Study On The Structure And Properties Of Ge-based Olivine And Phenakite Microwave Dielectric Ceramics

In recent years,as the frequency of wireless communication continues to increase,signal delays will become more serious,system loss and heat generation will also increase,and system stability gradually become worse.Therefore,the performance parameters of microwave dielectric ceramics（relative dielectric constantε_r,quality factor Q×f and resonance frequency temperature coefficientτ_f）,which are the key basic materials for manufacturing resonators,filters,and dielectric substrates,are proposed to be higher requirements.At present,the series of A₂BO₄ type olivine-structured ceramics（Mg₂Si O₄,Ca MgGeO₄）and phenakite-structured ceramics（Zn₂Si O₄,Zn₂Ge O₄）have received extensive attention due to their lowε_r and high Q×f.However,their relatively high sintering temperature limits the practical application.This thesis firstly introduced Li into the A site of the olivine-type A₂BO₄ compound to prepare Li₄Ge O₄ microwave dielectric ceramics at low temperature,with lowε_r and high Q×f,and the chemical compatibility of co-fired with Ag electrodes was studied;Secondly,two LiAGeO₄（A=Al,Ga）ceramics with phenakite structure were prepared,and the effect of Al/Ge or Ga/Ge order-disorder distribution on the microwave dielectric properties was studied.Finally,the influence of Ga substitution for Al on the phase composition,crystal structure and microwave dielectric properties of Li Al_1-xGa_xGe O₄（0≤x≤1）ceramics was studied.The specific results are as follows:（1）Li₄Ge O₄ ceramics with tetragonal olivine structure were prepared by solid-phase sintering method,and the space group is Bmmb（63）.At the optimal sintering temperature of 720℃,Li₄Ge O₄ ceramics,with the relative density 95.1%,the density3.01 g/cm³ possessed the best microwave dielectric properties:ε_r=6.99,Q×f=47,813GHz andτ_f=-159.2 ppm/℃.By introducing Li Ca₂Mg₂V₃O₁₂ with a positiveτ_f of 269ppm/℃,a temperature-stable composite ceramic 0.6Li₄Ge O₄+0.4Li Ca₂Mg₂V₃O₁₂with a near-zeroτ_f of+2.7 ppm/℃ was obtained.In addition,Li₄Ge O₄ ceramics and Ag electrodes have good chemical compatibility when co-fired at 700℃,which makes it a promising candidate for LTCC technology.（2）LiAGeO₄（A=Al,Ga）ceramics crystalized in hexagonal phenakite structure with oxygen tetrahedrons close-packed.However,there is a significant difference in atomic occupation of LiAGeO₄（A=Al,Ga）ceramics.For Li GaGeO₄,Ga andGeare randomly arranged at 9b site,and the space group is R（?）（148）;For Li AlGeO₄,Al/Ge are orderly distributed at 18f site with the radio of 1:1,and the space group is R3（146）.Li AlGeO₄ ceramic showed excellent microwave dielectric properties:ε_r=5.45,Q×f=70,832 GHz,τ_f=-34.4 ppm/℃ when sintered at 1100℃.Li GaGeO₄ ceramic showed excellent microwave dielectric properties:ε_r=6.07,Q×f=30,065 GHz,τ_f=-60.3ppm/℃ when sintered at 1075℃.Theε_th of LiAGeO₄（A=Al,Ga）ceramics are slightly lower thanε_r,which may be caused by the"ratting"effect of cations.The intrinsicε_r and intrinsic Q×f of LiAGeO₄（A=Al,Ga）ceramic samples were obtained by fitting the far-infrared reflectance spectrum as follows:ε_r=5.27,Q×f=110,312GHz andε_r=5.79,Q×f=57,452 GHz,and the infrared active vibration mode of 50-400 cm^-1 contribute 82.3%and 84.2%to the intrinsic microwave dielectric polarization,respectively.The composite ceramics withτ_f close to zero were obtained in0.84Li AlGeO₄+0.16Ti O₂（-3.14 ppm/℃）and 0.78Li GaGeO₄+0.22Ti O₂（+2.17ppm/℃）.（3）Hexagonal phenakite structure Li Al_1-xGa_xGe O₄（0≤x≤1）ceramics were prepared at 1000-1125℃ by traditional solid method to explore the influence of order-disorder transformation on the properties of microwave dielectric ceramics.According to the results of XRD and structural refinement,as the amount of Ga³⁺substitution increases,the intensity of the diffraction peak（211）gradually decreases,while the intensity of the diffraction peak（140）gradually increases,which indicates the change of crystal structure.When 0<x≤0.5,the substitution of Al³⁺by Ga³⁺destroy the orderly distribution of Al andGecations at the 18f site and the space group is R3（146）;when 0.5<x≤1,Al,Ga andGeare randomly distributed at the 9b site,the crystal structure has higher symmetry with space group R（?）（148）.Raman spectroscopy showed that the[Ge O₄]stretching vibrational mode gradually shifted to lower wave numbers with the increase of Ga³⁺substitution,when 0<x≤0.5,the[Al O₄]tetrahedral stretching vibration mode gradually broadens,indicating the decrease of cation ordering,when 0.5<x≤1,the anharmonic vibration is enhanced and the tensile vibration mode is significantly reduced due to the random distribution of Ga³⁺,Ge⁴⁺and Al³⁺at the 18f site.As x increases,theε_r of Li Al_1-xGa_xGe O₄（0≤x≤1）ceramics increases from 5.45 to 6.07,the Q×f value decreases from 70,832 GHz to 30,065 GHz,and theτ_f value increases from-34.4 ppm/℃ to-60.3 ppm/℃.