The dependence of nanoscale aluminum and water propellant combustion on acidity/basicity and rheology

David E Kittell, Purdue University


Over the past few years the combustion of nAl/H2O propellants has been widely studied, but further progress has been slowed for the following reason: the loosely correlated trends in combustion data are insufficient in guiding further research efforts and they cannot be used to significantly improve the Isp observed in static rocket motor tests. It was assumed that the different mixing techniques (hand, planetary and resonant mixers, duration and temperature) gave rise to the different burning rate measurements, but the influences from pH and rheology on nAl/H2O combustion were not considered. In this work it is found that the effects of pH on nAl/H2O propellants are profound, and correlate well with viscosity, low pressure deflagration limits, burning rate exponents, rocket motor performance, and result in better correlated burning rate data. In particular, the findings suggest that agglomeration on the length scale of the transition particle diameter between diffusion and kinetic burning can influence the pressure exponent. Agglomeration in mixtures is affected by electrostatic repulsion from charged H+ and OH− ions at the solid/liquid interface, and this is reflected through zeta potential and viscosity measurements at the different pH levels. Additionally, it is observed that pH appears to control the reaction kinetics during ignition as the propellant transitions from low-temperature oxidation (that is still highly exothermic) to combustion. Basic pH values accelerate the low-temperature oxidation rates, and as a result the low pressure deflagration limit decreases with increasing pH. Finally, coatings and surface modifiers are identified that can stabilize pH and yield more uniform dispersions. These coatings can improve mixing safety and prevent coagulation (pre-combustion mixture agglomeration) during the freezing process, unfortunately the best coating option (palmitic acid) is not suitable for efficient nAl/H2O propellant combustion because samples produce a hard, dense slug during the combustion. The findings of this work have direct application beyond the aluminum and water system, extending to the research of nano-particle addition to liquid fuels, slurries, gels, and for the aqueous processing of Al based nanothermites.




Pourpoint, Purdue University.

Subject Area

Aerospace engineering|Chemical engineering|Materials science

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