Effect of TiO2 doping on microstructure and microwave dielectric properties of CaNb2O6 ceramics

The TiO₂ doped CaNb₂O₆ ceramics were successfully prepared by solid state reaction in this work. The XRD and Rietveld refinement results of the CaNb_2-xTi_xO₆ (0≤x≤0.025) showed that Ti⁴⁺ entered the Nb⁵⁺ lattice site and replaced a part of Nb⁵⁺, forming a substitutional solid solution. The XPS results of the CaNb_2-xTi_xO₆ ceramics explained that Ti⁴⁺ entered the Nb⁵⁺ lattice position and caused the formation of oxygen vacancies. Dipoles caused by oxygen vacancies limited the long-range transition of electrons, resulting in electron pinning effect. The SEM results displayed that with the addition of Ti⁴⁺, the structure of CaNb_2-xTi_xO₆ ceramics densified rapidly. The dielectric constant (ε_r), resonant frequency (τ_f) and quality factor (Q×f) values were related to density, bond valence, anharmonic motion between atoms and crystallinity, respectively, which provided a theoretical basis for further regulating microwave dielectric properties. When the doping amount of Ti⁴⁺ increased from 0.005 to 0.02, the τ_f value increased from -62ppm/°C to -22ppm/°C. The CaNb_2-xTi_xO₆ ceramics with x=0.02 sintered at 1425 °C for 4 h had excellent microwave dielectric properties: ε_r = 15.76, Q×f =30397 GHz, τ_f =-22 ppm/°C.