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The nano-size effect, which indicates a drastic increase in conductivity in solid electrolyte materials of nano-scale microstructures, has drawn substantial attention in various research fields including in the field of solid oxide fuel cells (SOFCs). However, especially in the cases of the conductivity of ultra-thin films measured in an in-plane configuration, it is highly possible that the 'apparent' conductivity increase originates from electrical current flowing through other conduction paths than the thin film. As a systematic study to interrogate those measurement artifacts, we report various sources of electrical current leaks regarding in-plane conductivity measurements, specifically insulators in the measurement set-up. We have observed a 'great conductivity increase' up to an order of magnitude at a very thin thickness of a single layer yttria-stabilized zirconia (YSZ) film in a set-up with an intentional artifact current flow source. Here we propose that the nano-size effect, reported to appear in ultra-thin single layer YSZ, can be a result of misinterpretation.

The physical and electrical properties of sputtered YSZ thin films on various substrates were investigated. The in-plane electrical properties of the films were measured for evaluating YSZ thin film for co-planar SOFC electrolytes. The conductance measured on YSZ over Si substrates was significantly affected by the buffer layer thickness and exhibited higher values than that of YSZ on sapphire. This indicates that electrical leakage occurred through the substrate when Si substrates were utilized. Nevertheless, pure ionic conduction was observed in YSZ/ sapphire regardless of the film thickness. It implies that much care should be taken for the selection of substrate materials in measuring or utilizing in-plane conductivity, especially for high temperature applications.

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