The Theoretic Study For Morphology Control Of Hexogen In Solution

This dissertation is focused on the Hexogen(RDX)crystal morphology grown in solution.Through adopting the Density Function Theory and Molecular Dynamics simulation methods as well as the Gaussian and Material studio software as simulation tools,the morphology prediction model for RDX grown in solution combined the actual RDX crystallization process is established,and is used to interpret the experiment phenomenon.The object is to supply the theory basis and experimental guidance for RDX crystal morphology control technology.The geometry optimizations of RDX unit cell and molecular structure indicate that Compass is the optimal forcefield for modeling the RDX crystal.Via the prediction of BFDH and AE theory models,the RDX crystal growth morphology in vacuum is dominated by the(111),(020),(002),(200)and(210)faces,in which the morphology importance of(111)face is the most.The structure analysis of crystal surface shows that(111)face is morphological smooth,while(200)and(210)faces are morphological relative rough.The strong polar nitro groups are exposed to(111),(002),(200)and(210)surfaces,but(020)face is only observed to be exposure of the non-polar methylene groups.According to the polarity of crystal surface,(111),(002),(200)and(210)faces may be classified to the polar surfaces,while(020)face belongs to be the non-polar surface.The PBC analysis indicates that those surfaces have more than two interwined PBC chains,thus RDX important habit faces are morphological Flat planes.The Density function theory with implicit SMD model is performed to investigate the solvation effect of cyclohexanone,acetone,dimethyl formamide and dimethyl sulphoxide solvents on the quality and sensitivity of RDX crystal.The results indicate that compared with those in gas phase,the bond length of trigger bond(N-NO2)of RDX molecule in solvent phases has a large change,while the change of bond angle and dihedral angle of RDX molecule are small,and RDX molecule still keeps the AEE configuration.The natural population analysis shows that the salvation effect makes the more negative potentials concentrated on the nitro groups,while the positive potentials are mainly focused on the methylene groups.The calculation of structural parameters like trigger bond length,electrostatic potential balance parameters and nitro group charges reveal that the solvation effect may improve RDX insensitivity to a certain extent,but such effect has the upper limit.The calculated salvation free energies of RDX in acetone and dimethyl formamide are larger than those in cyclohexanone and dimethyl sulphoxide.It may be predicted that the qualities of RDX crystal produced from cyclohexanone and dimethyl sulphoxide are higher than that from acetone and dimethyl formamide.The theory prediction is coincided with the experimental results.The RDX growth interface model is constructed according to the actual solution environment,which mainly contains the RDX crystal surface phase determined by the crystal internal structures and the solution phase consisted by the massive solvent molecules.The molecular dynamics simulation method is empolyed to investigate the adsorption interactions between solution phase and RDX interfaces.The adsorption interactions between RDX polar faces and solvents are larger than those with non-polar face,in which(210)face has the largest interfacial adsorption energies.The interfacial adsorption interactions include the vdW interactions and electrostatic interfactions as well as the weak C-H…O hydrogen bond interactions,in which vdW interactions are dominated.By the surface quantitative analysis of molecular electrostatic potential,it is explained the contribution of electrostatic interactions to the acetone-RDX interface adsorption system is larger than the cyclohexanone-interface adsorption system,because the tendency of acetone molecule interacting with RDX molecule electrostatically is much stronger than cyclohexanone molecule.It is qualitatively predicted that if RDX crystallizes from polar solvents,the morphology importance of polar faces will increase,whereas the importance of non-polar(020)face probably diminishes.From the perspective of the thermodynamics and kinetics principles of crystal growth and combined the external growth conditions,through the specific association of the modified attachment energies with the growth rate of crystal surfaces,the crystal habit prediction model are established for RDX growth in solution.The RDX crystal morphologies in different solvent environments are modeled by means of molecular dynamics simulation method and the reliability of the prediction model are proved by comparing with the experiment results.It is indicated that when RDX grows from cyclohexanone solvent,all habit faces are appeared,in which(210)and(111)faces have the largest morphology importance,and the importance of(002),(200)and(020)faces are reduced.If RDX produces from acetone solvent,(210)face becomes the most morphologically important surface,(111),(002)and(200)faces are still remained,while(020)face is disappeared.The prediction results are reasonable in accordance with the observed growth habits of RDX crystal by the cooling crystallization experiments.The study of solvent diffusion shows that there may be existence of corresponding relation between the modified attachment energies and diffusion coefficients.The larger the solvent diffusion coefficients are,the smaller the modified attachment energies are,that the effect on RDX crystal morphology is stronger.The diffusion coefficients of acetone to RDX growth interfaces are larger than those of cyclohexanone,thus the solvent effect of acetone is stronger than that of cyclohexanone,resulted in the different crystallization morphologies of RDX,which is further to confirm the accuracy of the prediction model.