Farbraction And Properties Of Ba₃（VO₄）₂-based Low-permittivity And Low-temperature Fired Microwave Dielectric Composite

As the key materials of microwave substrate, dielectric antenna and othermicrowave components, the low-permittivity LTCC microwave dielectric ceramicshave been widely utilized in microwave communication systems. In thisdissertation, the near-zero Ï„fvalue materials were synthesized through combiningthe positive Ï„fvalue material （Ba₃（VO₄）₂） and negative Ï„fvalue materials(Zn_2-xSiO_4-xand LiMg_0.9Zn_0.1PO₄), to compose the Ba₃（VO₄）₂-Zn_2-xSiO_4-xandBa₃（VO₄）₂-LiMg_0.9Zn_0.1PO₄composite ceramics with low-temperature cofireability.The relationships between the processing methods, phase compositions,microstructure and the microwave dielectric properties of the composite have beeninvestigated by XRD, SEM, EDS, Raman Spectrum and network analyzer, etc. Themain contents are listed as following:First of all, the XRD results indicated that the Zn_1.87SiO_3.87powder withoutZnO secondary phase was prepared by controlling the ZnO deficiency with theZn/Si ratio of1.87:1. The SEM studies demonstrated the coexistence of Ba₃（VO₄）₂and Zn_1.87SiO_3.87phases in the （1-y）Ba₃（VO₄）₂-yZn_1.87SiO_3.87composite sinteredbodies, as further confirmed by XRD and Raman results. A slight amount ofsilicon-rich secondary phases may increase the dielectric loss of the composite, as aresult, the composite with40%Zn_1.87SiO_3.87sintered at1100oC exhibited desirablemicrowave dielectric properties of εr⁹.3，Q×f²4000GHz，and Ï„f^-0.4ppm/oC.Secondly, adding the B₂O₃into the0.5Ba₃（VO₄）₂-0.5Zn_1.87SiO_3.87compositeceramic can not only induce the formation of B₂O₃-SiO₂liquid phase where theB₂O₃reacted with the armorphous SiO₂in the composite, but also may reduce theamount of the silicon-rich secondary phase. As a result, the sintering temperatureand dielectric loss of the composite were significantly decreased. The3wt%B₂O₃doped0.5Ba₃（VO₄）₂-0.5Zn_1.87SiO_3.87ceramics can be well sintered at925oC andexhibit excellent microwave dielectric properties of Q×f⁴0800GHz and Ï„f⁰.5ppm/oC, which could be a good candidate material for LTCC applications.The （1-x）Ba₃（VO₄）₂-xLiMg_0.9Zn_0.1PO₄（x=0.4-0.65） composite ceramics wereprepared by the solid-reaction method. The XRD and SEM results indicate that theBa₃（VO₄）₂and LiMg_0.9Zn_0.1PO₄phases could coexist in the sintered body as themain phases, and the emergence of LiZnPO₄and Zn₃（VO₄）₂can also be detected in the Raman spectra. The content of these secondary phases was decreased bydecreasing the x value, in which the LiZnPO₄phase vanished when x≤0.45. Theemergence of some low melting point Zn₃（VO₄）₂phases （⁸60oC） can effectivelylower the sintering temperature of the composite ceramics, which makes theoptimal densification temperature drop to800oC. The near-zero Ï„fcould beachieved by adjusting the relative content of the two phases, and the0.55Ba₃（VO₄）₂-0.45LiMg_0.9Zn_0.1PO₄sintered at800oC exhibited desirablemicrowave dielectric properties of the Q×f⁶4500GHz, εr¹0, and Ï„f^-2.1ppm/oC, which would be a promising low-permittivity LTCC microwave dielectricmaterial.