Grain-Size Effects in YSZ Thin-Film Electrolytes

The transport properties of oxygen-ion conducting yttria-stabilized zirconia (YSZ)-featuring mean grain sizes from a few nm up to the mu m regime-were studied with regard to grain-size effects. Chemically homogeneous, 8.3 mol% YSZ thin films (thickness approximately 400 nm) were processed on single-crystal sapphire substrates by a sol-gel method. The mean grain size d of the thin films was systematically adjusted to 5 nm < d < 782 nm by (i) a rapid thermal annealing step for conversion into the oxide phase and (ii) a consecutive calcination step at 650 degrees C < T-cal (24 h) < 1400 degrees C for grain growth. The quality of the thin films was examined with respect to chemical homogeneity, crystal structure, grain-size, and grain-boundary properties. Total and specific conductivities of the thin films were characterized by means of electrical impedance spectroscopy at 200 degrees < T < 400 degrees C in ambient air, where a complex nonlinear least-squares approximation was applied to determine the bulk conductivity and the grain-boundary conductivity. Despite grain boundaries being free of second phases, oxygen transport was observed to be impeded by the grain boundaries as the specific grain-boundary conductivity was determined to be two orders of magnitude below the bulk conductivity for thin films with d > 36 nm. The transport properties of nanoscaled YSZ thin films (5 nm < d < 36 nm) were modeled by application of the brick-layer model indicating the absence of beneficial grain-size effects at the nanoscale.