The effect of Na+ concentration on the local structure of NaNO3-KNO3 and thermophysical properties with higher prediction accuracy: A molecular dynamics study

Abstract

Molten nitrates have been considered as attractive heat transfer and storage materials in concentrated solar power (CSP). In this paper, thermophysical parameters such as density, specific heat, viscosity and thermal conductivity of binary NaNO₃-KNO₃ with different Na ⁺ concentrations were calculated within the range of 600–800 K. The simulated values of thermophysical properties closely matched the experimental values. Particularly, the reverse non - equilibrium molecular dynamics (RNEMD) approach was employed to determine viscosity and thermal conductivity. The deviation of calculated thermal conductivity for Solar salt from the experimental value is less than 1 % with higher prediction accuracy. The average deviation of the simulated value of the viscosity from the experimental value is only 3.1 % in the temperature range of 500–700 K. The microscopic mechanisms of composition/temperature for macroscopic properties were elucidated from the local structure. An increase in temperature enhances the thermal motion of the ions and the system becomes loose, thereby causing a reduction in the density, viscosity and thermal conductivity of the system. As Na⁺ concentration rises, the bond lengths of N-O and N-N decrease, the structure of NO₃⁻ collapses, the system becomes more compact and the heat transfer distance is reduced, leading to an increase in density and thermal conductivity. Additionally, there is an enhancement of NO₃⁻ intramolecular interactions, which consequently results in an increase in the specific heat capacity. The diffusion activation energy and viscous activation energy are maximum in the system with 64 % Na⁺ concentration, indicating the lower mobility of the system at this composition.