R. Abraham

Preparation by in-situ reaction and physical characterization of Ag(Au) and Ag(Pd) sheathed (Tl, Pb, Bi) (Sr, Ba) 2Ca2Cu3O9−δ tapes

Superconducting tapes with high-purity substituted Tl(1223) powders (> 95 wt.%, Tc ∼120 K) were produced by a new method based on an in-situ reaction under high pressure of helium (25 bar He/0.5 bar O2). A Ag(20 wt.% Au) alloy showed to be a good sheath material for an in-tape reaction up to 990°C, even when involving partial melting of the oxide which favors the grain growth. Due to an interaction between Pd and the oxide, a Ag(6 wt.% Pd) alloy is much less suitable for tape preparation. Reproducible transport critical current densities of 9 kA/cm2 (77 K, 0 T) were obtained for Ag(Au) sheathed Tl(1223) tapes with in-tape-reacted powder, the highest value being 10 kA/cm2. The magnetic-field dependence of jc of such tapes was improved with respect to Ag sheathed tapes with pre-reacted Tl(1223) powders. Locally textured samples with up to 50 μm large plate-like grains were obtained after partial melting in Ag(Au) sheathed tapes.

Metallurgy, HRTEM, magnetic properties and specific heat of Tl2Ba2Cu1O6+δ 90 K-superconductors obtained by a new process

High-quality Tl2Ba2CuO6+δ (2201) compounds were synthesized using a high-pressure route. Starting from Tl2Ba2O5 and CuO, the reactions yielding either the orthorhombic or the tetragonal (superconducting) phases were carried out under 100 bar of oxygen or argon and helium, respectively. Preferential thallium losses are prevented and high sintering temperatures can be used to obtain pure samples with large 2201 grains. The experiments performed include metallographic examinations, X-ray diffraction, microprobe analysis, plasma emission spectroscopy, HRTEM, AC susceptibility, Meissner field cooling, hysteresis loops, normal-state susceptibility and specific heat measurements.

Preparation of pure Tl2Ba2CuO6±x: The contribution of phase equilibrium studies

The formation of the pure thallium-based cuprate Tl2Ba2CuO6+x (2201) using a convenient process in the ternary TIO1.5-BaO-CuO phase diagram is described. This process is based on the congruent formation of the oxide Tl2Ba2O5 to which CuO is mixed, constituting a quasi-binary system. In the resulting 2201 compound no trace of the main poisoning magnetic impurity BaCuO2 could be detected by AC susceptibility measurements. The orthorhombic to tetragonal transformation is asserted to be due to thallium deficiency which occurs during the heat treatments at a temperature depending on the oxygen partial pressure.

Structural characterization and superconducting properties of (Tl0.5Pb0.5)(Sr2−xBax)Ca2Cu3O9−δ

Abstract Samples of nominal composition (Tl0.5Pb0.5)(Sr2−xBax)Ca2Cu3O9−δ, 0 ≤ x ≤ 0.6, and (Tl0.6Pb0.2Bi0.2)(Sr1.8Ba0.2)-Ca2Cu3O9−δ were characterized by X-ray powder diffraction, scanning electron microscopy and AC susceptibility measurements. In the Bi-free samples, the highest weight fraction of the 1223 phase, 91.3%, was observed for x = 0.4. Above this Ba content, BaPbO3 perovskite appeared as main impurity phase. In the Bi-containing sample the weight fraction was even higher, 92.6%. The Ba content of the 1223 phase, as determined from X-ray diffraction data, was in agreement with the nominal composition of the samples. The unit cell parameters increased when the Sr atoms were progressively replaced by the larger Ba atoms. The refinement of the crystal structure of the 1223 phase revealed a significant off-centering of the Tl site. The distance from the splitted site to the ideal position on the four-fold axis was found to be ∼ 0.26 A for the Ba-containing samples, but only 0.07 A for the Ba-free sample. The distance from the Cu atom to the apical atom of the square pyramid formed by the oxygen atoms increased from 2.29 to 2.50 A as the Ba content was raised. The critical temperature remained between 116 and 120.5 K and was not significantly affected by changes in composition or structure.

A new elaboration process of the superconducting Tl2Ba2Cu1O6 phase with Tc=90K

Abstract We have synthesized high quality ceramic Tl 2 Ba 2 Cu 1 O 6+δ (2201) samples using the high pressure, high temperature route. At high oxygen content, the structure is orthorhombic and the samples are metallic but non-superconducting. Upon lowering the oxygen content, the symmetry changes from orthorhombic to tetragonal or pseudo-tetragonal. In the latter phase, the maximum superconducting critical temperature reaches 92 K. Optical micrographs show large 2201 grains and some traces of impurity phases like Tl 2 Ba 2 O 5 , Ba 2 Cu 3 O x and CuO. X-ray diffraction shows only the 2201 phase. Plasma emission spectroscopy indicates that the global sample stoichiometry is Tl:Ba:Cu=2:2:1. This analysis proves that the high pressure route effectively prevents the thallium evaporation. HREM investigations exclude the possibility of cation vacancies. Microprobe analyses (EDAX) show no variation of the cation stoichiometry between the 2201 grains. X-ray diffraction on a superconducting single crystal yields a refined composition Tl 1.94 Ba 2 Cu 1.06 O 6 .

