Ceramic Tl-oxide based superconductors reinvestigated by magnetic separation technique and SQUID measurements

Ceramic samples of Tl₂Ba₂Ca₂Cu₃O₁₀ (Tl-2223) and TlBa₂Ca₂Cu₃O₉ (Tl-1223) superconductor were resynthesized by using several preparation procedures and investigated by means of magnetic separation technique, allowing evaluation of the superconducting critical temperature of individual grains as small as 25 μm. Although an onset of diamagnetism detected by SQUID measurements was observed around 130 K in Tl-2223 and 134 K in Tl-1223, no grains with T_c > 125 K were found for Tl-2223 and with T_c > 130 K for Tl-1223. Furthermore, the volume fraction of superconductivity of such ceramics was less than 5% and 3% in best samples of Tl-2223 and Tl-1223, respectively. Here we provide a detailed comparison of results obtained by magnetic separation technique with those by SQUID applied to individual grains and bulk samples.     

Normal pressure synthesis of (Tl1−yCy)Ba2Ca3Cu4O12−δ superconductor

Single phase (Tl_1−yC_y)Ba₂Ca₃Cu₄O_12−δ (Tl_1−yC_y-1234) (y = 0, 0.25, 0.5 and 0.75) superconductor samples have been prepared by solid state reaction method. The FTIR absorption measurements have confirmed the substitution of carbon at thallium site in the charge reservoir layer, (Tl_1−yC_y)Ba₂O_4−δ. The electron micrographs of these samples have shown that the carbon substitution has improved the grain morphology of Tl_0.75C_0.25-1234 sample. The y = 0.25 was found to be the optimum carbon concentration to achieve higher superconducting transition temperature T_c[0] and improved grain morphology. The superconducting transition temperature of Tl_0.75C_0.25-1234 sample has been increased to 100 K whereas a decrease in the superconducting transition temperature of Tl_1−yC_y-1234 (y = 0.5 and 0.75) samples was observed. However, the magnitude of diamagnetism has been decreased in all the carbon substituted samples.     

Superconductivity in (Cu0.5Tl0.25Li0.25)Ba2Ca2Cu3−ySiyO10−δ samples

The (Cu_0.5Tl_0.25Li_0.25)Ba₂Ca₂Cu_3?ySi_yO_10?δ (y = 0, 0.25 0.5, 0.75, 1.0, 1.25) superconductor samples have been prepared by solid-state reaction method. The critical temperature and as well as the magnitude of diamagnetism is increased up to Si concentration y = 1.0, however, from the doping level y = 1.25 a decrease in the critical temperature along with the vanishing of the diamagnetism was observed. The carrier’s in the conducting CuO₂/SiO₂ planes were optimized by carrying out post-annealing in oxygen and an increase in the critical temperature was observed in all Si doped samples. The doping efficiency of Cu_0.5Tl_0.5Ba₂O_4?δ charge reservoir layer in (Cu_0.5Tl_0.25Li_0.25)Ba₂Ca₂Cu_3?ySi_yO_10?δ (y = 0, 0.25 0.5, 0.75, 1.0, 1.25) samples is enhanced by doping Li⁺¹ ion; as alkali metals are known to easily loose their outer most electron which could be supplied to CuO₂/SiO₂ conducting planes and would suppress the anti-ferromagnetism in the inner conducting planes. The FTIR absorption measurements have provided an indirect evidence of Si substitution at in CuO₂ planes.     

Spectrophotometric determination of oxygen content in calcium substituted RBCO (R=Eu,Gd, Er) superconductors

The oxygen stoichiometry in R_1−x Ca _x Ba₂Cu₃O _y (R=Eu, Er, Gd; x=0, 0.2, 0.25, 0.3) superconducting bulk samples was determined spectrophotometrically. The dependence of the critical temperature on the y-oxygen coefficient and the x-coefficient of the included calcium was studied.     

