Structure And Properties Of B-Site Ordered-Disordered Li-Based Spinel Microwave Dielectric Ceramics

With the advent of the era of the Internet of Everything,the high frequency,high performance and miniaturization of electronic communication devices have become the new development direction.As a key basic material for mobile communication devices—microwave dielectric ceramics need to have near zero resonant frequency temperature coefficient（τ_f）,high quality factor（Q×f）and low relative dielectric constant（ε_r）to achieve good temperature stability,high frequency selection and low latency.Spinel structure ceramics have received much attention for possessing lowε_r,high Q×f values,and tunableτ_f.The relationship among composition,structure and microwave dielectric properties of spinel structured ceramics need to further investigate.In this thesis,we investigate the connection between the crystal structure,vibrational spectra,sintering properties,chemical bonding properties and microwave dielectric properties of the disordered and B-site 1:3 ordered Li-based spinel ceramics,especially the cationic“compressed”and“rattling”effects in dielectric polarization and loss.The details are as follows:（1）In the Li₆A₇Ti₁₁O₃₂（A=Zn,Mg）ceramics of Chapter 3,Li₆Zn₇Ti₁₁O₃₂exhibits excellent microwave dielectric properties（Q×f=129,600 GHz,ε_r=20.7,τ_f=-45 ppm/℃）and low thermal expansion coefficientα_L=5.5 ppm/℃.Compared to Li₆Zn₇Ti₁₁O₃₂,Li₆Mg₇Ti₁₁O₃₂exhibits a more nearly zeroτ_f=-21 ppm/℃,but its Q×f（108,600 GHz）andα_L（4.9 ppm/℃）are lower andε_r（21.9）is higher.The bond valence theory demonstrates that the difference in the macroscopic dielectric properties of the two ceramics is due to the different“rattling”and“compressed”effects exhibited by the cations at their A-site（8c）,B1-site（4b）and B2-site（12d）.（2）In the Li₁₀ATi₁₃O₃₂（A=Zn,Mg）ceramics of Chapter 4,the microwave dielectric properties of Li₁₀Zn Ti₁₃O₃₂areε_r=28.23,Q×f=35,800 GHz,andτ_f=-17.06 ppm/℃,whereas those of Li₁₀Mg Ti₁₃O₃₂areε_r=29.23,Q×f=32,100 GHz,τ_f=-11.05 ppm/℃.The calculated bond valence reveals that almost all cations have a“rattling”effect and the expansion structure leads toτ_fclose to zero and lower values of Q×f.The bond ionicity and lattice energy of the Ti-O bond are much higher than that of the other bonds,indicating that the Ti-O bond is the main reason for theε_rand Q×f values.（3）In Chapter 5,Li Ga_5-xAl_xO₈（0≤x≤5）ceramics were prepared by replacing Ga³⁺in Li Ga₅O₈ceramic with Al³⁺.The increase in the amount of substituted Al³⁺tends to be accompanied by a decrease in the cell volume,and deterioration ofε_ras well as Q×f values,whileτ_fgradually moves toward near zero.The best microwave dielectric properties of Li Ga Al₄O₈ceramics at x=4 areε_r=8.35,Q×f=75,100 GHz,τ_f=-45.79 ppm/℃.The Q×f value is much larger than those of Li Al₅O₈ceramics,and the sintering temperature is lower,while theτ_fvalue is closer to zero compared to that of Li Ga₅O₈ceramics.（4）In Chapter 6,Zn_1-x(Li_0.5Ga_0.5)_xAl₂O₄（0≤x≤1）ceramics with disordered-ordered phase transition were prepared by replacing Zn²⁺in Zn Al₂O₄ceramic with[Li_0.5Ga_0.5]²⁺.The structure is disordered when x≤0.8,and the transition from disordered to ordered is completed when x=1.The maximum value of Q×f is obtained at x=0.6 in a completely disordered state with microwave dielectric properties ofε_r=8.66,Q×f=83,500 GHz,τ_f=-56.30 ppm/℃.