Columnar grain boundary coherence in yttria-stabilized zirconia thin film

This study elucidated the effects of coherence of grain boundary of 6 mol % Y2O3-doped ZrO2 (YSZ) thin films on ionic conductivity. The YSZ thin films were deposited with several orientation textures on MgO (100) and Al2O3 (102) substrates using metal-organic chemical vapor deposition (MOCVD). Impedance measurements revealed that the total ionic conductivity of the thin films was restricted by the columnar grain boundary. The orientation degree, defined by the average full width at half maximum (FWHM) of 100 pole of the YSZ thin films, mainly determines the ionic conductivity across the columnar grain boundary because of the degree of the crystallographic coherence. Films with a strongly oriented columnar structure showed ionic conductivity of about 30 times higher than that of nanocrystalline films having random orientation. The activation energy of the ionic conduction across the columnar grain boundaries simply increased concomitantly with decreasing degree of orientation of the columnar grains of the films. HRTEM analyses revealed that the small tilt angle grain boundary with low lattice defect density and with no second phase at grain boundaries showed superior properties. Consequently, a columnar structure with high coherence is preferred for use as a thin film ionic conductor.