Reactions between YBa2Cu3O7−δ and La2O3 and SrCO3

Solid state reactions at 925°C between the high-T _c ceramic superconductor YBa₂Cu₃O_7?δ and La₂O₃ and SrCO₃, respectively, mixed in various molar ratiosr=MeO_n/YBa₂Cu₃O_7?δ, were studied using X-ray powder diffraction and scanning electron microscopy. The reaction between YBa₂Cu₃O_7?δ and La₂O₃ yielded (La_1?xBa_x)₂CuO_4?δ, withx≈0.075?0.10. La_2?xBa_1+xCu₂O_6?δ, withx≈0.2?0.25 and La-doped (Y_1?xLa_x)₂BaCuO₅, withx≈0.10?0.15. Forr=3.0, Y-doped La₂BaCuO₅ resulted also. The reaction between YBa₂Cu₃O_7?δ and SrCO₃ yielded (Sr_1?zBa_z)₂CuO₃, withz≈0.1, Y₂(Ba_1?zSr_z)CuO₅, withz=0.1?0.15, and a nonsuperconducting compound with an approximate composition of Y(Ba_0.5Sr_0.5)₅Cu_3.5O_10±δ. At values ofr≤2.0, unsubstituted YBa₂Cu₃O_7?delta was found in the reaction products.     

Ba2Ca2Cu3Ox Precursor Effects on Synthesis of (Hg,Pb)Ba2Ca2Cu3O8+δ Superconductor

The influence of Ba₂Ca₂Cu₃O_x precursor on the synthesis and properties of (Hg,Pb)Ba₂Ca₂Cu₃O_8+δ has been examined. Fine homogeneous Hg-free precursor powder of Ba₂Ca₂Cu₃O_x of desirable phase composition was prepared by sol-gel method using EDTA acid as a complexing agent. A reproducible superconducting sample of Hg_0.8Pb_0.2Ba₂Ca₂Cu₃O_8+δ with fine-grained, dense microstructure, composed predominantly of (Hg,Pb)-1223 phase and with advantageous magnetic properties, was synthesized by high pressure crystallization in mercury environment of well-calcined Ba₂Ca₂Cu₃O_x precursor. This revised version was published online in July 2006 with corrections to the Cover Date.     

Techniques for obtaining far infrared transmission spectra of high Tc,superconductors and related synthetic precursors

The far infrared (FT-IR) spectra of Y₂O₃, CuO, BaO₂, YBa₂Cu₃O_7−σ and Tl₂Ca₂Ba₂Cu₃O₁₀ have been recorded in the range of 350-60 cm⁻¹. A variety of sample handling techniques have been utilized which include dispersion of the samples in nujol and fluorolube. Additionally, we present preliminary results of a technique of preparing thin films (1–20 μm thick) of these materials. These films do not require a substrate but are dispersed in a polyethylene matrix fused between two polyethylene plates. Such films are suitable for optical applications.     

Application of High Pressure-DTA for High Pressure-liquid Phase Epitaxy of Doped Mercury and Thallium HTS Formations

The fabrication method of superconducting thin films of compositions HgBa₂Ca₂Cu₃O_8+δ (Hg-1223) and Tl₂Ba₂CuO_y (2201) on single-crystalline SrTiO₃ and LaAlO₃ substrates is reported. The highest obtained T _c was 134 K and J _c over 10⁶ A cm^–2 at 77 K. High pressure DTA(HP-DTA) was applied to grow mercury- and thallium-based high-temperature superconducting crystals and thin films, to identify melting points of particular phases within these oxide systems and determine suitable processing conditions. The DTA system operates at the: maximum temperature of 1200°C, volume up to 5 cm³, working pressure up to 1.5 GPa and at a working atmosphere — inert gas with up to 25% oxygen. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ba2Cu3O5+δ as an intermediate phase during the synthesis of YBa2Cu4O8

We propose a reaction model for the synthesis of YBa₂Cu₄O₈ under normal pressure conditions, which contains 4 partial reaction steps. In a first step bariumnitrate and copperoxide react to Ba₂Cu₃O_5+δ. This substance will be formed for each mixtures Ba:Cu=2∶3...3∶2. The following two partial reaction steps are connected to Ba₂Cu₃O_5+δ, which reacts with Y₂O₃ and CuO to YBa₂Cu₄O₈ or decomposes to BaCuO₂ and CuO. In a last step parts of BaCuO₂ reacts with Y₂O₃ and CuO to YBa₂Cu₄O₈.     

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.     

