Columnar grain boundary coherence in yttria-stabilized zirconia thin film: effects on ionic conductivity

Abstract

This study elucidated the effects of coherence of grain boundary of 6 mol % Y 2 O 3 -doped ZrO 2 (YSZ) thin films on ionic conductivity. The YSZ thin films were deposited with several orientation textures on MgO (100) and Al 2 O 3 (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.

Chemical and structural effects on ionic conductivity at columnar grain boundaries in yttria-stabilized zirconia thin films

“…The logarithm of the ionic conductivity of all films decreases with the W-factor, as reported from a previous study. 21) This behavior is valid for all measurement temperatures of 600900°C. This result demonstrates that the ionic conductivity of oxygen ions is sensitive to the coherence of columnar grain boundaries.…”

“…In Fig. 4, two trends of dependence of ionic conductivity on the W-factor are apparent: one is that of YSZ/MgO and YSZ/Al 2 O 3 thin films (solid circles, squares, and triangles with solid lines); 21) the other is that of YSZ/SiO 2 thin films (open circles, squares, and triangles with dotted lines). The logarithm of the ionic conductivity of all films decreases with the W-factor, as reported from a previous study.…”

“…Figure 3 depicts that all films show linearity and no inflection of the plot for measured temperatures. Among them, YSZ/MgO and YSZ/Al 2 O 3 show conductivity characteristics that are mutually close, 21) on the (d) 500°C (Al 2 O 3 ), 21) and (e) 600°C (SiO 2 ).…”

“…21) Complex impedance measurements of 6 mol % Y 2 O 3 doped ZrO 2 (6YSZ) thin films were conducted with the columnar and nanocrystalline morphology with coherence of various sorts at the boundary. The ionic conductivity and the apparent activation energy of the migration of oxygen ions decreases and increases dependent on the tilt and rotation angle from the coherent grain boundary.…”

“…Details of the impedance spectroscopy were described in a separate paper. 21) A two-probe technique was used for measurement of the impedance spectra because of the adequately high impedance of YSZ. Pt wires were attached with Pt paste to the bilateral terminal of the samples and were sintered at 800°C in air.…”