High-dielectric-constant microwave dielectric in the (16-x-y)CaO-xSrO-yBaO-Li2O-Sm2O3-Nd2O3-TiO2 ceramic system

This article presents the successful synthesis and characterization of a novel high dielectric constant ceramic system with near-zero temperature coefficient for microwave applications. The system, based on the 5CaO-10SrO-BaO-9Li(2)O-10Sm(2)O(3)-2Nd(2)O(3)-63TiO(2) composition, was developed through a systematic modification of the non-stoichiometric CaO-Li2O-Sm2O3-Nd2O3-TiO2 (16:9:10:2:63) ceramic system using the conventional solid-state method. The initial focus of the study was on the effect of Sr substitution for Ca. By varying the Sr content from 1 to 11 mol% and sintering at temperatures ranging from 1120 to 1240 degrees C, it was observed that increasing Sr substitution led to a higher dielectric constant (epsilon(r)), while the quality factor multiplied by the resonant frequency (Q x f) decreased, and the temperature coefficient of resonant frequency (tau(f)) increased. Notably, at 1180 degrees C, the optimized composition with 11 mol% Sr substitution (CaO-SrO-Li2O-Sm2O3-Nd2O3-TiO2 = 5: 11: 9: 10: 2: 63) achieved a dielectric constant of 140 and an acceptable Q x f value of 1,100 GHz. However, the tau(f) value was measured as 49.7 ppm/degrees C, highlighting the need for further optimization for practical applications. To address the issue of the relatively high tau(f), 1 mol% of BaO was partially substituted for SrO. This substitution resulted in the formation of a Ba-substituted CSBLSNT phase, which successfully improved the tau(f) to near-zero while maintaining a moderate epsilon(r). The optimized composition, with a molar ratio of 5: 10: 1: 9: 10:2: 63 for CaO-SrO-BaO-Li2O-Sm2O3-Nd2O3-TiO2, was sintered at 1180 degrees C for 4 h. The resulting ceramic exhibited excellent dielectric properties: epsilon(r) = 132, Q x f = 1,200 GHz, tau(f) = 5.3 ppm/degrees C. The combination of a high dielectric constant and near-zero tau(f) value makes this ceramic system highly promising for the fabrication of smaller, more thermally stable microwave components.