Low Sintering Temperature Microwave Dielectric Ceramics and Composites Based on Bi_2O_3 -B_2O_3

摘要

Phase evolution, sintering behavior, and microwave dielectric properties of compounds within the Bi_2O_3-B_2O_3 phase diagram have been investigated. Two promising ceramics, Bi_6B_(10)O_(24) and Bi_4B_2O_9, were fabricated which densifled at 700℃ and 660℃ with permittivity, ε_r = 10 and 39, microwave quality factor, Qf_0 = 10800 and 2600 GHz, and temperature coefficient of resonant frequency, τ_f = -41 and -203 ppm/℃, respectively. The unusually large -ve τ_f, high ε_r and reasonable values of Qf_0 of Bi_4B_2O_9 coupled with its low sintering temperature suggested that it may be useful in the fabrication of temperature-stable composites with high ε_r, +ve ε_r end members. Composites were, therefore, fabricated of xBi_4B_2O_9 with (1-x)BaNd_2Ti_4O_(12) (xBB-BNT), and (1-x)[0.2CaTiO_3-0.8(Li_(0.5)Nd_(0.5))TiO_3] (xBB -CTLNT), respectively (x is in wt%). In each case, the sintering temperature to achieve > 95% theoretical density was reduced by ≥150℃ to 1200℃. Optimum properties were achieved for 0.075BB-BNT with ε_r = 86, Qf_0 = 5400 GHz, and τ_f = 4 ppm/℃, and for 0.04BB-CTLNT with ε_r = 127, Qf_0 = 2700 GHz, and τ_f = -4 ppm/℃. Unusually for composites, Qf_0 did not significantly deteriorate and ε_r increased markedly with respect to the high-permittivity end-member for composites with x < 0.1. Optimized compositions are suitable for applications in dielectrically loaded antennas and in COG high-frequency filter applications.