Evaluation of Homogeneous Arrhenius Reaction Kinetics for Energetic Materials in the High Explosive Initiation Time Experiment

The Arrhenius kinetics for the onset of exothermic reactions in energetic materials control many facets of their handling safety and performance. We report new measurements of the kinetics of thermal explosion at temperatures up to 700 K for the secondary explosives pentaerythritol tetranitrate (PETN), cyclotetramethylene tetranitramine (HMX), and hexanitrohexaazaisowurtzitane (CL-20). These data are obtained from the high explosive initiation time (HEIT) experiment, which rapidly heats small samples confined in hollow steel needles by the discharge of a high voltage fireset. The ignition and growth of deflagrations in the HEIT experiment are analyzed theoretically, which identify a high temperature regime that is associated with a small effective activation energy where the time-to-explosion is controlled by the deflagration velocity, and a low temperature regime with a large activation energy that is controlled by the induction time for the onset of reactions at the interface with the hot needle. Our theoretical model shows why the reaction rates obtained in the low temperature regime are consistent with predictions from independent reactive molecular dynamics simulations and experimental measurements for uniformly heated samples, despite the very inhomogeneous temperature profiles generated in the HEIT experiment prior to ignition. Since the HEIT experiment provides time-to-explosion data that is relevant to ignition in accident scenarios in the temperature interval between existing methods, including the one-dimensional time-to-explosion tests and reactive molecular dynamics simulations, we have derived and report improved estimates for the effective single-step Arrhenius kinetics of PETN, CL-20, and HMX.