Dopant Segregation and Space Charge Effects in Proton-Conducting BaZrO₃ Perovskites

Abstract

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 °C) with samples after extended annealing at high temperature (20 h at 1700 °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.

Theoretical modeling of defect segregation and space-charge formation in the BaZrO3 (210)[001] tilt grain boundary

“…Potential barriers around 0.65 V were obtained at temperatures below 900 K under wet conditions, which is consistent with experimental results [6,8,[11][12][13][14][15][16]. At these temperatures the excess core charge, and thereby the potential, is here found to be due only to protons.…”

“…6. The space-charge potential is about 0.65 V below 900 K under wet conditions which is consistent with experimental results [6,8,[11][12][13][14][15][16]. Under dry conditions the potential is slightly lower by a value of 0.05-0.1 V. A similar difference in the potential barrier height between dry and wet conditions was found experimentally by Shirpour et al [16].…”

“…In turn, the potential depletes the surrounding regions of mobile charge carriers thus limiting the conductivity [9,10]. The spacecharge model has been applied to BaZrO 3 by several research groups where the space-charge potentials have been determined based on both experiments [6,8,[11][12][13][14][15][16] and theoretical modeling [17][18][19][20][21].…”

Oxygen vacancy segregation in grain boundaries of BaZrO3 using interatomic potentials

“…Recent experiments show that dopant segregation to the grain boundary can increase the grain boundary conductivity significantly [10]. If dopants were mobile then they could enter the core and the surrounding spacecharge layers, and thereby reduce the space-charge potential.…”

“…Polycrystalline samples do however display significantly lower conductivity which is caused by resistive grain boundaries (GBs) [1,3,4]. This has recently been interpreted in terms of the space-charge model by several research groups [5][6][7][8][9][10][11][12][13]. This model assumes that positively charged defects, such as oxygen vacancies, accumulate at the GB core and thereby give rise to an electrostatic potential.…”