Elsevier

Thermochimica Acta

Volume 610, 20 June 2015, Pages 57-68
Thermochimica Acta

Thermal decomposition of ammonium perchlorate—A TGA–FTIR–MS study: Part I

https://doi.org/10.1016/j.tca.2015.04.025Get rights and content

Highlights

  • TGA–FTIR–MS study of ammonium perchlorate.

  • Decomposition was divided into low, intermediate, and high temperature regimes.

  • N2O and NO2 were the major species at low and high temperature regimes, respectively.

  • N2O, NO2, HNO3, and HCl were quantified to aid kinetic evaluation.

  • NO was not detected as a major product at any stage.

Abstract

The thermal decomposition of ammonium perchlorate has been studied using thermogravimetric analysis (TGA), coupled with Fourier transform infrared (FTIR) spectroscopy and electron ionization (EI) mass spectrometry (MS) of the evolved gases. The thermal decomposition could be demarcated into three distinct regimes, the low temperature decomposition (LTD) regime and the high temperature decomposition (HTD) regime, with an intermediate regime between the two, named as the intermediate temperature decomposition (ITD) regime. Using FTIR spectroscopy, N2O was detected as the primary species during the LTD regime, followed by HCl, NO2, and HNO3, in lesser quantities. On the contrary, NO2 was found to be the principal species, followed by almost equal concentrations of HCl, N2O, and HNO3 in the HTD regime. Other important species, such as H2O, Cl2, O2, etc., although observed by MS, were not quantified. NO could not be identified in appreciable quantities in any of the regimes. Based on the species detected during the present work, and previous research, a reaction scheme has been proposed for AP decomposition in the LTD and the HTD regimes.

Introduction

Ammonium perchlorate (AP) has been consistently utilized in the solid propellant industry for the past century due to its excellent oxidizing properties, compatibility with other solid propellant components, as well as ease of availability [1], [2]. The thermal decomposition of AP, and the corresponding chemical kinetic parameters are of interest to the propellant community for two broad reasons—the ability to predict the performance of composite propellants formulated from potential fuels and burn rate enhancers using simulation of one dimensional combustion of such strands, as well as the ability to predict the stability of various propellant mixtures with AP as an ingredient under thermal stress during storage and handling. Thus the decomposition behaviour of AP is required for simulating fast combustion under heating rates of the order of approximately 106 Ks−1, as well as for simulating accelerated ageing behaviour under heating rates of the order of approximately 5 K min−1.

The possibilities of existence of multiple reaction mechanisms involving endothermic and exothermic reactions and processes under two radically different heating rates makes it extremely challenging to elucidate the overall reaction chemistry of AP. Additionally, due to the complicated chemical structure of AP, the nature of its decomposition remains subject to modification by generations of researchers with advancements in diagnostic techniques. The chemical kinetics of the decomposition scheme, involving the activation energies and the pre-exponential factors of individual elementary reactions, are also subject to modification with each passing decade.

The slow decomposition of AP has been studied through the ages by several researchers, and reviewed by several authors [3], [4], [5], [6]. The most recent review by Boldyrev [6] summarized the research conducted on slow thermal decomposition of AP, and concluded that despite the advancement in the diagnostic techniques, the complete array of physico-chemical processes occurring on the microstructures formed over and inside the crystals of AP is still beyond the reach of researchers. It is well-established that the decomposition of AP follows two temperature dependant regimes, the first regime corresponding to the LTD reactions between approximately 215–330 °C, with a phase transition of AP from orthorhombic to cubic, and the second regime corresponding to the HTD reactions between approximately 330–420 °C [1], [2]. Sublimation also takes place along with decomposition, depending upon the experimental conditions.

The conflicting theories guiding the mechanism of LTD of AP were postulated as follows—the electron transfer mechanism involving an electron transfer from the perchlorate anion to the ammonium cation, the proton transfer mechanism involving a proton transfer from the ammonium cation to the perchlorate anion, and a thermal decomposition mechanism involving the breakup of the chlorine–oxygen bonds. One of the earliest studies on thermal decomposition mechanisms of AP by Bircumshaw and Newman [7] proposed the electron transfer theory with several pieces of experimental evidence in its favour. However, the proton transfer theory was conclusively proven to be the one actually dominating the process by several researchers, with arguments such as the identical composition of products of decomposition and sublimation, the retardation of the reaction by ammonia vapour, and acceleration of the reaction by perchloric acid, and the stabilizing effect of water on perchloric acid [6]. The review also enumerates several reactions that lead to the products observed in the LTD reactions, with the generation of ClO2 and its subsequent oxidation of ammonia and the ammonium cation being the premier steps. The HTD reactions were dominated by the sublimation of AP, and the reaction of ammonia and perchloric acid generated from the condensed phase in the neighbouring gas phase.

