Ionic conductivity of nanocrystalline yttria-stabilized zirconia: Grain boundary and size effects

Abstract

We report on the effect of grain size on the ionic conductivity of yttria-stabilized zirconia samples synthesized by ball milling. Complex impedance measurements, as a function of temperature and frequency are performed on 10 mol % yttria-stabilized zirconia nanocrystalline samples with grain sizes ranging from 900 to 17 nm. Bulk ionic conductivity decreases dramatically for grain sizes below 100 nm, although its activation energy is essentially independent of grain size. The results are interpreted in terms of a space-charge layer resulting from segregation of mobile oxygen vacancies to the grain-boundary core. The thickness of this space-charge layer formed at the grain boundaries is on the order of 1 nm for large micron-sized grains but extends up to 7 nm when decreasing the grain size down to 17 nm. This gives rise to oxygen vacancies depletion over a large volume fraction of the grain and consequently to a significant decrease in oxide-ion conductivity.

XANES and EXAFS study of the local order in nanocrystalline yttria-stabilized zirconia

“…12 It was reported that the conductivity associated to oxide ions traveling through the grain-boundary core from one grain to another one is similar for samples with grain sizes ranging from ∼1 μm to 17 nm, i.e., it is rather independent of the grain size (see Fig. 1, right axis).…”

“…More details about sample preparation and characterization were reported in previous works. 12,13 The mechanically alloyed sample is labeled as MA, and sintered samples as MA-1350, MA-950, MA-850, and MA-750, where the number indicates the sintering temperature. In addition we have also studied commercial powders of 10% mol YSZ (Tosoh-Europe), 5.4% mol YSZ (Alfa Aesar), and ZrO 2 (Alfa Aesar), hereafter labeled C-10, C-5, and C-Zr respectively.…”

“…5 The appearance of these space-charge effects in YSZ and its consequences over the oxygen ion conductivity in nanobulk YSZ or thin films has triggered a strong controversy during the last years. [6][7][8][9] While some authors reported an enhancement in the ionic conductivity in samples with nanometer size grains, 10 others have found no changes 11 or even a decrease of the ionic conductivity 12 in samples obtained with different synthesis methods, with different grain sizes, and even with different dopant concentrations.…”

F anion dynamics in cation-mixed nanocrystalline LaF3: SrF2

“…The situation resembles that of Durá et al [36] who studied oxygen ion conductivity in nanostructured ZrO 2 . The authors presented indications of an extended space-charge layer because of oxygen vacancy segregation in the grain-boundary core region.…”

“…Conductivities in the mS range at room temperature [34,35]. Space-charge and segregation effects might be used to explain the decrease in ionic conductivity when going from single crystals to nm-sized systems with high surface areas [36]. [34] would translate into very fast F exchange processes as reported for single crystals.…”