Preparation, Structure And Functional Properties Of Bismuth Ferrite-based Lead-free Ferroelectric Ceramics

Lead zirconate titanate（Pb（Zr,Ti）O₃,PZT）system has been developed for more than half a century since its discovery in1950s.By virtue of its excellent electrical properties,it has dominated the worldwide market of ferroelectric or piezoelectric materials in industrial applications.However,it is well known that,for typical commercial lead-based materials,the weight percent of lead oxide is usually more than 60%.In addition,in the process of material preparation,application and subsequent treatment and recycling,lead elements or lead-containing oxide would cause a serious hazard to human health as well as the environment.Since the beginning of the 21st century,with the increase of people’s awareness of environmental protection,relevant laws and regulations have been legislated to prohibit the use of lead in electronic devices,such as"The Restriction of the use of certain Hazardous Substances in Electrical and Electronic Equipment”（RoHS）and“Waste Electrical and Electronic Equipment”（WEEE）in Europe.Subsequently,various countries,including Japan,the United States,and China,have issued similar laws and regulations,which have imposed strict restrictions on the application of lead-containing electronic materials and devices.Therefore,the development of high performance lead-free ferroelectric and piezoelectric materials both has important scientific significance and substantial practical value.Researchers around the world have been energetically exploring and developing eco-friendly lead-free ferroelectric and piezoelectric materials.Up to now,the mainstream lead-free systems consist of BaTiO₃（BT）based solution,（K,Na）NbO₃（KNN）based solution and(Bi_0.5Na_0.5)TiO₃（BNT）based solution.Although some inspiring achievements have been accomplished in improving the their corresponding performance,such as piezoelectric,electrocaloric,and pyroelectric properties,the performance of these traditional lead-free ceramic solutions is still not comparable to lead-based piezoelectric ceramics to some extent owing to the intrinsic imperfection of these material system,such as low Curie temperature and spontaneous polarization,inferior stability and so on.Therefore,it is very necessary to explore new type of lead-free ferroelectric solutions.Recently,bismuth ferrite（BiFeO₃）,which can be considered as very promising lead-free materials to replace the lead-based counterpart,has been reported to exhibit large spontaneous polarization（¹00μC/cm²）along with high Curie temperature（⁸37°C）.In addition,the perovskite system could form a morphotropic phase boundary（MPB）with BaTiO₃ perovskite solution,similar to PZT solutions.In recent years,considerable efforts have been devoted to improve electrical properties in this BT-modified BFO solution.Based on the above background,our investigation is closely associated with BiFeO₃-BaTiO₃ based materials by means of ion modification and the addition of third component to improve the multiferroic,piezoelectric and energy storage properties of BiFeO₃-based lead-free ferroelectric ceramics.Furthermore,the negative electrocaloric effect was found in BiFeO₃-based polycrystalline ceramic system for the first time.The main research results are as follows:1,（1-x）BiFeO₃-x BaTiO₃ Binary System（1）With the increase of BT content,the ferroelectric rhombohedral distortion of the system became weak,the content of rhombohedral phase decreased,and the content of pseudocubic phase increased.In addition,it is also found that there is a locally heterogeneous heterostructure in the microscopic region for the components near the phase boundary（x=0.30,x=0.32）.（2）For the first time,a negative electrocaloric effect was found in（1-x）BiFeO₃-xBaTiO₃（x=0.28,0.30,0.32,0.34）polycrystalline ceramic system,and the authenticity of the negative electrocaloric effect was confirmed by DSC direct test method（?T=-2.0 K,x=0.32）,and further explored its possible physical mechanism.It may be related to the existence of antiferroelectric structure in the non-polar region inside the material.With the application of the electric field,the field-induced phase transition which is similar to antiferroelectric-ferroelectric transformation would take place,thus leading to the occurrence of negative electrocaloric effect.（3）The component x=0.34 has a large piezoelectric strain(d^*₃₃>340pm/V)in a wide temperature range（30-170°C）,and it is also found to possess a stable high piezoelectricity in the high temperature range.In addition,the piezoelectric coefficient(d₃₃*⁵00pm/V)is superior to the widely reported KNN-based lead-free piezoelectric ceramics,which provides a further development of lead-free actuator materials with high piezoelectric coefficient along with high temperature stability.（4）Drawing on the inspiration from lead-free relaxor ferroelectric materials,we introduce the Sr（Al,Nb）_0.5O₃ and Ba(Zn_1/3Ta_2/3)O₃ system to maintain local high polarization while introducing more local random fields to lower the remanent polarization in the system.Finally,high energy storage density（>2J/cm³）and energy storage efficiency（>80%）are obtained simultaneously under low electric field（<20kV/mm）,and the maximum value of 2.56J/cm³,which is maximum value of BiFeO₃ based ceramics reported previously.2,0.75BiFeO₃-0.25(Ba_0.85Ca_0.15)(Ti_0.90Zr_0.10)O₃ Binary System（1）Mn-modifid 0.72BFO-0.28BCZT multiferroic ceramics were fabricated via conventional solidstate reaction method.Enhanced ferroelectricity and large negative strain were obtained in Mn-modified BFO-based ceramics.Piezoresponse force microscope（PFM）analysis confirmed that Mn could regulate the ferroelectric domains greatly,and increased the amount of non-180°ferroelectric domains.Raman spectra demonstrated that Mn ions mostly occupied B-site,and strengthened the Jahn-Teller distortion,thereby enhancing ferroelectric and magnetic properties.Compared with the increase of the resistivity due to Mn addition in BFO-based materials,we suppose that the developed ferroelectric domain structure should be mainly responsible for the enhanced ferroelectric and strain properties.（2）Single phase 0.75BiFe_0.98Ga_0.02O₃-0.25(Ba_0.85Ca_0.15)(Zr_0.10Ti_0.90)O₃ multiferroic ceramics were prepared by conventional solid-state reaction with non-quenched process.The crystalline structure maintains a rhombohedral（R3c）perovskite phase.The ceramics exhibit excellent ferroelectricity with large remanent polarization（44μC/cm²）.PUND measurements clearly confirm that the large polarization arises from ferroelectric domain switching,ruling out the leakage contribution.Meanwhile,superior ferromagnetism（0.2emu/g）is also obtained from the samples at 300K.The observed macroscopic magnetization derives from the canting of the spins,eliminating the probable contribution of Fe-containing impurity phases.The pronounced enhancement of multiferroic characteristics makes this non-quenched BFO based materials a potential candidate for multiferroic application.