BSCF epitaxial thin films: Electrical transport and oxygen surface exchange

Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) shows promising oxygen transport properties for the use as oxygen separation membranes and cathode material for solid oxide fuel cells. In order to evaluate the electrical conductivity properties as well as surface exchange coefficients without the influence of grain boundaries present in polycrystalline bulk samples, high-quality epitaxial BSCF thin films on single crystal NdGaO3 (110) (NGO) substrates were prepared for the first time. This was achieved by pulsed laser deposition (PLD) using single-phase cubic targets sintered from powder prepared via the mixed oxide route. Structural and compositional properties of the thin films were verified by various X-ray diffraction techniques as well as by scanning electron microscopy (SEM). The electrical conductivity of the thin films was measured in dependence of temperature and compared to values obtained by measurements on polycrystalline bulk samples from the same powder as well as data from literature, showing different activation energies and higher general conductivity. Dependence of the conductivity on oxygen partial pressure has also been studied, revealing a rather small variation and indicating a possible phase change beginning to occur at oxygen partial pressures below 10−4 bar. Additionally, the surface exchange coefficient kChem of the purely surface exchange controlled samples was determined by a conductivity relaxation technique and yielded an activation energy of 93 kJ/mol.