Monofilamentary Ag/Bi:2212 tapes - Effects of silver powder addition

The Bi-Sr-Ca-Cu-O system has been one of the most studied superconducting ceramic materials for industry applications. The most of the studies with this aim are on silver/ceramic composites, due to the benefits and great compatibility of this metal with the oxide. Tapes made by the powder in tube (PIT) method have been successfully tested in pilot power plants in many countries but in Brazil. In this paper, 5, 10, and 20-wt% silver powders are introduced to compose the core of the tape along with the Bi:2212 ceramic powder. The results of electrical experiments are compared with those made with no silver addition Ag tapes. The best current density, at 60 K and no applied magnetic field, was found for the 10-wt% silver proportion, doubling the value obtained for the tape with no silver in the core.

An investigation on the effects of Ag addition in the powder — in tube processing of long multifilamentary (Bi,Pb) - 2223/Ag superconducting tapes

It is important that long superconducting tape must have desired strain tolerance (less reduction of Jc with applied strains) and stress tolerance (less reduction of JC in applied stresses) for its use as coils and magnets. Ag addition to the BPSCCO system has many advantages with its physical and chemical inertness to the system, reduces the processing temperature, and promotes the grain growth, grain alignment and connectivity. All these not only enhance the critical current density of the tapes but also improve the mechanical properties. But the published results show very much scattering on the type of Ag additive to be selected, method of addition and its optimum percentage. Also there are some negative reports in this regard. The present work has been undertaken to study the effect of silver addition in different forms (Ag powder, Ag2O, AgNO3) on the superconducting and mechanical properties of (Bi,Pb)-2223/Ag tapes and to find out a suitable form of Ag additive and its optimum percentage to have better superconducting and mechanical properties. Also it is the aim of the present work is to optimise the process parameters needed to prepare (Bi,Pb)-2223/Ag multifilamentary tapes of length ~ 12 m in solenoid and pancake coil forms with good critical current density and homogeneity of J C along the length of the tapes.

Effect of 4 mass% Ag addition on the thermal behavior of the Cu-9 mass% Al alloy

The effect of 4 mass% Ag addition on the thermal behavior of the Cu-9 mass% Al alloy was studied using differential scanning calorimetry (DSC), optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and X-ray diffractometry (XRD). The results showed that the presence of silver causes (Cu)-alpha+(alpha+gamma1)-->(Cu)-alpha+beta transformation to occur in two stages. In the first one, part of the produced beta phase combines with the precipitated Ag to give a silver-rich phase and in the second one the transformation is completed. The formation of this silver-rich phase seems to be enhanced at very low cooling rates.

Synthesis and characterisation of nanoscale magnesium oxide powders and their application in thick films of Bi2Sr2CaCu2O8

Nanoscale MgO powder was synthesized from magnesite ore by a wet chemical method. Acid dissolution was used to obtain a solution from which magnesium containing complexes were precipitated by either oxalic acid or ammonium hydroxide, The transformation of precipitates to the oxide was monitored by thermal analysis and XRD and the transformed powders were studied by electron microscopy. The MgO powders were added as dopants to Bi2SrCa2CuO8 powders and high temperature superconductor thick films were deposited on silver. Addition of suitable MgO powder resulted in increase of critical current density, J(c), from 8,900 Acm(-2) to 13,900 Acm(-2) measured at 77 K and 0 T. The effect of MgO addition was evaluated by XRD, electron microscopy and critical current density measurements. (C) 1998 Elsevier Science B.V.

Metal-insulator transition and giant magnetoresistance in La1-xMnO3-delta thin films and superlattices

The first fabrication of self-doped La1-xMnO3-delta films which are unique among the other La(1-x)M(x)MnO(3) (M = Ca, Ba and Pb) thin films showing giant magnetoresistance is reported. Ag-doped La0.7MnO3-delta films were grown on LaAlO3[100] substrates. These films show ferromagnetic and metal-insulator transition at 220 K and exhibit giant magnetoresistance (GMR) with Delta R/R(o) = 85% and Delta R/R(H) > 550%. Without silver addition these self-doped films are non-magnetic, Enhancement in GMR up to 8% has been observed in superlattices having alternate magnetic and non-magnetic La1-xMnO3-delta layers.

Kinetics of eutectoid decomposition in Cu-Al and Cu-Al-Ag alloys

The kinetics of eutectoid decomposition beta(1)' --> gamma(2) + (alpha + gamma(2)) in Cu-12.86 wt% Al and Cu-12.84 wt% Al-1.98 wt% Ag alloys was studied by hardness measurements, using the Johnson-Mehl-Avrami equation. The results indicate that the presence of silver seems to influence the nucleation rate and the activation energy of the reaction.

