| 1,1-Diamino-2,2-dinitroethylene(FOX-7)had excellent characteristics such as high energy,good thermal performance and mechanical sensitivity,and had good application prospects in insensitive explosives and propellants.However,the problems of FOX-7synthesized directly,such as irregular morphology,small and single particle size,wide particle size distribution and poor fluidity,resulting in poor application effects.Recrystallization was an important method to improve the crystal performance of explosives.In this paper,the crystallization kinetics and crystallization thermodynamics of FOX-7 in different solvents were deeply studied,and the crystallization mechanism of FOX-7 was obtained.Based on this,the cooling crystallization process was optimized to prepare FOX-7 particles with different morphologies.Firstly,Molecular dynamics simulation software was used to predict the morphology of FOX-7 in three binary mixed solvent systems with different volume ratios by modifying the adhesion energy model,and the results were compared with the experimental results.The results showed that the predicted crystal morphology of FOX-7 in the mixed solvent system of N-methylpyrrolidone-water,N,N-dimethylacetamide-water and γ-butyrolactone-water was basically consistent with the experimental results,The results showed that the molecular dynamics model was basically consistent with the actual situation.Secondly,the thermodynamic properties of FOX-7 crystallization were studied,and the solubility of FOX-7 in the mixed solvents of N-methylpyrrolidone-water,N,Ndimethylacetamide-water and γ-butyrolactone-water was predicted by Cosmologic software.The solubility was measured by FBRM dynamic monitoring method.Apelblat equation,Van’t Hoff equation and Yaws equation were used to correlate the solubility data according to the experimental results.The thermodynamic parameters,solid-liquid surface tension and surface entropy factors in the dissolution process of FOX-7 in different solvent systems were calculated.The results show that the COSMO-RS theory could accurately predict that the solubility of FOX-7 in different mixed solvents increased with the increased of temperature and decreased with the decreased of the mole fraction of the solvent.But there was a certain gap between the solubility and the actual solubility.Among the three correlation models,the Yaws model can relate the solubility data well.The surface tension of solid and liquid decreased with the increased of temperature,and the surface entropy factor is greater than 5,which indicated that the nuclear barrier of FOX-7 was higher.The crystal growth mechanism was spiral dislocation growth,and the crystal surface was very smooth.Thirdly,with the help of FBRM online monitoring method,the changes of stirring rate,cooling rate and initial saturation temperature during the cooling crystallization process on the number of particles and average particle size in the solution during the crystallization process were studied,and the curves of FOX-7 nucleation and growth rate with time were calculated by the method of moments,and the particle number and average particle size under different conditions were compared..The results showed that as the stirring rate increased,the number of FOX-7 particles precipitated in the solution increases,and the average particle size decreases.As the cooling rate increased,the number of FOX-7 particles precipitated in the solution increased,and the average particle size decreased;as the cooling rate increased,the median chord length of FOX-7 particles decreased.As the initial saturation temperature increased,the number of particles in the solution tended to increase,and the average particle size increased.Finally,the influences of solvent type,solvent-non-solvent ratio,initial saturation temperature,stirring rate,cooling rate,additives and the amount of additives on FOX-7morphology were studied by single factor experiment.The hexagonal,long and disk-like FOX-7 were prepared by orthogonal experiment.The cylindrical FOX-7 was optimized by orthogonal experiment.The crystal forms,thermal decomposition properties and impact sensitivity of FOX-7 with different morphologies were characterized.The results showed that the FOX-7 crystal with different morphologies did not change.The disc-shaped and cylindrical FOX-7 prepared have better thermal stability and reactivity than the raw materials.Long strip FOX-7 had high thermal stability and reactivity.Hexagon FOX-7 had higher thermal stability and less reactivity than raw materials.The impact sensitivity of FOX-7 with different morphologies had almost no change relative to the raw materials. |
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