Phase Transitions Of Sodium Azide Under Shear Stress And High Pressure

Exploring new types of high energy density materials（HEDM）is a long-term research hotspot in condensed matter physics.Because there are big energy differences between N-N single bond or N=N double bond and N≡N triple bond,huge energy will be released when high-energy polynitrogen compounds decompose,which is an important new kind of high-energy-density materials.In addition,polynitrogen compounds are also environmentally friendly materials,which have important application potential in in aerospace and national defense fields,and their synthesis studies have become the research frontiers of high-energy density materials.The application of high-pressure conditions,shear stress and laser heating technology to experiments to study changes in the structure and physical properties of materials has attracted widespread attention and has important scientific research significance in practical applications.In the study of high-pressure synthesis of nitrogen-rich energetic materials,exploring new ways to reduce the energy-containing high-pressure phase transition pressure and obtaining novel energetic structures are scientific problems to be solved.In response to the above problems,this thesis uses NaN₃as the experimental precursor,using rotating diamond anvil press and laser heating technology,combined with high-pressure in-situ Raman scattering spectroscopy technology,and carried out the application of shear stress under different pressure conditions.And high-temperature-induced high-pressure structural phase transition and chemical transformation of NaN₃.The specific research content and results of the thesis are as follows:1)The phase transition behavior of sodium azide（NaN₃）under high pressure shear was studied using high-pressure in-situ Raman scattering spectroscopy.The experimental results show that when NaN₃is pressurized up to 0.7 GPa,the rhombic R-3m-NaN₃（β-NaN₃）may transform into the monoclinic C2/m-NaN₃（α-NaN₃）.With the pressure upto 17.3 GPa,α-NaN₃begins to transform into tetragonal I4/mcm phase（γ-NaN₃）.Decompression experimental data showed thatγ-NaN₃can be stabilized at the highest pressure of 51.4 GPa in this experiment.At 13.7 GPa,the shearing operation was applied by rotating the anvil surface,and theα-NaN3 transform into theγ-NaN₃,indicating that the plastic shear stress can effectively reduce the pressure of the NaN₃phase transition.When the pressure was increased to 51.4 GPa and anvil surface was rotated by 45°,the Raman spectrum shows that the sample tends to become amorphous.After pressurizing to 49.6 GPa and rotating at 405°,the characteristic Raman vibration peak of azide disappears.Combining the comparative analysis of experimental and theoretical prediction of Raman spectra,we believe that NaN₃may transforms into amorphous phase-NaN₅.Upon decompression,the sample returns back toβ-NaN₃at ambient pressure.N₅ˉis bonded by low-ordered covalent bond with bond energy is between the N-N bond energy and N=N bond energy,which is considered as a new potential HEDMs.We use shear technology under high pressure to may transform NaN₃into amorphous phase-NaN₅at a pressure about 50GPa.This research provides a new method for synthesizing new nitrogen-rich and high-energy density materials under lower pressure and normal temperature conditions.2)The phase transition behavior of NaN₃under high temperature and high pressure combined with shear stress was studied by high pressure in-situ Raman scattering spectroscopy experiments.The experimental results show that the original characteristic vibration peak disappears when NaN₃is heated at 1800 K and 38.2 GPa with shear stress,and new vibration peaks which cannot be attributed to the precursor material were observed,proving that NaN₃is transformed into a new Na-N compound.According to the theoretical calculation results,the new Na-N compound was may identified as Cm-NaN₅.It can be stabilized down to 23.2 GPa.The experimental results reveal that NaN₃transforms into a new type of energetic polynitrogen compound under high temperature and high pressure conditions assisted with shear stress,which provides a new synthesis route for the polynitrogen HEDMs.