Study On Control Of LiGa₅O₈ Inverse Spinel Structure And Its Microwave Dielectric Properties

With the rapid development of 5G network,communication equipment is developing toward miniaturization and high frequency.Microwave dielectric ceramics with low dielectric constant（ε_r）,high quality factor（Q×f）and resonant frequency temperature coefficient near zero（τ_f）have been widely paid attention.Some spinel ceramics have been widely studied because of their lowε_r and high Q×f values.However,compared with perovskite composite microwave dielectric ceramics,the relationship between their composition,structure and properties（especially dielectric loss）is still lacking.This article selects octahedral position 1:3 orderly LiGa₅O₈ inverse spinel structure as the research object.Then,the sintering behaviors,crystal structure,vibrational spectrum and microwave dielectric properties of these spinel ceramics were characterized,and the relationships between composition,structure and properties investigated by XRD,SEM,Raman spectrum,Far-infrared reflective spectrum,TEM,XPS,and Ac impendence spectrum.The main results in this paper are as follows:（1）Cubic inverse spinel structured LiGa₅O₈ ceramics with the space group P4₃32 was prepared by sintering in the range of 1200-1300℃,Li⁺and partial Ga³⁺occupy the octahedral in a 1:3 long-range order and the remainder of the Ga³⁺ions are tetrahedrally coordinated.Optimum microwave dielectric properties were obtained for LiGa₅O₈ ceramic sintered at 1260℃withε_r=10.51,Q×f=127,040GHz,τ_f=-60.16 ppm/℃.Theε_r and Q×f of LiGa₅O₈ ceramics is negatively correlated with the Raman shift and FWHM of Raman mode at 424 cm^-1,respectively.The intrinsic dielectric properties of LiGa₅O₈ ceramic materials were further studied by far-infrared reflectance spectra.The fitting analysis of far-infrared reflectance spectra of LiGa₅O₈ shown that the active vibration modes in the low wave number range(100-600cm^-1)has a greater contribution to the intrinsic microwave dielectric polarization.The optimization of composite ceramic with near zeroτ_f can be obtained by two-phase composition of LiGa₅O₈ and CaTiO₃.Excellent microwave dielectric properties withε_r=12.79,Q×f=109,752GHz,τ_f=+4.07 ppm/℃were achieved in 0.94LiGa₅O₈–0.06CaTiO₃ sintered at 1260℃for 6h.（2）A series of Ga₂[Li_1+xGa₃]O_8+δ（0.05≤x≤0.125）ceramics were prepared by the traditional solid-state method.The optimal sintering temperature of all ceramic samples was1260℃,when x=0.125,the second phase LiGa O₂ appeared,so the maximum solid solubilities were between x=0.1～0.125.By calculating the intensity ratios of the superlattice diffraction peaks I_（110）/I_（220）and I_（211）/I_（422）,it is found that with the increase of x,the ratio first increases and then decreases.The larger the ratio is,the higher the degree of order of octahedral cations,indicating that the appropriate addition of Li₂O is beneficial to improve the degree of order of octahedral cations.The optimum microwave dielectric properties of the sample x=0.1 were obtained at 1260℃for 6 h:ε_r=10.78,Q×f=171,171GHz,τ_f=-56.47 ppm/℃.With the change of x,the change ofτ_f is negatively correlated with the change of bond valence and octahedral distortion of Ga1.Far-infrared reflectance spectra（FIR）confirms that the dielectric polarization in microwave band is mainly caused by ionic polarization,while dipole polarization and other polarization mechanisms almost do not exist.Through AC impedance analysis,as x increases gradually,the insulation performance is improved,and the relaxation activation energy and DC conductance activation energy of ceramics both increase first and then decrease,and their values are also relatively close.（3）Li(Zn_0.5Ti_0.5)_xGa_5-xO₈（1≤x≤5）spinel solid solution ceramics were formed by[Zn_0.5Ti_0.5]³⁺substitution of Ga³⁺to destroy the octahedral order of LiGa₅O₈.By XRD and crystal structure refinement found within the scope of replacing formed a single-phase cubic spinel compounds,the space group is changed from P4332 to Fd-3m,and the superlattice diffraction peak（110）,（210）,（211）,（221）and（310）disappeared,show that the obtained material from order to disorder,moreover TEM analysis results also show that the structure of the material from order to disorder.As the value of x increases,the bulk density of the material decreases,the average ionicity increases,and the lattice energy first increases and then decreases,indicating that the stability of the material system gradually increases and then decreases.With the increase of x value,the dielectric constant of the material increases,which can be attributed to the increase of ionicity andα_theo/V_m.Theτ_f value varies from-69.29 ppm/℃to-48.73 ppm/℃,which is the same as that of the bond valence at the octahedral position with the x value.Compared with LiGa₅O₈,the Q×f value of the entire Li(Zn_0.5Ti_0.5)_xGa_5-xO₈（1≤x≤5）is greatly reduced,which can be attributed to the decrease in the degree of order of the octahedron.When x varies from 1 to 5,Q×f value of the material gradually increases and then decreases,in x=1 and x=2 has lower Q×f value can be attributed to Ti base,with the increase of x value,lattice energy increases,intending to effectively restrain the Ti,so as to make the Q×f value increases.Therefore,the optimal dielectric properties of microwave are obtained at x=3:ε_r=16.46,Q×f=79,268GHz andτ_f=-53.36 ppm/℃.