Dependence of La2O3 content on the nonlinear electrical behaviour of ZnO, CoO and Ta2O5 doped SnO2 varistors

The effects of La2O3 on the properties of (Zn, Co, Ta) doped SnO2 varistors were investigated in this study. The samples with different La2O3 concentrations were sintered at 1400 degrees C for 2 h and their properties were characterized by XRD, SEM, I-V and impedance spectroscopy. The grain size was found to decrease from 13 pm to 9 gm with increasing La2O3 content. The addition of rare earth element leads to increase the nonlinear coefficient and the breakdown voltage. The enhancement was expected to arise from the possible segregation of lanthanide ion due to its larger ionic radius to the grain boundaries, thereby modifying its electrical characteristics. Furthermore, the dopants such as La may help in the adsorption of O' to O '' at the grain boundaries characteristics. (c) 2006 Elsevier B.V. All rights reserved.

The hydrogen evolution reaction on codeposited Ni-hydrogen storage intermetallic particles in alkaline medium

The hydrogen evolution reaction (HER) was studied on Ni-LaNi5 and Ni-MmNi(3.4)Co(0.8)Al(0.8) electrode materials in 1 mol dm(-3) NaOH solution. The steady-state polarization curves and electrochemical impedance spectroscopy experimental data showed a pronounced improvement in HER kinetics when these electrode materials were used. The electrochemical results are in accordance with the Volmer-Heyrovsky mechanism. The kinetic results indicate a more effective improvement in the Heyrovsky step, suggesting an electrocatalytic synergistic effect of the hyper-electronic character of the Ni and the hypo-electronic character of the rare-earth element on the electrode surface. (C) 2000 International Association for Hydrogen Energy. Published by Elsevier B.V. Ltd. All rights reserved.

Red-green-blue upconversion emission and energy-transfer between Tm3+ and Er3+ ions in tellurite glasses excited at 1.064 mu m

Red, green, and blue emission through frequency upconversion and energy-transfer processes in tellurite glasses doped with Tm3+ and Er3+ excited at 1.064 mum is investigated. The Tm3+/Er3+-codoped samples produced intense upconversion emission signals at around 480, 530, 550 and 660 nm. The 480 nm blue emission was originated from the (1)G(4)-->H-3(6) transition of the Tm3+ ions excited by a multiphoton stepwise phonon-assisted excited-state absorption process. The 5 30, 5 50 nm green and 660 mn red upconversion luminescences were identified as originating from the H-2(11/2), S-4(3/2) --> I-4(15/2) and F-4(9/2) --> I-4(15/2) transitions of the Er3+ ions, respectively, populated via efficient cross-relaxation processes and excited-state absorption. White light generation employing a single infrared excitation source is also examined. (C) 2003 Elsevier B.V. (USA). All rights reserved.

Biosotption of lanthanum using Sargassum fluitans in batch system

Separation and purification of lanthanum from other rare-earth (RE) elements are highly complex processes comprising several steps of extraction using organic solvents or ion-exchange resins at high costs. In order to study the biosorption process as an alternative for conventional lanthanum recovery, this work investigated some basic aspects of lanthanum-Sargassum biomass interactions in batch equilibrium contact. The dynamics of biosorption, influence of pH, and the desorption of this RE were investigated. Maximum biosorption coefficient (q(max)) increased from 0.05 at pH 2 to 0.53 mmol g(-1) at pH 5 for lanthanum sulfate. When lanthanum chloride was used, a higher q(max) at pH 5 (0.73 mmol g(-1)) was observed as compared to the sulfate salt (q(max) = 0.53 mmol g(-1)) at the same pH. Adsorption and desorption curves pointed out a complete recovery of metal adsorbed in the Sargassum fluitans biomass, showing a reversibility of this process and indicating the potential of biosorption for lanthanum removal and recovery. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Yttrium oxysulfide nanosized spherical particles doped with Yb and Er or Yb and Tm: efficient materials for up-converting phosphor technology field

This work reports on the preparation, structural and luminescent studies of nanosized up-converter phosphors Y2O2S:Yb(4%), Er(0.1%) and Y2O2S:Yb(4%), Tm(0.1%),both from polymeric and basic carbonate precursors. The precursors were submitted to a sulphuration process that was previously developed for oxysulfide preparation from basic carbonate. From XRD data, all phosphors presented the oxysulfide phase and the mean crystallite size estimated from the Scherrer formula in the range of 15-20 nm. Polymeric precursor leads to the smallest crystallite size independent on the doping ion. SEM and TEM results confirmed that basic carbonate leads to spherical particles with narrow size distribution and mean diameter of 150 nm, and polymeric precursor smaller spherical particles with diameter between 20 and 40 nm. Up-conversion studies under 980 nm laser excitation showed that Er-doped phosphors present strong green emission related to H-2(11/2), S-4(3/2) --> I-4(15/2) Er transitions as well as the red ones, F-4(9/2) --> I-4(15/2). Tm-doped samples show strong blue emission assigned to (1)G(4) --> H-3(6) and also the red ones, related to (1)G(4) --> F-3(4). Therefore, the sulphuration method was successfully applied to prepare nanosized and nanostructured blue and green up-converter oxysulfide phosphors starting from basic carbonate and polymeric precursors. (C) 2003 Elsevier B.V. All rights reserved.

