Results And Discussion

Microstructure Characterization: The SEM images show that the BSCCO-covered area is divided into two zones with visible morphological differences: namely a central zone (approximately 15% of the total area) and a rim zone, as shown in Figure 1.

In the central zone, the EDAX analysis reveals that the composition is primarily a combination of Zr, Bi, Sr, Ca, Cu and O. On the surface of this nominal composition, there are many needle-shaped objects, typically 10 nm in diameter and 200 pm long. These whiskers have a nominal composition of YO1O2, suggesting that yttria close to the interface region has reacted with the partially-melted BSCCO to form YCu02. Zirconia was rejected to the bulk BSCCO.

In the rim zone, we found a noticeable amount of almost spherical or ellipsoidal objects, typically 100-200 nm in diameter or length. EDAX spectra show that these objects are CuO. Underneath these objects is the most interesting feature at the interface. As shown in Figure 2, most of the rim zone was covered by a uniform but highly porous microstructure. The porous microstructure consists of rectangular crystals with a grain size distribution in the 0.5-2 pm range. EDAX spectra indicate that the composition of the crystals is predominantly Zr, Sr, Ca and O. The lack of Cu and Y in the porous microstructure agrees with the existence of CuO in the rim zone and YG1O2 in the central zone, indicating a significant Cu and Y segregation at the interface.

Figure 1: SEM surface morphology of a Figure 2: A highly uniform, porous micro-

BSCCO-coated YSZ sample showing con- structure in the rim zone showing a narrow trast between the rim and central zone. particle size distribution of 0.5-2.0 pm.

Figure 1: SEM surface morphology of a Figure 2: A highly uniform, porous micro-

BSCCO-coated YSZ sample showing con- structure in the rim zone showing a narrow trast between the rim and central zone. particle size distribution of 0.5-2.0 pm.

Complex Impedance Spectra: The complex impedance spectra obtained from two types of cell were used to derive kinetic properties of each system. A typical Nyquest plot was shown in Figure 3, comparing the results from the two types of cell at 300°C. Similar to those reported in the literature, the spectrum from the Ag|YSZ|Ag cell contains three semicircles: From the high frequencies, the first semicircle represents the contribution from the bulk conductivity; following by the second semicircle for the grain boundaries and the third for the Ag|YSZ interface. The spectrum from the BSCCO-coated cell, although similar in shape, exhibits some differences: The first semicircle seems to be smaller than that of the previous cell. Since the YSZ pellets were all prepared at the same time from the same composition and heat treatments, one would expect the bulk conductivity having the same value. The difference is therefore possibly attributed to the variation in the aspect ratio. If the assumption is valid, the second semicircle of the BSCCO-coated cell is then apparently larger than that of the cell with only Ag contact. From the asymmetric shape of the second semicircle of the BSCCO-coated cell, we conclude that the impedance contributions from both grain boundary and the BSCCO layer are convoluted in this semicircle. The differences in the second semicircles allowed us to calculate the specific resistance of the BSCCO layer.

Figure 3: Nyquest plots of the Ag|YSZ|Ag and Ag|BSCCO|YSZ|Ag cells in air at 300°C.

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