Stefan Bringuier, Nick Swinteck, Venkateswara Rao Manga, Pierre Lucas, Pierre Deymier, Krishna Muralidharan Dept. of Materials Science and Engineering University of Arizona Molecular dynamics study of viscosity and thermal conductivity NaCl-KCl-ZnCl 2 melts DE-EE Acknowledgments The research is supported by the Department of Energy under MURI Grant: DE-EE

Structure-transport phenomena interplay in the network-forming ZnCl 2 -based ternary liquids Stokes-Einstein relationship breaks down in these ZnCl 2 -based ternary liquids Thermal conductivity varies as: 0.59 ZnCl 2 – 0.32 KCl – 0.09 NaCl 1/NBC per tetrahedron

Motivation: ZnCl 2 -NaCl-KCl ternary molten salts as heat transfer fluids in concentrating solar power plants T=523 K 1- SS_NaCl-KCl#1+SS_NaCl-KCl#2+K 2 ZnCl 4, 2- SS_NaCl- KCl#1+K 2 ZnCl 4, 3- SS_NaCl-KCl#1+K 2 ZnCl 4 +Na 2 ZnCl 4, 4- Liquid+K 2 ZnCl4+Na 2 ZnCl 4, 5- Liquid+K 2 ZnCl 4, 6- Liquid+K 5 Zn 4 Cl 13 +K 2 ZnCl 4, 7- Liquid+K 5 Zn 4 Cl 13, 8- Liquid, 9- Liquid+Na 2 ZnCl 4, 10- Liquid+ZnCl 2, 11. Liquid+ZnCl 2 +KZn 2 Cl 5, 12- Liquid+KZn 2 Cl 5 and 13. SS_NaCl-KCl#2+K 2 ZnCl 4 ZnCl 2 KCl NaCl Target properties - Stable liquids between 523 – 1073 K - High Specific heat, Cp - Low Viscosity - High Thermal conductivity - Not Corrosive ZnCl 2 -NaCl-KCl are network forming liquids Objectives: - Predict transport properties from molecular dynamics simulations - Interplay between the structure and transport properties

Viscosity of the network forming liquids Non-Arrhenius behavior of the polymeric liquids – Vogel-Tamman-Fulcher equation Stokes-Einstein relation for the ZnCl 2 -based network forming liquids : Valid or breaks down?

Thermal conductivity of the network-forming ZnCl 2 -based ternary liquids Interplay between structure (the chain length) and the thermal conductivity - bridging and non-bridging chlorines as a function of T, X Na and X K Rationalize the different T-dependencies of thermal conductivity behavior - e.g. NBO/T in SiO2-melts is shown to be related to the thermal conductivity [Seetharaman et al.] (NBO/T – non-bridging oxygen per tetrahedron) [Seetharaman et al.] Fundamental of Metallurgy, edited by S. Seetharaman

Viscosity of the ternary liquids is best described by Vogel-Tamman-Fulcher equation [Nitta et al.] Electrochimica Acta 54 (2009) 4898 ZnCl 2 KCl NaCl : Exp fit [Nitta et al.]

Self-diffusion in the ternary liquids showed Arrhenius dependence on temperature 0.63 ZnCl 2 – 0.18 KCl – 0.19 NaCl 0.59 ZnCl 2 – 0.32 KCl – 0.09 NaCl 0.55 ZnCl KCl NaCl Zn +2 and Cl -1 diffusion coefficients are nearly same Arrhenius behavior of diffusion in the ternary network forming liquids

Stokes-Einstein relation breaks down in these ZnCl 2 -rich ternary liquids 0.63 ZnCl 2 – 0.18 KCl – 0.19 NaCl 0.55 ZnCl KCl NaCl 0.59 ZnCl 2 – 0.32 KCl – 0.09 NaCl The polymeric/network-forming liquids - Underlying mechanisms of diffusion? - Not same for diffusion and viscous flow

Activation energies of viscous flow and diffusion are different Characteristic of network forming liquids: - Stokes-Einstein relation not valid Underlying mechanisms of diffusion and viscous flow in the ternary liquids?

Ternary liquids (with X KCl > 0.15) meet the thermal conductivity for an optimal thermal fluid ( ≥ 0.58 W/m/K at 600 °C) KCl-rich liquids in the selected compositions exhibit increasing thermal conductivity with T ZnCl 2 KCl NaCl Thermal conductivity of NaCl-KCl-ZnCl 2 ternary liquids from molecular dynamics (MD) simulations

Network-structure and thermal conductivity correlation : their temperature dependence 1/NBC per tetrahedron Higher the NBC/T lower the chain length Phonon mean free path dependent on chain length

Summary The network forming ZnCl 2 -NaCl-KCl liquids do not obey the Stokes-Einstein relationship The viscosity exhibits VTF behavior at all ternary compositions investigated in this study Temperature dependence of the thermal conductivity correlates well with non-bridging chlorines per tetrahedron in the predominantly tetrahedral network structure of the liquid ZnCl 2 -NaCl-KCl ternary mixtures meet the targets on transport properties as required by high temperature heat transfer fluids in concentrating solar power plants