Broadband dielectric spectroscopy of standard and core-shell BaTiO3-NiO ceramic composites compared to the BaTiO3 ceramics

Using spark plasma sintering we have processed and studied two ceramic composites of 70 vol% BaTiO (BTO) and 30 vol% NiO with different topologies: standard and core-shell composites. The materials were characterized by X-ray diffraction and scanning electron microscopy and studied by quantitative broadband dielectric spectroscopy up to the infrared range using four different spectroscopic techniques in the 10-450 K temperature range. The spectra, which differed quite appreciably from each other, were fitted with the factorized dielectric function of generalized damped oscillators in the infrared-terahertz range and by a set of Cole-Cole relaxations at lower frequencies below the polar phonon frequency range. All three ferroelectric phase transitions were indicated by the maxima in the temperature dependent low-frequency permittivity, with no temperature shift compared to BTO. Using the dielectric functions of the BTO ceramics and NiO crystal, we modelled the effective dielectric functions of the composite using three models based on the effective medium approximation: Bruggeman, Lichtenecker and generalized brick model (coated spheres). The models were discussed on the basis of the degree of BTO-grain percolation through the composite, which strongly affects the effective BTO soft-mode behaviour. Qualitative agreement between the experimental and modelled effective dielectric functions was obtained, but small discrepancies indicate some interdiffusion between the BTO grains and NiO shells, which also affected the rather complex dielectric dispersion and conductivity spectra below the phonon frequency range.