Grain boundary blocking of ionic conductivity in nanocrystalline yttria-doped ceria thin films

Abstract

Nanocrystalline YDC thin films with grain-sizes ranging from 38 to 93-nm were prepared using pulsed laser deposition followed by thermal annealing. Ionic conductivity decreased up to four orders of magnitude as the grain size increased. Using energy-dispersive X-ray spectroscopy, we showed that the counter-intuitive reduction in conductivity with grain-size is likely due to dopant and impurity segregation near grain-boundaries. Spectroscopic evidence suggests that the blocking effect due to defect segregation is more dominant on ionic conductivity than the grainsizes.

Microstructure design for fast oxygen conduction

“…However, segregation of dopants could lead to higher binding energies that may prevent oxygen vacancies from participating freely in conduction. These theoretical results thus seem to agree with the recent experimental observations by An et al, 81 i.e., dopant segregation has a more pronounced impact on oxygen conductivity than grain boundary density. Therefore, to take leverage of microstructural properties, preventing dopant segregation may need to be kept in prime focus in the design of fast oxygen conducting materials.…”

“…The binding between dopants and oxygen vacancies in forming defective clusters could actually be an important parameter that may decide the ultimate fate of oxygen conductivity. Recent experimental work such as by An et al 81 on understanding the coupling between dopant segregationvacancy diffusion seems to be important and should to be aggressively pursued in future. As the grain size decreases, the concentration of dopants segregated at grain boundaries increases.…”

“…Later Bellino et al 80 reported an order of magnitude increase of conductivity in nanocrystalline yttria doped ceria (YDC) and samarium doped ceria (SmDC) compared to microcrystalline samples. An et al 81 have recently reported 4 orders of magnitude decrease with increasing grain size in nanocrystalline YDC. In contrast, Lee et al 82 and Maglia et al 83 reported no change in oxygen conductivity in nanocrystalline GDC and SmDC, respectively.…”

“…Very recently, An et al 81 performed experiments comparing the oxygen conductivity in the as-grown and annealed YSZ samples. They observed that the ionic conductivity decreased with the annealing time as the grain size grew from 10 to 50 nm at 1200°C after 10 h. The Arrhenius plot of the as-deposited, 1000°C annealed and 1200°C annealed samples is shown in Fig.…”

“…Thus, in view of these experimental observations, it appears that instead of the blocking effect of grain boundaries, larger focus may be required on preventing dopant segregation. 81,94 Theoretical calculations on oxygen energetics at grain boundaries and heterointerfaces illustrate the effect of dopant segregation on oxygen vacancy migration and binding energies. 94 The inset in Fig.…”