Theoretical Study On The Structure And Performance Of Nano-Al And Nano-AlH₃ Complex Energetic Materials

Aluminum-containing energetic materials are important in civil and military applications,but their combustion mechanism has not been well characterized,which hinders the further development of nano-sized aluminum energetic materials.In this thesis,the ReaxFF-lg reactive force field involving the CHONAl elements in the aluminum-containing energetic material system was developed based on the first-principles method.Using the force field,the molecular reaction dynamics simulation of nano-Al/explosive and nano-Al H₃/explosive composites were carried out.The initial reaction mechanism,energy release and product formation were studied.In addition,the crystal phase transition of aluminum hydride affected by Li-doping and the development of the coarse-grained molecular force field of polyacrylamide based on the quantum mechanical equation of state was studied.The main contents include the following parts:1.Development of ReaxFF-lg reactive force field for CHONAl energetic materialsAiming at the shortcoming that the current ReaxFF-lg reactive force field lacks Al-X（X=C,H,O,N）interactions,which cannot simulate the energetic composite materials containing aluminum,a set of CHONAl ReaxFF-lg force field parameters applicable to energetic materials were established.The chemical information for the ReaxFF-lg includes the bond dissociation and the bond angular bending potential energy surface,the density,equilibrium bond length,volume-energy curve of the crystals,and the interface adsorption energy of the aluminum-containing clusters.Compared with the experimental results and the results obtained by quantum mechanics method,the proposed reaction field can accurately simulate the formation and decomposition of aluminum-containing clusters in gas phase,the density and compressibility of aluminum-containing crystals,and the adsorption of gaseous molecules on the surface of alumina.2.Structure and properties of nano-Al/explosive compositesThe thermal decomposition of 2,4,6-Trinitrotoluene（TNT）,1,3,5-trinitro-1,3,5-triazinane（RDX）,1,3,5,7-tetranitro-1,3,5,7-tetrazocane（HMX）and Hexanitrohexaazaisowurtzitane（CL-20）on Al nanoparticles was examined by reactive dynamic simulations using a newly parameterized reactive force field with low gradient correction（ReaxFF-lg）.The partially passivated Al nanoparticles were constructed and mixed with TNT,RDX,HMX and CL-20 crystals and then the mixed systems were heated to high temperature in which the explosives were fully decomposed.The simulation results showed that the aluminized explosives undergo three main steps of thermal decomposition,which were denoted as"adsorption period"（0-20 ps）,"diffusion period"（20-80 ps）and"formation period"（80-210 ps）.These stages in sequence are the chemical adsorption between Al and surrounding explosive molecules（R-NO₂-Al bonding）,the decomposition of the explosives and the diffusion of O atoms into the Al nanoparticles,and the formation of the final products.In the first stage the Al nanoparticle decreases the decomposition reaction barriers of RDX(1.90 k J g^-1),HMX(1.95 k J g^-1)and CL-20(1.18 k J g^-1),respectively,and decreases the decomposition reaction barrier of TNT from 2.99 to 0.29 k J g^-1.Comparing with the crystalline RDX,HMX and CL-20,the energy releases are increased by 4.73-4.96 k J g^-1in the second stage.The numbers of produced H₂O are increased by 25.27%-27.81%and the numbers of CO₂ are decreased by 47.73%-68.01%in the third stage.These three stages are confirmed further by the evolutive diagram of the structure and temperature distribution for the CL-20/Al system.The onset temperatures（T_o）of generating H₂O for all the aluminized explosives decrease,while that of generating CO₂ for aluminized HMX and CL-20 increases,which are in accord with the experiment of aluminized RDX.3.Structure and properties of nano-Al H₃/explosive compositesUsing reactive molecular dynamics（RMD）method with the reactive force field（ReaxFF）framework,the thermal decomposition