Structure And Properties Of Zircon Vanadate Microwave Dielectric Ceramics With Low Permittivity

Microwave dielectric ceramics are the key fundamental materials for modern communication technology.With the development of 5G communication technology,AVO₄-type zircon structural ceramics with lowε_r have attracted extensive attention.In this thesis,based on the structural characteristics of tetragonal zircon,the series ceramics of ACe V₂O₈（A=Ca,Sr）,Ca RECe V₃O₁₂（RE=Nd,Sm）,and La REV₂O₈（RE=Nd,Sm）were designed and prepared by different cations are occupied in A-site.And the relationship between phase,microstructure,crystal structure,Raman spectrum,far-infrared reflectance spectrum,chemical bond characteristics and microwave dielectric properties have been carried out.To explore the influence and mechanism of the“rattling”or“compressed”effect of A and B positions on microwave dielectric properties,and to reveal the relationship between the composition,structure and performance of this series of ceramics.The specific contents are as follows:（1）The tetragonal zircon-type ACe V₂O₈（A=Ca,Sr）ceramics were prepared by compounding Ca²⁺and Sr²⁺with Ce⁴⁺to co-occupy the A-site,respectively.And their optimal sintering temperature are 1400°C with the optimum microwave dielectric properties were:ε_r=9.632 and 10.288,Q×f=22,170 and 24,498 GHz,τ_f=–53 and–46 ppm/°C.The measuredε_r are significantly higher than the theoretical values calculated by the C-M equationε_r（C-M）（7.65,8.14）due to the“rattling”effect of Ce⁴⁺at the A-site in the zircon structure,and the stronger“rattling”effect of Ce⁴⁺in Sr Ce V₂O₈ samples leading to a higherε_r and a near-zeroτ_f.In addition,the P-V-L chemical bonding theory shows that the Ce-O bond contributes the most to the ionicity（44.5%and 44.6%）and the lattice energy（29,120 and 29,370 k J/mol）is positively correlated with Q×f.（2）Ca RECe V₃O₁₂（RE=Nd,Sm）ceramics were prepared by compounding Ca²⁺and Ce⁴⁺with Nd³⁺and Sm³⁺to occupy the A-site with optimal sintering temperatures of 1225°C and 1200°C,respectively.The microwave dielectric properties wereε_r=11.85 and 11.33,Q×f=40,100 and 43,290 GHz,andτ_f=–27.2 and–22.4 ppm/°C.Theε_r is larger thanε_r（C-M）（9.41 and 9.47）due to the“rattling”effect of Ce⁴⁺in the A-site and V⁵⁺in the B-site.Meanwhile,Q×f is positively correlated with the relative density（95.83%and 96.97%）,and packing fraction（55.7%and 55.9%）and negatively correlated with the FWHM of the A_1g Raman mode.The intrinsic dielectric constants（11.85 and 11.33）were obtained by fitting and extrapolating the far-infrared reflection spectrum.Finally,the P-V-L chemical bond theory showed that the lattice energy（28,805 and 28,858 k J/mol）was positively correlated with Q×f,and the contribution of V-O bond to the lattice energy is dominant.（3）La REV₂O₈（RE=Nd,Sm）ceramics were prepared by partially replacing the A-site of La VO₄ with Nd³⁺and Sm³⁺,respectively,and the optimal microwave dielectric properties were obtained at 1300 and 1275°C,respectively:ε_r=10.02 and10.63,Q×f=28,862 and 26,735 GHz,τ_f=–41 and–44 ppm/°C.The Q×f values are positively correlated with the relative densities（96.89%and 95.92%）and negatively correlated with the FWHM of the Raman spectra.In addition,the compressed effect of the A site of La Sm V₂O₈ is stronger than that of La Nd V₂O₈,resulting in a lowerτ_f value.Finally,through the analysis of the P-V-L chemical bond theory,it is concluded that the ionicity is positively correlated withε_r,the lattice energy（27,936 and 27,910 k J/mol）is positively correlated with Q×f,and the contribution of V-O bond to the lattice energy is dominant.