Titanium-boron reactive composite powders with variable morphology prepared by arrested reactive milling

Abstract

Powders with compositions Ti·B and Ti·2B with components mixed on the nanoscale were prepared by arrested reactive milling as high energy density fuels. A planetary mill with hardened steel vials and milling media were used. Powders with irregular and spherical particle shapes were prepared. Spherical powders were prepared milling starting boron and titanium in presence of two immiscible liquids, hexane and acetonitrile, serving as process control agents. Prepared powders were characterized using electron microscopy, x-ray powder diffraction, nitrogen adsorption, thermal analysis, and ignition measurements. Average volumetric particle sizes for spherical powders could be controlled in a range from tens to hundreds of µm selecting milling conditions. All spherical powders were found to be porous, unlike reference irregularly shaped composite powders. Despite porosity, spherical powders packed at nearly the same bulk density as irregularly shaped powders. It was also observed that milling led to formation of poorly crystalline TiB in all milled materials. The amount of observed TiB as well as contamination by Fe from the milling tools increased at greater milling dose (e.g., energy transferred to the powders being milled from the milling tools). Substantial portion of the prepared composite materials (more than half) was amorphous. The amount of the formed TiB did not correlate with the reactivity of the prepared materials. All composite materials exhibited exothermic reactions at low temperatures when heated in both inert and oxidizing environments. The exothermic reactions were always stronger for spherical powders. Oxidation for spherical powders started sooner (at lower temperatures) compared to the irregularly shaped composites or to elemental metal fuels, boron and titanium. Similarly, spherical powders ignited at ca. 500 °C, a lower temperature than ignition of irregularly shaped powders with the same compositions.