Rare Earth Doped Na_0.5Bi_0.5TiO₃-BaTiO₃ Lead-free Piezoelectric Ceramics And Construction Of Ternary Lead-free Piezoelectric System

Piezoelectric materials are chemically stable,easy to prepare and widely used materials.The main piezoelectric materials are piezoelectric single crystals,piezoelectric ceramics,piezoelectric thin films,and organic-inorganic hybrid piezoelectric materials.Among them,piezoelectric ceramics include lead-based piezoelectric ceramics and lead-free piezoelectric ceramics.Lead-based piezoelectric ceramics are widely used in military,medical and other fields because of their excellent ferroelectric properties.As people become more aware of environmental protection,they realize that Pb O can cause harm to human body and pose a serious threat to the ecological environment.The research of high performance lead-free piezoelectric ceramics to replace lead-based piezoelectric materials in piezoelectric devices has become a high priority.Na_0.5Bi_0.5Ti O₃（NBT）,which has excellent ferroelectric properties and simple preparation process,has been a hot research topic.However,its poor piezoelectric properties and narrow sintering temperature range have limited its application in brakes and converters.How to modify Na_0.5Bi_0.5Ti O₃ so that its ferroelectric properties can be greatly improved has become the primary problem to be solved by researchers.In this thesis,the phase structure,ferroelectric and dielectric properties of Na_0.5Bi_0.5Ti O₃-Ba Ti O₃（NBT-BT）in the MPB range are studied in detail by introducing strong dielectric Ba Ti O₃ with Na_0.5Bi_0.5Ti O₃ as the substrate.The introduction of the rare earth element europium into Na_0.5Bi_0.5Ti O₃-Ba Ti O₃further enhances the electrical and fluorescent luminescence properties of the material.Finally,influenced by the design of ternary lead-based piezoelectric ceramic system,a new ternary piezoelectric solid solution Na_0.5Bi_0.5Ti O₃-Ba Ti O₃-Sr(Ta_0.5Al_0.5)O₃ is designed to further investigate the effects of Sr(Ta_0.5Al_0.5)O₃ on the ferroelectric,dielectric,and field strain of the substrate.A new idea for NBT-BT modification was provided.The following conclusions were obtained.The（1-x）Na_0.5Bi_0.5Ti O₃-x Ba Ti O₃（x=0.06,0.063,0.065,0.068）lead-free piezoelectric ceramics were synthesized by the conventional solid-phase synthesis method.All component samples exhibit pure chalcogenide phases（coexistence of tri-and tetragonal phases）.Scanning electron microscopy and EDS results show that the ceramic samples have smaller grain size,obvious grain boundaries,no porosity,dense structure and uniform element distribution.The re-established long-range ferroelectric order allows NBT-BT to exhibit more excellent ferroelectric properties.0.063BT fraction yields the maximum residual polarization P_r～35.3μC/cm² and the maximum dielectric constantε′～5062,and the coercivity field of 0.068BT is as low as 27.78 k V/cm.NBT-BT as a high dielectric,low dielectric loss piezoelectric ceramic It has the potential to be applied in the field of converters and transducers.Lead-free piezoelectric ceramics 0.937Na_0.5Bi_0.5-xEu_xTi O₃-0.063Ba Ti O₃（x=0.003,0.005,0.01,0.013,0.015,0.017,0.02）were synthesized by solid-phase synthesis.The introduction of Eu³⁺resulted in significant changes in the ferroelectric,dielectric and fluorescent luminescence properties of the substrates.The remanent polarization and coercive field of pure NBT-BT are P_r～29.24μC/cm² and Ec～39.33 k V/cm,respectively,at room temperature.the maximum remanent polarization P_r～38.02μC/cm² and the highest dielectric constant 6899 were obtained for NBE_0.003T-BT at room temperature.The maximum bipolar electric-field strain was obtained for NBE_0.015T-BT.In addition,the polarized NBE_0.015T-BT had the strongest fluorescence luminescence performance with an average lifetime of 785.98μs.The new ternary lead-free piezoelectric solid solution（1-x）(Na_0.5Bi_0.5)0.937 Ba0.063Ti O₃-x Sr(Ta_0.5Al_0.5)O₃（x=0,0.003,0.005,0.01,0.015,0.02）was synthesized by solid-phase synthesis.All component samples were pure perovskite structure with no heterogeneous phases generated.The remanent polarization and coercive field of pure Na_0.5Bi_0.5Ti O₃-Ba Ti O₃ at room temperature were P_r～35.23μC/cm² and Ec～42.46 k V/cm,respectively.The introduction of Sr(Ta_0.5Al_0.5)O₃ sharply affected the ferroelectric properties of the substrate.With the increase of Sr(Ta_0.5Al_0.5)O₃ doping,the whole system undergone a transition from the ferroelectric phase to the antiferroelectric phase.This process was accompanied by the appearance of double hysteresis loops for three components of the samples（0.003SAT,0.005SAT and 0.01SAT）.Among them,the highest dielectric constantε′～4300 was obtained for 0.005SAT.In addition,the positive biaxial strain of 0.015SAT was 0.53%at room temperature,corresponding to a high-field piezoelectric constant d^*₃₃～996 pm/V,which makes NBT-BT-SAT promising for a wide range of applications in high-performance brakes.