An efficient approach that combines short-term (minutes) highenergy dry ball milling and wet grinding to tailor the nano-and microstructure of Ni +Al composite reactive particles is reported. Varying the ball-milling conditions allows control of the volume fraction of two distinct milling-induced microstructures, that is, coarse and nanolaminated. It is found that increasing the fraction of nanolaminated structure present in the composite particles leads to a decrease in their ignition temperature (T ig ) from 700 and 500 K. Material with nanolaminated microstructure is also found to be more sensitive to impact ignition when compared with particles with a coarse microstructure. It is shown that kinetic energy (W cr ) thresholds for impact ignition, obtained for an optimized nanolaminated microstructure, is only 100 J. High-speed imaging showed that the impact-induced ignition occurs through formation of hot spots caused by impact. Molecular dynamic simulations of a model system suggest that impact-induced localized plastic deformation raises the local temperatures to ∼600 K, enough to initiate exothermic reactions. Analysis of the kinetics and reaction mechanism shows that the reason for low T ig and W cr for nanolaminated microstructure is the rapid solid-state dissolution of nickel in aluminum lattices.

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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.

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In this paper, we report that the thermal conductivity (TC) of heat transfer nanofluids containing Ni coated single wall carbon nanotube can be enhanced by applied magnetic field. A reasonable explanation for these interesting results is that Ni coated nanotubes form aligned chains under applied magnetic field, which improves thermal conductivity via increased contacts. On longer holding in magnetic field, the nanotubes gradually move and form large clumps of nanotubes, which eventually decreases the TC. When we reduce the magnetic field strength and maintain a smaller field right after TC reaches the maximum, the TC value can be kept longer compared to without magnetic field. We attribute gradual magnetic clumping to the gradual cause of the TC decrease in the magnetic field. We also found that the time to reach the maximum peak value of TC is increased as the applied magnetic field is reduced. Scanning electron microscopy images show that the Ni coated nantubes are aligned well under the influence of a magnetic field. Transmission electron microscopy images indicate that nickel remains attached onto the nanotubes after the magnetic field exposure.