Studying The Influence Of Phase Transformation And Cocrystal On The Stablitity Of Typical Energetic Materials Based On Isothermal Thermal Decomposition Method

The thermal stability of energetic materials with different solid forms(polycrystalline and co-crystal)is significantly different.Therefore,the study on the thermal stability of energetic materials with different solid forms has important guiding significance for their practical production and application.On the one hand,for HMX,CL-20 and other simple explosives with thermal crystallization phenomenon,the conventional isokinetic heating research methods,such as differential scanning calorimetry(DSC)and thermogravimetric method(TG),obtain the thermal decomposition kinetic parameters of the crystal form after crystallization,but can't obtain the thermal decomposition data of the crystal form actually used at room temperature;On the other hand,for the third generation explosive CL-20,although it has been reported in the literature that the thermal stability can be improved to a certain extent by forming eutectic with other explosives,the mechanism of improving the stability by eutectic is not clear,and only less decomposition information can be obtained by using traditional rapid heating methods such as DSC and TG.Based on this,the thermal decomposition processes of polycrystalline energetic materials CL-20 and HMX,co-crystal energetic materials CL-20/TNT,CL-20/NQ and their mixtures were studied by isothermal thermal decomposition method.The effects of different solid forms such as co-crystal and crystal form on the thermal stability of CL-20 and HMX were explored,the corresponding isothermal thermal decomposition kinetic parameters were obtained,and the possible isothermal thermal decomposition mechanism was revealed.The main work includes the following two aspects:(1)The effects of different crystal forms on the thermal decomposition properties of HMX and CL-20 before and after thermal transformation were studied by isothermal thermal decomposition method.Firstly,466-469 K was determined as the temperature range of?-?crystal transformation of HMX by in-situ on-line XRD.The isothermal thermal decomposition law of HMX before and after crystallization at 443-483 K was studied,and the variation curves of gas pressure released from HMX decomposition with time under different isothermal conditions were measured.Combined with the model fitting calculation,the dynamic parameters of two crystal forms of HMX before and after thermal transformation are obtained.The results show that,the decomposition of?-HMX in the temperature range of 443-466 K conforms to the mechanism function model 6,that is,Jander equation,activation energy Ea=114.15 k J·mol^-1;The decomposition of?-HMX in the temperature range of 469-483 K conforms to model 26,that is,the power function law,Ea=87.99 k J·mol^-1.The experimental study determined 443-453 K as the temperature range of CL-20?-?crystal transformation.Therefore,the isothermal thermal decomposition law of CL-20 before and after crystal transformation at 423-468 K is studied.The variation of gas pressure released by CL-20decomposition with time under different isothermal conditions is obtained.The model fitting calculation results show that the temperature range of 423-443 K is the isothermal thermal decomposition of?-CL-20 conforms to the mechanism function No.28,that is,the reaction order n=1/4,the activation energy Ea=118.82 k J·mol^-1;it is?-CL-20 in the temperature range of 423-443 K,and its isothermal thermal decomposition conforms to the mechanism function No.28,the activation energy Ea=77.74 k J·mol^-1.Therefore,the high-temperature evaluation results can't truly reflect the decomposition state of energetic materials in low-temperature environment,especially explosives with thermally induced crystalline phase transition.The thermal stability evaluation of energetic materials can‘t be generalized.Various changes caused by temperature should be investigated and discussed according to different conditions.(2)The thermal decomposition law and mechanism of CL-20/TNT and CL-20/NQ eutectic explosives were studied by isothermal thermal decomposition method.The variation of gas pressure produced by decomposition of CL-20/TNT co-crystal energetic material and its mixture with time was measured under the isothermal condition of 443 K-463 K,and the kinetic parameters and reaction mechanism function were obtained by combining calculation.It was found that there was an obvious inflection point on the pressure-time curve of CL-20/TNT co-crystal,while there was no inflection point in the mixture curve.The residues at different decomposition depths were characterized by FT-IR and SEM.The results show that the decomposition of CL-20/TNT co-crystal is coordinated by CL-20 and TNT,and the decomposition of CL-20 and TNT in co-crystal affects each other.CL-20/NQ co-crystal energetic material and its mixture with time was measured at 423-463 K isothermal condition,and the kinetic parameters of thermal decomposition and reaction mechanism function were obtained by combining calculation.The residues at different decomposition depths were characterized by FT-IR and SEM.The results showed that the decomposition rate of the mixture was faster than that of CL-20/NQ co-crystal.In conclusion,the formation of CL-20/TNT and CL-20/NQ co-crystal can improve the thermal stability to some extent.