The Investigation On The Phase-boundary Design And Electric Properties Of ?K,Na?NbO₃-based Lead-free Piezoceramics

Among all the lead-free piezoelectric systems,?K,Na?NbO₃?KNN?based ceramics,as a promising candidate for lead free piezoelectric ceramics,have been extensively investigated in the past ten years because of their good electric properties and high Curie temperature?TC?.In order to enhance the electric properties of KNN,the effective methods include constructing O-T phase coexistence or R-O phase coexistence near room temperature by shifting the orthorhombic-tetragonal phase transition temperature?TO-T?or rhombohedral-orthorhombic phase transition temperature?TR-O?to room temperature.Nevertheless,the properties of KNN based ceramics are still inferior to Pb-based ceramics.In recent years,some researchers have started to investigate the construction and modification of R-T or R-O-T phase boundary in KNN ceramics by shifting the TO-T and TR-O to room temperature simultaneously,the results show that the ceramics with this kind of phase boundary own dramatically enhancedpiezoelectric properties.Based on the idea of constructing a new phase boundary,the author studies in detail the influence of two new additives?Bi,Sm?_0.5Na0.5ZrO₃and?Bi,Me?_0.5?Na,K?_0.5ZrO₃?abbreviated as BSNZ and BMKNZ,Me3+=La3+,Nd3+,and Sm3+?on microstructure,phase boundary evolution,and electric properties in?K,Na?NbO₃?KNN?,?K,Na??Nb,Sb?O₃?KNNS?and?K,Na??Nb,Ta,Sb?O₃?KNNTS?ceramics.Adding BSNZ and BMKNZ into KNN,KNNS or KNNTS successfully realizes the R-T/R-O-T phase coexistence near room temperature,a series of KNN-based ceramics with excellent properties have been obtained.The research results are as follows:1.?1-x?K_0.48Na_0.52NbO₃-x(Bi_0.46Nd_0.04)(Na_0.82K_0.18)_0.5ZrO₃?abbreviated as KNN-xBNNKZ?,?1-x?K_0.48Na_0.52NbO₃-x(Bi_0.46La_0.04)(Na_0.82K_0.18)_0.5ZrO₃?abbreviated as KNN-xBLNKZ?,and?1-x?K_0.48Na_0.52NbO₃-x(Bi_0.45Sm_0.05)(Na_0.82K_0.18)_0.5ZrO₃?abbreviated as KNN-xBSNKZ?systems have been designed and prepared by using traditional solid sintering method.With the increase of BMNKZ content,the TO-T and TR-O are shifted to room temperature simultaneously.When the R-T phase coexistence or R-O-T phase coexistence appears near room temperature,the enhanced electric properties were obtained.The optimal compositions of three systems are listed below:KNN-xBNNKZ?x=0.05?,d33=308 pC/N,kp = 43%,?r=1205,tan?=3.8%,Pr=23.45 ?C/cm2,TC=331 °C;KNN-xBLNKZ?x=0.05?,d33=368 pC/N,kp = 44.7%,?r =1358,tan?=3.5%,Pr=22.8 ?C/cm2,Ec=12 k V/cm,TC=323 °C;KNN-xBSNKZ?x=0.05?,d33=342 pC/N,kp = 46%,?r =1476,tan?=4.3%,Pr=23.7 ?C/cm2,Ec=12 k V/cm,TC=328 °C.2.?1-x?K_0.48Na_0.52Nb0.96Sb0.04O₃-x(Bi_0.45Sm_0.05)(Na_0.82K_0.18)_0.5ZrO₃?abbreviated as KNNS-xBSNKZ?systems has been designed and prepared by using traditional solid sintering method.With the increase of BMNZ content,TO-T decreases quickly,TR-O increases slowly,When the R-T phase coexistence appears near room temperature,the enhanced electric properties were obtained.The optimal composition of the system is listed below:KNNS-xBSNKZ?x=0.045?,d33=480 pC/N,kp = 48%,?r =2180,tan?=3.6%,Pr=19.5 ?C/cm2,Ec=8.1 kV/cm,TC=258 °C.3.0.96K0.4Na0.6Nb0.97-xTa0.03SbxO₃-0.04(Bi_0.45Sm_0.05)Na0.5ZrO₃?abbreviated as KNNTSx-BSNZ?system has been designed and prepared by using traditional solid sintering method.With the increase of Sb content,the TO-T and TR-O are shifted to room temperature quickly.The R-T phase coexistence appears near room temperature when the Sb content changes from 0.02 to 0.07.The optimal composition of the system is listed below:KNNTSx-BSNZ?x=0.045?,d33=510 pC/N,kp = 44%,?r =2136,tan?=4.1%,Pr=20.4 ?C/cm2,Ec=7.85 k V/cm,TC=238°C.