Study On Composition,Structure Design And Piezoelectric Thermostability Of Na_0.5Bi_0.5TiO₃ Based Lead-Free Piezoelectric Ceramics

Sodium bismuth titanate has been widely studied due to relatively high remanent polarization（P_r～38μC/cm²）and Curie temperature（T_m～300℃）.It was regarded as one of the most promising materials to replace Pb（Zr,Ti）O₃（PZT）based materials.However,compared with traditional lead-based materials,there still exists a large performance gap such as large coercive field（E_c～73 k V/cm）,high conductivity during poling and low depolarization temperature（T_d）,which greatly restricted its practical application.Especially,low depolarization temperature restricted the use of materials at high temperature immensely.It has become hot pursuit of majority researchers to further improve the temperature stability so as to meet the practical needs.At present,most study by researchers focused on exploring the comprehensive properties of the ceramic system in different fields through ion doping,but they ignored the most basic piezoelectric properties.As a result,the commercialization of the material has been ineffective.To solve this problem obtain ceramics with high piezoelectric constant and favorable thermostability,this paper takes the depolarization temperature as the starting point and adopts different methods to study the relationship among structure,composition and performance.（1）A series of non-stoichiometric（Na Bi）_xBa_0.06TiO₃（N_xB_xT-6BT）ceramics were prepared by the conventional solid-state method.The effect of A-site non-stoichiometry on structural and electrical properties for prepared ceramics were investigated.The room temperature XRD and Raman spectra revealed the coexistence of Cc and P4bm phases for all samples.Experimental results in the present work showed that Bi³⁺and Na⁺deficiency can lead to a transformation from Cc to P4bm phase,following by the variations of piezoelectric coefficient d₃₃ and Cc phase fraction.Favorable thermostability for piezoelectric properties of the ceramics was observed by testing depolarization temperature（T_d）and XRD patterns.These results showed that the deficiency of Na⁺and Bi³⁺can significantly enhance T_d and extend the temperature regions for the application of NBT-6BT ceramics.（2）0.94Na_1/2Bi_1/2TiO₃-0.06BaTiO₃（NBT-6BT）perovskite structure ceramics doped by Nano-sized TiO₂,which stayed in the grain boundaries of NBT-6BT grains,were prepared by the solid-state method.NBT-6BT:xTiO₂（x=0,0.05,0.1,0.2,0.3）0-3 type composite structural ceramics were fabricated successfully and the effects of doped TiO₂on structure and piezoelectric properties were investigated in detail.The results show that some TiO₂ enter into the lattice of NBT-6BT matrix,which result in a decrease of Cc phase content and an enhancement of crystal symmetry.The increasing amounts of TiO₂significantly improve the depolarization temperature of NBT-6BT ceramics.After doping0.1 mole of TiO₂,the depolarization temperature improves by 88%.The dielectric loss tanδand piezoelectric coefficient d₃₃ are 0.044 and 99 p C/N respectively at this time,which indicates that this kind of ceramic is a new type of lead-free piezoelectric materials suitable for higher temperature range.（3）The composite ceramics of NBT-0.2KBT:xZnO（x=0,0.1,0.2,0.3,0.4）were synthesized by solid state reaction and the composite structure was characterized by X-ray diffraction,scanning electron microscopy and energy disperse spectroscopy.The dielectric and piezoelectric properties were also tested.The results show that NBT-0.2KBT:xZnO 0-3 type composite structure ceramics were fabricated successfully and Nano-sized ZnO stayed in the grain boundaries of NBT-0.2KBT grains.The inflection point of dielectric loss disappear after doping ZnO which indicate that doped ZnO can inhibit depolarization and improve the T_d obviously.After doping 0.2 mole of ZnO,the depolarization temperature improves by 75%（T_d=140℃）,which indicates that this method is also applicable to NBT-KBT ceramics.In addition,NBT-0.2KBT:0.2ZnO ceramic is a new type of lead-free piezoelectric materials suitable for higher temperature range.