Study On The Processing And Properties Of Vanadate Based Low-loss Microwave Dielectric Ceramics

Microwave dielectric ceramic has been applied in microwave frequency (300MHz-300GHz) circuits as dielectric material to serve multiple functions. With the rapid development of modern microwave mobile communication technology, as a key material for fabricating advanced substrates, dielectric antennas and other wireless communication devices, microwave dielectric ceramic has become a hot topic in the research field of dielectric materials throughout the world. In this paper, the conventional solid-state method was employed to prepare novel vanadate based ReVO4 (Re=La, Ce) and BaMg2V2Og microwave dielectric ceramics. The performance optimization of BaMg2V2O8 was completed with the utilization of ion substitution and non-stoichiometry methods. The effects of fabrication process, phase structure and microstructure on microwave dielectric properties were systematically investigated by XRD, SEM, EDS, XPS and vector network analyzer. The main contents are listed as follow:Firstly, the ReVO4 (Re=La, Ce) compounds as representatives of lanthanide vanadates and BaMg2V2O8 microwave dielectric ceramics were synthesized by solid-state method. The XRD, Rietveld refinement and SEM measurement results indicated that the as-sintered ReVO4 (Re=La, Ce) and BaMg2V2O8 samples exhibited single-phase structure, closely packed particles and identifiable grain boundaries. Excellent microwave dielectric properties could be obtained for LaVO4 (εr= 14.2, Qxf= 48197 GHz, τf=-37.9 ppm/℃), CeVO4 (εr= 12.3, Qxf= 41460 GHz, τf=-34.4 ppm/℃) and BaMg2V2O8 (εr= 12, Qxf= 156140 GHz, τf=-36 ppm/℃) samples when individually sintered at 850℃,950℃ and 900℃. The good performance and low-temperature sintering characteristic make the lanthanide vanadates and ultra-low loss BaMg2V2O8 promising ceramic materials for the application of low-temperature cofired ceramic technology (LTCC).Secondly, the Sr2+with smaller ionic radius than Ba2+was introduced to investigate the effect of A-site substitution on the microwave dielectric properties of BaMg2V2O8, with a designed formula of Ba1-xSrxMg2V2O8 (x=0-0.40). The XRD measurement results identified the formation of solid solutions within the composition range of the system. With increasing Sr2+ contents, the diffraction peaks shifted towards higher angles and obvious peak split could be seen, indicating enhanced anisotropy and increased lattice distortion. Appropriate Sr2+ substitution did not change much of the grain size, however, the grain refinement mechanism started to appear when x was beyond 0.15. The refined data demonstrated that Sr2+ of A-site increased the bond valance of A-O and then made the if value shifted towards positive direction, a near-zero τf and enhanced microwave dielectric properties of the matrix were thus obtained. The x=0.15 sample sintered at 915℃ demonstrated excellent microwave dielectric properties (εr, Qxf, τf being 13.3, 86640 GHz,-6 ppm/℃, respectively) and large potential in the application of LTCC-based materials.Finally, the ratio of Ba was adjusted to study the effect of non-stoichiometry on the phase structure, densification behavior and microwave dielectric properties of BaMg2V2O8, with a designed formula of Ba1+xMg2V2O8 (x=0-0.52). The XRD, SEM and EDS results indicated that excess Ba would introduce Ba3V2O8, which formed composite ceramic with BaMg2V2O8. Owing to the relative higher densification temperature and positive τf that Ba3V2O8 possesses, the sintering temperature of the composite system was slightly increased. A near-zero τf could be yielded by appropriately adjusting the relative amounts of the two phases. The x=0.44 sample exhibited good microwave dielectric properties of εr= 13.7, Qxf= 92580 GHz, τf=-5.3 ppm/℃ when sintered at 930℃, which is of application value as a LTCC microwave dielectric ceramic.