Microstructural effects on the electrical behaviour of SrTi0.95Nb0.05O3+δ materials on changing from reducing to oxidising conditions

Impedance spectroscopy has been used to investigate the electrical behaviour of SrTi0.95Nb0.05O3+δ materials upon changing from reducing to oxidising conditions. Dense materials respond very slowly to these step changes in the atmosphere. After an initial stage where the behaviour of relatively dense samples is essentially representative of an ideal resistor, the resistance increases slowly for at least up to three weeks. However, the response becomes much faster for porous samples. The electrical behaviour is thus discussed in terms of porosity. The impedance and modulus responses of the densest sample revealed two microstructural components which were interpreted in terms of a core-shell microstructure with resistive shells and conductive cores. Spectra obtained for a sample with intermediate density also revealed two components at short reoxidation times. At longer times the sample approaches a fully reoxidised condition and the core contribution is concealed even in modulus representations. Very porous samples appear to be reoxidised very fast, and only show a single arc in both types of representations (impedance and modulus plots), even at short times.