The influence of post-sintering re-oxidation treatment on dielectric response of dense and porous Ba0.70Sr0.30TiO3 ceramics

The effects of post-sintering re-oxidation treatment on dielectric relaxation, conductive and non-linear properties of dense Ba0.70Sr0.30TiO3 ceramics and its porous homologs were discussed. The ceramics were prepared by solid state reaction and sintered at 1450 °C/2 h. The porosity degree was induced by adding 35 vol% graphite as sacrificial pore forming agent into Ba0.70Sr0.30TiO3 powder. A comparative impedance spectroscopy study was performed for the (i) as-sintered and (ii) after a post-sintering re-oxidation thermal treatment of both types of ceramics. The impedance spectroscopy analysis revealed that as-sintered dense and porous Ba0.70Sr0.30TiO3 ceramics exhibit a giant relaxation with a shift of the transition temperature from ∼34 °C to above ∼117 °C, in the frequency range of (20 kHz, 1.5 MHz), and a decrease of dielectric permittivity with increasing porosity. The presumption of presence of oxygen vacancies in the grain boundary of as-sintered Ba0.70Sr0.30TiO3 ceramics is validated by electrical inhomogeneity as revealed by the existence of two components in the complex impedance spectra. After a post-annealing treatment at 1100 °C for 36 h, the dielectric response is completely changed: the permittivity vs. temperature dependences present maxima located at around Tm≈35 °C, with almost no frequency dispersion, the dielectric losses are dramatically reduced and the complex impedance spectra present a single arch component, confirming the electrical homogeneity for both dense and porous samples. For both the investigated samples it was found that zero field permittivity ε(0), and the values of the relative tunability are also affected by the re-oxidation treatment.