Evaluation of rare earth oxides doping SnO2.(Co1/4,Mn3/4)O-based varistor system

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Influence of the rare-earths oxides doped on the SnO2CoOMnO2Ta2O5 varistor system

The tin dioxide is an n-type semiconductor, which exhibits varistor behavior with high capacity of absorption of energy, whose function is to restrict transitory over-voltages without being destroyed, when it is doped with some oxides. Varistors are used in alternated current fields as well as in continuous current, and it can be applied in great interval of voltages or in great interval of currents. The electric properties of the varistor depend on the defects that happen at the grain boundaries and the adsorption of oxygen. The (98.90-x)%SnO2.0.25%CoO+0.75%MnO2+0.05%Ta2O5+0.05%Tr2O3 systems, in which Tr=La or Nd. Current-voltage measurements were accomplished for determination of the non-linear coefficient were studied. SEM microstructure analysis was made to evaluate the microstructural characteristics of the systems. The results showed that the rare-earth oxides have influenced the electrical behavior presented by the system. (C) 2002 Kluwer Academic Publishers.

Rare earth centers properties and electron trapping in SnO2 thin films produced by sol-gel route

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Dielectric response features and oxygen migration on rare earth modified lead titanate ferroelectric ceramics

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Hydrothermal rare earth elements mineralization in the Barra do Itapirapua carbonatite, southern Brazil: behaviour of selected trace elements and stable isotopes (C, O)

The Bura do Itapira pua carbonatite is located in southern Brazil and belongs to the Cretaceous Ponta Grossa alkaline-carbonatitic province related to the opening of the South Atlantic. The carbonatite complex is emplaced in Proterozoic granites and is mainly composed of plutonic magnesio- to ferrocarbonatite, with smaller amounts of subvolcanic magnesiocarbonatite. Hydrothermal alteration of the carbonatite has led to the formation of quartz, apatite, fluorite, rue earth fluorocarbonates, barite and sulfides in variable proportions. Trace element data, delta(13)C and delta(18)O are presented here, with the aim of better understanding the geochemical nature of hydrothermal alteration related to rare earth elements (REE) mineralization. The non-overprinted plutonic carbonatite shows the lowest REE contents, and its primitive carbon and oxygen stable isotopic composition places it in the field of primary igneous carbonatites. Two types of hydrothermally overprinted plutonic carbonatites can be distinguished based on secondary minerals and geochemical composition. Type I contains mainly quartz, rare earth fluorocarbonates and apatite as hydrothermal secondary minerals, and has steep chondrite normalized REE patterns, with Sigma(REE+Y) of up to 3 wt.% (i.e., two orders of magnitude higher than in fresh plutonic samples). In contrast, the Type II overprint contains apatite, fluorite and barite as dominant hydrothermal minerals, and is characterized by heavy REE enrichment relative to the fresh samples, with flat chondrite normalized REE patterns. Carbon and oxygen stable isotope ratios of Types I and II are elevated (delta(18)O + 8 to + 12 parts per thousand; delta(13)C - 6 to - 2 parts per thousand) relative to the fresh samples. Hydrothermally overprinted carbonatites exposed to weathering show even higher delta(18)O values (delta(18)O 13 to 25 parts per thousand) but no additional REE enrichment. The subvolcanic carbonatite has anomalously high delta(13)C of up to + 1 parts per thousand, which suggests crustal contamination through interaction with carbonate-bearing metasediments. (C) 1999 Elsevier B.V. B.V. All rights reserved.

Biosorption and desorption of lanthanum(III) and neodymium(III) in fixed-bed columns with Sargassum sp.: Perspectives for separation of rare earth metals

Rare earth (RE) metals are essentials for the manufacturing of high-technology products. The separation of RE is complex and expensive; biosorption is an alternative to conventional processes. This work focuses on the biosorption of monocomponent and bicomponent solutions of lanthanum(III) and neodymium(III) in fixed-bed columns using Sargassum sp. biomass. The desorption of metals with HCl 0.10 mol L-1 from loaded biomass is also carried out with the objective of increasing the efficiency of metal separation. Simple models have been successfully used to model breakthrough curves (i.e., Thomas, Bohart-Adams, and Yoon-Nelson equations) for the biosorption of monocomponent solutions. From biosorption and desorption experiments in both monocomponent and bicomponent solutions, a slight selectivity of the biomass for Nd(III) over La(III) is observed. The experiments did not find an effective separation of the RE studied, but their results indicate a possible partition between the metals, which is the fundamental condition for separation perspectives. (C) 2012 American Institute of Chemical Engineers Biotechnol. Prog., 2012

Influence of rare earth doping on the structural and catalytic properties of nanostructured tin oxide

Nanoparticles of tin oxide, doped with Ce and Y, were prepared using the polymeric precursor method. The structural variations of the tin oxide nanoparticles were characterized by means of nitrogen physisorption, carbon dioxide chemisorption, X-ray diffraction, and X-ray photoelectron spectroscopy. The synthesized samples, undoped and doped with the rare earths, were used to promote the ethanol steam reforming reaction. The SnO2-based nanoparticles were shown to be active catalysts for the ethanol steam reforming. The surface properties, such as surface area, basicity/base strength distribution, and catalytic activity/selectivity, were influenced by the rare earth doping of SnO2 and also by the annealing temperatures. Doping led to chemical and micro-structural variations at the surface of the SnO2 particles. Changes in the catalytic properties of the samples, such as selectivity toward ethylene, may be ascribed to different dopings and annealing temperatures.

