J.A. Labrincha

Cathode materials for intermediate temperature SOFCs

Abstract This work examines the electrical transport properties and electrode performance of two ceramic cathode materials (La0.84Sr0.16Co0.3Fe0.7O3 – LSCFe7 and La0.84Sr0.16Co0.7Fe0.3O3 – LSCFe3) prepared by solid state reaction from oxides and carbonates. Point-type electrodes pressed against the electrolyte material (GCO – Gd2O3 doped CeO2) were used to study the intrinsic performance of these electrode–electrolyte combinations. Pt electrodes were also used for comparison. For both ceramic cathodes a significant decrease in the cell ohmic contribution with increasing cathodic overpotentials suggests that the electrochemical reaction spreads from the triple phase boundary line to the electrode surface, a typical feature of mixed conducting electrode materials. Overall, the steady state cathodic polarization measurements performed at 800°C showed the LSCFe3 sample to be the best cathode material. LSCFe3 also had a higher conductivity than LSCFe7 (in the range 200 to 800°C), thus showing an overall highly promising performance.

A combustion synthesis method to obtain alternative cermet materials for SOFC anodes

Abstract Homogeneous mixtures of nanocrystalline powders of Ni–YSZ, (Ni,Co)–YSZ, (Ni,Fe)–YSZ, and (Ni,Cu)–YSZ were obtained by combustion synthesis from mixtures of molten nitrates and urea. Electrolyte/anode bilayers were then prepared by pressing a cermet layer onto a previously pressed yttria-stabilised zirconia (YSZ) electrolyte layer, at room temperatures, and then co-firing; this yields a porous cermet anode, and a dense YSZ layer with good adhesion. The anode layer remains sufficiently porous even after firing at temperatures up to 1450°C. This technique is thus suitable to obtain porous cermets for SOFC anodes with homogeneous distribution of the metallic and ceramic components. The effects of different parameters of this chemical preparation route, such as the furnace temperature, the amount of urea and the cooling rate, have been studied, together with the preparation of different compositions of cermets. These results have shown that the composition and the volume percentage of the metallic phases can be predicted. Easy control of the composition and a good reproducibility have been obtained. In terms of physical properties, nanometric particles with high specific surface area have been produced. The resulting electrodes co-pressed on the electrolyte have exhibited fine grains and porous microstructures, as well as a very good adherence to the electrolyte, even at high sintering temperature.

La2Zr2O7 formed at ceramic electrode/YSZ contacts

Strontium-doped LaCoO3 or LaMnO3 materials have been studied for use as cathodes for solid oxide fuel cells (SOFCs). This choice relies on the required properties and competitive cost. However, formation of reaction products under typical electrode-firing conditions may affect the performance of SOFCs. La2Zr2O7 was detected at YSZ/electrode interfaces. This reaction product was synthesized from powders and characterized to obtain a better understanding of its effects on cell performance. Its structural, thermal, and electrical properties are reported.

Applicability of the brick layer model to describe the grain boundary properties of strontium titanate ceramics

AbstractImpedance spectroscopy was used to evaluate the bulk and grain boundary resistances (R B and R gb ) of undoped strontium titanateceramicsinairandinN 2 .Thehighfrequencycontributionofthespectradoesnotdependsignificantlyontheaveragegrainsizeorgrainsize distributions, and was thus ascribed to the bulk behaviour. The grain boundary results obtained for several samples vary with theaverage grain size, and these e•ects nearly agree with predictions by a simple brick layer model. The deviations from this model mayincrease with decreasing temperature, and are somewhat greater in N 2 than in air. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Electrical resistance; Grain boundaries; Impedance spectroscopy; SrTiO 3 1. IntroductionThe barrier character of grain boundaries of strontiumtitanate ceramics play an important role on low tempera-ture applications such as multilayer capacitors. 1 Thisinterest thus prompted investigations to separate the bulkand grain boundary contributions by comparing impe-dance spectra obtained for single crystals, bicrystals, andceramic samples,

An alternative representation of impedance spectra of ceramics

Abstract Suitable approximations are derived to extract the relevant parameters of the bulk, grain boundary, and electrode contributions of impedance spectra from Z′ vs. Z″/f plots. Corrections are proposed for cases where the difference between bulk and grain boundary relaxation frequencies and/or the difference between grain boundary and electrode relaxation frequencies is relatively small. In these cases, the Nyquist plot (Z″ vs. Z′) shows overlapped arcs, but inspection of alternative Z′ vs. Z″/f representations may reveal contributions with relatively small differences in relaxation frequencies. These plots and the corresponding derivative dZ′/d(Z″/f) may be useful to interpret cases showing a significant distribution of grain boundary relaxation frequencies.

