Laser ignition of select hexanitrohexaazaisowurtzitane (HNIW or CL-20) cocrystals

Andrew W McBain, Purdue University


Energetic cocrystals are a new class of materials that consist of two or more reactive species in a unique crystal structure. The multicomponent crystal structure can possess significantly different properties than either component, and therefore it is possible that ignition phenomenon could be different than a physical mixture. In this paper we report the time to ignition and reaction dynamics of cocrystallized HNIW energetic materials under CO 2 laser heating. An effort was made to minimize the material used in comparison to previous laser ignition. 1:1 molar Trinitrotoluene (TNT): Hexanitrohexaazaisowurtzitane (CL-20) produced by slurry and precipitation methods and 1:2 molar cyclotetramethylene-tetranitramine (HMX):CL-20 produced by slurry and LabRAM methods were investigated in addition to the individual constituents and equivalent molar physical mixes for each. Cylindrical pellets with diameters of 3.2mm and heights between 1 and 2 mm were pressed of each material and a CO2 laser was used to ignite the samples at irradiances ranging from 310 to 1446 W/cm2. Visual light data was collected with a high speed camera and ultraviolet (UV) spectral data was collected with spectrometer coupled to a UV streak camera. Additionally, high speed schlieren imaging was performed to investigate the ignition dynamics prior to light generation. Ignition was characterized by both a ‘fully developed flame kernel’ and ‘first effect’ criteria in order for a more accurate comparison between materials. The cocrystallized materials were found to generate gas quicker than the constituents or a physical mixture although the times to the final flame kernel were less conclusive. Specific species identified with the spectrometer include OH and CN. Qualitatively the cocrystallized materials ignited similarly to their constituents. In particular the TNT:CL-20 cocrystals ignited similarly to CL-20 at low irradiances but resembled TNT at higher irradiances.




Son, Purdue University.

Subject Area

Mechanical engineering

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