Effect of process control agents used in mechanochemical synthesis on properties of the prepared composite reactive materials

The study explores synthesis and reactivity of new reactive materials prepared by ball milling. High-energy ball milling became a ubiquitous mechano-chemical tool to manufacture diverse powders, from pharmaceuticals or foods to alloys to new solid rocket propellants. It enabled a dramatic expansion of the range of chemical compositions obtainable; however, it did not so far, allowed one to fine-tune morphology or interfaces in the generated powders. It is shown in this work how different process control agents (PCAs) can serve to tune the powder morphology and reactivity. Commonly used as lubricants and cooling agents during milling, liquid PCAs can be used as an effective tool in modifying both chemistry and morphology of mechanochemically prepared reactive materials. For example, a polar, non-oxidizing fluid, e.g., acetonitrile, can reduce the size of aluminum particles, but more interestingly, it can modify their surface to enable new redox reaction pathways leading to accelerated ignition and combustion. Using such modified aluminum in a composite prepared by milling makes it possible to design unusual reactive materials. Materials with the same chemical compositions, and thus the same overall energy densities can be made with controllable reaction dynamics and tunable heat release. Thus, it becomes possible to separate the effects of chemical composition and interface structure on the reaction mechanisms and rates. An even more unusual capability of manipulating shapes and sizes of the synthesized powders is discovered in this study when liquid PCA comprises two immiscible fluids. A complex system including an emulsion combined with suspended particles is generated inside the milling vial. When such a system is milled, solid particles can be refined, mixed, and eventually accumulated inside the droplet phase. Thus, spherical solid aggregates are formed with narrow size distributions. Milling conditions can be found to tune size, density, and porosity of such spheres. Produced narrowly-sized spherical powders are attractive because of their dramatically improved flowability. The existing methods for synthesizing spherical powders (e.g., spray-drying, extrusion-spheronization, droplet-melting) are more expensive, time-consuming, and energy-intensive. Unlike milling, they cannot be employed to a diverse range of materials and the challenges associated with wide particle size distributions often are unsurmountable. Our approach has been validated experimentally for elemental (e.g., Al, B), alloyed (B-Ti, Al-Ti), ceramic (Fe2O3), organic (melamine), and composite (Al-CuO) spheres from materials with a broad range of initial particle sizes and mechanical properties. The average size of the particles could be selectable from 5 to 200 µm. Experiments also confirmed superior rheological properties of the prepared reactive powders and their enhanced reactivity. For future, this study can be expanded beyond reactive materials to discover a new generation of value-added materials for catalysts, adsorbents, and feedstock powders for additive manufacturing