Synthesis and structural investigations on TeO2-PbF2-CdF2 glasses and transparent glass-ceramics

New glasses have been prepared in the oxifluoride mixed system TeO2-PbF2-CdF2. Starting from pure TeO2 the addition of the fluorides leads to a decrease in the glasses characteristic temperatures. Also from Raman scattering results a structural evolution was observed where the number of structural units described as [TeO3] trigonal pyramids and [TeO3+1] polyhedra increases at the expense of the [TeO4] trigonal bipyramids supposed to exist in the TeO2, rich samples. Transparent glass ceramics were obtained from the glass with composition 80TeO(2)-10PbF(2)-10CdF(2), (mol%) with the PbTe3O7 crystalline phase being identified by X-ray diffraction and EXAFS measurements performed at the Te K, Cd K and Pb L-III edges. Also from Exafs measurements it is proposed that cadmium ions are preferentially surrounded by oxygen atoms although they were in a fluoride anion environment in the starting material. (C) 2002 Published by Elsevier B.V. Ltd.

Lead-cadmium oxyfluoride glasses and glass-ceramics

Glasses and glass-ceramics have been obtained in oxyfluoride systems involving lead and cadmium fluorides and one of the well-known glass former oxides SiO2, B2O3 and TeO2. Vitreous domains were established and a wide range of compositions including high heavy metal contents lead to stable glasses. Amorphous structures have been studied by short-range order spectroscopy techniques (Raman scattering and x-ray absorption) and molecular basic structures have been identified. Besides the usual oxides, the role of glass former could also be proposed for cadmium ions. Special attention has been paid for crystallization process. Cubic lead fluoride, cubic lead tellurite, tetragonal tellurium oxide and a solid solution of the type Pb1-xCdxF2 are obtained as crystallization products depending on the composition and temperature of heat treatments. Pb1-xCdxF2 solid solutions are well known superionic materials and obtaining this solid solution as a crystal phase could be very interesting for applications concerning ionic electrical conduction properties. The addition of rare earth ions led to the control of the crystallization process. In the presence of the nucleating ion only the cubic form beta-PbF2 was identified. Rare earth ions are present in the crystal phase and crystal-like spectroscopic properties were observed suggesting interesting applications for these perfectly transparent glass ceramics in photonics.

OXIDES AS HETEROGENEOUS PROMOTERS FOR LIQUID-PHASE HYDROCARBONYLATION REACTIONS WITH IODOCARBONYLRUTHENIUM CATALYSTS

Basic or acid oxides, used as heterogeneous promoters of carbonylruthenium catalysts in liquid-phase hydrocarbonylation reactions on oxygenated substrates, strongly affect the activity and selectivity of the catalytic system. Concurrent or successive reactions of simple carbonylation, homologation, hydrogenation to hydrocarbons, and etherification take place to varying extents. Carbonylation and etherification are favored by acid oxides and homologation and hydrogenation by basic oxides. This behavior is related to the formation and stabilization by the oxides of H+ and H- hydridocarbonylruthenium catalytic species, whose relative concentrations in solution depend on the nature of the oxide. Heterogeneous oxides are easily separated and recycled from the reaction mixture. Their use simplifies the catalytic system and allows one to direct the catalytic process toward the target product.

Intense red upconversion emission in infrared excited holmium-doped PbGeO3-PbF2-CdF2 transparent glass ceramic

Intense red upconversion emission around 650 nm in PbGeO3-PbF2-CdF2 transparent glass ceramic containing beta-PbF2:Ho3+ nanocrystals, is presented. The holmium-doped vitroceramic samples were excited by a 980 nm diode laser source. The 650 nm upconversion signal was assigned to the F-5(5) --> I-5(8) transition of holmium ions. Very low intensity signals around 490 and 540 nm corresponding to the F-5(2,3) --> I-5(8) and S-4(2), F-5(4) --> I-5(8) transitions, respectively, were also detected. The upconversion excitation mechanism was achieved through a combination of stepwise phonon-assisted multiphoton absorption, cross-relaxation processes involving pairs of holmium ions, and excited-state absorption. Using a diode laser pump source around 850 nm green upconversion emission around 540 nm was the observed predominant signal. (C) 2004 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.

XPS study of the corrosion protection of fluorozirconate glasses dip-coated with SnO2 transparent thin films

Tin oxide nanoparticles prepared by an aqueous sol-gel method were deposited by dip-coating on fluorozirconate glass, ZBLAN (53%ZrF4-20%BaF2-4%LaF3-3%AlF3-20%NaF) to improve its resistance against wet corrosion. The aqueous leaching of uncoated and SnO2-coated fluorozirconate glass was studied by X-ray photoemission spectroscopy (XPS) and it was shown that even an ultra thin tin dioxide film provides good protection of the glass surface against the bulk propagation of the hydrolytic attack.

