Mechanism of the organic fluoride effect on the formation of agglomerates and condensed products in the combustion of aluminised solid propellants

Abstract

The combustion process of Al particles in an aluminised solid propellant containing organic fluoride (SOF) has been investigated in comparison to that of a normal solid propellant (Sn). The agglomeration process of ignited Al particles on the burning surface was recorded by means of microscopic high-speed photography. The records showed that the ignited Al particles in propellant Sn tended to form large, homogeneous agglomerates on the burning surface, whereas the ignited Al particles in propellant SOF tended to form distinct, coherent yet discrete agglomerates with a flocculent structure. The combusting agglomerates were quenched with cold acetone to obtain the burning-interrupted agglomerates, and morphological and surface element distribution analyses were performed on these quenched agglomerates by SEM with EDX. The results showed the quenched agglomerates from Sn to be large (50–100 µm) and to have an obvious ‘cap–core’ structure, with the ‘cap’ and ‘core’ consisting mostly of aluminum oxide (Al2O3) and unreacted aluminum (Al), respectively. In contrast to those from Sn, the quenched agglomerates from SOF were spherical and much smaller (5–20 µm), with uniform dispersal of Al, O, and F on their surfaces. The solid agglomerate products from complete combustion of the propellants in an oxygen bomb were collected and further analysed by TEM, selected area electron diffraction (SAED), and XRD. The results indicated that the propellant Sn tended to form solid combustion products with a dense crystalline structure, whereas the propellant SOF tended to form solids with a more irregular inner structure of low crystallinity. The above results suggested that the ignition/agglomeration/combustion process of Al in SOF did not obey the prototypical model for aluminised solid propellants. The effect of fluoridation on Al particles during the combustion process of the solid propellant containing organic fluoride has been confirmed and is considered to be a crucial factor in giving rise to the observed phenomena.