Study On Structure And Microwave Dielectric Properties Of Zn₂TiO₄-Li₄Ti₅O₁₂ Ceramics With Spinel Structure

As a basic material in the field of wireless communication,microwave dielectric ceramics are widely used in wireless communication devices such as filters,dielectric resonators,and antennas.Microwave dielectric materials are required to have low dielectric constant,low sintering temperature,low dielectric loss and good chemical stability,to meet the requirements of miniaturized/miniaturized electronic information systems of electronic components and adapt to the characteristic frequency of the new generation of mobile communication technology for low temperature sintered ceramics（LTCC）.The Li₂O-Zn O-Ti O₂ system with spinel structure has the advantages of low cost,low loss,and low sintering temperature.By studying the LZT spinel system,the relationship between the spinel structure and the microwave dielectric properties was established,and the basis of the best composition of LZT and the optimal microwave dielectric properties were determined.And the addition with Sr₃（VO₄）₂ improved temperature stability and reduced sintering temperature.Finally,potential LTCC material for 5G was synthesized.The main results are following:（1）Zn₂Ti O₄ and Li₄Ti₅O₁₂ microwave dielectric ceramics were prepared by a high-temperature solid-phase method.During the sintering process,variations in the stoichiometric ratio of the two ceramics were found.The second phase Zn O appeared in ZT ceramics,and the LT ceramics underwent phase transformation due to Li volatilization at high temperature,which deteriorated the properties of the ceramics.The density of states and crystal formation energies of the two crystals were calculated by first-principles calculations.The low crystal stability,inferior to that of solid solution,was led by the weak interaction between the Zn-O bond in ZT crystal and the Li-O bond in LT crystal and the high crystal formation energy,which led to the appearance of the second phase and the volatilization of Li.Meanwhile,the interaction of Ti-O bonds was stronger than other bonds in the three structures,indicating the dominant role of Ti-O bonds on the dielectric properties.（2）（1-x）Zn₂Ti O₄-x Li₄Ti₅O₁₂（0.3≤x≤0.75）microwave dielectric ceramics were prepared by high-temperature solid-phase method.The relationship between intrinsic and extrinsic factors and microwave dielectric properties was established via P-V-L theory,vibrational spectroscopy and XPS.According to the characterization,the change in the dielectric constant（ε_r）was attributed to the increase in the average bond ionicity of Ti-O(Af_{i Ti-O})and the polar mode of lattice vibration;the change in the quality factor（Q×f）was caused by the average Ti-O lattice energy(AU_Ti-O)and change in oxygen vacancies;the increase in temperature coefficient of resonant frequency（τ_f）was caused by the increase in Ti-O bond energy.Li_0.6Zn_1.1Ti_1.3O₄ ceramics（x=0.45）were sintered at 1125℃,and the optimum microwave dielectric propertiesε_r=17.3,Q×f=76318 GHz andτ_f=-58 ppm/℃ were obtained.（3）Based on the previous work,Li_0.6Zn_1.1Ti_1.3O₄ was selected as the base in the LZT system.The addition of Sr₃（VO₄）₂ improved the temperature stability of ceramics and reduced the sintering temperature to 900℃.The multi-phase proportions were calculated by XRD refinement,and the theoretical properties were calculated by the mixing rule.The reaction between both two phases at 930℃was found by DSC and sintering shrinkage curve.The generations of Sr Ti O₃ and Li-Zn-V glass phases were observed via TEM and Raman spectroscopy,which reduced the sintering temperature and improved the temperature stability.And there was good chemical stability between the compound and Ag.In addition,the defect behaviour in the ceramics was studied by XPS and impedance spectroscopy.The defects in LZT-SV composites were oxygen vacancies,Ti³⁺and V⁴⁺.The migration of defects in single-phase ceramics is mainly the migration of oxygen vacancies,while in composites the migration mode is charge hopping between V_o^-,Ti⁴⁺-Ti³⁺sites and V⁵⁺-V⁴⁺sites.And generation and migration of defects occurred more easily in LZT-SV composites,leading to an increase of dielectric loss.Finally,0.7Li_0.6Zn_1.1Ti_1.3O₄-0.3Sr₃（VO₄）₂ ceramic sintered at 900℃showed the optimal properties:ε_r=17.8,Q×f=41891 GHz andτ_f=-4.4 ppm/℃.