Electrical microstructures of CaTiO3-Bi0.5Na0.5TiO3 microwave ceramics with high permittivity (εmax ∼487)

The (1-x)CaTiO3-xBi0.5Na0.5TiO3 [(1-x)CT-xBNT), x = 0.1–0.6] microwave ceramics were studied by traditional solid-state method. Crystal structures, microstructures, microwave dielectric properties and impedance analysis of the as-prepared ceramics were investigated systematically. Refinement of X-ray diffraction patterns from all of the samples were pure orthorhombic structure with Pbnm space group. The Raman spectra of (1-x)CT-xBNT were in accord with the previous reports except that some abnormal modes appeared in high-wavenumber of 1200 cm−1-3400 cm−1. The microstructures were dominated by even grains and the average sizes increased gradually with an increase in Bi0.5Na0.5TiO3 (BNT) content. The microwave dielectric constant, εr, improved from 184 to 487, and the quality factor, Q×f, decreased constantly from 6973 GHz to 157 GHz as the amount of BNT in (1-x)CT-xBNT increased. However, the temperature coefficient of resonant frequency, τf, was restricted to fluctuating from 680 ppm/°C to 820 ppm/°C due to that the oxygen octahedral was twisted. For analysis and modeling of impedance data of 0.7CT-0.3BNT ceramics, the most appropriate equivalent circuits were found to consist of a parallel resistor-capacitor-constant phase element (R-CPE-C). And the activation energy for grains, 1.8 eV, was different to that in the grain boundaries, 1.0 eV.