KINETICS OF THE DIRECT AND INVERSE Ⅳ→Ⅲ PHASE TRANSITIONS IN AMMONIUM NITRATE CRYSTAL LATTICE

摘要

Ammonium nitrate (AN) is the least expensive and the most available oxidizer for energetic compositions. AN has a considerable advantage before other oxidizers when the main requirements to the composition to be created are not their energetic parameters (like for explosives and propellants) but, in contrast, low burning temperature (2200K or so). Such characteristics are required for smokeless low-toxic (containing even practically no CO and NO in combustion products) gas generating compositions for vehicle safety airbags. Creation of stoichiometric (i.e. where O=2C+0.5H) compositions with such low burning temperatures is possible only if AN is used as oxidizer, any other known solid CHNO-oxidizers with appropriate level of stability may be used only for propellants with the combustion temperature T_c not less than 2700-2800K. However, presence of several phase transitions in AN crystal structure in temperature range from -50 to 100℃ finds serious difficulties in using such compositions. During long-term storage at variable temperature rupture of the composite structure occurs (due to phase transitions with volume change) and, hence uncontrollable change of burning law of the energetic material. The most dangerous among all phase transitions in AN crystal structure is the transition between phases Ⅳ→Ⅲ (it occurs at temperature close to 32℃) because this transition is accompanied with the maximum crystal volume change.