Preparation of Energetic Metastable Nano-Composite Materials by Arrested Reactive Milling M. Schoenitz, T. Ward and E.L. Dreizin

Highly metastable, nano-scale energetic materials were prepared by Arrested Reactive Milling (ARM). When reactive milling is carried out with materials systems suitable for SelfPropagating High Temperature Synthesis (SHS), reaction between the components occurs spontaneously and violently after a certain period of milling. In this research, metastable nanocomposites with high energy density, were prepared by arresting the milling process prior to the spontaneous reaction. Products thus obtained are powders with particle sizes in the 10-50 μm range. Individual particles are intimate mixtures of reactive components, comparable to Metastable Intermolecular Composites (MIC), with near theoretical maximum density. The time of arrest determines the degree of grain refinement and therefore the sensitivity to mechanical, electrical, or thermal initiation. Particle sizes of the product powders can be adjusted by appropriate choice of milling parameters. This paper describes the application of ARM to the material systems Al-Fe2O3 and Al-MoO3. After empirical determination of optimum milling parameters, the reactive composites are structurally characterized by electron microscopy and x-ray diffraction. First results of combustion tests are presented. INTRODUCTION Metallic additives to energetic formulations in propellants, explosives, or pyrotechnics are known to improve performance due to their high combustion enthalpies [1]. However, mainly slow kinetics and incomplete reactions limit the usefulness of metallic fuels. Metastable metalbased materials have been proposed to improve overall ignition and combustion rates [2]. Recently some of these materials have been prepared using mechanical alloying [3]; their combustion behavior showed improvements compared to pure metals [4-8]. The previously prepared energetic materials were Al-based metastable alloys with varying composition (Al-Mg, Al-Ti [4, 8]), where the interactions between the alloying components are characterized by relatively small energy effects. Continuous mechanical alloying gradually transforms these materials from macroscopic elemental blends to supersaturated solid solutions (Al-Mg) and metastable intermetallic phases (Al-Ti). In contrast, if sufficiently exothermic reactions between the components are possible, then mechanical alloying, or reactive milling will lead to spontaneous initiation of these reactions when certain conditions are met. Although criteria sufficient for this initiation to occur have not been rigorously defined, it has been suggested that for adiabatic reaction temperatures higher than 1800 K spontaneous reaction is likely [9, 10]. If milling of a given reactive system is arrested, at some stage before the spontaneous initiation would occur, an activated material is obtained. Such activated materials have been used as ingredients for combustion synthesis, giving rise to higher reaction rates, and resulting in more homogeneous final products [11]. In this paper, this approach is developed further, focusing on the activated materials rather than their reaction products. The practical objective is to develop energetic formulations with higher energy output and reaction rates. Preliminary constant volume aerosol explosion experiments on nanostructured B-Ti composites obtained by ARM have showed dramatically increased AA2.6.1 Mat. Res. Soc. Symp. Proc. Vol. 800 © 2004 Materials Research Society