| dc.description.abstract | The thesis embodies investigations carried out to understand the mechanism of thermal decomposition and explosion of some high?energy compounds of nitrogen. The importance of these compounds stems from the fact that the alkyl and aryl substituted ammonium nitrates and perchlorates, by virtue of having both the oxidizer and combustible groupings in the same molecule, are in general explosive and find extensive application in explosive and propellant formulations.
Ammonium nitrate is the major source of oxygen available for modern commercial explosives and ammonium perchlorate is the oxidizer of choice in solid rocket propellants. It is mainly due to these reasons their thermal properties have been thoroughly studied. A thermal study of the substituted ammonium nitrates and perchlorates, some of which have already been used as sensitisers and constituents of explosive compositions, has however not been paid due attention and therefore a systematic investigation into the thermal and explosive characteristics of these compounds has been undertaken.
The present programme consists of a study of thermal decomposition of methylammonium nitrates, arylammonium nitrates and arylammonium perchlorates. A survey of the work done to date on these compounds indicates that no systematic study has been attempted to understand the mechanism of decomposition and explosion. Though the compounds have been reported to be stable at room temperature, they explode when subjected suddenly to external stimuli like heat, impact etc. It is therefore interesting to make a thorough study on substituted ammonium nitrates and perchlorates to bring to light the following points:
(i) to evaluate the relative thermal stability as the substitution on the central nitrogen atom increases successively,
(ii) to examine the changes that the configurational stability of the cation brings about in the thermal stability,
(iii) to find the role of basicity of the central nitrogen atom in governing the thermal stability and in turn the decomposition and explosion characteristics,
(iv) to see the variation in explosive and thermal characteristics by varying the anion keeping the cation unchanged,
(v) to study the effect of additives on decomposition and explosivity, and
(vi) to establish the mechanism of thermal decomposition and explosion.
The differential thermal analysis (DTA), thermogravimetric analysis (TGA), mass spectrometry and explosion?delay measurements are the chief thermoanalytical techniques employed in the present work. The impact sensitivity of the compounds has been found out by falling?weight method. Chemical methods of analysis and spectroscopic techniques such as nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy have been used to characterise the compounds prepared during the present investigation.
The results obtained from the present study of the substituted ammonium nitrates and perchlorates can be summarised in the following paragraphs:
Decomposition temperatures of methylammonium nitrates as observed by DTA and dynamic TG are in the order:
monomethylammonium nitrate > dimethylammonium nitrate > trimethylammonium nitrate,
and thus the thermal stability decreases as the methyl substitution increases.
The extent of decomposition as observed from mass?spectral results on gaseous decomposition products increases with increasing substitution on the central nitrogen atom. The activation energy for the decomposition process has been determined by isothermal decomposition techniques. The overall decomposition has been explained to proceed via a dissociation step, which also constitutes the primary step in the process.
The thermal decomposition of tetramethylammonium nitrate is an interesting phenomenon in the light of its additional stability, acquired by the configurational compactness of the tetramethylammonium ion. A decomposition process involving a methyl?group transfer, resulting in the formation of trimethylamine and methyl nitrate prior to decomposition, has been found to be operative. Such a conclusion obtains support from the mass?spectral analysis of the gaseous decomposition products. The activation energy determined by isothermal gravimetry and isothermal mass spectrometry corresponds to the bond dissociation energy of the H?C–N bond. Results of the investigations on the kinetics of thermal decomposition of this compound are also presented.
The basicity of the nitrogen atom governs the thermal stability of the ring?substituted monoarylammonium nitrate. An electron?donating group on the ring increases the thermal stability whereas an electron?withdrawing group on the aryl ring decreases the thermal stability. Steric effects also play a major role in deciding the stability as is evident from studies of ortho?substituted derivatives. The decomposition temperatures as well as dissociation (sublimation) temperatures found out by DTA experiments at atmospheric pressure and at low pressure respectively show linear relationships when plotted against ?, the Hammett substituent constant. ? represents the electron?releasing or electron?withdrawing power of the substituent on the aryl ring. The activation energy for decomposition, found out by isothermal decomposition method, increases with the increasing basicity of the arylamine. The results indicate that a proton abstraction by the anion from the cation is the rate?controlling step.
Keeping in mind the gradation in thermal stability observed in arylammonium nitrates, the study has been extended to the ring?substituted arylammonium perchlorates. The perchlorates are more prone to explosion than the corresponding nitrates. Apart from thermal stability, the explosion sensitivity to heat and impact has also been found out. The activation energy for explosion shows a linear relationship with ?, besides the decomposition and dissociation temperatures. The compounds have impact?sensitivity values comparable to those of initiating explosives.
Transition?metal oxides bring about considerable sensitisation of the decomposition and explosion of the arylammonium perchlorates. The presence of transition?metal oxides, MnO?, CuO and Fe?O? (used in the present investigation) brings down the decomposition temperature of anilinium perchlorates significantly. The addition of these metallic oxides drastically increases the explosion sensitivity to impact and heat, the activation energy for explosion being much less compared to that of the pure compound. The sensitisation has been found to occur via an intermediate metal perchlorate–aniline formation, which facilitates the process of proton transfer and also hastens the decomposition or explosion by the heat liberated during the explosion of the metal perchlorate–aniline complex itself.
The appearance of an exotherm at atmospheric pressure and an endotherm at low pressure along with a sublimate in DTA experiments indicates that the decomposition of all these high?energy materials proceeds via a dissociation step. The identification of dissociation products in the mass spectrum also lends support to the argument. At atmospheric pressure, dissociation (endothermic) resulting in the formation of the N?base amine and the oxidising acid precedes the decomposition (exothermic), which actually is due to subsequent reactions between the amine and/or its decomposition products and the acid and/or its decomposition products. At low pressures, the dissociation products escape away from the hot reaction zone, and the residence time is not sufficient for decomposition to be complete; as a consequence, the endothermic process predominates. The kinetic data and the trends in decomposition and dissociation temperatures further establish that the mechanism involves proton/methyl?group transfer as the case may be, prior to decomposition. | |
