Constrained and non-constrained sintering of plasma sprayed zirconia based electrolytes

Kurzfassung

The current development in solid oxide fuel cells (SOFCs) is focused on reducing the operating temperature below 800 °C. The ionic conductivity of electrolytes shows a strong dependency on layer thickness and cell operating temperature. Though, the reduced operating temperature promotes durability of cells and decreases stringent demands on peripheral components, the ionic conductivity of electrolytes decreases following Arrhenius law. To solve this problem two different ways are possible: a) reducing the thickness of the conventionally used yttria-stabilised zirconia (YSZ) electrolyte by using nanostructured particles as feedstock or b) by using an electrolyte with improved ionic conductivity for intermediate temperature (IT)-SOFCs. In this paper, conventional and nanostructured YSZ electrolyte layers were prepared by plasma spraying. As all thermal sprayed coatings contain some porosity, which influences the cell performance, sprayed electrolyte layers were sintered in a second step. Conventional sintering of electrolytes is performed over several hours at temperatures above 1400 °C. Thin sprayed layers were sintered in the temperature range of 800 to 1520 °C. Thereby, the influence of particle size on sintering kinetics and microstructure development was analysed. Nanostructured YSZ particles after spraying maintained nanostructure. Nanostructure material assisted in enhancing the kinetics for sintering and grain growth. Coatings of both materials were under compressive stresses. However, it was observed that sintering of free-standing coatings differed from that of coatings on substrates which was explained by theory of constrained sintering. A detailed comparison of sintering behaviour under constrained and non-constrained conditions for conventional and nanostructured YSZ was developed. All constrained sintering measurements were performed in a dilatometer. Sintering properties, microstructure, and conductivity of sprayed and sintered YSZ electrolyte layers were investigated by scanning electron microscopy (SEM), 4-point dc method, and mercury intrusion porosimetry and image analysis. By comparison of nanostructured and conventional YSZ layers, differences in the sintering rate were determined. Lower thermal expansion rate of nanostructured compared to conventional plasma sprayed YSZ during heating above 900 °C was measured, indicating initiation of densification at lower temperature compared to conventional YSZ. A higher shrinkage rate of the nanostructured YSZ layer strongly suggests that observed differences in sintering rates are due to different particle sizes of the powder feedstock. Experimental shrinkage rates were assigned to different mass transfer effects according to the sintering model of Coble. Changes of the microstructural characteristics were identified using SEM. According to the constraint from the substrate, the nanostructured YSZ sample constrained sintered at 1000 °C has achieved a lower thickness compared to the free-standing sample sintered at 1520 °C. Comparing the values of both YSZ electrolytes sintered at 1325 °C to the ones of the as-sprayed layers, a faster sintering process of the nanostructured YSZ can be supposed. The lamellar microstructure in the as-sprayed samples was found to significantly reduce the electrical conductivity of the YSZ electrolytes, measured by 4-point dc method. Sintering of as-sprayed electrolyte layers at sufficient temperature increased the electrical conductivity, due to changes of the microstructure densification.