The grain size effect of La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 3-δ (LSGM) electrolyte thin film was investigated in order to clarify the mechanism of the change of ion conductivity of thin film. The film was deposited on a LaSrGaO 4 (LSGO) substrate by radio frequency magnetron sputtering and then annealed at 900 ο C or 1100 ο C to change the grain size in the film. The mean grain sizes in the films annealed at 900 ο C and 1100 ο C were 80 nm and 281 nm, respectively. The ion conductivity was measured by electrochemical impedance spectroscopy (EIS). The ion conductivity values of the film of the mean grain size of 80 and 281 nm at 800 ο C were σ=1.34 × 10 -2 and 1.81 × 10 -2 S/cm, respectively. The activation energy value of both films was 1.08 eV. The grain boundary conductivity obtained by applying the Brick-layer model was nearly two orders of magnitude lower than the bulk conductivity. The activation energy value of the grain boundary conductivity was 1.33 eV. The resistance to transport the ions across the grain boundary should be high. The higher conductivity of the 1100 ο C annealed film derives from the decrease of the high resistive grain boundary density.

In order to reveal the influence of substrate on the conductivity change of La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 3-δ (LSGM) thin film from the bulk, we prepared LSGM thin films on sapphire and the quartz substrates by radio frequency magnetron sputtering, and evaluated the crystallinity, the microstructure, and the conductivity. The lattice constant of 0.6 μm thick film was different between the substrates. Compared with the bulk, the lattice constant of the thin film on the sapphire substrate decreased 1.3%, and that on the quartz substrate increased 0.7%. The grains in the thin film on the quartz substrate were approximately 0.2μm in size, and those on the sapphire substrate were approximately 0.1μm in size. The conductivity was measured at 710°C by electrochemical impedance spectroscopy. The thin film on the quartz substrate was σ = 2.2 × 10 -3 Scm -1 and that on the sapphire substrate was 9.0 × 10 -4 Scm -1 . The reason why the thin film on the quartz substrate had higher ion conductivity than that on the sapphire substrate can be attributed to the higher density of the grain boundaries which govern the total resistivity of the film on quartz substrate than that on sapphire substrate.