Crystal structure, sintering behavior and microwave dielectric properties of Ca_xY_3−xAl_5−xTi_xO₁₂ (0 ≤ x ≤ 2.0) solid solution ceramics

A novel Ca_xY_3−xAl_5−xTi_xO₁₂ (0 ≤ x ≤ 2.0) solid solution with garnet structure was synthesized by solid-state reaction technique. Ca²⁺ and Ti⁴⁺ substituted for Y³⁺ and Al³⁺, respectively. Ca²⁺/Ti⁴⁺ doping reduced the sintering temperature of Ca_xY_3−xAl_5−xTi_xO₁₂ ceramics to 1450 °C, and it also improved corresponding densification process. Meanwhile, the doping strongly influenced microwave dielectric properties. For the samples sintered at 1550 °C for 3 h, the dielectric constant (ε_r) increased from 7.2 to 34.7 by increasing Ca²⁺/Ti⁴⁺ content, and at the same time the quality factor (Q × f) values increased from 12,000 to 45,200 GHz and then slightly declined to 41,000 GHz. The temperature coefficient of resonant frequency (τ_f) values of these samples increased monotonically from − 40 to + 31 ppm/°C by increasing Ca²⁺/Ti⁴⁺ content. Therefore, Ca_xY_3−xAl_5−xTi_xO₁₂ solid solution ceramics with x = 1.5 sintered at 1550 °C for 3 h showed optimum microwave dielectric properties: ε_r = 32.6, Q × f = 45,200 GHz and τ_f = + 7 ppm/°C. Ca_xY_3−xAl_5−xTi_xO₁₂ (0 ≤ x ≤ 2.0) solid solution ceramics with garnet structure were new promising aluminate ceramics for microwave dielectric application.