Exploration And Properties Research Of Yttrium Aluminum Garnet Microwave Dielectric Ceramics

With the booming development of aerospace,Io T,communication,medical and automotive drives,microwave dielectric ceramics（MWDC）are gaining a promising application as a basic device for the communication industry.Terahertz communication technology is an important direction for the development of future 6G communication technology.As the signal delay time is proportional to the square root of the relative dielectric constant,dielectric ceramics with low dielectric constant and high quality factor can minimise the delay in remote communication reception and meet the high frequency operating requirements of terahertz media.Therefore,the development of dielectric ceramics to meet the needs of ultra-high speed transmission has become the key to a new generation of communication technology.Based on the demand for low dielectric materials for terahertz communication devices,a series of yttrium aluminium garnet-based solid solutions were constructed through the substitution of rare earth elements and non-equivalent ions and defect chemistry,and the material’s stoichiometriy,defect types and concentrations,and phonon vibration modes were designed and regulated to obtain the desired performance,providing a solid experimental and theoretical basis for the development of microwave dielectric ceramic devices related to 6G communication.The experimental scheme was first designed based on the non-stoichiometriy of A-site（Y-site）and the doping of rare earth elements,while B-site（Al-site）heterovalent ion substitution study was carried out afterwards.In particular,an in-depth theoretical study was conducted on the mechanism of lattice defect response and dielectric property optimisation of Nb₂O₅-doped Y₃Al₅O₁₂（YAG）ceramics,revealing the essential mechanism by which microstructural properties including Raman vibrational modes,cell parameters,ion polarisation rates,bond lengths and bond energies affect the dielectric losses of Nb₂O₅-doped YAG ceramics.The microwave loss characteristics of YAG ceramics after A/B site substitution are quantitatively summarized by comparing theoretical calculations with experimental results.The aim is to provide an effective experimental and analytical method and theoretical support for improving the microwave dielectric properties of yttrium aluminium garnet ceramics.This dissertation focuses on the following:（1）In the study of the non-stoichiometry of Y₃Al₅O₁₂ ceramics,the changes in phase composition,lattice evolution,crystal defects and dielectric properties within the YAG ceramic system were carefully analyzed in excess of Y and Al elements,respectively,to demonstrate the importance of the Y/Al ratio on the performance of YAG ceramics.The relationship between the lattice evolution and the microwave dielectric properties of the YAG ceramics was investigated using five rare earth element ions doped with ions of La³⁺,Sm³⁺,Gd³⁺,Er³⁺and Yb³⁺.The SEM characterization showed that the grains of the Gd³⁺and Er³⁺doped YAG samples were overdeveloped,while the La³⁺doped ceramic grains were well aligned and uniform in size.In the discussion of the microwave dielectric properties,the P-V-L theory was introduced to establish a link between the lattice evolution and the microwave dielectric properties.Furthermore,Y_3-xLa_xAl₅O₁₂（0≤x≤0.09）ceramics were prepared based on the above experimental results to further confirm the optimum substitution ratio for La³⁺doped YAG.The effect of grain boundaries and lattice defects on the dielectric loss was visualized by transmission electron microscopy characterization analysis.The co-grained grain boundaries between the grains of Y₃Al₅O₁₂ ceramics modified by La₂O₃ doping effectively suppress the proliferation of intergranular dislocations,curb the negative effects of excessively wide grain boundaries on grain development and ensure a stable periodic arrangement of the lattice during high temperature sintering,helping YAG ceramics to achieve low dielectric constant and low loss microwave properties.（2）A study of the dielectric properties of Y₃Al₅O₁₂ ceramics with B-site heterovalent ion substitution was performed.First,light Mg²⁺ions are used to replace the B-site Al³⁺ions in YAG ceramics.On the one hand,the lighter mass of Mg²⁺ions enters the YAG lattice and completes the migration quickly as a lighter diffusion particle,helping to enhance the densification efficiency of the ceramics.On the other hand,the Mg²⁺ions replace the Al³⁺ions at the B-site,inducing oxygen vacancy defects in the lattice,allowing the ceramic to gain additional momentum in sintering and successfully lowering the phase formation temperature of the YAG ceramic.Then the sintering process,microscopic morphology and microwave dielectric properties of Y₃Al₅O₁₂ ceramics with the addition of TEOS（decomposed to Si O₂ at high temperatures）have been investigated successively.It has been demonstrated that the addition of TEOS triggers liquid-phase sintering,which promotes the dissolution and migration of solid-phase particles and thus improves the densification of YAG ceramics,thereby facilitating the improvement of their dielectric properties at lower temperatures.Finally,Nb₂O₅-doped YAG ceramics were investigated and the experimental results showed that Nb⁵⁺ions replaced Al³⁺ions at the B-site.However,due to the easy combination of Nb⁵⁺and Y³⁺ions at high temperature,YNb O₄secondary phase precipitates in the system with larger amounts of Nb₂O₅ doping.（3）Based on the analysis of Nb₂O5 in B-site doped YAG ceramics,the experimental protocol was further optimized to prepare Y-deficient and Nb-doped yttrium aluminium garnet ceramics,which successfully circumvented the appearance of the secondary phase of YNb O₄ and obtained YAG ceramic samples with excellent microwave dielectric properties.The YAG ceramics doped with appropriate amounts of Nb₂O₅ exhibited good crystallinity and crystal structure homogeneity.The Nb₂O₅ substitution induces modulation of the YAG crystal structure to generate a reconfigured superlattice,where the non-harmonic vibrations within the crystal are suppressed and the intrinsic losses are reduced,which is closely related to the ultra-high quality factor obtained for the ceramics.And Y_2.9Al₅Nb_0.1O₁₂ ceramics sintered at 1700°C for 6 hours exhibit excellent microwave properties:ε_r=10.99,Q×f=280,387 GHz,τ_f=-34.7 ppm/°C.Furthermore,four sets of control experiments were designed by adjusting the stoichiometric ratio of YAG to provide different doping environments for Nb⁵⁺ions with the chemical formulae S1:Y₃Al₅O₁₂,S2:Y₃Al₅O₁₂+1wt.%Nb₂O₅,S3:Y₃Al_4.955Nb_0.045O₁₂,S4:Y_2.955Al₅Nb_0.045O₁₂.The reaction mechanism of three defects in Nb₂O₅ heterovalently substituted yttrium aluminum garnets was analyzed in detail and its relationship with microwave dielectric properties was explored.