Premature thermal decomposition behavior of 3,4-dinitrofurazanfuroxan with certain types of nitrogen-rich compounds

Jiao Huang,Ru-fang Peng ,Bo Jin

State Key Laboratory of Environment-friendly Energy Materials,Southwest University of Science and Technology,Mianyang,621010,Sichuan,PR China

Keywords:DNTF N-H rich Nitrogen compounds Advanced thermal decomposition peak

ABSTRACT 3,4-Dinitrofurazanfuroxan(DNTF),as a high-energy-density material,features good thermal stability and wide applications.This study aimed to elucidate the thermal decomposition mechanism of DNTF combined with nitrogen-rich compounds containing N-H.The thermal stabilities of DNTF and its hybrid systems were investigated using differential thermal analysis/thermogravimetry (TG),vacuum stability test,and accelerating rate calorimetry under isothermal,non-isothermal,and adiabatic conditions,respectively.Results showed that the thermal stability and thermal safety of DNTF significantly decreased after combining with nitrogen-rich compounds containing N-H.Calculation results showed that the activation energy of the DNTF hybrid systems was significantly lower than that of DNTF.The TGIR was used to monitor the generation of fugitive gases during the thermal decomposition of the DNTF/5-aminotetrazole (5-ATZ) hybrid.Moreover,the nitrogen-rich molecules containing N-H interacted extensively with DNTF,and this interaction accelerated the thermal degradation of DNTF.

1.Introduction

High-energy-density materials[1-3]are widely used in defense science and technology for energy storage,weapons security,and high power.In 1987,the high-energy-density material CL-20 [4,5]was introduced to current all-nitrogen/nitrogen-rich compounds.The current research on high-energy-density materials is focused on nitrogen-rich compounds [6-9],which are widely studied because of their high nitrogen content,density,positive heat generation,burst performance,and thermal stability.

Introducing N-O coordination bonds in nitrogen-rich compounds can increase their density,oxygen balance,burst performance,sensitivity,and stability [10].3,4-Dinitrofurazanfuroxan(DNTF) is a typical coordination oxygen compound whose molecular structure contains furazan ring and oxidized furazan rings,which can form reactive oxygen atoms and thus increase its density[11-13].The low melting point and high density and energy of DNTF make it a key raw material for the study of high-performance explosives[14-17].Over the years,many scholars have studied the synthesis [18],physical properties [10,19-21],solubility [11,22],and applications[23,24]of DNTF.In 2010,Ren et al.[19]studied the fast thermal cracking of DNTF via T-Jump/Fourier-transform infrared spectroscopy and found that the fast-cracking products of DNTF are CO,CO2,NO,and NO2.Sinditskii et al.[25]studied the thermal decomposition of molten DNTF under isothermal conditions and found that it is close to that of HMX.The thermal decomposition of DNTF at different pressures and its interaction with a catalyst were investigated using differential scanning calorimetry (DSC) and thermogravimetric analysis (TG).Zhang [26]et al.studied the thermal stability of DNTF using accelerating rate calorimetry (ARC) and found that the initial exothermic decomposition temperature of DNTF under adiabatic conditions is 180.7°C,which indicates good thermal stability.Despite numerous studies on DNTF and some reports on the thermal decomposition and thermal stability of DNTF,there are few reports on the effects of nitrogen-rich compounds on the thermal decomposition behavior of DNTF.As a promising high-energy-density material,the compatibility of DNTF with other components in mixed explosives is crucial.Li et al.[27]investigated the compatibility of DNTF with TNT,TATB,NTO,LLM-105,ANPYO,DNT,and waxes through DSC and showed that DNTF is incompatible with LLM-105,ANPYO,DNT and waxes.In 2013,Li et al.[28]found that DNTF and DAAzF showed poor compatibility through DSC research.Poor compatibility will affect the safety and service life of mixed explosives,resulting in dangerous accidents.Therefore,it is of great significance to study the compatibility between DNTF and nitrogen rich compounds with specific structures.

In this work,we explored the thermal decomposition of the hybrid systems of DNTF and nitrogen-rich compounds containing active-H.The thermal decomposition behavior of the DNTF hybrid systems was evaluated using isothermal (vacuum stability test,VST),non-isothermal (differential thermal analysis (DTA)/TG),and adiabatic (ARC) analyses.Results showed that the decomposition peaks of DNTF advanced after combining with nitrogen-rich compounds containing active-H.The thermogravimetric-infrared (TGIR)technique was used to verify the presence of N-O compounds in the DNTF/5-aminotetrazole (5-ATZ) hybrid system at approximately 190°C.

