Thermal reaction studies and prediction of stoichiometry of pyrotechnic compositions using DSC and XRD methods
Introduction
Pyrotechnic flash compositions are generally made of finely divided metallic or non-metallic fuels and inorganic oxidizers [1,2]. These are mainly used to produce light and sound in various firework applications [2]. The pyrotechnic compositions are either binary or ternary mixtures. Ternary mixtures contain either mixed fuels or mixed oxidizers. In order to determine the potential effect of pyrotechnic compositions, the inherent thermal characteristic of the mixture and the level of confinement at which the mixture undergoes decomposition must be studied. When considering the chemical composition, stoichiometric mixtures would produce the desired output. The reaction under confined environment would be more rigorous than those under an unconfined environment [3]
A thorough understanding of the thermal reactions and residues of the pyrotechnic composition is required to arrive at the optimum stoichiometry [4]. In order to quantify this understanding, it is necessary to conduct a systematic experimentation using the Differential Scanning Calorimetry (DSC) along with X-ray diffraction (XRD) and Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDS). Although there have been studies on thermal reactions [[4], [5], [6], [7], [8], [9]] and residue analyses of the binary mixtures [4], there have been few studies on ternary mixtures [10,11]. In this study, a systematic experimental methodology has been employed to identify the reaction products by performing residue analysis using X-ray diffraction (XRD) and Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDS), and eventually derive the stoichiometric equations for the commonly used pyrotechnic ternary compositions comprising Potassium nitrate (KNO3), Aluminium (Al) and Sulphur (S).
The objectives of this research paper are to 1) study the inherent thermal characteristics of typical pyrotechnic flash compositions containing Potassium nitrate, Aluminium and Sulphur under various test conditions viz., confined (sealed) and unconfined (unsealed), 2) perform thermal studies by varying the weight proportion of the pyrotechnic compositions, and 3) perform residue analysis to identify the combustion products for the prediction of stoichiometry.
Section snippets
Materials
Reagent grade Sulphur of 99% purity was obtained from the TCI Chemicals (India) Pvt Limited and commercial grade Potassium nitrate and Aluminium flake powders were obtained from the firework factories in Sivakasi, Tamil Nadu, India.
The SEM-EDS characterization was carried out for KNO3 and Al as they were obtained from the commercial firework industries. The SEM-EDS results of KNO3 and Al are presented in Fig. 1, Fig. 2, respectively. There were no significant impurities present in the EDS.
Thermal analysis of KNO3/Al/S at confined and unconfined test conditions
Fig. 3 shows the thermal characteristics of KNO3/Al/S flash compositions under confined and unconfined experimental conditions. The continuous lines represent DSC results of the mixture under confined conditions, while the dotted line represents unconfined condition. No thermal events were identified up to 114 °C. The peak at 114 °C corresponds to the melting of Sulphur [12,13] and the peak at 132 °C corresponds to orthorhombic to rhombohedral transition of Potassium nitrate [3]. Immediately
Conclusions
Exothermic reactions were observed for ternary mixtures comprising KNO3/Al/S under confined conditions, whereas the same mixture was unreactive under unconfined conditions. The Sulphur in the ternary mixture played a significant role in the initiation of the reaction in the confined system. A maximum exothermic energy of 637 J/g was obtained for the stoichiometric mixture of 53% KNO3: 33% Al: 14% S, which was experimentally determined by residue analysis. This experimental methodology is useful
Acknowledgements
The authors are thankful to the Director NIT, Trichy and Armament research board (ARMREB), Defence Research and Development Organisation (DRDO) for funding this research work (Project number: ARMREB/CDSW/2015/172)
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