Nanoporous Silicon Combustion: Observation of Shock Wave and Flame Synthesis of Nano-particle Silica

The persistent hydrogen termination present in nanoporous silicon (nPS) is unique compared to other forms of nanoscale silicon (Si) which typically readily form a silicon dioxide passivation layer. The hydrogen terminated surface combined with the extremely high surface area of nPS yields a material capable of powerful exothermic reactions when combined with strong oxidizers. Here, a galvanic etching mechanism is used to produce nPS both in bulk Si wafers as well as in patterned regions of Si wafers with microfabricated ignition wires. An explosive composite is generated by filling the pores with sodium perchlorate (NaClO4). Using high-speed video including Schlieren photography, a shock wave is observed to propagate through air at 1127 +/- 116 m/s. Additionally, a fireball is observed above the region of nPS combustion which persists for nearly 3x as long when reacted in air compared to N-2, indicating that highly reactive species are generated that can further combust with excess oxygen. Finally, reaction products from either nPS-NaClO4 composites or nPS alone combusted with only high pressure O-2 (400 psig) gas as an oxidizer are captured in a calorimeter bomb. The products in both cases are similar and verified by transmission electron microscopy (TEM) to include nano- to micrometer scale SiOx particles. This work highlights the complex oxidation mechanism of nPS composites and demonstrates the ability to use a solid state reaction to create a secondary gas phase combustion.