Phase Stability in Calcia-Doped Zirconia Nanocrystals

Calcia-doped zirconia exhibits all of the polymorphism seen in the yttria-doped zirconia ceramics, but can be produced at lowered costs and in greater abundance due to the accessibility of Calcium precursors in comparison to Yttrium. Although with great challenges, there exists an opportunity to replace yttria with calcia in applications such as ionic conductors where phase stability is critical. There is a dearth of surface characterization to enable design and prediction of the polymorphism in nanoparticulate calcia-zirconia. With recent advances in water adsorption microcalorimetry, one can accurately probe surface energies of the four zirconia polymorphs: monoclinic, tetragonal, cubic, and amorphous. The surface energies can then be coupled with bulk enthalpies extracted from oxide melt drop solution calorimetry to create a nanocrystalline phase stability diagram similar to its bulk counterpart. We report here the surface and bulk thermodynamic data on polymorphs of calcia-zirconia with composition ranging from 0 to 20 mol% calcia and use it to build a nanophase diagram for this system. The effect of the humidity in the phase stability diagram trends is also addressed and demonstrated to minimize the effect of the surface energies in the overall polymorphism trends.