Ultra-low temperature co-fired ceramics with adjustable microwave dielectric properties in the Na₂O-Bi₂O₃-MoO₃ternary system: A comprehensive study

Herein, a series of microwave dielectric materials in the Na2O-Bi2O3-MoO3 ternary system, such as Na2MoO4, Na6Mo10O33, Na2Mo2O7, Bi2Mo3O12, Bi2Mo2O9, and Bi2MoO6, were studied via phase identification, microstructure characterization, spectral analysis and microwave dielectric property tests; all the samples of each component could be sintered below 750 °C. Three different kinds of bulk ceramics, x(NaBi)0.5MoO4-(1-x)Na2MoO4 (x = 0.2-0.6), x(NaBi)0.5MoO4-(1-x)Bi2MoO6 (x = 0.6, 0.8) and 0.5(NaBi)0.5MoO4-0.5Bi2Mo2O9, were considered as the main research objects in this work. The sintering temperature of the x(NaBi)0.5MoO4-(1-x)Na2MoO4 (x = 0.2-0.6) composite ceramics ranged from 600 °C to 640 °C; high-performance microwave dielectric properties with relative permittivities in the range of 8.9-16.2, Qf (quality factor) values in the range of 11 050 GHz-8,900 GHz, and τf values ranging from-27-11 ppm °C-1 could be obtained in these ceramics. For the x(NaBi)0.5MoO4-(1-x)Bi2MoO6 (x = 0.6, 0.8) composite ceramics, a near-zero τf value (-2.6 ppm °C-1) was achieved with the 0.8(NaBi)0.5MoO4-0.2Bi2MoO6 ceramic sintered at 720 °C with a middle-permittivity of 32.6 and a Qf value of 14570 GHz. The multiphase ceramic 0.5(NaBi)0.5MoO4-0.5Bi2Mo2O9 could be sintered at 680 °C; the dense sample had a relative permittivity of 35.7, a Qf value of 9780 GHz and a small positive τf value of 17 ppm °C-1. As this study shows, the Na2O-Bi2O3-MoO3 ternary system had a lot of new composite ceramics with low sintering temperatures and great microwave dielectric properties. All these materials could be regarded as candidates applicable to LTCC/ULTCC technology.