Calculation of Littlewood predominance diagrams for metal electrodeposition from molten chlorides using CALPHAD software and databases

Abstract

Littlewood predominance diagrams are maps of phase stability with a potential in the ordinate and a pO2− on the abscissa. pO2− is the negative logarithm of the activity of O2− ions.In the current work, a new method for calculation of Littlewood predominance diagrams is presented. The method is particularly suitable for software and databases constructed according to the CALPHAD (CALculation of Phase Diagrams) methodology. This enhances the consistency of the calculations and enables inclusion of elaborate descriptions of non-stoichiometric phases. The calculation is based on Gibbs energy minimization so that identification of relevant reaction equations is not needed as input.A thermodynamic database with all the relevant phases, including metals, chlorides, oxides and oxychlorides is a perquisite for the calculation. The calculation results depend on the thermodynamic modeling and data. However, the algorithm does not depend on the details of the thermodynamic models used. That is, the same calculation method is applied irrespective of the thermodynamic model chosen. Sample calculations are given for the simple Mg-Cl-O system and for the more-complex Ca-Ti-Cl-O system. We use thermodynamic datasets published in the literature, as well as a small CALPHAD database for the Ca-Ti-Cl-O system constructed in the course of the present work.