Metal based reactive nanocomposites prepared by cryomilling

METAL BASED REACTIVE NANOCOMPOSITES PREPARED BY CRYOMILLING by Shasha Zhang Aluminum is one of the most commonly used metal fuel additives for propellants, explosives, and pyrotechnics. Recent interest has been focused on replacements for aluminum as fuel additives to achieve higher combustion temperatures and stronger pressure pulses for applications in advanced munitions systems. Two applications are addressed in this work. In the applications for explosives designed to defeat stockpiles of chemical and biological weapons, it is of interest to develop multifunctional materials combining the high energy density of metal fuels with the biocidal activity of halogens. A challenge of this effort is to design and prepare powder-like Al-I2 materials which can be used as drop-in replacements for pure aluminum powders in aluminized energetic formulations. For another application, it is desired to tailor combustion dynamics of aluminum in order to fully exploit its high reaction energy by modifying its surface and structure. Hydrocarbons with good volatility and reactivity are selected as additives to composite aluminum-based powders to achieve improved combustion dynamics. For both applications, mechanical milling offers a scalable and versatile method for modifying aluminum. The mechanical milling-based approach is explored in this effort using milling at the liquid nitrogen temperatures, or cryomilling, which enables mixing aluminum with materials that are unstable or difficult to process at room temperatures. Two types of composite materials are prepared and characterized: Al-I2 and Al-hydrocarbon (where wax, low density polyethylene and cyclooctane were used as different hydrocarbon components). Powders prepared by cryomilling are evaluated using Thermogravimetry Analysis (TGA), Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD). TGA results suggest that iodine is bound to Al, while hydrocarbon is present as a coating on the surface of fine Al grains and composite particles. Very fine, nano-scale particles can be prepared for composites milled at the liquid nitrogen temperature. Ignition temperatures are determined at heating rates in the range of 2000 35000 K/s using an electrically heated filament. Constant volume explosion experiments are used to characterize combustion performance of the produced powders. Materials are fed into an oxygen-acetylene flame to observe their burning characteristics and to measure the combustion temperature. The burn time and temperature as a function of particle size are measured using a single particle combustion measurement. Al-I2 powders are supplied to University of Cincinnati for independent evaluation of the biocidal properties of their combustion products. Ignition temperatures of the prepared materials are substantially reduced compared to Al. Burn rates for individual particles are comparable or somewhat lower than for pure Al. Combustion temperatures for the prepared composites are close to those of pure Al. Independent tests show that Al-I2 materials added to hydrocarbon flame substantially improve inactivation of the aerosolized biologically viable spores. The experiments show that combustion dynamics of the prepared Al-hydrocarbon composites is improved compared to pure Al powders. METAL BASED REACTIVE NANOCOMPOSITES PREPARED BY CRYOMILLING