Diffusion in Molten Sodium Carbonate.

Area of Science:

• Materials Science
• Physical Chemistry
• Condensed Matter Physics

Background:

• Understanding ion diffusion in molten salts is crucial for applications like batteries and catalysis.
• Molten sodium carbonate exhibits complex behavior due to its ionic nature and high operating temperatures.
• Previous studies relied on tracer diffusion, necessitating direct measurement of ion mobility.

Purpose of the Study:

• To directly measure the self-diffusion coefficients of sodium and carbonate ions in molten sodium carbonate.
• To compare experimental results with molecular dynamics simulations.
• To investigate the influence of anion flexibility on molten salt properties.

Main Methods:

• Quasi-elastic neutron scattering (QENS) at 1143 K to probe ion dynamics.
• Analysis of scattering data to differentiate sodium and carbonate ion contributions.
• Molecular dynamics (MD) simulations using an advanced model with flexible anions and fluctuating charges.

Main Results:

• Experimental self-diffusion coefficients: D_Na = 4.5 × 10⁻⁵ cm²/s and D_CO₃ = 2.4 × 10⁻⁵ cm²/s.
• QENS data distinguished sodium ion diffusion at low wave vectors and carbonate ion diffusion at higher wave vectors.
• MD simulations, after temperature scaling based on the melting point, showed good agreement with experimental diffusion coefficients.

Conclusions:

• QENS provides accurate measurements of ion diffusion in molten sodium carbonate.
• Anion flexibility significantly impacts molten salt viscosity, leading to chain and complex structure formation upon cooling.
• The improved MD simulation methodology is suitable for studying complex molten salts.