Electrical properties of Nb-doped SrTiO3 ceramics with excess TiO2 for SOFC anodes and interconnects

The electrical properties of Nb-doped SrTiO3 ceramics with and without excess TiO2 were investigated for anodes and interconnects of SOFCs by a dc four-probe conductivity measurement. The microstructures and the grain boundary properties were revealed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy, SEM-electron backscatter diffraction pattern-orientation imaging microscopy, and high-resolution transmission electron microscopy. It was found that the conductivity of Nb-doped SrTiO3 polycrystal was semiconductor-like with positive temperature coefficient while the single crystal and the polycrystal with excess TiO2 showed the conductivity with negative temperature coefficient-like metals at SOFCs operating temperatures. The TiO2 second phases was found in the polycrystal materials with excess TiO2. No grain boundary segregation was found, and the excess TiO2 was found to stay at the triple grain junctions. The grain-boundary orientation was found mostly random either with or without excess TiO2. These results suggest that the excess TiO2 reduces the influence of the grain boundary on the conduction. (c) 2007 The Electrochemical Society.