Study On Propertv Optimization And Low- Temperature Sintering Of Mg₂SnO₄-based Microwave Dielectric Ceramics

With the rapid development of mobile telecommunication, microwave devices are required to be developed and optimized to meet the performance requirements. Microwave dielectric ceramics is the key material of circuit substrate, package and several passive component. The property optimization and low-temperature sintering of microwave dielectric ceramics has become a hot topic being studied.Mg2SnO4 ceramic is a well performance microwave dielectric ceramic, which has low relative dielectric constant and low dielectric loss. The researches reported to date focused on the modification of the dielectric properties of Mg2SnO4 via ion-doping and second-phase addition. However, several problems remained unresolved. (1) The microstructure of Mg2SnO4 ceramic sintered at around 1600℃ is porous, and the density is low, which might be caused by the volatilization of Sn. Also, such a high sintering temperature makes it unable to co-fire with Ag-Pd electrode. (2) The temperature coefficient of resonate frequency of Mg2SnO4 overruns the allowable range.In this study, sintering behavior and microwave dielectric properties of Mg2SnO4 was improved via nonstoichiometry and ion-doping; the influence of glass, low-melting compound and composite sintering aids on the sintering behavior, phase composition, microstructure and dielectric properties of Mg2SnO4-based ceramics was studied; CaTiO3 was introduced to modify the temperature coefficient of resonate frequency; the co-firing performance with Ag-Pd electrode was also investigated.1. Stoichiometric and non-stoichiometric Mg2SnO4 ceramics were prepared via solid-state route. Rietveld refinement of stoichiometric Mg2SnO4 indicated a reversed spinel structure, with O ions face-centered close packing, half of Mg2+in tetrahedral sites, the other half of Mg2+ and all of Sn4+ occupying octahedral sites. In Mg-accessed non-stoichiometric Mg2+xSnO4 ceramic, the accessed Mg occupied interstitial positions, which arouse lattice distortion that improved sintering behavior. Mg2.15SnO4 ceramics sintered at 1620℃ performed high dielectric properties:εr=7.42, Q×f=76800 GHz, τf=-50 ppm/℃.2. Effect of Al3+ doping on the crystal structure and dielectric properties of Mg2SnO4 ceramic was studies. Rietveld refinement results showed that Al3+ entered both tetrahedral and octahedral sites, which caused the distortion and tilting and lowed the symmetry of oxide polyhedron. Al3+doping improved the sintering behavior of MgSnO4 and the soaking time during sintering was decreased from 4 h to 1 h. Meanwhile, appropriate amount of Al3+ doping enhanced Q×f value due to greater density, and temperature coefficient of resonate frequency was lower due to higher cation bond valence. Mg1.755Al0.05Sn0.75O4 ceramics sintered at 1620℃ for 1 h performed high dielectric properties:εr= 7.42, Q×f= 76800 GHz, τf=-50 ppm/℃.3. Bismuthate and borosilicate glass aids were introduced to lower the sintering temperature of Mg2SnO4 ceramic. The effect of glass addition on phase composite, microstructure and dielectric properties were investigated. (1) Glass melted and wet grains during heating, which promoted the sintering process and lower the sintering temperature. The addition of 15 wt.%bismuthate glass or 20 wt.%borosilicate glass lower the sintering temperature from 1620℃ to 1350℃ or 1250℃ respectively. (2) New phases produced in glass add Mg2SnO4 ceramics affected dielectric properties. Q×f value decrease dramatically due to the present of Bi2O3 which enhanced porosity in bismuthate glass added ceramics. However, Q×f value was enhanced with the present of Mg2SiO4, which showed low dielectric loss itself in bosilicate glass added ceramics. (3) The dielectric loss of glass added Mg2SnO4 ceramics are large overall, because of the great amount of glass phase in them.4. LiF-Fe2O3-V2O5 composite sintering aid was used to lower the sintering temperature of Mg2SnO4. By adding LFV additive, the densification temperature of Mg2SnO4 ceramics was significantly lowered from 1620℃ to 1050℃. TEM image and EDS spectra indicated that an amorphous phase was formed in the intergranular region, and Fe3+ ions presents in Mg2SnO4 matrix. Mg2SnO4 with 4 wt.% LFV additive sintered at 1050℃ provided the attractive combination of dense microstructure and excellent dielectric properties:εr= 7.90, Q×f= 41,400 GHz and τf=-80 ppm/℃. In addition, the ceramic was compatible with Ag-Pd electrodes. With 20%addition of CaTiO3, the τf value was optimized to -5 ppm/℃.