Design, Preparation And Performance Optimization Of Low-Firing Microwave Dielectric Ceramics Based On Polynary Systems

With the development of wireless communications equipment for miniaturization, integration, the microwave dielectric materials should not only have appropriate dielectric constant（εr）, high quality factor（Q×f） and small temperature coefficient of the resonant frequency（τf）, but also exhibit a low sintering temperature（Ts）, which can be cofired with low- melting point conductors, meeting the demands for LTCC application. In this thesis, a new series of low sintering temperatures microwave dielectric ceramics with the composition of 5Mg_O-3Li₂O-4WO₃, Li_4x Mg_3（1-x）Al_6（1-x）Ti_5xO₁₂ and Li₄Ti₅O₁₂ were prepared by a solid-state reaction method. The low temperature sintering, processing optimization, sintering characteristics and performance adjustment of LiZnNbO₄ ceramic were also researched. Finally, the chemical compatibility between these compounds and Ag electrodes were also discussed.（1） A novel low sintering temperature microwave dielectric ceramic with the composition of 5MgO-3Li₂O-4WO₃ was prepared by the solid state reaction method. The phase structure, atoms occupation, sintering characteristics and microwave dielectric properties of the ceramic were researched by XRD, S EM and Microwave Network Analyzer. 5MgO-3Li₂O-4WO₃ ceramic not only had low sintering temperatures（≤840 oC）, but also obtained low relative permittivity ¹0.3, high quality factor ²0,537 GHz（f¹1 GHz）. Especially, the ceramics can be co-fired well with Ag electrode, which indicates that this material is a candidate for LTCC devices.（2） The influence of raw materials preparation on phase transition microstructure, sintering properties, and microwave dielectric properties was comparatively studied. LiZnNbO₄ ceramic obtained high quality factor（Q×f） at a low temperature range（950 1070 oC） by appropriate processing of the raw powders, which can promote the sintering ability of the ceramics and reduce its sintering temperature. A certain amount of CaTiO3 addition could adjust the τf of LiZnNbO4 ceramic. 0.9LiZnNbO_4-0.1CaTiO₃ sintered at 1070 oC obtained good microwave dielectric properties with （?）=22.9, Q×f=10,583 GHz（at ⁷.4 GHz） and τf =-27.5 ppm/ oC.（3） The phase structure, micromorphology and microwave dielectric properties of Li_4xMg_3（1-x）Al_6（1-x）Ti5xO12（0.2≤x≤0.8） microwave dielectric ceramics were investigated. Li_4xMg_3（1-x）Al_6（1-x）Ti_5xO₁₂ ceramics presented an excellent comprehensive performance with εr of 11.5²6.5, Q×f values of 7,102³0,191 GHz and τf values of-55.4⁺5.7 ppm/oC, and the sintering temperature could be reduced to 925 oC. Especially, Li_{4 x} Mg_3（1-x）Al_6（1-x）Ti_5xO₁₂ ceramics（x=0.6） sintered at 1075 oC shown moderate εr of 18.8, high Q × f values of ¹2,429 GHz（at ⁸ GHz）, and near-zero τf of-6.0 ppm/°C.（4） A novel low- firing microwave dielectric ceramic with the composition of Li₄Ti₅O₁₂ was fabricated through solid state reaction method. The phase structure, sintering ability, micromorphology of grains and microwave dielectric properties of ceramics were systematically investigated. X-ray diffraction data showed that Li₄Ti₅O₁₂ had a cubic spinel structure（JCPDS file No.00-049-0207）. The ceramic can be well sintered at 925 °C and exhibited optimal microwave dielectric properties with moderate （?） of 31.5, high quality factor（Q×f） of 29,530 GHz（at ⁷ GHz） and small τf of-15.0 ppm/°C. As the sintering temperature increased above 1025 °C, the elimination of TiO₂ phase could adjusted the τf value of mixed phases to near-zero.