Nonisothermal crystallization kinetics and microstructure evolution of calcium lanthanum metaborate glass

This paper reports results on the crystallization kinetics of 35.5CaO-7.25La(2)O(3)-57.25B(2)O(3) glass under nonisothermal conditions based on the studies carried out from the differential thermal analysis upon using various well-established models. The crystalline phases formed during the optimized ceramization process have been confirmed from the X-ray diffraction. The activation energies of the first (formation of CaLaB(7)O(13)) and second (formation of LaBO(3)) crystallization events have been estimated using the conventional methods of Kissinger, Augis-Bennett, Ozawa, and Matusita, and the results are found to be in good agreement with each other. The Avrami exponents that are determined by these models for the crystallization of CaLaB(7)O(13) and LaBO(3) are found to be in the range of (1.81-2.35) and (4.03-4.65), respectively. This indicates that the formation of CaLaB(7)O(13) is dominated by a surface crystallization, whereas LaBO(3) is formed by three-dimensional bulk crystallization with an increased rate of nucleation. This observation is further validated by microstructural investigation, which shows the formation of CaLaB(7)O(13) phase as a surface layer and a bulk crystallization of LaBO(3) in optimally ceramized samples

Enhanced Blue Emission from Transparent Oxyfluoride Glass Ceramics Containing Pr3+:BaF2 Nano-crystals

Transparent glass ceramics containing Pr3+:BaF2 nano-crystals in chemical composition of SiO2–BaF2–K2CO3–La2O3-Sb2O3 oxyfluoride glass systems have been prepared from melt quenching and with subsequent heat-treatment method. Luminescence and structural properties of these materials have been evaluated and the results are reported. Rietveld analysis of X-ray diffraction (XRD) patterns and investigation of transmission electron microscopy (TEM) confirmed the presence of BaF2 nano-crystals dispersed in the heat treated glass matrices. Measured UV-Vis-NIR absorption spectra have exhibited nine bands of the transitions 3H4 3P2, (1I6, 3P1), 3P0, 1D2, 1G4, 3F3,3F2, 3H6 and 3H5 from all the samples with non-degenerated 1I6 and 3P1 levels in the glass ceramics. The photoluminescence spectra show an enhancement in the intensities upon ceramization indicating the incorporation of Pr3+ ions in BaF2 nano-crystals which possess low phonon energy (346 cm-1). This has further been corroborated from the observation of a significant 3-fold increase in the relative intensity ratio of blue (3P0 3H4) to red (1D2 3H4, 3P0 3H6)emissions from glass-ceramics compared with the glass. It is due to a significant decrease of multiphonon non-radiative relaxation from 3P0 to 1D2 level of Pr3+ in glass ceramics. Time resolved spectra exhibit 3P0 level decays faster than 1D2 level

Sensitized red luminescence from Bi(3+) co-doped Eu(3+): ZnO-B2O(3) glasses

Photoluminescence properties of Bi(3+) co-doped Eu(3+) containing zinc borate glasses have been investigated and the results are reported here. Bright red emission due to a dominant electric dipole transition (5)D(0) -> (7)F(2) of the Eu(3+) ions has been observed from these glasses. The nature of Stark components from the measured fluorescence transitions of Eu(3+) ions reveal that the rare earth ions could take the lattice sites of C, or lower point symmetry in the zinc borate glass hosts. The significant enhancement of Eu(3+) emission intensity by 346nm excitation ((1)S(0)->(3)P(1), of Bi(3+) ions) elucidates the sensitization effect of co-dopant. The energy transfer mechanism between sensitizer (Bi(3+)) and activator (Eu(3+)) ions has been explained. (C) 2009 Elsevier B.V. All rights reserved

Efficient non-resonant energy transfer in Nd3+ - Yb3+ codoped Ba-Al-metaphosphate glasses

