Space-charge concepts on grain boundary impedance of a high-purity yttria-stabilized tetragonal zirconia polycrystal
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A detailed impedance analysis using the brick-layer model is performed on a high-purity yttria-stabilized tetragonal zirconia polycrystal (Y-TZP). Space-charge impedance is generally formulated and expressions for the respective space-charge models are therefrom derived depending on whether dopant ions are mobile or immobile. Pronounced yttrium segregation in Y-TZP is also considered in the analysis in that the dopant profile is assumed to be frozen from a high-temperature equilibrium distribution. Comparison with experimental observations shows that the electrically measured grain-boundary thickness corresponds to the Schottky-barrier width, slightly modified by the dopant segregation. The grain-boundary resistance is not consistent with any space-charge models and the strong defect interaction due to the yttrium enrichment is suggested to be mainly responsible.
I. INTRODUCTION
Fine grained microstructure (with grain size n(CPE2) > n(CPE3) > 0.5 are observed. The inductance effects represented by L in the equivalent circuit mainly result from the long (unshielded) platinum lead wires within the measurement rig. The inductance of the measurement setup in the present work amounts to approximately 5 × 10−5 Henry. A large inductance deforms the semicircular response of the RC circuit by shifting the data points downward by an amount proportional to the frequency.7 The effects become apparent in spectra when the resonance frequency of the RL series circuit RL(⳱R/L) is comparable to or lower than RC(⳱1/RC). Since Cb is approximately 2 × 10−12 F, the inductance effects become appreciable when Rb is less than approximately 2 × 104 ⍀. In the highfrequency end of the spectrum in Fig. 2(c) ( f > 106 Hz) the deviation of the measured points (in closed circles) from the simulated R–CPE response (in solid lines) can be noted. B. Material characterization 1. Bulk dielectric constant
The relative dielectric constant (⑀b) of 2Y-TZP estimated from bulk capacitance Cb is 106 (±2). Similar values can be estimated from the spectra in the previous reports.11,23 The dielectric constant values of YSZ range from 25 to 35,24 –26 which increase with decreasing yttrium content.26,27 The large dielectric constant value of Y-TZP appears thus qualitatively consistent even though the different crystal structures are involved. For YSZ the dielectric constant values from electrical characterization are almost the same as those from the infrared reflectivity, indicating that no other relaxation processes occur in the intermediate frequency range.24,25
J. Mater. Res., Vol. 16, No. 9, Sep 2001
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J-S. Lee et al.: Space-charge concepts on grain boundary impedance of a high-purity yttria-stabilized tetragonal zirconia polycrystal
Large complex defect associations in YSZ like (YZr⬘VO⭈⭈YZr⬘)× are considered electrically inactive due to the small associated charges.28 On the other hand, comparative mechanical and electrical loss measurements by Weller et al.28
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