Elaboration,structrure And Functional Properties Of BiFeO₃-based Ferroelectric Ceramics

Bismuth ferrite(Bi Fe O₃,abbreviated as BFO)is an attractive lead-free ferroelectric material owing to its high Curie temperature(T_c=825?)and large spontaneous polarization(?100?C/cm²).In addition,BFO ceramics with the addition of other perovskites Ba Ti O₃(BT)and Pb Ti O₃(PT)can form the morphotropic phase boundaries(MPB),similar to the PZT solutions.Bi Fe O₃-Pb Ti O₃(BF-PT)based ceramics with high Curie temperature are considered to be a very promising high-temperature piezoelectric material.However,two main factors restrict their applications in the field of high-temperature piezoelectric devices.This behavior can be related to BF-PT system in the vicinity of the MPB where a large tetragonality(c/a)occurs,enhancing the internal compatibility stresses which suppress the domain switching.The other reason is that high leakage current and dielectric loss make it difficult to be proper electrical poled for ceramics.In recent years,many reports have devoted to the enhancement of the piezoelectric properties through doping modification and preparation technology,mainly due to the dense microstructure,the formation of phase boundaries,and the reduction of the leakage current.It is critical to understand the interplay between the composition(ionic radius,electronegativity),structure(phase boundary,lattice distortion),defect(oxygen vacancy,Fe³⁺/Fe²⁺)and electrical performance(ferroelectricity,piezoelectricity,conductivity)for the BF-PT-based high-temperature piezoelectric ceramics.On the other hand,considering the large spontaneous polarization and the adjustable pseudo-cubic phase structure of BFO ceramics,BF-BT-based ferroelectric ceramics have great application prospects in the field of dielectric energy storage.Exploring and developing the energy storage application of BFO-based ferroelectric ceramics is a valuable research.Based on the research background above,our investigation is closely associated with Bi Fe O₃-based ferroelectric ceramics.Doping modification was used to improve the ferroelectric and piezoelectric properties of Bi Fe O₃-Pb Ti O₃-based piezoelectric ceramics.At the same time,the defect pinning and the leakage current mechanisms of Bi Fe O₃-based ceramics were also studied.In addition,the energy storage property of lead-free Bi Fe O₃-Ba Ti O₃-based ferroelectric ceramics was also studied.The main results are as follows:(1).It can be found that the different lead raw materials have a significant influence on the electrical properties of BF-PT-based piezoelectric ceramics.A small amount of Bi-rich second phase is precipitated at the grain boundaries of BF-PT-based ceramics prepared by Pb₃O₄.In addition,there are high concentration of oxygen vacancies and Fe²⁺in the samples prepared by Pb₃O₄,resulting in significantly higher leakage current than the samples prepared by Pb O.This study not only confirms that the high leakage current of BF-PT-based piezoelectric ceramics is due to the fluctuation of Fe³⁺and the formation of oxygen vacancy,but also will be helpful to the suitable lead raw material for preparation of BF-PT-based piezoelectric ceramics with high resistivity.(2).A novel ferroelectric(0.76-x)Bi Fe O₃-0.24Pb Ti O₃-x Ba(Sn_0.2Ti_0.8)O₃ ternary system was prepared by a solid-state reaction method.It is shown that the incorporation of large ionic radius Ba²⁺at A-site and nonferroelectric-active Sn⁴⁺at B-site generates a decrease in the tetragonality(c/a),which enhances the domain switching.Moreover,a wide multiphase coexistence region is formed with the content of BST ranging from0.13 to 0.28.The enhanced piezoelectric coefficient(d₃₃?200 p C/N)is achieved while maintaining a high Curie temperature(T_c?500°C)and a high depolarization temperature(T_d?450°C)for the composition of 0.6BF-24PT-0.16BST,which is superior to other BF-PT-based high-temperature piezoelectric ceramics.(3).The comparison of the dielectric and ferroelectric properties for the poled and unpoled BF-PT-BST ceramics was studied.It is found that pinning of domain wall due to the defects such as Fe²⁺/Fe⁴⁺and oxygen vacancies,causes the P-E loops are not saturated for the unpoled BF-PT-BST ceramics.After poling,the P-E and S-E loops are displaced along the E axis with the internal bias field,which is related to the formation of the defect dipole.The defect dipole is not easy to turn with the direction of the electric field.By increasing the temperature or decreasing the frequency of the electric field,the oxygen vacancies can be moved along the direction of the electric field,which leads to a decrease of the internal bias field.In addition,with the decrease of Bi Fe O₃ content,the difference in ferroelectric properties gradually disappeared and the internal bias field decreases for the unpoled and poled BF-PT-BST ceramics.This may be related to the decrease of the concentration of charged defects for the ceramics.(4).The influence of Mn doping on the conductivity,dielectric and ferroelectric properties of 0.7Bi Fe O₃-0.3Pb Ti O₃ ceramics was systematically studied.It is found that the leakage current of pure 0.7BF-0.3PT ceramics is huge,which severely limits its use as piezoelectric devices.In addition,the dielectric loss-temperature curve of pure0.7BF-0.3PT ceramics shows a relaxation peak in the temperature range of 30-250?and the frequency range of 100 Hz-1 MHz.The calculated activation energy shows that the relaxation peak is related to the doubly-ionized oxygen vacancies.A small amount(0.15 wt%)of Mn doping can significantly reduce the leakage current of BF-PT ceramics,which is related to the decrease of the oxygen vacancy concentration for the ceramics.However,when the content of Mn increases(0.45 wt%),the leakage current of ceramics increases instead,which may be related to the more carrier charges caused by the fluctuation in the valence of the B-site ion(Mn,Fe).(5).Lead-free BF-BT-based ceramics with fine-and coarse-grains were fabricated by the conventional solid-state reaction process.The high-energy ball milling technique was utilized to obtain the fine-grained ceramics.The grain size effects on the dielectric,ferroelectric and energy storage properties of Bi Fe O₃-Ba Ti O₃-based ceramics were investigated.The fined-grained BFBT-BMN ceramics exhibit the broadening of?-T curve and the slim P-E loops,corresponding to a low remanent polarization P_r.The recoverable energy storage density(W_re)of the fine-grained BFBT-BMN ceramics greatly increased by 8 times from 0.16 J/cm³ to 1.27 J/cm³,compared to the coarse-grained BFBT-BMN ceramics.This study shows that the reduction of grain size is an effective method to improve the energy storage properties of Bi Fe O₃-based ferroelectric ceramics.