Temperature investigation of infrared-to-visible frequency upconversion in erbium-doped tellurite glasses excited at 1540 nm

Temperature investigation of infrared-to-visible frequency upconversion in erbium-doped tellurite glasses excited by CW laser radiation at 1540 nm and under cryogenic temperatures is reported. Intense upconversion emission signals around 530, 550 and 660 nm corresponding to the H-2(11/2), S-4(3/2), and F-4(9/2) transitions to the I-4(15/2) ground state were generated and studied as a function of the laser intensity and temperature. The upconversion excitation mechanism of the Er3+ ions emitting energy levels was accomplished via stepwise multiphoton absorption. The green upconversion luminescence exhibited a fivefold intensity enhancement when the temperature of the sample was varied in the range between 5 and 300 K. A maximum green upconversion intensity was attained around 120 K and a steady decreasing behavior for higher temperatures up to 300 K was observed. A model based upon conventional rate equations was used to model the observed temperature evolution of the upconversion luminescence. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Infrared-to-visible frequency upconversion in Pr3+/Yb3+- and Er3+/Yb3+-codoped tellurite glasses

Upconversion luminescence and thermal effects in Pr3+/Yb3+- and Er3+/Yb3+-codoped 60TeO(2)-10GeO(2)-10K(2)O-10Li(2)O-10Nb(2)O(5) tellurite glasses excited by CW infrared radiation at 1.064 mum is reported. Generation of intense green and red fluorescence emission in Er3+/Yb3+-codoped samples and appreciable upconversion luminescence in the wavelength region of 450-680 nm in Pr3+/Yb3+-codoped samples is observed. Temperature-induced enhancement of X12 in the upconversion efficiency in Er3+/Yb3+- and X10 in the Pr3+/Yb3+-doped samples is demonstrated. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Red, green, and blue upconversion luminescence in ytterbium-sensitized praseodymium-doped lead-cadmium-germanate glass

Blue, green, red, and near-infrared upconversion luminescence in the wavelength region of 480-740 nm in Pr3+/Yb3+-codoped lead-cadmium-germanate glass under 980 nm diode laser excitation, is presented. Upconversion emission peaks around 485, 530, 610, 645, and 725 nm which were ascribed to the P-3(0)-H-3(J) (J = 4, 5, and 6), and P-3(0)-F-3(J) (J = 2, 3, and 4), transitions, respectively, were observed. The population of the praseodymium upper P-3(0) emitting level was accomplished through a combination of ground-state absorption of Yb3+ ions at the F-2(7/2), energy-transfer Yb3+(2F(5/2))-Pr3+(H-3(4)), and excited-state absorption of Pr3+ ions provoking the (1)G(4)-P-3(0) transition. The dependence of the upconversion luminescence upon the Yb3+-concentration and diode laser power, is also examined, in order to subsidize the proposed upconversion excitation mechanism. (C) 2004 Elsevier B,V. All rights reserved.

Thermally enhanced cooperative energy-transfer frequency upconversion in terbium and ytterbium doped tellurite glass

Infrared-to-visible frequency upconversion through cooperative energy-transfer and thermal effects in Tb3+/Yb3+-codoped tellurite glasses excited at 1.064 mum is investigated. Bright luminescence emission around 485, 550, 590, 625 and 65 nm, identified as due to the D-5(4) --> F-7(J) (J= 6, 5, 4, 3, and 2) transitions of the terbium ions, respectively, was recorded. The excitation of the D-5(4) emitting level of the Tb3+ ions is assigned to cooperative energy-transfer from pairs of ytterbium ions.. The effect of temperature on the upconversion process was examined and the results revealed a fourfold upconversion enhancement in the 300-500 K interval. The enhancement of the upconversion process is due to the temperature dependence of the Yb3+-sensitizer absorption cross-section under anti-Stokes excitation. A rate-equation. model using multiphonon-assisted absorption for the ytterbium excitation combined with the energy migration effect between Yb-Yb pair, and Tb3+ ground-state depopulation via multiphonon excitation of the F-7(J) excited states describes quite well the experimental results. (C) 2003 Elsevier B.V. B.V. All rights reserved.

Phosphate functionalization of spongiolite surface

Spongiolite from Mato Grosso do Sul (Brazil), natural inorganic composite constituted of silica needles, was treated with concentrated phosphoric acid at high temperatures. Superficial coating of the needles was proved to be constituted of silicon diphosphate, a compound offering six-coordinated silicon sites. Owing to the affinity of three charged ions to phosphate groups, this coating acts as specific adsorbent for the rare earth elements which prefer octahedral coordination (starting from samarium, yttrium included). The uptake of lanthanum and neodymium are significantly lower due to different coordination tendencies. Lanthanide fixation upon silica with PO4 groups anchored on its surface may be useful in the manufacturing of special phosphate-silicate glasses. (C) 2003 Elsevier B.V. All rights reserved.