The BaO-TlO1.5 system

Abstract The equilibrium phase diagram of the binary system BaO-TlO 1.5 under an oxygen pressure of 1 atm was studied by means of differential thermal analysis coupled with a thermobalance, X-ray diffractometry and plasma emission spectroscopy. Four intermediate phases have been identified: Tl 2 BaO 4 , Tl 6 Ba 4 O 13 , Tl 2 Ba 2 O 5 and TlBa 2 O 3.5 . The congruent melting of Tl 2 Ba 2 O 5 offered a chance to prepare the superconducting cuprate Tl 2 Ba 2 CuO 6 avoiding the formation of BaCuO 2 , a poisoning phase for reliable normal state studies at low temperatures.

Tl/Pb and Sr/Ba cuprates of type 1212: compositional effect on the purity and on the superconducting properties

Abstract The effect of Sr/Ba and Tl/Pb substitutions has been investigated for Tl 1− x Pb x (Sr 1− y Ba y ) 2 CaCu 2 O 7− δ (0≤ x and/or y ≤1) compounds produced by conventional solid state reaction. The observed phases were characterized by X-ray powder diffraction, scanning electron microscopy and their nominal composition has been controlled by plasma emission spectroscopy. A tentative single phase field is proposed at T ≤960°C. For x =0.5, the highest weight fraction of the (Tl 0.5 Pb 0.5 )(Sr/Ba)-1212 phase was found to be 96.4% and 94% when Ba increases from 0 to 0.2. It drastically decreases when Ba content is over 0.3. For Ba over 0.3, (Tl/Pb)(Sr/Ba)-1223 and perovskite (Tl/Pb)(Sr/Ba)O 3 are observed as main impurities and their content increases with Ba stoichiometry. The unit cell parameters of (Tl 0.5 Pb 0.5 )(Sr/Ba)-1212 phase increase when Sr atoms are progressively replaced by Ba atoms. Structural refinement leads to conclude that 10 at% of Tl would be substituted on Ca sites and less than 2 at% of Ca could be substituted on Sr sites. The distance between the Cu atom and the apical O atom of the square pyramid increases only slightly when the Ba content raises. The critical temperature remains between 80 and 90 K and seems to be affected by the substitution Sr/Ba.

Quaternary system SrO-CaO-BaO-CuO at 950 °C: I. Phase relationships in the limiting ternary system SrO-CaO-CuO

Knowledge of the succession of phases in dependence on composition and temperature is essential in order to control the formation of pure superconducting thallium-based cuprates (Tl/Pb){sub m}Sr{sub 2}Ca{sub n{minus}1}Cu{sub n}O{sub x}. Mixtures (Sr/Ba)O-CaO-CuO are used as precursors in their elaboration. In the first stage of their investigation, the ternary system SrO-CaO-CuO has been reinvestigated at 950 C, in order to determine precisely the phase relationships in the ternary subsystem. In the second stage, the study will be extended to the quaternary BaO-SrO-CaO-CuO system. The present paper reports the results concerning the polyphased fields in the ternary system under isothermal conditions at 950 C under 1 bar of oxygen. A comparison with literature data shows that new domains are observed. Solid phases mentioned in the literature are confirmed, but the observed three phases equilibria are not the same.

Quaternary system SrO-CaO-BaO-CuO at 950 °C: II. Phase equilibria related to the substitution Sr/Ba in (Sr/Ba)2Ca2Cu2Cu3O7 and (Sr/Ba)2CaCu2O5 samples

Abstract A partial study of the solid-solid equilibria in the quaternary system BaOSrOCaOCuO has been undertaken in order to determine the observed phases and their evolution when Ba is substituted for Sr in mixtures (Sr/Ba)2CaCu2Ox and (Sr/Ba)2Ca2Cu3Ox. The temperature was fixed at 9SO °C and samples were elaborated under 1 bar of flowing oxygen. It has been shown that two four-phased domains exist in this quantemary system, including the following phases: ( Ba /Sr/Ca)CuO2, ( Sr/Ca /Ba)2CuO3, ( Sr/Ca /Ba)14Cu24Ox and Ba4CaCu3O8 substituted Sr/Ca for the one and ( Ba/Sr /Ca)CuO2, ( Sr/Ca /Ba)2CuO3, ( Sr/Ca /Ba)24Cu24Ox and ( Sr/Ca /Ba)CuO2 for the other.

Growth and characterization of (Tl,Pb)(Sr,Ba)2Ca2Cu3Oy single crystals

Abstract (Tl,Pb)(Sr,Ba)2Ca2Cu3Oy single crystals were grown by a self flux method. We find that the crystal growth strictly depends on the starting composition, on optimizing the melting temperature by suitable additives and on the choice of the cooling rate. The crystals were characterized by X-ray diffraction, EDX, SEM and SQUID magnetometry. The supercon-ducting transition temperatures and the best transition widths obtained for as-grown crystals are 105.0–111.2 K and 3–6 K, respectively. Layer by layer growth patterns, but no spiral growth, were observed by SEM. Annealing effects were studied in O2 or Ar atmosphere and led to improvements of Tc and the transition width only following annealing in Ar. Annealing at temperatures above 600°C results in a decomposition of the crystals into the insulating oxides Tl2O and (Sr,Ba)CaCu.