Spherical boron powders prepared by mechanical milling in immiscible liquids

Spherical boron powders with sizes ranging from 4 to 100 μm were prepared by ball milling a commercial boron powder. A planetary mill was used with a process control agent consisting of two immiscible fluids: hexane and acetonitrile. The spherical powders formed when the volume of the loaded boron was close to that of acetonitrile in presence of excess hexane. Particle sizes were measured using low-angle laser light scattering, they were examined using scanning electron microscopy, and their specific surface area was measured using nitrogen adsorption. The size distributions of the prepared spheres were much narrower than is common for solids processed by high-energy milling. Spherical particles were fragile; their mechanical stability was improved when a small amount of Fluorel®, a fluorocarbon binder, was preliminarily dissolved in acetonitrile. Experiments showed that the prepared spherical powders exhibited a higher bulk density and much better flowability than starting boron powder. Process-property relations were explored by systematically varying milling parameters and analyzing their effect on the size distribution of the formed spheres. The size of spherical particles was found to decrease with the decrease in the amount of acetonitrile used; it was also decreased with an increase in milling time, number of milling balls, and speed of milling.