Thermoanalytical studies of the processing of bulk and thin film BSCCO highT c superconductors by the Edita-Gel route

The decomposition of EDTA gel precursors for BSCCO superconductor manufacture has been studied using STA (TG/DTA) and dilatometry in conjunction with FTIR. Ther thermoanalytical data are discussed in relation to the sequence of phase formation necessary for the production of the 2223 BSCCO superconducting phase. Thin film preparation of Bi-based highT _c superconductors have been carried out on MgO (100). Grain orientation of oxide thin films has been investigated. Well orientated 2212 grains have been achieved, with the (001) planes parallel to the substrate. The EDTA-gel method has been modified by the addition of glycerol to achieve the appropriate viscosity for spin coating. STA has been used to study the decomposition of these modified gels for the formation of thin films.     

Production of bscco bulk high <Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> superconductors by sol-gel method and their characterization by ftir and XRD techniques

The production of bulk high T _c superconducting phase (2223) by EDTA-gel (ethylenediaminetetraacetic acid) techniques has been investigated. It is shown that close control of pH is necessary for the production of a well-complexed precursor which allows subsequent decomposition in two stages at 300 and 800°C. The problem of carbonate formation was investigated experimentally and solved. Precursors are characterised by Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) and the sintering behaviour was monitored by dilatometry. At least three different phases Bi₂Sr₂Ca_x−1Cu_xO_8+y (BSCCO); where x=1, 2, 3 were identified within superconducting pellets using XRD, named as Bi₂Sr₂CuO₇ (2201), Bi₂Sr₂CaCu₂O₉ (2212) and Bi₂Sr₂Ca₃O₁₀ (2223). The superconducting properties of the sintered samples were studied by vibrating sample magnetometer (VSM). Transition to a superconducting state around 80 K appeared in samples (sintered at 845°C) containing the Bi₂Sr₂Ca₁Cu₂O_y (2212) phase. Liquid phase sintering of the samples aided the formation of Bi₂Sr₂Ca₂Cu₃O_x (2223) phase at high temperature (860°C), which showed a superconducting transition temperature of 108 K.     

Development of superconducting phases in BSCCO and Ba-BSCCO by sol spray process

The effects of barium substitution for Bi or Sr sites on the growth of superconducting phases have been studied. The sol spray process has been used to synthesis the Bi-Sr-Ca-Cu-O (BSCCO) and Ba-BSCCO homogeneous ceramic powders. Thermogravimetric (TG), differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) techniques were employed to characterize the synthesized materials. The electrical resistance was measured by classical d.c. fourprobe technique. It has been observed that sol spray process has affected the physico-chemical properties of the materials and also avoid the use of chelating agent as in the case of sol gel process. In addition to the Bi₂Sr₂CaCu₂O₈ (2212) phase Ba doped specimens also contained Bi₂Sr₂Ca₂Cu₃O₁₀ (2223), BaBiO₃, BaCuO₂ and CuO phases. The results revealed that the specific effect of barium-doping on either sites (Bi or Sr) seems to avoid the formation of higher volume fraction of the low T _c phase and promoting the formation of BiBaO₃, BaCuO₂ and CuO along with formation of a high T _c 2223 phase. The substitution of Ba on either sites (Bi or Sr) lower the sintering temperature for the formation of high T _c (small volume fraction) however, the Ba doped specimens also contained non-superconducting phases.     

Time and temperature-based study for the production of high T C phase by sol-gel technique in Pb-bscco system

The high T_C superconducting phase Bi₂Sr₂Ca₂Cu₃Ox (2223) in the Pb-BSCCO system has been produced by EDTA-gel processing using nitrate solutions. The precursor has heated in two stages, at 300 and 800°C each for 2 h, to avoid the burning of the important species involved in the final product. The effects of time (6 to 48 h) and temperature (845 and 855°C) on the formation of the 2223 phase have been studied by sintering the samples in air. Thermal analysis (TG/DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and a vibrating sample magnetometer (VSM) have been employed to investigate the powder produced at different stages of decomposition, oxidation and formation of sintered materials from the powders. The volume-fraction of the 2223 phase at 845°C increases with time, the maximum value of the 2223 phase was obtained at 120 h. It has been observed that the formation of the high T_C phase is remarkably enhanced at the temperature of the endothermic peak of the DTA curve. The best result has been obtained in the sample sintered for 24 h at the temperature 855°C (endothermic peak). This also indicated that at 855°C, the large volume-fraction of 2223 phase with T_C 113 K grew in short time and as the sintering time increased, it decomposed into the Bi₂Sr₂CaCu₂Ox (2212) phase and other phases.     

