Role of metal oxide nanomaterials on thermal stability of 1,3,6,-threenitrocarbazole
Seied Mahdi Pourmortazavia*, Mehdi Rahimi-Nasrabadib, Hossein Raib, Abdollah Javidanb
aFaculty of Material and Manufacturing Technologies, Malek Ashtar University of Technology, Tehran, Iran
bNano Science Center, Imam Hossein University, Tehran, Iran
Abstract:
1,3,6- threenitrocarbazole (TENT) is usually utilized in the formulation of composite explosives. This secondary explosive is used to reduce the sensitivity of the composite and also enhance the stability of the explosive composites. In this investigation, thermal behavior of TENT in the form of pure and nanocomposite explosives was examined by i.e., differential scanning calorimetery (DSC) and thermogravimetery (TG) as common thermal analysis techniques. Thermoanalytical data revealed that thermal decomposition of pure TENT is significantly different from the investigated nanocomposites. The results confirm that pure TENT decomposed wholly in a single stage at the temperature range of 400-450 ºC. Though, adding of the nanoparticles to the TENT powder leads to higher thermal stability in comparison with the pure TENT. Decomposition kinetic of the pure TENT and the nanocomposites were studied by non-isothermal DSC at diverse heating rates. The resulted thermokinetic and thermodynamic parameters for the thermal decomposition of pure TENT were compared with the nanocomposites.
Key Words: Thermal behavior; Nanocomposites; Thermokinetic; 1,3,6-threenitrocarbazole; Energetic compositions.
To whom correspondence should be addressed: S. M. Pourmortazavi, P.O.Box 16765-3454, Tehran, Iran. Fax: 0098212936578, E-mail:
1. Introduction
1,3,6-threenitrocarbazole (TENT) has the chemical structure shown in Fig. 1. This nitroaromatic compound might be used as the secondary explosive [1,2]. Since Second World War, TENT has been used by Germany as black powder inheritor due to the non-corrosive and non-humidity properties of this explosive. Nowadays, TENT is used in the formulation of explosive composites in the presence of 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX) and 1,3,5-trinitro-1,3,5-triazinane (RDX) for reducing the sensitivity and enhancing the stability of the explosive compositions [2,3]. Furthermore, TENT could be used in car airbags as the catalyst and initiator of decomposition [4]. Also, TENT in civil industry is applied as an insecticide [3,4].
Usually, organic/inorganic nanocomposites comprise one or more nanomaterials. The nanocomposites have been widely explored during recent years [5-9]. Nanomaterials, depending on the number of dispersed particle dimensions in the nano-scale, may be categorized to three major groups, i.e., nanoparticles, nanolayers and nanotubes [10-13]. The main inorganic nanomaterials, which are used widely in nanocomposites as the fillers, are includes nanotubes (mainly carbon nanotubes or CNTs), the layered silicates (e.g., montmorillonite and SiO2), metal oxides (i.e., Al2O3, TiO2, Fe2O3), pure or alloyed metals nanoparticles (i.e., Au, Ag), the semiconductors (i.e., PbS, CdS), and also the nanodiamond [5,14]. These nanoparticles are utilized to enhance the physical and mechanical properties of the organic and polymer compounds in the diverse composites [15]. Meanwhile, the influence of nanoparticles on improving the flame retardancy and thermal degradation of nanocomposites is well known [16].
Thermal analysis methods have significant roles in the study of energetic materials not only for sympathetic the kinetics of their thermal decomposition, but also for assessing the information on the potential hazards during the storage, processing, handling, and also approximating their shelf-life [17-20]. Furthermore, thermokinetic data provide valuable information on thermal stability and life expectation of the energetic materials during the storage period [21-22].
In of this work, thermal behavior of pure TENT was characterized by the use of thermogravimetery (TG) and differential scanning calorimetery (DSC) techniques and then, the special effects of two different nano-sized inorganic compounds, i.e., Al2O3 and TiO2 nanoparticles and carbon nano-tubes (CNTs) on the thermal stability of TENT were studied. Furthermore, thermokinetic and thermodynamic parameters correspond to the decomposition of pure TENT and both nanocomposites were calculated. To the best our knowledge, there is no report available on the thermal degradation kinetic of TENT.
2. Experimental
2.1. Materials and instruments
N-hexane as the wetting solvent was purchased from Merck. TENT was synthesized as reported previously [23] and the product of synthesis reaction was recrystallized three times from acetone (purity of 99.7%). Nanomaterials, i.e., titanium oxide and aluminum oxide nanoparticles with the average particle size of respectively 40 and 50 nm were prepared in the material laboratory by flame combustion technique.
