Effect of CuO-based oxide additives on Bi₂O₃-ZnO-Nb₂O₅ microwave ceramics

Different kinds of CuO-MO_x mixtures were used to investigate their role in the microstructure evolution, sintering densification and microwave properties of Bi₂(Zn_1/3Nb_2/3)₂O₇ (BZN) ceramics. Without the addition of CuO-MO_x mixtures, pure BZN ceramics have to be sintered above 900°C. However, a comparable bulk density to that of pure BZN ceramics sintered at 950°C can be achieved for 2 wt% 0.29BaCO₃-0.71CuO-added at 850°C and 0.15CuO-0.85MoO₃-added BZN ceramics at 800°C, which was attributed to the liquid-phase sintering. A higher dielectric constant along with a smaller Q factor was obtained in 0.29BaCO₃-0.71CuO-added BZN ceramics because of the formation of the BaBi₂Nb₂O₉ phase. On the other hand, a larger Q factor of ∼ 1500 can be obtained at about 850°C for 1 wt% 0.15CuO-0.85MoO₃-added BZN ceramics. However, as the 0.2CuO-0.8GeO₂ mixture was used, a partial transformation of pseudo-tetragonal Bi₂(Zn_1/3Nb_2/3)₂O₇ to cubic (Bi_1.5Zn_0.5)(Zn_0.5Nb_1.5)O ₇ phase was induced possibly because both Zn²⁺ and Ge⁴⁺ have similar ion radius and electronegativity. With increasing 0.2CuO-0.8GeO₂ content, a decrease in microwave properties was also obtained due to an inhibition of sintering densification since a eutectic liquid phase was not formed.