Solidification kinetics of alpha versus gamma formation of aluminum oxide (Al2O3) particles in solid propellant rocket exhausts : an analytical approach

The objective of this research was to provide insight into the solidification kinetics of aluminum oxide (AI₂O₃) particles formed in the exhaust of solid rocket motors (SRM) which use aluminized propellants. The research was motivated by the observation that particles collected from SRM exhausts often show more than one crystalline phase of AI₂O₃. The larger particles tend to be composed of the stable alpha phase while the smaller particles tend to be composed of gamma or one of the other metastable phases. The various phases of AI₂O₃, have different properties and crystalline structures. The existence of multiple phases is important in applications where particle density, chemical properties, and optical properties need to be considered. It is not important to motor performance since the enthalpy difference between the phases is insignificant. Although much is known in the field of ceramics about the solidification kinetics of AI₂O₃, this knowledge had not been applied to rocket exhaust products in a systematic manner. Thus, an analytical method was devised which combined the solidification kinetics of AI₂O₃ and the output from industry standard nozzle and plume codes. This method was used to investigate the dependence of the crystal phase of solid AI₂O₃ particles upon various parameters. The kinetic rate analysis indicates that the rapid temperature quench occurring in a gas dynamic nozzle expansion and free expansion into the atmosphere is capable of producing significant amounts of metastable aluminas. It also shows a direct correlation between crystal phase and particle size, the degree of undercooling, motor type, and altitude.