The microwave dielectric properties and the microstructures of Bi(Nb, Ta)O₄ ceramics

The microwave dielectric properties and the microstructures of (1 - x)BiNbO_4-xBiTaO₄ ceramics (x = 0, 0.15, 0.2, 0.4, and 0.6) doped with 0.5 wt% CuO are investigated. Ta is substituted for Nb in order to adjust the temperature coefficient of the resonant frequency (τ_f) and improve the quality factor (Q). All BiNb_(1-x)Ta_xO₄ compositions can be sintered to achieve a 96% theoretical density below 960°C. The sintering temperatures of BiNb_(1-x)Ta_xO₄ increase with increasing of x values. The dielectric constants (ε_r) of all compositions saturate at 44-45 in spite of the Ta content. Q × f values of 13000-22000 (GHz) are obtained for all compositions when the sintering temperatures are in the range of 920-940°C. By increasing the amount of the doped Ta, the temperature coefficient of the resonant frequency τ_f can be adjusted from a positive value of + 15 ppm/°C to a negative value of -30 ppm/°C. The BiNb_(1-x)Ta_xO₄ ceramics can be applied to multilayer microwave devices with low sintering temperatures.