Dopant segregation and space charge effects in proton-conducting BaZrO3 perovskites

In a humidified atmosphere, acceptor-doped BaZrO3 perovskites exhibit a high bulk proton conductivity, but the total conductivity is severely decreased by the blocking character of the grain boundaries. In our study, we compare rapidly densified Y- and Sc-doped BaZrO3 ceramics (Spark Plasma Sintering, 5 min at 1600 degrees C) with samples after extended annealing at high temperature (20 h at 1700 degrees C). Under these conditions, the dopants become mobile, resulting in a strong grain boundary conductivity enhancement, although no grain growth occurs. This increase is accompanied by a significant increase in dopant concentration in the grain boundary region, as evidenced by transmission electron microscopy. The correlation between the electrical properties of grain boundaries and their chemical composition is consistent with the interpretation in terms of the space charge model with a positive excess charge in the grain boundary core and adjacent proton depletion zones.