Pressure dependence of thermal decomposition of ammonium nitrate oxidizer

Abstract

There is an increasing interest in research and development of ecofriendly solid propellant systems which can replace the existing ones in applications particularly where the rockets have to be fired in close proximity ofthe personnel and smokeless HCI-free burning is desired. Ammonium nitrate (AN) is receiving wide attention as a possible substitute for ammonium perchlorate oxidizer in view of the hazardous nature of other candidates like nitramines and non-availability of substances like ammonium dinitramide in requisite quantities besides the cost effectiveness. The phase transitions and initial endothermic decomposition of ammonium nitrate, however, pose problems in its application in composite solid rocket propellants. The influence of pressure on phase transitions, melting and thermal decomposition characteristics of AN is required to be systematically studied in order to have an insight in the combustion mechanism of AN containing propellants. Pressure differential scanning calorimetric (PDSC) studies have consequently been conducted on pure AN under ambient conditions and upto a maximum inert pressure of 68.03 atm. The shifts in the phase transition peaks, melting and decomposition temperatures with pressure have been studied. It has been observed that thermal decomposition of AN remains an endothermic process upto a prepare of 20.41 atm and thereafter becomes exothermic in nature. Phase transitions, melting and decomposition processes are found to be pressure sensitive and the associated peaks register a positive temperature shift bearing a relationship with crystal structure in two distinct pressure segments corresponding to endo- and exothermic decomposition.