Advantages of electrospray fabrication on the surface, microstructure and combustion characteristics of energetic composites of Al powders encapsulated in PVDF/WO₃ hybrid shells

Electrospray deposition was used to create concentric polyvinylidene fluoride/tungsten oxide (PVDF/WO₃) shells on aluminum (Al) powders spanning two size regimes (nano-Al, ≈ 87 nm; micro-Al, ≈ 20 μm). Transmission-electron microscopy shows that nano-Al acquires a uniform 5.5 nm-thick polymer/oxide coating, whereas micro-Al develops a discontinuous fibrous layer several hundred nanometers thick. These contrasting morphologies govern the energetic response. By comparing nano- and micro-scale Al cores and contrasting electrosprayed and ball-milled composites, the study elucidates how shell morphology governs the transient thermal processes of laser ignition, heat release and flame development. In laser-ignition tests, the nano-Al electrosprayed composite (NC) exhibited the shortest ignition delay of 42 ± 3 ms and reached a peak flame temperature of 2500–2700 °C, producing a narrow, self-sustaining jet; the corresponding ball-milled sample (NM) showed an ignition delay of 85 ± 5 ms and peaked at ~1500 °C, while the micro-Al electrosprayed material (MC) had an ignition delay of 156 ± 8 ms, plateaued at ~1107 °C, and failed to maintain combustion. The ball-milled micro-Al sample (BM) could not be ignited under the same conditions. Differential-scanning-calorimetry combined with Kissinger analysis revealed markedly different apparent activation energies for the first exothermal process (NC: 171 kJ/mol; NM: 63 kJ/mol; MC: 91 kJ/mol; ball-milled micro-Al, MM: 21 kJ/mol), underscoring the sensitivity of the initial Al–WO₃ reaction to coating continuity and diffusion distance. Overall, electrospraying reduces ignition delay by approximately 50 % compared to ball-milled samples, intensifies heat release, and maximizes flame temperature when the aluminum core is in the nano-scale regime. The resulting activation energies, ignition delays, flame speeds and peak temperatures provide thermal-design data for high-temperature heat-source layers in micro-thrusters, gas generators and other compact thermal-engineering devices, in addition to advanced propulsion or pyrotechnic applications.