(247f) Theoretical Investigation of Thermal Oxidation of Carbon-Coated Aluminum Nanoparticles Using the Reaxff Reactive Force Field

The thermal oxidation of aluminum nanoparticles (ANPs) has been widely investigated for solid-fuel rockets due to their high enthalpy of oxidation during the combustion process. Essentially, ANPs have an accelerated reaction rate because of an increased surface area-to-volume ratio as compared to larger particles (e.g., a micron size), making them attractive in reactive additives for a solid propellant. Unfortunately, the increased reaction rate of ANPs also leads to a critical issue where excessive oxide layers on bare ANPs can be formed and grown prior to the combustion process. As a result, the energy release per unit mass during thermal oxidation significantly decreases because the naturally-formed oxide layer serves as a dead weight. In order to overcome this challenge, a carbon coating on bare ANPs has been proposed because of the several following advantages: (a) carbon is non-reactive at low temperature, but reactive at elevated temperatures; (b) carbon-coated ANPs have a hydrophobic nature; and (c) carbon can serve as an additional fuel-constituent. However, growth kinetics of the oxide layer on carbon-coated ANPs are not yet fully understood. In addition, a direct comparison of surface chemistry in bare and carbon-coated ANPs has not been done. In this work, we investigate the surface chemistry of the thermal oxidation of carbon-coated ANPs as compared to bare ANPs using the ReaxFF reactive force field. For this purpose, a ReaxFF description for Al/C interactions was developed, and then ReaxFF-molecular dynamics (MD) simulations were performed to investigate the growth kinetics of the oxide layer on carbon coated ANPs. Our results show that during the thermal oxidation of carbon-coated ANPs with ramping (300 - 1200 K), the oxidation rate can be delayed at a low temperature as compared to bare ANPs, whereas the oxidation rate can be accelerated at elevated temperatures (T > 700 K) because the carbon coating locally peels off at the outer surface of the ANPs. These results are consistent with experimental literature and provide insights into the role of the carbon coating during the ANP oxidation.