Crystal structure, electrical conductivity, thermal expansion and compatibility studies of Co-substituted lanthanum strontium manganite system

摘要

La_(0.76)Sr_(0.19)Mn_(1-x)Co_xO_(3+ -delta), LSMCo_x (0 < = x < = 1) perovskite oxides were synthesized by conventional ceramic route. The effect of Co substitution for Mn on the crystal structure, electrical conductivity and thermal expansion properties was investigated. XRD indicated rhombohedral symmetry for the studied compositions at 1673 K. The lattice parameters so determined showed significant reduction in cell volume, which is attributed to smaller ionic radii of Co~(3+) ions. The results of electrical conductivity data indicated that the conductivity mechanism is by thermally activated hopping of small polarons between localized states corresponding to Mn or Mn and Co sites of different valence value. The conductivity decreases at all temperatures up to 40 mol percent Co substitution while the energy of activation increases. This is possibly due to an increase in Jahn-Teller distortion, at an extent higher than the increase of the concentration of charge carriers. Thermal expansion coefficient values in the series increase with increasing Co content which has been explained on the basis of the changes in the spin states of the Co ions and the consequent changes in the ionic size with temperature. Solution route synthesis produces fine-size particles with better properties, consequently one composition from the above having enhanced requisite properties, viz. La_(0.76)Sr_(0.19)Mn_(0.8)Co_(0.2)O_(3+ -delta) was synthesized by sol-gel route. The sol-gel synthesized compound had crystallite size of approx30nm at 1173 K obtained using Scherrer's equation. Thus the potential of these compounds as cathodes for solid oxide fuel cells (SOFCs) have been evaluated. As Ce_(0.8)RE_(0.2)O_(2-delta) (RE = Sm, Gd) are being investigated for their use as electrolytes in SOFCs, their mechanical compatibility as well as chemical compatibility with the potential cathode material from the above LSMCo_x series was also studied.