Effects of Postdensification Annealing on Microwave Dielectric Properties of Ba([Mg _{1− x} Co _x ] _1/3 Nb _2/3 )O ₃ Ceramics

Effects of postdensification annealing on microwave dielectric properties of Ba([ M g _{1− x} C o _x ] _1/3 Nb _2/3 )O ₃ ceramics have been investigated. The 1:2 ordered complex perovskite structure is obtained in the entire composition range of the as‐sintered samples, and the ordering degree decreases with increasing x . With increasing x , the dielectric constant (ε _r ) increases, the temperature coefficient of resonant frequency (τ _f ) shifts toward negative and can be tuned to near‐zero for x = 0.6, while the Q × f value decreases. With postdensification annealing at 1400°C in air for 24 h, the ordering degree increases, and Q × f value can be significantly improved without obvious change in ε _r , while the composition for obtaining near‐zero τ _f shifts to x = 0.8. Annealing at 1375°C and 1425°C in air for 24 h does not improve microwave dielectric properties more than annealing at 1400°C. A good combination of microwave dielectric properties of Ba([Mg _{1− x} Co _x ] _1/3 Nb _2/3 )O ₃ ceramics after postdensification annealing at 1400°C in air for 24 h has been obtained for x = 0.8: ε _r = 31.7, Q × f = 76,900 GHz, τ _f = 3.3 ppm/°C.