Shilong Zhao

Structural evolution and enhancement of luminescence in the Eu-doped oxyfluoride glass ceramics containing NaGdF4 nanocrystals

Cubic-to-hexagonal phase evolution, narrow crystalline size distribution and enhancement of luminescence are simultaneously achieved by modifying the ratio of SiO2/B2O3 content in the Eu3+-doped oxyfluoride borosilicate glass ceramics containing NaGdF4 nanocrystals. The mechanisms of phase evolution of NaGdF4 nanocrystals are discussed on the basis of IR spectra, viscosity, XRD and TEM data. The results indicate that the stresses developed during the thermal treatment process play an important role in the phase evolution, which provides an experimental evidence of pressure-induced disorder-to-order phase evolution during the crystallization process for the first time. It is also found that the excitation and emission spectra as well as the luminescence lifetimes of the Eu3+ ions in the oxyfluoride glass ceramic are sensitive to the crystal lattice of NaGdF4.

Luminescence properties of novel emission-tunable NaSr(4−x−y)Bax(BO3)3:yEu2+ phosphors for white light emitting diodes

A series of emission-tunable NaSr(4−x−y)Bax(BO3)3:yEu2+ phosphors have been prepared by a conventional solid-state reaction method. The structures of NaSr(4−x−y)Bax(BO3)3:yEu2+ have been investigated by Rietveld refinement of the X-ray diffraction (XRD) patterns. The results indicated that the as-prepared samples showed the same crystal structure of NaSr4(BO3)3 with a cubic unit cell and space group of Iad. With the increase of Ba2+ concentration, the Sr2+ sites were replaced by Ba2+ completely and the lattice parameter of the unit cell increased from a = b = c = 15.0710 A to 15.7266 A. Both emission spectra and decay curves of NaSr3.98(BO3)3:0.02Eu2+ and NaBa3.98(BO3)3:0.02Eu2+ showed the existence of two different Eu2+ emission centers named Eu1 and Eu2. Eu2 was six-coordinated and Eu1 was eight-coordinated of oxygen. With the increase of Eu2+ concentration in the NaSr3−yBa(BO3)3:yEu2+ sample, the emission intensity increased and reached a maximum at y = 0.02. Then the concentration quenching phenomenon emerged due to the electric dipole–dipole interaction. Upon the cation substitutions (Sr2+ for Ba2+) in the NaSr(4−x−y)Bax(BO3)3:yEu2+ host, the emission peaks of Eu2+ blue-shifted from 609 nm to 544 nm and the thermal stability decreased, which was ascribed to the change of the covalency and the crystal field strength that the 5d orbital of the Eu2+ ion experiences. The CIE chromaticity coordinates of the obtained phosphors can be continuously tuned from orange-red (0.4795, 0.4070) to yellow-green (0.3432, 0.4665) by adjusting the Ba2+ concentration. The results demonstrate that the emission-tunable NaSr(4−x−y)Bax(BO3)3:yEu2+ phosphors have a potential application for white light emitting diodes (w-LEDs).

Preparation and photoluminescence of Eu-doped oxyfluoride borosilicate glass ceramics

Abstract Eu-doped transparent oxyfluoride borosilicate glass ceramics containing Ba 2 GdF 7 nanocrystals were prepared by controlling crystallization of melt-quenched glass fabricated under a reductive atmosphere. In the oxyfluoride borosilicate glass ceramics, the mean crystal size of Ba 2 GdF 7 nanocrystals was about 30 nm, which could be observed by X-ray diffraction (XRD) and transmission electron microscopy analysis. The photoluminescence spectra of the samples excited at 392 nm showed that, besides the characteristic sharp emissions of Eu 3+ ions, a very intense broadband emission of Eu 2+ ions centered at 450 nm appeared. The photoluminescence intensity of Eu 3+ and Eu 2+ ions in the glass ceramics was much stronger than that in the as-made precursor. The long decay lifetimes of Eu 3+ and Eu 2+ ions evidenced the partitions of Eu 3+ and Eu 2+ ions into the Ba 2 GdF 7 nanocrystals. The energy transfer from Gd 3+ ions to Eu 3+ and Eu 2+ ions was confirmed by the excitation and emission spectra.

