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  • br Composition and characterization Kolev

    2018-11-15


    Composition and characterization Kolev and Tzonev presented the results of their practical solutions to these problems in two types of solid state thermobaric explosives [41]. They have air blast TNT equivalent of about 2.5 times and metal fragmentation capabilities similar to that of TNT. Both types of compositions mentioned are thermally stable, cheap and technologically accessible for mass production. A widely used fuel in energetics is the micrometer-sized aluminum. However, performance of propellants, explosives, and pyrotechnics could be significantly improved if its ignition barriers could be disrupted. Sippel et al. reported the morphological, chemical and thermal characterization of fuel-rich aluminum-polytetrafluoroethylene (70–30 wt%) (Al-PTFE) reactive particles formed by high and low energy milling [42]. Average particle sizes of their samples ranged from 15 to 78 µm; however, the specific surface areas of the particles ranged from approx. 2–7 m2g−1 due to milling induced voids and cleaved surfaces. The SEM and energy dispersive spectroscopy revealed a uniform distribution of PTFE, providing nanoscale mixing within the particles. The combustion enthalpy was found to be 20.2 kJ g−1, though a slight decrease (0.8 kJ g−1) results from extended high energy milling due to α-AlF3 formation (note that PTFE is present). For high energy mechanically activated particles, differential scanning calorimetry in argon atmosphere exhibited a strong peak standing for the exothermic pre-ignition reaction that onsets near 440 °C and accompanied by a second, more dominant exotherm that onsets around 510 °C. Scans in O2-Ar atmosphere have indicated that, unlike physical mixtures, more complete reaction occurs at higher heating rates and the reaction onset is drastically reduced (approx. 440 °C). The simple flame tests reveal that these modified Al-polytetrafluoroethylene particles light readily unlike micrometer-sized aluminum. Safety testing also shows that these particles possess high electrostatic discharge (89.9–108 mJ), impact (>213 cm), and friction (>360 N) ignition thresholds. The data imply that these particles may be useful for reactive liners, thermobaric explosives, and pyrolants. In particular, the altered reactivity, large particle size and relatively low specific surface area of these fuel-rich particles make them an interesting and suitable replacement for aluminum in solid propellants. This work clearly shows that mechanical activation of fuel rich Al-PTFE mixtures can result in micrometer-sized Al-PTFE composite particles with increased reactivity. The authors have observed that use of mechanical activation process results in nanoscale mixing of reactants with reaction behavior similar to that of nAl-nPTFE. Notably, high or low energy mechanical activation results in significant shift of primary exotherm onset from 600 °C to 440 °C in 6XHis heating and from 540 °C to 440 °C in the presence of O2 [42]. For composite particles formed with high energy mechanical activation, differential scanning calorimetry in O2-Ar indicates that, unlike physical mixtures or those particles formed under low energy mechanical activation, more complete reaction occurs. At higher heating rates the reaction onset is also drastically reduced (approx. 440 °C). Furthermore, results suggest that at aerobic heating rates, greater than 50 K min−1, nearly complete heat release happens approximately at 600 °C instead of at higher temperature. While mechanical activation drastically alters the reactivity of these particles, they are relatively insensitive to electrostatic discharge (ESD), friction initiation and impact. In addition to having significantly modified reaction behavior, the enthalpy of combustion of mechanically activated particles was found to be as high as 20.2 kJ g−1, so that it is approximately 60% higher than the measured combustion enthalpy of nAlnPTFE mixtures. Additionally, the large (15 to 78 µm) average particle size and moderate specific surface areas (2 to 6.7 m2g−1) of composite particles suggest that they will be far more useful than nanoparticles in high solids loaded energetics and may age more favorably than nanoparticle mixtures. Their expectation is that further reduction of particle specific surface area helps improvement of aging characteristics which may be achieved by adding a small amount of binder (e.g., Viton A) during the milling process or through crash deposition after mechanically activated particle formation. The conjecture is that a lower fraction of PTFE may also prove to be advantageous for some applications. These micrometer-sized activated fuel particles with modified ignition and reaction characteristics are a promising alternative to nanoparticle solid propellant additives such as nAl. With these particles, the authors expect similar propellant performance improvement and particles becoming less detrimental to propellant rheological and mechanical properties. When used as a replacement in solid propellants, these particles may ignite far below the ignition temperature of micrometer-sized aluminum (>2000 °C) and the expectation of the authors is that with these particles they may decrease ignition delay, agglomerate size, and reduce condensed phase losses as well as lead to increased heat output and enhanced burning rates [42]. Use of these fuel-rich Al-PTFE composite particles in structural energetics (e.g., reactive liners), incendiaries, flares and other energetics could also likely lead to better performance, far exceeding that of energetics which are made from physical mixtures of micrometer- or nanometer-sized particles. Now, efforts have been focused on the use of other fluorocarbon oxidizers. Study of the ignition and combustion of these activated fuel particles at high heating rates is interesting too. Additionally Sippel et al. have been working to incorporate these materials into solid and hybrid propellants [42].