Estudo da influência de adições de prata na cinética de precipitação e no comportamento eletroquímico da liga Cu-6%Al

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

A review of the effect of the addition of hydrogen in the selective catalytic reduction of NOx with hydrocarbons on silver catalysts

Research is progressing fast in the field of the hydrogen assisted hydrocarbon selective catalytic reduction (HC-SCR) over Ag-based catalysts: this paper is a review of the work to date in this area. The addition of hydrogen to the HC-SCR reaction feed over Ag/Al2O3 results in a remarkable improvement in NO (x) conversion using a variety of different hydrocarbon feeds. There is some debate concerning the role that hydrogen has to play in the reaction mechanism and its effect on the form of Ag present during the reaction. Many of the studies use in situ UV-Vis spectroscopy to monitor the form of Ag in the catalyst and appear to indicate that the addition of hydrogen promotes the formation of small Ag clusters which are highly reactive for NO (x) conversion. However, some authors have expressed concern about the use of this technique for these materials and further work is required to address these issues before this technique can be used to give an accurate assessment of the state of Ag during the SCR reaction. A study using in situ EXAFS to probe the H-2 assisted octane-SCR reaction has shown that small Ag particles (containing on average 3 silver atoms) are formed during the SCR reaction but that the addition of H-2 to the feed does not result in any further change in the Ag particle size. This points to the direct involvement of H-2 in the reaction mechanism. Clearly the addition of hydrogen results in a large increase in the number and variety of adsorbed species on the surface of the catalyst during the reaction. Some authors have suggested that conversion of cyanide to isocyanate is the rate-determining step and that hydrogen promotes this conversion. Others have suggested that hydrogen reduces nitrates to more reactive nitrite species which can then activate the hydrocarbon; activation of the hydrocarbon to form acetates has been proposed as the key step. It is probable that all these promotional effects can take place and that it very much depends on the reaction temperature and feed conditions as to which one is most important.

Influence of grain boundaries on the vibrational properties of silver nanowires

Metal and semiconductor nanowires (NWs) have been widely employed as the building blocks of the nanoelectromechanical systems, which usually acted a resonant beam. Recent researches reported that nanowires are often polycrystalline, which contains grain boundaries (GBs) that transect the whole nanowire into a bamboo like structure. Based on the larger-scale molecular dynamics (MD) simulations, a comprehensive investigation of the influence from grain boundaries on the vibrational properties of doubly clamped Ag NWs is conducted. It is found that, the presence of grain boundary will result in significant energy dissipation during the resonance of polycrystalline NWs, which leads a great deterioration to the quality factor. Further investigation reveals that the energy dissipation is originated from the plastic deformation of polycrystalline NWs in the form of the nucleation of partial dislocations or the generation of micro stacking faults around the GBs and the micro stacking faults is found to keep almost intact during the whole vibration process. Moreover, it is observed that the closer of the grain boundary getting to the regions with the highest strain state, the more energy dissipation will be resulted from the plastic deformation. In addition, either the increase of the number of grain boundaries or the decrease of the distance between the grain boundary and the highest strain state region is observed to induce a lower first resonance frequency. This work sheds lights on the better understanding of the mechanical properties of polycrystalline NWs, which benefits the increasing utilities of NWs in diverse nano-electronic devices.

In situ observation of structural changes in polycrystalline silver catalysts by environmental scanning electron microscopy

Morphology changes induced in polycrystalline silver catalysts as a result of heating in either oxygen, water or oxygen-methanol atmospheres have been investigated by environmental scanning electron microscopy (ESEM), FT-Raman spectroscopy and temperature programmed desorption (TPD). The silver catalyst of interest consisted of two distinct particle types, one of which contained a significant concentration of sub-surface hydroxy species (in addition to surface adsorbed atomic oxygen). Heating the sample to 663 K resulted in the production of 'pin-holes' in the silver structure as a consequence of near-surface explosions caused by sub-surface hydroxy recombination. Furthermore, 'pin-holes' were predominantly found in the vicinity of surface defects, such as platelets and edge structures. Reaction between methanol and oxygen also resulted in the formation of 'pin-holes' in the silver surface, which were inherently associated with the catalytic process. A reaction mechanism is suggested that involves the interaction of methanol with sub-surface oxygen species to form sub-surface hydroxy groups. The sub-surface hydroxy species subsequently erupt through the silver surface to again produce 'pin-holes'.

