Discussion of “use of solid electrolyte galvanic cells to determine the activity of CaO in the CaO-ZrO2 system and standard gibbs free energies of formation of CaZrO3 from CaO and ZrO2”

Abstract

In a recent article, Tanabe and Nagata[1] have used Ca4P2O9 (4CaOzP2O5) as a solid electrolyte in a cell designed to measure the activity of CaO in the system CaOZrO 2 between 1572 and 1877 K. The article is interesting, not only because of new data obtained on an important system, but also because the authors have tried to extend the current experimental capabilities for measuring the activity of CaO to higher temperatures than is possible with the well-established CaF2 solid electrolyte.[2,3,4] At temperatures above 1300 K, CaF2 begins to soften and it is difficult to use the material in solid-state cells. The only other solid electrolyte suitable for measuring the activity of CaO at higher temperatures is Ca b-alumina. The b-alumina phase is metastable in the CaO-Al2O3 system. It is stabilized by the addition of MgO, with the MgO/CaO molar ratio varying from 0.7 to 0.8. However, calcium b-alumina is not in equilibrium with CaO. Hence, pure CaO cannot be used as a reference electrode, despite claims to the contrary by Kumar and Kay[5] and Rog and Kozlowska-Rog.[6] Several intermediate aluminates form at the interface between the electrolyte and the CaO reference electrode.[7] Thus, the development of new solid electrolytes for measuring the activity of CaO at T . 1300 K is important for the development of the theoretical basis of extractive metallurgy and high-temperature ceramic processing. Earlier work by Tanabe et al.[8] had established that Ca4P2O9 is an ionic conductor with transference number of Ca2+ ion close to unity in the temperature range 1373 to 1873 K and oxygen partial pressure range from 105 to 1025 Pa.