Sprayed and Constrained-Sintered Zirconia Based Electrolytes

Abstract

The current development in solid oxide fuel cells (SOFCs) is focused on reducing the operating temperature below 800°C. The reduced operating temperature promotes durability of cells and decreases stringent demands on peripheral components. However, with lower temperature the ionic conductivity of electrolytes decreases as described by Arrhenius law. Thickness reduction of the conventionally used yttria-stabilized zirconia (YSZ) electrolyte by using nanostructured particles as feedstock is a possibility to avoid this problem. Another possibility is the use of an electrolyte with improved ionic conductivity for intermediate temperature (IT)-SOFCs. Scandia-stabilized zirconia (ScSZ), owing to its high ionic conductivity, is of great interest as a potential low temperature electrolyte. Conventional sintering of electrolytes is performed over several hours at temperatures above 1400°C. In this paper, an alternative mean is suggested to produce thin hermetic YSZ and ScSZ based electrolytes after sintering at around 1000°C. Layers of ScSZ and of nanostructured YSZ were deposited on metal substrates using plasma spraying. The YSZ particles after spraying maintained nanostructure. The coatings of both materials were under compressive stresses. Both factors assisted in enhancing the kinetics for sintering and grain growth. However, it was observed that the 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 ScSZ and nanostructured YSZ was developed. All sintering measurements were performed in a temperature range of 800 to 1200 °C in a dilatometer. Sintering properties, microstructure, and conductivity of sprayed and sintered YSZ and ScSZ electrolyte layers were investigated. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were applied to analyze YSZ and ScSZ powder feedstock, sprayed and sintered layers. Porosity of sintered free-standing electrolyte layers was determined by Hg-porosimetry. Conductivity measurements were performed by 4-point dc method.