Tailoring the mechanical and combustion performance of B/HTPB composite solid fuel with covalent interfaces

The development of high-performing solid fuels with desired mechanical and combustion properties is critical to future air-breathing propulsion systems for space exploration and hypersonic navigation. Boron (B)/hydroxyl-terminated polybutadiene (HTPB) composite has been studied for this purpose due to the high energy density (heat of combustion) of B and the appropriate processability and mechanical properties of HTPB. However, the weak interface between B and HTPB results in weakened mechanical properties, agglomerated B particles, and slow and inefficient combustion, especially for composites with high B loading (30 wt% and above). In this study, the effect of interface between B and HTPB on the combustion and mechanical performance of high-loading B/HTPB composites was investigated by surface functionalization of B particles. Three interfacial characteristics were compared: polar (pristine B)/nonpolar (HTPB), nonpolar (hydrocarbon-functionalized B)/nonpolar (HTPB), and covalently bonded (amine-functionalized B/HTPB) interfaces. It was found that both covalently bonded and nonpolar/nonpolar interfaces effectively reduced the aggregation of B particles in the HTPB matrix, even with up to 45 wt% B loading, thus promoting the combustion efficiency and burning rate. Moreover, covalently bonded interfaces in B/HTPB composites led to strain-hardening behaviors, resulting in enhanced strength, ductility, and toughness. This work highlights the significance of interface engineering in B/HTPB composites for the efficacy and safety of future air-breathing solid-fueled propulsion devices.