Electrical Conduction And Strain Properties Of BiFeO₃-BaTiO₃-Based Lead-Free Piezoelectric Ceramics

Piezoelectric ceramics possess excellent electromechanical coupling properties,which is able to transform between electrical energy and mechanical energy.As the core component of some sensors,actuators and transducers,piezoelectric ceramics have been widely used in many fields like aerospace,energy exploration,automotive,medical and consumer electronics,etc.The performance of piezoelectric ceramics is critical to piezoelectric actuators.Lead zirconate titanate（PZT）-based piezoelectric ceramics have large electrostrain value and good strain temperature stability,which occupy more than 90%of the market share of piezoelectric materials.However,PZT-based piezoelectric ceramics contain the toxic element lead.With the increasing emphasis on environmental protection and human health in modern society,lead-free piezoelectric ceramics have become a research hotspot in the field of piezoelectricity.In recent years,various types of lead-free piezoelectric ceramic systems have received widespread attention,including barium titanate（BaTiO₃）-based,bismuth sodium titanate(Bi_0.5Na_0.5TiO₃)-based,potassium sodium niobate(K_0.5Na_0.5NbO₃)-based and bismuth ferrite（BiFeO₃）-based ceramics.However,most types of lead-free piezoelectric ceramics have the problems of low Curie temperature and low depolarization temperature,which is unfavorable for the temperature stability of their piezoelectric performance.BiFeO₃-BaTiO₃ ceramics that formed by the solid solution of BiFeO₃ and BaTiO₃ possess both high Curie temperature and excellent strain performance,making them a promising lead-free piezoelectric ceramic system.However,previouly reports showed that the room temperature resistivity is only～10⁷Ωcm for pure BiFeO₃-BaTiO₃ ceramics,which is 4 to 7 orders of magnitude lower than that of traditional PZT-based ceramics.This is detrimental to the practical application of the ceramic systems.As the conduction mechanism of BiFeO₃-BaTiO₃ceramics remains unclear,which prevents the targeted mediation of their conductivity.In addition,BiFeO₃-BaTiO₃ ceramics also have some problems such as insufficient electrostrain value and poor strain temperature stability.The unipolar electrostrain value of BiFeO₃-BaTiO₃ ceramics is about 0.18%（@6k V/mm）,which is still lower than that of PZT-based ceramics.The rate of change of the electrostrain for some BiFeO₃-BaTiO₃ ceramics is higher than 100%in the range from room temperature to150℃,which shows poor temperature stability.Aiming at the problems of unclear conduction mechanism,insufficient strain value and poor temperature stability of electrostrain,the following research have been carried out in this paper:（1）A series of BiFeO₃-BaTiO₃ ceramics with different contents of BaTiO₃ were prepared.These BiFeO₃-BaTiO₃ ceramics have a Curie temperature above 430℃,and a maximum Curie temperature of 640℃.The maximum piezoelectric coefficient of130 p C/N was also obtained,which show excellent properties.（2）Furthermore,AC impedance spectrum measurements with different atmosphere and Seebeck coefficient measurements at high temperatures suggests that BiFeO₃-BaTiO₃ ceramics are mixed conductors of oxide-ion and electron hole,and their main charge carrier is p-type hole.By annealing in nitrogen atmosphere,hole can be suppressed and the conductivity of the ceramics can be reduced by about an order of magnitude.（3）Using a combination of donor W doping and quenching treatment,here we obtained BiFeO₃-BaTiO₃-based piezoelectric ceramics with both low conductivity and large elestrostrain.Compared to the untreated one,the conductivity value of the ceramics was reduced by nearly two orders of magnitude.Their strain value was also increased by 40%,and the highest unipolar strain reaches 0.25%.Research on phase structure and properties indicated that the improvement of these properties may be related to the increased contents of ferroelectric phases with polar structure.（4）Based on this,by forming a solid solution with relaxor ferroelectrics with n-type conduction mechanism,to reduce the content of hole carriers in pure BiFeO₃-BaTiO₃ ceramics.At the same time,by using the relaxor properties of the third components,to promote the transformation into relaxor ferroelectrics,and reduce their strong ferroelectricity at high temperatures,to obtrain BiFeO₃-BaTiO₃-based ceramics with both low conductivity and good stability of electrostrain.The conductivity of a series of the ceramics was reduced by two orders of magnitude,and the rate of change of strain value is reduced to 13%from room temperature to 175℃.The solid solution with relaxor ferroelectrics with negative strain-temperature coefficients can provide a feasible solution for obtaining high temperature piezoelectric ceramics with good temperature stability.（5）To increase the strain value of BiFeO₃-BaTiO₃ ceramics,the third component Sr_0.8La_0.1□0.1TiO_2.95 that substituted with rare earth La was added into the solid solutions.As a result,a maximum strain value of 0.45%（@10k V/mm）was obtained.The highest normalized piezoelectric coefficient reaches 600 pm/V.The dielectric peak temperature（T_m）of all the ceramics are higher than 300℃,which show excellent comprehensive performance.The shifted bipolar strain curve and the abnormal displacement of the Raman vibration peaks indicated that the improved electrostrain may be related to the offset strain field introduced by solid solutions.In this study,we investigated the electrical conduction and strain behavior of BiFeO₃-BaTiO₃-based piezoelectric ceramics.The electrical conduction mechanism was revealed,and several methods for reducing electrical conductivity and improving strain performance were also provided.Here we provide a good basis for the preparation of lead-free piezoelectric ceramics with low conductivity,large strain,and high temperature stability.