Altering combustion of silicon/polytetrafluoroethylene with two-step mechanical activation

Though silicon (Si) has been widely considered as a reactive fuel, the ability to tune its ignition and combustion characteristics remains challenging. One means to accomplish this may be mechanical activation (MA), which has been shown to be effective with aluminum fuels. In this work, a two-step MA process was developed to prepare fuel-rich Si/polytetrafluoroethylene (PTFE) composite reactive powders. The process consisted of cryogenic milling, followed by high intensity milling at room temperature. This resulted in particle refinement of the hard, brittle silicon particles and also dispersion within the more ductile PTFE matrix. Surface area of the as-milled powder was found to be moderate, ranging from 1.75 ± 0.06 m2 g−1 (44/51 Si/PTFE) to 5.37 ± 0.10 m2 g−1 (90/10 Si/PTFE) and was driven by the fraction of refined Si powder not dispersed in a PTFE matrix. Combustion enthalpies ranged from 15.6 ± 0.4 kJ g−1 (44/51 Si/PTFE) to 21.8 ± 2.2 kJ g−1 (90/10 Si/PTFE) and were higher than a physical mixture of the precursors. Combustion experiments showed that burning rates ranged from 1.6 to 2.1 mm s−1 and combustion temperatures (as measured from gray body emission) ranged from 1708 to 1889 K. The combustion performance of MA Si/PTFE was comparable to mixtures prepared with nanoscale and nanoporous silicon powders, indicating that the reactivity of silicon fuel had been successfully altered inexpensively without many of the major drawbacks associated with using high specific surface area powders.