Surface characterization of energetic materials using point-to-point contact FTIR-ATR

Surface characterization of energetic materials and their components has been a useful tool for resolving many research and quality control issues. An important application for gun propulsion is the understanding of plasma-propellant interactions (PPIs) in order to exploit plasma ignition, which offers the potential for rapid, controlled ignition of high-energy density charges. Toward this end, surface characterization using Fourier-transform infrared (FTIR) microscopy has been performed on propellants exposed to plasma in extinguished closed-bomb and open-air plasma experiments. These experiments are being performed to elucidate the properties of plasmas that enhance energy transfer and ignition, ultimately with the goal of designing an electrothermal-chemical (ETC) igniter-propellant charge for the next-generation weapon for the Future Combat System. However, analytical techniques capable of low detection limits for propellant degradation products are needed for characterizing the nature (e.g., radiation, convective heat transfer, hot particle flow, etc.) and distribution (in a packed bed or charge to ensure uniform and repeatable ignition) of plasma interaction with the propellant. Knowledge of such phenomena is valuable for validating simulations of plasma dynamics and optimizing igniter design. Reflectance spectroscopy has been used previously in our laboratory in closed-bomb studies to characterize the morphology and chemical composition of the grains from extinguished closed-bomb and open-air plasma experiments, but degradation products had proved difficult to detect. Recently, the same samples were reanalyzed using point-to-point contact attenuated total reflectance (PPC-ATR) in order to improve diagnostics for plasma-propellant interaction. Commercially available attachments have been designed to exert greater pressure on the sample for greater contact with the ATR crystal, improving sensitivity. With PPC-ATR, it was possible not only to detect but to identify the components of JA2 most likely associated with plasma degradation. Decomposition products were limited to less than a millimeter into the sample, involved secondary carbon denitration, and apparently did not migrate, strongly suggesting that plasma-propellant interaction involved denitration- of nitrocelluose (NC). A chemical species with infrared absorption at ∼1570 cm^-1 is apparently specific to plasma ignition (i.e., not observed with conventional ignition) and could serve as a marker compound for the extent of plasma interaction in charges with ETC igniters.