Research On The Preparation And Performances Of Low-Permittivity Microwave Dielectric Ceramics And BST-Based Ferroelectric Ceramics By Gelcasting

Based on studying lots of relevant research papers and the requirements of our research programs, in this dissertation, the0.9Al2O3-0.1TiO2,90wt.%(0.75ZnAl2O4-0.25TiO2)-10wt.%MgTiO3(ZTM) low-permittivity microwave dielectric ceramics, and45wt%Ba0.6Sr0.4TiO3-55wt%MgO (BSTM) ferroelectric ceramics were prepared by gelcasting. The effects of different preparation methods on the phase compositions, microstructures and relevant properties of the prepared ceramic materials were systematically investigated.Based on the simple introductions for the rheology properties of ceramic slurry and the solidification theory of acrylamide gelcasting system, the effects of pH value, dispersant and solids loading on the rheology properties of ceramic slurry and environmental temperature, initiator and catalyst contents, and monomer concentration on the solidification rate of ceramic slurry were all investigated.The0.9Al2O3-0.1TiO2ceramics were prepared by aqueous gelcasting and dry pressing. The effects of different preparation methods on the phase compositions, microstructures and microwave dielectric properties of0.9Al2O3-0.1TiO2ceramics were investigated. With adopting proper powder calcining temperature and continuous and slow cooling treatment, the Al2TiO5secondary phase is eliminated, and there are only Al2O3and TiO2phases. Compared with dry pressing, there are fewer pores, larger grains and more uniform grain distribution in the0.9Al2O3-0.1TiO2ceramics prepared by aqueous gelcasting. Moreover, adopting aqueous gelcasting to prepare0.9Al2O3-0.1TiO2ceramics could effectively improve their microwave dielectric properties. Based on stepped drying and slow binder removal, some intact, large dimension and complex shaped0.9Al2O3-0.1TiO2components were prepared by aqueous gelcasting.The ZTM ceramics were prepared by aqueous gelcasting and dry pressing. The effects of different preparation methods on the phase compositions, microstructures and microwave dielectric properties of ZTM ceramics were investigated. The results demonstrate that the phase compositions of the ZTM ceramics are the same no matter what preparation method is adopted. Compared with dry pressing, there are fewer pores, bigger grains and more uniform grain distribution in the ZTM ceramics prepared by aqueous gelcasting. Furthermore, adopting aqueous gelcasting to prepare ZTM ceramics could not only improve their microwave dielectric properties effectively, but also decrease their sintering temperature in some extent. Based on stepped drying and slow binder removal, the ZTM microwave dielectric antenna substrates were prepared by aqueous gelcasting, and then a GPS antenna was fabricated, which meets the requirements of real application.The BSTM ferroelectric ceramics were prepared by aqueous gelcasting, semi-aqueous gelcasting and dry pressing. The effects of different preparation methods on the phase compositions and microstructures of the BSTM ceramics were investigated. The results demonstrate that the phase compositions of the BSTM ceramics are the same no matter what preparation method is adopted. Meanwhile, the microstructures of the BSTM ceramics prepared by aqueous and semi-aqueous gelcasting are all uniform.The BSTM ceramics were prepared by non-aqueous gelcasting and dry pressing. The effects of different preparation methods on the phase compositions, microstructures and dielectric properties of the BSTM ceramics were investigated. The results demonstrate that there is no influence on the phase compositions whatever preparation method is adopted. Compared with dry pressing, the microstructures of the BSTM ceramics prepared by non-aqueous gelcasting are more uniform. Moreover, adopting non-aqueous gelcasting to prepare the BSTM ceramics could acquire the ones with more stable dielectric properties and could improve their dielectric properties in some extent.The BSTM ceramic powders were prepared by aqueous gelcasting-assisted solid-state method (AGASSM) and solid-state method (SSM). The effects of different ceramic powder preparation methods on the phase compositions and microstructures of the ceramic powders were investigated. Subsequently, adopting the dry pressing method to form samples, the effects of different ceramic powder preparation methods on the phase compositions, microstructures and dielectric properties of the BSTM ceramics were investigated. The results demonstrate that the BSTM ceramic powders prepared by AGASSM are more uniform with smaller (Dav=1.00μm) and more sphere-like particles than those prepared by SSM. The phase compositions of the prepared BSTM ceramic powders and the BSTM ceramics from the prepared ceramic powders are the same no matter what ceramic powder preparation method is adopted. Compared with SSM, the microstructures of the BSTM ceramics from ceramic powders prepared by AGASSM are more uniform. Furthermore, adopting AGASSM to prepare ceramic powders could not only improve the dielectric properties of the BSTM ceramics, but also decrease their sintering temperature.The BSTM ceramic powders were prepared by improved aqueous gelcasting-assisted solid-state method (IAGASSM), AGASSM and SSM. The effects of different ceramic powder preparation methods on the phase compositions and microstructures of the BSTM ceramic powders were investigated. Subsequently, adopting the dry pressing method to form samples, the phase compositions, microstructures and dielectric properties of the BSTM ceramics were investigated. The results demonstrate that the BSTM ceramic powders prepared by IAGASSM are the most uniform with the smallest particles (Dav=0.83μm) than those prepared by SSM and AGASSM. The phase compositions of the prepared BSTM ceramic powders and the BSTM ceramics from the prepared ceramic powders are the same no matter what ceramic powder preparation method is adopted. Compared with SSM and AGASSM, the microstructures of the BSTM ceramics from ceramic powders prepared by IAGASSM are the most uniform. Moreover, compared with SSM and AGASSM, adopting IAGASSM to prepare ceramic powders could not only improve the dielectric properties of the BSTM ceramics dramatically, but also decrease their sintering temperature.