HP-DSC study of energetic materials. Part I. Overview of pressure influence on thermal behavior
Graphical abstract
Introduction
One of the first reports devoted to thermal analysis at elevated pressure was published in 1923 [1]. Since then the method has been extensively developed and nowadays HP-DSC – high pressure DSC – is routinely used in several fields, e.g. to study oxidation of oils [2]. Main advantage of HP-DSC technique application to this process is the suppression of evaporation leading to increased sensitivity of the onset detection [3].
Literature contains only limited data on energetic materials decomposition under elevated pressures [4], [5], [6], [7], [8], [9], [10], [11]. Most of these results have been obtained using DSC with sealed containers [5], [6], [7]. Hermetic containers must withstand pressure of formed gaseous products causing pans complexity and consequently lowering sensitivity. Another problem is that the pressure inside container is unknown and cannot be controlled. An alternative way to study DSC at elevated pressure is to use device with controlled pressure inside the furnace, i.e., HP-DSC. Previously, this technique was used to investigate the cyclotrimethylenetrinitramine (RDX) thermolysis with focus on the heat effect dependency on pressure [5]. For ammonium dinitramide (ADN) thermal decomposition have been studied only in range 0.1–6 MPa [6], [7]. Vargeese [10] studied ammonium perchlorate thermolysis up to 1.2 MPa pressure and observed some noticeable changes compared to 0.1 MPa. Recently, the decomposition of some explosives, including cyclotetramethylene-tetranitramine HMX, have been investigated at pressures 0.1–5 MPa and the DSC peak temperature shift on pressure reported [11].
But how can environment pressure affect thermoanalytical measurements? In general, the elevated pressure has three-fold effect on the results of thermoanalytical measurements. Firstly, it suppresses the evaporation, the effect widely used in chemical industry to characterize thermal hazards of solvents or peroxides [12]. The example from energetic materials field—a study of compatibility of trinitroazetidine (TNAZ) with other compounds [13].
Secondly, the melting temperature is also affected by pressure. This dependency is commonly described in form of Simon equation [14]:where Tm – melting temperature, a and b – parameters related to material structure. The dependency (Eq. (1)) defines the critical pressure level to initiate the self-heating regime of decomposition, which is important, in particular, for hazard evaluations, such as sensitivity to mechanical stimulus [15].
Thirdly, the pressure increase can affect the process kinetics. The rate of chemical reaction is usually expressed in form [16]:where k(T) represents rate constant, which in turn is typically parameterized through the Arrhenius equation, f(α)—model of reaction. The dependency on pressure is usually omitted.
The present paper, first in series, examines the effect of pressure within the considerably expanded pressure range of 0.1–14 MPa on vaporization, melting, and decomposition kinetics of three widely known materials, i.e. high explosive RDX, oxygen-deficient trinitrotoluene (TNT) and oxidizer ADN. Selected energetic materials represent compounds with negative and positive oxygen balance, therefore both inert and oxidative environment pressures has been applied. Thus, the main idea of the study is to show the capabilities of HP-DSC method to study energetic materials.
Section snippets
Materials
TNT has been recrystallized twice prior to use and grounded with a pestle in mortar. RDX powder was used as-received with particle size about 200 μm. Ammonium dinitramide have been treated at 50 °C during 12 h to remove the moisture and then stored in desiccator. ADN powder consists of spherical particles with an average diameter of 180 μm.
Experimental methods
Thermal behavior has been studied with DSC 204HP (Netzsch) apparatus. Samples in closed aluminum pans with pierced lids were heated with rates of 0.5–20 K/min
General pressure influence on thermal behavior
DSC curves for linear heating of RDX sample at 0.1 MPa are shown in Fig. 1a: endothermal peak corresponding to the sample melting is observed at Tm = 204.2 ± 0.3 °C (averaged of 21 runs), followed by the immediate decomposition with strong exothermal effect. The average heat effect of melting is found to be ΔHm = –124 ± 14 J/g, which is lower than one reported by Hall, i.e., 160 J/g [17], apparently due to the partial overlapping of melting with decomposition process. With the pressure increase up to 5 MPa
Conclusions
Presented results intended to show the pressure influence on thermal behavior of energetic materials. Using RDX, TNT and ADN as objects, the melting, decomposition kinetics and thermolysis heat effect have been analyzed at pressures 0.1–14 MPa both in inert and oxidative environments. Results reveal the melting temperature to increase with pressure almost linearly (0.12–0.15 °C/MPa). The thermolysis kinetics appears to be affected by pressure too. Three situations have been proposed:
- (1)
evaporation
Acknowledgments
The study was funded by Russian Foundation for Basic Research, according to the research project No. 16-33-60162 mol_a_dk.
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