Study On Structure Regulation And Performance Optimization Of Vanadate Microwave Dielectric Ceramics Based On LTCC Application

With the rapid development of low temperature co-fired ceramics?LTCC?technology,more attention has been focused on finding microwave dielectric ceramics with low sintering temperature,excellent microwave dielectric properties and good chemical compatibility with Ag.Some vanadate ceramics have been reported to have low sintering temperature and low loss,but none of them has good comprehensive microwave dielectric properties.Therefore,in this paper,we try to change the composition of some known V-based ceramics to obtain better integrated microwave dielectric properties.Specific methods include solid solution formation,second phase composition and non-stoichiometric ratio.The main research results of XRD,SEM,EDS,IR,Raman spectroscopy,XPS and vector network analyzer are as follows:?1?Firstly,the microwave dielectric properties of Ba₃V₂O₈ were investigated by introducing Sr²⁺with a smaller radius than Ba²⁺ions.The design composition was Ba_3-x-x Sr_x?VO₄?₂?0?x?3?.XRD and Raman analysis show that the system is a single hexagonal solid solution in the range of 0?x?3.With the increase of Sr²⁺content?x?,the densification temperature of the system decreases from 1450^oC to 1150^oC,and the grains gradually change from spherical to cylindrical,the transgranular cracks disappear and the density increases.XRD refinement data show that Sr²⁺substitution reduces the stacking rate of crystals,and the bond valence and bond strength of A-O bond decrease linearly,resulting in a slight decrease in?_r and Q×f.The increase of distortion of tetrahedron at B site[VO₄]reflects the increase of lattice distortion,which deteriorates the value of?_f to+63.5 ppm/^oC.Secondly,low temperature sintering of Sr₃?VO₄?₂ ceramics and compensation of its resonant frequency temperature coefficient to zero are realized by two methods:I.A series of Sr_3-yV₂O_8-y?y=0-1?two-phase composite ceramics are obtained by in-situ adjusting Sr:V molar ratio.Because the second phase Sr₂V₂O₇ is contrary to the?_f of the matrix material,the increase of V content promotes the formation of the second phase,leading to the continuous negative change of?_f.When y=0.6,nearly zero?_f value can be obtained for multiphase ceramics,and the dielectric properties of the samples sintered at950 ^oC are as follows:?_r=9.8,Q×f=27,550 GHz,?_f=1.1 ppm/^o C.II.The microwave dielectric properties of the material were improved by adding a small amount of glass phase B₂O₃,in which Sr₃V₂O₈+1 wt%B₂O₃ was sintered at 900^oC with a?_f value of+47.4 ppm/^oC.It is a LTCC microwave dielectric material with excellent properties.?2?According to the classical harmonic oscillator model,reducing the total ionic polarizability of the system or increasing the unit cell volume are effective strategies to reduce the intrinsic loss.A series of Gd_1-xBi_xVO₄ ceramics?0?x?0.8?with low sintering temperature and rich dielectric constant were obtained by substituting Bi³⁺for Gd³⁺in zircon structured GdVO₄ ceramic.Firstly,the phase composition and phase transition of the binary system were systematically studied.The binary system of Gd_1-xBi_xVO₄ was divided into:I-tetragonal solid solution region?x?0.6?;II-monoclinic and tetragonal solid solution coexistence region?0.6?x?0.8?.From the point of view of crystal structure and cell parameters,the transformation process of each phase region in binary phase diagram is described in detail,and the mechanism of?_f value changing from negative to positive in I-tetragonal solid solution region is emphatically studied.Combining far infrared reflection spectra and Raman spectra,the variation of optical frequency dielectric constant and the difference of lattice vibration in different phase composition were determined.Photoelectron spectroscopy?XPS?confirms that the dielectric loss of V⁴⁺,oxygen vacancies and electronic defects caused by partial reduction of V⁵⁺at higher temperatures?>1000 ^oC?is an important source of dielectric loss.A series of potential microwave dielectric ceramics have been developed in this binary system:Gd_0.79Bi_0.21VO₄ sintered at900 ^oC,?_r=25,Q×f=15,000 GHz,?_f=+0.5 ppm/^oC,which doesn't react with Ag;with the further increase of Gd³⁺,?_f becomes more and more positive,reaching a maximum value of+130.5 ppm/^oC at Gd_0.21Bi_0.79VO₄,and then gradually decreasing,and Gd_0.28Bi_0.82VO₄ sintered at 760 ^oC shows excellent properties of?_r=58,Q×f=25,000 GHz,?_f=1.5 ppm/^oC.?3?In order to solve the problem that ceramics containing only one V-based component still have high sintering temperature,a new type of low melting point ceramics was synthesized in Bi₂O₃-V₂O₅ double low melting point oxide system.A series of BiMVO₅?M=Ca,Mg?microwave dielectric ceramics with sodium mica structure were synthesized.Dense ceramics can be sintered at lower temperatures?740-850 ^oC?.Raman spectroscopy is used to study their structure and microwave dielectric properties.BiCaVO₅crystallizes into orthogonal structure,space point group is Pbca,and can be densified at lower temperature to 820 ^oC,showing good microwave dielectric properties:?_r=15.7,Q×f=55,000 GHz,?_f=71 ppm/^o C;BiMgVO₅ crystallizes into monoclinic structure,space group is P21/n,and can be densified at lower temperature to 800^oC,showing good microwave dielectric properties:?_r=18.55,Q×f=71 ppm/^oC;BiMgVO₅ crystallizes into monoclinic structure,space group is P21/n,and can be densified at lower temperature to800^oC with Q×f=86,860 GHz,?_f=65 ppm/^oC.The effect of holding time on the synthesis of single-phase compounds was also studied.The phase purity was sensitive to time.Moreover,for practical industrial applications,both BiCaVO₅ and BiMgVO₅ via CaTiO₃ to adjust the?_f value,and do chemical compatibility tests with Ag.