Energetic Characteristics and Reaction Mechanism of Hydrogenated Magnesium Nanoparticles: The Role of Condensed-Phase Reaction.

Abstract

Understanding the reaction mechanism of energetic composites is crucial for tuning their reactivity and energy release. Although magnesium hydride nanoparticles (NP) have shown tremendous potential as high-performance reactive materials due to their high combustion enthalpy, the fundamental energy release mechanism and kinetics are yet to be explored. In this work, nonthermal plasma processing is implemented to hydrogenate magnesium nanoparticles, which are prepared via in-flight gas condensation of Mg vapor. Nanoparticle-based metals face multiple challenges, such as loss of nanostructure or sintering at high temperatures before combustion and the presence of a native oxide layer, which acts as the kinetic barrier to reaction. Magnesium has the advantage of high vapor pressure, allowing it to resist sintering; however, Mg must still diffuse out through the oxide layer, which is the rate-limiting step for ignition to take place. Our experiments revealed that upon the desorption of hydrogen, magnesium hydride leaves behind a fresh metallic magnesium surface, which undergoes a solid-state reaction, unlike Mg NPs, for which ignition initiation depends on the outward diffusion of Mg released from the core. The ignition temperature is significantly lowered from 690 °C for Mg nanoparticles to 480 °C for hydrogenated Mg nanoparticles with ∼9-fold reactivity enhancement.