Low Dielectric Constant Polyanion-type Microwave Dielectric Ceramics

Recently, the rapid development of Low-Temperature Co-fired Ceramic（LTCC） and Monolithic Microwave Integrated Circuit（MMIC） are raising demands of the fabrication of microwave dielectric ceramics with low dielectric constant（ er）, high quality factor（ Q × f） and nearly zero temperature coefficient of resonant frequency（ tf）. Meanwhile, the low sintering temperature of ceramics and the chemical capability between ceramics and metal electrode are also required. In present thesis, the fabrication and dielectric properties of polyanion type Mg₂SiO₄-LiMgPO₄ and LiMnPO₄ microwave dielectric ceramics are investigated and the temperature coefficient is adjusted to near zero by incorporating rutile phase of TiO₂ with a positive tf. The reaction dynamics, chemical capability between ceramics and rutile phase or Ag electrode at sintering temperature, together with the dielectric relaxation are systematically investigated.Mg₂SiO₄-LiMgPO₄ powders were synthesized using the conventional solid-state reaction method in present work. The result shows that, the complete solid solutions of（1-x）Mg₂SiO₄-x LiMgPO₄（x = 0.0 1.0） could be obtained by incorporating（Li+P5+） into（Mg₂+Si4+） in Mg₂SiO₄ at appropriate sintering temperature. The phase constitution of 0.5Mg₂SiO₄-0.5LiMgPO₄ is found to be dependent on firing temperature. The experimental result shows that（1-x）[Mg₂SiO₄-x LiMgPO₄]（x = 0.5, 0.7, 0.9） is chemical compatible to the rutile phase at sintering temperature. The larger LiO6 octahedron with larger ion radius of Li+ will compress the MgO6 octahedron and it can also strengthen the Mg-O bond. Mg₂+ is hardly released to react with TiO₂ and the chemical capability is improved. The optimized microwave dielectric properties for 0.35Mg₂SiO₄-0.35LiMgPO₄-0.3TiO₂ ceramics sintered at 1050°C show low dielectric constant（11.4）, high quality factor（31,800 GHz）, and low temperature coefficient of resonant frequency（-4 ppm/°C）.LiMnPO₄ with olivine structure is usually widely studied as a promising cathode material for lithium batteries. In present work, LiMnPO₄ ceramic has been found to have potential application on low-temperature co-fired ceramic technology. Single phase of LiMnPO₄ was synthesized by one-step solid state reaction using ammonium polyphosphate（APP） as PO₄ precursor. The reaction dynamics in different synthesis temperatures are also investigated. Dense LiMnPO₄ ceramic with more than 95% theory density can be obtained at 750°C, and it can be cofired with silver electrode. The dielectric measurement shows the dielectric relaxation which obeys the universal law with fractional exponent. The dielectric spectra are well fitted with the equivalent circuit capacitance parallel with constant phase degree. The optimized microwave dielectric properties for LiMnPO₄ ceramics sintered at 750°C show low dielectric constant（8.1）, and high quality factor（44,224GHz） and it can also be co-fired with Ag. Dense ceramic with low temperature coefficient of resonant frequency（|tf | ≤ 5 ppm/°C） can be sintered at 900 975°C by doping with TiO₂.