The impact of crystal morphology on the thermal responses of ultrasonically-excited energetic materials

Abstract

The ability to detect explosive materials may be significantly enhanced with local increases in vapor pressure caused by an elevation of the materials' temperature. Recently, ultrasonic excitation has been shown to generate heat within plastic-bonded energetic materials. To investigate the impact of crystal morphology on this heating, samples of elastic binder are implanted with single ammonium perchlorate crystals of two distinct shape groups. Contact piezoelectric transducers are then used to excite the samples at ultrasonic frequencies. The thermal responses of the crystals are recorded using infrared thermography, and the rate of heating is estimated. Surface temperature increases up to 15 °C are found to arise after 2 s of excitation, with much higher heating levels expected near the inclusions themselves as demonstrated by the chemical decomposition of some crystals under favorable excitation conditions. The rates of heat generation are compared to various crystal morphology features through 2D estimates of length scale, perimeter and irregularity. It is observed that crystals grown in the lab, featuring sharp geometric facets, exhibit a higher probability of significant heat generation than inclusions with more spherical shapes. However, no statistical link is found between the rates of heat generation and the crystal morphology in those samples that do generate significant heating, likely because variations in surface roughness cannot be entirely eliminated during experimentation. It is hoped that this study will lead to a better understanding of the nature of heat generation in energetic materials from ultrasonic sources.

Localized Heating Near a Rigid Spherical Inclusion in a Viscoelastic Binder Material Under Compressional Plane Wave Excitation

“…It is also important to note that the particle morphology of the embedded inclusion has been experimentally shown to greatly affect the stress distributions as well as the heating rates associated with the ultrasonic excitation of similar energetic systems [29]. Since the model with a spherical inclusion does not account for those stress concentrations associated with irregular particle morphologies, the predictions shown in the present work may represent a conservative estimate of the heating magnitudes in these systems.…”

“…The generation of such "hot spots" is difficult to observe experimentally, due to the extreme pressures, temperatures, and short time scales associated with these processes. Recently, ultrasonic excitation has been used to generate hot spots within samples consisting of an elastic binder material with embedded single energetic particles, or crystals [26][27][28][29]. This experimental investigation of ultrasonically induced hot spots has been proposed as a valuable technique to investigate corresponding processes during detonation events.…”

Localized Heating due to Stress Concentrations Induced in a Lossy Elastic Medium via the Scattering of Compressional Waves by a Rigid Spherical Inclusion

“…), may then dominate the thermal behavior of the system at later times in this process. It is also important to note that the particle morphology of the embedded inclusion has been experimentally shown to greatly affect the stress distributions as well as the heating rates associated with ultrasonic excitation of similar energetic systems [23]. Since the model with a spherical inclusion does not account for those stress concentrations associated with irregular particle morphologies, the predictions shown in the present work may represent a conservative estimate of the heating magnitudes in these systems.…”

“…The generation of such hot spots is difficult to observe experimentally, due to the extreme pressures, temperatures, and short time scales associated with these processes. Recently, ultrasonic excitation has been used to generate hot spots within samples consisting of an elastic binder material with embedded single energetic particles, or crystals [20][21][22][23]. This experimental investigation of ultrasonically-induced hot spots has been proposed as a valuable technique to investigate corresponding processes during detonation events.…”

Vibration-Induced Heating of Energetic Materials: A Review

“…8 right), which continued up to the point of ignition. A subsequent paper [109] noted that less spherical particles produced greater heat generation, and that localised heating may also arise from a 'focussed-viscoelastic' effect, and/or frictional heating of interfaces.…”