of nano-Al H₃/TNT and nano-Al H₃/CL-20 composites were investigated to develop new high explosives.The binding energies and relaxed densities of the composites show that the compatibility of nano-Al H₃/TNT is better than nano-Al H₃/CL-20,which is confirmed by the DFT calculations.The pyrolysis simulations showed that the potential barrier of nano-Al H₃/TNT(1.14 k J g^-1)and nano-Al H₃/CL-20(0.72 k J g^-1)are smaller than pure TNT(1.99 k J g^-1)and pure CL-20(1.01 k J g^-1),respectively.This indicates the catalytic effect of Al H₃ nano particle on the decomposition of TNT and CL-20.The catalytic effect was confirmed by the density functional theory（DFT）calculations of the R-NO₂ rupture of TNT and CL-20molecules on Al H₃ surfaces.Different heating rates results show sthat the dependence of the simulation results on the heating rate is small.Moreover,nano-Al H₃/TNT and nano-Al H₃/CL-20release 1.07 and 1.97 k J g^-1 more energies and generate 33.8%and 14.0%more total gas products than pure TNT and CL-20,respectively.Therefore,Al H₃ nano particles can improve the detonation performance and specific impulse for TNT and CL-20.These are in accord with the effect of Al H₃ on nitromethane（NM）and ammonium perchlorate（AP）/hydroxyl-terminated polybutadiene（HTPB）propellants in the experiments.As a result,the nano-Al H₃/TNT and nano-Al H₃/CL-20 composites can be promising candidates of new high explosives.4.Crystal phase transition properties of aluminum hydrideIn order to solve a contradiction between early theoretical prediction and experiments concerningγ→αphase transition of aluminum hydride,models of Li-doped Al H₃ were constructed and investigated theoretically.Thermodynamic calculations show that theγ→αtransition of pure Al H₃ absorbs energy,and the changes of Gibbs free energy are in range of1.74-1.99 k J mol^-1 at 298-380 K.These are opposite to the experimental fact that theγ-toα-phase transition takes place at 380 K.However,the changes of enthalpy and Gibbs free energy inγ→αphase transition of Li-doped Al H₃ are negative.The doping of Li decreases the activation energy ofγ→αtransition and introduces more metastable states between them.As the doping content increases,both the changes of enthalpy and Gibbs free energy（ΔHγ→αandΔGγ→α）decrease.The experimentalΔHγ→αvalue(-2.83 k J mol^-1)is between those of doped Al H₃ with1/23 and 1/11 Li-contents(-0.87 and-5.62 k J mol^-1 for Al₂₃Li H₇₀ and Al₁₁Li H₃₄,respectively).Heat capacity C_P（T）increases as the Li-doping content increases.The C_P（T）of Al₂₃Li H₇₀ is consistent with the experiments.Considering the thermodynamic evidences and the experiment conditions for Al H₃ preparation,the aluminum hydride synthesized by the reaction of Li Al H₄+Al Cl₃ are probably Li-doped with Li content of 1/23.The changes of enthalpy and Gibbs free energy,as well as the activation energy forγ→αphase transition can be increased if the Li-doped Al H₃ is purified.5.Development of a coarse-grained molecular force field of polyacrylamideA new coarse-grained（CG）molecular dynamics force field was developed for polyacrylamide（PAM）based on fitting to the quantum mechanics（QM）equation of state（EOS）.In this method,all nonbond interactions between representative beads are parameterized using a series of QM-EOS,which significantly improves the accuracy in comparison to common CG methods derived from atomistic molecular dynamics.This CG force-field has both higher accuracy and improved computational efficiency with respect to the OPLS atomistic force field.The nonbond components of the EOS were obtained from cold-compression curves on PAM crystals with rigid chains,while the covalent terms that contribute to the EOS were obtained using relaxed chains.For describing PAM gels,water-PAM interaction parameters were developed by fitting with the EOS of water-PAM adsorption structures.The new CG-PAM force field reproduces the EOS of PAM crystals,isolated PAM chains,and water-PAM systems,and successfully predicts such experimental quantities as density,specific heat capacity,thermal conductivity and melting point.