Mechanistic understanding of the participation effect of carbon in the combustion of PTFE/n-Al: Combining experimental verification with reactive molecular dynamics simulation

Abstract

Owing to the high energy density, polytetrafluoroethylene/aluminum nanoparticles (PTFE/n-Al) has been considered as the promising composite in energetic applications, but the limited combustion efficiency restricts its practical uses. Undoubtedly, for the further improvement of combustion efficiency, mechanistic understanding of the combustion of PTFE/n-Al is fundamentally necessary. However, the participation effect of non-fluorine species (i.e., carbon) on combustion has been overlooked, leading to the lack of generalized framework for combustion mechanism. By using a combined experimental verification and reactive molecular dynamics (RMD) simulation approach, this work focuses on the participation effect of carbon and involved reaction mechanism in the combustion of PTFE/n-Al. The thermal reactivity and combustion performance of four PTFE/n-Al composites with F/Al molar ratio of 1.0 (R1), 2.0 (R2), 3.0 (R3) and 4.0 (R4) have been investigated by thermal analyses (TGA-DSC-MS) and combustion experiments (flame images, heat of reaction and condensed combustion products). The heat of reaction decreases in the order of R2 (7.89 kJ/g), R1 (7.67 kJ/g), R3 (7.33 kJ/g) and R4 (6.55 kJ/g), and a similar trend has been found for the flame intensity. Moreover, the carbon deposition percentage (%) in combustion of R1, R2, R3 and R4 reaches 76.5%, 76.4%, 90.2% and 123.3%, which shows a negative effect on combustion performance. Based on experimental results, the RMD simulations for the reactions of n-Al and Al core with trafluoroethylene (TFE) and related fluorocarbon atmospheres have been performed. It is found that, going from R1 to R4, the elevated active fluorocarbon species play a double-edged sword role in combustion. On one side, they exhibit a promoting role in accelerating the crack of Al2O3 shell, the dispersion and heat releasing kinetics of Al core. On the other side, they also result in extensive carbon deposition on unreacted n-Al, showing a hindering role in further reaction and lowering combustion efficiency. Accordingly, in experiments, optimal combustion performance of PTFE/n-Al has been observed with suitable composition (R2). Finally, the generalized reaction mechanism involving participation effects of carbon and fluorine has been proposed for PTFE/n-Al, which is beneficial for perfecting combustion mechanism and improving combustion efficiency of fluorocarbon/metal composites.