Dielectric Properties of Pd/Y8Z Composites
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Figure 1: Schematic behaviour of the electrolyte near the electrode surface.
429 Mat. Res. Soc. Symp. Proc. Vol. 575 ©2000 Materials Research Society
small. In contrast with two parallel metal plates, the same amount of charge on the electrodes will result in a very high capacity. Between the two space charge regions the electric field strength is zero and therefore no potential drop exists in this region. So this part does not contribute to the overall capacity of the sample, which implies that the thickness of the capacitor does not influence its capacity as long as the distance between the electrodes is larger than twice the double-layer thickness. In fact a capacity can be obtained by placing electrodes anywhere on the surface of the sample. EXPERIMENTAL Cubic yttria stabilised zirconia (TZ-8Y, Tosoh company) and palladium(II)oxide (anhydrous, 99.9% Alfa) powders were separately suspended in water. The powder concentration was taken as low as possible to obtain maximum yield while at the same time particle agglomeration was prevented. Darvan C (ammonium polymethacrylicacid) was used as dispersant. For the palladium oxide a minimum of 15 wt% (dry weight base) Darvan C was added, which completely dispersed the powder. The same amount of Darvan C was added to the zirconia suspension in order to prevent agglomeration after mixing with the palladium oxide supension. The suspensions were milled with 2 mm zirconia balls for 24 hours. After milling the milling balls were removed and flushed with a Darvan C solution with the same concentration as the suspensions to prevent coagulation. To obtain a homogeneous mixture the suspensions were added to a stirring vessel in which they were mixed at approximately 1000 rpm for 1 hour. The final powder was obtained by spray-drying the suspension in a Bitchi 190 mini spray dryer after which it was powdered in a ceramic mortar. This dry powder was subsequently heated at 550 'C for 2 hours to remove all organic residues. The powder was uniaxially prepressed at approximately 750 bar after which it was isostatically pressed at 4000 bar. All compacts (except the ones for dilatometer experiments) were sintered at 1480 'C for 6 hours with heating and cooling rates of 2 and 1 °C/min, respectively. The sintering behaviour was investigated with a Netzsch 402 E dilatometer. The compacts were cut and polished after which they were further characterised by scanning electron microscopy and impedance spectroscopy. The microstructure of yttria stabilised zirconia and composites varying in palladium content were investigated with Scanning Electron Microscope (Jeol, JSM-5800at 20 kV). RESULTS Dilatometer Experiments The isostatically pressed compacts reach a green density of about 55 %. Cubic zirconia starts to sinter at approximately 900 'C and reaches a maximum sinter speed at 1250 'C. After 2.5 hours of sintering at 1460 °C the zirconia is fully dense. The palladium-zirconia composites (Pd/Y8Z) start to sinter at 900 'C and reach a maximum sinter speed between 1200 and 1250 °C. The maxim
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