C. Christenn, S. A. Ansar, German Aerospace Center (DLR), Stuttgart, Germany
Solid oxide fuel cells (SOFCs) electrochemically convert chemical energy into electrictrical power. A SOFC consists of a porous cathode, a gas tight oxygen ion conductive electrolyte, and a porous anode. The ionic conductivity of usually used yttria-stabilized zirconia (YSZ) electrolyte shows a strong dependency on layer thickness and cell operating temperature. Usually SOFCs operate in a temperature range of 900 to 1000 °C. The current development in SOFCs is focused on reducing the operating temperature below 800 °C. Reduction of cell temperature leads to decrease of ionic conductivity of electrolytes 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 IT-SOFCs. 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. 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, 4-point dc method, and mercury intrusion porosimetry and image analysis.
Summary: Plasma sprayed conventional and nanostructured yttria-stabilized zirconia electrolyte layers were constrained and non-constrained sintered.
