Microwave dielectric properties of 0.7(Sr_yCa_1−y)TiO₃–0.3(Sm_xNd_1−x)AlO₃ ceramics with near-zero temperature coefficient

The microstructures and the microwave dielectric properties of 0.7(Sr_yCa_1−y)TiO₃–0.3(Sm_xNd_1−x)AlO₃ (7SCT–3SNA) ceramics system prepared by conventional solid-state route were investigated. X-ray powder diffraction results showed solid solutions with the perovskite structure were completely formed within the entire observation x range, and the unit cell volume of 0.7CaTiO₃–0.3(Sm_xNd_1−x)AlO₃ (7CT–3SNA) ceramics decreased with increasing x value. The substitution of Nd³⁺ by Sm³⁺ could enhance the densification of 7CT–3SNA ceramics, and consequently improve the dielectric properties of the ceramics. When x = 0.75, specimens exhibited excellent microwave dielectric properties with ε _r  ~ 44.52, Q × f value ~40,110 and τ _f value ~−1 ppm/°C. For further improving the microwave dielectric properties of the samples, Ca²⁺ in 0.7CaTiO₃–0.3(Sm_xNd_1−x)AlO₃ were partially substituted by Sr²⁺. The results showed that the replacement of Ca²⁺ by Sr²⁺ effectively enhanced the densification of specimens. By optimizing the composition, the ceramics with y = 0.01 in 0.7(Sr_yCa_1−y)TiO₃–0.3(Sm_0.75Nd_0.25)AlO₃ (7SCT–3SNA) showed high relative density and dense and uniform microstructures as well as well-developed grain growth with superior microwave dielectric properties: ε _r  ~ 45.38, Q × f value ~41,730 and τ _f value ~−0.2 ppm/°C.