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daidongfa

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虫号: 1019899
注册: 2010-05-16
性别: GG
专业: 水泥与耐火材料

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Abstract. Er3+/Yb3+ codoped pyrochlore-type yttrium titanate (Y0.985-xEr0.015Ybx)2Ti2O7 (x=2.5 %, 5.0 %, 7.5 %, 10.0 % and 12.5 %) nanopowders were successfully prepared by the Pechini-type sol−gel process with citric acid as chelating ligands and polyethylene glycol as cross-linking agent. The preparation process was monitored by X-ray diffraction, thermal-gravimetric differential thermal analysis and Fourier transform-infrared experiments and the microstructures and morphologies of as-prepared samples were characterized by transmission electron microscopy and high resolution transmission electron microscopy images. Detailed proofs showed that the samples annealed at 800 C for 1 h exhibit good crystallinity, dispersibility and narrow size distribution with average size of 20-30 nm. For all the Er3+/Yb3+ codoped pyrochlore-type (Y0.985-xEr0.015Ybx)2Ti2O7 nanoparticles, bright visible upconversion emission for all the samples can be clearly observed with naked eyes under 980 nm excitation at room temperature. As for amorphous (Y0.91Er0.015Yb0.075)2Ti2O7 samples, upconversion spectra exhibit the two broad erbium emission bands centered at approximately 546 nm and 660 nm, which are ascribed to the 4S3/2→4I15/2 and 4F9/2→4I15/2 transitions, respectively, and relatively weaker broad emission bands at about 524 nm, which is attributed to 2H11/2→4I15/2 transitions. As the annealing temperature increases, there exits Stark splitting effects, and the upconversion luminescence intensities increase, whih was attributed to the removal of the hydroxyl groups and to the decrease of the surface area caused by the growth of the nanoparticles. There exists the optimal Yb3+ ion concentration of 7.5 mol % doping in (Y0.985-xEr0.015Ybx)2Ti2O7 nanocrystals which can give rise to a maximum upconversion luminescence intensity. These are the efficient upconversion nanophosphors.
3.4 TEM spectra
Transmission electron microscopy (TEM) was employed to obtain direct information about the size and structure of the as-prepared Er3+ /Yb3+ cooped (Y0.91Er0.015Yb0.075)2Ti2O7 nanocrystals. The typical TEM results of (Y0.91Er0.015Yb0.075)2Ti2O7 nanocrystals prepared at 800C for1 h are shown in Fig. 5. From Fig. 5(a), it can be found that quasi-spherical nanopowders have good dispersibility, and the average size estimated from TEM is around 20-30 nm, which is quite consistent to above-mentioned XRD results. The insert in Fig. 5(a) shows the corresponding selected-area electron diffraction (SAED) patterns, which is the characteristic rings of nano-polycrystals. In Fig. 5b, the typical high-resolution transmission electron microscopy (HRTEM) images indicate the clear and regular crystal lattice distance suggesting that highly crystalline (Y0.91Er0.015Yb0.075)2Ti2O7 were formed. The lattice fringes with the interplanar distance of 0.292 nm obtained from the HRTEM images was attributed to the (222) plane of the cubic structure.
4. Conclusion
Abstract. Er3+/Yb3+ codoped pyrochlore-type yttrium titanate (Y0.985-xEr0.015Ybx)2Ti2O7 (x=2.5 %, 5.0 %, 7.5 %, 10.0 % and 12.5 %) nanopowders were successfully prepared by the Pechini-type sol−gel process with citric acid as chelating ligand and polyethylene glycol as cross-linking agent. The samples annealed at 800 C for 1 h exhibit good crystallinity, dispersibility and narrow size distribution with average size of 20-30 nm; And also, there exists the optimal Yb3+ ion concentration of 12.5 mol % doping in (Y0.985-xEr0.015Ybx)2Ti2O7 nanocrystals which can give rise to a maximum upconversion luminescence intensity. The increase of the upconversion luminescence intensities with the calcination temperature was attributed to the removal of the hydroxyl groups and to the decrease of the surface area caused by the growth of the nanoparticles.

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