Reactivity of metastable solids to heat

Abstract

The thesis embodies investigations on the thermal decomposition of barium azide and the thermal decomposition and sublimation of ammonium perchlorate (AP). A lot of work has been done on the thermal decomposition of these metastable materials. In spite of this, the decomposition mechanisms of these materials cannot be considered well understood. Further, this study has been undertaken to understand the role played by various types of lattice imperfections in determining the chemical reactivity of solids.

Significance of Barium Azide Barium azide belongs to an important class of explosives, namely the azides. The thermal decomposition of heavy metal azides like Pb(N?)? cannot be studied easily over a wide temperature range because of their intrinsic instability. Therefore, it is worthwhile to study the thermal decomposition of barium azide, which is a mild explosive, in a wide temperature range to throw light on the decomposition mechanism of explosive azides.

Significance of Ammonium Perchlorate AP is widely used as an oxidizer in solid rocket propellants. Though the thermal decomposition of AP has been studied extensively, its decomposition mechanism still remains controversial. The effect of pre-treatments like doping could provide additional insight into the decomposition mechanism. Sublimation of AP has not received as much attention as its decomposition. Understanding the sublimation process is important because the thermal decomposition of AP is complicated by concurrent sublimation. Further, it is desirable to suppress sublimation (an endothermic process) in AP-based propellants. A comparative study of the effect of pre-treatments on thermal decomposition and sublimation will hopefully clarify the decomposition mechanism of AP. While the effect of pre-treatments on thermal decomposition has been studied earlier, the present work focuses on sublimation of pre-treated AP in detail.

Results on Thermal Decomposition of Ba(N?)? The Arrhenius plot in the temperature range 120–190°C for the thermal decomposition of single crystals of anhydrous barium azide, both pure and doped, shows a break (knee).

Activation energy in the low-temperature range: 23 ± 1 kcal/mol Activation energy in the high-temperature range: 36.8 kcal/mol

The knee temperature depends on the type and concentration of the dopant:

Cl? impurity: No effect on decomposition rate or knee temperature Na? or CO?²? impurity: Shifts knee to higher temperatures and sensitizes decomposition Al³? impurity: Lowers knee temperature and desensitizes decomposition

The results are explained by a diffusion-controlled mechanism, with Ba²? interstitials as the diffusing species.

Low-temperature activation energy corresponds to diffusion of interstitials High-temperature region likely corresponds to the intrinsic region where Ba²? interstitials are self-generated due to thermal effects

Pre-compression results in sensitization of decomposition, possibly due to increased lattice imperfections. Pre-treatments (doping and pre-compression) do not alter the decomposition mechanism.

Results on Thermal Decomposition and Sublimation of AP

Thermal decomposition behavior of PO?³? doped AP is analogous to that of SO?²? doped AP: both initially sensitize decomposition, followed by desensitization. Mechanism involves diffusion of ClO?? ions as the rate-determining step. Doping with SO?²? or PO?³? increases anion vacancies, facilitating ClO?? diffusion. At higher concentrations, dopants precipitate on imperfections, poisoning decomposition sites. Activation energy studies show doping does not alter the decomposition mechanism.

Pre-treatments markedly affect sublimation kinetics:

Doping and pre-heating: Desensitize sublimation Pre-compression: Sensitizes sublimation

Crystal strain plays a significant role in sublimation kinetics. Sublimation rate increases with decreasing particle size until a critical size is reached, after which the rate decreases (explained by Mampel’s theory). Earlier studies show:

Ca²? impurity: Desensitizes decomposition SO?²? impurity: Sensitizes decomposition Pre-heating: Sensitizes thermal decomposition

Present results show:

Doping AP with Ca²?, SO?²?, or PO?³? desensitizes sublimation Pre-heating lowers sublimation rate

These divergent behaviors indicate that the rate-determining steps in decomposition and sublimation are different. Sublimation of AP proceeds through a proton-transfer mechanism, whereas thermal decomposition does not involve proton transfer as the rate-determining step.