Abstract
Micron-diameter aluminum particles (μAl) are highly reactive when combined with a solid oxidizer. However, μAl powder is less reactive in a gaseous air environment, where oxygen is the only available oxidizer, unless very well dispersed. While enthalpy of oxidation for Al is high, many variables influence the viability of harnessing stored chemical energy within a single Al particle whose reaction is diffusion controlled. One way to enhance Al particle reaction is to coat the particle surface with a condensed phase oxidizing agent that is in immediate contact with the particle surface to promote diffusion reactions. Fluorocarbons such as perfluorocarboxylic acids have been used to enhance Al combustion for nanoscale Al (nAl) particles because fluorinated species are also reactive with the Al2O3 passivation shell surrounding the Al core particle. This study extends previous work on nAl toward μAl particles coated with perfluorotetradecanoic acid (PFTD) (F3C(CF2)11CO2H) and then characterizes the μAl-PFTD thermal reactivity. Samples were prepared with varying PFTD concentrations ranging from 0 to 20 mass percent, and experiments were performed using thermogravimetric analysis and high-speed infrared imaging on powder samples. Results show the PFTD-coated μAl particles provide higher apparent reaction temperatures (by about 500 °C) and longer burn times at elevated temperatures (by about 20%). Higher concentrations of PFTD tend to produce more gas-generating reactions with particles ejecting from the loose powder pile, but 9% PFTD coating on μAl particles provides a good balance of stable reactivity with high-temperature reactions (~ 1800 °C) and high-temperature burn times (~ 18 s). The higher temperatures for PFTD-coated μAl particles are attributed to the increased (nearly double) energy produced in the formation of AlF3 (56.10 kJ g−1) compared to Al2O3 (30.98 kJ g−1); both of these product species are identified in XRD analysis. In addition, the formation of AlF3 may reduce the melting and phase transition temperature of Al2O3 by several hundred degrees and contribute to catalytically activating the Al reaction. Overall, coating μAl particles with a fluorinated polymer facilitates reaction in an air environment, even for small concentrations of PFTD coating.
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Acknowledgements
The authors are grateful for support from DTRA under award HDTRA1-15-1-0029 and partial support by ARO under award W911NF-17-10387. We would also like to thank Dr. Daniel Unruh at Texas Tech University Chemistry Department for assistance with XRD analysis and Drs. R. Carl Brothers and R. Wilson at Indian Head Naval Surface Warfare Center for preparation of the materials investigated in this study.
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Campbell, L.L., Hill, K.J., Smith, D.K. et al. Thermal analysis of microscale aluminum particles coated with perfluorotetradecanoic (PFTD) acid. J Therm Anal Calorim 145, 289–296 (2021). https://doi.org/10.1007/s10973-020-09742-4
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DOI: https://doi.org/10.1007/s10973-020-09742-4