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

Abstract

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–9Li₂O–10Sm₂O₃–2Nd₂O₃–63TiO₂ 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 °C, it was observed that increasing Sr substitution led to a higher dielectric constant (ε_r), while the quality factor multiplied by the resonant frequency (Q × f) decreased, and the temperature coefficient of resonant frequency (τ_f) increased. Notably, at 1180 °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 × f value of 1,100 GHz. However, the τ_f value was measured as 49.7 ppm/°C, highlighting the need for further optimization for practical applications. To address the issue of the relatively high τ_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 τ_f to near-zero while maintaining a moderate ε_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 °C for 4 h. The resulting ceramic exhibited excellent dielectric properties: ε_r = 132, Q × f = 1,200 GHz, τ_f = 5.3 ppm/°C. The combination of a high dielectric constant and near-zero τ_f value makes this ceramic system highly promising for the fabrication of smaller, more thermally stable microwave components.