Thermodynamic properties of liquid sodium under high pressure

Huaming Li,Yongli Sun,Mo Li,Xiaoxiao Zhang

doi:10.1063/1.4980023

Abstract

Acquiring reliable thermodynamic properties in liquid metals at high pressure and temperature is still a challenge in both experiment and theory. Equation of state (EoS) offers an alternative approach free of many of the difficulties. Here using the EoS of a power law form we obtained the thermodynamic properties of liquid sodium under pressure along the isothermal lines, including isothermal buck modulus, thermal expansion coefficient, Grüneisen parameter, and Anderson-Grüneisen parameter. The results are in excellent agreement with available experimental data measured by a piezometer at high temperature and high pressure and sound velocity measurement with pulse-echo technique. We found that the pressure derivative of the isothermal bulk modulus at zero pressure is a monotonic function of temperature and has a value around 4. In addition, unexpected crossing points were found in the isobaric thermal expansion coefficient and Grüneisen parameter; and a minimum in the isobaric heat under isothermal compression was also observed. While some of these detailed predictions are yet to be confirmed by further experiment, our results suggest that the power law form may be a more suitable choice for the EoS of liquids metals.

Highlights

Alkali metals are generally considered as the simplest metal systems because there is only a single s electron in their valence band
Since liquid Na is a simple dense liquid thermodynamically,[1] such complex behavior under high pressure certainly leads to new curiosity that demands better description of the various thermodynamic properties important for technological application and basic understanding
We show that the power law form of Equation of state (EoS) can predict isothermal bulk modulus, volume and other thermodynamic properties such as isobaric thermal expansion coefficient, internal pressure, entropy, enthalpy, internal energy, isobaric heat capacity, isochoric heat capacity, heat capacity ratio, adiabatic bulk modulus, sound velocity, Gruneisen parameter, and Anderson-Gruneisen parameter

Summary

Introduction

Alkali metals are generally considered as the simplest metal systems because there is only a single s electron in their valence band. They are still the object for many technological applications and basic scientific research. Due to its large thermal and electrical conductivity at high temperature liquid sodium (Na) is used as coolant in reactors and conductor of electric current.[1,2] Since liquid Na in these applications is usually under very high pressure and how pressure affects the melting curve and other thermodynamic properties has been a focus of engineering and basic scientific research.[3] In a recent experiment by X-ray diffraction of Na up to pressures of 130 GPa, a maximum melting temperature around 1000K at 31 GPa has been observed in a body centered cubic phase of Na,[4] which is unexpected for the simple metal. While the calculated electronic configurations and atomic structures offer detailed explanations, the different time and length scales in the calculations from those in experiment impose additional limitations that need to be resolved.[5,6]

Results

Discussion

Conclusion