Phase formation and microstructure in Re1-x CaxBa2Cu3Oz (Re=Y, Eu, Er; x=0, 0.2, 0.3) superconducting ceramics

We have investigated the influence of Ca-substitution and different rare earths on the microstructure of RE_1-x Ca_xBa₂Cu₃O_7-δ (RE= Y, Eu, Er; x=0, 0.2, 0.3) superconducting ceramics. Scanning electron microscopy, X-ray microanalysis and energy dispersive spectroscopy have been used to study the microstructure and the chemical composition of the samples. A correlation was established between the polycrystalline microstructure and phase formation depending on the additive content. We observed that calcium is distributed uniformly in the crystals. The formation of minor impurity phases improved the sintering conditions.     

151Eu and57Fe Mössbauer and X-ray diffraction study of high temperature Tl-containing superconductor

⁵⁷Fe and¹⁵¹Eu Mössbauer spectroscopy and X-ray diffractometry measurements were performed with europium and iron containing Tl₂(Ca_0.8Eu_0.2)Ba₂(Cu_0.98Fe_0.02)₂O₈ and EuBa₂Cu₃O_7-d high temperature superconductor samples in order to study the effect of europium incorporated in a Tl-containing high temperature superconductor. Significant differences have been found between the⁵⁷Fe Mössbauer spectra of Tl-containing samples with and without Eu or between the¹⁵¹Eu Mössbauer spectra of Tl-containing and 1-2-3 type superconductors. The results can be interpreted by assuming that Eu can be localized at the Ca site in the Tl-containing superconductor.     

Über Erdalkalimetalloxothallate. II. Darstellung und Kristallstruktur von Ba2Tl2O5

On Alkaline Earth Metal Oxothallates. II. Preparation and Crystal Structure of Ba₂Tl₂O₅ Ba₂Tl₂O₅ was prepared and investigated by X-ray single crystal methods (space group D? Pcmn, a = 6.264, b = 17.258, c = 6.05 Å) Ba₂Tl₂O₅ is isotypic with Ca₂Fe₂O₅.     

Sol-Gel Synthesis Route for the Preparation of Y(Ba1−x Sr x )2Cu4O8 Superconducting Oxides

Superconducting oxide ceramics of composition Y(Ba_1–x Sr _x )₂Cu₄O₈ (x = 0.00, 0.10, and 0.20) have been prepared by a simple sol-gel method based on the complexation of metal ions in aqueous medium, by chelating acetate and tartarate ligands. Homogeneous sols were obtained by complexing copper ions with tartaric acid, which prevented the flocculation of copper acetate during the gelation process. Single-phase bulk samples were obtained after firing the mixed-metal polymeric acetate-tartarate precursor to 835°C for 70 h in flowing oxygen atmosphere. Thermal decomposition of the gels was studied by thermogravimetry. Effect of strontium substitution on the properties of the compounds was studied by X-ray powder diffraction, electron microscopy, X-ray photoemission spectroscopy and resistivity measurements. These data indicate that nearly monophasic Y(Ba_1–x Sr _x )₂Cu₄O₈ superconducting samples were obtained for x < 0.30. The Sr-doping in the YBa₂Cu₄O₈ (Y-124) phase shows a pronounced effect on the superconducting properties enhancing the critical temperature from 78 K (for the non-substituted sample) to 88 K (for Y(Ba_1–x Sr _x )₂Cu₄O₈).     

Oxygen permeation and phase structure properties of partially A-site substituted BaCo0.7Fe0.225Ta0.075O3-δ perovskites

Ba_0.9R_0.1Co_0.7Fe_0.225Ta_0.075O_3-δ (BRCFT, R = Ca, La or Sr) membranes were synthesized by a solid-state reaction. Metal cation Ca²⁺, La³⁺ or Sr²⁺ doping on A-site partially substituted Ba²⁺ in BaCo_0.7Fe_0.225Ta_0.075O_3-δ oxides, and its subsequent effects on phase structure stability, oxygen permeability and oxygen desorption were systematically investigated by XRD, TG-DSC, H₂-TPR, O₂-TPD techniques and oxygen permeation experiments. The partial substitution with Ca²⁺, La³⁺ or Sr²⁺, whose ionic radii are smaller than that of Ba²⁺, succeeded in stabilizing the cubic perovskite structure without formation of impurity phases, as revealed by XRD analysis. Oxygen-involving experiments showed that BRCFT with A-site fully occupied by Ba²⁺ exhibited good oxygen permeation flux under He flow, reaching about 2.3 mL·min⁻¹ ·cm⁻² at 900 °C with 1 mm thickness. Of all the membranes, BLCFT membrane showed better chemical stability in CO₂, owing to the reduction in alkalinity of the mixed conductor oxide by La doping. In addition, we also found the stability of the perovskite structure under reducing atmospheres was strengthened by increasing the size of A-site cation (Ba²⁺>La³⁺>Sr²⁺>Ca²⁺).     

