B site doped strontium titanate as a potential SOFC substrate

Excellent thermal and mechanical stability coupled with low cost have attracted interest in the application of the cubic perovskite SrTiO₃ as a substrate material in supported SOFC designs. For such designs increased substrate conductivity is beneficial. A method of improving conductivity is by cation substitution. Due to the constraint of electro-neutrality, oxygen ion vacancies can be generated in strontium titanate by successful substitution of tetra-valent titanium ions with divalent metal ions (M) to produce materials of stoichiometry SrTi_(1−x)M_xO_(3−x). By raising the intrinsic oxygen vacancy concentration in this manner there is an increase in available hopping sites. The increase in vacant sites facilitates oxygen transport through the crystal and hence increases the potential for oxide ion conductivity. The synthesis of such materials was carried out by standard solid-state techniques using calcium and magnesium as dopants. B site solubility limits for both species were obtained by powder X-ray diffraction. The conductivity behaviour of successful phase pure compounds was investigated using AC impedance spectroscopy and four point DC measurements across a range of pO₂ values. The B site solubility limit for magnesium was found to lie between 5 and 7 %. SrTi_0.95Mg_0.05O_2.95 exhibited increased conductivity and reduced activation energy for conduction as compared to undoped strontium titanate. DC measurements for the same material confirmed the increased p-type behaviour of the system associated with magnesium doping at high oxygen partial pressures. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France Oct. 1–7, 2000.