| This dissertation investigates the electromagnetic (EM) scattering of microwave excitations in energetic composites, specifically RDX (cyclotrimethylene trinitramine). Of interest is the presence of electric field peaks in these mediums with insight into the potential of a microwave neutralization system for explosives. Such mediums are characterized by large numbers of crystals with sizes on the scale of hundreds of microns, a small fraction of the wavelength of the incident microwave excitation, and an explosive device comprised of many millions of crystals. Furthermore, the critical features in the initiation process are electric field peaks that have a size that is a further fraction of that of the crystals, and durations which are picoseconds long. The number and size of crystals, and the required time and space resolutions make exhaustive re-creation and EM simulation of these mediums extremely time-consuming and infeasible without abstraction. This work develops new, efficient and accurate methods to reduce the time required to performEM simulation of large volumes of energetic composites.;The crystals are subwavelength in size, suggesting a description of the structure using an effective permittivity, thus characterizing the complex scattering medium with a single parameter. This permittivity is determined for a variety of structures varying in crystal number and complexity. It is found that more detailed investigations via full-wave EM simulations do not yield significantly improved results over well-known analytic models. In addition, very little variation of the effective permittivity is found between different arrangements and types of crystals, in contrast to previous studies.;In efforts to improve simulation efficiency while maintaining accurate prediction of peak field behavior, a range of abstractions of crystals is considered to determine the minimum complexity which correctly models field peaking. These abstractions are compared to the more realistic structure of non-uniform crystals placed using a physics based script. It is determined that a medium of randomly rotated and positioned cubes is the best model, offering improved speeds while maintaining the majority of the field localization phenomenon. The presence of hotspots is seen to be heavily influenced by local geometry, with the presence of multiple corners and edges resulting in the highest fields. Peak fields eight times that of the incident excitation are observed.;Finally, a coupling of the electric field behavior into thermal heating is performed via a simulation of dielectric heating and thermal conduction. Locations of high temperature in an RDX-estane composite are tracked over 20 ms for a 1 MV/m sinusoidal excitation, with peak temperature increases of over 75 K. It is observed that while locations of highest electric field do display higher than average temperatures, the higher dielectric loss of the estane binder results in the regions of highest temperature being locations where binder density is high. It is found that regions of high binder density have higher temperatures overall. Still, the highest field locations generally correspond to areas of high temperature. With the compounded effects of electric field and temperature, these locations will have a lower threshold for initiation. than would be suggested by the separate effects.;The cubic crystal abstraction is shown to work well from the perspective of predicting high peak fields, as well as displaying thermal behavior in agreement with previous observations. The success of this abstraction offers significantly improved runtimes or expanded scope for future investigations of energetic materials and may be adapted to other methods of excitation, such as purely thermal, acoustic or mechanical stress, or a combination of coupled sources. |
