The Effect of Heating Rate on Combustion of Fully Dense Nanocomposite Thermite Particles

The effect of heating rate on the combustion pathways of fully dense, micron-sized, nanocomposite thermite particles is examined. Stoichiometric nanocomposite thermites utilizing aluminum or zirconium as fuels were prepared by arrested reactive milling with several oxidizers (WoO₃, MoO₃, CuO, Fe₂O₃, and Bi₂O₃). The powders were ignited using both electrostatic discharge (ESD) and CO₂ laser beam. The particles ignited directly by ESD were heated at ca. 10⁹ K/s while the particles ignited by laser were heated at ca. 10⁶ K/s. Additional reference laser ignition experiments were performed with particles of pure Al and Zr. Optical emission of burning particles was recorded. For both Al- and Zr-based thermites, burn times of particles ignited by the laser were slightly longer than for similarly sized particles of pure Al and Zr, respectively. This suggests that both Al- and Zr-based thermite particles heated by laser lose their nanostructure immediately upon their ignition. Burn times and temperatures were compared for the particles of the same materials ignited using different heating rates. For Zr-based thermites, ESD generated burning particle clouds, with many particles ignited with a substantial delay and heated much slower than those ignited by ESD directly. Thus, direct comparisons were possible only for the Al-based thermites, for which both laser and ESD ignition yielded individual burning particles. It is observed that the ESD-ignited Al-based thermite particles burn much faster than the same particles ignited by the laser. The difference in the burning regime of particles heated at different rates is interpreted observing that the rate of reaction in a particle heated up to the Al melting point is substantially higher in the ESD-heated particle compared to the same particle heated by the laser. After Al melts, the nanostructure of the composite particles may be lost; when this occurs the extent of thermite reaction in the ESD-heated particle is much greater than that in the particle heated by the laser.