(632a) Combustion of Mg-S and Zr-S Reactive Nanocomposite Powders Heated to Ignition at Different Rates

Recently, it was observed that the burn rates of nanocomposite aluminum-based thermite powders prepared by arrested reactive milling change substantially depending on the heating rate achieved during their ignition. Micron-sized powder particles ignited by passing through a focused CO2 laser beam are heated at about 106 K/s. Such particles burn during several ms, which is slightly longer than the burn times of the similarly sized particles of pure aluminum. The burn times are reduced to hundreds of microseconds for same powder particles ignited by an electro-static discharge (spark), for which the heating rate is estimated to be close to 109 K/s. It was proposed that the much higher burn rates observed for the particles heated very rapidly are caused by essentially volumetric heterogeneous reaction occurring at all interfaces between reactive components within the nanocomposite structure. Thus, the nanocomposite structure is preserved upon ignition, at least partially. Conversely, when the heating rates are lower, the nanocomposite structure is lost upon ignition, so that the combustion is controlled by the external surface of the molten particle. In this work, the concept described above is tested for a different type of nanocomposite materials, mixing metal as a fuel and sulfur as an oxidizer. Such materials have high reaction enthalpy and generate potentially biocidal combustion products. Reactive nanocomposite powders were prepared by arrested reactive milling using elemental sulfur and ball milling it separately with two different metal fuels, zirconium and magnesium. Milling conditions are identified for each composition to prepare fully dense composites with components mixed on the nanoscale. As in previous work, the powders are ignited using a CO2 laser beam and electrostatic discharge. Emission traces of the ignited materials are captured using a single PMT filtered at the 568 nm wavelength. Time-resolved spectra of the ignited materials are also taken using a 32-channel spectrometer covering the visible range of wavelengths. Partially burned materials are captured and examined using electron microscopy. Results will be presented and discussed in this talk.