Oxidation reaction kinetics of HTPB-boron carbide/polytetrafluoroethylene formulations as a solid fuel

Abstract

A solid-fuel ramjet engine powered by boron carbide (B4C) can generate nearly the same amount of energy as boron. However, the inherent oxide layer on the B4C surface restricts the energy released during its oxidation. In this study, we examine the effect of polytetrafluoroethylene (PTFE) on the oxidation of B4C and the removal of the oxide layer during the oxidation reaction. The B4C/PTFE binary composite powder was prepared using a ball milling process at three different concentrations (5:20, 10:20, and 15:20). Hydroxyl-terminated polybutadiene (HTPB) loaded with binary composite powder was manufactured by vacuum casting technique. The pure HTPB and HTPB/PTFE fuels were manufactured as reference formulations. The B4C/PTFE binary composite powder was characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), and high-resolution scanning electron microscope (HRSEM). The thermal oxidation characteristics of prepared fuel formulations were studied using the thermogravimetric technique. The kinetics of the oxidation reaction was studied using both isothermal and non-isothermal methods. The thermogravimetric curves showed that the oxidation reaction of HTPB-based fuel occurred in four steps. When B4C/PTFE binary composite powder is added to HTPB, it decomposes faster and at a higher rate. The decomposed fluorine species of PTFE significantly improved the oxidation reaction of composite powder and increased the energy release rate of B4C. The kinetic studies of oxidation suggested that the addition of B4C/PTFE binary composite powder into HTPB lowered the activation energy (Ea: S1 > S2 > S3 > S4 > S5) required to prompt the oxidation reaction. Based on thermal and kinetic results, a potential oxidation promotion mechanism of B4C/PTFE composite powder was proposed. The initial stages of B4C oxidation resulted in a glassy layer of B2O3 oxide forming at the interface of B4C and PTFE powder. As a result of the oxidation reaction between B4C/PTFE and O2, CF4, Trifluoroboron (BF3) and CO2 were generated. At high temperatures, the oxide shell ruptured, allowing the O2 to diffuse into the core of the B4C particle and releasing a large amount of heat energy.