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Modification of the reactivity of micrometer-sized aluminum through inclusion of low levels of poly(carbon monofluoride) (PMF) using mechanical activation (MA) is reported. Resulting composite particle combustion enthalpy, average particle size, and specific surface area depend on MA intensity, duration, and inclusion level, and range from 18.9 to 28.5 kJ g−1, 23.0 to 67.5 μm, and 5.3 to 34.8 m2 g−1, respectively. Differential scanning calorimetry experiments in O2/Ar indicate that MA reduces the exotherm onset from 555 to 480 °C (70/30 wt-%). Particles are sensitive to electrostatic discharge stimulus (11.5-47.5 mJ) but not to impact (>213 cm) or friction (>360 N) and some low energy MA particles are ignitable by optical flash. With their altered reactivity and high combustion enthalpy, these nanofeatured, micrometer-sized particles may have use as replacements for aluminum in energetic applications. [ABSTRACT FROM AUTHOR]

Micrometer-sized aluminum is widely used in energetics; however, performance of propellants, explosives, and pyrotechnics could be significantly improved if its ignition barriers could be disrupted. We report morphological, thermal, and chemical characterization of fuel rich aluminum-polytetrafluoroethylene (70-30 wt-%) reactive particles formed by high and low energy milling. Average particle sizes range from 15-78 μm; however, specific surface areas range from approx. 2-7 m2 g−1 due to milling induced voids and cleaved surfaces. Scanning electron microscopy and energy dispersive spectroscopy reveal uniform distribution of PTFE, providing nanoscale mixing within 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. For high energy mechanically activated particles, differential scanning calorimetry in argon shows a strong, exothermic pre-ignition reaction that onsets near 440 °C and a second, more dominant exotherm that onsets around 510 °C. Scans in O2-Ar indicate that, unlike physical mixtures, more complete reaction occurs at higher heating rates and the reaction onset is drastically reduced (approx. 440 °C). Simple flame tests reveal that these altered Al-polytetrafluoroethylene particles light readily unlike micrometer-sized aluminum. Safety testing also shows these particles have high electrostatic discharge (89.9-108 mJ), impact (>213 cm), and friction (>360 N) ignition thresholds. 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 replacement for aluminum in solid propellants. [ABSTRACT FROM AUTHOR]

Aluminum particles are widely used as a metal fuel in solid propellants. However, poor combustion efficiencies and two-phase flow losses result due in part to particle agglomeration. Recently, engineered composite particles of aluminum (Al) with inclusions of polytetrafluoroethylene (PTFE) or low-density polyethylene (LDPE) have been shown to improve ignition and yield smaller agglomerates in solid propellants. Reductions in agglomeration were attributed to internal pressurization and fragmentation (microexplosions) of the composite particles at the propellant surface. Here, we explore the mechanisms responsible for microexplosions in order to better understand the combustion characteristics of composite fuel particles. Single composite particles of Al/PTFE and Al/LDPE with diameters between 100 and 1200 µm are ignited on a substrate to mimic a burning propellant surface in a controlled environment using a CO 2 laser in the irradiance range of 78–7700 W/cm 2 . The effects of particle size, milling time, and inclusion content on the resulting ignition delay, product particle size distributions, and microexplosion tendencies are reported. For example particles with higher PTFE content (30 wt%) had laser flux ignition thresholds as low as 77 W/cm 2 , exhibiting more burning particle dispersion due to microexplosions compared to the other materials considered. Composite Al/LDPE particles exhibit relatively high ignition thresholds compared to Al/PTFE particles, and microexplosions were observed only with laser fluxes above 5500 W/cm 2 due to low LDPE reactivity with Al resulting in negligible particle self-heating. However, results show that microexplosions can occur for Al containing both low and high reactivity inclusions (LDPE and PTFE, respectively) and that polymer inclusions can be used to tailor the ignition threshold. This class of modified metal particles shows significant promise for application in many different energetic materials that use metal fuels. [ABSTRACT FROM AUTHOR]

Though silicon (Si) has been widely considered as a reactive fuel, the ability to tune its ignition and combustion characteristics remains challenging. One means to accomplish this may be mechanical activation (MA), which has been shown to be effective with aluminum fuels. In this work, a two-step MA process was developed to prepare fuel-rich Si/polytetrafluoroethylene (PTFE) composite reactive powders. The process consisted of cryogenic milling, followed by high intensity milling at room temperature. This resulted in particle refinement of the hard, brittle silicon particles and also dispersion within the more ductile PTFE matrix. Surface area of the as-milled powder was found to be moderate, ranging from 1.75 ± 0.06 m 2 g −1 (44/51 Si/PTFE) to 5.37 ± 0.10 m 2 g −1 (90/10 Si/PTFE) and was driven by the fraction of refined Si powder not dispersed in a PTFE matrix. Combustion enthalpies ranged from 15.6 ± 0.4 kJ g −1 (44/51 Si/PTFE) to 21.8 ± 2.2 kJ g −1 (90/10 Si/PTFE) and were higher than a physical mixture of the precursors. Combustion experiments showed that burning rates ranged from 1.6 to 2.1 mm s −1 and combustion temperatures (as measured from gray body emission) ranged from 1708 to 1889 K. The combustion performance of MA Si/PTFE was comparable to mixtures prepared with nanoscale and nanoporous silicon powders, indicating that the reactivity of silicon fuel had been successfully altered inexpensively without many of the major drawbacks associated with using high specific surface area powders. [ABSTRACT FROM AUTHOR]

Abstract: In solid propellants, aluminum is widely used to improve performance, yet theoretical specific impulse is still not achieved largely because of two-phase flow losses. Losses could be reduced if aluminum particles quickly ignited, more gaseous products were produced and if upon combustion, aluminum particle breakup occurred. To explore this, tailored, fuel-rich, mechanically activated composite particles (aluminum/polytetrafluoroethylene, Al/PTFE 90/10 and 70/30wt.%) are considered as replacements for reference aluminum powders (spherical, flake, or nanoscale) in a composite solid propellant. The effects on burning rate, pressure dependence, and aluminum ignition, combustion, and agglomeration are quantified. Using microscopic imaging, it is observed that tailored particles promptly ignite at the burning surface and appear to breakup into smaller particles, which can increase the heat feedback to the burning surface. Replacement of spherical aluminum with Al/PTFE 90/10wt.% does not significantly affect propellant burning rate. However, Al/PTFE 70/30wt.% increases the pressure exponent from 0.36 to 0.58, which results in a 50% increase in propellant burning rate at 13.8MPa. This increased pressure sensitivity is consistent with more kinetically controlled combustion that occurs from smaller burning metal particles near the surface. Combustion products were quench collected using a new, liquid-free technique at 2.1 and 6.9MPa and were measured. Both Al/PTFE 90/10 and 70/30wt.% composite particles reduce the coarse product fraction and diameter. The most significant reduction occurs from 70/30wt.% particle use, where average coarse product diameter is 25μm, which is smaller than the original, average particle size and is also smaller than the 76μm products collected from reference spherical aluminized propellant. This is a 66% decrease in agglomerate diameter or a 96% decrease in volume compared to agglomerates formed from reference spherical aluminum. Smaller diameter condensed phase products and more gaseous products will likely decrease two-phase flow loss and reduce slag accumulation. [Copyright &y& Elsevier]