Insight into cooling agent addition on combustion activity and mechanism of catalyzed 5AT-Sr(NO3)2 based propellant

Thermal decomposition characteristics and combustion behaviors of 5-amino-1H-tetrazole-strontium nitrate (5AT-Sr(NO3)2) propellant are of great essence in determining fire-fighting efficiency of the novel Halon alternative technology-solid propellant gas generators (SPGGs). This study will seek to find the impacts on combustion mechanism of adding two kinds of coolants, aluminum hydroxide (Al(OH)3) and calcium carbonate (CaCO3) into 5AT-Sr(NO3)2 based propellant with the employments of experimental measurement and theoretical analysis. Results show that apart from three similar reactions occurring around the close temperature ranges for three different propellants, an endothermic reaction stemming from the breakdown of Al(OH)3 forms when adding Al(OH)3, and two extra exothermic reactions representing the reaction between CaCO3 and HCN, and the generation of isocyanato exist with CaCO3 addition. Especially, according to traditional Kissinger method calculation, great variations can be seen in the activation energy except for the 5AT decomposition reaction. With the presence of Al(OH)3 or CaCO3, the propellant's theoretical outlet temperature has been reduced by ∼300 °C, while experimental combustion temperature at different sampling points are lowered approximately by 342-417 °C. With the employment of closed-bomb test, it is discovered that the burning rate in the relatively-low pressure zone is mastered by condensed-phase thermal decomposition reaction of 5AT, whereas that in the relatively-high pressure area is dominant by the redox reaction. Unlike Al(OH)3, addition of CaCO3 could reduce the pressure exponent effectively. The specific shapes of burning surface are found to determine the flame shapes for three 5AT-Sr(NO3)2 based propellants, which turn out be parallel, divergent, and converging profile, respectively. Besides, a gas flow model for 5AT-Sr(NO3)2 based propellant is proposed in this study, which can well illustrate the burning rate differences from the perspective of porous structure for propellant particles. Results of this study have implications concerning designs for 5AT-Sr(NO3)2-based propellants widely used in SPGGs apparatus, which can provide a potential approach for making a novel 5AT-Sr(NO3)2-based propellant with low combustion temperature as well as low burning rate pressure exponent.