Influence of the oxygen adsorbed on tin varistors doped with Co, Mn and Cr oxides

The non-ohmic properties of the 98.90% SnO2+(1-x)%CoO+0.05% Cr2O3+0.05% Nb2O5+x% MnO2 varistor system (all of them in mol %), as well as the influence of the oxidizing and reducing atmosphere on this system were studied in this work. Experimental evidence indicates that the electrical properties of the varistor depend on the defects that occur at the grain boundary and on the adsorbed oxygen species such as O''(2), O'(2), O in this region. Thermal treatments at 900 degreesC in oxygen and nitrogen atmospheres indicated such a dependence with the values of the non-linearity coefficient (alpha) increasing under oxygen atmosphere, being reduced in nitrogen atmosphere and restored after a new treatment in oxygen atmosphere, presenting a reversibility in the process. EDS analysis accomplished by SEM showed the distribution of the oxides in the varistor matrix. (C) 2002 Kluwer Academic Publishers.

Label-free DNA detection based on modified conducting polypyrrole films at microelectrodes

A label-free electrochemical detection method for DNA hybridization based on electrostatic modulation of the ion-exchange kinetics of a polypyrrole film deposited at microelectrodes is reported. Synthetic single-stranded 27-mer oligonucleotides (probe) have been immobilized at 2,5-bis(2-thienyl)-N-(3-phosphorylpropyl)pyrrole film formed by electropolymerization on the previously formed polypyrrole layer. The 27- or 18-mer target oligonucleotides were monitored via the electrochemically driven anion exchange of the inner polypyrrole film. The performance of the miniaturized DNA biosensor system was studied in respect to selectivity, sensitivity, reproducibility, and regeneration of the sensor. Control experiments were performed with a noncomplementary target of 27-mer DNA and 12 base-pair mismatched 18-mer sequences, respectively, and did not show any unspecific binding. Under optimized experimental conditions, the label-free electrochemical biosensor enabled the detection limits of 0.16 and 3.5 fmol for the 18- and 2 7-mer DNA strand, respectively. Furthermore, we demonstrate reusability of the electrochemical DNA biosensor after successful recovery of up to 100% of the original signal by regenerating the DNA label-free electrode with 50 mM HCl at room temperature.

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.

Effect of natural rubber latex on the conducting state of polyaniline blends determined by Raman spectroscopy

Blends possessing the elastomeric properties of natural rubber (NR) and the conducting properties of conducting polymer (polyaniline, PANI) were obtained, which are promising for further application in deformation sensors. Blends containing 20% (v/v) of PANI in 80% of NR latex were fabricated by casting in the form of free-standing films and treated either with HCl or with corona discharge, which lead PANI to its conducting state (doping process). Characterization was carried out by Raman spectroscopy, d.c. conductivity and thermogravimetric analysis. Evidence for chemical interaction between PANI and NR was observed, which allowed the conclusion that the NR latex itself is able partially to induce both the primary doping of PANI (by protonation) and the secondary doping of PANI (by changing the chain conformation). Further improvement in the primary doping could be obtained for the blends either by corona discharge or by exposing them to HCl the electrical conductivity reached in the blends was dependent on the doping conditions used, as observed by Raman scattering. Copyright (C) 2003 John Wiley Sons, Ltd.

NMR study of ion-conducting organic-inorganic nanocomposites poly(ethylene glycol) - Silica - LiClO4

Hybrid organic-inorganic ionic conductors, also called ormolytes, were obtained by dissolution of LiClO4 into silica/poly(ethylene glycol) matrices. Solid-state nuclear magnetic resonance (NMR) was used to probe the inorganic phase structure (Si-29) and the effects of the temperature and composition on the dynamic behavior of the ionic species (Li-7) and the polymer chains (H-1 and C-13). The NMR results between -100 and +90 degrees C show a strong correlation with ionic conductivity and differential scanning calorimetry experiments. The results also demonstrate that the cation mobility is assisted by segmental motion of the polymer, which is in agreement with the results previously reported for pure poly(ethylene oxide), PEG, electrolytes.

An investigation of metal oxides which are photoluminiscent at room temperature

Amorphous thin films, based on different network formers, were processed by a soft chemical process called the polymeric precursor method. The resultant amorphous metal oxides, displayed intense photoluminescence (PL) at room temperature. Heat treatment increases the PL intensity of these materials. Theoretical ab initio calculations are correlated with the observed experimental trends. (C) 2004 Elsevier B.V. All rights reserved.