Effect of B₂O₃ on the microstructure and microwave dielectric properties of Ba(Mg_1/3Ta_2/3)O₃ ceramics

Ba(Mg_1/3Ta_2/3)O₃ (BMT) ceramics have excellent microwave dielectric properties, but they are difficult to sinter because of their high sintering temperature of 1650 °C. However, in this study the addition of B₂O₃ to BMT ceramics enabled sintering to be performed at temperatures as low as 1300 °C. The BaB₄O₇ phase, which melts at approximately 900 °C, may have been responsible for the decrease of the sintering temperature. The presence of Ba₃Ta₅O₁₅ second phase was observed in the BMT ceramics containing a large amount of B₂O₃. The relative density and dielectric constant (ε_r) were considerably increased with the addition of a small amount of B₂O₃. The Q-value was also increased when a small amount of B₂O₃ was added, but was decreased when the B₂O₃ content exceeded 0.5 mol%. The decrease of Q × f was explained by the presence of the Ba₃Ta₅O₁₅ second phase. Excellent microwave dielectric properties of Q × f = 195,000 GHz, ε_r = 24 and τ_f = 4.74 ppm/°C were obtained by adding 0.5 mol% B₂O₃ to BMT ceramic and sintering at 1500 °C for 6 h.