稠密流体氮高温高压相变及物态方程

Phase transition and equation of state of dense liquid nitrogen at high temperature and high pressure

氮的高温高压物态方程以及相图对于研究和制备高能量密度含能材料至关重要.本文采用基于密度泛函理论的分子动力学模拟方法,研究了液氮的高温高压行为,给出900—25000 K,2—200 GPa区间流体氮的物态方程以及组分、相态变化.在上述相空间,观察到流体氮分子相-聚合物相以及聚合物-原子相的相变发生.获得的液氮Hugoniot理论曲线与实验结果吻合较好,发现30—60 GPa区间Hugoniot曲线的软化与分子-聚合物流体相的相变有关;在60 GPa后Hugoniot曲线变陡峭与流体氮进入聚合物相区有关.

Nitrogen is the main reaction and detonation product of energetic materials.Therefore,studying the equation of state and phase transition of nitrogen at high temperature and high pressure is very important in evaluating the energy characteristics of energetic materials,especially in designing a new-generation nitrogenrich energetic materials.Using density functional molecular dynamics simulation method,we calculate the pressure,internal energy and chemical components of fluid nitrogen in a temperature range of 900–25000 K and a pressure range of 2–300 GPa.The negative changes of pressure with temperature on isochores are observed under the temperature and pressure conditions of 3000–10000 K and 20–80 GPa.As the temperature increases,the pressure drop is caused by the collapse of nitrogen molecules.This phenomenon is related to the phase transition from molecular fluid nitrogen to polymerized fluid nitrogen.The triple bond in the molecule breaks and a polymer forms,which is connected by single and double bonds with neighboring atom.We also study the equation of state along Hugoniot curve under impact loading.The obtained Hugoniot curve is in good agreement with the experimental results.It is found that the softening of the experimental curve in a range of30–60 GPa is related to the decomposition of nitrogen molecules and the formation of polymeric nitrogen.