High‐Speed, microwave ignition: Antenna igniters with frequency and bandwidth selectivity

Abstract

This effort demonstrates the development of (1) dielectric breakdown igniters and (2) bridgewire igniters powered by half‐wavelength dipole microwave microstrip antennas with bandwidth selectivity for use in standoff ignition applications. Dielectric igniters utilized a dielectric epoxy filled with up to 30 wt. % nanothermite cast between the gap of two quarter wavelength microstrips. Bridgewire igniters employed a wire soldered across the gap of two quarter wavelength microstrips. A variety of different bridgewire materials were tested in diameters ranging from 76 μm to 203 μm. The ignition delay of resonant microwave igniters is measured from high‐speed video observation of the microwave illumination process within a free‐space anechoic microwave cavity driven by a 2 kW continuous magnetron source. Dielectric breakdown igniters produced ignition delays ranging from 400 ms to 850 ms. Ignition delay decreased with higher loading of thermite. Bridgewires were found to be highly efficient and measured ignition delays ranged from 17 ms to 271 ms (24.4 J to 457.1 J ignition energy). Smaller diameter bridgewires and high resistivity metals were found to produce the shortest ignition delays. We demonstrate through S‐parameter measurements the bandwidth selectivity of bridgewire igniters and show that control of dipole geometry enables tuning of igniter center frequency and bandwidth and demonstrate experimentally the ability of bridgewire igniters to reject accidental ignition from an off frequency high power microwave field. The tunability, bandwidth selectivity, and low energy requirements, in particular of bridgewires, make them attractive for a number of new and challenging ignition applications.

On‐demand microwave growth of porosity within a granular composite energetic material: Void formation via a dielectric loss phase change binder additive for propellant burning rate control

“…These possibilities include electromagnetically ignitable thermites [14], throttling of a propellant burning rate through alkali dopant-promoted microwave energy absorption to the gas phase propellant flame [15][16][17], the microwave eddy current heating of a wired propellant via microwave irradiation to allow for microwave burning rate control of the propellant [18,19]. Additional possibilities include electromagnetic switching of pyrotechnic light emission color and chromaticity [20,21], high-speed, low energy microwave antenna igniters with tunable ignition frequency and acceptance bandwidth [22]. Finally, the development of graphene oxide (GO)-doped thermites with thermally switchable electromagnetic absorption [23], where the as-fabricated GO-doped thermite remains highly microwave reflective and resistant to microwave ignition until it is mildly heated and GO undergoes conversion to reduced graphene oxide (r-GO), making the thermite highly microwave absorptive and enabling rapid ignition by microwave in � 10-100 ms.…”