Composition Modulation And Electrical Properties Of ?Ba,Sr??Ti,Zr? Tio₃ Ceramics

As a famous ferroelectric and piezoelectric material,barium titanate?BT?as early as in 1942 was synthesized by the scientists of United States and Soviet Union.Pure BT ceramics have a moderate d33 of?190 pC/N,while this number for PZT ceramics is up to 500-600 pC/N due to the effect of a morphotropic phase boundary created by a proper Zr/Ti atomic ratio.Electromechanical performance of ferroelectric ceramics has a close relationship with their phase structures.In recent years,much efforts have been spent to construct rhombohedral-orthorhombic?R-O?,orthorhombic-tetragonal?O-T?and tetragonal-rhombohedral?T-R?phase boundaries near room temperature in BaTIO3?BT?-and?K,Na?NbO3?KNN?-based lead-free ferroelectric ceramics.These efforts are mostly concentrated in obtaining discrete PPT or MPB regions near room temperature in the temperature-composition?T-x?diagrams.This can be achieved,for example,by taking advantage of the transition behavior in BT ceramics doped with some larger B-site ions,where the O-T and R-O PPT regions are usually well defined,and are separated by a sizable composition difference.From the experience,it came naturally to place the composition to the proximity of a transition between two ferroelectric phases to achieve high piezoelectricity in a BT-based solid solution ceramic.In the periodic table of elements,Sr is right above Ba and Zr right under Ti.Therefore,either element can form complete solid solutions with BaTiO₃.In this work,firstly,we report dielectric,piezoelectric and ferroelectric properties of perovskite Ba_0.95Sr_0.05Zr_xTi?1-x?O₃ ceramics?BSZT,x=0.05,0.10,0.15,0.20?synthesized by using a solid-state reaction method.It is demonstrated that when the Zr concentration was 5 at%and the sintering temperature 1400?;the BSZT ceramic gives the best overall electromechanical performance,including a large remnant polarization of?12.1 ?C/cm2,large dielectric constants(?_r^peak?12600,?_r^RT?3500)and an excellent d33 value of-355 pC/N.Curie temperature of this ceramic is about?100?.We also study the polymorphic phase transitions?PPT?in ferroelectric Ba_0.95Sr_0.05Zr_xTi_1-xO₃?BSZT,x=0.01-.10?ceramics synthesized by using a solid-state reaction method.The doping elements and composition ratios were selected in order to create adjoining phase boundaries near room temperature,hence to achieve a broadened peak of piezoelectric performance with respect to composition.Based on an optimized design of the polymorphic phase transition?PPT?temperature-composition?TPPT-x?relationships,we successfully merged the Orthorhombic-Tetragonal?O-T?and Rhombohedral-Orthorhombic?R-O?PPT regions near room temperature in the BSZT ceramics.It was revealed that the two adjacent PPT regions near room temperature showed different characteristics in property enhancement.However,due to the proximity of the phase boundaries,Ba_0.95Sr_0.05Zr_xTi_1-xO₃ ceramics in a fairly broad range of composition?0.02?x?0.07?have shown excellent piezoelectric properties near room temperature,including a large piezoelectric constant?312 pC/N ?d33?365 pC/N?and a high electromechanical coupling coefficient k_p?0.42?k_p?0.49?.This approach can be applied to other ferroelectric ceramics with multiple doping elements,where a concerted effect can be obtained to bring two phase boundary compositions close to each other near the temperature of application interests,and,therefore,an enhanced electromechanical performance can be achieved and stabilized in a fairly broad composition range.