MECHANICAL PROPERTIES OF Sc???Ce????Zr????O? ELECTROLYTE MATERIAL FOR INTERMEDIATE TEMPERATURE SOLID OXIDE FUEL CELLS

Scandia doped zirconia has been considered a candidate for electrolyte material in intermediate temperature Solid Oxide Fuel Cells (SOFCs) due to its high ionic conductivity, chemical stability and good electrochemical performance. The aim of this study is to determine the mechanical properties of SCZ, ie. zirconia (ZrO?) doped with Scandia (Sc?O?) and small amount of ceria (CeO?) that are important for reliability and durability of the components manufactured from SCZ. The SCZ was prepared from powder by uniaxiall cold pressing at subsequent sintering at 1550 ?C for 4 hours. The density and porosity of the sintered samples was measured following the ASTM Standard C20-00 for alcohol immersion method. A pure cubic phase of SCZ sample was identified by X-ray diffraction (XRD) at room temperature. Quantitative compositional analyses for Zr, Sc, Ce, Hf and Ti were carried out on a Cameca SX50 electron microprobe with wavelength-dispersive spectroscopy (WDS) and energy-dispersive spectroscopy (EDS). Scanning Electron Microscopy (SEM) images were acquired using both secondary electron (SE) and back-scattered electron (BSE) detectors. WDS and EDS analysis also revealed that Zr, Sc, Ce, Hf and Ti are relatively homogeneously distributed in the structure. The average grain size of sintered SCZ samples was measured to be 4 ?m. Thermal expansion at different temperatures for the SCZ ceramic was determined using Thermal Mechanical Analyzer, and the instantaneous Coefficient of Thermal Expansion (CTE) was found to be 8.726?10?? 1/?C in the in 25-400 ?C temperature range. CTE increases monotonically with temperature above 400 ?C to 1.16?10?? at 890 ?C, most likely as a result of thermo-chemical expansion due to an increase in oxygen vacancy concentration. Room temperature Vickers hardens of 12.5 GPa was measured at loads of 1000 g, while indentation fracture toughness was found to vary from 2.25 to 4.29 MPa m???, depending on the methodology that was used to calculate fracture toughness from the length of the median corner cracks. Elastic moduli, namely Young and shear moduli were determined using Resonance Ultrasound Spectroscopy (RUS). It was found that elastic moduli decreases with temperature in non-linear manner, with significant drop in the 300-600 ?C temperature range, the same temperature range in which loss modulus determined by Dynamic Mechanical Analyzer exhibits frequency dependant peaks. The high loss modulus and significant drop in elastic moduli in that temperature regime is attributed to the relaxation of doping cation-oxygen vacancies clusters. The flexural strength in 4-point bending was measured at room temperature, 400 ?C, 600 ?C and 800 ?C. and the results were analyzed using Weibull statistics. It was found that flexural strength changes with temperature in a sigmoidal way, with the minimum strength at around 600 ?C. Non-linear decrease in strength with temperature can be traced back to the changes in elastic moduli that are caused predominately by relaxation of oxygen vacancies.