Preparation And Properties Of AGe₄O₉（A=Sr,Ba）based Microwave Dielectric Ceramics With Low Permittivity

The rapid development of 5G mobile communication technology puts forward higher requirements on microwave dielectric ceramics with low permittivity（ε_r）,high quality factor（Q×f）and near-zero temperature coefficient of resonant frequency（τ_f）.The research object of this thesis was to investigate the crystal structure,microstructure and microwave dielectric properties of AGe₄O₉（A=Sr,Ba）ceramics by XRD,SEM and far-infrared reflectivity spectra.The relationship between structure and microwave dielectric properties was evaluated with respect of bond length,bond valence and octahedral distortion.Theτ_f value can be tuned by the“rattling”effect of the A-site cation.The sintering temperature of AGe₄O₉（A=Ba,Sr）ceramics was successfully reduced by adding a sintering aid,and the ceramics were co-fired with Ag powder to explore the chemical compatibility of ceramics with Ag electrodes.Finally,theτ_f of the ceramics were adjusted by forming composite as well as cation substitution.The main results of the thesis are as follows:（1）High relative density AGe₄O₉（A=Ba,Sr）ceramics were fabricated by the conventional solid reaction method.According to the results of XRD and Rietveld refinement,the AGe₄O₉（A=Ba,Sr）ceramics have hexagonal structure with space group P321.BaGe₄O₉ ceramics sintered at 1060℃ possessed the optimum comprehensive performances withε_r～9.8,Q×f～78,400 GHz,andτ_f～-44.2 ppm/℃,compared toε_r～11.2,Q×f～62,500 GHz,andτ_f～-11.7 ppm/℃ for Sr Ge₄O₉ ceramics sintered at 1100℃.The intrinsicε_r of the AGe₄O₉（A=Ba,Sr）ceramics fitted by far-infrared reflectivity spectra were 8.32 and 10.1,and the intrinsic Q×f values were91,923 GHz and 81,383 GHz,respectively.Sr Ge₄O₉+2 wt.%B₂O₃ ceramics sintered at 920 ℃ for 4 h exhibited excellent microwave dielectric properties:ε_r～10.8,Q×f～46,100 GHz andτ_f～-13.1 ppm/℃,which could be co-fired with Ag electrodes at 920℃.（2）To understand the significant difference on microwave dielectric properties between AGe₄O₉（A=Ba,Sr）ceramics,hexagonal Ba_1-xSr_xGe₄O₉ solid solution ceramics were prepared by Sr²⁺substitution for Ba²⁺.With the gradual increase of Sr²⁺content in Ba_1-xSr_xGe₄O₉ceramics,the A-site bond valence monotonously decreases,indicating that the“rattling”effect of A-site cation increases.The deviation（Δ（%））between the“observed”polarizability(α_obs)and“theoretical”polarizability(α_theo)of the ceramics increases.The corrected permittivity(ε_corr)of AGe₄O₉（A=Ba,Sr）ceramics was higher than the theoretical permittivity(ε_th)by 21.2%and 37.3%,respectively.Moreover,the enhancement of the“rattling”effect causes the larger measuredε_r and lower Q×f value of Sr Ge₄O₉ compared to BaGe₄O₉,and theτ_f of Sr Ge₄O₉ was closer to zero.Meanwhile,the distortion of[Ge O₆]octahedral in Sr Ge₄O₉（Δ（×10⁴）=1）was smaller than that of BaGe₄O₉（Δ（×10⁴）=12.7）,and the Ge-O bond valence of distorted[Ge O₆]octahedral in Sr Ge₄O₉(V_Ge-O=4.36617)was larger than that of BaGe₄O₉(V_Ge-O=4.30181).The above results indicate that adjusting the“rattling”effect of the A-site cation might provide a new idea for optimizing the performance of lowε_r microwave dielectric ceramics.