Study On The Electrostrain-induced Properties Of Donor Ion Doped BNT-based Lead-free Piezoelectric Ceramics

Piezoelectric ceramics are used in a wide range of applications due to their ability to convert mechanical and electrical energy into each other.However,most of the piezoelectric ceramics used today are lead-based piezoelectric ceramics,as we all know,lead-containing piezoelectric ceramics produce substances that are hazardous to human health and pollute the environment during the preparation process.As people pay more attention to environmental issues,the need for lead-free piezoelectric ceramics has become increasingly urgent.In recent years,many scholars have conducted a lot of research on lead-free piezoelectric ceramics,among which,BNT based lead-free piezoelectric ceramics is considered as one of the most likely materials to replace lead based piezoelectric ceramics due to its excellent ferroelectric,such as high piezoelectric constants(d₃₃),high electromechanical coupling coefficient（k_p）and Curie temperature（T_m）.In this thesis,BNKT-BT ternary lead-free piezoelectric ceramic matrix was prepared by traditional solid-phase sintering,and Nb₂O₅ was used as dopant for B-site doping to investigate the effect of Nb⁵⁺doping on the structure and properties of BNKT-BT-x Nb ceramics.According to XRD patterns,the material changes from the coexistence of tetragonal and rhombohedral phase phase to pseudo-cubic phase due to the addition of Nb⁵⁺.The shrinking P-E loops and the broad peak of the maximum dielectric constant indicate that the doping of Nb⁵⁺causes the transition from ferroelectric phase to relaxation phase.With the increase of Nb⁵⁺content,the T_m of the material shifts to a higher temperature,the P-E curves becomes more elongated,and the positive piezoelectric coefficient d₃₃ of the material decreases from205 pC/N to 33 pC/N.The long-range ferroelectric order of the material is destroyed,and the relaxation degree increases.When x=0.0125,the electroinduced strain of the material reaches the maximum value of 0.38%,and the corresponding inverse piezoelectric coefficient d₃₃^*=S_max/E_max=633 pm/V.Subsequently,the BNKT-BT-x Nb ceramic was aged and it was found that the maximum strain value of the sample improved from 0.38%to 0.41%and the hysteresis was significantly reduced,from 57%to 34%.[0.852(Bi_0.5-xSr_xNa_0.5)-0.11(Bi_0.5K_0.5)-0.038Ba]Ti_（1-x）Ta_xO₃ceramics were sintered by traditional solid-state method.According to the phase structure curve,Ta⁵⁺and Sr²⁺have been totally dissolved in BNKT-BT matrix and formed a stable solid solution without other formed phase,and the material has undergone the transformation from the coexistence of both rhombohedral and tetragonal phase to a single pseudo cubic phase.The disappearance of the T_F-Rpeak under the dielectric constant curve indicates that the doping of Ta⁵⁺and Sr²⁺destroyed the long-range ferroelectric order of the ceramic matrix,and the material has changed from ferroelectric phase to relaxation phase at room temperature,which is confirmed by the shrining of the P-E loops and the disappearance of negative strain at room temperature.When x=0.005,the sample achieves a large strain of 0.46%at 7 kV/mm,an improvement of28%compared to B-site doping alone,at which point the d₃₃^* is 657 pm/V.Based on the experiments in the previous two chapters,the ceramic component BNKT-BT-0.5TaSr with the highest electrostrain performance was selected and added to the ferroelectric phase ceramic seed crystal to make a relaxation/ferroelectric ceramic to improve the overall performance of the large electrostrain ceramic.The XRD results shows that both of them have been completely solidified and forming a stable perovskite structure.The SEM image shows that the large grains are surrounded by small grains of relaxor phase grains,indicating the formation of a relaxor/ferroelectric phase composite ceramic.According to the P-E curves of the material,it can be seen that as the amount of BNT seed crystal added increases,the P-E loops of the material is gradually expansion,showing the characteristics of ferroelectric phase.In addition,the E_dep of the material decreases,indicating the decrease of the material strain drive field.When the content of BNT seed crystal is 5wt%,the samples can still maintain high values of d₃₃^* under low electric field,417,530 and 583 pm/V at 4,5 and6 kV/mm respectively,which is all higher than those without the addition of seed crystal.Moreover,at 6 kV/mm,the strain hysteresis decreases from 61%to 35%,which shows that the combination of relaxor andferroelectric materials is an effectively method to reduce the strain driving field and hysteresis of BNT based ceramics.