First Principles Molecular Dynamics Study On The Physical Properties Of Warm Dense Methane Fluid

Hydrogen rich chemical with high hydrogen content is a good material for energy storage,and methane is the simplest saturated hydrocarbon in hydrogen rich system.Methane is also major component of the giant planets such as the mantle,Uranus and Neptune.The internal temperature range and pressure range of these planets are 2000K-8000 K and 20 GPa-600 GPa,respectively.Under high temperature and high pressure,it is important to explore its electronic structure,change of phase states(including solid,liquid,gas and plasma phase etc.),dissociation of methane and atomic recombination and its influence on physical properties(including thermal,electrical and optical properties etc.)It is of great scientific significance for people to understand the properties of condensed matter under extreme conditions.It is also helpful to solve some important problems in modern natural science and engineering technology,such as energy issues,celestial evolution,metal hydrogen etc.In this paper,the first principles simulation based on density functional theory is used to study the equation of state,dissociation phase and conductivity at high temperature and pressure.The research content includes the following several aspects:1.By using the first principles calculation based on density functional theory,the state equation of the fluid methane is explored in density range of 0.6-2.5 g/cm3,temperature from 1000 K to 8000 K,and the corresponding pressure range of 3.6 GPa-332 GPa.Through the isochore of different density,we observed that the pressure stagflation or reduce in some area except 0.60 g/cm3,in other words((?)P/(?)T)?0.In addition,the fitting formula is obtained by fitting the calculated data.2.The situations of chemical composition and atomic bonding were analyzed by the pair correlation functions(PCFs)and fluid structures.We also confirm the decomposition products of the methane under extreme conditions,which consist of saturated hydrocarbons such as ethane,butane,and saturated hydrocarbons such asalkenes,alkynes,eventually forming some long alkane chains and some hydrogen or free hydrogen atoms.However,diamond—structure carbon is not observed in the dissociation of system.These areas of the pressure stagflation or reduce is closely connected with chemical decomposition process of the methane.3.By calculating the variation laws of electron energy band and DC conductivity with temperature and pressure,the nonmetal-metal transition occurs under high temperature and pressure.The critical temperature of the nonmetal-metal transition is5000 K,and the corresponding pressure is about 8 GPa.We observe that dominant contributions to a high DC conductivity come from free hydrogen atom and the boned carbon atoms in C-C chain by analyzing the partial density of state(PDOS).Finally,we give the phase transition curves of nonmetallic-metal transition.Molecular dynamics simulations based on density functional theory calculation have been used to study thermodynamic properties of warm dense methane,including the equation of state,chemical decomposition and nonmetal-metal transition under high temperature and high pressure.