Study On Construction And Performance Of Micro-nano Hierarchical Structure Of Ammonium Nitrate Explosives

Due to the excellent comprehensive performances,1,1-diamino-2,2-dinitroethylene（FOX-7）,cyclotetramethylene tetranitramine（HMX）,and hexanitrohexaazaisowurtzitane（CL-20）have been the research hotspo.Micro-nano hierarchical structure is a composite structure in which the structural units on micro-scale are built from the secondary nano-scale structural units.The idea of micro-nano hierarchical structure are introduced into the energetical materials to built new structural energetical materials.The energetic materials with hierarchical micro/nanostructures not only have the advantage of nano-energetic materials,such as lower sensitivity,higher energy release rate and combustion efficiency,but also overcome the shortcomings of nano-energetic materials of easy aggregation,bad formability and compatibility et al.Consequently,FOX-7,HMX and CL-20 were taken as research objects in this thesis to obtain their micro-nano hierarchical structures applying template method.Scanning electron microscope（FE-SEM）,powder X-ray diffraction（XRD）,infrared spectroscopy（IR）,high performance liquid chromatography（HPLC）,differential scanning calorimetry（DSC）,thermogravimetry（TG）and other modern analytical testing techniques are applied for structure and performance characterizations,and the main research contents are as follows:（1）Polyhemiaminal（PHA）aerogels was prepared by high temperature polymerization of 4,4-diaminodiphenyl ether（ODA）and paraformaldehyde（POM）in N-methylpyrrolidone（NMP）,and the morphology,structure and thermal properties of the as-prepared PHA aerogels were characterized.The results show that the volume density of PHA is 0.1052g·cm^-3,the specific surface area of PHA is 67.18 m²·g^-1,and the porosity of PHA is 90%,which indicates PHA aerogels is an ideal template material for constructing micro-nano hierarchical structure.（2）PHA-HMX composite was firstly prepared by using PHA aerogel as template through evaporation crystallization method.The HMX（nano-HMX）with three-dimensional porous nano structure was obtained by removing the template with 10%dilute sulfuric acid.The effects of different solvents,solution concentrations and crystallization times on the crystallization of HMX were investigated.The FE-SEM results show that the PHA’s three-dimensional porous nano structure is fine preserved in nano-HXM.The XRD test indicates that the crystal formof as-prepared HMX wasβ-type and its average grain size is92.40 nm calculated by Scherrer formula.The purity of nano-HMX is 99.0%determined by HPLC method,indicating the almost complete removing of the PHA template.Compared with the raw-HMX,the exothermic peak of the nano-HMX is delayed by 0.5°C,the decomposition enthalpy is increased by 31 J·g^-1,and the apparent activation energy is increased by 58.11 kJ·mol^-1,indicating the improvement of nano-HMX’s thermal stability and decomposition efficiency.The H₅₀ of the nano-HMX increases from 36 cm to 66.43 cm,indicating great reduce of the impact sensitivity.（3）The CL-20（mn-CL-20）sample was obtained by the similar method with the HMX.The FE-SEM results show that the PHA’s three-dimensional porous nano structure disappears in mn-CL-20,in which there are a few nano particles.The XRD test indicates that the crystal form of mn-CL-20 is changed fromεtoβ.The thermal analysis results show that the decomposition peak temperature of mn-CL-20 is advanced from 246°C to 236.22°C,and the thermal decomposition weight loss rate is increased by 6%compared with theβ-CL-20.The H₅₀ of the as-prepared mn-CL-20 increases by 4 cm compared with theβ-CL-20,indicating the impact sensitivity decrease of as-prepared mn-CL-20.（4）The FOX-7（nano-FOX-7）with three-dimensional porous nano structure was obtained by the similar method with the HMX.The FE-SEM results show that the PHA’s three-dimensional porous nano structure is nearly completely preserved in nano-FOX-7.The XRD test indicates that the polymorph of as-prepared nano-FOX-7 is the same as the raw-FOX-7（α-type）and its average grain size is 83.68 nm calculated by Scherrer formula.The transformation peak and low temperature decomposition peak of FOX-7 are respectively delayed by 11.3°C and 21.3°C,the nano-FOX-7’s exotherm is almost completely concentrated at the pyrolysis peak of 291.0°C.Compared with the raw FOX-7,the decomposition enthalpy increases from 1309 to 1421 J·g^-1,and the apparent activation energy increases by 31.46%(from 408.80 to 537.42 kJ·mol^-1),which explains that the energy release efficiency and thermal stability of the nanostructure FOX-7 are greatly improved.Using the PHA aerogel as a template,the results of the construction of micro-nano hierarchical structure for three typical nitramine explosives show that the PHA template is more suitable for FOX-7 explosive with strong intramolecular and intermolecular hydrogen bonds.The template effect is more pronounced and the performance improvement is more significant.Next,PHA aerogel has also a good template effect for HMX,but the most unsatisfactory effect for CL-20.Not only does the particle size fail to reach the nanometer level,the three-dimensional porous structure obviously collapses,but also the crystal form changes fromεtoβ.The results show that the construction of micro-nano hierarchical structure using PHA aerogel as a template has certain selectivity for nitramine explosives.