Protonic conduction in La2Zr2O7-based pyrochlore materials

Abstract Electrochemical pumping of oxygen can be used to adjust the oxygen partial pressure without changing the total content of hydrogen containing species (H2+H2O), and this is suitable for studying materials with proton conductivity selectively dependent on water vapour partial pressure. This method was used to demonstrate that lanthanum zirconate based materials possess a significant protonic conductivity contribution at temperatures lower than about 1000°C, and to revise the defect chemistry model. Y for Zr substitution enhances the electron hole, oxygen ion and protonic conductivity contributions, as expected for addition of an acceptor species. The effective mobility of oxygen vacancies, and the equilibrium constant for formation of anti-Frenkel defects were evaluated by taking into account the oxygen ion conductivities in Y-containing and undoped lanthanum zirconate.

Development of glass-ceramics by sintering and crystallization of fine powders of calcium-magnesium-aluminosilicate glass

Abstract Natural raw materials normally used in the ceramic and glass industry were studied for the production of calcium–magnesium–aluminosilicate glass or glass-ceramic materials. Sintering and crystallization processes of fine powders of parent glass with chemical composition (wt.%) 46.00 SiO 2 , 15.90 Al 2 O 3 , 1.20 Fe 2 O 3 , 0.42 TiO 2 , 23.50 CaO, 9.37 MgO, 0.04 Na 2 O, 0.98 K 2 O, 1.95 P 2 O 5 and 0.35 CaF 2 were studied. Crystallization kinetics of glass-ceramics was carefully examined by DTA, XRD, and SEM techniques and by dilatometric studies. The desired sequence of events, i.e. nucleation, sintering and devitrification occurred by heat-treating the glassy powder. However, crystallization of the parent glass did not follow phase diagram predictions, since anomalous appearance of akermanite phase was detected along with expected anorthite and diopside precipitation. A reasonable explanation for this unexpected observation is given.

PHOTOCATALYTIC DECOLOURATION OF ORANGE II BY ZNO ACTIVE LAYERS SCREEN-PRINTED ON CERAMIC TILES

In this work ZnO layers have been deposited by screen-printing in common ceramic tiles. These layers were characterized and tested for the photocatalytic degradation of the organic dye Orange II in aqueous solutions, using a batch photoreactor either under visible light provided by a Philips ML-160 W lamp or under direct exposure to sunlight. For sake of comparison, ZnO suspensions have also been evaluated for similar reacting conditions. The influence of experimental parameters such as (i) firing temperature of the printed layer; (ii) layer thickness; and (iii) operation time have been investigated. Screen-printed ZnO layers obtained in optimal processing conditions showed photocatalytic activity comparable to aqueous ZnO suspensions. The maximal attenuation degree is over 70% and decolourisation rate, assuming that reaction kinetics follows a pseudo-first order rate law, is over 0.015 min(-1). Thus these ZnO-layered ceramic tiles can be regarded as an alternative to photocatalytic suspensions of the same material with the advantage of avoiding the removal of the photocatalyst.

Permeability of diatomite layers processed by different colloidal techniques

Abstract Recently, efforts have been directed towards the development of ceramic filters for water treatment systems in which the microstructure is tailored to the application, is well characterised and is reproducible. This work reports on the use of slip and tape casting techniques, as well as a new “direct consolidation technique”, to obtain porous diatomite layers for filtration purposes. Diatomite has been chosen because of its low price, abundance, and intrinsic properties such as high porosity and small grain size. Layers prepared by tape casting and lamination, with a bending strength of 57 MPa, were almost 3 times stronger but less permeable than those produced by slip casting, due to the presence of binders and to the lamination step that promoted particle rearrangement. However, both forming techniques gave relatively low permeable layers due to fine (0.25–0.6 μm) and monomodal pore size distributions. The use of starch granules (30–50 wt%) as pore former and consolidator agents enabled to increase the permeability of sintered bodies for almost one order of magnitude as a result of the increasing amount and average size of pores. As expected, porosity and average pore size values show a direct dependence on the starch content.

Removal of nickel from aqueous solutions by clay-based beds

The use of spray-dried natural clays for the removal of nickel ions in aqueous solution is discussed in this paper. Two different clays were tested and their mineralogical composition and physical characteristics related to efficiency of nickel removal. In particular, relevant parameters including flow rate and overall removal efficiency were evaluated for 4.8 cm depth filters. The removal efficiency of the tested clays was generally similar or greater than that of granular activated carbon (GAC). The mineralogical composition of the clays, particularly the presence of montmorilonite, might explain this behaviour. Higher cation exchange capacity and development of surface negative charge on clay particles in contact with water also contribute to this promising performance, despite the lower available specific surface area of the spray-dried grains in comparison with granular activated carbon. The calcination of pressed pellets made of contaminated filter grains gives consolidated ceramic bodies that can be used in the fabrication of common products like wall or floor tiles, where nickel ions remain firmly attached and inertized.