Directional solidification of ZrO2–BaZrO3 composites with mixed protonic–oxide ionic conductivity

The synthesis and characterization of directionally solidified Y-doped zirconia–barium zirconate eutectic materials by the laser floating zone technique (LFZ) is presented in the search for novel mixed ionic conductors by protons and oxide ions. Emphasis is placed on the control of the LFZ conditions that promote the solubilization of Y in both eutectic phases, and how the dopant influences the ionic conductivity. Fast growth under constitutional supercooling conditions leads to the development of ZrO2–dendrites above the solidification interface, which are dipped into a finer-interpenetrated network of ZrO2 and BaZrO3. Yttrium-doping of the ZrO2 fluorite-type lattice is favored by the high thermodynamic stability of BaZrO3. However, even low Y-substitution levels in BaZrO3 lead to dominating protonic conduction in the composite under humidified atmospheres below 500 °C. At higher temperatures, the proton incorporation is unfavorable, and the total conductivity of the composite is determined by the hole conduction in the Ba(Zr,Y)O3, and/or oxide–ion conductivity through the (Zr,Y)O2.