composite solid propellants, widely used in space and military missions [1], consist of two major components, i.e., a gas-producing solid oxidizer and an organic and metale fuel. Perchlorates, e.g., ammonium perchlorate (AP), and nitrates are common oxidizers in composite solid propellants [2, 3]. The oxygen balance of AP is up to +34%, which indicates its highest effective oxygen content among the oxidizers. However, the combustion of AP-based propellants generates significant amount of HCl [4]. Emission of HCl cause to produce a large amount of smoke in the tail of the rocket or missile. Moreover, due to its toxicity, the emission of HCl is an environmental concern [5, 6].
AN, which has extensive applications in the area of nitrogen fertilizers and explosives [7], is of interest as a potential replacement for AP, due to its environmentally innocuous products [8, 9], and positive oxygen balance (+20%) [10]. The advantages of AN-based propellants can be expressed as their low cost, smooth burning without smoke and producing of high volume of gases [11]. Using the AN in large missile motors, despite these privileges, is restricted due to some disadvantages such as the room temperature volume changing phase transformations, adsorption of moisture, low energy, and low burning rate [12-15]. The phase transitions of AN which occur near room temperature, are accompanied by a remarkable volume expansion and result in crack formation in the propellant grain [16].There exist five polymorphic phases of AN, which appear at other temperatures. Among different phase transitions of AN, the ANIV ↔ ANIII phase transition at 32 ℃ causes severe problems in the utilization of AN in specific practical applications [17-19]. To overcome this weakness, phase stabilization of AN has been carried out by adding some additives, including metal oxides [20-23], alkali nitrates and carbonates [24, 25], metal-organic frameworks (MOFs), especially the MOF-199 [26], organic compounds [27], potassium nitrate [28], cellulose derivatives [29], and amino acids (glycine and alanine) [30].
Zeolitic imidazolate frameworks (ZIFs) [31, 32], have the desired properties of both zeolites and MOFs. The ZIF-8 is an attractive functional material, owing to its relatively high thermal and chemical stability, easse of synthesis, and unique behavior in adsorption and transportation processes[32, 33]. Herein, the thermal behavior of AN in the presence of the ZIF-8, as a hybrid metal-organic additive, was investigated. In this study, ZIF-8 as an active catalyst, has been used to phase stabilization and thermal decomposition of AN, simultaneously. It is expected that the ZIF-8, due to its porous structure and Lewis/Bronsted acid sites, dispersed metallic sites, and relatively high thermal and chemical stability, could improve the thermal behavior of AN and remove its inappropriate phase transitions.