Influence of ZnO addition on microstructure and proton electrical conductivity of BaZr0.8Y0.2O3-δ ceramics

Sintering aids are widely used to promote densification and grain growth for electrolytes based on yttrium-doped barium zirconate. However, there are some discrepancies in the literature about the influence of these sintering aids on the microstructure development. Some authors consider that ZnO remains on grain boundaries, forming an amorphous phase that promotes sintering, and others proposed that ZnO forms a solid solution with barium zirconate. Even considering different mechanisms, it was proposed that ZnO addition compromised protonic conductivity. In this work BaZr0.8Y0.2O3-delta (BZY20) was prepared by conventional oxide mixture (solid state sintering), adding ZnO as sintering aid. We proposed a mechanism for the ZnO actuation on the microstructure development, by the formation of a liquid phase during sintering and formation of a vitreous phase throughout grain boundaries during cooling. This could be the reason for poor protonic conductivity in comparison to the undoped BZY20 electrolytes. The proposed mechanism was established through the scanning electron microscopy analyses and electrical conductivity measurements under several different atmospheres by impedance spectroscopy. High density samples were obtained by using ZnO, but with compromised electrical conductivity compared to the undoped samples.