Investigation of the mechanical properties of aluminum titanate (Al 2TiO 5) doped NI-YSZ solid oxide fuel cell anodes

Abstract

Recently, there has been growing interest in anode supported Solid Oxide Fuel Cells (SOFCs) because of improved single cell performance. In these systems, the anode layer is the thickest and provides the mechanical strength of the stack. Nickel-yttria stabilized zirconia (Ni-YSZ) composites are widely used as anode material but, in contrast to the vast amount of data available on their electrochemical properties, little data on the mechanical performance exists. This dissertation work focuses on the use of secondary materials added to traditional Ni-YSZ anodes to enhance SOFC anode mechanical performance. Small amounts of, aluminum titanate (Al2TiO5, ALT), added to the NiO-YSZ system during the manufacturing process, results in a material that is over 50% stronger than the native Ni-YSZ. Samples with different geometries have been fabricated and tested in uniaxial and biaxial strength testing apparatuses. Advanced microscopy techniques and Weibull statistical analyses have been used to properly characterize the mechanical performance, the failure mechanism and to elucidate chemical compositions. This work has found that the enhanced strength resulting from ALT is related to the development of secondary phases: Al2O3 reacts with NiO to form NiAl2O4 while TiO2 preferentially reacts with YSZ to form a solid YSZ framework defined as the 'rough phase' that add stiffness to the system and persists upon reduction. The mechanical behavior of reduced samples has been related to the partial reduction of NiAl2O4 which results in the formation of Ni nanoparticles within an Al2O3 matrix ('small particle phase'). This phase is characterized by a high strength interface while adding ductility and crack deflection ability to the system. ALT was also found responsible for changing the Ni-YSZ system failure mechanism from an intergranular to a transgranular fashion indicating the material toughness increased. During cyclic operational testing, ALT has potential for mechanical stabilization through porosity development with secondary phase formation. Testing of ALT anodes with YSZ electrolyte material showed increased strength over similar native assemblies. This dissertation work lays the foundation for future research into the effects of ALT doping on the SOFC system and how this material could be tailored for even further increases in strength.