Impact of Thermochemical Treatments on Electrical Conductivity of Donor-Doped Strontium Titanate Sr(Ln)TiO 3 Ceramics.

The remarkable stability, suitable thermomechanical characteristics, and acceptable electrical properties of donor-doped strontium titanates make them attractive materials for fuel electrodes, interconnects, and supports of solid oxide fuel and electrolysis cells (SOFC/SOEC). The present study addresses the impact of processing and thermochemical treatment conditions on the electrical conductivity of SrTiO 3 -derived ceramics with moderate acceptor-type substitution in a strontium sublattice. A-site-deficient Sr 0.85 La 0.10 TiO 3-δ and cation-stoichiometric Sr 0.85 Pr 0.15 TiO 3+δ ceramics with varying microstructures and levels of reduction have been prepared and characterized by XRD, SEM, TGA, and electrical conductivity measurements under reducing conditions. The analysis of the collected data suggested that the reduction process of dense donor-doped SrTiO 3 ceramics is limited by sluggish oxygen diffusion in the crystal lattice even at temperatures as high as 1300 °C. A higher degree of reduction and higher electrical conductivity can be obtained for porous structures under similar thermochemical treatment conditions. Metallic-like conductivity in dense reduced Sr 0.85 La 0.10 TiO 3-δ corresponds to the state quenched from the processing temperature and is proportional to the concentration of Ti 3+ in the lattice. Due to poor oxygen diffusivity in the bulk, dense Sr 0.85 La 0.10 TiO 3-δ ceramics remain redox inactive and maintain a high level of conductivity under reducing conditions at temperatures below 1000 °C. While the behavior and properties of dense reduced Sr 0.85 Pr 0.15 TiO 3+δ ceramics with a large grain size (10-40 µm) were found to be similar, decreasing grain size down to 1-3 µm results in an increasing role of resistive grain boundaries which, regardless of the degree of reduction, determine the semiconducting behavior and lower total electrical conductivity of fine-grained Sr 0.85 Pr 0.15 TiO 3+δ ceramics. Oxidized porous Sr 0.85 Pr 0.15 TiO 3+δ ceramics exhibit faster kinetics of reduction compared to the Sr 0.85 La 0.10 TiO 3-δ counterpart at temperatures below 1000 °C, whereas equilibration kinetics of porous Sr 0.85 La 0.10 TiO 3-δ structures can be facilitated by reductive pre-treatments at elevated temperatures.