RESUMO
Ultrashort-pulse laser-induced breakdown spectroscopy (LIBS), specifically using a femtosecond laser, has certain advantages over longer-pulse, nanosecond-duration lasers, in that they typically have kilohertz repetition rates and reduced background noise along with little-to-no laser-plasma interaction, all of which lead to a better chance of detecting LIBS signals from trace particles. In this work, femtosecond-LIBS is investigated for the detection of metallic particles in the hot flame zone of solid propellant strands burning in the atmosphere. The metallic particles doped into the solid propellants were aluminum (Al), copper, lead, lead stearate, and mercury chloride, which are all either typically found in energetic formulations as additives or impurities. Using an 80-fs-pulse-duration, amplified Ti:Sapphire laser operating at 1000 Hz, single-shot concentration measurement experiments were performed. The femtosecond-LIBS apparatus could detect all metallic additives, whereas a previous nanosecond-LIBS scheme with comparable conditions was able to detect only higher concentrations of Al. The single-shot concentration study, conducted with the Al-doped propellants, indicated that there is a linear relationship between the percentage of laser shots detecting a LIBS signal and the mass percentage of Al initially present in the strands. The present results illustrate the advantages of using a femtosecond laser over a nanosecond laser for LIBS detection during energetics material reactions.
RESUMO
Numerous metals and metal compounds are often added to propellants and explosives to tailor their properties such as heat release rate and specific impulse. When these materials combust, these metals can be released into the air, causing adverse health effects such as pulmonary and cardiovascular disease, particulate-matter-induced allergies, and cancer. Hence, robust, field-deployable methods are needed to detect and quantify these suspended metallic particles in air, identify their sources, and develop mitigation strategies. Laser-induced breakdown spectroscopy (LIBS) is a technique for elemental detection, commonly used on solids and liquids. In this study, we explored nanosecond-duration LIBS for detecting airborne metals during reactions of solid propellant strands, resulting from additives of aluminum (Al), copper, lead, lead stearate, and mercury chloride. Using the second harmonic of a 10-ns-duration 10-Hz, Nd:YAG laser, plasma was generated in the gas-phase exhaust plume of burning propellant strands containing the target metals. Under the current experimental conditions, the ns-LIBS scheme was capable of detecting Al at concentrations of 5%, 10%, and 16% by weight in the propellant strand. As the weight percentage increased, the LIBS signal was detected by more laser shots, up to a point where the system transition from being nonhomogeneous to a more-uniform distribution of particles. Further measurements and increased understanding of the reacting flow field are necessary to quantify the effects of other metal additives besides Al.
RESUMO
The prospect of extending existing metal-ceramic composites to those with the compositions that are far from thermodynamic equilibrium is examined. A current and pressure-assisted, rapid infiltration is proposed to fabricate composites, consisting of reactive metallic and ceramic phases with controlled microstructure and tunable properties. An aluminum (Al) alloy/Ti2AlC composite is selected as an example of the far-from-equilibrium systems to fabricate, because Ti2AlC exists only in a narrow region of the Ti-Al-C phase diagram and readily reacts with Al. This kind of reactive systems challenges conventional methods for successfully processing corresponding metal-ceramic composites. Al alloy/Ti2AlC composites with controlled microstructures, various volume ratios of constituents (40/60 and 27/73) and metallic phase sizes (42-83 µm, 77-276 µm, and 167-545 µm), are obtained using the Ti2AlC foams with different pore structures as preforms for molten metal (Al alloy) infiltration. The resulting composites are lightweight and display exceptional mechanical properties at both ambient and elevated temperatures. These structures achieve a compressive strength that is 10 times higher than the yield strength of the corresponding peak-aged Al alloy at ambient temperature and 14 times higher at 400 °C. Possible strengthening mechanisms are described, and further strategies for improving properties of those composites are proposed.
