Study On Physical Properties Of Energetic Materials Under Extreme Conditions

In high pressure science research,we can change the molecular distance and internal energy by applying high pressure and high temperature(or low temperature),and then explore the physical and chemical properties of condensed matter to reveal the general law of matter.Considering the significance of energetic materials in national defense,military,and other fields,understanding the structure transformation,electron transition,and decomposition mechanism of energetic materials under extreme conditions of high temperature and high pressure can effectively guarantee the safety and effectiveness of the synthesis process transportation,storage,and application,as well as guidance for the development of new energetic materials.With the advancement of high pressure technology,some previously theoretically predicted energetic materials,such as metal hydrogen and metal nitrogen,have been synthesized under high temperature and high pressure conditions.The use of high pressure experimental technology not only expands scientific research methods,but also opens up new avenues and ideas for the synthesis of new substances.In this thesis,the evolution and thermal decomposition mechanism of crystal structure and electronic structure of new energetic material LLM-105(1-oxide-2,6-diamino-3,5-dinitropyrazine)were studied under extreme conditions which include high pressure,low temperature as well as high temperature.The formation of metal hydrogen and hydrogen rich compounds,as well as the structural phase transition of hydrogen and C-S-H hydrogen rich compounds are investigated under high pressure.This thesis is divided into six chapters,which are as follows:The first chapter briefly introduces the technical mechanism,research methods,and related research progress of high pressure science research,as well as the research purpose and significance of this thesis.In chapter 2,the structural phase transition of LLM-105 is investigated under various pressures and temperatures,the change of the hydrogen bond network in the crystal is examined,and the influence of crystal morphology on the phase transition process of LLM-105 is discussed.The results of high pressure XRD and Raman spectra show that LLM-105 undergoes structural phase transition at 26.5 GPa.The results of XRD,Raman and infrared spectrum show that LLM-105 does not have a phase transition prior to decomposition temperature.Using infrared spectroscopy and theoretical calculations,it was discovered that the hydrogen bond network strengthens during the pressurization and cooling processes.High pressure Raman spectra,on the other hand,show that the smaller the size of the LLM-105 crystal,the less likely it is to undergo structural phase transition.The third chapter focuses on the optical absorption and fluorescence emission of LLM-105 under high pressure and low temperature.The high pressure fluorescence spectrum revealed that the fluorescence of LLM-105 increased significantly under pressure,reaching a maximum at 9.2 GPa,then decreasing under pressure and disappearing at 26.4 GPa.The fluorescence is strongest at low temperatures when the pressure is 10.2 GPa.The high-pressure absorption spectra show that the band gap decreases linearly with increasing pressure,and the displacement rate decreases at 10.5 GPa.The first principle calculation reveals that the molecular configuration distortion occurs at 10.5 GPa and that the electronic structure changes.When the structure transition occurs in 26.5 GPa,the band gap of LLM-105 decreases by about 0.08 eV.In Chapter 4,the decomposition process of LLM-105 at high temperatures and pressures,as well as the dissolution recrystallization process in water,are investigated,and the formation of a transparent crystal product is discussed.High temperatures and high pressure experiments demonstrate that the temperature at which LLM-105 decomposes rises as the pressure rises.At ambient pressure,the separation of the nitro group and ring can initiate the thermal decomposition process.At high temperatures and pressures below 1 GPa,LLM-105 can dissolve in water and form a new acicular crystal after cooling.In Chapter 5,the phase transition behavior of hydrogen and C-S-H hydrogen rich systems under high pressure is discussed.High pressure hydrogen Raman spectra show the appearance of a new lattice vibration mode near 109 GPa,which is thought to be a structural phase transition.CH4,H2S,and H2 mixed crystals were successfully obtained during the synthesis of the C-S-H hydrogen rich system,and their structural phase transition was studied using Raman spectroscopy.The results show that there are structural phase transitions at 16.4 GPa,20.1 GPa,30.8 GPa and 34.7 GPa.After 34.7 GPa,the Raman mode of S-H bond disappeared,indicating the S-H covalent bond weakens under high pressure.The findings of the research are summarized in Chapter 6.