Co-doped Bi Research Articles

Energy transfer and up-conversion in Bi 2TeO 5 crystals co-doped with Yb 3+ and Tm 3+

Yb + Tm co-doped Bi 2TeO 5 single crystals were grown by the Czochralski method. Optical absorption and luminescence spectra of the crystals were determined and interpreted. The dynamics of the Tm and Yb excited states were followed by luminescence decay curves. Up-conversion processes were detected after 980 nm diode laser excitation, that resulted in an 800 nm and a weaker 475 nm emission corresponding to the 3H 4 → 3H 6 and 1G 4 → 3H 6 transitions of Tm 3+. The excitation power dependence of the integrated intensity of the up-converted emission implied two-step excitation process including Yb–Tm energy transfer for the 800 nm emission, and three-photon excitation process for the 475 nm up-converted emission. Examination of absorption and emission spectra of Yb and Tm ions revealed that the consecutive energy transfer steps did not fulfill the resonance conditions, and they have to be assisted by phonon emission.

Effect of WO 3 on the spectroscopic properties in Er 3+/Yb 3+ co-doped bismuth–borate glasses

The spectroscopic properties of Er 3+/Yb 3+ co-doped Bi 2O 3–B 2O 3–WO 3 (BBW) glasses were analyzed and discussed. The effect of WO 3 content on the absorption spectra, the Judd–Ofelt parameters Ω t ( t=2, 4, 6), emission spectra and the lifetime of the 4I 13/2 level and the quantum efficiency of Er 3+: 4I 13/2→ 4I 15/2 transition were also investigated. With the substitution of WO 3 for B 2O 3, the measured lifetime of the 4I 13/2 level and the quantum efficiency of Er 3+: 4I 13/2→ 4I 15/2 transition increase from 0.98 to 1.31 ms and from 38.2% to 49.2%, respectively. The effective width of emission band and the emission cross-section both decrease slightly. And the emission spectra is analyzed via the different curve ( σ e− σ a) of BBW glasses, the influence of OH − is also discussed.

Transient behaviour of dense catalytic membranes based on Cu- and Co-doped Bi 4V 2O 11 (BIMEVOX) in the oxidation of propene and propane

ME-doped γ-Bi 4V 2O 11 (BIMEVOX) oxides are highly oxide ion conducting materials and this property may be profitably used in selective oxidation of hydrocarbons. The catalytic properties of BICUVOX and BICOVOX when shaped as dense membranes displayed in catalytic dense membrane reactor are examined in the oxidation of propene and of propane. Mirror-polished BICUVOX and BICOVOX membranes studied previously were poorly active for propene oxidation because of a small number of active sites but showed an excellent stability and reproducibility (lasting more than 1 month) during which products of mild oxidation (acrolein, hexadiene) and CO were formed. Membranes with depolished surfaces exhibit high conversions of propene (up to 60 mol%), and also of propane (up to 20 mol%) but – contrary to mirror-polished membranes – a complex transient behaviour is observed during which syngas production occurs. The membrane polarisation followed by in situ solid electrolyte potentiometry shows that the oxygen reservoir is far higher than expected on the reaction side which is separated (by the membrane) from the oxidising side where (diluted) oxygen is reduced to O 2− specie. The influence of oxygen partial pressure on the catalytic performance suggests that the electronic conductivity of the material is limiting the oxygen flux through the membrane, and thus is determining the catalytic properties and transient behaviours of depolished membranes.

EFFECT OF THE Fe AND Zn CODOPED ON THE INTERGRANULAR PROPERTIES FOR (Bi1.6Pb0.4)(Sr1.8Ba0.2)Ca2(Cu1-x-yZnxFey)3Oz SUPERCONDUCTING SYSTEM

Systematic measurements of the AC susceptibility as a function of temperature and magnetic field amplitude H ac for the Fe and Zn codoped ( Bi 1.6 Pb 0.4)( Sr 1.8 Ba 0.2) Ca 2( Cu 1-x-y Zn x Fe y)3 O z (x = 0.00;0.02 and y = 0.00;0.01) system are reported. Above H ac = 200 A/m, the linear dependence of T p for maximum of imaginary χ′′(T) peak as a function of AC field amplitude agree with Muller critical state model. The intergrain pinning force density decreases in doped and codoped samples.

Co-doped Bi 2Sr 2CaCu 2O x: Anderson localization and superconductivity

Cobalt-doping of Bi 2Sr 2CaCu 2O x beyond a critical level of 1.5–1.7 at % produces both a large decrease in the superconducting transition critical temperature and all of the classic symptoms of Anderson localization. These include: a negative slope of the ab-plane resistivity vs temperature, no dispersing states near the Fermi level and a decrease of the photoemission signal in the same region.

Co-doped Bi 2Sr 2CaCu 2O x: Anderson localization and superconductivity

Cobalt-doping of Bi 2Sr 2CaCu 2O x beyond a critical level of 1.5–1.6 at % produces both a large decrease in the superconducting transition critical temperature and all of the classic symptoms of Anderson localization. These include: a negative slope of the abplane resistivity vs temperature, no dispersing states near the Fermi level and a decrease of the photoemission signal in the same region. The correlation between Anderson localization and high-temperature superconductivity at low doping levels supports a non-local interaction for the paring mechanism, such as antiferromagnetic spin fluctuations.

Traps and recombination centers in pure and Co-doped Bi 12SiO 20 crystals

Pure and Co-doped (0.36-1.8 mol% Co) Bi 12SiO 20 crystals have been studied. The temperature dependence of the dark and photoconductivity, thermostimulated currents and the spectral dependence of the photoconductivity have been measured. Exponentially distributed traps situated close to the conduction band bottom have been found in pure BSO crystals. It was found that Co atoms act as donors in BSO crystals, and two new traps at about 0.24 and 0.45 eV appear. Based on the observed thermal quenching of the photocurrent an identical energy position of the “slow” recombination centers has been determined in both type of crystals. The defects responsible for the “shoulder” absorption (at about 2.7 eV below the conduction band) have been found to play the role of these “slow“ recombination centers.