Elsevier

Thermochimica Acta

Volume 521, Issues 1–2, 10 July 2011, Pages 125-129
Thermochimica Acta

Rapid-heating of energetic materials using a micro-differential scanning calorimeter

https://doi.org/10.1016/j.tca.2011.04.015Get rights and content

Abstract

A micro-differential scanning calorimeter (μ-DSC) was employed to study the thermal decomposition of organic energetic materials at high heating rates. Heating rates from 1900 to 65,000 K/s were explored, which are many orders of magnitude higher than traditional DSC, but much closer to the conditions these materials would experience in their application. Temperature calibration was done by heating Sn, KNO3, and KClO4 at the desired heating rates to determine the temperature profile at each rate. The samples studied were 5-amino-1H-tetrazole, 5-amino-1-methyl-1H-tetrazolium dinitramide, 1,5-diamino-4-methyl-1H-tetrazolium dinitramide, and 1,5-diamino-4-methyl-1H-tetrazolium azide, which comprise a new class of high-nitrogen containing energetic materials. Activation energies determined using the Kissinger method, are much lower than those reported for the same materials under low heating rates indicating that other decomposition mechanisms are in play at high heating rates.

Highlights

► Used a μ-DSC device capable of rapid heating rates of up to 107 K/s. ► Rapid heating rate studies may probe different mechanistic pathways. ► Studied reaction of novel tetrazole containing energetic materials. ► Used heating rates that are many orders of magnitude greater than traditional DSC. ► Materials showed much lower activation energies under rapid heating conditions.

Introduction

Differential scanning calorimetry (DSC) is often used as a characterization technique to determine the thermal behavior of energetic materials (EMs). As new organic EMs are formulated, this is an important test to determine thermal stability and ignition temperature. Recently, new organic energetic materials are being created to not only improve performance and stability, but also to reduce their environmental impact. These “green” energetics are generally high-nitrogen content ionic salts which have N2 gas as a primary reaction product and create the majority of their energy from high heats of formation. Conversely, classic organic explosives (TNT, RDX, etc.) create energy through oxidation of a carbon backbone [1], which often results in gaseous carbon containing reaction products. Many ionic salts have a tetrazole-containing cation, which is primarily responsible for the high-nitrogen content of the material. Two common cation structures are 5-amino-1H-tetrazole, and 1,5-diaminotetrazole which have 82.3 wt.% and 84 wt.% nitrogen, respectively [2], while the anion can be composed of a variety of different structures.

Several groups have characterized high-nitrogen energetics via DSC and related experiments. 5-Aminotetrazolium nitrate [3], 1,5-diamino-4-methyl-1H-tetrazolium nitrate [4], 1,5-diamino-4-methyl-1H-tetrazolium dinitramide [4], 1,5-diamino-4-methyl-1H-tetrazolium azide [4], and 5-aminotetrazolium dinitramide [5], are just a few examples of ionic salts that have been characterized by standard DSC methods. In addition to decomposition temperature, activation energy can be calculated using varied heating rate experiments and either an iso-conversion technique like that by Ozawa [6] or the Kissinger method [7]. Ma et al. performed this experiment for 5-aminotetrazolium nitrate from 2 to 25 K/min and obtained an activation energy of 303.2 kJ/mol using the Ozawa method and 311.0 kJ/mol using the Kissinger method [3]. In a similar DSC experiment at heating rates of 2–40 K/min, Fischer et al. determined the activation energy of several ionic salts and found them to be in the range of ∼101–138 kJ/mol using both the Ozawa and Kissinger methods [4]. Relative to heating rates commonly experienced during combustion of energetic materials, traditional DSC heating rates are many orders of magnitude lower. This is relevant as higher heating rates may lead to different mechanistic steps in the reaction. For example, it is established that as the heating rate increases, the activation energy tends to decrease. Heating experiments for Ge2Sb2Te5 films showed the activation energy for film crystallization decreased by more than a factor of four over a heating rate range of (3–500) K/min.

To achieve very high heating rates that approach those that are relevant to energetic materials, new classes of DSC devices are necessary. These new devices based on MEMS fabrication methods offer very fast response due to the small thermal mass of both the heaters and the sample [8], [9], [10], [11]. A μ-DSC device previously developed at the National Institute of Standards and Technology (NIST) is capable of heating rates up to 1 × 107 K/s [11], and was previously used to explore phase changes in Ni/Si thin films [12]. In the present study this device is used to investigate the decomposition temperature and activation energy of several high-nitrogen energetic salts and results are compared with that of traditional DSC.

Section snippets

Experimental

The main diagnostic tool in this study is a silicon based μ-DSC device [11] (66 × 240 × 3.3 μm) shown in Fig. 1, where the center rectangular section is the heated platform. The heated platform can be split through the vertical axis into two halves; one holds the sample and the other serves as a reference during heating. Each section has a poly-silicon heater, which is driven by a function generator (Tabor Electronics WS8102) [13] that supplies a linear voltage ramp. An aluminum poly-silicon

Results and discussion

The decomposition temperature of each material at four different heating rates is shown in Fig. 3. The error bars represent the standard deviation at each temperature point. We expect that the primary error in this experiment is due to either inconsistencies in drop size and placement, or non-uniformities in the heating surface's temperature profile. Either of these issues would cause slight variations in sample temperature throughout the experiment.

In varied heating rate experiments it is

Conclusion

Decomposition of energetic materials are nominally studied using traditional DSC tools which employ heating rates many orders of magnitude lower than that experienced in a real world application. In this study we employ a μ-DSC to study the thermal decomposition of organic energetic materials at high heating rates. Four high nitrogen-containing energetic materials were chosen for the study, for which we found a consistently, and significantly lower activation energy than that found at low

Acknowledgements

The authors thank Professor T.M. Klapötke of Ludwig-Maximilians University in Munich, Germany, for providing the samples used in this study. This work was supported by the Army Research Office and the Army Research Laboratory.

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