(501c) Filled Spherical Composite Particles Prepared By Mechanical Milling

Spherical powders are deemed highly desirable owing to improved flowability and ease in their handling and processing in energetic, pharmaceutical and other formulations exploiting composite materials and structures. Spherical micro particles are most commonly prepared by cooling droplets of melts. This method however cannot be employed for materials susceptible to heating, such as most energetic materials, for which the structure may be compromised during the processing. Other existing techniques include extrusion, spheronization, and emulsion polymerization, applicable to a relatively narrow range of soft materials. Hollow particles, such as colloidosomes, can be prepared from Pickering emulsions. However, filling them to achieve high density is difficult and involves preparing complex double emulsions. In this study, we introduce a simple, versatile, cost effective and readily scalable technique of synthesizing densely filled spherical powders by mechanical milling. Powders of different materials, including elemental Al, B, and Ti, oxide Fe₂O₃, as well as reactive composites Al·CuO, Al·Fe₂O₃, and Al·B were ball milled in presence of two immiscible fluids (hexane and acetonitrile) serving as a liquid process control agent. Emulsification of acetonitrile takes place during milling. The emulsion is stabilized by particles adsorbing at the liquid interface, so that Pickering emulsion forms. Most of the solid remains suspended in the continuous phase, hexane. It is hypothesized that upon high shear and stress interaction between suspended particles and Pickering emulsion droplets caused by the milling media, particles from the suspension penetrate and get trapped inside the emulsion droplets. Eventually, densely filled spherical particles form upon going through repeated destabilization and repairing of the droplet/continuous fluid interfaces laden with particles. Effects of liquid to solid ratio and milling time have been explored in this study thus far. Upon recovery of the powders, the surface morphologies and particle sizes are characterized using scanning electron microscopy (SEM). The sizes of the forming spheres are observed to decrease with the increasing milling time. The study aims at elucidating the mechanism of spherical particle formation during high-energy milling. In future, effect of different milling parameters, such as milling media size and type, energy introduced during milling, and presence of surfactants on the particles shapes and density will be investigated. The applicability of this milling technique to preparing spherical powders of a wide range of materials involving different immiscible fluid systems will also be explored.