Experimental Aspects

The 10 mol% yttria-stabilized zirconia were prepared by our collaborators in the Advanced Materials Program in the Australian Nuclear Science and Technology Organization (ANSTO) in the form of sintered and polished circular pellets. The YSZ powder was obtained from TOSOH, Japan. The YSZ powder was first pressed uniaxially with a 0.5 ton of force in a 10 mm diameter die to form pellets. They were then isostatically pressed under 200 MPa to form green body. The pellets were sintered at 1400°C in air for 2 hours, air cooled, followed by polishing with 1 pm diamond paste on both sides of the pellets to the desired thickness, ranging from 0.5 to 2.0 mm.

The Bi2Sr2CaCu208 (BSCCO) electrode was prepared separately by solid state reaction. The starting materials were Bi2C>3, SrCOi, CaC03 and CuO. They were thoroughly mixed and heat treated under the following conditions: 820°C for 12 hours; 820°C for 12 hours (repeated); and, 840°C for 12 hours; to obtain the final composition. The repeated sintering and grinding procedure was to ensure a complete reaction and high degree of homogeneity. The BSCCO electrode was coated on one side of the YSZ pellets. The BSCCO powder was first mixed with an organic binder (ethyl cellulose), a lubricant (glycerol) and a solvent (ethanol) to form a paste. A layer of the BSCCO paste (approximately 500 |im thick) was applied to the YSZ pellet surface and allowed to dry for 12 hours. The dried samples were then processed by the following steps: The sample was heated at 150°C hour' to reach 890°C. The sample was then held at 890°C for 4 hours before subjected to a cooling at 300°C hour1 to room temperature. The sintering at 890°C was able to partially melt the BSCCO compound, creating an effective bonding with YSZ.

Metallic silver made from an in-house silver paste was sintered either onto BSCCO as the contact bonding material or onto YSZ as the counter electrode. The electrical contact was made by attaching a thin Ag foil (0.025 mm thick) on each side of the pellet with a Ag wire (0.25 mm in diameter). The silver paste was made from fine metallic silver powder (1 pm in particle size) and the same organic additives used in the preparation of BSCCO. The following process was used in binding Ag to the cell: The cell assembly was heated at 60°C hour-' to 880°C, held at that temperature for 4 hours and then cooled at 300°C hour-' to room temperature. Each Ag contact area is approximately 8 mm in diameter, giving an apparent surface area of about 0.5 cm2.

Two different cell configurations were constructed for complex impedance measurements: Type (A)- three cells with the Ag|BSCCO|YSZ|Ag configuration. One used a 0.5 mm thick YSZ pellet and the other two 1.0 mm. Type (B)- one cell with the Ag|YSZ(0.5 mm)|Ag configuration. The conductivity was determined by complex impedance spectroscopy between 200 and 700°C in the frequency range of 10 mHz to 1 MHz using a Solartron 1260 frequency response analyzer. All the measurements were carried out in air inside a temperature-controlled chamber.

The microstructure of the BSCCO|YSZ interface was characterized by scanning electron microscopy (SEM, Zeiss 962) and the chemical composition by energy dispersive x-ray fluorescence spectroscopy (EDAX, Oxford Instruments Link ISIS system) using one of the Type (A) cells. The Ag contact on the BSCCO electrode was removed to allow SEM examination.

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