Preparation And Characterization Of Ba_0.991Bi_0.006TiO₃@Nb₂O₅Co₃O₄-La₂O₃@ZnO-B₂O₃-SiO₂ Ceramics With A Fine-grained Microstr

The development trends for electronic machine have emphasized miniaturization, large capacity, high reliability, high stability and low cost. Its important parts, multilayer ceramic capacitors?MLCC?, came into being and gradually turned to the miniaturization, high capacity, application in wide temperature range. BaTiO₃ with a perovskite structure has been widely used in MLCCs. Because the temperature stability of the pure BaTiO₃ ceramics is very poor, the dielectric constant at Curie temperature is high and that at room temperature is very low, the pure BaTiO₃ ceramics can not applied in a wide temperature range. BaTiO₃ ceramic powders have been commonly prepared by conventional solid-state reaction method and the size of powder particles is large, so it can not meet the requirements of application in wide temperature range and fine grain. Moreover, due to the high sintering temperature of the traditional BaTiO₃ dielectric ceramics, the ceramic grains unusually grow up, and the sintering temperature is too high to save energy and protect environment. The article coated Nb₂O₅-Co₃O₄-La₂O₃ onto the surface of Ba_0.991Bi_0.006TiO₃ particles uniformly and obtained the dense and fine-grained X8R-type MLCC dielectric ceramic material; prepared a ZnO-B₂O₃-SiO₂ shell on the surface of Ba_0.991Bi_0.006TiO₃ nanoparticles uniformly by means of the sol-precipitation method and obtained the X8R-type MLCC dielectric ceramic material with a low sintering temperature, a high dielectric constant and a fine-grained microstructure. The main contents of this paper are as follows:1. The certain amount of Nb₂O₅ and Co₃O₄ ?Nb/Co=2:1? and different amounts of La₂O₃ are coated onto the surface of Ba_0.991Bi_0.006TiO₃ particles uniformly via precipitation method and prepared Ba_0.991Bi_0.006TiO₃@Nb₂O₅-Co₃O₄-La₂O₃ ?BBT@NCL? nanoparticles with a "core-shell" structure. The effects of the coating amount of La₂O₃ and sintering temperature on the microstructure, phase constitution and dielectric properties of BBT@NCL ceramics system were also investigated. The experimental results show that: the particles coated with Nb₂O₅-Co₃O₄-La₂O₃ have obvious perovskite structure, Nb₂O₅ and Co₃O₄ phase, but La₂O₃ has not detected because of little coating quantity. The particles after coating have good monodispersity, the layer of Nb₂O₅-Co₃O₄-La₂O₃ is clear, and the thickness is about 9 nm, which is a typical "core-shell" structure. The ceramics sintered at 1220? have dense microstructure and the average size of grains is 280 nm. When the coating amount of La₂O₃ is 0.3 mol%, the dielectric constant at room temperature of BBT@NCL ceramics is 2862, the dielectric loss is 0.0067, and TCC is between-15%?+15%, so it meets the X8R standard.2. In order to study the effects of ZnO-B₂O₃-SiO₂ coating layer on BBT@NCL nanoparticles and ceramics, Ba_0.991Bi_0.006TiO₃ is chose as the simple material of core and coated with ZnO-B₂O₃-SiO₂ via the sol-precipitation method, prepare Ba_0.991Bi_0.006TiO₃@ ZnO-B₂O₃-SiO₂ ?BBT@ZBSO? nanoparticles and then study the effects of the coating amount of ZnO-B₂O₃-SiO₂ and sintering temperature on the microstructure, phase constitution and dielectric properties of BBT@ZBSO ceramics system. The experimental results show that:the BBT@ZBSO particles have pure perovskite structure and no second phase. The Ba_0.991Bi_0.006TiO₃ particles after coating have good monodispersity, the layer of ZnO-B₂O₃-SiO₂ is clear, which is a typical "core-shell" structure. Among the samples sintered at 1140?, the sample coated with 4 wt% ZnO-B₂O₃-SiO₂ has the best microstructure, the sample coated with 5 wt% ZnO-B₂O₃-SiO₂ has the highest dielectric constant ?2050? and the lowest dielectric loss ?0.0060?.3. ZnO-B₂O₃-SiO₂ sintering aid is coated onto the surface of BBT@NCL nanoparticles uniformly via the sol-precipitation method and reduces the sintering temperature of BBT@NCL ceramics. The effects of the coating amount of ZnO-B₂O₃-SiO₂ and sintering temperature on the microstructure, phase constitution and dielectric properties of BBT@NCL@ZBSO ceramics system are investigated. The experimental results show that: the phase composition of BBT@NCL@ZBSO nanoparticles includes clear perovskite structure, Nb₂O₅ and Co₃O₄ phase, but has not detected out La₂O₃ and ZnO-B₂O₃-SiO₂ due to the little amount and low preparation temperature. The BBT@NCL particles after coating have good monodispersity and the layer of ZnO-B₂O₃-SiO₂ is clear. When sintered at 1100?, BBT@NCL@ZBSO ceramics samples show a clear perovskite structure, accompanied by the detection of Nb₂O₅, CO₃O₄ and the impurity phase Ba₂TiSi₂O₈. As the coating amounts of ZnO-B₂O₃-SiO₂ increases gradually, the porosity of the samples decreased significantly, density increased, and the dielectric constant of the samples increased first and then decreased. When the coating amount of ZnO-B₂O₃-SiO₂ is 4 wt%, the dielectric constant at room temperature of BBT@NCL@ZBSO ceramics is 2841, the dielectric loss is 0.0044, TCC is between -15%?+15% and meet X8R standard. With the increased sintering temperature, the impurity phase Ba₂TiSi₂O₈ gradually appeared; the density of ceramic gradually improved, ceramic grain also grew up. The exceedingly high temperature will lead to abnormal grain growth and poor dielectric properties.Therefore, the experiment obtained the dielectric ceramic material of X8R-type MLCC with a low sintering temperature, a high dielectric constant and a fine-grained microstructure by coating Nb₂O₅-Co₃O₄-La₂O₃ and ZnO-B₂O₃-SiO₂.