Anti-sintering behavior and combustion process of aluminum nano particles coated with PTFE: A molecular dynamics study

The characteristic of easy sintering of aluminum nanoparticle (ANP) limits its application in solid propellants. Coating ANP with fluoropolymer could effectively improve its combustion performance. To find out how the coating layer inhibits sintering and promotes complete combustion of particles from an atomic view, a comparative study has been done for bare ANP and PTFE coated ANP by using reactive molecular dynamics simulations. The sintering process is quantified by shrinkage ratio and gyration radius. Our results show that, at the same heating rate and combustion temperatures, bare ANPs are sintered together after the temperature exceeds the melting point of aluminum but the decomposition of PTFE coating layer pushes particles away and increases reaction surface area by producing small Al–F clusters. The sintering of ANPs which are heated in PTFE is alleviated compared with particles heated in oxygen, but particles still sinter together due to the lack of intimate contact between PTFE and alumina surface. The effect of temperature on the combustion of PTFE coated ANPs is also studied from 1000 to 3500 K. The number density analysis shows the particles will not be sintered at any temperature. Aluminum fluoride prefers diffusing to the external space and the remained particles are mainly composed of Al, C and O. Fast ignition simulations are performed by adopting micro canonical ensemble. With the expansion of aluminum core and the melting of alumina shell, bare ANPs are sintered into a liquid particle directly. For PTFE coated ANPs, the volatilization of gaseous aluminum fluoride products continually endows particles opposite momentum.