Preparation And Dielectric Properties Of Strontium Barium Titanate Based Ceramics

Barium strontium titanate(Ba_1-xSr_xTi O₃,BST)is a complete solid solution of stro ntium titanate and barium titanate,which belongs to perovskite structure（ABO₃）.It ha s the advantages of high dielectric constant,low dielectric loss and adjustable Curie te mperature.It is widely used in the fields of dielectric energy storage,dynamic random memory,microwave phase shifter and multilayer ceramic capacitor,etc.it has good r esearch value and application prospect.In this dissertation,Ba_0.6Sr_0.4Ti O₃ ceramic powder was prepared by oxalate precipi tation method.The crystals of ceramic powder were tested by X-ray diffraction,scanni ng electron microscope,Fourier transform infrared spectroscopy,thermogravimetric-diffe rential thermal analysis and other test methods.The structure and microscopic morphol ogy were characterized.The reaction process of oxalate precipitation method and the i nfluence of reaction conditions such as solution p H and calcination temperature on the microscopic morphology of the powder were studied.The Ba_0.6Sr_0.4Ti O₃ powder with uniform particle size,good dispersion and high purity was selected as the matrix mate rial,and Cu（NO₃）₂ and Mg（NO₃）₂ were used as coating materials.The chemical precip itation method and room temperature were systematically studied.The effect of solid p hase preparation of composite powder and reaction conditions on the structure and mo rphology of the powder,and the formation mechanism and influencing factors of the s urface coating were discussed respectively.On this basis,Ba_0.6Sr_0.4Ti O₃@Cu O and Ba_0.6Sr_0.4Ti O₃@Mg O multiphase ceramics were prepared by using powders prepared by chemical precipitation and room temperature solid phase as raw materials,and the sint ering mechanism and microstructure were studied.Influence of dielectric properties.Fi nally,as a comparison,the sintering process and dielectric properties of copper-calcium titanate composite barium titanate ceramics were studied.The specific conclusions are as follows:（1）The results show that Ba_0.6Sr_0.4Ti O₃ powder with good morphology and high phase purity can be obtained by controlling the p H value of the reaction solution to 3.5 and the calcination temperature to 850℃.（2）Using Cu（NO₃）₂·3H₂O and Ba Ba_0.6Sr_0.4Ti O₃ powders prepared by the above c oprecipitation method as raw materials,BST@xwt%Cu O composite ceramic powders w ith different coating amounts were prepared by precipitation method and room tempera ture solid phase method.On this basis,BST@Cu O composite powder was used as the raw material,pressed at 12-18M pa molding pressure,and sintered at 1280℃for 2hours to prepare BST@Cu O ceramics.The analysis of the dielectric properties test res ults showed that due to the high temperature The diffusion part of Cu²⁺enters the po sition of Ti⁴⁺in the perovskite structure,causing lattice distortion.At the same time,t he liquid phase Cu O generated during the sintering process is at the position of the g rain boundary,causing the interface polarization to increase the dielectric constant of t he ceramic and the dielectric loss.Also increased.The results of the dielectric perfor mance tests at different frequencies indicate that both groups of BST@Cu O ceramics have good frequency stability and dielectric loss at room temperature.（3）Using Mg（NO₃）₂·6H₂O and the previously prepared Ba_0.6Sr_0.4Ti O₃ powder as raw materials,BST@xwt%Mg O composite ceramic powders with different coating amo unts were prepared by precipitation method and room temperature solid phase method.XRD and SEM analysis proved that Mg O was coated on the surface of BST grain s.The composite powders with different coating amounts of BSTxwt%Mg O composite powders prepared by precipitation method and room temperature solid phase method w ere used as raw materials.After being compressed in the pressure range of 12-18M pa,they were sintered at 1280°C for 2 hours to produce multiphase ceramic chips.Mg O has the effect of refining grains and increasing the density of ceramics,and also has the effect of promoting grain growth and producing large-sized grains,so the pro per coating amount can better improve the density of complex ceramics.SEM analysis shows that,compared with the two preparation methods,the ceramic chips prepared by the precipitation method have higher density and more uniform and fine grains.Th e dielectric impedance test results show that the composite ceramic sheets of each co mponent of the two preparation processes have very good frequency stability.Since th e dielectric constant of magnesium oxide is lower than BST,in most cases,coating a certain amount of Mg O will reduce the ceramic dielectric constant.Comparing the diel ectric properties of the composite ceramic prepared by the precipitation method and th e room temperature solid phase method,the composite ceramic of the room temperatur e solid phase method has better frequency stability and higher dielectric constant,and the composite ceramic of the precipitation method has a lower Dielectric loss.（4）The（1-x）Ba Ti O₃-x Ca Cu₃Ti₄O₁₂（x=0.05,0.1,0.15 and 0.2）compound cera mics was prepared by the conventional solid state reaction processing with two-step m ethod in order to optimize its dielectric properties.And the phase structure,morpholog y and dielectric properties of the compound ceramics were investigated.The results in dicate that single phase of Ba Ti O₃ interacts with Ca Cu₃Ti₄O₁₂ at 1200°C,and thus f orms the Ba₄Ti₁₂O₂₇and Ca Ti O₃ phases.And with the content of Ca Cu₃Ti₄O₁₂ increasi ng,the grain size increases followed by the inhomogeneous microstructure of two clas ses of different grain formed.One class has the small,square type grains and the oth er has the large rod-like grains.Also,with the Ca Cu₃Ti₄O₁₂added,the dielectric const ant and its temperature stability increase accompanied by the emergence of diffused fe atures around 100°C.