2.Experimental section

2.1.Materials

DNTF with a purity of 99.4% was obtained from the Chinese Academy of Engineering Physics.5-ATZ and tetrazole (TZ) were purchased from Macklin.1,2’-diamino-5,5’-bistetrazole (DABTZ)[29],5,5’-bistetrazole (BTZ) [30],and diamino-furazan (DAF) [31]were synthesized independently as previously described.The molecular structures of the related compounds are shown in Fig.1.

Fig.1.Molecular structure of DNTF and six nitrogen-rich compounds.

2.2.Experimental equipment and conditions

All DTA curves were obtained on a WCR 1/2D instrument (Beiguang Hongyuan Instrument Co.,Ltd.,Beijing) under an air atmosphere using a ceramic crucible presentation sample with a 1:1 mass ratio of DNTF and nitrogen-rich compounds.TG was executed on an SDT Q160 (TA Instrument Co.,USA) with a heating rate of 10°C/min.An ARC 245 instrument (NETZSCN,Germany) was utilized to examine the self-exothermic phenomena of the samples under adiabatic conditions after a heating-wait-search mode [32].The operating temperature was 60-350°C,and the test sample volume was approximately 30 mg.The test sample ball was composed of titanium with a mass of 3.0543 g,a heating rate of 5°C/min,and an exothermic threshold of 0.02°C/min.The differential scanning calorimetry (DSC) was operated on a Q200 (TA Instrument Co.,USA) under a nitrogen (N2) atmosphere.TG-IR was recorded on an STA449F5-INVENIO R (NETZSCN,Germany) instrument with a heating rate of 10°C/min and under a N2atmosphere.A laboratory-made vacuum stability device was employed to examine the variation in pressure of the samples with time under isothermal conditions at a test temperature of 100°C and a test volume of 20 mg.

3.Results and discussion

DTA and TG results showed that the decomposition peaks of DNTF combined with nitrogen-rich compounds containing N-H were 40°C-120°C earlier than those of DNTF.VST results indicated that the thermal stability of the DNTF hybrid systems was significantly inferior to that of DNTF,and ARC results demonstrated that the exothermic peak of the DNTF hybrid systems was significantly lower than that of DNTF under adiabatic conditions.TG-IR data revealed a distinctive interaction between DNTF and nitrogen-rich compounds containing N-H.

3.1.DTA measurements

Fig.2 shows the DTA plots of six nitrogen-rich compounds containing N-H bonds in an air atmosphere at 20°C.The thermal decomposition temperatures of the DNTF hybrid systems were significantly advanced compared with those of the single systems.Fig.3 displays the temperature difference between the DNTF hybrid systems and the DNTF single system under identical condition.The decomposition temperatures of the DNTF hybrid systems were all immensely advanced,ranging from 40 to 110°C.

Fig.2.(a)-(f) DTA curves of six nitrogen-rich compounds at 20 °C/min heating rate.

Fig.3.Decomposition temperature and temperature difference of DNTF and its hybrid systems under the same conditions.

Fig.4 shows the DTA test curves for DNTF and its hybrid systems at different heating rates under air atmosphere.As shown in Fig.4(a),the melting point of DNTF at atmospheric pressure ranged from 107.9°C to 113.5°C,and its thermal decomposition peak maximum was 260.9-282.4°C,which is consistent with the results reported in the literature[33,34].Fig.4(b)-Fig.4(g)show the DTA curves of DNTF combined with six nitrogen-rich compounds containing N-H at different heating rates.The melting point peak of DNTF appeared in the DTA curve of the DNTF hybrid system.An obvious decomposition peak that is smaller than the decomposition peak of the single nitrogen-rich compound was also found,but the decomposition peak of the single nitrogen-rich compound was not observed.Among these samples,DNTF/AMTZ and DNTF/DAF have two distinct decomposition peaks compared with other samples.

Fig.4.Non-isothermal DTA curves of DNTF and its hybrid systems at different heating rates (air atmosphere).

