ReviewThermal decomposition of ammonium perchlorate
Introduction
Properties of ammonium perchlorate in general and particularly its thermal decomposition have been a subject of extensive literature including reviews [1], [2], [3], [4], [5].
It was revealed during investigation of thermal decomposition of ammonium perchlorate that the process is characterized by a number of features which were of purely scientific interest irrespective of applied problems at which these investigations had been aimed at first. It turned out that the character of thermal decomposition of ammonium perchlorate below phase transition at 240 °C and above this point is different.
Low-temperature thermal decomposition of ammonium perchlorate turned out to be extremely sensitive to the “biography” of a crystal, the presence of admixtures in it, preliminary mechanical treatment and irradiation. It was also revealed that addition of simple and complex oxides, which are typical catalysts of oxidation–reduction processes, has a very strong effect on the rate of thermal decomposition. It was established that low-temperature thermal decomposition of ammonium perchlorate proceeds not completely and runs through the formation and growth of reaction nuclei composed of unreacted NH4ClO4. Since no solid products are formed during thermal decomposition of ammonium perchlorate, a usual scheme of the process auto-localization due to catalysis by a solid product of the reaction could not be accepted; therefore, another scheme of feedback leading to auto-localization of the process, specific for ammonium perchlorate, had to be searched for. Below the phase transition at 240 °C reaction arrest after the decomposition of one-third of ammonium perchlorate is also unusual.
High-temperature thermal decomposition of ammonium perchlorate proceeds till completion; no dry residue is formed.
In the very beginning of study, the mechanism of thermal decomposition of ammonium perchlorate was not clear. There were at least three points of view on the elementary stages from which the process of thermal decomposition starts. According to one of these opinions, decomposition is considered to start with electron transfer from the anion to cation similarly to the case of heavy metal azide decomposition. According to another one, the decomposition process starts with the rupture of chlorine–oxygen bond, similarly to thermal decomposition of perchlorates of alkaline metals. According to the third point of view, thermal decomposition of ammonium perchlorate starts with proton transfer from the cation to anion, similarly to decomposition of all the ammonium salts.
Section snippets
Phase transition
Ammonium perchlorate is a white crystalline substance. Two most important crystal modifications of ammonium perchlorate are: orthorhombic (region of existence: T < 240 °C) (Fig. 1) and cubic one, which is stable at temperatures T > 240 °C.
Heat of phase transition of ammonium perchlorate from orthorhombic to cubic modification is 11.3 kJ/mol [6].
The unit cell of the orthorhombic modification contains four molecules. The unit cell parameters are: a = 0.9202, b = 0.5816 and c = 0.7449 nm. Four oxygen atoms form
General information about thermal stability and the composition of products formed under thermal decomposition of ammonium perchlorate
Ammonium perchlorate is considered to be rather stable chemical compound. Its decomposition starts at 130 °C. At this temperature, ammonium perchlorate is decomposed about 10 times slower than ammonium nitrate. However, thermal stability of ammonium perchlorate is strongly dependent on the method of obtaining crystals, preliminary treatment and storage time after obtaining the crystals [46].
The data reported by Dode [47], [48] should be considered as being among the earliest data on the chemical
The general characterization of the process topography
The early investigations into thermal decomposition of ammonium perchlorate showed that clearly exhibited localization and auto-localization of the process take place. In other words, thermal decomposition of ammonium perchlorate starts not over the whole crystal volume and not on the whole surface but in some sites near the outer and inner surfaces of a crystal. Having started at this active centres of a crystal, the reaction propagates near the place where it has already started, that is, the
Mechanism of thermal decomposition of ammonium perchlorate
It has already been mentioned above that different points of view on thermal decomposition of ammonium perchlorate existed.
According to one of them, the decomposition is a consequence of electron transfer from anion to cation, that is, the decomposition of ammonium perchlorate proceeds similarly to the case of thermal decomposition of azides, fulminates and other similar compounds.
One of the first versions of such a mechanism of that proposed by Bicromshaw and Newman [16]. According to
The effect of irradiation on thermal decomposition of ammonium perchlorate
The use of ammonium perchlorate as a component of solid propellants was one of the reasons of increased attention to radiation chemical processes taking place during its irradiation, and changes in the reactivity of ammonium perchlorate under irradiation.
The mechanism of processes occurring during the radiolysis of ammonium perchlorate was investigated in a number of works. According to their results, excitation of ions occurs under the action of irradiation both in the cation and in the anion
The influence of additives on thermal decomposition of ammonium perchlorate
A specific feature of thermal decomposition of ammonium perchlorate is extremely high sensitivity to the action of various additives, either entering the lattice (homophase) or forming their own phase (heterophase). These additives can accelerate or decelerate the process of low-temperature thermal decomposition, which is important for the storage of ammonium perchlorate and mix compositions based on it. The additives may affect the deflagration delay time. It was assumed that there existed a
The effect of electric field on thermal decomposition of ammonium perchlorate
Since the edge components of dislocation in ion crystals are charged, an attempt was made in [187], [188] to study the effect of the constant electric field on thermal decomposition of ammonium perchlorate. The results of experiments carried by the authors of [187], [188] showed that a constant electric field with electrostatic intensity 0–10 kV/cm affects the character of thermal decomposition development in the crystal: anisotropy of reaction increases due to prevailing growth of nuclei along
The effect of crystal size and mechanical treatment on thermal decomposition of ammonium perchlorate
Anomalous dependence of the rate of thermal decomposition on crystal size was noticed even in the earliest works on thermal decomposition of ammonium perchlorate [16]. It turned out that with a decrease in the size of crystallites the rate of thermal decomposition passes through some optimal value; that is, as the specific surface area of ammonium perchlorate increases, no increase in rate is observed unlike the usual case of topochemical processes. A reason of such a specific dependence is
Conclusion
Studies of the thermal decomposition of ammonium perchlorate are interesting not only because of the application of the ammonium perchlorate as a component of solid propellants. The reaction provides a unique example of the auto-localization of thermal decomposition of a solid, when no solid products are formed, and the feed-back during the reaction is due to the secondary reactions, in which gaseous reaction products are involved.
As a consequence, a study of the mechanism of this reaction
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