Low Dielectric Constant Microwave Dielectric Ceramics

With the rapid development of microwave communication and radar technologies, researches on microwave dielectric resonant materials have been drawn toward high band of microwave frequency application. Exploiting and researching microwave dielectric materials with low permittivities (<10), high Q value and near-zero resonant frequency temperature coefficient has attracted much attention. It has been the key issue to study the loss mechanism and develop low loss dielectric materials. Materials with a permittivity less than 10 always have high Q values but large negative resonant frequency temperature coefficients, and it is necessary to modify the structure and microwave properties to get near-zero resonant frequency temperature coefficient. In this thesis, effects of crystal structure and microstructure upon dielectric properties are investigated in order to modify the microwave dielectric characteristics of low permittivity materials.Modification of Mg₂SiO₄ microwave dielectric ceramics has been comprehensively investigated by Ti substitution for Si. During the synthesizing process of Mg₂SiO₄ ceramics, MgSiO₃ is usually observed as second phase. The introduction of Ti could suppress MgSiO₃ effectively. The dielectric constant of Mg₂ (Si_1-xTi_x)O₄ ceramics increases from 6.8 to 8.1 with increase of x, and Qf value is improved prominently. While, the resonant frequency temperature coefficient is not improved by Ti substitution. The optimum properties are obtained at x=0.1: ε_r=7.4, Qf =73,760 GHz and Ï„_f= -60 ppm/℃.In the Mg₂SiO₄-Zn₂SiO₄ ceramic systems, the limited solution could be detected for the composition at the vicinity of two end-members, and foresterite and willemite co-existed between x=0.1 and x=0.9. The microwave properties of Mg₂SiO₄ are deteriorated due to both Zn₂SiO₄ and Zn evaporation. On the contrary, the introduction of Mg could improve the microwave dielectric properties of Zn₂SiO₄, The optimum property could be obtained at x=0.6: ε_r=6.6, Qf==95,650GHz, Ï„_f= -60ppm/℃.By controlling nonstoichiometric ratio of Mg/Si, MgSiO₃ secondary phase is suppressed, and Mg₂SiO₄ with single phase could be synthesized when Mg/Si ratio equals to 2.05. Due to the suppression of MgSiO₃, Qf value of Mg₂SiO₄ ceramics isincreased from 54,820 GHz to 114,730 GHz, and the resonant frequency temperature coefficient is improved from -63ppm/℃ to -58ppm/℃. The optimum properties of Mg₂SiO₄ ceramics are obtained: ε_r =7.5, Qf= 114,730GHz, Ï„_f= -59ppm/℃.Al₂O₃ was used as model material to explore the affecting factors upon dielectric loss. Y₂O₃ dopant could enforce the densification of Al₂O₃ ceramics and restrict grain growth. Due to the discrepancy of ionic radius, Y³⁺ is difficult to enter the lattice of corundum to form solid solution, the second phase Al₅Y₃O₁₂ is formed. Therefore, the outer factors primerily affect the microwave properties. The dielectric loss is increased from 8.4×10^-5 to 2.2×10^-4, but optimumizing grain size is advantageous to improve Q values.α-Al₂O₃ powders with various grain size and crystal morphologies are synthesized by glycothermal method. Compared with hydrothermal method, glycothermal method lowered the synthesizing temperature and pressure of reaction of systems without any additives. α-Al₂O₃ grains begin to form at 290℃, and transformation from precursors to α-Al₂O₃ is completed at 300℃ during a suitable time period. The orders of growth rate of α-Al₂O₃ facet are {0001} <{112|-3|-}={112|-3|-}<{112|-0}. The growth rate of the facet {0001} is the slowest, so that the facet {0001} is the feature shape of α-Al₂O₃ to show up. The crystal particles grow in layer by linkage of [AlO₆] octahedrons along c-axis, based on the facet of {0001}.