Structural dependence of reactivity in iron oxide/Al nano-thermites

Abstract

Iron oxide-based thermites are classical energetic materials, yet the influence of their precursor hydration states and morphological evolution on combustion dynamics remains poorly understood. In this study, three highly related iron oxides with distinct hydration degrees and morphologies—hematite (HEM), goethite (GEO), and ferrihydrite (FER)—were synthesized and assembled with nano-aluminum (nano-Al) to investigate their thermite reaction mechanisms. Comprehensive characterizations and combustion tests reveal a counterintuitive ‘spatial hindrance’ and a ‘double-edged sword’ effect of hydration. Despite possessing the largest theoretical specific surface area (200 m2 g−1), the extensively hydrated and heavily agglomerated FER acts as a severe thermal sink, yielding the poorest combustion performance. Conversely, the acicular GEO containing moderate structural hydroxyls (-OH) forms an open 3D interpenetrating network with nano-Al, maximizing the effective solid-solid interfacial contact. Furthermore, the localized release of water vapor from GEO triggers an in-situ ‘micro-explosion,’ dynamically enhancing convective mass transfer by shattering the alumina passivation shell. Consequently, the GEO/Al system delivers extraordinary energetic performance, achieving a maximum heat release of 476.11 J g−1 and a peak pressure of 303.16 kPa. This work provides profound insights into the structural design of metal-oxide energetic materials beyond conventional surface area metrics.