A combined environmental scanning electron microscopy and Raman microscopy study of methanol oxidation on silver(I) oxide

The techniques of environmental scanning electron microscopy (ESEM) and Raman microscopy have been used to respectively elucidate the morphological changes and nature of the adsorbed species on silver(I) oxide powder, during methanol oxidation conditions. Heating Ag2O in either water vapour or oxygen resulted firstly in the decomposition of silver(I) oxide to polycrystalline silver at 578 K followed by sintering of the particles at higher temperature. Raman spectroscopy revealed the presence of subsurface oxygen and hydroxyl species in addition to surface hydroxyl groups after interaction with water vapour. Similar species were identified following exposure to oxygen in an ambient atmosphere. This behaviour indicated that the polycrystalline silver formed from Ag2O decomposition was substantially more reactive than silver produced by electrochemical methods. The interaction of water at elevated temperatures subsequent to heating silver(I) oxide in oxygen resulted in a significantly enhanced concentration of subsurface hydroxyl species. The reaction of methanol with Ag2O at high temperatures was interesting in that an inhibition in silver grain growth was noted. Substantial structural modification of the silver(I) oxide material was induced by catalytic etching in a methanol/air mixture. In particular, "pin-hole" formation was observed to occur at temperatures in excess of 773 K, and it was also recorded that these "pin- holes" coalesced to form large-scale defects under typical industrial reaction conditions. Raman spectroscopy revealed that the working surface consisted mainly of subsurface oxygen and surface Ag=O species. The relative lack of sub-surface hydroxyl species suggested that it was the desorption of such moieties which was the cause of the "pin-hole" formation.

Influence of oxidation and reduction conditions upon the morphology of silica-supported polycrystalline silver catalysts

The effect of oxidation and reduction conditions upon the morphology of polycrystalline silver catalysts has been investigated by means of in situ Fourier-transform infrared (FTIR) spectroscopy. Characterization of the sample was achieved by inspection of the νas(COO) band profile of adsorbed formate, recorded after dosing with formic acid at ambient temperature. Evidence was obtained for the existence of a silver surface reconstructed by the presence of subsurface oxygen in addition to the conventional family of Ag(111) and Ag(110) crystal faces. Oxidation at 773 K facilitated the reconstruction of silver planes due to the formation of subsurface oxygen species. Prolonged oxygen treatment at 773 K also caused particle fragmentation as a consequence of excessive oxygen penetration of the silver catalyst at defect sites. It was also deduced that the presence of oxygen in the gas phase stabilized the growth of silver planes which could form stronger bonds with oxygen. In contrast, high-temperature thermal treatment in vacuum induced significant sintering of the silver catalyst. Reduction at 773 K resulted in substantial quantities of dissolved hydrogen (and probably hydroxy species) in the bulk silver structure. Furthermore, enhanced defect formation in the catalyst was also noted, as evidenced by the increased concentration of formate species associated with oxygen-reconstructed silver faces.

The facile formation of silver dendritic structures in the absence of surfactants and their electrochemical and SERS properties

In this work a simple approach to the creation of highly dispersed electrocatalytically active silver microstructured dendrites on indium tin oxide in the absence of any surface modification or surfactant is presented. It is found that the addition of low concentrations of supporting electrolyte to the AgNO3 solution dramatically influences the morphology of electrodeposited silver which is independent of both the anion and the cation employed. The silver dendrites are characterized by SEM, XRD, XPS as well as by cyclic voltammetry under alkaline conditions. It is found that the surface oxide formation and removal processes are significantly influenced by the microstructured morphology of the silver electrodeposits compared to a smooth macrosized silver electrode. The facile formation of dendritic silver microstructures is also shown to be beneficial for the electrocatalytic oxidation of both formaldehyde and hydrazine and oxygen reduction. The formation of a continuous film of dendritic silver is also investigated for its SERS activity where the connectivity between the individual dendrites is found to be particularly important.

Complementary surface-enhanced resonance raman spectroscopic biodetection of mixed protein solutions by chitosan- and silica-coated plasmon-tuned silver nanoparticles

Silver nanoparticles with identical plasmonic properties but different surface functionalities are synthesized and tested as chemically selective surface-enhanced resonance Raman (SERR) amplifiers in a two-component protein solution. The surface plasmon resonances of the particles are tuned to 413 nm to match the molecular resonance of protein heme cofactors. Biocompatible functionalization of the nanoparticles with a thin film of chitosan yields selective SERR enhancement of the anionic protein cytochrome b5, whereas functionalization with SiO2 amplifies only the spectra of the cationic protein cytochrome c. As a result, subsequent addition of the two differently functionalized particles yields complementary information on the same mixed protein sample solution. Finally, the applicability of chitosan-coated Ag nanoparticles for protein separation was tested by in situ resonance Raman spectroscopy.