Thermal calibrations of hypervelocity capture in aerogel using magnetic iron oxide particles

Due to its extremely high porosity and the nanoscale filaments that make up its structure, aerogel is an excellent material for the capture of hypervelocity, micron-sized particles. A great deal of the kinetic energy of a particle is converted to thermal energy during the capture process, altering or even destroying components of the particle. The studies described here were conducted using aggregate projectiles made up of magnetic sub-micron hematite particles in an attempt to directly measure the temperatures experienced by fine particles during hypervelocity capture in aerogels. When these particles are heated to a temperature above their Curie temperature (675 °C) during the capture, they lose their magnetization. Thus, by impact testing these particles in aerogels at different velocities, we were able to determine if individual components of these aggregate particles were heated to a temperature greater than their Curie temperature by observing their magnetization. After impact testing, the particles were extracted from the aerogel, thin sectioned, and observed using atomic and magnetic force microscopy, as well as, electron paramagnetic resonance. Terminal particles for impacts at or above 4.5 km/s were still magnetic, while those from the track walls were not. Even terminal particles captured at 6.6 km/s were still magnetic. Iron oxide particles coated with silica, to mimic extraterrestrial materials, from track walls captured at 5.47 km/s were still magnetic. The study also demonstrated that aggregate projectiles can survive the forces they are subjected to during hypervelocity launch in a light gas gun.