（3）Ba Ti_xGe_4-xO₉（0≤x≤1）series ceramics were prepared at 1060～1180 ℃ by partially replacing Ge⁴⁺with Ti⁴⁺to adjust theτ_f of BaGe₄O₉.With the increase of Ti⁴⁺substitution,theε_r of the ceramics increased,Q×f decreased andτ_f closed to zero.When 0≤x≤0.6,hexagonal Ba Ti_xGe_4-xO₉ solid solution ceramics with space group P321 were synthesized by Ti⁴⁺entering into the[Ge O₆]octahedron.In the range of 0.6（27）x（27）1,the hexagonal structure（space group P-6c2）of Ba Ti Ge₃O₉ appears gradually.When x=1,Ba Ti Ge₃O₉ ceramics with smallτ_f value and good performance were obtained:ε_r～11.3,Q×f～19,750 GHz andτ_f～-16.8 ppm/℃.Composite ceramics were formed by adding Ti O₂ to BaGe₄O₉,in which 0.6BaGe₄O₉+0.4Ti O₂ceramics sintered at 1080 ℃ showed a near-zeroτ_f of+6.4 ppm/℃ with lowε_r～12.4 and high Q×f～57,900 GHz.（4）To further improve the Q×f value of BaGe₄O₉,Ba Si_xGe_4-xO₉（0≤x≤1.5）ceramics were prepared in the temperature range of 1060～1180 ℃.As the amount of Si⁴⁺increased,theε_r of the ceramics became small,theτ_f increased in the direction of close to zero.The Q×f values reached the maximum at x=0.1 and then decreased.When x=0.1,the Ba Si_0.1Ge_3.9O₉ ceramics were obtained,and the microwave dielectric properties were:ε_r～9.5,Q×f～95,890 GHz andτ_f～-42 ppm/℃.When 0≤x≤0.1,Si⁴⁺entering the[Ge O₄]tetrahedron,and the hexagonal solid solution Ba Si_xGe_4-xO₉ceramics with space group P321 were formed.In the range of 0.1（27）x（27）1.5,the hexagonal Ba Si_1.5Ge_2.5O₉ phase appeared with space group P-6c2.Si⁴⁺and Ge⁴⁺occupied the[BO₄]tetrahedral position together at the ratio of 1:1,and the content of the Ba Si_1.5Ge_2.5O₉ phase increased with the increase of x value.When x increased to1.5,the BaGe₄O₉ phase disappears,and the Ba Si_1.5Ge_2.5O₉ ceramics were obtained and exhibitedε_r～8.0,Q×f～11,060 GHz andτ_f～-16 ppm/℃.（5）The effect of Si⁴⁺and Ti⁴⁺substitution for Ge⁴⁺at B-site on the structure and performance of Sr Ge₄O₉ were studied,respectively.Sr B_xGe_4-xO₉（B=Si,Ti）ceramics were prepared in the temperature range of 1100～1180 ℃.As the x increased from 0to 0.4,the Q×f value of the Sr B_xGe_4-xO₉（B=Si,Ti）ceramics decreased,and theτ_fincreased in the direction of close to zero.Theε_r of Sr Si_xGe_4-xO₉ decreased while theε_rof Sr Ti_xGe_4-xO₉ increases due to the different ionic polarizabilities of Si⁴⁺,Ge⁴⁺and Ti⁴⁺.When 0≤x≤0.4,hexagonal structure with space group P321 were formed for Sr B_xGe_4-xO₉（B=Si,Ti）solid solution ceramics in which Si⁴⁺entered into the[BO₄]tetrahedron while Ti⁴⁺entering the[BO₆]octahedron.When x=0.4,a near-zeroτ_f was achieved for both ceramics,and the microwave dielectric properties wereε_r～9.3,Q×f～10,550 GHz andτ_f～-3.5 ppm/℃ for Sr Si_0.4Ge_3.6O₉,andε_r～12.8,Q×f～12,560GHz andτ_f～-2.8 ppm/℃ for Sr Ti_0.4Ge_3.6O₉,respectively.