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

The combustion process of Al particles in an aluminised solid propellant containing organic fluoride (S_OF) has been investigated in comparison to that of a normal solid propellant (S_n). 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 S_n tended to form large, homogeneous agglomerates on the burning surface, whereas the ignited Al particles in propellant S_OF 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 S_n 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 S_n, the quenched agglomerates from S_OF 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 S_n tended to form solid combustion products with a dense crystalline structure, whereas the propellant S_OF 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 S_OF 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.