Spectroscopic investigations on Er3+/Yb3+-doped oxyfluoride glass ceramics containing YOF nanocrystals

Spectroscopic properties of Er3+/Yb3+-doped transparent oxyfluoride borosilicate glass ceramics containing YOF nanocrystals were systematically investigated. X-ray diffraction (XRD) confirmed the formation of YOF nanocrystals in the glassy matrix. Based on the Judd-Ofelt theory, the intensity parameters Ωi (i=2, 4, 6), spontaneous emission probability, radiative lifetime, radiative quantum efficiency and the effective emission bandwidth were investigated. The upconversion luminescence intensity of Er3+ ions in the glass ceramics increased significantly with the increasing crystallization temperature. The transition mechanisms of the green and red upconversion luminescence were ascribed to a two-photon process, and the blue upconversion luminescence was a three-photon absorption process.

Sintering Behavior and Microwave Dielectric Properties of MgTiO3 Ceramics Doped with B2O3 by Sol-Gel Method

The B 2 O 3 -doped MgTiO 3 powders and ceramics have been prepared by sol-gel method using Mg(NO 3 ) 2 6H 2 O, Ti(C 4 H 9 O) 4 and H 3 BO 3 as the starting materials. The sintering behavior and microwave dielectric properties of ceramics prepared from powders with different particle sizes were investigated. The gels were calcined at 650, 700, 750, 800, 850 and 900 °C and the derived particle sizes of powders were 20–30 nm, 30–40 nm, 40–60 nm, 60–90 nm, 90–120 nm and 120–150 nm, respectively The nanoparticles with the size of 30–60 nm benefited the sintering process with high surface energy whereas nanoparticles with the size of 20–30 nm damaged the microwave dielectric properties due to the pores in the ceramics. The addition of B 2 O 3 used as a liquid sintering aid reduced the sintering temperature of MgTiO 3 ceramic, which was supposed to enter the MgTiO 3 lattice and resulted in the formation of (MgTi) 2 (BO 3 )O phase. The B 2 O 3 -doped MgTiO 3 ceramic sintered at 1100°C and prepared from the nanoparticles of 40–60 nm had compact structure and exhibited good microwave dielectric properties: ɛ r =17.63, Q × f =33,768 GHz τ f =−48×10 −6 °C −1 .

Structure, luminescence and temperature sensing in rare earth doped glass ceramics containing NaY(WO4)2 nanocrystals

Novel rare earth doped glass ceramics containing NaY(WO4)2 nanocrystals were fabricated for the first time. The appearance of sharp diffraction peaks and well-resolved lattice fringes certifies the precipitation of NaY(WO4)2 nanocrystals with high crystallinity. After the crystallization process, significant changes in the photoluminescence emission spectra and fluorescence lifetime of Sm3+ ions are observed, which are ascribable to the enrichment of Sm3+ ions in the highly disordered NaY(WO4)2 nanocrystals. Under 980 nm excitation, characteristic green and red upconversion emission signals were detected and the enhanced upconversion luminescence of Er3+ ions in the glass ceramics was attributable to the incorporation into the low energy phonon NaY(WO4)2 nanocrystals. Based on the dependence of upconversion intensity on the excitation power, the upconversion mechanism of Er3+–Yb3+ ions was proposed. The temperature-dependent fluorescence intensity ratio (FIR) of the thermally-coupled 2H11/2 and 4S3/2 energy levels was determined at a low power density of 0.4125 W cm−2. The maximum temperature sensitivity is 146 × 10−4 K−1 at 523 K, which is mainly attributed to the highly disordered structure of NaY(WO4)2 nanocrystals and exhibits promising potential for optical temperature sensors.

