J. P. Singh

Mechanical and superconducting properties of sintered composite YBa2Cu3O7−δ tape on a silver substrate

Flexible sintered composite YBa2Cu3O7−δ tapes on a silver substrate have been fabricated by the tape casting technique followed by appropriate sintering. The sintered composite tape can be bent into an arc of a 27‐mm‐diam circle without breaking. This represents approximately 2% strain, as compared with a typical YBa2Cu3O7−δ superconductor material without silver, which fractures at a strain of ∼0.1–0.2%. The silver addition does not adversely affect the superconducting properties of the composite.

Sintering of YBa2Cu3O7−x compacts

Abstract Heating of YBa 2 Cu 3 O 7− x compacts above about 930°C is shown to induce liquid formation. Presence of the liquid phase results in excellent densification, but limited superconducting properties. Sintering below 930°C occurs primarily by solid-state diffusion. Although the density of these samples is low, the superconducting properties are similar to those of the dense materials produced via liquid-phase sintering. The highest current densities (≈ 500 A/cm 2 ) have been obtained in these solid-state sintered samples.

Electrical conductivity and chemical diffusion in sintered YBa2Cu3Oy

Abstract Sintered specimens of the superconducting ternary oxide, YBa2Cu3Oy, were equilibrated in air or in pure oxygen in the temperature range of 293–932 °C. The effect of chemical diffusion on the conductivity shows that the reduction/oxidation processes when oxygen is incorporated on or removed from lattice sites (p-type conduction). Using measured conductivity data and oxygen stoichiometries, the concentration and mobility of holes were calculated.

Role of p O2 in microstructural development and properties of YBa 2 Cu 3 O x superconductors

An evaluation of the effects of oxygen partial pressure (pO2) on sintering behavior and the resulting microstructure of YBa2Cu3Ox (YBCO) indicates that sintering kinetics are enhanced at reduced pO2. The density of specimens sintered at 910 °C increased from 79 to 94% theoretical when pO2 was decreased from 0.1 to 0.0001 MPa. It is believed that increase in density with decrease in pO2 is the result of enhanced sintering kinetics, due probably to increased defect concentration, decreased activation energy of the rate-controlling species, and possibly the presence of a small amount of liquid phase. Sintering at 910 °C resulted in a fine-grain microstructure, with an average grain size of ≍4 μm. Such a microstructure results in reduced microcracking. Consequently, strength as high as 191 MPa is achieved. Reduced microcracking may have important implications for developing microstructures with improved critical current density.

Calcination of YBa2Cu3O7−x powder☆

YBa2Cu3O7−x was synthesized by mixed-oxide reaction of Y2O3, BaCO3, and CuO. The starting constituents were mixed, followed by multiple steps of calcination with interposed grinding. Heating rapidly to 900°C was shown to minimize formation of liquid during calcination while still producing a phase-pure powder. Slow heating to 900°C or fast heating to 950°C resulted in formation of a liquid phase deleterious to YBa2Cu3O7−x synthesis.

Effects of thermomechanical treatment on phase development and properties of Ag-sheathed Bi(Pb)−Sr−Ca−Cu−O superconducting tapes

The effects of thermomechanical treatment, starting composition, and Ag addition on microstructure and corresponding critical current density (Jc) of Ag sheathed Bi1.8Pb0.4Sr2Ca2.2Cu3Ox (BSCCO) superconductor tapes made by a powder-in-tube technique have been studied. It was observed that for sintering in a 10% flowing oxygen atmosphere, 835-840 degrees C is the optimum processing temperature range for improved Jc. Above this temperature range, although the grain alignment increases, the content of 2223 phase decreases, resulting in a corresponding decrease in Jc. The tapes were subjected to repeated thermomechanical treatment of pressing and sintering. These thermomechanical treatments resulted in increased grain size and alignment, and increased content of 2223 phase, leading to increased Jc. X-ray analysis suggests that the 2212 phase reacts with non-superconducting phases, such as CaCuO2, (SrCa)2CuO3, CaO, and CuO, to form the 2223 phase. Furthermore, addition of Ag to a superconducting BSCCO core slightly improved Jc without adversely affecting the transition temperature of a Ag-sheathed BSCCO tape.

Influence of processing methods on residual stress evolution in coated conductors

Several processing methods are under study for deposition of different layers of YBa2Cu3O7−x- (YBCO-) coated conductors. The effect of these processing techniques on residual stress evolution in thin films of yttria-stabilized zirconia (YSZ) and YBCO was evaluated by measurement of the residual stresses using x-ray diffraction (XRD). The YSZ films (textured and nontextured) were deposited on Hastelloy C substrates by ion-beam-assisted deposition (IBAD), and the YBCO films were deposited on lanthanum aluminate (LaAlO3) substrates by pulsed laser deposition (PLD) and sol–gel techniques. The measured residual stresses in the YSZ films (both textured and nontextured) were more compressive than the calculated thermal mismatch stress between Hastelloy C and YSZ, apparently due to intrinsic compressive stresses induced in the YSZ films during IBAD processing. In addition, a lower compressive residual stress was measured in the textured YSZ film compared to the nontextured film because of a reduction in the intrinsic compressive stress in the textured film. PLD processing of YBCO films on LaAlO3 substrate resulted in a lower tensile residual stress (in the YBCO film) than the calculated thermal mismatch stress between YBCO and LaAlO3. This difference is attributed to the generation of intrinsic compressive stresses in the YBCO film during PLD, in a manner similar to IBAD. In comparison to IBAD and PLD, sol–gel processing apparently generated negligible intrinsic stresses, resulting in a good agreement between the measured residual stress in the YBCO film and the calculated thermal mismatch stress between YBCO and LaAlO3.

Enhancement of critical current density and strain tolerance in Ag-sheathed BSCCO (2223) tapes by continuous silver reinforcement

We have used a modified powder-in-tube technique in various configurations to demonstrate significant improvement in the transport properties and strain tolerance of Ag-sheathed BSCCO superconducting tapes. In this process, the BSCCO core has been reinforced with a silver rod placed concentrically within the silver tube and then processed into thick tapes by established metal-working techniques. A noticeable improvement in critical current density, , was observed as a result of this modification. This increase is attributed to increased texturing, decrease in thickness of BSCCO layers, increase in the length of the Ag/BSCCO interface, and decrease in the volume fraction of the superconducting core which is less effective in contributing towards the overall critical current of the tape. Further improvements have been made to this configuration by incorporating another ring of superconductor into the core. The improved configurations led to a significant increase in over that of monolithic BSCCO tapes. Critical current densities of at 77 K and self-field have been consistently achieved in thick tapes after 150 h of sintering. Analysis of critical current through various sections of the tape indicates that 80% of the critical current is carried by the centre section of the core within which the Ag rod is located. The bend strain tolerance of the tapes containing a silver core was found to be significantly better than that of the monolithic tapes. At an applied surface bend strain of 1.0%, about 98% of initial was retained in Ag-reinforced tapes, compared with only 54% in monolithic tapes.

Direct imaging of current sharing in (BiPb)2Sr2Ca2Cu3Ox composite conductors

Using a high-resolution magneto-optical imaging technique, we study the flow of transport currents around cracks in monofilament (BiPb)2Sr2Ca2Cu2Ox composite conductors. Shunting through the Ag clad is directly observed. The current in the silver decays exponentially with distance from the crack as expected in a one dimensional distributed resistance model. At 77 K, the transfer length, λ, is approximately 0.56 mm, implying an interface resistivity of 7.5×10−8 Ω cm2. This transfer length is found to increase slightly with increasing temperature.

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.