TG/DSC studies were carried out using a simultaneous thermal analyzer (STA) instrument comprising thermogravimetric analyzer and differential scanning calorimetery attachment. TG/DSC experiments were carried out at either heating rates of 10, 15, 20, and 25 °C.min-1 from 40 °C up to the end of the decomposition reaction. Thermal analyses were performed at the nitrogen atmosphere at 1 bar and purging flow rate of 50 ml min-1, while 3 mg of the samples was placed in an alumina pan with a perforated cover.
2.2. Preparation procedure of nanocomposites
The energetic nanocomposites were prepared by dispersion of the nanomaterial (10% with respect to the weight of TENT) in the 10 ml of wetting solvent (n-hexane) and its sonication during 10 min. The TENT powder was then added to the container involving nanomaterial / hexane and the mixture was again sonicated during 20 min. The resulted homogeneous mixture was subjected to the solvent evaporation in order to remove the n-hexane from the TENT. Thus, the vessel containing the mixture was coupled to the vacuum pump to evaporate the n-hexane from the mixture.
3. Results and discussion
3.1. Characterization of synthesized polymers
The prepared energetic samples were characterized by the SEM technique to determine the dispersion of the nanoparticles through the sample. Figure 2a presents the SEM images of titanium oxide and aluminum oxide nanoparticles prepared via flame combustion technique. As seen in this figure, the prepared titanium oxide and aluminum oxide nanoparticles have the average particle size of about 40 and 50 nm, respectively. On the other hand, Fig. 2b shows the SEM image corresponds to the obtained energetic nanocomposites. As seen in the images of the prepared Al2O3/TENT and TiO2/TENT nanocomposites, the spherical nanoparticles are dispersed through the sample.
3.2. Thermal properties of pure TENT and nanocomposites
The TG/DSC curves of the pure TENT are presented in Fig. 3. As seen, the DSC curve of TENT reveals no considerable change in thermal pattern of the energetic material up to 290°C. This curve exhibits an endothermic phenomenon near 299.7°C which is corresponding to the melting of the TENT. Thereafter, thermal decomposition of TENT was carried out during a sharp exothermic peak at the temperature of 412◦C. Thermal decomposition of TENT was along with a mass loss about 100%, which is exposed in the TG curve of the sample. From the DSC curve of the sample in Fig. 3, it could be established that there is a long interval between TENT fusion temperature (299.7°C) and its rapid thermal decomposition at 412◦C.
The TG/DSC curves of the TENT-TiO2 nanocomposite showed no thermal phenomenon prior to the melting of TENT near 300°C. Thereafter, two exothermic peaks above fusion temperature were observed, which are corresponding to thermal decomposition of the TENT. These decomposition steps continued at about 650°C until the decomposition of the sample is completed, while, TG curve established completeness of the sample decomposition. In fact, this energetic nanocomposite decomposed at first at the peak temperature of 432.8 °C. However, the second decomposition was occurred during an exothermic phenomenon at peak temperature of 551.7 °C. On the other hand, TG curve of the TENT-TiO2 sample revealed a continuous mass loss during these exothermic steps with about 85% decrease in the total mass of sample.
TG/DSC curves of the TENT-Al2O3 confirmed that thermal behavior of the nanocomposite is comparable to the TENT-TiO2 sample. However, the DSC curve of this sample exhibits an endothermic peak and two exothermic peaks at the temperatures of 429.5 and 540.2 ºC. Meantime, the endothermic peak appeared at 294.3 °C is responsible for the fusion of TENT in the nanocomposite, while the TG shows no decrease in the mass of sample at this temperature. The first exothermic peak observed at 429.5 ºC originates from initiation of thermal decomposition of the TENT-Al2O3 nanocomposite and the next peak (at 540.2 °C) is responsible for the final stage of nanocomposite decomposition. Furthermore, the TG curve of the TENT-Al2O3 sample exhibits an incessant 88% total mass loss during the exothermic phenomena. Table 1 gives a summary TG/DSC data about thermal behavior of the studied energetic samples.
