Key Engineering Materials Vols. 368-372

Abstract: Spherical SiO2 particles were coated with Ca2Y8(SiO4)6O2:Eu3+ phosphor layers through a sol-gel process. The results of XRD (X-ray diffraction) analysis indicated that the phosphors crystallized completely at 1000oC. AFM study revealed that the average grain size is 500 nm. In Ca2Y8(SiO4)6O2:Eu3+ spherical phosphors , the Eu3+ showed its characteristic red emission at 612 nm(5D0-7F2) upon excitation into its charge transfer band of Eu3+-O2- at 242nm.

Abstract: The luminescent properties of SrAl2O4 based materials were studied systematically, which emit long persisting phosphorescence with high brightness and show greater chemical stability than the traditional ZnS:Cu,Co phosphor. In order to obtain the better dispersed powders, SrAl2O4:Eu2+, Dy3+ was synthesized by a dipping process. The as-received samples are yellow-green in color and display better dispersion than those synthesized by the co-deposition method. The optimized conditions of this dipping process were explored. The luminescent properties and the crystal structure were studied in detail.

Abstract: A new method for preparing long afterglow phosphor and properties had been studied in the paper. In the new process the activated Al-Sr alloy powder hydrolyzes in pure water. The hydrolysates contain the mixture of Sr(OH)2.8H2O and Al(OH)3 in main, included a little Al(OH)3, AlOOH, SrCO3, and 3SrO.Al2O3. 6H2O. After heat treatment at 700°C for 1 hour, the mixture of SrO and α-Al2O3, γ-Al2O3 were formed. As a precursor, the hydrolysates after heat treatment had been used to prepare long after phosphor Eu, Dy co-doped SrAl2O4. The crystal structure of hydrolysates and Eu, Dy co-doped SrAl2O4 was analyzed by XRD. The luminescent properties of long afterglow material were studied. The Results showed the emission spectra of Eu and Dy co-doped SrAl2O4 lied near 520nm, resulted from the character transition of 4f5d-4f of Eu2+ ion. The method can improve afterglow properties dramatically. The initial luminescent intensity of long afterglow material reached 18cd/m2, and the duration time lasted 46 h.

Abstract: Ultrafine cerium-doped GSAG phosphor powders are prepared by a solution combustion process using glycine and urea as fuel. Single-phase cubic GSAG:Ce crystalline powder is obtained by calcining the as-synthesized amorphous materials at 800oC and no intermediate phase is observed. Transmission electronic microscope morphology shows that the resultant GSAG:Ce powders have uniform size and good homogeneity. The photoluminescence spectra of Ce3+ substituted for Ce3+ in GSAG has been measured on samples calcined at 1000oC.

Abstract: Y2SiWO8:Dy3+ phosphors were prepared through a sol-gel process. XRD and photoluminescence spectra were used to characterize the resulting phosphors. The results indicated that the phosphors crystallized completely at 1000°C. In Y2SiWO8:Dy3+ phosphors, the Dy3+ showed its characteristic yellow emission at 483nm (4F9/2-6H5/2) and 575nm (4F9/2-6H13/2) upon excitation into 275nm.

Abstract: Rare earth phosphors of Ca0.85(W1-xMox)O4:Eu0.15 (x=0.00-0.50) were synthesized via solidstate reaction in air. The lattice parameters strongly depended on the x value, which was analyzed by the XRD patterns. Microstructures of the powders were observed by SEM. The phosphors could be effectively excitated by ultraviolet (UV) light (250-300nm) and blue light (464nm), which were showed with the excitation and emission spectra. With the increasing of x value, moreover, another emission peak of the sample which was excitated by UV light appeared and shifted to short wavelength, from 597 to 545 nm. The luminescence properties of the phosphors and energy transfer mechanism were also discussed.

Abstract: Hexagonal NaYF4 crystals co-doped with different concentration of Yb3+ and Er3+ ions were synthesized via hydrothermal reaction route. The samples doped with 30mol% of Yb3+ and 1mol% of Er3+ showed the highest upconversion efficiencies under 980nm excitation. Powder X-ray diffraction (XRD) analyses indicated that the samples were pure hexagonal phase. The morphology of the powder was observed by scanning electron microscopy (SEM). With the excitation of a 980nm diode laser, strong green emissions were obtained at 522, 530, 542 and 551nm in addition to weak red emission at 654nm. The green and red emissions were attributed to the transitions of 4S3/2, 2H11/2 → 4I15/2 and 4F9/2 → 4I15/2 of Er3+ ion, respectively. The additions of Yb3+ ion as sensitizer can notably enhance the upconversion efficiencies from infra-red to visible. The upconversion mechanism of the emission for the samples was also discussed.

Abstract: Hexagonal NaYF4:Ln3+ (Ln3+=Yb3+ and Tm3+) was prepared via solid-state synthesis route. The synthesized powders were identified with X-ray diffraction patterns. Hexagonal phase was obtained by heating the powders at 550°C in reducing atmosphere, and the ions of Yb3+ and Tm3+ were completely incorporated into the lattice of hexagonal NaYF4 in the doping concentration range. Microstructure of the phosphor was observed by scanning electron microscopy (SEM). Intense blue (1G4→3H6, 1D2→3F4) and weak red (1G4→3F4) upconversion emission of Tm3+ ions with the energy transferred from Yb3+ ions were observed in the phosphor excited with a 980nm continuous wave laser diode. The intensity of blue emission was effectively enhanced by suitable doping of Tm3+ and Yb3+ ions, and strongest blue emission was obtained with the concentration of 0.2mol% Tm3+ and 40mol% Yb3+ doped.

Abstract: The transparent nanocrystalline MgAl2O4 spinel ceramics were synthesized at lowtemperature and high-pressure conditions with low-cost nano-sized MgAl2O4 powder. The sintering characteristics of transparent nano-ceramics were investigated at 500~700oC under 2 ~ 5 GPa. The optimal sintering condition for preparing transparent nano-ceramics was determined. The microstructure and phase composition of powder, as well as the microstructures, morphologies, optical properties, densities and mechanical properties of synthetic ceramics were investigated. The grain sizes of the synthetic ceramics are less than 100 nm, far smaller than those of usual transparent micron-ceramics, and the average grain size depends on the pressure and temperature. The transmittance at the saturation plateau can near 80%. The relative densities of all samples are less than 99%, however, they are highly transparent. The toughness, derived from energy dissipation, of the transparent nano-ceramics was investigated and was compared with that of transparent micron-ceramic.

Abstract: Ultrafine MgAl2O4 powders were synthesized by a nitrate-citrate sol-gel combustion process using Al(NO3)3·9H2O, Mg(NO3)2·6H2O and C6H8O7·H2O as initial materials. The result of X-ray diffraction (XRD) analysis indicated that single-phase MgAl2O4 powder could be obtained at 850 oC, which is much lower than that needed in solid-state reactions. Transmission electron microscopy image showed that the as-prepared powder was in the nano scale and little agglomerate exited. It is concluded that this process need shorter time, lower temperature, and simple equipments.