Cyril Chiang

Formation mechanism of 110 K high-Tc superconducting phase in the Ca- and Cu-rich Bi-Pb-Sr-Ca-Cu-O system

Abstract The starting compositions of Bi 1.7 Pb 0.4 Sr 2− x Ca 2+ x Cu 3.6 O y ( x ⩾0) have been studied to illustrate the formation mechanism of 110 K superconducting phase. It was proposed that a reactive liquid-phase-aided sintering process is responsible for the promotion of 110 K phase in the Pb-doped Bi-Sr-Ca-Cu-O system. The Ca-rich stoichiometry containing more Ca 2 PbO 4 in calcined powder would result in more reactive liquid phase during the sintering period. The acceleration of the formation of 110 K phase and large grain growth were observed. A nearly monophasic 110 K superconducting phase can be easily obtained within 20 h when appropriate stoichiometry and sintering conditions were met.

Preparation of high purity 110 K phase in the (Bi, Pb)-Sr-Ca-Cu-O superconductor system using a solution method

Abstract A solution method has been demonstrated to purify the 110 K superconducting phase in the (Bi, Pb)-Sr-Ca-Cu-O system. Oxalic acid was used to chelate metal cations from the nitrates. The amount of chelating agent impacts the cation absorption in the precursor. The higher purity 110 K superconducting phase can be obtained in the shortest time and at the lowest sintering temperature. X-ray diffraction, resistivity and magnetization measurements all indicated positive evidence. The process can be expedited when the stoichiometry was enriched by calcium and copper. The preparation technique is considered valuable to purify multi-phase superconductors for mass production.

Accelerated formation of the three Cu‐O layered Tl‐(Pb,Bi)‐Sr‐Ca‐Cu‐O compound

Bulk samples containing nearly single phase of the three Cu‐O layered compound in the Tl‐Pb(Bi)‐Sr‐Ca‐Cu‐O system have been readily synthesized. By partially substituting Ca for Sr, an accelerated formation of the desired three Cu‐O layered phase was observed when the Ca/Sr ratio reached up to 2.4/1.6. The phase purity was clearly evidenced by x‐ray diffraction and magnetization measurement. The synthesis is valuable for further application interests.

Structural investigation of oxygen-deficient perovskite CaMnO2.75

Perovskite CaMnO3 and its oxygen-deficient derivatives have been prepared from solid solution precursor with a calcite-type structure. CaMnO3−x phases are constructed from MnO6 octahedra and MnO5 square pyramids. From X-ray powder data CaMnO2.75 75 was indexed on an orthorhombic cell with a = 5.35(1)A, b = 20.96(2)A and c = 7.47(1)A. Rietveld refinement of time-of-flight neutron diffraction data was attempted for several models containing 50 mol% MnO5 and 50 mol% MnO6 based on MnO6 octahedra and MnO5 square pyramids, but the results were inconclusive. Simulation patterns illustrate the intrinsic difficulty for structural comprehension.

Flexible and superconducting properties of sintered Bi-Pb-Sr-Ca-Cu-O tapes

Flexible Bi1.81Pb0.43Sr1.71Ca2.14Cu3Ox superconducting tapes have been fabricated by the powder‐in‐silver tubing technique followed by appropriate sintering. The sintered tape, Jc≳104 A/cm2 at 77 K, can be bent into an arc with a 0.9‐cm radius of curvature without deteriorating the transport properties at liquid nitrogen temperature. This indicates that the superconducting a‐b plane of the Bi‐based tape has a strain of 0.05%–0.1%.

Preparation of high-purity Tl-based 1223'' superconductor phase by modified Pechini process in water solution

Because of the mixed metal valance state of thallium in a Tl‐based superconductor, a nearly single ‘‘1223’’ phase can be readily prepared by a modified Pechini method (US Patent 3 330 697). Tl2O3 and other metal nitrates are dissolved in an aqueous solution plus a certain amount of oxalic acid for chelating the metal cations. The desired phase shows zero resistance at 114 K and the phase purity is identified by x‐ray diffraction. Characteristics of superconductivity also are evidenced by the Meissner effect. The preparation technique is considered valuable to purify multiphase superconductors for mass production.

Synthesis of the three Cu-O layered Tl-(Pb,Bi)-Sr-Ca-Cu-O compound

A three Cu-O layered superconducting compound in the Tl-(Pb,Bi)-Sr-Ca-Cu-O system has been prepared by different processes. These include the use of excess Ca and Cu for off-stoichiometric composition, and the increase in Ca/Sr ratio for modified stoichiometric composition. A nearly single phase of 1223 superconductor can be obtained within three hours of heat treatment using the modified stoichiometric composition, whereas the intermediate phase (Ca,Sr)2CuO3 was remarkably accompanied in other synthetic processes. Observations were also corroborated by X-ray diffraction and magnetization measurements.

Comparative study to enhance the phase formation of Tl-based superconductor with three CuO layers

Abstract Characteristics of the “1223” phase in the (Tl, Bi, Pb)SrCaCuO superconductor system have been enhanced using an over-stoichiometric “1234” precursor (Tl 0.64 Pb 0.2 Bi 0.16 )Sr 2 Ca 3 Cu 4 O y . The quality of the 1223 phase heavily depends on the synthetic condition of the 1234 precursor. The precursor synthesized from Sr 2 Ca 2 Cu 3 O y yields the most “1223” phase after 920°C sintering for 10 h, whereas a remarkable amount of the “1212” phase containing two CuO layers was accompanied in other synthetic courses. Although some solid solution impurities are observed, they are not inevitable with a Ca-rich composition. Quality of the superconducting phase was examined by X-ray diffraction, resistivity and magnetization measurements.

Formation of (Tl, Pb, Bi)Sr1.6Ca2.4Cu3Oy superconducting phase with Tc = 105 K by nitrates conversion

Abstract In the study of (Tl, Pb, Bi)-Sr-Ca-Cu-O systemm, a nitrate conversion method was developed to prepare (Tl, Pb, Bi)Sr1.6Ca2.4Cu3Oy (1223 phase) with high yield in short reaction time. Also, the well mixing of precursors including Tl2O3 could be achieved avoiding a two-stage reaction. The studies of individual reactant indicated that all the reactants were nitrates except the formation of thallium oxide. Moreover, a quick oxidation from Tl+ to Tl3+ was speculated after carrying out a titrametric method. X-ray diffraction investigation as well as physical characterizations confirmed the formation of 1223 phase from as-prepared precursors. This method provides a possible and simple route to make the 1223 sample in large scale.

Mechanism for thallium-based superconductors to form multi-CuO2 layers

Abstract In ceramic superconductor critical temperature ( T c ) is normally seen to increase with the increasing number of CuO 2 layers. From bismuth to thallium the most notable change occurs when phases with double layers convert to the corresponding tripled ones. Using XRD and SEM/EDS we will demonstrate the multi-layer formation mechanism for (Tl, Bi, Pb)-Sr-Ca-Cu-O system, in which CuO 2 layers gradually accumulate via a transient liquid phase. Such a liquid phase is clearly observed on the sample quenched from high temperatures, and is believed to accelerate the desired phase formation