Research On MgO-CaO-SiO₂-based Low Dielectric Constant Microwave Dielectric Ceramics

Both Mg₂SiO₄ and CaSiO₃ ceramics are good microwave dielectric ceramics with low dielectric constant. However, sintering temperature of Mg₂SiO₄ ceramic is high and the formation of MgSiO₃ which produced in the preparation process of Mg2SiO₄ ceramics is difficult to eliminate. The Mg₂SiO₄ ceramics also possesses a large temperature coefficient of resonant frequency. The practical application of CaSiO₃ ceramics is also affected by the narrow sintering temperature range of CaSiO₃ system. In this paper, sintering properties and dielectric properties of Zn²⁺ doped Mg2SiO₄ and Co²⁺ doped Mg₂SiO₄ ceramics were studied. yMg₂SiO₄-(1-y)CaSiO₃ was prepared and the effects of sintering aids on its sintering property and dielectric properties were investigated. The results are described as follows:(1) (ZnxMg1-x)SiO₄ and (CozMg1-z)SiO₄ ceramics were prepared. The results revealed that part of the added ZnO dissolved in Mg₂SiO₄ crystal, while Bi_1.5Zn_0.5(Zn_0.5Nb_1.5)O₇, ZnNb₂O₆ were formed by part of ZnO reacting with sintering aids. In (Co_zMg_1-z)SiO₄ ceramics, Co²⁺ replaced Mg²⁺ and completely dissolved in Mg₂SiO₄ crystal. By comparison, the dielectric loss of (Co_zMg_1-z)SiO₄ ceramics was lower than (Zn_xMg_1-x)SiO₄ ceramics. Sintered at 1250℃for 3 hours, (Zn_0.075Mg_0.925)₂SiO₄ ceramics possessed good dielectric properties:ε_r=7.9, Q=7353(1.8MHz),Ï„_c=55.8ppm/℃, (Co0.025Mg0.975)2SiO₄ ceramics possessed good dielectric properties:ε_r=7.7, Q=8850(1.8MHz),Ï„_c=50.4ppm/℃.(2) yMg₂SiO₄-(1-y)CaSiO₃ ceramics with the different Mg/Ca were prepared. When the ratio of Mg/Ca was less than 0.5, the main crystalline phases were CaSiO₃, Ca₂Mg(SiO₇), and CaMgSi₂O₆ . As the value of Mg/Ca ratio increased, CaSiO₃ phase disappeared and the sintering temperature of the sample increased. Sintered at 1290℃for 3 hours, 2Mg2SiO₄-8CaSiO₃ ceramics possessed optimum dielectric properties:ε_r=8.6, Q=9278(2MHz),Ï„_c=47.5ppm/℃.To improve the sintering properties and optimize dielectric properties, Nb₂O₅ and Bi₂O₃ complex sintering aid was added to 2Mg₂SiO₄-8CaSiO₃ ceramics. By adding different quantities and different Bi/Nb, it was found that the sintering performance was improved effectively. The sintering temperature range was widened and the sintering temperature was reduced, but the ratio Bi/Nb and the content of Nb₂O₅ and Bi₂O₃ should be control properly. When Bi/Nb=1.5, 1.45wt% Bi₂O₃ and 0.55wt% Nb₂O₅ doped 2Mg₂SiO₄-8CaSiO₃ ceramics can be sintered at 1230℃and has good dielectric properties:ε_r=9, Q=8333(2MHz),Ï„_c=52.1ppm/℃. However, when the value of Bi/Nb is 4, the serious volatility of Bi₂O₃ makes the samples of high porosity, which cause the low density and bring severely damages in sintering properties.