Sintering behavior and microwave dielectric properties of Bi₃(Nb_1-xTa_x)O₇ solid solutions

Solid solutions of Bi₃(Nb_1-xTa_x)O₇ (x = 0.0, 0.3, 0.7, 1) were synthesized using solid state reaction method and their microwave dielectric properties were first reported. Pure phase of fluorite-type could be obtained after calcined at 700 °C (2 h)^-1 between 0 ≤ x ≤ 1 and Bi₃(Nb_1-xTa_x)O₇ ceramics could be well densified below 990 °C. As x increased from 0.0 to 1.0, saturated density of Bi₃(Nb_1-xTa_x)O₇ ceramics increased from 8.2 to 9.1 g cm^-3, microwave permittivity decreased from 95 to 65 while Q_f values increasing from 230 to 560 GHz. Substitution of Ta for Nb modified temperature coefficient of resonant frequency τ_f from -113 ppm °C^-1 of Bi₃NbO₇ to -70 ppm °C^-1 of Bi₃TaO₇. Microwave permittivity, Q_f values and τ_f values were found to correlate strongly with the structure parameters of fluorite solid solutions and the correlation between them was discussed in detail. Considering the low densified temperature and good microwave dielectric proprieties, solid solutions of Bi₃(Nb_1-xTa_x)O₇ ceramics could be a good candidate for low temperature co-fired ceramics application.