C. Coddet

Influence of slurry characteristics on the morphology of spray-dried alumina powders

Abstract Among all the parameters that must be managed to produce plasma sprayed coatings with controlled properties, the powder feedstock is a crucial point. The shape, density, morphology, composition and size distribution are the major characteristics of the powders that influence the coating properties and depend on the way of production. In the present work we focused on the spray drying which is a convenient process for producing sprayable ceramic powders. A systematic study has been performed to determine how the alumina slurry formulation (e.g. dispersant level, pH, binder addition) affects the granule characteristics. Aqueous slurries consisting of 30 vol.% of alumina particles, 0–1.2 wt.% ammonium polyacrylate as a dispersant and 15 wt.% styrene–ester acrylic copolymer as a binder were investigated using settling experiments and rheological analyses. Correlations appeared between sediment volume, zeta potential, yield stress and, viscosity of the slurries. At pH 4 and for 0.08 wt.% added PAA, the maximum flocculated suspension (without binder) is achieved as shown by the highest values of the sediment height (ratio of 0.85), yield stress and viscosity and the lowest zeta potential value. In contrast, at pH 9 and with the same dispersant concentration, the slurry, which is characterised by the lowest sediment ratio (0.45) and viscosity as well as no yield stress, is considered fully dispersed. Several spray dryer operating conditions were also investigated. Granules prepared from pH 4 slurry (flocculated) have solid, spherical shapes while the ones obtained from pH 9 slurry (dispersed) are hollow. It appears clearly that a close link exists between the floc structure and the granules morphology.

Characterization of YSZ solid oxide fuel cells electrolyte deposited by atmospheric plasma spraying and low pressure plasma spraying

Yttria doped zirconia has been widely used as electrolyte materials for solid oxide fuel cells (SOFC). Plasma spraying is a cost-effective process to deposit YSZ electrolyte. In this study, the 8 mol% Y2O3 stabilized ZrO2 (YSZ) layer was deposited by low pressure plasma spraying (LPPS) and atmospheric plasma spraying (APS) with fused-crushed and agglomerated powders to examine the effect of spray method and particle size on the electrical conductivity and gas permeability of YSZ coating. The microstructure of YSZ coating was characterized by scanning electron microscopy and x-ray diffraction analysis. The results showed that the gas permeability was significantly influenced by powder structure. The gas permeability of YSZ coating deposited by fused-crushed powder is one order lower in magnitude than that by agglomerated powder. Moreover, the gas permeability of YSZ deposited by LPPS is lower than that of APS YSZ. The electrical conductivity of the deposits through thickness direction was measured by potentiostat/galvanostat based on three-electrode assembly approach. The electrical conductivity of YSZ coating deposited by low pressure plasma spraying with fused-crushed powder of small particle size was 0.043 S cm−1 at 100 °C, which is about 20% higher than that of atmospheric plasma spraying YSZ with the same powder.

Effect of in-flight particle temperature on the microstructure and gas tightness of atmospheric plasma-sprayed YSZ coating

The microstructure, gas tightness and ionic conductivity of plasma-sprayed yttrium stabilised zirconia coating is evaluated. Results show that the gas permeability and ionic conductivity of coating was improved by increasing the in-flight particle temperature.