Structure, Ferroelectric, Piezoelectric And Photoluminescent Properties Of Rare Earth-doped Bi_0.5Na_0.5TiO₃-based Piezoelectric Lead-free Ceramics

Piezoelectric ceramic materials is one of the most important functional material and has been widely used in actuator, sonar and energy converter. However, most of commercial piezoelectric ceramics are Pb-containing materials. The strong toxicity of Pb O has caused serious environment problems. Therefore, many science researchers have focused on developing lead-free ceramic materials with excellent properties. On the other hand, with the development of the miniaturization and mulfifunctionalization, rare earth-doped ferroelectric-photoluminesct multifunction materials have attraction considerable attention in recent years.In this thesis, we developed and prepared rare earth(Dy³⁺, Sm³⁺, Tm³⁺)-doped BNT based ceramics by a conventional ceramic technique, and the structure ferroelectric, piezoelectric and photoluminescence properties of the ceramics were systematically inveatiagated. The main results are listed as following:（1）Lead-free ceramics of 0.94(Bi_1-xDy_x)_0.5Na_0.5TiO₃-0.06 Ba TiO₃ were prepared by an ordinary sintering method and their structure, ferroelectric, piezoelectric and photoluminescent properties were studied. All the ceramics possess a pure perovskite structure with the coexistence of the rhombohedral and tetragonal phase. The partial substitution of Dy³⁺ for Bi³⁺ enhanced effectively the piezoelectricity of the materials. The optimum piezoelectricity is obtained at x = 0.025, giving d33 = 190 p C/N and kP = 37.2%, respectively. After the addition of Dy³⁺, excellent photoluminescence of the ceramics is induced. The Dy-doped ceramics exhibit strong emissions at ⁴78 nm（blue） and ⁵75 nm（yellow）, corresponding to ⁴F_9/2 �' ⁶H_15/2 and ⁴F_9/2 �' 6H_13/2 of Dy³⁺ transitions, respectively. The optimum photoluminescence of the ceramics is reached at x = 0.025. The ceramics with the doping of 2.5 mol% Dy³⁺ exhibit simultaneously good piezoelectricity, strong ferroelectricity and excellent photoluminescence, suggesting a promising application in the electro-optic devices.（2）0.94(Bi_1-xSm_xNa)_0.5TiO₃-0.06 Ba TiO₃（x = 0-0.05） lead-free ceramics were synthesized by a conventional solid-state reaction route and the microstructure, piezoelectric, ferroelectric and photoluminescent properties of the ceramics were investigated. The ceramics can be well sintered at 1200 °C for 2 h. All the ceramics exhibit a pure perovskite structure with the coexistence of rhombohedral and tetragonal phases. After the addition of a small amount of Sm³⁺（x ≤ 0.035）, the piezoelectricity is enhanced with d33 increasing from 147 p C/N to 172 p C/N, while excess Sm³⁺（x = 0.035–0.05） degrades significantly the piezoelectricity with d33 decreasing from 172 p C/N to 18 p C/N. Under the excitation of 406 nm, three emission peaks located at 563 nm(⁴G_5/2-⁶H_5/2), 597 nm(⁴G_5/2-⁶H_7/2), and 645 nm(⁴G_5/2-⁶H_9/2) are observed. Under the emission of 564 nm, the ceramics with Sm³⁺ exhibit five excitation peaks at 407, 420, 464, 440 and 479 nm corresponding to ⁶H_5/2-⁴F_7/2, ⁶H_5/2-（6P, 4P）5/2, ⁶H_5/2-4G9/2, ⁶H_5/2-⁴I_11/2,13/2 and ⁶H_5/2-⁴M_15/2, respectively. Both the intensities of the ⁴G_5/2�'⁶H_5/2 emission peak and ⁶H_5/2�'⁴F_7/2 excitation peak first increase and then decrease with increasing x. And the highest intensity was obtained at x = 0.035. The average lifetime τav decreases from 1.0869 ms to 0.7850 ms with increasing x.（3）Lead-free ceramics of(Bi_1-xTm_x)_0.5(Na_0.85K_0.15)_0.5TiO₃ were prepared by a conventional ceramic technique and the effects of Tm doping on the microstructure, ferroelectric, piezoelectric and photoluminescent properties were studied. All the ceramics possess a pure perovskite structure and Tm³⁺ diffuses into the Bi_0.5(Na_0.85K_0.15)_0.5TiO₃ lattices to form a solid solution with the coexistence of the rhombohedral and tetragonal symmetry. The addition of Tm³⁺ leads to the significant change in the grain morphology and size for the(Bi_1-xTm_x)_0.5(Na_0.85K_0.15)0.5TiO₃. The grain size increases and then gradually decreases with the increasing x. The partial substitution of Tm³⁺ for Bi³⁺ enhanced effectively the piezoelectricity, giving d33 = 155 p C/N at x = 0.04. While the excess Tm³⁺ degrades significantly the piezoelectricity. Under the excitation of 980 nm, the Tm³⁺-doped ceramics shows three up-conversion emission peaks at 486nm、550nm and 649 nm, which can be attributed to the ¹G₄�'³H₆, ¹G₄�'³F₄ and ³ F ₂₃�'³H₆ transitions of Tm³⁺.