Mesoscale observations of the thermal decomposition of energetic composites under ultrasonic excitation

Abstract

Polymer bonded explosives (PBXs) have exhibited localized heating and, in some cases, subsequent reactions in response to ultrasonic excitation. The objectives of this work are to investigate the conditions for, and locations of, hot spot initiation of energetic crystals embedded within a polymer binder subjected to periodic mechanical excitation from a contacting transducer operating at 210.5 kHz. Crystal and binder interactions and events such as delamination, solid-solid phase change, and gas production were observed in real time via optical microscopy. We conclude that there are two main pathways of heat generation which are capable of driving an explosive to decomposition in the systems of interest: frictional heating from a delaminated and moving binder interface and viscoelastic heating in the binder near an embedded crystal. Formulations that address the vibration initiation sensitivity of PBX composites require knowledge of the key internal heat generation mechanisms. The results included here indicate that improving binder adhesion to energetic crystals or improving crystal morphology to reduce heating during cyclic loading may only address one of the available pathways of energy dissipation and that binder and crystal selection should be done concurrently. Furthermore, the results presented herein appear to indicate that rounded particles, in contrast to faceted crystals, with strong adhesion to the binder are expected to result in decreased heating rates under ultrasonic excitation.