Thermal decomposition and combustion of ammonium dinitramide (review)

A comprehensive review of thermal decomposition and combustion of ammonium dinitramide (ADN) has been conducted. The basic thermal properties, chemical pathways, and reaction products in both the condensed and gas phases are analyzed over a broad range of ambient conditions. Detailed combustion-wave structures and burning-rate characteristics are discussed. Prominent features of ADN combustion are identified and compared with other types of energetic materials. In particular, the influence of various condensed- and gas-phase processes in dictating the pressure and temperature sensitivities of the burning rate is examined. In the condensed phase, decomposition proceeds through the mechanisms ADN → NH₄NO₃ + N₂O and ADN → NH₃ + HNO₃ + N₂O, the former mechanism being the basic one. In the gas phase, the mechanisms ADN → NH₃ + HDN and ADN → NH₃ + HNO₃ + N₂O are prevalent. The gas-phase combustion-wave structure in the range of 5-20 atm consists of a near-surface primary flame followed by a dark-zone temperature plateau at 600-1000°C and a secondary flame followed by another dark-zone temperature plateau at 1000-1400°C. At higher pressures (60 atm and above), a final flame is observed at about 1800°C without the existence of any dark-zone temperature plateau. ADN combustion is stable in the range of 5-20 atm and the pressure sensitivity of the burning rate has the form r _b = 20.72p ^0.604 [mm/sec] (p = 0.5-2.0 MPa). The burning characteristics are controlled by exothermic decomposition in the condensed phase. Above 100 atm, the burning rate is well correlated with pressure as r _b = 8.50p ^0.608 [mm/sec] (p = 10-36 MPa). Combustion is stable, and intensive heat feedback from the gas phase dictates the burning rate. The pressure dependence of the burning rate, however, becomes irregular in the range of 20-100 atm. This phenomenon may be attributed to the competing influence of the condensed-phase and gas-phase exothermic reactions in determining the propellant surface conditions and the associated burning rate.