Investigation Of Intrinsic Dielectric Loss Mechanism Of Medium-low Dielectric Constant Materials With Rare Earth Cations

In the era of 5G communications,the improving demand of huge markets of electronic components has accelerated the development of the exploration and application of microwave dielectric ceramics.From the ongoing expansion of the database of ceramics with different permittivity to the summary of the factors affecting microwave dielectric properties,abundant investigations have emerged to conclude the mechanism of dielectric loss.With the features of diverse compounds,various structures,and different properties,it is difficult to break through the bottleneck of how to adjust the microwave dielectric properties with a fast,accurate,and precise method.Adjusting experimental parameters is valid to avoid the influence of external factors on microwave dielectric properties.Whereas,the fundamental way to effectively tune the dielectric properties is to analyze the different properties caused by the crystal structure,understand the origin of dielectric loss in different systems,and break the limitations of existing theory to establish a universal theory for guiding the experiment on improving the microwave dielectric properties.For ceramic systems with different compositions and structureses,bond characteristics can be quantified by complex chemical bond theory（namely P-V-L theory）based on the typical structure.The variation of chemical bond characteristics is used to explain the change in microwave dielectric properties.However,it is a onefold evaluation of bond characteristics and their effects on microwave dielectric properties.The accuracy and the reliability of the calculated results should be further verified.For example,the density of states results based on the first principle theory can support the conclusion of the P-V-L theory.Considering the contribution of chemical bonds to the lattice stability and the quantified dielectric loss ascribed to phonon mode,these results provide information about the origin of dielectric loss.This dissertation aims to analyze the origin of the dielectric loss of three types of crystal structures（NdNbO₄,Y₂MgTiO₆,Gd₂Zr₃（MoO₄）₉）with various complexity.In those systems,the calculated results of the first principle theory and P-V-L theory are mutually confirmed.The results of lattice stability and lattice vibration spectrum after fitting establish the relationship between chemical bond characteristics and microwave dielectric properties.And then,the doping site for improving the properties of the materials is proposed.To verify the universality and reliability,this method is applied to three different chemical formulas（ReBO₄,Re₂ATiO₆,and Re₂Zr₃（MoO₄）₉）to explain the influence of chemical bonds characteristics on the microwave dielectric properties.Herein,the main contents and results of this dissertation are listed as follows:（1）For NdNbO₄ ceramics,the predicted doping site is summarized after analyzing the chemical bond characteristics and their effects on the lattice stability according to P-V-L theory and the first principle theory.The calculated results show that the Nb-O bond has a more significant influence on microwave dielectric properties.The co-doped cations(A_xW_1-x,A=Mg,Al,Zr;A_xMo_1-x,A=Mg,Al,Zr,Si)are designed to substitute for Nb,and the measured properties verify the correctness of the prediction of tuning the dielectric loss.Meanwhile,for the chemical general formula ReBO₄（B=Nb,Ta）,the calculated data of the P-V-L theory illustrate that the variations of ionicity of Re-O andεr are similar.The drop in lattice energy of the ReNbO₄ system is consistent with the decline of quality factors.The lattice energy of Ta-O bonds makes the main contribution to the total lattice energy for ReTaO₄ and its variation is consistent with Q×f values.（2）There are three different coordination polyhedrons in the Y₂MgTiO₆ system.After analyzing the results of chemical bond characteristics and the lattice vibrational spectrum,the predicted doping site for reducing the dielectric loss is the Y-O bond,due to its major contribution to the dielectric polarization,lattice energy,and dielectric loss.In the experiment,equivalent Nd³⁺ions and Sm³⁺ions were used to substitute for Y³⁺.It has been demonstrated that the prediction of the doping site in Y₂MgTiO₆ ceramics is correct,after analyzing the calculated bond characteristics of doped ceramics and the fitted results of the far-infrared reflectivity spectrum along with the terahertz spectrum.For the chemical general formula Re₂ATi O₆（A=Mg,Co,Zn）,the calculated data of P-V-L theory clarify that the ionicity of the different ceramics follow the same sequence as f_i（Re-O）>f_i（Ti-O）>f_i（A-O）.Theεr of the system is consistent with the overall trend of the ionicity of Re-O,while both the lattice energy and the Q×f values vary in the same direction.（3）For the low-dielectric Gd₂Zr₃（MoO₄）₉ ceramics with double molybdate structure,the ionicity of different bonds follows the order of f_i（Gd-O）>f_i（Zr-O）>f_i（Mo-O）based on P-V-L theory,which is similar to the bonding environment of the density of states result according to the first principle theory.For the chemical formula Re₂Zr₃（MoO₄）₉,the ionicity of Re-O and the average ionicity of the system are consistent with the variation ofεr as the radius of the rare earth ions increase,while the variations of total lattice energy and Q×f values are identical.