Thermal behavior and non-isothermal decomposition kinetics of 3，4-bis(3'，5'-dinitrophenyl-1'-yl)furoxan ①

ZHANG La－ying，ZHU Xin－hua，QING Hui，HUO Huan，YI Jian－hua，BAI Juan，DING li

(1.Xi＇an Modern Chemistry Research Institute，Xi＇an 710065，China;2.Military Representatives Office of PLA in 845 Factory，Xi＇an 710302，China)

0 Introduction

Furazan energetic compounds are pivotal in the field of energetic material because of their unique properties［1］.The furazan－based energetic derivatives are of higher energy density，higher standard enthalpy of formation(ΔHf)，higher nitrogen content，and good heat resistance，because of these advantages，they are a potentially valuable type of energetic compounds［2－4］.DNTF(3，4－dinitrofurazanfuroxan)is a representative of such compounds，and many experiments and investigations have been carried out by researchers throughout the world in regards to its synthesis，molecular structure，thermal behavior and explosive performance，together with its applications in solid propellants［5－10］.

For explosives，the density and detonation velocity can be improved 0.06 ～0.08 g·cm－3and 300 m·s－1，respectively.When the nitryl in their benzene ring systems is substituted by oxidized furazan［11］.In order to obtain a better understanding of the chemical stability of DNTF and seek out novel energetic materials，a new furazan－based energetic derivative—3，4－Bis(3＇，5＇－dinitrophenyl－1＇－yl)furoxan(BDPF)was designed and synthesized［12］.In this paper，the BDPF＇s thermal behavior was investigated by means of DSC，PDSC and TGDTG，and the non－isothermal decomposition reaction kinetics.

1 Experimental

1.1 Sample

BDPF was prepared according to the reported method［12］.Analysis calculation for C14H6N6O10:C 40.15，H 1.494，N 19.68;found(%)C 40.19，H 1.435，N 20.10.IR(KBr)ν:3 085，1 544，1 351，1 614，1 462，99 cm－1.1H NMR(DMSO － d6，500 MHz)δ:8.951 ～8.947(d，J=2.0 Hz，1H)，8.902 ～8.899(d，J=1.5 Hz，1H)，8.822 ～ 8.802(dd，J=10.0 Hz，1H)，8.595 ～8.574(dd，J=10.5 Hz，1H)，8.240 ～8.233(d，J=3.5 Hz，1H)，7.762 ～7.745(d，J=8.5 Hz，1H);13C NMR(CD3CN)δ:153.3，149.6，149.3，147.4，147.1，135.0，134.1，129.2，129.0，124.3，121.6，121.0，120.9，113.1(14C);Purity，＞99%.

The molecular structure of BDPF is shown in Fig.1.

Fig.1 Scheme of the BDPF molecular structure

1.2 Thermal decomposition conditions

The differential scanning calorimetry(DSC)curves were obtained by a 204 HP differential scanning calorimeter(Netzsch，Germany)under dynamic flowing nitrogen gas(purity，99.999%).The conditions of high－pressure DSC analyses were:N2flowing rate，50.0 cm3·min－1;heating rate，10 K·min－1;sample mass，0.7 mg;pressure，0.1，2.0，5.0 MPa;the non－isothermal DSC curves at 0.1 MPa were analyzed at the heating rates of 5.0，10.0，15.0，20.0 K·min－1.The DSC curves obtained under the same conditions are consistent with each other，indicating that the reproducibility of the tests is satisfactory.

Thermogravimetry and differential thermogravimetry(TG－DTG)experiments for BDPF were performed by using a model TA2950 apparatus(TA，USA)under a nitrogen atmosphere(flow rate:100.0 mL·min－1，sample mass:1.0 mg，heating rate:10.0 K·min－1，temperature range:293～723 K).

2 Results and discussion

2.1 Thermal behavior

Typical DSC and TG/DTG curves of BDPF are shown in Fig.2 and Fig.3.

Fig.2 DSC curve of BDPF at a heating rate of 10 K·min－1

Fig.3 TG-DTG curves of BDPF ata heating rate of 10 K·min－1

From the DSC curve，it is observed that the thermolysis process of BDPF could be divided into three stages.The initial stage is a melting process of BDPF，and the extrapolated onset temperature(Te)，peak temperature(Tm)and melting enthalpy(ΔHm)of this stage obtained at the heating rate of 10 K·min－1are 498.7 K，500.7 K and 100.8 J·g－1，respectively.The next two stages are the exothermic reaction processes.The first exothermic decomposition process occurs at the temperature range of 502.6 ～ 597.7 K with a mass loss of about 47.78%while the latter exothermic decomposition process occurs at 636.5 ～757.5 K with a mass loss of about 13.13%.

