Grain‐Size Effects in YSZ Thin‐Film Electrolytes

Abstract

The transport properties of oxygen‐ion conducting yttria‐stabilized zirconia (YSZ)—featuring mean grain sizes from a few nm up to the μm 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°C≤Tcal (24 h) ≤1400°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°≤T≤400°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.