Upconversion luminescence in transparent glass ceramics containing beta-PbF2 nanocrystals doped with erbium

Frequency upconversion luminescence in erbium-doped PbGeO3-PbF2-CdF2-based transparent glass ceramics (TGC) under 980 nm infrared excitation is investigated. Upconversion emission signals around 410, 525, 550, 660, and 850 nm were generated and identified as due to the H-2(9/2) H-2(11/2), S-4(3/2), and F-4(9/2) transitions to the I-4(15/2) ground-state, and S-4(3/2)-I-4(13/2), respectively. The erbium ions excited-state emitting levels were populated via a combination of stepwise ground-state absorption (GSA), excited-state absorption (ESA), and cross-relaxation processes. The results also disclosed that both blue (410 nm) and red (660 nm) upconversion emission signals in the transparent glass ceramic sample presented twice as much intensity as compared to its vitreous counterpart. (C) 2003 Elsevier B.V. All rights reserved.

Blue cooperative luminescence properties in Yb3+ doped GeO2-PbO-Bi2O3 vitreous system for the production of thin films

Germanate glasses are of interest for optoelectronic applications because they combine high mechanical strength, high chemical durability and temperature stability with a large transmission window (400 to 4500 nm) and high refractive index (2.0). GeO2-PbO-Bi2O3 glasses doped with Y-b(3+) were fabricated by melting powders in a crucible and then pouring them in a brass mold. Energy Dispersive Spectroscopy showed that the glass composition has a high spatial uniformity and that the Yb concentration in the solid sample is proportional to the Yb concentration in the melt, what was confirmed by absorption measurements. Intense blue emission at 507 nm was observed, corresponding to half of the wavelength of the near infrared region (NIR) emission; besides, a decay lifetime of 0.25 ms was measured and this corresponds to half of the decay lifetime in the infrared region; these are very strong indications of the presence of blue cooperative luminescence. Larger targets have been produced to be sputtered, resulting in thin films for three dimensional (3D) display and waveguide applications. (c) 2006 Elsevier B.V. All rights reserved.

Poole-Frenkel effect in Er doped SnO2 thin films deposited by sol-gel-dip-coating

Electrical properties of Er-doped SnO2 thin films obtained by sol-gel-dip-coating technique were measured. When compared to undoped tin dioxide, rare-earth doped films present much higher resistivity, indicating that Er3+ presents an acceptor-like character into the matrix, which leads to a high degree of electric charge compensation. Current-voltage characteristics, measured above room temperature for Er-doped films, lead to non-linear behavior and two conduction regimes. In the lower electric field range the conduction is dominated by Schottky emission over the grain boundary potential barrier, which presents an average value of 0.85 eV. Increasing the applied bias, a second regime of conduction is observed, since the Poole-Frenkel coulombic barrier lowering becomes a significant effect. The obtained activation energy for ionization is 0.67 eV. (C) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Energy-transfer frequency upconversion in neodymium-sensitized praseodymium-doped PbGeO3-PbF2-CdF2 glass excited at 810 nm

Visible frequency upconversion emission through resonant energy-transfer involving neodymium and praseodymium ions in PbGeO3-PbF2-CdF2 glass excited by a semiconductor laser at 8 10 nm is investigated. Luminescence emission centered around 485, 530, 610, and 645 nm, which correspond to the P-3(0) -> H-3(4), P-3(1) + I-1(6) -> H-3(5), P-3(0) -> H-3(6) and P-1(0) -> F-3(2) transitions of praseodymium ions, respectively, are observed. The upconversion excitation of the Pr3+ ions excited-state emitting levels was accomplished by means of an ion-pair interaction involving ground-state absorption, multiphonon relaxation, and excited-state absorption of pump photons at 8 10 nm by the Nd3+ (I-4(9/2) -> H-2(9/2), F-4(5/2); F-4(3/2) -> P-2(1/2)) and direct energy-transfer to Pr3+ ((4)G(11/2) + K-2(11/2), H-3(4) -> I-4(9/2), P-3(1) + I-1(6)). The dependence of the upconversion emission intensity upon the excitation power, and neodymium concentration are also examined. (c) 2004 Elsevier B.V. All rights reserved.

Optical characteristics of Er3+-Yb3+ doped SnO2 xerogels

Er3+ doped SnO2 xerogels have been obtained from aqueous colloidal suspensions. Emission and excitation spectra were obtained and allowed the identification of two main families of sites for Er3+. In the first one Er3+ substitutes for Sn4+ in the SnO2 cassiterite structure. In the second Er3+ are found adsorbed at the SnO2 particle surface. For the first family of sites the technological important infrared Er3+ emission about 1.5 mum is efficiently excited through absorption at the SnO2 conduction band at 3.8 eV. on the other hand the emission due to adsorbed ions appears inhomogeneously broadened by the statistical distribution of sites available for Er3+ ions at the surface of the particles. Moreover it is not excited by the host. The emission of this second family of sites could be also excited by an energy transfer mechanism involving Yb3+ ions also adsorbed a posteriori at particles surface. Results are compared with spectra obtained for Eu3+ doped samples. (C) 2002 Elsevier B.V. B.V. All rights reserved.