Bi ₂O ₃-MoO ₃ Binary system: An alternative ultralow sintering temperature microwave dielectric

Preparation, phase composition, microwave dielectric properties, and chemical compatibility with silver and aluminum electrodes were investigated on a series of single-phase compounds in the Bi ₂O ₃-MoO ₃ binary system. All materials have ultralow sintering temperatures <820°C. Eight different xBi ₂O ₃-(1-x)MoO ₃ compounds between 0.2≤x≤0.875 were fabricated and the associated microwave dielectric properties were studied. The β-Bi ₂Mo ₂O ₉ single phase has a positive temperature coefficient of resonant frequency (TCF) about +31 ppm/°C, with a permittivity ε _r=38 and Q _f=12 500 GHz at 300 K and at a frequency of 6.3 GHz. The α-Bi ₂Mo ₃O ₁₂ and γ-Bi ₂MoO ₆ compounds both have negative temperature coefficient values of TCF∼-215 and ∼-114 ppm/°C, with permittivities of ε _r=19 and 31, Q _f=21 800 and 16 700 GHz at 300 K measured at resonant frequencies of 7.6 and 6.4 GHz, respectively. Through sintering the Bi ₂O ₃-2.2MoO ₃ at 620°C for 2 h, a composite dielectric containing both α and β phase can be obtained with a near-zero temperature coefficient of frequency TCF=-13 ppm/°C and a relative dielectric constant ε _r=35, and a large Q _f∼12 000 GHz is also observed. Owing to the frequent difficulty of thermochemical interactions between low sintering temperature materials and the electrode materials during the cofiring, preliminary investigations are made to determine any major interactions with possible candidate electrode metals, Ag and Al. From the above results, the low sintering temperature, good microwave dielectric properties, chemical compatibility with Al metal electrode, nontoxicity and price advantage of the Bi ₂O ₃-MoO ₃ binary system, all indicate the potential for a new material system with ultralow temperature cofiring for multilayer devices application.