Subsequently,several important kinetic parameters of DNTF and its hybrid systems were calculated using the Kissinger and Ozawa iterative methods on the basis of the relationship between the exothermic peak temperature and the heating rate to analyze the difference in energy required for the molecules to reach activation.This type of method considers the slow variation ofH(μ) andQ(μ)with μ,without the limitation of μ range [35].By iterating several times to a reasonableEvalue satisfyingEi-Ei-1＜100 J/mol,the relevant equations are expressed as follows:

(Kiterative method)

(Oiterative method).where β denotes the heating rate in°C/min,Ais the pre-exponential factor,Eis the activation energy,Tis the peak temperature,andRis the ideal gas constant.The DTA data at different heating rates are substituted into Eq.(1) and Eq.(2),respectively,and the fitting curves obtained are shown in Fig.S1.The correlation coefficients (R2) between the fitted curves and the experimental points were greater than 0.98,and the linear relationships were good.The apparent activation energies of DNTF and its hybrid systems were calculated after 2-4 iterations of the above method,and the results are exhibited in Table 1,which directly shows that the advanced (E) of DNTF (149.6 kJ/mol) is significantly larger than that of its hybrid systems under the same test conditions.In addition,the smaller activation energy of the DNTF hybrid systems compared with DNTF indicated that the energy required to reach thermal decomposition was lower in the hybrid systems than in the single system.

Table 1Kinetic parameters of DNTF and its hybrid systems calculated using the iterative method of equal conversion rates.

The DSC curves of DNTF and its hybrid systems at different heating rates (5°C/min,10°C/min,15°C/min,20°C/min) are represented in Fig.5.The obtained kinetic parameters by the equal conversion Kissinger-Akahira-Sunose (KAS) method [36-39]to understand the changes in their decomposition process.The relevant equations are as follows:

Fig.5.Non-isothermal DSC curves of (a) DNTF;(b) DNTF/DAF and (c) DNTF/AMTZ systems at different heating rates (N2 atmosphere).

(KAS method)

where βiis heating rate,Eα is activation energy,Tα is the temperature at arbitrary conversion values,Ris ideal gas constant.The activation energy is shown in Table 2 when the temperature of different conversion is substituted into the equation and the linear fittings (Fig.S2) are performed.The activation energy of DNTF did not change obviously at the conversion rate of 0.1-0.5,but increased obviously at the quasi-conversion rate of 0.6-0.8,and then decreased.In contrast to DNTF,the activation energy of DNTF/DAF and DNTF/AMTZ system decreased at first and then increased,and the thermal degradation rate decreased at first and then increased,which indicated that the reaction process changed greatly with temperature and the decomposition process is complex.

Table 2Kinetic parameters of DNTF and its hybrid systems calculated using the KAS method.

3.2.Vacuum stability tests under isothermal conditions

The amount of gas produced under the same conditions is usually a criterion for stability evaluation[40].The vacuum stability(VST) [41]of DNTF and its hybrid systems under isothermal conditions was investigated using the method based on gas production.As shown in Fig.6(a),the decomposition pressure and gas production rate of the DNTF hybrid systems under the isothermal condition of 100°C were significantly higher and faster,respectively,than those of DNTF with the extension of time.In specific,the gas production per unit mass of DNTF,DNTF/5-ATZ,and DNTF/TZ were 3.21 mL,17.98 mL,and 152.05 mL after 1440 min,respectively.The smaller the gas yield,the better the stability [42].In the present study,the nitrogen-rich compounds containing N-H significantly increased the gas production rate and gas yield of DNTF through thermal decomposition.Thus,the stability of the DNTF hybrid systems was considerably lower than that of DNTF.

Fig.6.(a)Isothermal VST of DNTF and its hybrid systems at a constant temperature of 100 °C(mass: 30 mg);(b) Deflation volume per unit mass of the sample at a constant temperature of 100 °C for 24 h.

3.3.TG measurements

A non-isothermal TG test under nitrogen atmosphere was performed on the DNTF hybrid systems,and the results are shown in Fig.7(a)-Fig.7(f).As shown in Fig.7(a),the temperature test range of 65-500°C(test range)had only one step of weight loss,and the maximum weight loss temperature was 190.89°C,indicating that the DNTF/5-ATZ hybrid system underwent one-step thermal decomposition.This finding agrees with the DTA test results.The other DNTF blends also started to lose weight after 100°C,and the maximum weight loss also occurred before 200°C.For the TG-DTG plot of the DNTF/BTZ samples (Fig.7(e)),a significant secondary weight loss occurred at 264.4°C,which belongs to the thermal decomposition peak of BTZ.This phenomenon may be attributed to the small amount of sample and the lumpy form of BTZ samples in the test.

Fig.7.(a)-(f) Non-isothermal TG-DTG curves of DNTF hybrid systems in a N2 atmosphere (HR: 10 °C/min).

