Impedance Spectroscopy Investigation on the Low-Temperature Degradation of Tetragonal Zirconia: Influence of Measurement
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ABSTRACT Impedance spectroscopy has been used for the characterization the low-temperature degradation of yttria-stabilized tetragonal zirconia polycrystals in a non-destructive assay. Superficial transformations resulting from annealing in air lead to the appearance of an extra relaxation in the impedance spectra which evolves with degradation extent. It has been demonstrated that in situ electrical characterization cannot be performed because the electrode layers retard the degradation phenomenon. The ideal temperature/time condition to record spectra is also discussed. INTRODUCTION Impedance spectroscopy (IS) is a relatively new and powerful non-destructive tool for microstructural characterization of moderately conductive polycrystalline ceramics. Since the pioneering work by Bauerle in 1969 [1], it has been widely used for electrical characterization of zirconia ceramics, as well as other solid-oxide electrolytes [2,3]. The advantage of IS is its ability to separate intragrain, grain-boundary and electrode contributions. The grain-boundary response is believed to be due to the blocking of the ionic carriers at the interfaces of the grains and isolating second phases, generally rich in Si, precipitated in these regions. This blocking concept has been extended to others impermeable regions such as pores and cracks [4]. Impedance spectra are strictly related with the microstructure of ceramic materials and therefore it may be used as a probe to accompany microstructural evolution. This non-destructive technique has been employed, for example, to monitor both sintering process [5,6] and phase transformation [7-10], and also to determine the localization of the second phase [10-13] and solute segregation [11,13,14]. Zirconias containing tetragonal phase have been extensively studied because of their high strength and high fracture toughness [15,16]. These superior mechanical properties are attributed to the stress-induced phase transformation from metastable tetragonal to stable monoclinic occurring near propagating cracks [16]. Yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) has received special attention due to its excellent mechanical properties, good sinterability, and high conductivity at moderate temperatures [17]. Although such characteristics make Y-TZP a good candidate for application as an electrolyte material in solid oxide fuel cells, when the ceramic pieces are annealed or used at low temperatures (60'-500'C), particularly in humid environments, their mechanical properties are strongly deteriorated [18]. This so-called "low-temperature degradation" has been widely documented [18-30]. It is believed to be caused by the spontaneous tetragonal to monoclinic transformation at the surfaces without applied stresses. These transformations cause volume 87 Mat. Res. Soc. Symp. Proc. Vol. 591 ©2000 Materials Research Society
expansion with appearance of both micro and macrocracks on the surfaces. The degradation of the electrical properties has also been verified [9,10]. During preliminary experi
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