Molecular dynamics simulations of the thermodynamic properties and local structures on molten alkali carbonate Na2CO3

Abstract

Molten carbonate salts as phase change materials have received particular attention for high-temperature thermal energy storage and heat transfer applications due to desirable thermal characteristics such as wide operating temperature range, low causticity and excellent thermal stability. In this study, molecular dynamics (MD) simulations were performed on molten alkali carbonate Na2CO3 based on an effective pair potential model, a Born-Mayer type combined with a Coulomb term. The temperature dependences of thermodynamic properties including the density, thermal expansion coefficient, specific heat capacity, shear viscosity, thermal conductivity and ion self-diffusion coefficient were simulated in detail from 1150 to 1500 K, which was all difficult to achieve from experiments on account of high-temperature extreme conditions. The simulation results were in satisfactory agreement with experimental data available with high calculation accuracies. In particular, both the equilibrium molecular dynamics (EMD) and non-equilibrium molecular dynamics (NEMD) simulations were tried to calculate the shear viscosity and thermal conductivity, and the NEMD method turned out to be more suitable for molten alkali carbonate salts with higher calculation accuracies of 15.11% and 22.48% for shear viscosity and thermal conductivity separately. Moreover, the radial distribution functions (RDF) and coordination number curves of the molten salt were characterized to explore the temperature dependences of macroscopic properties from microscopic view, and the results suggested that the changes of thermodynamic properties with temperature were induced by the distance changes between Na2CO3 particles. Besides, it could be concluded that the structure of CO32− was inferred reasonably to be ortho-triangular pyramid from the comprehensive analysis of local structures including the angular distribution functions (ADF).