P.T. Wu

Superconductivity and the metal-semiconductor transition in the septenary oxide system, (Tl0.5Pb0.5)(Ca1−yYy)Sr2Cu2O7−δ

Abstract The septenary compound (Tl0.5Pb0.5)(Ca1−yYy)Sr2Cu2O7−δ exhibits the highest superconducting transition temperature (110 K) yet observed in the TlCaBa2Cu2O7−δ (1122) structure type. This complex, six-blend combination of metallic elements which make up the material is, however, compensated by a relatively simply crystal structure, which bears many similarities to that of 90 K superconductor YBa2 Cu3O7−δ. In this note we report some important features of the cation-substitution chemistry of the title compound, drawing attention to the fundamental changes in the electronic properties of the (Tl0.5Pb0.5) (Ca1−yYy)Sr2Cu2O7−δ system as Ca2+ is replaced by Y3+. Superconductivity is observed over the homogeneity range y = 0–0.5, with the superconducting transition temperature showing a maximum (108 K) at y = 0.2. Across the homogeneity range y = 0.6–1.0, the material undergoes a metal-semiconductor transition.

Accelerated formation of 110 K high Tc phase in the Ca-and Cu-rich Bi-Pb-Sr-Ca-Cu-O system

The crystal structure and superconducting properties of the Bi‐Pb‐Sr‐Ca‐Cu‐O system with Ca‐ and Cu‐rich nominal composition were investigated. A nearly single‐phased 110 K high Tc superconductor can be obtained with 852 °C/20 h sintering from the starting composition of Bi1.7Pb0.4Sr1.6Ca2.4Cu3.6Oy. X‐ray diffraction patterns, resistivity measurement, diamagnetic susceptibility results, and scanning electron micrographs all indicate that the Ca‐ and Cu‐rich nominal composition would result in better superconducting properties than those of Ca:Sr=1:1 Bi‐Pb‐Sr‐Ca‐Cu‐O compounds in a much shorter sintering time.

High transport critical currents and flux jumps in bulk YBa2Cu3Ox superconductors

A preferentially oriented bulk YBa2Cu3Ox superconductor was prepared by the liquid phase method. A continuous dc current carrying capacity exceeding 120 A with current density higher than 37 300 A/cm2 at 77 K has been obtained. The high current effects causing fracture of the sample were studied in relation to the thermal diffusion properties. In magnetization measurements, flux jumps were observed for currents in the sample a‐b plane below the temperature of about 20 K. The jumps are more closely spaced with decreasing temperature or increasing field sweep rates.

Superconductivity with Tc(zero) above 105 K in Tl-containing septenary oxides with Y1Ba2Cu3Oy-like structure

Abstract A family of high- T c superconductors with T c(zero) above 105 K is described. The compounds are seven-element blends with the general formula Tl 0.5 Pb 0.5 Ca 0.8 A 0.2 Sr 2 Cu 2 O x ( A = Y ), and rare-earth elements La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb). The results represent one of the first cases where chemical substitution has significantly raised T c in a known compound. The septenary compounds are tetragonal in structure with almost identical lattice parameters to those of now-famous Y 1 Ba 2 Cu 3 O y structure. The samples were highly reproducible, stable and remarkably homogeneous both in composition and structure. The preparative conditions were found to be much less stringent than those of other copper-based high- T c superconductors.

Synthesis and Characterization of High-Tc Superconducting Oxides by the Modified Citrate Gel Process

High-Tc superconducting Y-Ba-Cu-O, Bi-Ca-Sr-Cu-O, and Tl-Ca-Ba-Cu-O materials were synthesized using a modified citrate precursor process. Ethylenediamine was used to neutralize the metal citrate solution and facilitate the complex formation. Highly homogeneous and viscous gel without any precipitation was obtained. In the Tl-Ca-Ba-Cu-O system, the Ca-Ba-Cu-O precursor was first prepared by the sol-gel method, then mixed with thallium oxide for final sintering. The complex formation was monitored by UV/VIS spectrophotometry. The thermal decomposition of gels and the formation of superconducting phases had been studied using IR, DTA, TGA, and XRD. This process is highly reproducible and leads to powders with excellent homogeneity and small particle size for easy sinterability.

