Info

25 nm

<200 nm

Particle shape

spherical

spherical

spherical

Company

Baker, USA

TOSOH, J

Forschungszentrum Jülich, G

The characteristics of the starting powders are presented in Table 1. Lanthanum-Strontium-Manganese perovskite (LSM) with the optimized composition Lao65Sr03Mn03 was used as electroactive cathode material because of its high electric conductivity and enhanced compatibility with the electrolyte (8 mol% Yttria Stabilized Zirconia, 8YSZ) (1,2). A mixture of 8YSZ- and NiO-Powder was used for the anode. After reduction to nickel the nickel amount was 40 vol%, disregarding the porosity. The powders, which were agglomerated in the initial state, had to be deagglomerated and stabilized to prevent reagglomeration in the spray suspensions. Otherwise, the desired fine electrode microstructure - a maximum of particle fineness and a porosity of about 30% - cannot be achieved. Deagglomeration was obtained by milling the spray suspension which consisted of the powder, a suitable dispersant and a carrier (ethanol, water or mixtures thereof) for at least 24 hours until the particle size was less than 1 pm. We used high molecular weight polyelectrolytes as dispersants, which stabilizes the powder particles sterically and also electrostatically in hydrous solvents due to their dissociation. These dispersants also act as binders for the green coating. Their stabilizing effect was verified immediately after milling. Therefor the viscosity of the spray suspensions was measured with a rotation viscosimeter (Haake RV20). Wet Powder Spraying has been successfully applied for several applications (3,4). The suspension is sprayed on the substrate by means of a paint gun. Part of the carrier evaporates during the spraying process. Due to the remaining carrier present, the viscosity of the suspension remains low for a sufficient period of time so that local variations in the layer thickness can be compensated by flow processes. After complete drying binder bridges are formed between the particles and towards the substrate ensuring sufficient green strength. Removal of the binder and sintering may be performed in one step. Because of the higher viscosity and density of the powder suspensions compared to paints, the spray gun had to be modified so the spray jet could be varied and reproducibly adjusted for each powder suspension. A computer-controlled X-Y system was installed to guide the gun during spraying ensuring high reproducibility. The required coating area is achieved by masks covering the uncoated areas of the substrate. Layer thickness can be varied by the spray parameters, and by superposing several layers. The minimum thickness depends on the powder particle size. SOFC-Cathodes 90x90 mm2 and 240x240 mm2 were produced for the SOFC concept of Forschungszentrum Jülich, which is described elsewhere (5). Overlapping spraying has been applied. Spraying is performed continuously while the gun advances and retreats. The microstructure was evaluated by applying the line intersection method on SEM micrographs. For this purpose, the intercept lengths of at least 50 particles per phase were averaged for each specimen. The thickness of the unsintered coatings was measured with a 3-d coordinate measuring system, and plastic foils with I00±1 pm thickness were used as substrates. By means of impedance spectroscopy and potentiodynamic current-potential-tests, the electrochemical performance of the electrodes was determinated. A schematic diagram of the cell design for the electrochemical measurements can be found in (6).

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