Research On Preparation And Dielectric Mechanism Of Li₂ZnTi₃O₈ Microwave Dielectric Ceramic

With the rapid development of the low-temperature cofired ceramics（LTCC） technology, the research on the novel microwave dielectric ceramics, which have low raw price, low sintering temperature, high dielectric constant, low dielectric loss, near zero temperature coefficient of resonant frequency and chemical compatibility with the meatal electrodes, have attracted much attention. The Li2 Zn Ti3O8 microwave dielectric ceramic with relatively low sintering temperature is chosen as research subject in this dissertation, and the relationships among preparing methods（containing solid state reaction and molten salt method）, sintering technology（conventional solid state reaction method and reaction-sintering method）, phase compositions, microstructures and microwave dielectric properties have been investigated by X-ray diffraction（XRD）, X-ray photoelectron spectroscopy（XPS）, Raman spectroscopy, scanning electron microscopy（SEM）, transmission electron microscopy（TEM） and network analyzer, etc. Finally, the Li2 Zn Ti3O8 ceramics with low sintering temperature and excellent microwave dielectric properties were prepared and their dielectric mechanisms were discussed.The effects of sintering process on the microstructures and microwave dielectric properties of Li2 Zn Ti3O8 ceramics prepared by the conventional solid state reaction method were studied. Furthermore, the relationships among molecular polarizabilities, packing fractions, valence of Ti element, bond valences and bond strengths of cations, and microwave dielectric properties of Li2 Zn Ti3O8 ceramics were thoroughly discussed. The optimal sintering process parameters were determined at calcining temperature of 900 °C, sintering temperature of 1075 °C and heating rate of 3°C/min. Li2 Zn Ti3O8 ceramic sintered with this optimal sintering process exhibited the optimal microwave dielectric properties of εr=26.6, Q×f=83563 GHz, ηf=-12.4 ppm/°C.The lattice vibration modes of Li2 Zn Ti3O8 ceramic were classified and assigned, and the relationships between lattice vibrations and microwave dielectric properties of Li2 Zn Ti3O8 ceramics were investigated. The dielectric constant and Q×f value of the ceramic were related to the peak position and FWHM of A1g（1） mode in Raman spectroscopy. The conduction spectra and dielectric loss spectrum at room temperature were obtained by the AC impedance spectroscopy. Futrthermore, the dielectric loss mechanisms at different frequency range for Li2 Zn Ti3O8 ceramic were studied.Li₂ZnTi₃O₈ ceramics were successfully synthesized by molten salt method, and their sintering characteristic, microstructures and microwave dielectric properties were thoroughly investigated. Compared to the solid state reaction, smaller sizes and higher activity powders were obtained by Na Cl-KCl, Li Cl and Zn Cl2 molten salt method, which could decrease the reaction and sintering temperature, at the same time, also deteriorated the microwave dielectric properties of Li2 Zn Ti3O8 ceramics. Li2 Zn Ti3O8 ceramic calcined at 600 °C in Zn Cl2 molten salt was well densified at 975 °C, and exhibited good microwave dielectric properties with εr=26.68, Q×f=67724 GHz, ηf =-11.5 ppm/°C.The effects of sintering process parameters on the phase compositions, sintering characteristics, microstructures and microwave dielectric properties of Li2 Zn Ti3O8 ceramics prepared by reaction-sintering were studied, and the variations of the grain shapes and Q×f values were discussed by order-disorder phase transition of Li2 Zn Ti3O8. With the sintering temperature increased from 975 °C to 1075 °C, the Li2 Zn Ti3O8 phase in the ceramic totally translated from order phase to disorder phase, which made the grain shape changed from equiaxed to tetragonal and the Q×f value obviously decreased. Li2 Zn