3.4.Thermal analysis results of adiabatic conditions by ARC

The thermal decomposition of DNTF and its hybrid systems under adiabatic conditions was investigated to assess the safety of the samples during storage[43-45](Fig.8).As shown in Fig.8(a),the initial setting temperature of DNTF was 60°C,and DNTF decomposition did not occur at this temperature.After several heating-waiting-searching operation cycles,the exothermic effect of DNTF was detected at 206.1°C,and the temperature rise rate was 0.024°C/min at this time.The DNTF hybrid systems showed an identical trend to DNTF,but the initial self-exothermic temperature of the DNTF hybrid systems was significantly lower than that of DNTF,and the duration of the exothermic reaction was greatly shortened,as shown in Table 3.These results indicated that the initial decomposition temperature of DNTF at the maximum temperature rise rate is 212.8°C,which indicated high thermal stability.By contrast,the decomposition temperatures of the DNTF hybrid systems at the maximum temperature rise rate were between 110°C and 170°C,which suggested their poor thermal stability under adiabatic conditions.

Table 3Decomposition characteristics parameters of DNTF and its hybrid systems in the self-acceleration phase(adiabatic).

Fig.8.ARC temperature and pressure versus time of (a) DNTF;(c) DNTF/5-ATZ;(e) DNTF/DAF;(g) DNTF/AMTZ;(i) DNTF/BTZ and (k) DNTF/TZ;ARC temperature and self-heating rate versus time of (b)DNTF;(d) DNTF/5-ATZ;(f) DNTF/DAF;(h) DNTF/AMTZ;(j) DNTF/BTZ and (l) DNTF/TZ.

3.5.TG-IR characterizations

TG-IR can be used to rapidly and intuitively analyze the structure and decomposition mechanism of the thermal decomposition product,thereby allowing to elucidate the mechanism of action of effective escape gas [46,47].In the present study,the TG-IR technique was employed to investigate the thermal decomposition behavior of DNTF/5-ATZ in a N2atmosphere at a heating rate of 10°C/min,and the experimental results are represented in Fig.9.Fig.9(a) depicts the TG curves of the samples in the weight loss phase at temperatures ranging from 60 to 500°C.The sample remained in a thermally stable state after melting,and no weight loss was observed.The weight loss started at 176.1°C and reached a maximum temperature of 185.1°C,which differed from our pretest TG experiments.This difference can be attributed to the different test sample amounts and instruments used.The 3D TG/IR spectra of DNTF/5-ATZ during thermal degradation are shown in Fig.9(b),and the results indicate that the sample decomposition began before 200°C.The IR spectra at 150.1°C,188.6°C,195.4°C,280.3°C,and 353.9°C were obtained and are presented in Fig.9(c).The main gas products of DNTF/5-ATZ at 195.35°C are affiliated with N2O (2248 cm-1),and NO (1801,1945 cm-1),HCN (712,333 cm-1),and NH3(966 cm-1)[48-51]gases were also produced at the same time,which is consistent with the previous thermal analysis and test results,further proving that the thermal decomposition temperature of DNTF/5-ATZ mixed system is ahead,which directly indicates that DNTF interacts with 5-ATZ,possibly because H protons produced during the decomposition of small-molecule compounds accelerate the decomposition of DNTF,and H free radicals also promote the decomposition of small molecular compounds themselves,resulting in mutual promotion after mixing,showing inferior thermal stability.

Fig.9.(a)TG-DTG curves of DNTF/5-ATZ in a N2 atmosphere(HR:10 °C/min);(b)3D TG/IR spectra of DNTF during thermal decomposition;(c)FT-IR spectra of DNTF/5-ATZ during thermal decomposition at different temperatures;(d) IR absorbance profiles of off-gases.

4.Conclusions

The main decomposition temperature of DNTF ranges from 260 to 280°C.The thermal decomposition temperature of DNTF combined with nitrogen-rich compounds containing N-H bonds is significantly lower than that of DNTF alone.Calculation results show that the activation energy of the DNTF hybrid system is lower than that of DNTF,and the non-isothermal TG-DTG curves divulge that the weight loss of the DNTF hybrid system is a one-step process.The results of the adiabatic self-acceleration study indicate the poor safety of DNTF and its hybrid systems.The thermal decomposition of DNTF/5-ATZ is further corroborated by TG-IR,indicating that an interaction occurs between DNTF and nitrogen-rich compounds containing N-H,and this interaction accelerates the thermal decomposition of DNTF.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors appreciate the financially sponsor of the Natural Science Foundation of China(Grant No.51972278),the Outstanding Youth Science and Technology Talents Program of Sichuan (Grant No.19JCQN0085),and the Open Project of State Key Laboratory of Environment-friendly Energy Materials (Southwest University of Science and Technology,Grant No.21fksy19).

Appendix A.Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.dt.2022.07.002.