Synthesis And Thermal Decomposition Properties Of Hierarchically Porous Carbon Based Nanocomposite Energetic Materials

Compared with the traditional energetic materials,nanoenergetic materials have unique characteristics in terms of thermal decomposition,combustion and detonation.However,due to the large specific surface area and poor stability,nanoenergetic materials are easy to agglomerate and cause the reduction of reactivity,which seriously affects their practical applications.Therefore,how to make energetic materials stable at the nanoscale with controllable performance is a challenge for the design and preparation of nanoenergetic materials.In this thesis,a new idea is proposed to maintain the dimensional stability of nanoenergetic materials through the space-confinement of specific hierarchically porous channels.The commonly used oxidizer ammonium perchlorate(AP)and the promising oxidizer hexanitrohexaazaisowurtzitane(CL-20)in solid propellants are chosen as the research objects,and an energetic materials confined nanocomposite structure based on three-dimensional hierarchically ordered macro-/mesoporous carbon(3D HOPC)materials is designed innovatively.The unique composite structure confines energetic materials inside the hierarchically porous carbon scaffold at the nanoscale,which can not only maintain the stable nano-activity of energetic materials,but also regulate the thermal decomposition properties of energetic materials by precisely controlling the composition of nanocomposites.In this nanocomposite structure,3D HOPC plays multiple roles:1)As a support,energetic materials can be confined into its porous channels at the nanoscale,so as to have a higher decomposition rate at a lower temperature and form the compact interfacial contact with catalysts.2)As a catalyst,the high specific surface area brought by its multi-scale pore structure can provide plentiful reaction active sites,and its good pore connectivity can shorten the diffusion path of gas products produced form the decomposition of energetic materials.3)As a catalyst support,the introduction of highly active nanoparticles can further enhance its ability to regulate the thermal decomposition properties of energetic materials.4)its remarkable electrical and thermal conductivity can promote electron transfer and heat conduction during the thermal decomposition process of energetic materials,as well as provide a large amount of combustion heat for the reaction.Under the integrated effects of these factors,the thermal decomposition temperature of AP and CL-20 can be significantly reduced and the reaction heat release can be greatly increased by constructing the hierarchically porous carbon based nanocomposite energetic materials,which makes AP and CL-20 more in line with the development of solid propellants with high energy,high burning rate and low signature properties.In order to construct the nanocomposite structure with energetic materials confined,the hierarchically porous carbon scaffold must has highly ordered porous structure and good pore connectivity.Herein,3D HOPC scaffolds with adjustable pore structure parameters are synthesized through a soft/hard dual-templating approach.The ordered macropores are arranged in a face-centered cubic structure and well-connected to each other by small windows,and the macropore walls are composed of mesopores.All of the obtained 3D HOPC scaffolds possess high specific surface area(574?1061 m²/g)and large pore volume(0.86?1.76 cm³/g).This structure allows energetic materials to easily enter the interiors of the carbon scaffold to achieve nanoscale loading,and shows the potential for high catalytic activity.Reducing the high-temperature decomposition temperature and increasing the apparent specific heat release of AP has become an effective way to improve the burning rate of AP-based solid propellants.In this thesis,AP-confined AP/HOPC nanocomposites are prepared via the precise control of the solvent drop-evaporation process.AP nanocrystals(26.2?69.0 nm)are successfully space-confined into the 3D HOPC scaffold and their distribution and size are influenced by AP loading amount.When AP loading amount is 80wt.%,3D HOPC shows the best catalytic performance in reducing the high-temperature decomposition temperature of AP from 440.9°C to 315.4°C,decreasing the activation energy from 176.4 k J/mol to 130.8 k J/mol,and unprecedentedly increasing the heat release from 371J/g to 3765 J/g,which achieves the purpose of generating higher heat release at lower decomposition temperature.The stable catalytic activity of 3D HOPC is confirmed by repeated thermal catalytic reactions.Furthermore,highly active nanoparticles are introduced into the 3D HOPC scaffold to further reduce the high-temperature decomposition temperature of AP.Herein,?-Fe₂O₃nanoparticles are in-situ encapsulated inside the 3D HOPC scaffold via a dual-templating based multi-component co-assembly method,so that the active nanoparticles can maintain their high catalytic activity and stability in the obtained HOPC/Fe₂O₃composite scaffold.In the AP-confined AP/HOPC/Fe₂O₃nanocomposites,Fe₂O₃nanoparticles(3.8?10.6 nm)are highly dispersed in the pore walls of the carbon scaffold,and AP nanocrystals(24.2?44.6 nm)are homogeneously supported on the surface of the pore walls,which greatly improves the uniformity of the three composite components and then facilitates the thermal decomposition reaction.Owing to the synergistic effect between 3D HOPC and highly dispersed Fe₂O₃nanoparticles,HOPC/Fe₂O₃composite scaffold exhibits significantly better catalytic activity than single 3D HOPC.The high-temperature decomposition temperature of AP can be reduced from 440.9°C to 280.5°C,which is 29.5°C lower than the 310.0°C catalyzed by 3D HOPC,and the high heat release(2114 J/g)can be obtained at the same time,so as to achieve the aim of further reducing the high-temperature decomposition temperature of AP without sacrifice to the decomposition heat release.In view of the low heat release and high signature of AP,CL-20,which is expected to replace AP due to its high energy and low signature,is selected as the research object in this work.The reduction of the thermal decomposition temperature of CL-20 can decrease the pressure exponent of the propellants,thus improving the applications of CL-20 in solid propellants.Here,CL-20-confined CL-20/HOPC nanocomposites are synthesized though solvent evaporation-induced dispersion process and solvent drop-evaporation process.In these nanocomposites,CL-20 nanocrystals are homogeneously space-confined into the 3D HOPC scaffold with a particle size of 9.2?26.5 nm.It is found that the thermal decomposition properties of CL-20/HOPC nanocomposites are affected by CL-20 loading amount,as well as the surface and macropore size of 3D HOPC.Compared with single mesoporous structure and macroporous structure,3D HOPC has the best catalytic performance for the thermal decomposition of CL-20 in reducing the decomposition peak temperature from 247.0°C to 174.8°C and lowering the activation energy from 165.6 k J/mol to 115.3 k J/mol.In addition,3D HOPC also has high catalytic activity for the thermal decomposition of other high energy oxidizers used in solid propellants.The above works have creatively explored the application potential of 3D HOPC,a new type of carbon material,in regulating the thermal decomposition properties of energetic materials,which not only expands new insight to design high-performance catalysts for solid propellants,but also provides abundant theoretical and experimental basis for the further applications of high energy oxidizers represented by AP and CL-20 in solid propellants.