Microwave dielectric ceramics in Li₂O-Bi₂O ₃-MoO₃ system with ultra-low sintering temperatures

A series of compounds in the Li₂O-Bi₂O ₃-MoO₃ ternary system were investigated with regard to the preparation, phase composition, microwave dielectric properties, and chemical compatibility with silver (Ag) and aluminum (Al) electrodes. All the ceramics in this work have sintering temperatures lower than 750°C. The sintering behaviors and microwave dielectric properties of three single phases Li ₂MoO₄, (Li_0.5Bi_0.5)MoO₄, and Li₈Bi₂Mo₇O₂₈ bulk ceramics, were of particular focus in this investigation. The Li₂MoO₄ ceramic can be sintered to a high density at 540°C/2 h with a relative permittivity ∼5.5, a Q × f value of 46 000 GHz, and a temperature coefficient of resonant frequency (TCF) of ∼-160 ppm/°C. The (Li _0.5Bi_0.5)MoO₄ ceramic has a scheelite structure and the largest relative permittivity of 44.4 among the ceramics studied in this work with a sintering temperature around 560°C, a Q × f value of 3200 GHz, and a large positive TCF of ∼+245 ppm/°C. The Li ₈Bi₂Mo₇O₂₈ ceramic could be sintered at 540°C and has a relative permittivity of 13.6, a Q × f value of 8000 GHz, and a small negative TCF value of ∼-59 ppm/°C. From the X-ray diffraction analysis of cofired ceramics, the Li₂MoO₄ ceramic does not react with either Ag or Al powders. The Li₈Bi ₂Mo₇O₂₈ ceramic reacts with Ag but not with Al at its densification temperature. The (Li_0.5Bi_0.5)MoO ₄ ceramic was found to strongly react with Ag powder and to a limited extent with Al powders. From this study, the Li₂O-Bi ₂O₃-MoO₃ ternary system has a number of attractive new materials with low sintering temperatures, high-performing microwave dielectric properties, chemical compatibility with both Ag and Al metal electrodes, nontoxicity, and low-cost constituents. All these materials can be included in the new field of ultra-low-temperature cofiring dielectrics for multilayer applications.