Diffusion Behavior of Cd²⁺, Ni²⁺, Co²⁺ and Cr⁶⁺ Ions in Molten Sodium Borates

Abstract

Diffusion behavior of Cd²⁺, Ni²⁺ Co²⁺ and Cr⁶⁺ ions in molten sodium borates (Na₂O: 15.7-38.7mol%) was investigated by constant current voltammetry (chronopotentiometry) with platinum electrodes in the temperature ranges 720° to 1040°C.
Arrhenius plots of diffusion coefficients of these ions break at higher temperature than the respective liquidus in many cases of Na₂O-B₂O₃. Isothermal diffusion coefficients show minima at about 20mol% Na₂O content. There is no apparent relation between the diffusion coefficients of these ions and their ionic radii.
The activation energies for diffusion were 25 to 35kcal/mol at higher temperature region and 35 to 45kcal/mol at lower temperature region. These values are higher than the values for electric conduction but lower than the values for viscous flow of the base glasses. There is no apparent relation between the activation energies for diffusion of these ions and their “Ion-Oxygen Attraction”.
Stokes' equation does not hold between diffusion coefficient and viscosity, also between electric conduction and viscosity in molten sodium borates.
Diffusion coefficients decrease with increasing total number of oxygen ions per unit volume in the base glasses.