Oxygen exchange study in phase transformations of superconducting bismuth cuprates (Pb2212 and Pb2223)

The oxygen exchange between the condensed phase(s) and the gas phase was studied as a function of temperature for superconducting phases of the system (Bi,Pb)-Sr-Ca-Cu-O.An oxygen probe apparatus allowed confirmation that the variation in oxygen composition in the Pb2212 and Pb2223 superconducting phases is a reversible phenomenon on heating and cooling. It was demonstrated that the mass loss of both phases, for the 905 and 980°C isotherms, was due to the oxygen composition variation and to PbO loss (this latter phenomenon begin irreversible).     

Sol-gel preparation and fluorine substitution of BiSCCO high-T c superconductors

Sol-gel citrate technology was used to prepare BiSCCO 2223 and 2212 superconducting phases. Fluorine in the initial mixture was shown to enhance considerably formation of the 2223 phase. Fluorine substitution of up to three F atoms per Bi(Pb)SCCO 2223 formula was achieved by treatment of the sample in 80 Torr ClF₃ atmosphere. A number of experimental methods have been used to characterize the samples and to trace the phase transformations in the sol-gel process: DTA/TG, XRD, atomic absorption, potentiometry with fluoride selective electrode, magnetic susceptibility measurements.Dedicated to Prof. Menachem Steinberg on the occasion of his 65th birthdayThe authors are thankful to Dr. I. Felner for the magnetic measurements. J. H. Greenberg is grateful to the Ministry of Absorption and L. Ben-Dor to the Ministry of Science and Arts for material support.     

高价离子掺杂对Bi系成相和结构的影响(II)

In our previous work we have found that the 2223 phase is formed more rapidly and the structural stability can be enhanced by doping high valence cation(Sn~(4+)、Sb~(5+)、V~(5+)、W~(6+)、Mo~(6+) etc.) in the Bi(Pb)SrCaCuO system. In this paper the effects of high valence cations doped on the phase formation and the crystal structure are discussed further.The Aurivillus building and rocksalt building unit coexist in the Bi double layers. The structural variety strongly affects the structural stability. Owing to Pb~(2+) addition the probability of Aurivillus building is increased, the stability of 2223 phase can be promoted. However, the structure is still not stable, since the addition of Pb~(2+) has decreased the valence state of Bi site and the oxygen positions are not fully occupied. By doping high valence cation, especially, when the valence of Pb_x~(2+)M_y~(n+) equals to +3 the structure of the 2223 phase can be stabilized. This result has been observed with examining elemental content of 2223 phase.The melting point of the grain boundaries are much lower after doping high valence cations, therefore the ion diffusion is more rapid and acceleration of the 2223 phase formation is observed.     

Sol-gel route for obtaining bismuth high T c superconducting oxide compounds

Bismuth high T _c superconducting oxide compounds were obtained by sol-gel method using acetate precursors. A study of the decomposition of quasi-amorphous acetates has been carried out to establish the conditions of superconducting phase formation. A comparison of the acetate sol-gel route with the oxalate coprecipitation method (previously used by the authors) reveals comparable results. Both methods require much shorter periods of thermal treatment for the formation of superconducting phases, as compared with the solid state reactions. The sol-gel route leads to better results from this point of view.     

Carbon Species and Precise Elemental Analysis of Precursors for Superconducting (Bi,Pb)2Sr2Ca2Cu3Ox Tapes

 The chemical composition of BiPbSrCaCuO precursors prepared by different syntheses has to be checked analytically before further application to production of superconducting (Bi,Pb)₂Sr₂Ca₂Cu₃O_x tapes. Carbon species in concentrations below 0,1%(w/w) were observed to be possibly a source of gas evolving reactions during the heat treatment procedures for the superconducting phase formation. A method that allows the temperature controlled detection of CO₂ resulting from reactions of various carbon species was developed based on an commercial C/H₂O-analyser with IR gas detectors. By means of this method it is possible to relate the carbon content to the conditions of synthesis and technological steps of the tape production. The phase formation mechanism of the superconducting (Bi,Pb)2223 phase from a multiphase precursor strongly depends on its elemental composition. Titrimetric and ICP spectrometric methods that allow the determination of the content of the main components Bi, Pb, Ca, Sr, and Cu with high precision and accuracy were developed and their results were compared. The best results were obtained with a combined chemical-spectrometric procedure. The relative standard deviations (RSD) for all metallic elements do not exceed 1%. The analytical results of the main components and specific trace determinations are essential for the functioning of superconducting tapes.     

