Structure,Ferroelectric/Piezoelectric And Luminescent Properties Of Rare Earth-Modified CaBi₄Ti₄O₁₅-Based Multifunctional Ceramics

In recenet years, multifunctional materials of rare earth-doped ferroelectrics have attracted much attention due to their simultaneous existence of electrical and luminescent properties. Among these ferroelectrics, bismuth oxide layer structure ferroelectrics have been instensively studied because of their high Curie temperiture, strong anisotropy and low dielectric constant. As one of the m = 4 member of the bismuth layer materials, CaBi₄Ti₄O₁₅ possesses high Curie temperiture of 790 oC and low aging rate. However, the application of the materials is limited due to poor piezoelectricity. Rare earth ions doping is an important approach to improve the electrical properties of CaBi₄Ti₄O₁₅ ceramics by substituting the Ca/Bi ions in A-site by rare earth ions. Besides, the addition of rare earth ions can also induce excellent luminescent properties. Therefore, it is necessary to develop new bismuth-layer structure materials that possess good temperature stability, ferro-/piezoelectricity and luminescence. In this thesis, single rare earth ion(Eu³⁺) and complex ions(Li_0.5Ln_0.5)²⁺,（Ln = Dy, Ho） consisting of rare earth ions Ln³⁺ and Li+ are introduced to CaBi₄Ti₄O₁₅ ceramics and new system of CaBi_4-xEu_xTi₄O₁₅, Ca_1-x(Li_0.5Dy_0.5)_x/2Bi₄Ti₄O₁₅ and Ca_1-x(Li_0.5Ho_0.5)_x/2Bi₄Ti₄O₁₅ were developed and synthesised, and the effects of ions doping on phase structure, microstructure, electrical and optical properties of the ceramics were studied systematically. The main contents and conclusions are listed as following:（1） we prepared new CaBi_4-xEu_xTi₄O₁₅ multifunctional ceramics by a traditional solid state reaction method, and the effects of Eu³⁺ doping on phase structure, microstructure, electrical and optical properties of the ceramics were studied. All the ceramics possess pure Aurivillius-type structure with m = 4 and the addition of Eu³⁺ ion inhibits the grain growth. As x increases, the ferroelectricity and piezoelectricity of the ceramics decreases, while the luminescence gradually increases. Under the excitation of 465 nm light, the ceramics exhibit strong emission peak centered at 590 nm、595 nm and 616 nm, which corresponds to the transition of the ⁵D₀�'⁷F₀、⁵D₀�'⁷F₁ and ⁵D₀�'⁷F₂ level in Eu³⁺ ion.（2） Novel Ca_1-x(Li_0.5Dy_0.5)_x/2Bi₄Ti₄O₁₅ multifunctional ceramics have been fabricated using a conventional sintering method. The composition dependences on phase structure, microstructure, ferroelectric, piezoelectric, luminescent properties and the variation of electrical and luminescent properties with temperature were investigated systematically. The crystal structure of the ceramics is transformed from the bismuth-layer structure with m = 4 to m = 3 as x increases. The ceramic with x = 0.2 obtains high resistivity（R = 3.0×1011 W·cm）,improved ferroelectricity（Pr = 9.92 μC/cm²）, piezoelectricity(d33 = 1_0.5 p C/N) and luminescence. Under the excitation of 451 nm light, the ceramics exhibit strong blue and yellow emission peak centered at 484 nm and 574 nm, respectively. This is because of the transition of ⁴F_9/2�'⁶H_15/2 and ⁴F_9/2�'⁶H_13/2 levels in Dy³⁺ ion. The ceramic with x = 0.2 possesses excellent temperature stability of electrical and luminescent properties, exhibits high depolarization temperature of 800 oC, and the ferroelectricity of the ceramic increases with temperature increasing while luminescence decreases.（3） We fabricated new multifunctional ceramics of Ca_1-x(Li_0.5Ho_0.5)_x/2Bi₄Ti₄O₁₅ by using traditional sintering method. The effects of composition on the structure, microstructure, ferroelectric/piezoelectric/dielectric properties, down/up-conversion luminescent properties and temperature/water on the electrical/luminescent properties of the ceramics were studied systematically. The gradual addition of complex ions makes the crystal structure of the ceramics be changed from four layered bismuth-layer structure to three layered structure. The ceramic with x = 0.1 exhibits high resistivity（R = 4.51×1011 W·cm）,strong ferroelectricity（Pr = 9.03 μC/cm2）,good piezoelectricity（d33 = 10.2 p C/N）, high Curie temperature（TC = 814 oC） and enhanced luminescent properties. Under the excitation of 451 nm light,strong green emission peak centered at 545 nm is observed, which corresponds to the transition of the ⁵S₂�'⁵I₈ level in Ho³⁺ ion. Under the near-infrared excitation of 980 nm at room temperature, a strong red up-conversion emission band located at 660 nm is observed, arising from the transition of ⁵F₅�'⁵I₈ level in Ho³⁺ ion. In addition, The ceramic with x = 0.1 exhibits excellent temperature stability of electrical and luminescent properties, as temperature increases, the ceramic obtains the increased ferroelectricity, high depolarization temperature of 800 oC and thermal quenching temperature above 250 oC. Moreover, the ceramic exhibits also superior water resistance performance.