Insight into the viscosity enhancement ability of Ca(NO3)2 on the binary molten nitrate salt: A molecular dynamics simulation study

Abstract

Molten salt is the most common used thermal energy storage material in Concentrating Solar Power plants. To lower the melting point of the most used binary mixture of NaNO3-KNO3 (60–40 wt%), Ca(NO3)2 is a wildly used addition in many advanced molten salt formulas. However, many Ca(NO3)2 containing mixtures suffer from high viscosity because of the viscosity enhancement ability of Ca(NO3)2. In this paper, the structure and transport properties of molten salt mixtures with different amount of Ca2+ is calculated with molecular dynamics simulation method. The used force field is validated with experimental viscosity data. It is found that with the addition of Ca2+, viscosities of the mixtures increase and the self-diffusion coefficients of all the ions decrease. The reason of the viscosity enhancement ability of Ca2+ is that, the strong Ca2+–NO3– “bond” caused strong Ca2+ coordination shell, making NO3– hard to escape from the coordination shell. The coordination structure of Ca2+ tends to form network structure even with the minimum Ca2+ addition. The network structure grows with the addition of Ca2+, and restricts the mobility of all the ions in the mixture.