Solid state phase transition and vapor pressure studies in ammonium nitrate-potassium nitrate binary system

The solid-state phase transitions in ammonium nitrate (NH₄NO ₃) and potassium nitrate (KNO₃) solid solutions and the equilibrium NH₄NO₃-KNO₃ (AN-KN) phase diagram have been determined. The phase transitions and phase diagram were determined by using the differential scanning calorimetry (DSC) and high temperature X-ray diffractometry. Samples of several different compositions were made for these analyses in a special “Dry Room” with very low humidity. In the X-ray diffraction experiments, the samples were heated on Pt-Rh strip and LaB6 or Si was added for internal calibration. Equilibrium phase diagram was also calculated by using the “FactSage” computer program. A single (AN III) phase region without any phase transitions between 293 to 373 K was observed for compositions between 5 to 25wt% KNO₃ in NH₄NO₃ that is critical for air bag gas generators. The higher temperature KNO₃ (KN I) phase has a wide stability range, from 100%KNO₃ to 20%KNO₃ solution. There is one eutectic, two eutectoids, and two peritectoids in this phase diagram. Two newly discovered solid-state phases were found in the mid-composition range of AN-KN solid solutions. Details of phase equilibria and lattice expansions during heating have been determined. Phase diagram calculations show a reasonable match of the phase boundaries. The total vapor pressures as well as the average molecular weights of pure ammonium nitrate and 16% KNO₃ solid solution were measured at various temperatures by the torsion-Knudsen effusion method. The partial pressures of NH₄NO₃ (PNH₄NO ₃), NH₃ (PNH₃), and HNO₃ (PHNO ₃) have also been determined.