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The Preparation and Their Photoluminescence Properties of the Doped Niobium Calcium （CaNb2O6） Fluorescence Powders and Effects of Different Additives on the Morphology and Luminescence of Bismuth-doped Calcium Niobate Nanocrystals

Author: ShiGuangLi
Tutor: PengZiFei
School: Shanghai Normal University
Course: Inorganic Chemistry
Keywords: Photoluminescence CaNb2O6 Bi3+ K+, Na+, Li+, Mg2+, Al3+ Blue-white light Nano-materials Crystal growth
CLC: TQ422
Type: Master's thesis
Year: 2010
Downloads: 82
Quote: 0
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Abstract


For several decades, people concentrate on an improvement that the fluorescence powders give out light strength. In the present paper, adding Bi3+ ions increases the luminescence of CaNb2O6 and the co-doping of the K+ ions significantly enhances the luminescence of CaNb2O6:Bi3+ via a high temperature solid state reaction. Under a 315nm excitation wavelength, the fluorescent intensity of CaNb2O6:1% Bi3+ with 1/15 K+ is very high, which has been rarely observed in the other similar products. At the same time, in recent years, nano-materials have been a hot study field. So nano-CaNb2O6:1% Bi3+ was synthesized via a sol-gel combustion method and the nano-crystals with the different morphologies were obtained. Besides this, we analyzed effects of different additives on the morphology and luminescence of bismuth-doped calcium niobate nanocrystals. The mechanisms of these effects were also investigated. Therefore, the paper launches the quest and research from the two respects following:1. The synthesis and photoluminescence properties of the doped niobium calcium (CaNb2O6) fluorescence powders were studied. CaNb2O6:1% Bi3+ phosphors with 1/n A+(n=2,5,10,15,20) (A+ stands for alkali metal) were synthesized by a high temperature solid-state reaction. From the XRD results of CaNb2O6:1% Bi3+ and CaNb2O6:1% Bi3+ with 1/n K+(n=2,5,10,15,20), it was found that the diffraction peaks of the seven samples can be indexed as an octahedral phase, consistent with the standard data of CaNb2O6 (JCPDS No.39-1392, orthorhombic, space group Pbcn) and a small shift towards the larger 2θside is observed, indicating that there is an expansion of the crystal lattice compared to CaNb2O6:1% Bi3+. It can be seen from the FESEM that KCl served as a cosolvent. In the ultraviolet-visible diffuse reflection spectra, there are two absorption peaks. One is at about 290 nm and the other is at around 340 nm. With incorporating Bi3+ and K+ into CaNb2O6, the two peaks both decrease, but the peaks around 290 nm fall dramatically, which reveals that the energy transfer from CaNb2O6 to Bi3+ happens. Under a 254 nm ultraviolet excitation, the emission spectra consist of two parts:one is a weak emission component at around 360 nm and the other is a strong broad band from 380-700 nm with a maximum at 463 nm. Under a 315 nm excitation wavelength, the emission spectra show a broad and strong band from 350 to 650 nm, which is primarily related to the 1S0→3P1 and 1S0→1P1 transition of the Bi3+ ion. In addition, the PL intensity increases with an increasing amount of K+ dopant. The maximum intensity occurs at n Bi3+/k+=1:15.The effects of doping Mg2+ or Al3+ on the luminescence properties of CaNb2O6:1% Bi3+,1/5 K+ were investigated. From their excitation and emission spectra, it can be observed that both Mg2+ and Al3+ improve the luminescent intensity of CaNb2O6:1%Bi3+,1/5 K+. But the effects of Mg2+ and Al3+ ions are smaller than K+ ions. However, under a 315 nm excitation wavelength, the luminescent intensity of (CaNb2O6:1% Bi3+ with 1/5 K+ with Mg2+) and (CaNb2O6:1% Bi3+ with 1/5 K+ with Al3+) is nearly as strong as the above optimized material, CaNb2O6:1% Bi3+ with 1/15 K+, which has been rarely observed in the other similar products.The photoluminescence properties of incorporating Bi3+, Mg2+ and Al3+ into CaNb2O6 were analyzed. Under a 254 nm excitation wavelength, the emission bands of CaNb2O6:Mg2+ and CaNb2O6:Al3+ are broader than that of CaNb2O6:Bi3+. It can be seen that arrangement of extranuclear electrons, ionic radius, vibration of the crystal and vibration energy of phonon have different impacts on the luminescence.2. Nanoparticles of bismuth-doped calcium niobate with different morphologies were synthesized via a sol-gel combustion method. By adjusting the amount of different additives, a number of interesting nano-crystalline forms, such as spherical, flower-like and rod-like, were synthesized. The as-synthesized nano-particles are spherical or nearly spherical when n Nb/CA amounts to 1:3. However, with the increase of n Nb/CA to 1:4, a significant change takes place in the state of particle aggregation. Most of the nano-particles are gathered together like flowers. When K+ ions were added into the sol, rod-like nano-crystals with diameters of about 60 nm and lengths of up to about 300-400 nm are produced. Therefore, K+ ions play a crucial role in the formation of rod-like nano-particles. The emission spectra of nanoparticles of bismuth-doped calcium niobate with different morphologies are composed of two parts:one is a weak emission component centered at around 360 nm, another is a strong band from 410 to 650 nm with a maximum at 463 nm under 254 nm exited and show a broad emission region from 350 to 650 nm under a 315 nm excitation.

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