The effects of electromagnetic radiation on energetic materials

An electromagnetic exposure chamber was designed and built to analyze the effects of exposing energetic materials, like thermites and secondary explosives, to electromagnetic waves. The electromagnetic exposure chamber can safely deliver electromagnetic power in the range of microwaves between 0.8 and 4.2 GHz to an energetic load. Diagnostics were developed to study energy absorption at different frequencies of microwave radiation by observing electromagnetic heating using an infrared camera. Future experiments with the electromagnetic exposure chamber could be used to understand how microwaves couple into various monomolecular explosives and metal-metal oxide composites for possible explosive detection techniques or for heating samples to ignition. For baseline experiments, three samples were placed in the electromagnetic exposure chamber and heating was observed and analyzed. The samples consisted of two borosilicate samples, one coated with graphite paint, and a compressed sample of flake graphite pressed to 69% of its bulk density. Results are in agreement with the theories of microwave heating and verify the functionality of this experimental design. This study investigates the effects of coupling microwaves to different particle sizes of a metal aluminum fuel and an iron (III) oxide oxidizer that make up the reactants of a thermite mixture used in energetic applications. An electromagnetic exposure chamber exposed compacted samples of each reactant to microwaves at a frequency of 3.3 GHz and in-situ 2-D spatial temperature measurements were captured from the surface of the sample to quantify microwave heating. It was shown that for the non-conductive oxidizer (Fe2O3), decreasing the particle size increased the microwave absorption because of the increase in effective surface area. In contrast, decreasing the conductive fuel (Al) particle size resulted in decreased microwave absorption because of the ratio of the particle size to the skin depth. The volume of aluminum participating in the microwave absorption is less in the compact consisting of smaller particles because of the higher alumina content. This research contributes to current interest in microwave heating powdered compacts with experimental results that aid in the safe handling and use of energetic composites. Methods for heating and igniting thermite mixtures are always evolving to make ignition systems safer, more efficient, and reliable. Recently researchers have employed microwave technology in igniting energetic materials when studies showed that metals could be heated with microwaves as long as the sample consisted of a powdered form of the metal; either in a loose configuration or powdered compact. This study investigates how adding a graphite susceptor to thermites consisting of aluminum (Al) and iron (III) oxide (Fe2O3) affects microwave coupling to the sample and also how the additive affects the combustion of the thermite. Results show that graphite enhances microwave coupling up to a quantity of 10% by mass of the thermite and then reduces heating at higher percentages once reaching a percolation threshold. The thermite mixture showed a decline in flame propagation when the percent mass of graphite exceeded 1%.