Research On The Preparation And Properties Of Li And Mg-based Microwave Dielectric Ceramics With Low Permittivity

With the development of microwave communication technology and radar system,the communication frequency is also further to develop to the high frequency range,the demands for the microwave dielectric ceramics with low relative permittivity（ε_r<20）and high quality factor（Q?f）have been greatly increased.In this paper,low relative permittivity ceramics,such as 3MgO-Al₂O₃-3TiO₂、MgAl₂Ti₃O₁₀、CaMgSiO₄ and CaWO₄-2Li₂WO₄,were synthesized by the traditional solid-state reaction method..Furthermore,the sintering behavior,phase structure,microstrucutre and microwave dielectric properties were also been investigated.（1）3MgO-Al₂O₃-3TiO₂ ceramics were synthesized by a conventional solid-state reaction method.The phase composition,microstructure and microwave dielectric properties of ceramics were researched using XRD,SEM and Network Analyzer.The experimental results show that the 3MgO-Al₂O₃-3TiO₂ ceramic contained the coexistence of three phases,includin g MgAl₂O₄,MgTiO₃ and MgTi₂O₅.After sintering at 1350 ^oC for 4 h,the ceramics showed good comprehensive performances:ε_r can reach 15.4,Q×f can increase to 91000GHz,whileτ_f is about-55.1 ppm/^oC.A small amount of CaTiO₃ was added to adjust theτ_f of3MgO-Al₂O₃-3TiO₂ ceramics.The 0.8（3MgO-Al₂O₃-3TiO₂）-0.2CaTiO₃ ceramics sintered at1325 ^oC shown excellent microwave dielectric properties withε_r=17.6,Q×f=36606 GHz,τ_f=4.63 ppm/^oC.（2）MgAl₂Ti₃O₁₀0 microwave dielectric ceramics were prepared by the conventional solid-state reaction method.The phase structure,microstructure and dielectric characteristics of MgAl₂Ti₃O₁₀0 ceramics were studied.The MgAl₂Ti₃O₁₀0 ceramic belongs to low permittivity microwave dielectric ceramics.The ceramics sintered at 1400 ^oC for 4 h obtained optimum performances with relative permittivity（ε_r）of 14.9,high quality factor（Q×f）of 26450 GHz,a negative temperature coefficient of resonant frequency（τ_f）of-83.7 ppm/^o C.（3）The CaMgSiO₄ microwave dielectric ceramics were also synthesized via conventional solid state reaction method.The crystal structure,microstructure and microwave dielectric characteristics of ceramics were investigated.The CaMgSiO₄ ceramics could be well densified at 1350 ^oC for 4 h and exhibited favorable microwave dielectric properties with a low relative permittivity（ε_r=7.05）,a relatively high quality factor values（Q×f=62,500 GHz）and a negative temperature coefficient of resonant frequency（τ_f）of-62.90ppm/^oC,indicating that the CaMgSiO₄ ceramics might be a promising candidate for high-frequency devices fabrication.（4）A low firing microwave dielectric ceramic with the composition of CaWO₄-2Li₂WO₄were synthesized by the solid-state reaction method.The effects of sintering temperature on the crystal structure,microstructure and microwave dielectric performances of ceramics were investigated.The experimental results show that the CaWO₄-2Li₂WO₄ ceramics had not only low sintering temperature（≤750 ^oC）,but also obtained optimum microwave dielectric performances at the sintering temperature of 740 ^oC with low relative permittivity（ε_r）of 6.10,high quality factor（Q×f）of 62400 GHz.The CaWO₄-2Li₂WO₄ ceramics were chemically compatible with silver（Ag）electrode,these results indicate that CaWO₄-2Li₂WO₄ ceramics can be a candidate in low temperature co-fired ceramic（LTCC）devices.