Ammonium Perchlorate Structural Evolution Leading to Decomposition Cessation

Ammonium perchlorate (AP) is ubiquitous in solid rocket propellant but has been the cause of catastrophic accidents due to its decomposition. As such, it is of great importance to understand the decomposition mechanism from a scientific and public perspective. Despite this importance, the cause for partial decomposition and its particle size dependence for orthorhombic AP have not been fully resolved since the early investigations ca. 1950. This study applies in situ synchrotron X-ray diffraction to quantify the structural evolution of AP during isothermal decomposition. The final crystalline coherence lengths are 79.9, 106.2, and 148.8 nm for the [201], [210], and [002] directions, respectively, and are independent of particle size. Crystal orientation and diffraction peak asymmetry values at the end of decomposition are also independent of the starting AP particle size. These findings suggest that the energy required to further increase the surface area from pore growth and cause additional dislocations is greater than the energy released from the reaction, thus limiting decomposition. The cessation of decomposition is caused by stress relief by slip after a critical dislocation density (at most 10⁸ cm^–2) is reached. Further, the particle size dependence on the extent of decomposition before cessation is a result of smaller particles reaching this critical dislocation density quicker, thereby limiting additional decomposition sites from forming. The effects of crystal structure and symmetry on decomposition directional dependence are discussed. This work provides an explanation of AP decomposition phenomena not well understood from more than 60 years of investigations.