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The effects of CuO and TiO 2 additives on the microstructure and microwave dielectric properties of Al 2 O 3 ceramics were investigated. Al 2 O 3 ceramics with CuO and TiO 2 additions can be well sintered to achieve 93∼98% theoretical densities below 1,360°C due to Ti 4 Cu 2 O liquid phase sintering effect. The Qf values decreased with increasing CuO and TiO 2 content, due to the formation of the second phase Ti 4 Cu 2 O. However, the varying behaviors of the dielectric constant (ɛ r ) and temperature coefficients (t f ) were associated with phase constitutions, as a result of the change of CuO and TiO 2 content. The t f can be shifted close to 0 ppm/°C by controlling the content of CuO and TiO 2 . The specimens with 0.5 wt.% CuO and 7 wt.% TiO 2 sintered at 1,360°C for 4 h showed ɛ r of 11.8, Qf value of 30,000 GHz, and t f of −7 ppm/°C.

A new ZnO–SiO2 microwave dielectric ceramic system with Li2CO3 and Bi2O3 composite additives was studied. It was difficult to acquire dense ceramics at 1380 °C without sintering aids using the ZnO–SiO2 system. With 5 wt % Li2CO3 and 4 wt % Bi2O3 composite addition, the ZnO–SiO2 system can be sintered well at approximately 900 °C. The dielectric constant of ZnO–SiO2 system decreased with increasing SiO2 content and Q×f value acquired a maximum at a certain SiO2 content. The samples of ZnO–0.6SiO2 with Li2CO3 and Bi2O3 addition sintered at 910 °C for 2 h showed excellent properties such as εr=6.65, Q×f=33,000 GHz (at 11 GHz), and τf=-70 ppm/°C.

The effects of LiF and ZnO–B2O3–SiO2 (ZBS) glass combined additives on phase composition, microstructures, and microwave dielectric properties of Ca[(Li1/3Nb2/3)0.84Ti0.16]O3−δ (CLNT) ceramics were investigated. The LiF and ZBS glass combined additives lowered the sintering temperature of CLNT ceramics effectively from 1150° to 880°C. The main diffraction peaks of all the specimens split due to the coexistence of the non‐stoichiometric phase (A) and stoichiometric phase (B), which all possess CaTiO3‐type perovskite structures. The transformation from A into B became accelerated with the increase of LiF or ZBS content. ZBS glass restrained the volatilization of lithium salt, which greatly affected the microstructures and microwave dielectric properties. CLNT ceramics with 2 wt% LiF and 3 wt% ZBS sintered at 900°C for 2 h show excellent dielectric properties: ɛr=34.3, Q×f=17 400 GHz, and τf=−4.6 ppm/°C. It is compatible with Ag electrodes, which makes it a promising ceramic for low‐temperature cofired ceramics technology application.

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