Laser ignition of CL-20 (hexanitrohexaazaisowurtzitane) cocrystals

Energetic cocrystals are a new class of materials that consist of two or more energetic molecules in a crystal structure. The multicomponent cocrystals can possess significantly different properties than either component, and therefore it is possible that their ignition behavior could be different than a physical mixture. In this paper, we report the time to ignition and reaction dynamics of various energetic materials, including select cocrystal materials under CO2 laser heating. An effort was made to minimize the amount of material used in comparison to previous laser ignition studies due to the limited availability of materials. The 1:1 molar Trinitrotoluene (TNT): Hexanitrohexaazaisowurtzitane (CL-20) produced by slurry or precipitation methods and 1:2 molar cyclotetramethylene-tetranitramine (HMX):CL-20 produced by slurry or resonant mixing methods were investigated along with the individual constituents and equivalent molar physical mixes. Pressed cylindrical pellets with diameters of 3.2 mm and heights between 1 and 2 mm were ignited at irradiances ranging from 310 to 1446 W/cm2. Visual imaging was performed with a high speed camera and ultraviolet (UV) spectral data were collected with a spectrometer coupled to a streak camera. Additionally, high speed schlieren imaging was performed to investigate the ignition dynamics prior to light generation. Specific species identified with the spectrometer include OH and CN, which coincide with the observation of the secondary flame as indicated from schlieren and first-light in the visual video record. All the cocrystallized materials ignited in a similar manner to CL-20 with comparable times to secondary flame formation, but the cocrystallized materials were found to have a shorter onset to gasification than the constituents or physical mixtures.