Towards understanding the catalytic properties of lead-based ballistic modifiers in double base propellants

Lead-based ballistic modifiers are additives in double base propellants (DBPs) which render the burn rate insensitive to changes in pressure within a defined pressure range, thus imparting greater control of combustion conditions. In-coming European legislation will soon ban the use of lead in propellant formulations, however, and few suitable candidate replacement materials are currently available. In an effort to understand better the unique properties offered by lead-based modifiers, we present a first-principles computational study on Pb, PbO, PbO2, SnO2 and Bi2O3, all of which have been investigated experimentally as ballistic modifier materials. Our study demonstrates that various quantifiable properties exist for the lead-based materials. Overall, they have narrower electronic band gaps, lower surface energies and lower surface work functions than the lead-free systems, indicating a greater propensity to form stable chemical surfaces with higher catalytic activity. We also show that of the set, only Pb and α-PbO can support the formation of a weakly bound layer of amorphous carbon, a key experimental observable in the burning of DBPs.