The PDSC curves of the BDPF at 0.1，2.0，5.0 MPa is shown in Fig.4.It is shown that the thermal decomposition process of BDPF is slightly affected by the ambient pressure.The main peak temperature of the exothermic change of BDPF is increased when the pressure is raised，which is 547.3，548.7 and 548.8 K at 0.1，2.0 and 5.0 MPa，respectively.The heat release of BDPF thermolysis is also increased with the increasing pressure，which is 1 995，2 332 and 2 529 J·g－1at 0.1，2.0 and 5.0 MPa，respectively.

Fig.4 DSC curves of BDPF at different pressure at a heating rate of 10 K·min－1

2.2 Non-isothermal decomposition reaction kinetics

In order to obtain the kinetic parameters(the apparent activation energy(Ea)and pre－exponential factor(A))of the exothermic decomposition reaction for BDPF，a multiple heating method(Kissinger＇s method［13］and Ozawa＇s method［14］)was employed at 0.1 MPa.The Kissinger and Ozawa equations are listed as follows:

Kissinger＇s Eq.:

Ozawa＇s Eq.:

The above mentioned values determined by Kissinger＇s method and Ozawa＇s method are listed in Table 1.Based on the calculated results，the apparent activation energy obtained by Kissinger＇s method is consistent with that obtained by Ozawa＇s method，and their linear correlation coefficients(r)are close to 1.So，the results are credible.In Table.1，subscript k data is obtained by Kissiger＇s method;subscript O data is obtained by Ozawa＇s method.

Table 1 Original data and calculated values of kinetic parameters for the exothermic decomposition reaction for BDPF determined from the DSC curves at various heating rates

By substituting the original data，βi，Tiand αi，i=1，2，3，…，from the DSC curves into Eq.(2)，the values of Eafor any given value of α were obtained and shown in Fig.5 and the Ozawa plots at varying conversions for the degradation are shown in Fig.6.The fitted straight lines are nearly parallel，which indicates that this method is applicable to our system in the conversion range studied.This fact suggests that a single reaction mechanism is operative.Then the values of Eawere steadily distributed in the α range of 0.02 ～0.70.The values were also used to check the validity of the activation energy obtained by the other methods.

The integral Eq.(3)～ (6)are cited to obtain the values of Ea，A and the most probable kinetic model function f(α)from a single non－isothermal DSC curve using the original data in the α range of 0.02 ～0.70［15］。Coats－Redfern equation:

MacCallum－Tanner equation:

?atava－?esták equation:

Agrawal equation:

where G(α)is the integral model function，dα/dt is the rate of conversion;T is the temperature at time t;α is the conversion degree.

Fig.5 Eavs α curve for the decomposition of BDPF by Ozawa＇s method

Fig.6 Ozawa plots at varying conversions for the degradation

Forty－one types of kinetic model functions in Ref.［15］and the original data tabulated in an additional table are put into Eq.(3)～ (6)for calculation，respectively.The kinetic parameters and the probable kinetic model function together with their appropriate values of linear correlation coefficient(r)，and standard mean square deviation(Q＇)，are presented in Table 2.The values of Eaand logA obtained by the four equations present some deviation，and the mean values are in approximately agree with those obtained by the Kissinger＇s method and Ozawa＇s method.Therefore，a conclusion can be drawn that the most probable kinetic model function of the main exothermic decomposition process of BDPF is classified asequation No.15［15］，andaccording to the unanimity rule of calculation results to each model equation，substituting f(α)withwith 180.66 kJ·mol－1and A with 1015.26s－1in Eq.(7):

The kinetic equation of the exothermic decomposition reaction may be described as:

Table 2 Calculated values of thermolysis kinetic parameters of the title compound

The value(Te0)of Tecorresponding to β→0 obtained by Eq.(9)taken from Ref.［16］is 498.6 K.

where n and m are coefficients.

The critical temperature of thermal explosion(Tb)obtained by Eq.(10)taken from Ref.［16］is 510.6 K.

where EOis the value of E by Ozawa＇s method.

3 Conclusion

(1)The thermal decomposition process of BDPF can be divided into three stages，including one melting process and two exothermic decomposition processes.The first decomposition process occurs at 502.6 ～ 597.7 K，with a mass loss of about 47.78%，while the second process occurs at 636.5 ～757.5 K，with a mass loss of about 13.13%.The peak temperature of exothermic change increase with the pressure increase，and the heat release during this process increase gradually with rising pressure.

(2)The thermolysis kinetics of BDPF was studied under non－isothermal conditions by DSC methods，and the governed kinetic equation of the exothermic process is obtained as followsexp(－2.173 0 ×104/T)，and the critical temperature for its thermal explosion is 510.6 K.

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