Approximation to the laser floating zone preparation of high temperature BSCCO superconductors by DSC

Differential scanning calorimetry (DSC) has been used to simulate laser fusion of ceramic precursors to prepare BSCCO high temperature superconducting materials. At fast heating rates, typical of the laser floating zone, the energy required for complete melting of the precursor increases with the Bi to Sr+Ca ratio, in agreement with the critical current results obtained. At low heating rates, solid-state reactions involving ionic rearrangements take place before incongruent melting. These changes can be divided into three processes and can be quantified from calorimetric measurements. Lower heating rates and isothermal experiments allow detailed visualisation of the ionic arrangements taking place. The results prove that the mechanism of the final phase formation is dependent on the fabrication procedure.     

超导相(Bi1.7Pb0.2In0.1)Sr2Ca2Cu3O10的制备和晶体结构研究

The superconducting phase(Bi_(1.7)Pb_(0.2)In_(0.1))Sr_2Ca_2Cu_3O_(10) with on-set T_c near 112K and zero resistivity at 95K has been prepared. The Unit cell is. body-centered tetragonal, space group I4/mmm, a=0.3.817(1) nm, c=3.704(1) nm. Chemical substitution of Pb and In in Bi—Sr—Ca—Cu—O series results in the growth of 2223 phase. The superconductivity and the formation mechanism are associated with the structural features.     

Synthesis and characterization of high temperature superconductors

The high temperature superconductors of the system Bi-Pb-Sr-Ca-Cu-O show promising properties and large scale applications can be considered. The (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀₊ (Bi,Pb(2223)) phase is attractive because of its high superconducting temperature (110 K) and high current transport capabilities.Simultaneous DTA/TG has proved an excellent tool for synthesizing, characterizing these high temperature superconductors, confirming phase diagrams and finding the optimal annealing temperature for Bi,Pb(2223)/Ag tapes in order to obtain high critical current densities around 30 kA cm^–2 at 77 K and 0T. The identification of the main DTA peaks of measurement of different phase mixtures made it possible to observe the formation of an intermediate phase Bi,Pb(2212) during the formation of Bi,Pb(2223).     

Optimization of the Sintering Process by DSC for the Preparation of High-Temperature Superconductors

The optimum calcination and sintering conditions for the preparation of the desired phases with the proper stoichiometry of the bulk high-temperature superconductors YBa₂Cu₃O_7-x (YBCO) and Bi₂Sr₂CaCu₂O_y> (BSCCO) were investigated by differential scanning calorimetry. DSC analysis on the corresponding superconducting coatings, electrophoretically deposited, indicated that the sintering and annealing procedure was suitable for the production of stable, strong and adherent coatings. X-ray diffraction analysis, Raman spectroscopy and magnetic measurements confirmed the improved properties of the superconducting powders and coatings obtained. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal analysis of formation of sialons by carbothermal reduction of clays

The formation at high temperatures of phases such as β′,X, and O′-sialon by carbothermal reduction of clays has been studied using simultaneous thermal analysis (STA) and thermomechanical analysis (TMA). Two reducing agents, silicon carbide and carbon, were used, with either kaolinite or bentonite. The progress of reaction was followed by interrupted and continuous thermal analysis procedures using STA and TMA and the changes correlated with the phases observed by X-ray diffraction at different stages. The conversion of kaolinite to β′-sialon is characterised by the dehydration and transformation of kaolinite independent of the nitriding atmosphere below 1400°C. At higher temperatures nitriding commences. Similar behaviour is observed with bentonite transforming to lower substitution sialon phases. The weight changes are correlated with oxidation/reduction reactions and mechanisms are suggested to explain the observed phenomena. The data obtained will permit the optimization of thermal cycles for batch production of sialon powders from natural minerals.     

Rapid Formation of the Textured (Bi,Pb)2(Sr,Ca)4Cu3Oy Superconductor by Multiple Pressing and Sintering Process

A superconducting (Bi, Pb)₂(Sr, Ca)₄Cu₃O₂ material has been prepared by tbe oxalate coprecipitation method. The high-Tc 110 K phase was effectively promoted by application of an intermediate grinding and pressing between the sintering heat treatments. The powder X-ray diffraction shows the existence of single high-Tc and high-textured microstructures with strong (001) reflections within a short sintering time ( 18 h). The morphology of the sintered samples was examined by scanning electron microscopy (SEM). Also, the kinetics of the formation of the high-Tc phase were investigated by the isothermal method. A possible mechanism of the formation of high-Tc 2223 phase was proposed.     