An efficient Nd3+ ® Yb3+ energy transfer in a new series of alkali-free bariumalumino-metaphosphate glasses with a transfer efficiency reaching up to 95% has been reported here. It is due to the effective phonon assistance arising with the excellent matching of the present host phonon energy with the energy mismatch between Nd3+ (4F3/2) and Yb3+ (2F5/2) excited levels. The energy transfer microparameters for Nd3+ ® Yb3+ forward and back energy transfer are estimated from the spectral data analysis. A parameter, ET (= Nd Yb DA C − / Nd Nd DA C − ) is proposed as a quantitative measure of sensitization ability per unit loss of the donor. The parameter is found to be highest for the presently reported barium-alumino-metaphosphate glasses

Luminescence properties of dual valence Eu doped nano-crystalline BaF2 embedded glass-ceramics and observation of Eu2+ -> Eu3+ energy transfer

Europium doped glass ceramics containing BaF2 nano-crystals have been prepared by using the controlled crystallization of melt-quenched glasses. X-ray diffraction and transmission electron microscopy have confirmed the presence of cubic BaF2 nano-crystalline phase in glass matrix in the ceramized samples. Incorporation of rare earth ions into the formed crystalline phase having low phonon energy of 346 cm-1 has been demonstrated from the emission spectra of Eu3+ ions showing the transitions from upper excitation states 5DJ (J = 1, 2, and 3) to ground states for the glass-ceramics samples. The presence of divalent europium ions in glass and glass-ceramics samples is confirmed from the dominant blue emission corresponding to its 5d-4f transition under an excitation of 300 nm. Increase in the reduction of trivalent europium (Eu3+) ions to divalent (Eu2+) with the extent of ceramization is explained by charge compensation model based on substitution defect mechanisms. Further, the phenomenon of energy transfer from Eu2+ to Eu3+ ion by radiative trapping or reabsorption is evidenced which increases with the degree of ceramization. For the first time, the reduction of Eu3+ to Eu2+ under normal air atmospheric condition has been observed in a BaF2 containing oxyfluoride glass-ceramics system

Thermolysis-Driven Growth of Vanadium Oxide Nanostructures Revealed by In Situ Transmission Electron Microscopy: Implications for Battery Applications

Understanding the growth modes of 2D transition-metal oxides through direct observation is of vital importance to tailor these materials to desired structures. Here, we demonstrate thermolysis-driven growth of 2D V2O5 nanostructures via in situ transmission electron microscopy (TEM). Various growth stages in the formation of 2D V2O5 nanostructures through thermal decomposition of a single solid-state NH4VO3 precursor are unveiled during the in situ TEM heating. Growth of orthorhombic V2O5 2D nanosheets and 1D nanobelts is observed in real time. The associated temperature ranges in thermolysis-driven growth of V2O5 nanostructures are optimized through in situ and ex situ heating. Also, the phase transformation of V2O5 to VO2 was revealed in real time by in situ TEM heating. The in situ thermolysis results were reproduced using ex situ heating, which offers opportunities for upscaling the growth of vanadium oxide-based materials. Our findings offer effective, general, and simple pathways to produce versatile 2D V2O5 nanostructures for a range of battery applications

Time resolved spectra and energy transfer analysis of Nd3+-Yb3+-Er3+ triply-doped Ba-Al-metaphosphate glasses for an eye safe emission (1.54 μm)

This paper reports on the development and systematic analysis of energy transfer mechanisms in Nd3+-Yb3+-Er3+ co-doped new series of barium-alumino-metaphosphate glasses. The time resolved fluorescence spectra of Nd3+ in triply doped Ba-Almetaphosaphte glasses have revealed that, Yb3+ ions could function as quite efficient bridge for an energy transfer between Nd3+ and Er3+ ions. As a result, a fourfold emission enhancement at 1.54 μm of Er3+ ions has been achieved through an excitation of 4F5/2 level of Nd3+ at 806 nm for the glass having 3 mol% Yb3+ with an energy transfer efficiency reaching up to 94%. Decay of donor (Nd3+) ion fluorescence has been analyzed based on theoretical models such as Inokuti-Hirayama, Burshtein (migration) and Yokota-Tanimoto (diffusion) and corresponding energy transfer parameters have been discussed. Primarily, electrostatic dipole-dipole (s ~ 6) interactions are found to be responsible for the occurrence of energy transfer process in theses glasses