3.3. DSC curves and decomposition kinetic studies
DSC curves of the pure TENT, TENT-Al2O3, and TENT-TiO2 samples at various heating rates are given in the Fig. 4. As expected [24], the samples exhibit a similar trend and showed the higher peak temperatures by raising the DSC heating rate. The kinetic parameters of thermal decomposition of pure TENT, and the nanocomposites, i.e., TENT-Al2O3, and TENT-TiO2 were computed through the non-isothermal DSC data obtained at different heating rates (i.e., 10, 15, 20, and 25ºC/min). Arrhenius parameters (i.e., activation energy and frequency factor) correspond to thermal decomposition of the studied nanocomposites were calculated by ASTM E698 [25]. This was carried out by using the presented data in Table 2 and through the plotting Ln(β.Tm-2) against 1/Tm; while β and Tm are respectively the DSC heating rates and maximum DSC peak temperatures. The resulted plots were three straight lines for the pure TENT (r = 0.995), TENT-Al2O3 (r = 0.999), and TENT-TiO2 (r = 0.998). These regression coefficients confirm that the mechanism of thermal decomposition of the studied energetic samples at the examined heating rates undergoes no deviation during their thermal decomposition [25, 26]. Thereafter, the activation energies of thermal decomposition of the energetic samples were acquired from the slopes (-Ea/R) of these lines. The calculated activation energies were utilized to predict the logarithm of frequency factor, log (A/S-1), by the following equation as proposed in the ASTM E698:
A= β (Ea/RTm2) exp (Ea/RTm) (1)
The results of calculations on activation energy and frequency factor for the energetic nanocomposites as well as pure TENT are shown in Table 3. Meantime, Starink method was utilized to calculate the activation energy (Ea) values of the studied samples. In this thermokinetic method, activation energy might be calculated by plotting Ln(β.Tm-1.92) vs. 1/Tm. The Starink method as well as the ASTM has a potential to predict the value of activation energy without an accurate awareness about the mechanism of reaction as proposed in follows [27, 28]:
Ln(β/Tm1.92)+ 1.0008Ea/RTm= C (2)
The presented data for the studied samples in Table 2 was used to plot the Ln(β/Tm1.92) against the reciprocal of maximum peak temperature. The results were straight lines with the linearity coefficient (r) of 0.996 for pure TENT, 0.999 for TENT-Al2O3, and 0.998 for TENT-TiO2. These results confirm the absence of any variation in the thermal decomposition mechanism of the studied samples over the utilized heating rates [26]. Thus, values of the frequency factor (A) correspond to the activation energy of the samples were obtained by this method as proposed by equation (1). The computed values of the Arrhenius parameters by both methods, i.e., ASTM and Starink are given in Table 3. The resulted data reveal that the activation energy for all samples obtained by Starink method is slightly higher than those of ASTM. Meantime, similarity in the values of activation energy for the samples establish the good agreement between the both utilized methods, i.e., ASTM with Starink.
Thereafter, the thermodynamic parameters concern with the activation of the decomposition process of the studied energetic samples were predicted by the subsequent expressions; while, the input was thermokinetic parameters resulted in by ASTM and Starink methods shown in Table 3. The thermodynamic parameters correspond to the activation of thermal decomposition reaction of the samples (i.e., free energy of activation (ΔG#), enthalpy of activation (ΔH#), and entropy of activation (ΔS#)) were computed by subsequent equations [29-31]:
(3)
(4)
(5)
While, in the equation (3), υ=KBT/h (KB is the Boltzmann constant and h is the Plank constant). Table 3 gives these calculated thermodynamic parameters for the pure TENT and the studied energetic nanocomposites. The resulted thermodynamic values are valuable since these data obtained based on the maximum peak temperatures (Tm) of the DSC curves. In fact, the data characterize the highest decomposition rate of the pure TENT and the nanocomposites.
3.4. Decomposition reaction rate constants
The reaction rate constants (k) for decomposition reaction of the pure TENT and nanocomposites were determined using equation (6), where the mechanisms of the decomposition reactions were assumed as the first- order [32, 33]:
Log k = Log A – Ea/2.3RT (6)
The decomposition reaction rate constant (k) for the studied samples were computed based on the values of activation energies (Ea) and frequency factors (A) obtained by ASTM and Starink methods at the temperature of 25ºC. The resulted values of log k for pure TENT, TENT-Al2O3, and TENT-TiO2 are presented in Table 3. Comparison of the pure TENT reaction rate constant with the TENT-Al2O3 and TENT-TiO2 reaction rate constants exhibited that decomposition reaction rate constant of pure TENT is slightly lower than that calculated for the TENT- TiO2. However, this parameter for TENT-Al2O3 nanocomposite is considerably lower than pure TENT and TENT-TiO2. The lower reaction rate constant for TENT-TiO2 confirms that this nanocomposite has a higher half-life rather than pure TENT and TENT-TiO2 in the identical storage conditions.
3.5. Critical ignition temperature
Critical ignition temperature (Tb) is defined as the lowest temperature for a specific compound that might be heated without undertaking thermal runaway. Determination of this parameter is vital for energetic compounds in order to sate handling and storage [34, 35]. This temperature might be predicted using combustion theory and utilizing the thermokinetic data of the compound, i.e., activation energy, pre-exponential factor, and heat of thermal decomposition reaction. In this work, thermal ignition temperature (Tb) for the pure TENT and nanocomposites of TENT-Al2O3 and TENT-TiO2 was estimated by means of equations (7) and (8) [34]:
(7)
(8)