（4）In this chapter,Li(Zn_0.5Ge_0.5)_xGa_5-xO₈（0.5≤x≤2）microwave ceramics were prepared by the traditional solid-phase method,investigate the effects of[Zn_0.5Ge_0.5]³⁺substitution of Ga³⁺on crystal structure,microstructure and microwave dielectric properties.Through XRD and crystal structure refinement,it is found that a single-phase cubic spinel compound is formed in the substitution range,the space group is changed from P4₃32 to Fd-3m,and the second phase LiGa Ge O₄ appears when x=2,so the maximum solid solubilities were between x=1.5～2.As the value of x increases,the density of the material decreases,the average ionicity first increases and then decreases,and the lattice energy first increases and then decreases,indicating that the stability of the material system is gradually increasing and then decreasing.Compared with LiGa₅O₈,the Q×f value of the Li(Zn_0.5Ge_0.5)_xGa_5-xO₈（0.5≤x≤2）material system is greatly reduced,which can be attributed to the decrease in the degree of order of the octahedron.In the range of 0.5<x<2,the value of Q×f increases first and then decreases,which is because the lattice energy increases first and then decreases.The optimum microwave dielectric properties of the sample x=1 are as follows:ε_r=10.89,Q×f=77,471GHz andτ_f=-68.5 ppm/℃when sintered at 1050℃for 6 h.[Zn_0.5Ge_0.5]³⁺substitution of Ga³⁺,and the sintering temperature of the material was reduced from 1260℃to 1050℃.In order to further reduce the sintering temperature,adding low melting point oxide B₂O₃ to reduce the sintering temperature of ceramic materials,when adding 2wt.%B₂O₃ to Li(Zn_0.5Ge_0.5)Ga₄O₈ ceramics,the sintering temperature is reduced to 940℃,and the Ag electrode are chemically compatible.In order to adjust theτ_f value of the material,Li Ca₂Mg₂V₃O₁₂ and Li(Zn_0.5Ge_0.5)Ga₄O₈ are used to form composite ceramics.In 0.79 Li(Zn_0.5Ge_0.5)Ga₄O₈–0.21 Li Ca₂Mg₂V₃O₁₂ composite ceramics,the value ofτ_f was successfully adjusted to zero.Using 0.79 Li(Zn_0.5Ge_0.5)Ga₄O₈–0.21Li Ca₂Mg₂V₃O₁₂–2wt.%B₂O₃ as the dielectric substrate,a patch antenna under 5.2GHz was designed,which can realize 5GHz Wi-Fi applications.（5）Li(Li_1/3Ti_2/3)_xGa_5-xO₈（1≤x≤5）series spinel microwave dielectric ceramics were formed by[Li_1/3Ti_2/3]³⁺substitution of Ga³⁺.Through XRD and crystal structure refinement,it is found that the single-phase cubic spinel compounds with Fd-3m space group are formed in the range of 1≤x≤2.When 2.5≤x<3,it is the two-phase mixing zone of Fd-3m and Imma.At x=3,LiGa Ti O₄ ceramics with both tetrahedral and octahedral positions of 1:2 are obtained,with orthogonal crystal structure with space group of Imma.When 3.5≤x<4.5,it is a two-phase mixing region of Imma and P4₃32.When 4.5≤x≤4.75,the P4₃32 phase ceramics have a cubic crystal structure,in which the tetrahedral and octahedral positions are disordered.When x=5,a cubic structure with space group Fd-3m is formed,and the tetrahedral position and octahedral position are disordered.With the increase of x value,the theoretical density and bulk density of decreased,and the optimal sintering temperature decreased gradually,indicating that[Li_1/3Ti_2/3]³⁺substituted for Ga³⁺was beneficial to reduce the density and sintering temperature of the material.The permittivity of single cell ion increases with the increase of its polarizability.The value of Q×f first decreases from 127,040GHz at x=0,then increases from x=1,reaches a maximum of 103,365GHz at x=3,and then decreases gradually.The main reason for this phenomenon can be attributed to the decrease of the order degree of polyhedral.When 0<x<3,the order degree of octahedron changes from 1:3 to disordered structure.With the increase of x,the order degree of octahedron gradually changes from disordered to 1:2 order,thus increasing the value of Q×f.With the further increase of the amount of substitution,the order degree of the polyhedron is destroyed again,making the original 1:2 order become disordered structure again,so that the value of Q×f decreases again.With the increase of[Li_1/3Ti_2/3]³⁺content,the dielectric constant increases,and the corresponding resonant frequency temperature coefficient is gradually improved.The optimum dielectric properties of microwave in the range of 1≤x≤5 are obtained at x=3:ε_r=15.632,Q×f=103,365GHz andτ_f=-53.74ppm/℃.