Structure-activity Relationship Of Sr_1.2Ca_0.8Nb₂O₇-based And PbNb₂O₆-based High-Temperature Piezoelectric Ceramics

High curie temperature piezoelectric materials are widely used in various fields.The development of piezoelectric materials with excellent performance and high Curie temperature is one of the current hotspots in scientific research.The performance of solid materials is closely related to its crystal structure.Exploring the relationship between the structure and physical properties of materials is conducive to the design of more excellent functional materials.In this paper,Sr_1.2Ca_0.8Nb₂O₇（SCNO-0.8）and PbNb₂O₆（PN）two kinds of high curie temperature piezoelectric materials are selected.X-ray diffraction and electron diffraction are used to study the relationship between their structure and properties,which provides a theoretical basis for the design of high temperature piezoelectric materials with high performance.It mainly includes the following three contents:The crystal structure of Sr_1.2Ca_0.8Nb₂O₇（SCNO-0.8）was analyzed using single crystal X-ray diffraction and three-dimensional electron diffraction（3D ED）techniques.The results showed that the superstructure of SCNO-0.8 could not be observed using single crystal X-ray diffraction.However,By using 3D ED,it can be determined that the NbO₆ octahedron and Sr/Ca atoms in SCNO-0.8 structure have different torsional and position deviation characteristics in different layers perpendicular to the direction of a.This demonstrates that the application of 3D ED technique provides an effective method for studying superstructures resulting from the ordered arrangement of light atoms in subsequent research.Non-stoichiometric Pb_1-xNb₂O_6-x（PN_x,x=0,0.02,0.04,0.05,0.08,0.10）ceramics were constructed based on A-site Pb element deficiency,and the changes of PN in crystal structure were analyzed and compared after deficiency.The experimental results show that the grain size of PN ceramics after Pb element deficiency is reduced by nearly 4 times compared with the original size,which inhibits the abnormal growth of grains and achieves dense sintering of PN ceramics.When Pb deficiency is 5 mol%,the piezoelectric constant d₃₃ is 58 p C/N,which is 13%higher than that of pure PN,and has a high Curie temperature.At the same time,it is proved by PEDT that Pb deficiency occurs at the A1 position of the pentagonal void.Pb deficiency is helpful to stabilize the orthorhombic ferroelectric phase structure at room temperature.Furthermore,by introducing Li⁺into Pb-deficient PbNb₂O₆(PN_0.05)with a shortage of 5 mol%Pb,ceramic compositions of Pb_0.95Li_xNb₂O_5.95+x/2（PLN_x,x=0.04,0.06,0.08,0.10）were designed and prepared.Analysis of the powder X-ray diffraction（XRD）patterns confirmed the incorporation of Li⁺into the PbNb₂O₆ lattice.Compared with PN_0.05,the grain size of PLN_x did not show significant changes,but the piezoelectric constant d₃₃ of the samples decreased while the Curie temperature T_Cincreased.It shows that the doping of Li⁺does not improve the characteristics of tungsten bronze structure ceramics with large coercive field and difficult to be completely polarized.