Influence of interstitial UO22+ doping on the valence control of Eu and energy transfer to substitutional Eu3+ and Sm3+ in SrBPO5.

Controlling the valence mixing of Eu3+/Eu2+ and energy transfer between activator ions in solid solution is an important process to improve the efficiency and specificity of phosphors. In this work, the structural and optical properties of stillwellite type SrBPO5 doped with uranium/europium/samarium, as prepared by conventional solid-state reaction synthesis, were investigated. PXRD studies and Rietveld analysis were carried out to determine the structure, phase purity, and coordination environment of the dopants in the host matrix. Samarium existed only as a trivalent cation in SrBPO5 synthesised in an air atmosphere, whereas europium exhibited abnormal reduction and Eu2+ co-existed with Eu3+. Unlike Eu and Sm, which replaced Sr at its sites, uranium gets stabilised as UO22+ in the interstitial vacant space in the SrBPO5 lattice. Uranium co-doping strongly influenced the Eu valence distribution by favouring the re-oxidation of Eu2+ to Eu3+. Possible mechanisms of Eu abnormal reduction in SrBPO5 and Eu valence control based on the electron transfer from substitutional Eu2+ to interstitial UO22+ are discussed. The photoluminescence and time-resolved photoluminescence properties of Eu/Sm/U doped SrBPO5 were investigated systematically. Uranium co-doping significantly enhanced the emission intensities of trivalent Eu and Sm through the exchange-type energy transfer. Besides, the Eu3+ luminescence intensity was further amplified in the presence of uranium due to the partial oxidation of Eu2+ to Eu3+. By controlling the uranium to Eu concentration ratio, the Eu-SrBPO5 phosphor could be tuned to different CIE coordinates.

SrBPO5: Ce3+, Dy3+ - A cold white-light emitting phosphor.

A single-component white-light emitting phosphor SrBPO5: Ce3+, Dy3+ with high color purity, good quantum efficiency and high thermal stability was prepared through the conventional high temperature solid state reaction. PXRD studies confirmed its phase purity. The suitability of Strontium borophosphate as a host for phosphor was confirmed through DFT calculations. The presence of Ce3+ along with Dy3+ in this host resulted in efficient energy transfer from Ce3+ to Dy3+ through a non-radiative multipole-multipole mechanism leading to the enhancement of Dy3+ luminescence towards white light emission. Lifetime decay and time-resolved emission studies confirmed the energy transfer along with multisite occupancy of Ce3+. In addition to energy transfer, the thermal stability of the phosphor was confirmed through temperature-dependent photoluminescence studies and the particle shape, size, uniformity of dopants of the phosphor was studied using the SEM and EDX spectroscopy.

Probing the optical properties and luminescence mechanism of a UV-emitting SrBPO5 :Ce3+ phosphor.

Ce-doped (1 × 10-5 to 3.0 mol%) SrBPO5 phosphors were synthesized using a conventional solid-state reaction route at 1273 K in an air atmosphere. Phase and morphology of the samples were studied from powder X-ray diffraction (XRD) patterns and scanning electron microscope (SEM) micrographs, respectively. The band gap energies of the pure and Ce-doped SrBPO5 phosphors were calculated from the recorded diffuse reflectance spectra. Photoluminescence (PL) and Ce3+ lifetime were recorded at 300 and 77 K. Photoluminescence lifetime measurements revealed two-lifetime values for Ce3+ at both 300 K (17 and 36 nsec) and 77 K (12 and 30 nsec), suggesting the presence of two different environments around Ce3+ . Time-resolved emission spectroscopy (TRES) studies confirmed the presence of Ce3+ in two different environments. In addition, SrBPO5 :Ce exhibited intense UV emission, signifying its possible use as an efficient sensitizer for solid-state lighting applications. The effect of γ-irradiation on PL was also determined. Thermally stimulated luminescence (TSL) glow curves of the γ-irradiated phosphor, along with trap parameters, dose-response, and the possible TSL mechanism were also investigated. Positron annihilation lifetime spectroscopy was carried out to probe defects present in undoped and Ce-doped SrBPO5 .

Dosimetric investigations of Tb3+-doped strontium silicate phosphor.

Tb(3+)-doped SrSiO(3) phosphor synthesised by co-precipitation technique exhibits intense green emission due to cross-relaxation phenomena between Tb(3+) ions. Dosimetric properties of this phosphor have been investigated using thermoluminescence (TL) technique. A dosimetrically useful glow peak observed was at 581 K along with a linear dose response over the wide dose range (100 mGy-4 Gy). TL parameters such as trap depth (E), frequency factor (s) and the order of kinetics (b) are determined by different methods such as Chen's peak shape, initial rise, isothermal decay and variable heating rate methods. Results of these methods are compared and reported in this study.

Biofouling and microbial corrosion problem in the thermo-fluid heat exchanger and cooling water system of a nuclear test reactor.

This article discusses aspects of biofouling and corrosion in the thermo-fluid heat exchanger (TFHX) and in the cooling water system of a nuclear test reactor. During inspection, it was observed that >90% of the TFHX tube bundle was clogged with thick fouling deposits. Both X-ray diffraction and Mossbauer analyses of the fouling deposit demonstrated iron corrosion products. The exterior of the tubercle showed the presence of a calcium and magnesium carbonate mixture along with iron oxides. Raman spectroscopy analysis confirmed the presence of calcium carbonate scale in the calcite phase. The interior of the tubercle contained significant iron sulphide, magnetite and iron-oxy-hydroxide. A microbiological assay showed a considerable population of iron oxidizing bacteria and sulphate reducing bacteria (10(5) to 10(6) cfu g(-1) of deposit). As the temperature of the TFHX is in the range of 45-50 degrees C, the microbiota isolated/assayed from the fouling deposit are designated as thermo-tolerant bacteria. The mean corrosion rate of the CS coupons exposed online was approximately 2.0 mpy and the microbial counts of various corrosion causing bacteria were in the range 10(3) to 10(5) cfu ml(-1) in the cooling water and 10(6) to 10(8) cfu ml(-1) in the biofilm.

Fluorimetric estimation of DNA content using sensitized lanthanide fluorescence.

Lanthanide fluorescence enhancement on complexation with calf thymus DNA was studied in aqueous solution. The DNA sensitized and enhanced fluorescence of terbium and europium by nearly two orders of magnitude. By applying this ligand sensitized lanthanide fluorescence enhancement, DNA could be estimated at 10 ppb level. Further, effect of addition of TOPO in Triton X-100 micellar medium to Tb-DNA complex in solution was also studied. On addition of TOPO, no synergistic terbium fluorescence enhancement was observed.

Synergistic fluorescence enhancement of Tb with aromatic monocarboxylic acids and TOPO-Triton X-100: role of Triton X-100 in synergism.

Fluorescence enhancement of Tb3+ has been studied, using substituted benzoic acids as ligands. These ligands enhance the fluorescence of Tb3+ by about three orders of magnitude. The enhancement is observed due to ligand sensitized fluorescence. The fluorescence of Tb3+ in these complexes is synergistically enhanced by the addition of trioctylphosphine oxide (TOPO) in Triton X-100. The synergism displayed by TOPO was observed only when Triton X-100 micellar medium was used. TOPO dissolved in SDS/DTAB micellar medium did not result in synergistic fluorescence enhancement of Tb3+. These observations are discussed in detail and the synergism displayed by TOPO is rationalized on the basis of the structure of the surfactants.