Effects of B-site substitution and annealing on the structural and microwave dielectric properties of CaTiO₃ ceramics

CaTi_1−x (Al_0.5Ta_0.5) _x O₃ (0.2 ≤ x ≤ 0.55) ceramics were synthesized by a conventional solid-state reaction method, and the effects of B-site substitution and annealing temperature on the structural and microwave dielectric properties of CaTi_1−x (Al_0.5Ta_0.5) _x O₃ ceramics were investigated. The XRD patterns reveal the formation of single solid solution phase with orthorhombic structure for all studied compositions (0.2 ≤ x ≤ 0.55) and the unit cell volume decreases linearly with an increment of x. The SEM results suggest no evidence of secondary phase. The permittivity (ε_r) and temperature coefficient of resonant frequency (τ _f ) decrease obviously with an increase of x. And the substitution of (Al_0.5Ta_0.5)⁴⁺ for Ti⁴⁺ improves the quality factor (Q × f). The permittivity of sintered samples decreases from 85.47 to 38.38 and the temperature coefficient of resonate frequency values (τ _f ) varies from 211.78 to −38.37 ppm/°C with x increasing from 0.2 to 0.55. Moreover, the Q × f values of the sintered samples are obviously affected by the annealing temperature. Optimized microwave dielectric properties with ε_r ~ 47.94, Q × f ~ 21602.45 GHz, τ _f  ~ 5.25 ppm/°C are achieved at x = 0.45 when sintered at 1580 °C and annealed at 1450 °C for 15 h, suitable for use in satellite broadcast systems due to high frequency selectivity.