Preparation Of HMX Micro/Nano Hierarchical Structure By Liquid-Liquid Interface Crystallization

The hierarchical micro/nanostructure means a composite structure that the structure unit of micron scale is assembled by the nano-building blocks.The composite structure has the dual characteristics of both microstructure and nanostructure materials,and can produce many physicochemical properties different from the pure micron-or nano-structure materials due to the novel synergistic effects.The micro/nano hierarchical structure concept is applied to construct the new structure energetic materials,which not only has the advantage of nano-energetic materials,but also overcomes the shortcomings of nano-energetic materials of easy aggregation,bad formability et al.At the same time,the coupling effect of micro/nano structure s will endow the energetic materials with new properties.Therefore,it can provide a new and effective way for the modification of energetic materials and the development of new energetic materials.In addition,it is also of great significance to improve the storage and application safety of energetic materials,and the operational performance of weapons and to develop new weapons.Construction of the micro/nano hierarchical structure for the high-energy ammonium nitrate explosives,a great challenge is the selection for the preparation method and the control of the structure/morphology due to its high sensitivity and weak intermolecular interaction.In this work,the liquid–liquid interfacial crystallization was employed,and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine(HMX)was chosen for the research object.Based on the design of the interface phase and the control of the crystallization process,the micro/nano hierarchical structure HMX(MN-HMX)was successfully prepared.The structure and properties were characterized,and the formation mechanism of the micro/nano structure was analyzed and discussed.The main research contents are as follows:(1)The design of liquid-liquid interface: in this paper,the solvent was selected according to the physical parameters of the solvent,such as density,polarity parameter,solubility parameter,the mutual solubility of the binary solvent,the solubility of HMX in different solvents,and then the ratio between binary solvents was determine,so the interface phase can be designed.The solvent used in this paper is HMX’s good solvent dimethyl sulfonate(DMSO)and the other solvent is dichloromethane(CH2Cl2),and the ratio between the two solvents is 1:4.(2)Preparation of HMX micro/nano hierarchical structure : Applying the liquid-liquid interfacial crystallization method,the dandelion-like MN-HMX was successfully prepared through controlling the crystallization process and screening of crystallization parameters at 20 o C in the incubator for 2 days.(3)Structure and property characterization: Scanning electron microscopy(SEM)images showed that the dandelion-like MN-HMX is assembled by 100~600 nm HMX particles;the laser particle size analyzer test indicated that the particle size distribution is narrow and the median diameter of D50 is 339.7 nm,far less than the size of raw materials of HMX(D50=93.1 m),and the results are consistent with SEM images;X-ray diffraction(XRD)and Fourier transform infrared spectroscopy(FTIR)analysis suggested that the prepared MN-HMX is not composed of a single phase,but both β the δ phases;the results of high performance liquid chromatography(HPLC)displayed that the purity of the prepared HMX is 99.2%;the results of surface area tester showed that the surface area of the prepared MN-HMX is 2.17 m2/g,and further supported that the particle size of MN-HMX is smaller than that of the raw material HMX.Simultaneous thermal analysis instrument(DSC)test results illustrated that the exothermic decomposition peak of MN-HMX is basically the same as that of the raw HMX,and the decomposition enthalpy(ΔH=2404.0 J·g-1)is close to the raw materials(ΔH=2525.0 J·g-1),which show that the thermal properties of HMX are almost unchanged after the microstructure changes,but MN-HMX presents much higher activation energy(280.82 KJ ·mol-1)than that of the raw HMX(258.90 KJ·mol-1).Namely,MN-HMX is more difficult to decompose under thermal stimulation.Sensitivity test results showed that the characteristic drop heights H50 of MN-HMX is 31.3 cm,and the percentage of friction explosion is 100 %,higher than those of the raw materials,which presents that MN-HMX is more sensitive to mechanical stimulation.The main reason for the increase in sensitivity should be attributed to the presence of δ-HMX.The surface morphology of MN-HMX prepared by the addition of surfactant span-80 has changed and there are β and δ crystalline phases,but the purity decreases slightly and the decomposition enthalpy decreases.(4)Formation mechanism of micro/nano hierarchical structure: Based on the basic principles of interfacial chemistry and crystallography,through the in situ observation of the crystallization process combined with the characterization of structure and properties,the growth mechanism of dandelion-like MN-HMX was obtained,which was mainly controlled by th e diffusion rate of the solvent and the crystal growth rate based on the interface concentration gradient.