Local order around rare earth ions during the devitrification of oxyfluoride glasses

Erbium L-3-edge extended x-ray absorption fine structure (EXAFS) measurements were performed on rare earth doped fluorosilicate and fluoroborate glasses and glass ceramics. The well known nucleating effects of erbium ions for the crystallization of cubic lead fluoride (based on x-ray diffraction measurements) and the fact that the rare earth ions are present in the crystalline phase (as indicated by Er3+ emission spectra) seem in contradiction with the present EXAFS analysis, which indicates a lack of medium range structural ordering around the Er3+ ions and suggests that the lead fluoride crystallization does not occur in the nearest neighbor distance of the rare earth ion. Molecular dynamics simulations of the devitrification process of a lead fluoride glass doped with Er3+ ions were performed, and results indicate that Er3+ ions lower the devitrification temperature of PbF2, in good agreement with the experimental results. The genuine role of Er3+ ions in the devitrification process of PbF2 has been investigated. Although Er3+ ions could indeed act as seeds for crystallization, as experiments suggest, molecular dynamics simulation results corroborate the experimental EXAFS observation that the devitrification does not occur at its nearest neighbor distance. (c) 2008 American Institute of Physics.

Ligand-rare-earth ion energy transfer in coordination compounds. A theoretical approach

A theoretical approach to the energy transfer process that occurs between a ligand and a rare-earth ion in luminescent complexes is presented. A discussion on the energy transfer mechanisms involved and on the associated selection rules is made. Numerical estimates are also presented.

Luminescent properties of some rare earth phosphates

Luminescent properties of scandium and yttrium phosphates are discussed and mechanisms involving their emissions proposed.

Effects of synthesis and processing on supersaturated rare earth-doped nanometric SnO2 powders

This paper presents and discusses some of the results of the effects of processing on rare earth-doped nanosize SnO2. Several relevant factors that may influence the characteristics of the final product are studied. The influence of two preparation routes and two heat-treatment conditions on the incorporation of dopants is investigated. The route whereby a soluble salt is used as the dopant source is found to provide the highest degree of dopant incorporation, even under the least favorable heat-treatment conditions.

Clustering of rare earth in glasses, aluminum effect: experiments and modeling

Luminescent spectra of Eu3+-doped sol-gel glasses have been analyzed during the densification process and compared according to the presence or not of aluminum as a codoping ion. A transition temperature from hydrated to dehydroxyled environments has been found different for doped and codoped samples. However, only slight modifications have been displayed from luminescence measurements beyond this transition. To support the experimental analysis, molecular dynamics simulations have been performed to model the doped and codoped glass structures. Despite no evidence of rare earth clustering reduction due to aluminum has been found, the modeled structures have shown that the luminescent ions are mainly located in aluminum-rich domains. The synthesis of both experimental and numerical analyses has lead us to interpret the aluminum effect as responsible for differences in structure of the luminescent sites rather than for an effective dispersion of the rare earth ions. (C) 2004 Elsevier B.V. All rights reserved.

Precipitation of rare earth phosphates from H3PO4 solutions

A study of several factors has been carried out in order to determine their influence on rare earth phosphates precipitation from H3PO4 solutions obtained after the treatment of the Kola phosphate rock.

Rare earth doped synthetic opals and inverse opals

Monodisperse spheres of silica and latex were obtained by a surfactant free styrene polimerization and the Stober method respectively. Controlling settling either by centrifugation or by dip-coating colloidal crystals could be obtained. Silica inverse opals were prepared by using the latex colloidal crystals as templates and TEOS/ethanol solution. Eu3+ containing silica spheres were obtained dispersing silica spheres in Eu(NO3)(3) isopropanol solutions. Emission spectra suggest the formation of an amorphous Eu3+ containing phase well adhered at the spheres surface. The utilization of solutions of trifluoroacetates salts of Pb2+ and Eu3+ was observed to destroy the silica spherical pattern when samples are treated at 1000degreesC. In that case nanocrystals of PbF2 and amorphous silica were obtained after heat treatment.

Rare Earth Doped SnO(2) Nanoscaled Powders and Coatings: Enhanced Photoluminescence in Water and Waveguiding Properties

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