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