Phase Transition Of Two Typical Energetic Materials Under High Pressure

In the absence of external stimuli,the physical and chemical properties of a substance are the same,and the aggregate state that has a clear boundary with other substances is called a phase.Under the action of external stimuli(such as pressure,temperature),matter will change from one phase to another.This process is called phase transition.When a substance undergoes a phase change,its microstructure will undergo a sudden change,which will affect the physical properties and macroscopic properties of the substance.The use of phase transitions is of great significance to the study of the structure of matter.In thermodynamics,pressure and temperature are two very important parameters.Pressure can change the arrangement of atoms,the distance between atoms,the electron orbital structure,and the crystal structure.For crystals of energetic materials,pressure can change the structure of the crystals,thereby inducing phase transitions.In this paper,two nitramine energetic materials,HMX and RDX,are selected.RDX(cyclotrimethylene trinitramine-C₃H₆N₆O₆),also known as cyclonite,is a powerful explosive with powerful explosive power.HMX(Cyclotetramethylenetetranitramine-C₄H₈N₈O₈),or Octogen,is a homologue with RDX.Both HMX and RDX are polymorphic compounds,they have the same chemical group,and have similar molecular structures.In this paper,through diamond anvil high pressure technology combined Raman spectroscopy with and synchrotron radiation technology,the phase transition of these two energetic material crystals under high pressure is studied.1.This article analyzes the RDX and HMX low-frequency in-situ high-pressure Raman spectra and the RDX synchrotron radiation spectrum.The results show that as the pressure increases,most of the Raman peaks move to the high wave number range.At 4.6 GPa,the original Raman peaks at 228 and 413 cm^-1 disappeared,and the spectrum showed obvious discontinuities.This indicates that the phase transition from?-RDX to?-RDX starts at 4.6 GPa,and ADXRD confirms that this is a reversible phase transition process.When a pressure-induced phase transition occurs,the pressure-induced response of each functional group in the molecule changes accordingly.This will cause a significant change in the frequency-pressure slope of the Raman peak.At pressures of 11 and 19.3 GPa,significant discontinuities in the frequency pressure curve are observed.At 11GPa,HMX changes from?-HMX to?-HMX.When the pressure increases to 19.3 GPa,HMX changes from?-HMX to?-HMX.The Raman spectrum after pressure drop is measured and analyzed to find this phase transition process.It is also reversible.High pressure can promote the close packing of energetic materials,resulting in a reduction in the length of intermolecular hydrogen bonds.The shorter the chemical bond,the more energy can be stored.2.Using Raman spectroscopy to study the phase change of HMX in the range of150?3200 cm^-1 under high pressure and variable temperature.The high-pressure experiment results show that as the pressure increases,the Raman displacement moves to the high-frequency region;the bandwidth of most Raman bands gradually widens,and the intensity of the spectral lines gradually decreases.At 12 GPa,the deformation mode of the NO₂ ring splits as the pressure increases,and at higher pressures,a wider envelope is formed.Two phase transitions were observed near 12 and 27.5 GPa,entering the?and?phases,respectively.After the pressure is relieved,by analyzing the Raman spectrum of the sample after the pressure is reduced,the phase change is reversible in this pressure range.The results of the temperature-changing experiment show that a phase change is observed near 468 K and enters the delta phase.The XRD measurement of?-HMX and the sample whose temperature dropped to room temperature after temperature change experiment showed that the phase change is irreversible.Research on energetic materials under extreme environments is conducive to analyzing the structure and performance of energetic materials,and then judging the impact on macroscopic characteristics,fully understanding the physical and chemical properties of energetic materials under extreme conditions,and making it ideal for manufacturing sensitivity and the safety of explosives is very important.These results can be used to improve the safety and detonation performance of energetic materials.