Influence of grain size on impedance spectra and resistance degradation behavior in acceptor (Mg)-doped BaTiO₃ ceramics

The effect of acceptor concentration in (Mg)-doped BaTiO₃ on the resistance degradation behavior was contrasted between coarse- and fine-grain samples with ∼90 and ∼0.8 μm in size, respectively. Both cases showed similar trends that the time to degradation decreased systematically with the increase of acceptor concentration, however the absolute rates are very different for a given dopant concentration. Although the grain size and grain boundaries are important, it is also shown through an impedance analysis that the degradation behavior depends on the grain conductivity (σ_g) and ionic transference number (t_ion) as evaluated by the Warburg impedance. Under the condition of the same nominal acceptor concentration, fine-grain samples showed much lower grain conductivity (σ_g) but little differences in the grain boundary conductivity (σ_gb) than that of coarse-grain samples. The ionic transference number (t _ion) was also much smaller in fine-grain samples. These results show that the actual effective acceptor concentration on being ionically compensated to enhance oxygen vacancies becomes smaller with the decrease of grain size. This fact coupled to the increase of the number of grain boundaries accounts for the improved degradation behavior with the decrease of grain size.