure range that the sample could not be aligned with a low power laser without ignition. The burn rate for that one polymorph could only be measured at pressures above and below that unique pressure. This anomalous ignition threshold is discussed with respect to the matrix of possible polymorphs, most of which have not been isolated in the laboratory. The changes in behavior, color and reaction rates of all samples are discussed with respect to possible implications to chemistry at high pressure