Electromagnetic studies of explosives at ISM (industrial, scientific, medical) frequencies
Microwave frequency electromagnetic properties are critical for understanding and predicting the heating and ignition behavior of explosives. In this work complex permittivity measurements are reported from 1 to 20 GHz for a variety of neat and plastic bonded explosives measured by the circular cavity technique. These measurements were then used with analytic techniques to develop and understand a theoretical microwave applicator, dependent on high local electric fields. From these assumptions, a practical cavity was then designed using COMSOL 4.3 Multiphysics finite element analysis software in order to simulate a low power (100 W), high electric field microwave applicator. Times to 100°C were predicted for all materials with appropriate thermal properties using both analytic approximations and COMSOL simulations. Using available chemical kinetic parameters, times to ignition were predicted for PBX9501, PETN, TATB, and HMX. For proposed applicators at 2.45 and 10 GHz, ignition may occur at less than 10 seconds or less than 1 second exposure, respectively. These predictions show that neat explosive materials can be effectively heated in short time scales through direct microwave heating without a need for absorptive binders or inclusions. The addition of a binder leads to highly localized heating in the absorptive binder at much shorter timescales. Such heating enables fundamental studies of materials, such as the exploration of localized ignition and hot spot formation. Experimentally, the designed cavity at 2.45 GHz was then used to observe rapid microwave heating of a plastic bonded, sugar mock explosive material (PBS9501). Heating to the point of vaporization occurred in less than 10 s effective exposure time, subject to pulsed exposure.
Son, Purdue University.
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