Effects Of Composite Ion Doping On The Structure And Properties Of BaBi₄Ti₄O₁₅-Bi₄Ti₃O₁₂ Intergrowth Bismuth Layered Piezoelectric Ceramics

Bismuth layered piezoelectric ceramics have many advantages such as high Curie temperature,good fatigue resistance and environmental friendliness.Thus,bismuth layered piezoelectric ceramics are widely used at high temperature.However,the electric polarization is confined by the layered structure,resulting in poor piezoelectric activity,which limits its practical applications.So it is necessary to develop bismuth layered piezoelectric ceramics with high Curie temperature and excellent piezoelectric properties.In this work,BaBi₄Ti₄O₁₅-Bi₄Ti₃O₁₂（BBT-BIT）intergrowth bismuth layered piezoceramics with high spontaneous polarization strength were investigated.The electrical properties of the ceramics were optimized by ions doping.The experiment results are as follows:（1）BaBi₈Ti_7-x(Cu_1/3Nb_2/3)_xO₂₇(BBT-BIT-x(Cu_1/3Nb_2/3))ceramics with superlattice structure were synthesized by traditional solid-state method.The effect of Cu/Nb co-doping on the structure and electrical properties were investigated.The results showed that the electrical properties of BBT-BIT-x(Cu_1/3Nb_2/3)ceramics were effectively improved.With the increase of x,the dielectric loss decreases and then increases,while the Curie temperature increases（T_c:483℃～494℃）.Meanwhile,the piezoelectric constant and residual polarization increase and then decrease.As x increases from 0 to0.035,the piezoelectric property improved from 6.8 pC/N to 18 pC/N,and the residual polarization intensity increases from 7.55μC/cm~2to 8.25μC/cm~2,respectively.In addition,BBT-BIT-0.035(Cu_1/3Nb_2/3)ceramics have good thermal stability.Its piezoelectric constant was 14 pC/N after annealing at 400℃ for 30 min,which is 82.4%of the value at room temperature.It is indicated that the Cu/Nb doped BBT-BIT ceramics have the ability to work at high temperature.（2）The Li/Ce co-doped BaBi₄Ti₄O₁₅-Bi₄Ti₃O₁₂（BBT-BIT）piezoelectric ceramics were prepared by conventional solid-state method.The results showed that all the samples had a single orthorhombic phase structure,indicating that Li/Ce dopants had successfully dissolved into the lattice.When the Li/Ce doping content was in the range of0.15～0.3,the electrical performance can be improved obviously.The piezoelectric constant d₃₃increased from 6.8 pC/N to 21.4 pC/N and the Curie temperature T_cincreased from 483℃ to 544℃.Among all the ceramics investigated,Ba_0.7(Li_0.5Ce_0.5)_0.3Bi₈Ti₇O₂₇has the best comprehensive properties:d₃₃=20.2 pC/N,T_c=540℃,tanδ=0.53%,E_a=1.13 eV,2P_r=10.5μC/cm~2.Meanwhile,the hysteresis loop of the BBT-BIT-0.3(Li_0.5Ce_0.5)sample sintered in an oxygen atmosphere was asymmetric,which may be caused by defects produced by the Li/Ce co-doping.Besides,The d₃₃value of 18.5 pC/N was achieved at 450℃ for 30 min,which is 91.6%of the value at room temperature,indicating the good thermal stability.Besides,the sample also showed high impedance,which can be attributed to the decrease of the concentration of oxygen vacancies.Our results suggest the Li/Ce co-doped BBT-BIT intergrowth piezoelectric ceramics are promising candidates for high temperature applications.（3）The Ba_（1-x）(Na_0.5Ce_0.5)_xBi₈Ti₇O₂₇piezoelectric ceramics were investigated to further study the effect of A-site doping on the piezoelectricity and Curie temperature.The results show that the electrical properties of BBT-BIT ceramics are significantly improved by Na/Ce co-doping.The sample with doping content x=0.3 exhibits the most excellent piezoelectric constant d₃₃of 21.8 pC/N and a high Curie temperature T_cof541℃.In addition,the remnant polarization 2P_rwas improved from 16.8μC/cm~2to20.2μC/cm~2.And the d₃₃is 20.6 pC/N after annealing at 450℃for 30 min,indicating that the ceramics had good thermal stability.Therefore,Na/Ce co-doping provides a new modification scheme for BBT-BIT ceramics,which plays a positive role in practical application.