First example of indium as a practical alternative to thallium in high-Tc superconductors

Abstract The substitution of indium for thallium in the material Tl x Pb 1− x Ca 1− n Y n Sr 2 Cu 2 O y has been carried out. A stable and reproducible T c(mid-point) of circa 60 K for the new material, In 0.3 Pb 0.7 Ca 0.8 Y 0.2 Sr 2 Cu 2 O y was observed in both electrical resistivity and magnetization measurements. The X-ray diffraction pattern shows that the superconducting phase can be indexed on the basis of the tetragonal thallium-based 1122-type compound.

Preparation of HIGH-Jc superconducting oxide in the Bi-Sr-Ca-Cu-O system by oxalate gel processing

Abstract The high- T c superconducting oxide in the Bi-Sr-Ca-Cu-O system, with superconducting transition temperature at about 80 K, has been fabricated by the sol-gel method using oxalic acid and nitrates as starting materials. Compared to the conventional solid-state reaction method, this chemical technique offers less fabricating time and lower sintering temperature to get a homogeneous superconductor. Thermal analysis, infrared spectra and X-ray diffraction data indicate that the mechanisms of decomposition and phase transformation of specimens prepared at various temperatures are greatly different.

The fabrication and characterization of superconducting Tl-Pb-Ca-Pr-Sr-Cu-O compounds with Y1Ba2Cu3Oy-like structure and Tc (zero) up to 106 K

Bulk superconductivity with T c (zero) up to 106 K in a Tl 0.5 Pb 0.5 Ca 0.8 Pr 0.2 Sr 2 Cu 2 oxide with Y 1 Ba 2 Cu 3 O y -like structure was observed. A single phase, tetragonal structure with a = 0.382 ± 0.001 and c = 1.200 ± 0.001 nm and space group P4/mmm, was found in Tl 0.5 Pb 0.5 Ca 0.8 Pr 0.2 Sr 2 Cu 2 oxide samples by X-ray diffraction. T c (zero)s were found to be 78, 96, 106, 98, 45 and 25 K for Tl 0.5 Pb 0.5 Ca 1− x Pr x Sr 2 Cu 2 oxide samples with x = 0, 0.1, 0.2, 0.3, 0.5 and 0.7, respectively. For x = 1, the samples exhibited semiconducting behavior. All sintered samples were found to be remarkably homogeneous both in composition and structure. They were also exceptionally regular in microstructure and highly reproducible and stable. Although the six-metal blend makes the system appear to be rather complicated composition-wise, it is compensated by the relatively simple crystal structure. Furthermore, the preparation conditions are much less stringent than those for other copper-based high- T c superconductors.

Epitaxial growth of high Tc superconducting Y‐Ba‐Cu‐O thin films on (001)MgO by a chemical spray pyrolysis method

A chemical spray pyrolysis method has been applied to grow epitaxial films of a high Tc Y‐Ba‐Cu‐O compound (YBCO) on (001)MgO. Films as thin as 0.6 μm in thickness was found to exhibit excellent superconducting transition behavior. For films up to 2 μm in thickness, typical values of Tc onset, Tc zero and transition width (90%–10%) were measured to be 82, 76, and 1.5 K, respectively. Both plan‐view and cross‐sectional transmission electron microscopy revealed that the orientation relationships between the epitaxial films and the substrate are [001]YBCO//[001]MgO and (110)YBCO//(200)MgO. Twins, which may be perceived as domains that are rotated 90° along the c axis of the thin films with respect to the substrate, were found to be copiously present. The influences of the configuration of the oriented growth of overlayer thin films on the superconducting properties are addressed. The advantages of the chemical spray pyrolysis in producing superconducting thin films are outlined.

Epitaxial Growth of High-Tc Bi-Ca-Sr-Cu-O Superconducting Layer by LPE Process

A Bi-Ca-Sr-Cu-O epitaxy layer has been successfully synthesized by the liquid-phase epitaxial (LPE) process on the MgO(001) substrate. X-ray diffraction patterns and SEM surface morphology analysis indicated a highly preferred orientation. This film showed superconductivity with onset and zero resistivity temperature of 110 K and 81 K, respectively.