Time-resolved spectroscopy of initiation and ignition of flash-heated nanoparticle energetic materials

Nanotechnology has brought a great deal of excitement to research in energetic materials (EMs). Nanoparticle EMs have high densities of stored energy and the potential for multifunctionality. Here we discuss research on fundamental mechanisms of initation and ignition of EM with Al or B fuel nanoparticles and Teflon ^AF or nitrocellulose (NC) oxidizer. Polybutadiene (PB) was also used as an inert. The thin-film samples were confined between two windows and were activated by flash-heating the metal nanoparticles with picosecond laser pulses. Reactions of isolated nanoparticles with their surroundings were studied by measuring ablation thresholds. A shock-induced polymer dissociation model was needed to explain the growth of the reaction volume surrounding a flash-heated fuel particle. Thicker oxide passivation layers confined the nanoparticle allowing the pressure to build up to higher values during flash-heating. Initiation, as the onset of chemical reactivity, was probed using time-resolved Raman or infrared to monitor disappearance of nitrato (ONO ₂) of NC or CF ₂, CF ₃,or CFO of Teflon ^AF. Ignition, the onset of energy-releasing processes, was studied by analyzing time-dependent spectra of emission bursts. In flash-heated Al/Teflon, initiation occurs in ∼50 ps and ignition occurs in ∼100 ps. In B/Teflon, ignition occurs in ∼200 ps. Transient opacity measurements of Al/Teflon suggest that chemical reactivity beyond the initial exothermic formation of AlF occurs within ∼5 ns.