Microstructures and microwave dielectric properties of (1 − x)Sr_0.2Na_0.4Sm_0.4TiO₃–xLnAlO₃ (Ln = Nd, Pr and Sm) ceramic systems

The microstructures and microwave dielectric properties of the (1 − x)Sr_0.2Na_0.4Sm_0.4TiO₃–xLnAlO₃ samples (where Ln stands for Nd, Pr and Sm, x = 0.2–0.3) were studied. The XRD result showed that the (1 − x)Sr_0.2Na_0.4Sm_0.4TiO₃–xLnAlO₃ (Ln = Nd, Pr and Sm) solid-solutions with single orthorhombic perovskite phase could be formed in the componential range of 0.2 ≤ x ≤ 0.3. With an increasing of Ln³⁺ content, the dielectric constant (ε _r ) decreased due to the smaller dielectric polarizability of LnAlO₃ than that of Sr_0.2Na_0.4Sm_0.4TiO₃. With increasing x from 0.2 to 0.3, the quality factor (Q·f) firstly increased and then decreased for the specimens with PrAlO₃, because the Q·f values were strongly depended on grain sizes. Moreover, either increasing the A-site bond valence or decreasing the B-site bond valence led to a decrease in the temperature coefficient of the resonant frequency (τ _f). Typical values of ε _r ~ 52.7, Q·f ~ 9700 GHz (at 4.33 GHz) and τ _f ~ 1.5 ppm/°C were obtained for the (1 − x)Sr_0.2Na_0.4Sm_0.4TiO₃–xSmAlO₃ (x = 0.25) ceramic sintered at 1470 °C for 4 h.