Microwave dielectric properties of low-loss (Zn_1-xCo_x)₃Nb₂O₈ ceramics for LTCC applications

The phase relation, crystal structure and microwave dielectric properties of (Zn_1-xCo_x)₃Nb₂O₈ ceramics were investigated. The limit of (Zn_1-xCo_x)₃Nb₂O₈ solid solutions was approximately x = 0.4. With an increasing x, the corrected dielectric constant decreased and the temperature coefficient of resonant frequency (τ_f) increased. The τ_f value is strongly dependent on the tilting of the oxygen octahedral, which can be evaluated by the B-site bond valence. However, a tremendous loss reduction can be achieved by a small amount of Co substitution due to the increase in relative density and packing fraction. The B₂O₃ and CuO addition can effectively lower the sintering temperature of (Zn_0.95Co_0.05)₃Nb₂O₈ without seriously deteriorating the dielectric properties. For LTCC applications, the 4 wt% B₂O₃-CuO doped (Zn_0.95Co_0.05)₃Nb₂O₈ ceramic sintered at 930 °C is a good choices because of its excellent dielectric properties (ε_r ∼ 20.75, Q × f ∼ 105,000 GHz and τ_f ∼ -83.7 ppm/°C) and the chemical compatibility with silver.