Luminescent properties of Tb3+ doped high density borogermanate scintillating glasses

Abstract Tb3+-doped high density borogermanate glasses were prepared by melt-quenching method. Their physical and luminescent properties including differential thermal analysis (DTA), density, transmittance spectra, photoluminescence, and X-ray excited luminescence spectra were investigated. The densities of the glasses doped with Tb3+ were in the range from 5.690 to 6.086 g/cm3. Under UV and X-ray excitations, the glasses showed intense green emissions. The lifetimes of Tb3+ doped borogermanate glasses were in the range from 1.597 to 1.869 ms. The results indicated that Tb3+ doped borogermanate glasses could be scintillator candidate used in X-ray detection application.

A Point Temperature Sensor Based on Upconversion Emission in Er3+/Yb3+ Codoped Tellurite-Zinc-Niobium Glass

Er3+/Yb3+ codoped tellurite-zinc-niobium (TZNb) glass was prepared by the melt-quenching method and used for the construction of a point all-fiber temperature sensor. The glass thermal stability and network structural properties were studied by differential thermal analysis and Raman spectrum, respectively. High glass transition temperature is beneficial to widen the working temperature range. The dependence of fluorescence intensity ratio (FIR) of green upconversion emissions on the surrounding temperature from 276 to 363 K was experimentally investigated and the maximum temperature sensitivity is 95 × 10−4 K−1 at 363 K. Strong green upconversion emission, broad temperature measurement range and high sensitivity indicate this point temperature sensor is a promising optical device for application on optical temperature sensing.

Influence of excitation power and doping concentration on the upconversion emission and optical temperature sensing behavior of Er 3+ : BaGd 2 (MoO 4 ) 4 phosphors

Generally, the effects of excitation power and dopant concentration on the optical temperature sensing behaviors of rare earth (RE) doped materials based on the fluorescence intensity ratio (FIR) technique are disregarded. In this paper, Er3+: BaGd2(MoO4)4 phosphors with different concentrations were fabricated by the high temperature solid-state reaction method. The results show that the variation of FIR (2H11/2/4S3/2) with excitation power is not only related to the laser-induced heating effect, but also the diverse power-dependences of 2H11/2 and 4S3/2 levels. Consequently, the temperature calibration curves change at different excitation power densities. When the calibration curve obtained at a low power density is applied to estimate the temperature of the object excited at a high power density, a large overestimate of the temperature rise induced by the optical heating effect can be caused. Besides, the temperature sensing sensitivity depends on the Er3+ doping concentration, which increases first with concentration to a maximum and then reduces. The maximal absolute sensitivity is ~110.5 × 10−4 K−1 in 5mol% Er3+: BaGd2(MoO4)4 phosphor, which is among the highest values of RE ions doped phosphors based on thermally coupled levels recorded before.

A portable all-fiber thermometer based on the fluorescence intensity ratio (FIR) technique in rare earth doped TeO2–WO3–La2O3–Na2O glass

A portable all-fiber thermometer based on the FIR technique in Er3+/Yb3+ co-doped TeO2–WO3–La2O3–Na2O (TWLN) glasses was designed and its temperature sensing performance in the temperature range of 293–569 K was determined in detail. The high glass transition temperature (Tg = 439 °C) of TWLN glass contributes to broadening the range of temperature measurement. The spectroscopic analysis of Eu3+ ions and Judd–Ofelt analysis of Er3+ ions reveal that the rare earth ions are situated in a low symmetry environment in TWLN glass, which suggests that a large FIR change and high temperature sensitivity are expected. Intense green upconversion emission in Er3+/Yb3+ co-doped TWLN glass fiber was easily observed under 980 nm excitation at a low pumping power (1.4 mW). The maximum temperature sensitivity of the portable all-fiber thermometer reached 86.7 × 10−4 K−1 at 553 K. Experimental results showed that the absolute errors were around ±1 K within the temperature range from 285 to 563 K.