Preparation,Structure Control Engineering,Luminescence And Electrical Properties Of Dy³⁺/Ca²⁺ Doped Na_0.5Bi_0.5TiO₃-Base Ferroelectrics

With the rapid development of functional materials and devices towards integration,miniaturization and multi-function,it is very attractive to design and explore new materials with multi-functional coupling effects in single-phase materials.Rare earth ion-doped perovskite ferroelectric materials have attracted much attention because of their unique photoluminescence and intrinsic ferroelectric/piezoelectric properties,and have great potential in new multi-functional sensors,memory and interdisciplinary applications.The perovskite structured system sush as sodium bismuth titanate(Na_0.5Bi_0.5TiO₃,abbr as BNT)has high spontaneous polarization,Curie temperature,low phonon vibration energy and lead-free properties,so it is considered to be the most potential lead-free ferroelectric material to replace lead-based materials.However,BNT has high coercive field and electrical conductivity at room temperature,and Na⁺and Bi³⁺are easy to volatilize during the preparation process,resulting in poor piezoelectric properties.In order to address the above problems,the main strategy is to construct phase boundaries in BNT by chemical modification（ion replacement or solid solution）to improve the piezoelectric/ferroelectric properties.For this purpose,rare earth ions doping can not only enhance the intrinsic piezoelectric and ferroelectric properties,but also give rise to the good fluorescence properties.In this thesis,BNT-based ceramics were prepared by traditional solid reaction method.The effects of A-site doping on the phase structure,fluorescence properties,ferroelectric and field strain properties of BNT:x Dy,（1-x）BNT-x CT and 0.9BNT-0.1CT ceramics were systematically studied by means of X-ray diffraction（XRD）,ultraviolet visible absorption spectroscopy（UV-Vis）,fluorescence spectroscopy（PL）,hysteresis loop（P-E）and strain electric field loop（S-E）.The main objectives and results are as follows:（1）To investigate the effect of A-site Dy³⁺doping on the fluorescence and electric-field induced strain of BNT:x Dy（x=0,0.003,0.005,0.01,0.03,0.05）ceramics.It shows the best remanent polarization（55μC/cm²）in BNT:0.005Dy ceramics.The single-phase BNT:0.03Dy ceramics have the strongest fluorescence properties and maximum electric field-induced strain（0.25%）.（2）To reveal the effect of A-site Ca²⁺substitution on the phase structure,dielectric properties and electric field-induced strain of（1-x）BNT-x CT（x=0,0.04,0.07,0.08,0.10,0.12,0.14,0.15,0.20）solid solution ceramics.After Ca²⁺doping,the unit cell parameters decrease,and the crystal structure changes from rhombohedral phase to orthorhombic phase.The two-phase coexistence region of（1-x）BNT-x CT solid solutions was determined to be the morphotropic phase boundary region of the solid solution by XRD,GSAS refinement and Raman spectra.0.86BNT-0.14CT solid solution has the best dielectric constant（1100）.Ca²⁺doping can effectively reduce the depolarization temperature,Curie temperature,coercive field and remanent polarization intensity of the solid solution.0.9BNT-0.1CT solid solution has the largest electric field-induced strain（0.30%）.（3）To understand the effect of A-site Dy³⁺doping on the phase structure,fluorescence properties,dielectric properties and electric field induced strain of 0.9BNT-0.1CT:x Dy（x=0,0.001,0.003,0.005,0.02,0.03,0.04,0.05）solid solution ceramics.After Dy³⁺doping,the unit cell parameters of 0.9BNT-0.1CT:x Dy solid solution decrease,and the crystal structure changes from rhombohedral phase and orthorhombic phase to orthorhombic phase.The two-phase region of 0.9BNT-0.1CT:x Dy solid solution forms a morphotropic phase boundary by XRD and GSAS.After Dy³⁺doping,the coercive field and remanent polarization of 0.9BNT-0.1CT:x Dy solid solution decrease gradually.Among them,0.9BNT-0.1CT:0.02Dy solid solution has the strongest luminous intensity.There is a large electric field-induced strain（0.36%）in 0.9BNT-0.1CT:0.003Dy solid solution.It has high remanent polarization and fluorescence enhancement properties（24%）in 0.9BNT-0.1CT:0.001Dy solid solution,indicating that remanent polarization may help to improve the luminescence properties of 0.9BNT-0.1CT:x Dy solid solution.