Ti3O8 ceramic sintered at 1025 °C for 6 h exhibited dense microstructures and good microwave dielectric properties of εr =25.8, Q×f =77100 GHz, ηf =-12.4 ppm/°C.The effects of B2O₃, Bi2O₃, Zn O-B2O₃-Si O2, and Zn O-La2O₃-B2O₃ sintering aids on the phase composition, sintering characteristics, microstructures and microwave dielectric properties of Li2 Zn Ti3O8 ceramics have been investigated. All the sintering aids could effectively decrease the sintering temperature of Li2 Zn Ti3O8 ceramics to about 925 °C, and the low temperature sintering mechanisms included the liquid phase sintering and defect activated sintering. The sintering aids affected the dielectric constants, Q×f values and ηf values of Li2 Zn Ti3O8 ceramics, which were mainly depended on the characteristics of sintering aids. In general, Bi2O₃ and Zn O-La2O₃-B2O₃ obviously decreased the Q×f values of the ceramics, and the B2O₃ and Zn O-B2O₃-Si O2 did not decrease the the Q×f values remarkably. Li2 Zn Ti3O8 ceramic with 1.0 wt% B2O₃ sintered at 925 °C for 4 h exhibited good microwave dielectric properties of εr =24.96, Q×f =49600 GHz, ηf =-11.3 ppm/°C. Li2 Zn Ti3O8 ceramic with 0.75 wt% ZBS glass sintered at 925 °C for 4 h exhibited better microwave dielectric properties of εr=25.61, Q×f=51615 GHz, ηf=-11.78 ppm/°C.The effects of Ca Ti O₃, Ba3（VO4）2 and Li2 Ti O₃ on the sintering characteristics and microwave dielectric properties of Li2 Zn Ti3O8+0.75wt%ZBS ceramics were investigated. The results shown that Ca Ti O₃ doping obviously decreased the Q×f values of the ceramics, but the Ba3（VO4）2 and Li2 Ti O₃ could improve the microwave dielectric properties of the ceramics. Ba3（VO4）2 doping could form the low melt point phase of Zn3（VO4）2 and Li Zn VO4 in the ceramics, which decreased its sintering temperature, and increased its relative density, thereby enhancing its Q×f value. 0.85Li2 Zn Ti3O8-0.15Ba3（VO4）2 +0.75wt%ZBS ceramic sintered at 850 °C for 4 h exhibited good microwave dielectric properties of εr =22.30, Q×f =53966 GHz, ηf =-2.4 ppm/°C. Although Li2 Ti O₃ doping is not beneficial to the denfication of the ceramics, Li2 Ti O₃ doping could greatly enhance its Q×f value, which was related to the decrease of the ordering domain size and stabilization of ordering-induced domain boubdaries by the partial substitution of Zn2+ ionic. 0.6Li2 Zn Ti3O8-0.4Li2 Ti O₃ +1.0wt%ZBS ceramic sintered at 900 °C for 4 h exhibited optimal microwave dielectric properties of εr =25.4, Q×f =86400 GHz, ηf =-1.0 ppm/°C, and had good chemical compatibility with Ag, which is a novel LTCC material for multilayer devices application.The effects of ionic substitution on the structures and microwave dielectric properties of Li2 Zn Ti3O8 ceramics were studied. The results shown that small amount of Co2+ ionic substitution obviously increased the Q×f values of Li2 Zn Ti3O8 ceramics, and the Q×f values decreased with increasing the amount of Co2+ ionic substitution, which was related to the packing fraction of the atoms. Small amount of Ni2+ ionic substitution decreased the Q×f values of Li2Zn1-xNixTi3O8 ceramics, which was caused by the increment of electron conduction resulting from Ni2+ ionic. The Q×f values of Li2Zn（Ti1-xSnx）3O8 ceramics increased with increasing amount of Sn4+ ionic substitution, and then decreased after reaching the maximum value at x=0.08, which was depened on the bigger average grain size and second phase of Sn O2. When the amount of Nb5+ ionic substitution below 20%, the Q×f values of Li2Zn（Ti1-xNbx）3O8 increased with increasing x value, which were related to restrained formation of Ti3+ ionic and oxygen vacancies. The substitution mechanism of the ceramic with 20% Nb5+ doping transformed to equivalence substitution, which caused the formtion of the second phase Li Zn Nb O4, thereby obviously decreasing the dielectric constant, Q×f value and ηf value.