Explosive versus Thermite Behavior in Iron(0) Aerogels Infiltrated with Perchlorates

Monolithic nanoporous iron was prepared via carbothermal reduction of interpenetrating networks of polybenzoxazine and iron oxide nanoparticles. Excess carbon was burned off at 600 °C in air, and oxides produced from partial oxidation of the Fe(0) network were reduced back to Fe(0) with H ₂ at different temperatures (temp) ranging from 300 to 1300 °C. Samples were carbon-free, for temp > 400 °C also oxide-free, and are referred to according to the final H ₂ -reduction temperature as Fe-temp. Fe-temp monoliths were infiltrated with perchlorates, dried exhaustively and were ignited with a flame in open air. Most experimentation was conducted with LiClO ₄ . Depending on temp, monoliths fizzled out (≤400 °C), exploded violently (500-900 °C) or behaved as thermites (≥950 °C). Samples sealed in evacuated tubes did not explode, while if sealed under N ₂ the explosive effect was intensified. Thus, explosive behavior was attributed to rapid heating and expansion of gas filling nanoporous space. However, although that condition was necessary for explosive behavior, it was not sufficient. Based on SEM, particle sizes via N ₂ sorption, electrical conductivity measurements and mechanical strength data under quasi-static compression, it was concluded that the boundaries between the three types of behavior after ignition were associated with (a) mild sintering (fizzling/explosive boundary at around 500 °C); and, (b) melting-like fusion of skeletal nanoparticles (explosive/thermite boundary at around 950 °C). Overall, mechanically weaker networks fizzled out; too strong behaved as thermites; networks of intermediate strength exploded. For thermite behavior in particular, other factors may be also at play, such as a combination of reduced porosity, a substoichiometric amount of LiClO ₄ and a slower heat release rate. The latter was supported by TGA data in O ₂ and was attributed to a slower rate of oxidation of progressively thicker nanostructures as the H ₂ -reduction temperature increased.