Kinetics of thermal decomposition of cubic ammonium perchlorate

The methods of thermogravimetric analysis (TGA) and differential scanning calorimettry (DSC) have been used to study the thermal decomposition of ammonium perchlorate (AP). TGA curves obtained under both isothermal and nonisothermal conditions show a characteristic slowdown at the extents of conversion, alpha = 0.30-0.35. DSC demonstrates that in this region the process changes from an exothermic to an endothermic regime. The latter is ascribed to dissociative sublimation of AP. A new computational technique (advanced isoconversional method) has been used to determine the dependence of the effective activation energy (E-alpha) on alpha for isothermal and nonisothermal TGA data. At alpha > 0.1, the E-alpha dependencies obtained from isothermal and nonisothermal data are similar. By the completion of decomposition (alpha --> 1) the activation energy for the isothermal and nonisothermal decomposition respectively rises to similar to 110 and similar to 130 kJ mol(-1), which are assigned to the activation energy of sublimation. The initial decomposition (alpha --> 0) shows the activation energy of 90 kJ mol(-1) for the isothermal decomposition and 130 kJ mol(-1) for the nonisothermal decomposition. The difference is explained by different rate-limiting steps, which are nucleation and nuclei growth for isothermal and nonisothermal decompositions, respectively.