C. A. Youngdahl

Synthesis of phase-pure orthorhombic YB2Cu3Ox under low oxygen pressure

Abstract Reaction of Y 2 O 3 , BaCO 3 and CuO for 4 h at 800°C in flowing O 2 with a total pressure of about 2.7×10 2 Pa, followed by cooling in O 2 at ambient pressure, has produced phase-pure orthorhombic YBa 2 Cu 3 O x . Keeping the ratio of O 2 to evolved CO 2 above 50 was necessary to ensure phase purity. The resultant powder yielded pressed and sintered pellets with improved super-conducting properties.

Microstructure and associated properties of YBCO superconductors prepared by melt-processing techniques

From the standpoint of applications, melt-processed bulk YBa2Cu3Ox (YBCO) superconductors are of considerable interest. In this paper, we studied the microstructure and levitation force of melt-processed YBCO, YBCO plus Y2BaCuO5, and YBCO+Pt. Large single-crystal samples, grown by a seeding technique, were also studied. The levitation force was highest in melt-processed samples made by the seeding technique.

Y2BaCuO5 as a Substrate for YBa2Cu3Ox

Bulk Y2BaCuO5 has been found to have a higher stiffness but a lower thermal expansion than YBa2Cu3Ox. Y2BaCuO5 exhibits semiconducting behavior at room temperature. Composite tapes consisting of layers of Y2BaCuO5 and YBa2Cu3Ox cracked upon sintering because of large differences in shrinkage rate. Addition of 15 volume % Ag to the YBa2Cu3Ox strengthened the material and prevented cracking.

Application of sinter-forged Bi-2223 bars to 1500-A a.c. power utility service as high-frequency current leads in a 77-4 K temperature gradient

Abstract Two assemblies of Pb-BSCCO 2223 superconductor bars were produced for use in ac connections between utility system lines at room temperature and a fault-current limiter operating at 4 K. Each assembly, consisting of four parallel bars arranged within a 100 mm-diameter boundary, delivered 1500 A (peak), 50–60 Hz ac through the 77-4 K range of the temperature gradient while dissipating 5000 A cm−2 at 4 K; magnetic field sensitivity was relatively small. Although thermal conductivity tests showed values much higher than those found in the literature for polycrystalline Pb-BSCCO 2223 made by other processes, this undesirable result was counterbalanced by the relatively high values of critical current, which enabled the use of bars with smaller cross-sectional areas. Typical 50-Hz ac power losses at 77 K in each bar (with a cross-sectional area of 0.54 cm2) were 1 mW cm−1 of length at 310 A and 1.75 mW cm−1 at 375 A. Losses were much smaller at 4 K. Bars were 25 cm long, including ends that carried silver contacts sinter forged to the ceramic, with a 21 cm silver-free length in the thermal gradient between the fixed-temperature terminals. Thermal efficiency of the assemblies was assessed by helium boiloff tests.

Performance characterizations of Bi-2223 composite powder-in-tube conductor elements☆

Abstract A 30 MW superconducting magnetic energy storage (SMES) system for electric utility application is currently under design by Babcock & Wilcox. High-temperature-superconductor (HTS) current leads have been designed to reduce the refrigeration load of the SMES systems 16 kA capacity electrical connection between the 300 K power conditioning system and the 4 K low-temperature-superconductor energy storage magnet. Each lead employs an array of 18 parallel HTS conductor elements. Each conductor element consists of a sintered stack of Bi-2223 HTS powder-in-tube tapes, each having an Au-alloyed Ag sheath. Laboratory performance characterization of preproduction conductor elements included critical current with applied field at 77 and 60 K, thermal and electrical cycling effects, critical current vs axial strain, and end-connection electrical resistivity. Details of the conductor element application and construction are described. The methodology and results of the performance characterization are presented. The testing demonstrates mechanical robustness and electrical stability required for long-term practical application in the SMES system.

Shape forming high-Tc superconductors

Before the potential of high-temperature superconductors can evolve from the realm of science fantasy to the marketplace of practical reality, materials scientists and engineers must first develop viable methods for fabricating these revolutionary materials into a wide assortment of functional shapes and sizes.

High-Tc superconductors: fabricating technologies and future perspectives

Processing methods for production of bulk high-{Tc} superconductors are reviewed. Conductors made by conventional forming and sintering methods can generally carry only small current densities. Melting and powder-in-tube methods yield superior conductors, but further improvements are necessary before these superconductors can be employed in most large-scale applications. 36 refs., 6 figs.

Synthesis of ceramic superconductors under low oxygen pressure

We have developed a process for synthesizing orthorhombic YBa2Cu3Ox (123) superconducting powders by calcination of the precursor powder under reduced total oxygen pressure. Because a single calcination at 800°C for 4 h in flowing oxygen at a pressure of 2 mm Hg results in essentially phase-pure material, total calcination times have been drastically reduced. At liquid nitrogen temperature, sintered pellets made from this powder have critical current densities of ∼1000 A/cm2 in zero applied magnetic field.

Microstructure and Electrical Properties of Bulk High-Tc Superconductors

Bulk forms of the superconductors Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O generally have a low critical current density (Jc). At 77 K in zero magnetic field, values greater than about 103 A cm−2 have seldom been obtained. The most notable exceptions are for melt-textured rods and filaments,1–3 for which Jc values often exceed 104 A cm−2. Values of Jc approaching 104 A cm−2 have also been reported for wires and tapes processed in Ag tubes.4–6 Little information about the microstructures of these composite conductors has emerged. This work was undertaken to examine the relationships between processing, microstructure, and electrical properties for high-temperature superconductors processed as pellets or Ag-clad tapes.

Reactive sintering of Bi 2 Sr 2 CaCu 2 O x superconductors

Three phase assemblages were used to produce Bi2Sr2CaCu2Ox (2212) during sintering: a mixture of 20% Ca-rich 2212+80%2212, partially synthesized 2212, and Bi2Sr2CuOx (2201)+(1/2 Ca2 CuO3+1/2 CuO) (denoted 0011). The mixture of 2201+0011 produced highly pure 2212 within 50 h of heating in air at ≈850°C. Ag tubes were filled with a mixture of 2201+0011 and worked into tapes by a powder-in-tube process. Heat treatments produced microstructures consisting of small, highly textured 2212 grains. Tc values were ≈70K. Transport Jc values at 4.2K were ≈104 A/cm2.