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The electrical properties of bulk and grain boundaries of Ce x Y 0.2-x Sc 0.6 Zr 3.2 O 8-δ (0 ≤ x ≤ 0.2) have been investigated as a function of temperature and oxygen partial pressure by application of impedance spectroscopy. All samples have been prepared by means of a sol-gel combustion process. The ionic conductivities of the bulk are in the range from 0.03 to 0.04 S cm -1 at 700°C in air. In the case of ceria-doped samples a significant decrease of the ionic conductivities of both bulk and grain boundary regions is observed under reducing conditions [p(O 2 ) < 10 -15 bar, 700°C] which may be interpreted in terms of association or clustering of oxygen vacancies. In addition, ageing studies have been performed on scandia-stabilized zirconia, doped with 10 mol% Sc 2 O 3 and 1.0 mol% CeO 2 , in reducing atmosphere (1%-H 2 /Ar) at 700°C by application of a dc four-point technique according to van der Pauw and ac measurements.

EXPERIMENTALThe powders were prepared by a sol-gel combustion method [12]. Scandia powders (TTE Metals, 99.9% purity) were dissolved in nitric acid (Fisher Scientific, 70%) and hydrogen peroxide (Fisher Scientific, N30% wv). Separately, cerium (III) nitrate hexahydrate (Aldrich, 99% purity), AbstractThe conductivity behaviour of the (Ce x Y 0.2-x )Sc 0.6 Zr 3.2 O 8-d (0 ≤ x ≤ 0.2) system and composition Ce 0.04 Y 0.02 Sc 0.67 Zr 3.27 O 7.66 have been investigated as function of time using impedance spectroscopy. All samples were prepared by sol-gel and combustion process, sintered at 1500 °C for 12 h and the densities obtained were between 92 and 97%. The electrical measurements were performed at 600 °C. The conductivity values were fairly stable during the first 1800 h of the experiment but after this time the conductivity decreases. For some compositions of the system a semi-circle is detected through time, with capacitance values of an order of magnitude lower, 10 -8 F/cm. This semi-circle becomes well defined with time. After the experiment, SEM pictures show that grain boundary is well defined and an increase of pore inside grains, in some cases the surface is damaged showing cracks and fissures indicating microstructure deterioration. Keywords: scandia-zirconia, ac impedance, ageing. Resumo

Polycrystalline CaCu3Ti4O12 ceramics were prepared by solid state reactions by spark plasma sintering (SPS) technique. In this study, the effects of the dwell temperature on structural, microstructural and dielectric properties of CaCu3Ti4O12 ceramics have been investigated. Powder mixtures were calcined at 900°C for 18 h before SPS consolidation. The dwell temperatures were 850, 900, 915 and 930°C. Sintered pellets were characterized by X-ray diffraction, scanning electron microscopy and impedance spectroscopy. X-ray diffraction patterns show evidences of a single-phase perovskite-type structure. The calculated lattice parameter is 7.40 Å. The hydrostatic density increases slightly with increasing dwell temperature. Scanning electron microscopy observations revealed a heterogeneous microstructure for all samples. The dielectric loss remains constant over a wide temperature range. The obtained permittivity is approximately 103 at 1 kHz. The increase of the dwell temperature is found to produce a brittle ceramic.