Stefan Bringuier, Nick Swinteck, Venkateswara Rao Manga, Pierre Lucas, Pierre Deymier, Krishna Muralidharan

Dept. of Materials Science and EngineeringUniversity of Arizona

Molecular dynamics study of viscosity and thermal conductivity NaCl-KCl-ZnCl2 melts

DE-EE0005942

AcknowledgmentsThe research is supported by the Department of Energy

under MURI Grant: DE-EE0005942

Structure-transport phenomena interplay in the network-forming ZnCl2-based ternary liquids

Stokes-Einstein relationship breaks down in these ZnCl2-based ternary liquids

Thermal conductivity varies as:

0.59 ZnCl2 – 0.32 KCl – 0.09 NaCl

1/NBC per tetrahedron

Motivation: ZnCl2-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+K2ZnCl4, 2- SS_NaCl-KCl#1+K2ZnCl4, 3- SS_NaCl-KCl#1+K2ZnCl4+Na2ZnCl4, 4- Liquid+K2ZnCl4+Na2ZnCl4, 5- Liquid+K2ZnCl4, 6- Liquid+K5Zn4Cl13+K2ZnCl4, 7- Liquid+K5Zn4Cl13, 8- Liquid, 9- Liquid+Na2ZnCl4, 10- Liquid+ZnCl2, 11. Liquid+ZnCl2+KZn2Cl5, 12- Liquid+KZn2Cl5 and 13. SS_NaCl-KCl#2+K2ZnCl4

ZnCl2 KCl

NaClTarget properties- Stable liquids between 523 –

1073 K- High Specific heat, Cp- Low Viscosity- High Thermal conductivity

- Not Corrosive ZnCl2-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 ZnCl2-based network forming liquids : Valid or breaks down?

Thermal conductivity of the network-forming ZnCl2-based ternary liquids

• Interplay between structure (the chain length) and the thermal conductivity

- bridging and non-bridging chlorines as a function of T, XNa and XK

• 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

ZnCl2KCl

NaCl

0.6-0.2-0.2: Exp fit[Nitta et al.]

Self-diffusion in the ternary liquids showed Arrhenius dependence on temperature

0.63 ZnCl2 – 0.18 KCl – 0.19 NaCl0.59 ZnCl2 – 0.32 KCl – 0.09 NaCl

0.55 ZnCl2 - 0.4 KCl - 0.05 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 ZnCl2-rich ternary liquids

0.63 ZnCl2 – 0.18 KCl – 0.19 NaCl

0.55 ZnCl2 - 0.4 KCl - 0.05 NaCl

0.59 ZnCl2 – 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 XKCl > 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

ZnCl2 KCl

NaCl

Thermal conductivity of NaCl-KCl-ZnCl2 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 ZnCl2-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

• ZnCl2-NaCl-KCl ternary mixtures meet the targets on transport properties as required by high temperature heat transfer fluids in concentrating solar power plants