Phase, microstructural characterization and dielectric properties of Ca-substituted Sr5Nb4TiO17 ceramics

The effect of Ca substitution for Sr on the phase, microstructure and microwave dielectric properties of the Sr5−x Ca x Nb4TiO17 composition series was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), an LCR meter, and vector network analyzer. Below 1450 °C, Sr5−x Ca x Nb4TiO17 (x = 1, 2, 3, or 4) compositions formed single-phase Sr4CaNb4TiO17, Sr3Ca2Nb4TiO17, Sr2Ca3Nb4TiO17, and SrCa4Nb4TiO17 ceramics, respectively. At x = 0 and 5, Sr5Nb4TiO17 and Ca5Nb4TiO17 formed, but along with Sr2Nb2O7 (at x = 0) and CaNbO3 and CaNb2O6 (at x = 5) secondary phases. Above 1450 °C, all the compositions formed two-phase ceramics. At low frequencies, a phase transition was observed in the composition Sr5Nb4TiO17. The substitution of Ca for Sr enabled processing of highly dense Sr2Ca3Nb4TiO17, with er ~ 53.4, τf ~ −6.5 ppm/°C and Q u × f o ~ 1166 GHz. Further investigations are required to improve the quality factor of these ceramics for possible microwave applications.