Estimation of melting (decomposition) heat of some compounds in the Y−Ba−Cu−O system

A thermodynamical method for the estimation of decomposition heat in a crystal state, incongrous and congruous melting of compounds with the use of temperature dependencies of total entropies of compounds was suggested. Entropies and heats of phase transformation of YBa₂Cu₃O₆, YBa₂Cu₃O₇, YBa₂Cu_3.5O_7.5, YBa₂Cu₄O₈, YBa₂Cu₅O₉, YBa₄Cu₃O_8.5, Y₂BaO₄, Y₂Ba₂O₅, Y₂Ba₄O₇, Y₄Ba₃O₉, YCuO₂, Y₂Cu₂O₅, Y₂BaCu₂O₅, Ba₂CuO₃, BaCuO₂, BaCu₂O₂, Ba₃Cu₅O₈ were calculated. Data, obtained by the authors earlier, are discussed.     

A Mixed Ionic and Electronic Conducting Dual‐Phase Membrane with High Oxygen Permeability

To combine good chemical stability and high oxygen permeability, a mixed ionic‐electronic conducting (MIEC) 75 wt % Ce_0.85Gd_0.1Cu_0.05O_2?δ‐25 wt % La_0.6Ca_0.4FeO_3?δ (CGCO‐LCF) dual‐phase membrane based on a MIEC–MIEC composite has been developed. Copper doping into Ce_0.9Gd_0.1O_2?δ (CGO) oxide enhances both ionic and electronic conductivity, which then leads to a change from ionic conduction to mixed conduction at elevated temperatures. For the first time we demonstrate that an intergranular film with 2–10 nm thickness containing Ce, Ca, Gd, La, and Fe has been formed between the CGCO grains in the CGCO‐LCF one‐pot dual‐phase membrane. A high oxygen permeation flux of 0.70 mL min^?1 cm^?2 is obtained by the CGCO‐LCF one‐pot dual‐phase membrane with 0.5 mm thickness at 950 °C using pure CO₂ as the sweep gas, and the membrane shows excellent stability in the presence of CO₂ even at lower temperatures (800 °C) during long‐term operation.     

Remarkable Oxygen‐Evolution Activity of a Perovskite Oxide from the Ca2−xSrxFe2O6−δ Series

Herein in we report the unprecedented catalytic activity of an iron‐based oxygen‐deficient perovskite for the oxygen‐evolution reaction (OER). The systematic trends in OER activity as a function of composition, defect‐order, and electrical conductivity have been demonstrated, leading to a methodical increase in OER catalytic activity: Ca₂Fe₂O_6?δ<CaSrFe₂O_6?δ<Sr₂Fe₂O_6?δ. Sr₂Fe₂O_6?δ also has the highest electrical conductivity and a unique type of defect‐order. In conventional experiments using glassy carbon electrode, Sr₂Fe₂O_6?δ shows better OER activity than the current state of the art catalysts, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ and RuO₂. It also offers a high intrinsic electrical conductivity, which allows it to act as a catalyst without the need for glassy carbon electrode or carbon powder. Pure disks of this material exhibit an outstanding activity for OER, without any additives or need for electrode preparation.     

Preparation of Y2Ba4Cu7O15-δ Superconductor without High Oxygen Pressure

The synthesis of bulk Y₂Ba₄Cu₇O_15-δ superconductor at atmospheric oxygen pressure via solid state sintering is reported. Temperature ranging from 860 to 890 °C as well as time interval over 2 to 15 days were used to investigate the formation of the Y₂Ba₄Cu₇O_15-δ phase. A time-temperature profile characterizing the conditions for the preparation of Y₂Ba₄Cu₇O_15-δ phase suggests the optimal condition to be sintering at 890 °C for over 10 days. Detailed results of X-ray diffraction, electrical resistivity, iodometric titration and magnetization measurements are described.