Temperature quenching of Cr3+ in ASc(Si1-xGex)2O6 (A=Li/Na) solid solutions

Blue absorbing near infrared (NIR) emitting phosphors are a promising class of materials for phosphor converted NIR LEDs, which can be used in compact NIR spectrometers. Preferably, these phosphors have a broad emission spectrum and show negligible luminescence quenching at LED operating temperatures (100 °C). Here, we investigated ASc(Si1-xGex)2O6 (A = Li/Na, x = 0,0.2,0.4,0.6,0.8,1) solid solutions doped with Cr3+ to tune and optimize the emission maximum and bandwidth to cover the full 700–1100 nm range. With increasing Ge content an emission redshift was observed, along with emission band broadening at intermediate Ge/Si ratio, which is explained by disorder around Cr3+ in the second coordination sphere (mixed Si/Ge). Temperature dependent emission spectra and luminescence decay curves were measured between 90 K and 670 K to determine the quenching temperature TQ. With increasing Ge content TQ drops from 550 K to below 400 K. Interestingly, Cr3+ emission in the highly symmetric site in LiScSi2O6 shows a strongly temperature dependent lifetime before thermal quenching sets in. DFT calculations on LiScSi2O6 indicate that asymmetric vibrations at the Sc site are involved and calculated phonon energies were confirmed by measuring FTIR. Our study indicates that a solid solution is a promising way to increase the emission bandwidth. However, with increasing Ge content TQ decreases. An optimum Ge-content in LiSc(Si1-xGex)2O6:Cr3+ is x = 0.2–0.4 as it redshifts the NIR band maximum close to 900 nm and offers a FWHM bandwidth around 180 nm, while keeping the thermal quenching temperature high enough for application in NIR-LEDs

Luminescence Temperature Quenching in Mn2+ Phosphors

Narrower band red and green emission in phosphor-converted white light-emitting diodes (wLEDs) can improve the efficacy and color gamut in lighting and display applications. A promising luminescent ion is Mn2+ that can have both narrowband green (tetrahedral coordination) and red (octahedral coordination) emission. Unlike in earlier lighting applications of Mn2+ phosphors, temperature quenching is important in wLEDs. Insight into the thermal quenching behavior of Mn2+ luminescence is lacking. Here systematic research is reported for a variety of Mn2+-doped phosphors; a huge variation in the luminescence quenching temperature T50, ranging from 50 K for Mn2+ in ZnTe to 1200 K in MgAl2O4, is revealed. The value T50 shows a positive correlation with the bandgap of the host, but no correlation with the full width half maximum (FWHM) of the emission band, indicating that thermally activated photoionization, not thermal crossover, is the operative quenching mechanism. This is confirmed by thermally stimulated luminescence (TSL) measurements that show a rise in TSL signal following photoexcitation at temperatures around T50 providing evidence that quenching is correlated with generation of free charge carriers. Based on these findings, as a design rule is obtained that for temperature-stable Mn2+ luminescence in (high power) LEDs a wide-bandgap host material is required

Energy transfer based efficient infrared emission at 1.57 mu m from Yb3+-Er3+ codoped Zinc-borobismuthate glasses

An intense NIR emission at 1.57 mu m is reported in a newly synthesized Yb3+-Er3+ codoped Zinc-borobismuthate glasses. The energy transfer efficiency is found to be 78% for 1 mol% Yb2O3 and 0.5 mol% Er2O3 concentrations. The transfer process follows a static energy transfer mechanism at a low donor concentration (0.1 mol%); whereas, it gets accelerated by donor energy migration at 1 mol% Yb2O3. In this glass system, the energy back-transfer has found to be negligible from estimated critical interaction distances for direct (Yb3+ -> Er3+) and back (Yb3+ <- Er3+) energy transfer phenomena. The observed efficient NIR emission at 1.57 mu m has been explained based on the effective energy transfer from Yb3+ ions accompanied by rapid depopulation of I-4(11/2) excited level of Er3+ ions through non-radiative multi-phonon relaxation processes, which has also restricted the excited state absorption (ESA) from this level leading to no upconversion suggesting the potentiality of present glass host for proficient NIR luminescence. (c) 2012 Elsevier B.V. All rights reserved