Study On Preparation Technology And Physical Properties Of Bismuth Ferrite-based Piezoelectric Ceramics

Bismuth ferrite(BiFeO3)has a high Curie temperature(approximately 825 ?)and a large remanent polarization(above 100 ?C/cm2).It is a kind of rare material both with ferromagnetic and ferroelectric properties above room temperature.Therefore,this material is important with regard to piezoelectric,ferroelectric and magnetoelectric coupling research.However,BiFeO3 has a high coercive field,is difficult to prepare as a pure phase,and is typically challenging to obtain sufficient polarization,in this material by applying an external electric field.In recent years,studies regarding BiFeO3 ceramic systems have primarily focused on the related solid solution systems.These studies have shown that the piezoelectric properties of this material can be significantly improved by doping with trace elements,or by creating morphotropic phase boundaries.It is also evident that the overall performance of BiFeO3 ceramics could be further improved by using new preparation methods,such as rapid liquid,microwave or two-step sintering.Even so,there have been few reports concerning these techniques.The present work comprised an exploratory study of methods of preparing BiFeO3 ceramics,and examined the effects of temperature gradient and substrate-assisted sintering on the self-polarization of these materials.The results demonstrate that self-polarized BiFeO3 ceramics having significant piezoelectric characteristics can be obtained using specific techniques,and a mechanism for this self-polarization effect is proposed.In addition,the effects of the step-by-step synthesis method on the properties of BiFeO3 piezoelectric ceramics were also explored,and materials in the(1-x)BiFeO3-xBaTiO3 system were found to be superior to those synthesized using the traditional method.BiFeO3 ceramics were shown to exhibit strong macroscopic self-polarization when produced via the temperature gradient sintering method at the optimal temperature.The direction of the macroscopic spontaneous polarization in these materials was found to consistently point from the high temperature zone of the specimen to the low temperature zone.Assessments of the piezoelectric constant,d33,of samples sintered under different conditions demonstrated that the highest room temperature d33(28 pC/N)was obtained at sintering temperatures slightly below the Curie temperature of 825 °C.After sintering at 810 ?,this material also showed a room temperature thickness electromechanical coupling factor of 38%and a dielectric loss of 2.9%.Applying an annealing temperature of 800 ? did not change the room temperature d33 of the sample,indicating that the macroscopic self-polarization effect in this BiFeO3 ceramic was highly stable in response to temperature changes.This work determined that the d33 value was raised to 90 pC/N at 370 ?,representing a threefold increase from the room temperature value.The effects of employing different sintering substrates on the self-polarization of BiFeO3 ceramics were also examined.Specimens were sintered on Pt substrates in standard and temperature gradient furnaces,and strong macroscopic self-polarization was observed following the application of sintering temperatures slightly lower than the Curie temperature after treatment in either furnace.In contrast,when using Ag and Fe substrates,only the sample sintered in the temperature gradient furnace exhibited significant macroscopic self-polarization.These results showed that intimate contact with the Pt substrate was required to produce macroscopic self-polarization in the BiFeO3.The absence of self-polarization when using the Ag and Fe substrates may be related to the relatively low work functions of these metals.The(1-x)BiFe03-xBaTiO3(x=0.20?0.25?0.275?0.29?0.31?0.33?0.35?0.40)system ceramics were prepared using a solid state reaction method in conjunction with pre-synthesized BaTiO3 and BiFeO3 as the raw materials.The resulting samples were examined in detail to ascertain their crystal structures and dielectric,ferroelectric and piezoelectric properties.The morphotropic phase boundary was found to appear at x= 0.29.Compared with the traditional preparation method,the step-by-step synthesis applied in this work significantly improved the overall performance of these materials.The specimen for which x= 0.29 showed a Pr value of 30 ?C/cm2,d33 of 172 pC/N,kp of 28%,TC of 500 0C and tand of 5.4%.