Research On Structure Characterization And Evolution Of Explosive Crystals Based On SAXS

Explosive crystals will inevitably produce defects during the growth process.Under the conditions of unexpected high temperature(fire,earthquake,sudden climate change),the thermal effect will also cause the crystal to produce pores,cracks,phase transitions,thermal expansion and thermal decomposition.These changes will change the physicochemical properties of the explosive crystals.The microstructure damage of the explosive crystals caused by itself has a higher shock wave sensitivity.In order to optimize the stability and function of explosives,it is necessary to explore the microstructure of explosive crystals.Small-angle X-ray scattering(SAXS)is a technology for obtaining information on the nanoscale structure of materials.Due to its non-destructive testing and good statistical advantages,it has a broad application prospect in the field of energetic materials microstructure testing.In this thesis,two typical explosives,HMX and CL-20,are selected as research objects,and a method for characterizing the nano-scale void structure of crystals based on SAXS is proposed.In-situ WAXS(Wide-angle X-ray scattering),in-situ SAXS and other technologies were used to study the evolution of pore structure under thermal stimulation of CL-20.The main work of the paper is as follows:1、The scattering intensity of the sample obtained by the small-angle experiment is a relative intensity,which is affected by external factors such as exposure time and sample size.The absolute scattering intensity is only related to the structure of the sample and has nothing to do with other factors,so correcting the absolute scattering intensity is very important for us to understand the true structure of the sample.The correction principle of absolute scattering intensity is given,and the calculation method of parameters such as volume fraction and specific surface of the scatterer and the operation process of data processing software are introduced in detail.2、Due to the influence of crystal surface effects,SAXS has a deviation in the characterization of the crystal microstructure.In order to solve the influence of crystal surface effect on the internal defect signal,the microstructure of Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine(HMX)crystal was characterized by soaking the sample in the matching solution.We found that the absolute scattering intensity,specific surface and volume fraction of the sample scatterer in the matching solution are significantly lower than the initial sample,which solves the influence of the crystal surface effect on the test results.Comparing the scattering results of the samples in different electron density matching solutions,it was found that the best scattering data can be obtained in the GPL-107 perfluoropolyether(PFPE)matching solution,and the same law was obtained by controlling experiment with 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane(CL-20)crystal.The scattering invariants Q of the samples in matching solutions with different electron density were fitted,and the true pore volume fraction of samples was calculated based on the fitted results.The density calculated from the theoretical density and void volume fraction of the sample is basically the same as the true density of the sample obtained from the PDDA,which proves that the method and data are correct.In this work,we provide a method to obtain the correct information of crystal microstructure.3、In order to understand the evolution law of micro-defects of CL-20 explosive crystals under heating conditions,the specific surface,volume fraction and size distribution of the pore structure during phase transition and high-temperature storage of CL-20 were studied by using WAXS and SAXS.Firstly,the absolute scattering intensity of the scattering data is corrected.The results showed that the number and size of pores in CL-20 powder do not change significantly during the heating process before phase transformation.At 170℃,CL-20 began to undergo ε-γ phase transition,and the number of pores of various sizes increased significantly.It is worth noting that the number of pores with small size(radius 9-21nm)changes particularly significantly.The phase transition was completed at 200°C,and γ-CL-20 was stored.Affected by the thermal decomposition process,small size pores appeared first.As the storage time increased,the small size pores gradually grew and fused into the medium size(radius 21-52nm)and large size(radius 52-64nm)pores.