In-situ Synthesis And Dielectric Properties Of Ferroelectric/dielectric Composite Ceramics

Ba0.5Sr0.5Ti O3-Mg O(BST-M), Ba0.5Sr0.5Ti O3-Mg2 Ti O4(BST-MT) and Ba0.5Sr0.5Ti O3-Mg2 Si O4(BST-MS) ferroelectric/dielectric composite ceramics have been prepared in-situ via wet chemical methods. The influence of various preparation process and component on the microstructure and dielectric properties of BST based composite ceramcis has been investigated, and the growth mechanism in-situ process has been analyzed. The influence of component and structure on microwave and dielectric tunable properties of BST based composite ceramcis has been studied to establish a relationship between variations in structure and perperties and percolation effect. The mian conclusions are as follows:BST-Mg O composite nanopowders have been fabricated in-situ at low calcining temperature of 700 oC via a citrate-nitrate combustion method. After that, the BST-Mg O composite ceramics with Mg O particles homogeneously dispersed in BST particles has been prepared throught the conventional electronic ceramics process. The amount of Mg2+ that entered into BST lattice increased with the increase of Mg O volume fraction(?M), which resulted to the formation of F centers. the permittivity(ε) of BST in the composites was strongly dependent on ?M, which was defined by the relation Mfbea???BST?. Also, the high percolation threshold of ?M=0.6 was observed, which was well in agreement with the theoretical predictions of the modified effective medium approximation(MEMA) coupled with the spherical inclusion model.BST-Mg2XO4(X=Ti, Si) composite powders with pure BST and Mg2XO4 phases could not have been fabricated in-situ below the calcining temperature of 1000 oC via a citrate-nitrate combustion method. However, BST-Mg2XO4(X=Ti, Si) composite ceramics with pure BST and Mg2XO4 phases could have been obtained at the sintering temeprature of 1300 oC. BST? in the composites decreased with increasing of the volume fraction of microwave dielectics(?), which satisfied the relationship fbea???BST?. Compared with BST? in the BST-Mg2 Ti O4 system, BST? markedly decreased with increasing of ? in the BST-Mg2 Si O4 system and the percolation behavior disappeared, owing to the more amount of Mg2+ incorporation into BST lattice resulting from the fusion of Si O2 during the sintering process of 1300 oC.BST-Mg O composite nanopowders with homogeneous size were sythsized in-situ at low calcining temperature of 700 oC via a coprecipitation method in ethanol solution. The alkaline precursors was good for the precipitation of Mg2+ but not conducive to the formation of BST and Mg O phases. For the BST-Mg O composite systems prepared by p H=2 precursors, as the concentration of Mg O was increased, the Curie temperature still remained unchanged, while the dielectric anomalous peaks of cubic-tetragonal phase transition were suppressed and braodened. Accordingly, both permittivity and tunability were decreased, while microwave loss still kept below 0.005.