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Composite cathode materials were prepared by mixing La0.83Sr0.17Ga0.83Mg0.O-17(2.83) (LSGM) and La0.8Sr0.2MnO3 (LSM) powders fired at 1300 degrees C. Several compositions were set up containing 1, 5, 25, 50, 75 wt.% of LSM. Their microstructure and electrical behaviour were investigated by XRPD, SEM/EDS and EIS. In composites containing 50 and 75 wt.% of LSNI, the electronic contribution to conductivity is predominant, then there is only a single point at the low frequency end of the Nyquist plot. On the contrary, in the composites with up to 25 wt.% of LSNI, there is a significant amount of ionic transport, then the IS spectra show complex features: at least three different arcs can be devised and their interpretation depends upon temperature. LSGM bulk and grain boundary conductivity, as well as interface polarization between the ionic (LSGM) and electronic (LSM) phases can be separated at temperatures below 600 degrees C; total LSGM contribution, i.e. bulk plus grain boundary, LSGM-LSM interface and electrode polarizations are attributed above 600 degrees C. (c) 2006 Elsevier B.V. All rights reserved.

The recent interest in the (Sr, Mg) doped LaGaO3 family compounds as electrolyte for Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFCs) has suggested the study of the system alpha-Al2O3/La-0.8,Sr-0.2,Ga0.8Mg0.2O3-delta as a possible new ionic conductor composite. The studies concerning the electrical and in i micro structural characterization of the system alpha-Al2O3/La0.8Sr0.2Ga0.8Mg0.2O3-delta, in a wide compositions range, are reported and discussed.

Mixtures of alpha-Al2O3 and La0.8Sr0.2Ga0.8Mg0.2O3-delta, with alpha-Al2O3 concentrations ranging from 1 to 20 wt.%, were uniaxially and isostatically pressed and finally sintered at 1500 degreesC for 4 h. The phase composition and microstructure were investigated by X-ray powder diffraction and scanning electron microscopy techniques. The electrical properties were studied by complex impedance spectroscopy in a wide range of temperatures (200-800 degreesC) in air. In the sintered bodies pure alpha-Al2O3 was not detected; the partial substitution of Al for Ga was suggested entailing the formation of the solid solutions of perovskite-type (La0.8Sr0.2)(Ga1-x-yAlxMgy)O3-delta and La0.8Sr0.2Ga1-xAlx O3-delta. alpha-Al2O3 added to La0.8Sr0.2Ga0.8Mg0.2O3-delta inhibited the grain growth of the majority phase. Under isothermal conditions, the electrical conductivity decreases with increasing alumina content, while the activation energies increase. (C) 2004 Elsevier B.V. All rights reserved.

Composites prepared by mixing La0.83Sr0.17GaO0.83Mg0.17O2.83 (LSGM) and La0.8Sr0.2MnO3 (LSM) powders (1-x)LSGM-xLSM (x=0, 0.01, 0.05, 0.25, 0.50, 0.75 and 1), have been studied. Composites have been sintered at 1300 degrees C in air for 2 h, and their relative densities vary in the range 89-85%. Characterization includes phase analysis based on X-ray powder diffraction data taken at room temperature after thermal treatments at temperatures suitable for IT-SOFCs, scanning electron microscopy for detecting the grain size, the porosity and the homogeneity of the samples, and complex impedance spectroscopy measurements of the electrical conduction properties in the temperature range 300-800 degrees C. The total conductivity was about 2 x 10(-2) S cm(-1) at 800 degrees C for samples with 1% and 5% LSM; such low conductivity is consistent with the values found in the literature for pure LSGM sintered at 1400 degrees C and may account for the low relative densities and the small grain size of the samples. Present results have been compared with those obtained by electrical measurements and EDX analysis on LSGM thick films sandwiched between LSM electrodes. In both experiments there was no evidence of the formation of new phases. (c) 2005 Elsevier Ltd. All rights reserved.

The simulated and the experimental electrical conductivity of ionic conductor composites (Al(2)O(3)/yttria-stabilized zirconia) either containing different amounts (5 and 50 wt.%) of alumina or having the same amount of insulating phase (10 wt.%) with different grain sizes are presented and compared. A digital image-based modelling procedure to simulate the electrical behaviour of the composites was used here. The method works, generating. by the Voronoi tessellation technique in which a genetic algorithm is used, two-phase polyhedral microstructures and then converting them into a random electrical network. The real and imaginary part of the electrical network impedance was computed by the transfer matrix method. The model is able to reproduce the experimental results well

Al2O3/8YSZ composites with a wide range of alumina compositions, from 0 to 50 wt.%, have been prepared and characterised. The electrical properties and the microstructure were studied by complex impedance spectroscopy and scanning electron microscopy techniques, respectively. Simple models derived from effective medium theories were used to predict the qualitative trend of electrical conductivity versus Al2O3 Volume fraction and a comparison with the experimental data is reported. (C) 2002 Elsevier Science Ltd. All rights reserved

Electrical properties of yttria-stabilised zirconia/alumina (YSZ/Al(2)O(3)) composites were tentatively foreseen by simulating their complex impedance spectra. A digital image-based model was developed to describe the electrical conduction process in polycrystalline mono- or multi-phase materials. The microstructure of each polycrystalline sample was reconstructed using the Voronoi tessellation technique (in fact, ad hoc modifications with respect to the original algorithm were introduced) and it was then converted into a three-dimensional electrical network according to a set of well-defined rules. The network was solved via an iterative procedure for different frequency values and successively the Nyquist plot generated. The complex impedance spectra of different composites were simulated considering the alumina content (5,10 and 15 %wt) as a parameter; both the bulk and grain boundary electrical conductivity were calculated and the Arrhenius plots obtained. Experimental and simulated results were compared and discussed

To simulate impedance spectra of yttria-stabilised zirconia/alumina composites a digital-image based model has been developed. The computer simulation generates the polycrystalline microstructure of the material by exploiting the Voronoi tessellation technique. Then a 3-D network of cubes is superimposed onto the simulated polycrystalline microstructure and the edges of each cube substituted with discrete RC parallel electrical circuits. The composite is, in such a way, converted into a three-dimensional electrical network whose impedance has been calculated at different frequency values. The numerical solution has been performed via an iterative method based on a matrix non-linear recursion relation. Results of the simulation are presented and compared with experimental data

In order to improve the mechanical properties of YSZ, to be used as electrolyte in SOFC, its composite ceramics have been suggested as possible candidates. The present paper reports electrical conductivity measurements, performed by the complex impedance spectroscopy technique on cubic yttria stabilized zirconia (7.54mol% Y2O3)/alumina (5-10-15wt%) composite ceramics. The experiments, performed at constant oxygen partial pressure (0.21 atm) in the temperature range 523-1123 K, allow separation of the bulk and the grain boundary electrical resistance. In all specimens, the bulk conduction process presents a mechanism variation at about 870 K. At higher temperatures the activation energy decreases with increasing the A1(2)O(3) content, while the behaviour is opposite at the lower temperatures. The total conductivity shows only one slope, in the temperature range investigated, with activation energies increasing with the Al2O3 content. Grain boundary conductivities present a change in the slope of the Arrhenius plots. The results are compared with those obtained on polycrystalline YSZ (7.54mol% Y2O3) specimens