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A new experimental technique was developed to directly observe the onset of shock initiation in powder mixtures contained in recovery capsules over time scales ranging from hundreds of nanoseconds to at least hundreds of milliseconds. Simultaneously with a thermocouple embedded in the test mixture to monitor bulk temperature changes, a photomultiplier tube detected light emissions produced by the reacting sample. A window/optic fiber system was developed that remained intact and did not move during the experiment. A polycarbonate window was placed into a steel recovery capsule, which was held solidly in place in a steel anvil designed to protect the fiber optic and to prevent motion of the window. Samples of as-blended 5Ti+3Si powders and ball-milled mixtures 5Ti+3Si were tested in this setup. In all experiments, a weak light emission peak was observed upon shock passage followed by much more intense light beginning a few milliseconds to a few hundreds of milliseconds later. The intense light emissions occurred at approximately the same time as a bulk temperature increase measured with the thermocouple. These results suggest that only a very small fraction of the sample was initiated by the shock. [ABSTRACT FROM AUTHOR]

Strong and dense structural reactive materials may be produced by mixing aluminum powders with heavy metal-oxide powders (such as Bi2O3, PbO, Pb3O4, I2O5, etc.). The addition of certain additives to such mixtures, such as V2O5 and B2O3, can lower the softening point of the oxide mixture below the melting point of aluminum. This could lead to the fabrication of dense and non-porous aluminum-metal oxide structural materials. The shock sensitivity of aluminum-metal oxide mixtures was investigated in this work. The minimum shock initiating pressure was obtained for various porous and non-porous aluminum-metal oxide mixtures using the shock recovery technique. Since most reactions of Al in metal oxide mixtures produce little pressure and material velocity changes but large increases in temperatures, thermocouples were used to observe the bulk reaction onset, which relates to the overall reaction rate, in those mixtures. The mixtures tested were found to be very sensitive to shock initiation and their reaction rates were found to be very fast, compared to other types of reactive powder mixtures. Finally, the addition V2O5 and B2O3 additives or the addition of liquid heptane (to fill the pores) did not significantly lower the sensitivity or reaction rates of the mixtures investigated. [ABSTRACT FROM AUTHOR]

Equimolar mixtures of manganese powder and sulfur at different starting densities were tested in two different types of steel recovery capsules in order to study the shock initiation phenomenon in Self-Propagating High-Temperature Synthesis (SHS) mixtures. Two different sizes of Mn particles were used for these experiments, <10 μm and -325 mesh (<44 μm). This mixture was selected due to the large exothermic heat release of the manganese-sulfur reaction (214 kJ/mol), which causes the reaction to be self-sustaining once initiated. The test samples were placed in planar recovery capsules and a strong shock was delivered via the detonation of a charge of amine-sensitized nitromethane. Various shock strengths were achieved by placing different thicknesses of PMMA attenuator discs between the explosive charge and the capsule. The results confirmed that shock-induced reactions can be produced in highly non-porous mixtures. It was also found that shock interactions with the side walls of the recovery capsule can play a significant role in the initiation. [ABSTRACT FROM AUTHOR]

Previous tests carried out in planar recovery capsules showed that strong shock waves (incident pressures 3–7 GPa), generated by a charge of amine-sensitized nitromethane and attenuated by a PMMA layer, can initiate reactions in non-porous equimolar mixtures of manganese and sulfur. The current study focused on the onset of these reactions using time-resolved temperature measurements. A photomultiplier-based two-color pyrometer was used to record sample temperatures shortly after the passage of the shock. An experimental complication encountered when studying shock-induced reactions in porous energetic materials using pyrometry, i.e. intense light due to large localized heating, can be mostly eliminated if a non-porous sample is studied. Further, in order to increase the reliability of the pyrometry results, the reactive test mixture (Mn-S) was chosen to have a high heat of reaction (214 kJ/mol) and thus a high reaction temperature, and baseline tests were performed with inert melt-cast mixtures of WS2 and sulfur. [ABSTRACT FROM AUTHOR]

Equimolar mixtures of manganese powder and sulfur were melt-cast into solid pellets in order to study the mechanism of shock-enhanced reactivity in non-porous heterogeneous mixtures. This mixture was selected due to the large exothermic heat release of the manganese-sulfur reaction (214 kJ/mol), which causes the reaction to be self-sustaining once initiated. The test samples were placed in planar recovery ampoules and a strong shock was delivered via the detonation of a charge of amine-sensitized nitromethane. Various shock strengths were achieved by placing different thicknesses of PMMA attenuator discs between the explosive charge and the ampoule. The results confirmed that shock-induced reactions can be produced in highly non-porous mixtures. Indeed, the critical shock pressure that caused ignition of the mixture in the ampoule was found to be in the range 2.2–3.0 GPa (pressures were estimated using LS-DYNA simulations of samples with 100% TMD). [ABSTRACT FROM AUTHOR]

The possibility of self-sustained solid-solid detonations (SSD) in Mn+S mixtures has been investigated. Charges 50 mm in diameter under different degrees of confinement were used. The initiation charges used include nitromethane, tetryl, and C-4. No self-sustained SSD were observed in the present study. Neither did the degree of confinement nor the type or weight of the initiation charge appear to make a significant difference in the results obtained. The shock and reaction front are initially coupled but decay rapidly. Decoupling occurs after about 25 mm of propagation when the shock has decayed to about 2.5 km/sec which corresponds to the estimated sound speed of the reactant. The lack of significant feedback of the chemical energy released to the shock via work done by the expansion of the solid products is probably the cause for failure to obtain SSD in Mn+S. [ABSTRACT FROM AUTHOR]