Enhancing the combustion of silicon nanoparticles via plasma-assisted fluorocarbon surface modification

• <10 nm silicon nanoparticles are nucleated and grown using a low-temperature plasma. • A fluorocarbon polymer shell is applied to the particles before exposure to air. • The polymer shell greatly delays the growth of the native oxide layer. • The exothermic reaction between the silicon core and the fluoropolymer shell enhances combustion. • The fluorocarbon-coated particles show a lower ignition temperature and an increased pressurization rate. Nanostructured silicon has great potential as an energetic material due to its high energy density, close to that of aluminum; however, its combustion kinetics are strongly affected by the presence of a native oxide layer. As the particle size is reduced, the material behavior is dominated by surface effects, highlighting the need for new approaches to fabricate silicon nanoparticles and precisely control their surface chemistry. Here, we describe the use of low-temperature plasma to nucleate and grow < 10 nm silicon particles. The resulting aerosol is passed through a second plasma reactor, to which a fluorocarbon monomer is supplied. The second plasma initiates the growth of a polymeric shell onto the silicon particles, resulting in a highly conformal coating. Chemical analysis confirms that the polymer shell is compositionally similar to polyvinylidene fluoride (PVDF). We find that the fluorocarbon shell acts as an artificial passivation layer that significantly delays the onset of oxidation in air. In addition, the direct contact between the fluorocarbon shell and silicon core lowers the ignition temperature compared to the physical mixtures of silicon nanoparticles and PVDF. It leads to a higher pressurization rate and a lower combustion time when tested in a combustion cell. This is consistent with the combustion process being initiated by the exothermic interfacial reactions between the silicon core and the fluorocarbon shell. Mass spectroscopy confirms this hypothesis because silicon fluoride is produced only for the core–shell structure, and not for a physical mixture of silicon and PVDF. This study provides an example of a new processing technique capable of controlling the interfacial chemistry of small nanoparticles with beneficial effects on their combustion.