First-principles Study On High-pressure Phase Transition And Equation Of State Of Dense Nitrogen Under High Temperature

Nitrogen is a familiar element and and the configuration of the extra-nuclear electron is 2s~22p~3.The possible bonding modes between nitrogen atoms are single bond(N-N),double bond(N=N),and triple bond(N?N).Nitrogen exists in the form of diatomic molecules at room temperature and atmospheric pressure,and N?N is one of the most stable chemical bonds.But the triple bond between the nitrogen atoms will be partially or completely broken to form polymeric nitrogen or atomic(plasma)nitrogen under the high temperature and high pressure.Nitrogen is also one of the important components in energetic materials,and gaseous nitrogen is the main explosion product of nitrogen-enriched energetic materials.As a result,understanding the phase transition and the equation of state of nitrogen at high temperature and high pressure is great important to evaluate the energy characteristics and external work capacity of fluid nitrogen rich energetic materials.And studying the equation of state and phase transition mechanism in a wider area is also helpful to construct the equation of state of explosive detonation products and the manufacture of high-energy materials.Molecular dynamics simulation method of density functional theory is used to explore the dissociation phase transition of fluid nitrogen at high temperature and high pressure and its influence on the equation of state in this study;and the reasonable analyzed and explanation are given for the experimental data of the equation of state of shock Hugoniot.The results content of this article are described from the following aspects:1.This study has obtained equations of state of isothermal and 200 data points.We also gained 12 isotherms at 2000 K-20000 K,and the density and pressure over the range of 1.03 g/cm~3-5.20 g/cm~3and 2 GPa-300 GPa.The results show that the isotherms show a phase transition characteristic.It is found that the isotherm has a first-order phase transition characteristic(((?)P/(?)?)_T?0)of discontinuous change in pressure in the range of T?4000 K,which is caused by the dissociation of nitrogen molecules;and the characteristic weakens as the temperature increases.At the higher temperature,the characteristic is not observed but the transition occurred.Therefore,the critical temperature of the first-order phase transition is estimated to be between 4000 K and5000 K.In addition,an abnormal phenomenon that the internal energy of the isotherms decreases with the increase of density is observed in the high temperature(T?15000 K)and low density area,and it is found that this is related to the phase transition of the atomic fluid phase.Through the data fitting,we have given the analytical expressions of the equation of state for each interval.2.The structural evolution characteristics of fluid nitrogen,including the dissociation of nitrogen molecular and atomic bonding,were analyzed by means of the radial distribution function,the variation of composition and the sampling configuration.The result shows that the triple bond of the nitrogen molecule was broken,it will recombine to form a polymer connected by single and double bonds or directly form an isolated nitrogen atom.If the number of polymeric is dominant in the dissociation phase,the phase of dissociation is the polymeric fluid phase,and vice versa is the plasma phase(Atomic fluid phase).In the studied pressure range,it is found that the triple bond of molecules is broken to form a single-double bond-connected nitrogen cluster in the phase transition interval(((?)P/(?)?)_T?0),the system occurs a transition from molecular fluid to polymer fluid at 2000 K?T?5000 K.With the increase of temperature,in the low pressure range of 5 GPa-15 GPa,the system is the molecular fluid phase within the temperature range of 5000 K?T?10000 K,and the system is already the polymer fluid phase at 10000 K?T?15000 K instead of the molecular fluid phase.Then it is observed that the system is in the atomic fluid phase in the interval of 5 GPa-27 GPa at temperature of 15000 K?T?20000 K.As the density increases,the isolated atom occur polymerization,which causes the internal energy to decrease,and the system undergoes a transition from atomic fluid phase to polymerized fluid phase.In this paper,two phase transition curves of molecular fluid-polymerized fluid-atomic fluid are obtained,and the study area is divided into three phase regions(molecular fluid phase,polymer fluid phase,and atomic fluid phase),so the fluid nitrogen phase diagram is obtained.4.We study the dissociation of liquid nitrogen and its influence on the Hugoniot of the equation of state under impact loading.The Hugoniot curves obtained is in good agreement with the experimental results,and it is found that the softening of the curves is closely related to the formation of nitrogen cluster polymers by the dissociation of nitrogen molecules.Finally,the phase transition curve of molecular fluid to polymeric fluid is given,so the phase diagram of fluid nitrogen is obtained.