Low firable BiNbO₄ based microwave dielectric ceramics

The sintering behavior, the microstructures and the microwave dielectric properties of Bi(Nb, Ta)O₄ ceramics with different amount of CuO additions were investigated. The CuO additive, appeared at grain boundary and acted as a sintering aid, could effectively lower the sintering temperature of BiNbO₄ ceramics. However, too many CuO additions (> 1 wt.%), too long a soaking time (> 3 h) or too high a sintering temperature (> 960°C) would cause abnormal grain growth resulted in the degradation of densities and dielectric properties of BiNbO₄ ceramics. To investigate the microstructures and the dielectric properties of Bi(Nb, Ta)O₄ ceramics, 0.5 wt.% CuO addition was selected as a proper sintering aid to reduce the sintering temperature. The dielectric constant ε_r of BiNb_(1-x)Ta_xO₄ ceramics was not significantly changed with Ta substitution and saturated at 44-45 for dense ceramics. The obtained quality values (Q × f) ranged from 4000 to 21 000 (GHz) were found to be functions of the sintering temperatures and the amount of Ta substitution. The τ_f values were shifted toward negative direction and became more negative with the increase of Ta content. Zero temperature coefficient of resonator frequency could be obtained by properly adjusting the Ta content.