Sprayed and Constrained-Sintered Zirconia Based Electrolytes

Kurzfassung

The current development in solid oxide fuel cells (SOFCs) is focused on reducing the operating temperature below 800 °C. 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-stabilized zirconia (YSZ) electrolyte by using nanostructured particles as feedstock or b) by using an electrolyte with improved ionic conductivity for intermediate temperature (IT)-SOFCs. Conventional sintering of electrolytes is performed over several hours at temperatures above 1400 °C. In this paper, plasma sprayed YSZ electrolyte layers were sintered under constraint and non-constraint conditions in the temperature range of 800 to 1520 °C. Thereby, the influence of particle size on sintering kinetics and microstructure development was analysed. By comparison of nanostructured YSZ and conventional YSZ layers, differences in the sintering rate were determined. Lower dL/L0 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. It was observed that the sintering of free-standing coatings differ from that of coatings on substrates which was explained by theory of constrained sintering. Changes of the microstructural characteristics were identified using scanning electron microscopy. According to the constraint from the substrate, the YSZ_nano 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.