Influence of microstructure on the electrical properties of iron-substituted calcium titanate ceramics

CaTi0.8Fe0.2O3-delta ceramics with grain sizes that varied from 2 to 10 mum were obtained and studied using SEM, TEM, Mossbauer spectroscopy, impedance spectroscopy, and electrochemical oxygen permeability measurements. Smaller grains developed a core-shell microstructure that consisted of a pure CaTiO3 core and an iron-rich microdomain structure at the shell. The effect of grain size on electronic conductivity was negligible. The ionic conductivity was higher for the ceramics with core-shell grains, which suggested that fast oxygen transport along microdomain Walls may have occurred. For the homogeneous ceramics, the ionic conductivity decreased with decreased grain size, in which case the grain boundary represented an additional resistance, probably because of the depletion of oxygen vacancies.