Mechanochemically prepared reactive and energetic materials: a review

Reactive and energetic materials are typically metastable and are expected to transform into thermodynamically favorable reaction products with substantial energy release. Preparation of such materials by mechanical milling is challenging: They are easily initiated by impact or friction. At the same time, milling offers a simple, scalable, and controllable technology capable of mixing reactive components on the nanoscale. In most cases, for reactive materials milling should be interrupted or arrested to preserve the metastable phases. Arrested reactive milling was exploited to prepare many inorganic reactive materials, including nanocomposite thermite, metal-metalloid, and intermetallic systems. Prepared materials are fully dense composites with unique properties, combining high density with extremely high reactivity. Different milling devices were used to prepare reactive materials and an approach was developed to transfer the process conditions between different mills. Different milling protocols, such as milling at cryogenic temperatures or staged milling can be used to prepare hybrid reactive materials with different components mixed on different scales; it was also used to tune the particle size distributions of metal-based reactive material powders. Metal-halogen composites were prepared, with metal matrix stabilizing a halogen (e.g., iodine) at temperatures substantially exceeding its boiling point. Mechanochemically prepared reactive materials can be classified based on the energy of reaction between components and the energy of oxidation of the bulk material composition. Work on mechanochemical preparation of reactive and energetic materials is reviewed with the focus on unique properties and ignition and combustion mechanisms of the mechanochemically prepared reactive materials. An ignition mechanism for nanothermites involving preignition reaction leading to a gas release preceding rapid temperature rise is discussed. A combustion mechanism is also discussed, in which the nanostructure of the mechanochemically prepared material is preserved despite the very high combustion temperatures.