Thermal Residual Stress and Biaxial Strength of (Y2O3)0.08(ZrO2)0.92 / (Sc2O3)0.1(CeO2)0.01(ZrO2)0.89 Multi-Layered Electrolytes for Intermediate Temperature Solid Oxide Fuel Cells

Abstract

8 mol % Y2O3-ZrO2 [YSZ] has been widely used as electrolyte material in SOFCs. (Sc2O3)0.1(CeO2)0.01(ZrO2)0.89 [SCSZ] has higher ionic conductivity in the intermediate temperature range (600oC - 800oC) compared to YSZ, but YSZ has better chemical and phase stability. In this work YSZ and SCSZ were used in developing layered electrolytes with a unique design to incorporate both materials, resulting in an electrolyte with enhanced ionic conductivity and improved robustness. The design involved placing SCSZ layers between two outer YSZ layers, so as to produce four- and six-layered electrolytes. Tape casting, lamination and pressureless sintering techniques were used in the development of the electrolytes. Due to the mismatch of the coefficient of thermal expansion between the two materials, thermal residual stresses arise between the layers. These stresses contribute to an enhancement of the ionic conductivity of the layered electrolytes. In addition, the compressive residual stress significantly affects the mechanical properties of layered electrolytes, and improves the electrochemical performance. Biaxial flexure strength was measured using ring-on-ring strength testing at room temperature and 800oC. A finite element method was employed to calculate the maximum principal stress at fracture. The results showed that the layered YSZ/SCSZ/YSZ electrolytes have improved flexure strength at both room temperature and 800 oC because of the appearance of compressive residual stresses in the outer YSZ layers of the electrolyte. The calculated compressive stress values were also verified using Weibull statistics of strength data measured at room temperature.