The products of decomposition of AP in LTD and HTD regimes have been identified by several research groups [8], [9], [10], [11], and were a mixture of various species, i.e. H2O, O2, N2, NO, NO2, N2O, Cl2, HCl, ClO2, ClO, HOCl, HClO2, ClO3, HClO3, HClO4 etc. Recent efforts were concentrated towards the utilization of the hyphenated techniques, namely TGA–MS and TGA–FTIR spectroscopy [12], [13]. The complexity of the reactions occurring during the LTD and the HTD regimes has precluded the development of detailed chemical kinetic mechanisms for both the regimes. However, the kinetics of decomposition was studied by several research groups by assuming that the processes were governed by known models or by using various model-free approaches. Recent studies by Lang and Vyazovkin [14] utilizing TGA–DSC methods on various crystal sizes of AP as well as pellets, showed that the activation energy of the LTD regime was approximately 120 kJ/mol, which began to drop to approximately 60 kJ/mol as the mass loss reached 20%. The activation energies were found to vary between 95 and 145 kJ/mol for HTD regime, and were speculated to be due to sublimation, although the exact mechanism behind the variation of the activation energies with sample types was unclear. The studies by Zhu et al. [13] also demarcated the decomposition process into the LTD and the HTD regimes, however, with the distinction of introducing a third regime between them, where the activation energy was found to increase from the lowest calculated value to the constant value of the HTD regime. The values of the activation energies for the LTD regime, the HTD regime, and the intermediate regime were 52, 142, and 218 kJ/mol respectively. Two reactions were proposed to explain the behaviour of the observed species in the gas phase, especially the dominance of N2O over NO2 in the LTD regime, and the dominance of NO2 over N2O in the intermediate regime.

The preceding discussion shows that although detailed chemical kinetic mechanisms (DCKMs) exist for explaining the thermal decomposition of AP in the gas phase, condensed phase reactions occurring in the solid and the liquid phases have been treated as global reactions till date, in which AP decomposes to form products in unrealistic single step mechanisms or semi-global mechanisms involving three to four steps, based on slow heating rates in thermogravimetric analysers. Only a few studies conducted by Brill and co-workers [15], [16] under faster heating rates to observe the products formed during the fast pyrolysis of AP approximate the behaviour at the thin fizz zone which comprises of liquid and gas phases at the interface of the gas phase and solid phase during combustion of AP. The semi global mechanism shown in Eqs. (1)–(4) currently being used by researchers for simulating steady state combustion using the Beckstead–Derr–Price model [17], may be summarized [18] as Scheme 1.

Therefore, there is a necessity for developing a detailed chemical kinetic mechanism (DCKM) with appropriate activation energies and pre-exponential factors for both LTD and HTD regimes of decomposition of AP. The current endeavour is focused on generating adequate data for aiding the development of such DCKMs. The technique chosen for this purpose was the hyphenated technique of TGA–FTIR–MS. Several species detected from the FTIR spectra were also quantified using a non-linear least squares method to serve as additional input for the development and validation of the DCKMs. The mass spectra, though uncalibrated, served as a valuable corroboration of the data obtained from the FTIR spectra. AP was also heated under three distinct heating rates to extract the variation of global activation energies and pre-exponential factors with the reacted mass fraction using iso-conversional methods and the kinetic compensation effect.

Section snippets

Experimental

The decomposition of AP was studied using the well-established technique of TGA, coupled with an FTIR spectrometer and MS. The two diagnostic methods have been found to be synergistic in nature, with the entire range of the product gases revealed by either of the instruments. The AP used in this study was provided by the High Energy Materials Research Laboratory, India, without any further processing or purification. Owing to the hygroscopic and agglomerating nature of AP, the particles were

Data processing

The activation energies corresponding to the distinct decomposition regimes were calculated using the model-free iso-conversional Kissinger–Akahira–Sunose [19] method, formulated for non-isothermal heating studies. The degree of conversion (α) of AP as a function of time is defined by Eq. (5):α(t)=m0m(t)m0mfwhere m(t) represents the mass of the sample at a time t, m0 and mf are the masses of the sample at the beginning and at the end of the process, respectively. The Kissinger–Akahira–Sunose

TGA data and kinetic analysis

The TGA and DTG plots of AP crystals subjected to a heating rate of 15 K min−1 are presented in Fig. 2. The curves are obtained after averaging three repeatable experimental runs with similar masses.

The mass-loss curve demonstrates that the decomposition process of AP may broadly be divided into two regimes—the first decomposition regime, occurring from 260 °C to approximately 330 °C accompanied by a 30% mass loss, and the second decomposition regime, occurring from 330 °C to 420 °C, accompanied by

Conclusions

The decomposition behaviour of a commonly used oxidizer in composite propellant formulations, ammonium perchlorate, was analysed under non-isothermal slow cook-off conditions using the hyphenated TGA–FTIR–MS technique. The samples of ammonium perchlorate used in this study with a particle size distribution between 200 and 250 μm, had numerous surface anomalies owing to prior heat treatment at low pressures. The mass loss studies revealed that the LTD regime of decomposition commenced with

Acknowledgements

We would like to acknowledge the valuable assistance of Mr. Ashutosh Jadhav in processing the mass spectrometric data. We would like to thank High Energy Materials Research Laboratory for providing ammonium perchlorate samples. The SEM micrographs were taken at the Sophisticated Analytical Instrument Facility of IIT Bombay. The financial grant for the TGA–FTIR–MS setup, provided by the Industrial Research and Consultancy Centre, IIT Bombay, through the RIFC scheme, is gratefully acknowledged.

References (28)

  • L.L. Bircumshaw et al.

    Thermal decomposition of ammonium perchlorate

    Proc. Roy. Soc.

    (1954)
  • G.A. Heath et al.

    Mass spectrometric study of the thermal decomposition of ammonium perchlorate

    Trans. Faraday Soc.

    (1964)
  • V.R.P. Verneker et al.

    Mass spectrometric study of the thermal decomposition of ammonium perchlorate

    J. Chem. Phys.

    (1967)
  • O.P. Korobeinichev et al.

    Investigation of rapid processes in the thermal decomposition of ammonium perchlorate with a transit time mass spectrometer

    Bull. Acad. Sci.

    (1969)
  • Cited by (81)

    View all citing articles on Scopus
    View full text