| In recent years, low temperature co-fired ceramics (LTCC) have been widely received people's attentions, because it could reduce the size of devices and achieve the integration. Meanwhile, low dielectric constant (≤25), high Q value (>10000 GHz) microwave dielectric ceramics have been widely investigated because of the reduction of delay signals in microwave device and make the size of wireless system to achieve integration. Mg4Nb2O9(MN) microwave dielectric ceramics have the corundum structure and good microwave dielectric properties:low dielectric constant (εr=12.8) and high quality factor (Q×f=190 000 GHz). However, the high sintering temperature (> 1400癈) and the poor temperature coefficient of resonant frequency (τf=-77 ppm/℃) of Mg4Nb2O9 ceramics make them virtually useless for commercial applications.In this thesis, in order to reduce the sintering temperature of Mg4Nb2O9 ceramics, low melting point oxide and low melting glass was added. It was investigated that the sintering additives content had far-reaching influence upon sintering character, phase structure, microstructure and microwave dielectric properties of Mg4Nb2O9 ceramics. The principal experiment results were showed as follows:(1) Addition of 20 wt% TiO2 and x wt% LiF (1.0≤x≤8.0) can effectively reduce sintering temperature of Mg4Nb2O9 ceramics from 1400℃to 750℃due to formation of a eutectic objects in the grain boundary by addition of nanometer TiO2 and LiF (80 nm),which plays the role of liquid phase. Besides, ions substitution,chemical reaction through the lattice diffusion or interface migration to provide good wetting environment, resulting in the increase of ion diffusion rate and sintering rate. It shows that grain sizes of Mg4Nb2O9 ceramics added with 20 wt% TiO2 and 8.0 wt% LiF increase with sintering temperature. The formation of MgTiO3 grew up in the microwave Mg4Nb2O9 ceramics has a very big effect on dielectric properties. All the samples sintered at 800℃show the best density with the maximum Q×f of 32 000 GHz and the maximum relative dielectric constantεr of 15.7. The lattice defects, porosity and second phase MNT ceramics have a bigger effect on dielectric loss than the influence of grain boundaries. MN-20 wt% TiO2-8.0 wt% LiF ceramics sintered at 750℃performed the best microwave dielectric properties:εr=15.6, Q×f=25 000 GHz,τf=-56 ppm/℃. The ceramics materials and Ag have good chemistry compatibility.(2) Using standard ceramics technology, Mg4Nb2O9 microwave dielectric ceramics doping PbO-B2O3-SiO2 (PBS) glass was prepared. Sintering behaviors, phase structure, microstructure of Mg4Nb2O9 doped with PBS were studied. Experimental results show that the doping PBS in 9 ceramics can effectively improve the resonant frequency temperature coefficientτf value, which concerned with the fonmation of Pb1.5Nb2O6.5.When 4.0 wt% PbO-B2O3-SiO2 added in Mg4Nb2O9, the sintering temperature is reduced to 975℃, the Mg4Nb2O9 at low temperature sintering densification was realized, and it has good microwave dielectric properties (εr=13.8, Q×f =37 382 GHz,τf=-16.9 ppm/℃), which is expected to become a new generation of low-temperature sintering substrate materials.(3) 0.6 MN-0.4 SrTiO3 (MNS) microwave dielectric ceramics with LiF was prepared, studies the different contents of MNS ceramics sintering doping behavior, phase structure, microstructure and the influence of microwave dielectric properties. Experimental results show that a certain amount of SrTiO3 and LiF co-doping result in the formation of the second phase Sr(Mg1/3Nb2/3)O3 in MN ceramics. Add 7.0 wt% LiF can effectively prompt sintering densification of MNS ceramics at the low temperature (800℃). Porosity is the main reason for effect of this ceramics dielectric constant changed. The second phase Sr(Mg1/3Nb2/3)O3 can attribute to Q×f value andτf value of MN ceramics. MNS ceramics with 7.0 wt% LiF sintered at 800℃owned well microwave dielectric properties:εr =26.5,Q×f=6 073 GHz,τf=-12 ppm/℃. |
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