Comparison of the Efficiency of the One and Two-Step Process for the Production of BSCCO (2212) Powders by Means of Analytical Techniques

 The efficiency of the production of the high temperature superconducting powders Bi₂Sr₂CaCu₂O_8+x (BSCCO 2212) using the solid state reaction in one or two step processes under different thermal treatment was compared by means of different modern analytical techniques. Through the same techniques the optimization of the production of the production of Bi-2212 powder produced by the two step process, was achieved. X-ray diffraction analysis (XRD) and Raman spectroscopy were used to characterize the products for their stoichiometry and phase-purity. The grain size of the powders was observed by scanning electron microscopy (SEM), while their superconducting properties were tested by electrical DC-resistivity and magnetic susceptibility measurements using a superconducting quantum interference device (SQUID). It resulted that the two step process gives a high quality BSCCO 2212 superconducting powder with T _c = 85 K, in a shorter time and with a greater recovery rate than the one step process. Received May 3, 1999. Revision April 27, 2000.     

Study of Fe Addition on the Thermal Decomposition of Coprecipitated Oxalates for the Bi-based Superconductor Synthesis

This work introduces results obtained during the preparation of a Bi-based material with superconducting properties by oxalate coprecipitation. The influence of Fe presence on the precursors thermal stability and on the superconducting phases formation mechanism are presented. The thermal decomposition and the stability in air of FeC₂O₄×2H₂O and also of the components mixture were studied by DTA/TG. It was evidenced that iron oxalate decomposes at the lowest temperature compared to the decomposition temperatures of the individual oxalates. XRD, IR and TEM/ED studies were approached to investigate the individual oxalates and the mixture coprecipitates for the high-T _c superconducting material synthesis. This revised version was published online in August 2006 with corrections to the Cover Date.     

The effect of calcination conditions on the superconducting properties of Y−Ba−Cu−O powders

Calcination conditions of the precursor powders, i.e. temperature, type of atmosphere and duration, were determined with a view to obtain superconducting powders with the most advantageous physico-chemical properties. Investigated were powders in the Y?Ba?Cu?O system prepared by the sol-gel method. Thermogravimetric examinations of the powders have revealed that the decomposition kinetics of BaCO₃ determines the formation rate of the superconducting YBa₂Cu₃O_7?x (‘123’) phase. It follows from the decomposition kinetics of BaCO₃ that the process is the most intensive in argon, whereas in static air and oxygen it is the slowest. The phase composition analysis (XRD) and low-temperature magnetic susceptibility measurements of the calcinated powders, confirm the above mentioned changes in the decomposition kinetics. The reaction of barium carbonate can be completed if the calcination process is conducted at the temperature of 850°C for 25 h, yielding easily sinterable powders for obtaining single-phase superconducting bulk samples with advantageous functional parameters.     

高价离子掺入Bi(Pb)-Sr-Ca-Cu-O体系对成相和结构的影响

The effects of high valence cations M~(n+) (n≥4) doped on the phase formation and the structure stability have been investigated. In the crystal structure X_2Sr_2Ca_2Cu_3O_(10)(X = Bi_(1-x-y)Pb_xM_y), the high valence cations M~(n+), Bi~(3+), and Pb~(2+) were deter-mined to statistically distribute at the bismuth positions. The bismuth valence is raised and the positions of oxygen atoms in the (XO_2X) double layers are fully occupied. Conseqently, The structural stability is increased. The phase 2223 was found to form more rapidly by doping and the single phase with T_c(0)>110 K can be obtained more easily.     

Ru上有氧条件下氨分解的动力学研究

IthasbeenshownthatRuisvalidforthesyn thesisanddecompositionofammonia[1,2 ] .FurtherstudyofammoniaadsorptionanditsdecompositionproductsdesorptiononRuwillbeimportant .Previ ousstudiesofammoniaadsorptiononRumainlyfo cusedontheammoniasynthesisandhydrogenpro ductionintheabsenceofoxygen[3] ,onlyafewinves tigationsonammoniadecompositioninthepresenceofoxygenhavebeenreported[4 ,5] ,andtheeffectofad sorbedoxygenontheratesofammoniadecompositionandproductformationonRuarestillnotwellunder stood .Inthispa…