Modification Of BCZT-based Lead-free Piezoelectric Ceramics By Microstructure And Defect Regulatio

In recent years,lead-free piezoelectric ceramics is the focus of research in the field of functional materials.Lead-free piezoelectric materials such as K_0.5Na_0.5Nb O₃（KNN）,Bi_0.5Na_0.5Ti O₃（BNT）and Ba Ti O₃（BT）based ceramics have been thoroughly studied and gradually applied in the field of electronics industry.Among these piezoelectric materials,BCZT ceramics is a typical lead-free piezoelectric ceramics based on BT.Because of its low porosity,good piezoelectric coefficient and low dielectric loss,it is considered as one of the candidate materials to replace PZT.However,the piezoelectric coefficient and Curie temperature of BCZT ceramics need to be further improved.In this paper,(Ba_0.9Ca_0.1)(Zr_0.15Ti_0.85)O₃（BCZT for short）as matrix was used to prepare lead-free piezoelectric ceramics with different doping components and proportions by doping modification and regulating sintering atmosphere,etc.The defects,crystal structure,phase composition,microstructure,electrical properties and relaxation properties of piezoelectric ceramics were investigated.Finally achieve the purpose of improving the comprehensive performance.The main research content of this paper is as follows:1.Y-doped BCZT-based lead-free piezoelectric ceramics（BCZT-x Y）were prepared by solid phase method.The results show that Y³⁺firstly occupies the B site and generates oxygen vacancy for ion charge compensation.Appropriate oxygen vacancy is conducive to mass transfer effect,effective grain growth,grain boundary migration and internal grain stress release,which effectively increases the Curie temperature of ceramics.Secondly,according to the XRD finishing results,the lattice constant c/a of the sample increases,which reflects the lattice distortion of the crystal structure,which is conducive to the improvement of electrical properties.In addition,there is a quasi-homomorphic phase boundary（MPB）of O-T coexistence at room temperature,and the polyphase structure has more spontaneous polarization directions,which reduces the barrier of domain wall motion and facilitates the rotation of domain walls,thus effectively improving the ferroelectric and piezoelectric properties of BCZT-x Y piezoelectric ceramics.When the doping content is 0.15 mol%,the comprehensive electrical properties of the ceramics are the best,where d₃₃=412 p C/N,k_p=0.535,ε_r=4068,tanδ=2.3%,T_C=103°C.2.Nb doped BCZT-based lead-free piezoelectric ceramics（BCZT-x Nb）were prepared by solid phase method.The results showed that Nb⁵⁺occupied the B site of the BCZT lattice as a donor,leading to a decrease in oxygen vacancy concentration.The reduction of oxygen vacancy makes the crystal structure more stable,which is conducive to reducing the leakage current during polarization and significantly reducing the loss.Nb⁵⁺doping causes the lattice stretching of Nb O₆ octahedron,promotes the formation of T phase at room temperature,the ceramic sample has O-T phase coexistence at room temperature,the polyphase structure has more spontaneous polarization direction,promotes the rotation of ferroelectric domains when polarized,and thus improves the piezoelectric and ferroelectric properties of BCZT-x Nb ceramics.When the doping amount x=0.1 mol%,the ceramics have the best comprehensive electrical properties(d₃₃=450 p C/N,k_p=0.526,ε_r=4082,tanδ=2.4%,T_C=93°C).3.(Ba_0.85Ca_0.15)(Ti_0.90Zr_0.10)O₃+x Y³⁺+x Nb⁵⁺（BCZT-x（Y+Nb））lead-free piezoelectric ceramics have been prepared by solid phase method.The results showed that the Ba₂YNb O₆ phase with double perovskite structure is formed in the ceramics,which significantly increases the lattice distortion due to its large lattice parameters.Second,XRD and TEM results show that R-O-T phase tricoexistence of quasi-homotype phase boundary.Thirdly,microscopic SEM,TEM analysis and density test results showed that the doped samples have fuller grain growth and more regular microstructure arrangement than the undoped samples,which is conducive to grain boundary migration and promotes the formation of dense microstructure.These three reasons together lead to the significant improvement of piezoelectric properties of Y and Nb co-doped ceramics.The experimental results show that the Curie temperature does not decrease with the increase of piezoelectric constant,but increases effectively.When the mass fraction of BCZT-x（Y+Nb）is x=0.1 mol%,the performance of the sample is the best,where d₃₃=667 p C/N,k_p=0.58,ε_r=5656,tanδ=0.022,P_r=12.8μC/cm²,E_C=2.17 k V/cm,T_C=92 ℃.4.On the basis of the previous three chapters,the sintering process of BCZT-0.1mol%（Y+Nb）ceramics was explored in different atmosphere（air,oxygen,nitrogen,vacuum environment）.The XPS results show that the oxygen vacancy formation energy in oxygen,air,argon and vacuum decreases successively.Oxygen sintering plays an important role in oxygen compensation,which can stabilize the crystal structure and increase the Curie temperature（T_C=102 ℃）.The oxygen vacancy in argon promoted the mass transfer effect,and the sample grain was full,which helped to improve the performance.However,the leakage current increases when the concentration of oxygen vacancy increases greatly in vacuum,which is not conducive to directional deflection of domain and leads to poor performance.In addition,the XRD results show that R-O-T phase coexistence exists in the air and oxygen sintered ceramic samples,forming a polyphase coexistence structure with more spontaneous polarization directions.Finally,the vacuum sintered samples were calcined in 800 ℃ air for oxygen-supplemental sintering.It was found that the oxygen vacancy concentration decreased with the extension of oxygen-supplemental time,the grain was further grown,and the piezoelectric properties were enhanced to a certain extent.The samples sintered in oxygen have good comprehensive electrical properties,especially the Curie temperature,(d₃₃=547 p C/N,k_p=55%,ε_r=5355,P_r=10.63μC/cm²,E_C=1.89 k V/cm,tanδ=0.02,T_C=102 ℃).