To assess a Ni-clad melt-processed YBa2Cu3Oy (YBCO) conductor approach, a systematic study was made on commercially available YBCO powder mixed and co-sintered with nickel oxide (NiO). Our research effort has focused on the effect of nickel on the critical current density in the YBCO/Ni composites. Four series of YBCO samples mixed with 0.3, 0.7, 1, 1.3, 2 and 3 wt% of high purity NiO were prepared, and optimized conditions for the melt process were established. These samples were characterized by resistivity, AC susceptibility and DC magnetization measurements. The highest magnetization critical current density (Jcm∼3×105 A/cm2 at T=4.2K and H=0 was observed in melt-processed samples with ∼ 1 wt% Ni (or -1.3 wt% NiO) subjected to a cooling rate of 2°C/hr from 1040°C to 980°C. This Jcm was found near the middle of the Ni-solubility range (about 2 wt% Ni is the solubility limit of Ni in YBCO) which may be attributed to the local Ni/Cu compositional fluctuation leading to an increased pinning force density in these composites.

Melt-texturing of YBaCuO is generally processed on substrates. Due to its high chemical activity, the non-stoichiometric liquid, mainly due to the barium element, generally reacts with the substrate which is therefore subjected to play a role in the crystallization of YBaCuO and in the development of the texture. The particular case of Y2O3, which is a reacting but non-polluting substrate, is treated in details. The comparison of textured YBaCuO in similar conditions but on different substrates: Al2O3, Y2O3 and MgO gives some insight in the texturing process.

The mechanical behaviors of flux pinning in GdxY1−xBa2Cu3Oy with various concentration of Gd were studied comparatively by means of a vibrating reed technique. Both the internal friction and the modulus increased with enhanced flux pinning due to the increasing of Gd content in GdxY1−xBa2Cu3Oy. It was in accordance with the results of the magnetic hysteresis measurements using a vibrating sample magnetometer. These results indicated that the substitution of magnetic rare-earth ions is a practicable method for improving the flux pinning in high Tc superconductor and the mechanical measurement of flux pinning as a judgement for the superconductivity of sample was proved.

In the past two years, it has been demonstrated that applied electric fields can be used to modulate significantly the normal state resistivity, the critical temperature and the critical current density of high-Tc films. Whereas earlier publications focussed on experimental aspects of high-Tc field effects, this contribution will discuss the conditions that are required to induce superconductivity on the surface of insulators. In addition, evaluations for performance limits of potential high-Tc field effect devices will be presented.

The structural, electrical and magnetic properties are compared for three superconducting Tl-Ca-Ba-Cu-O thin films prepared by different deposition and sintering protocols. One film containing a mixture of Tl2Ca2Ba2Cu3Ox and Tl2Ca2Ba2Cu3Oy structural phases has the best superconducting properties. Deposition of a Tl-free Ca-Ba-Cu-O precursor film followed by sintering in Tl-O vapor may be the preferred protocol to obtain a single structural phase.

A current controlled HTS switch (CCSS) has been fabricated using the Si-YBCO reactive patterning technique. The Si films were deposited on MgO substrates by e-beam evaporation and patterned via a lift-off process. The YBCO films were deposited thereafter by pulsed laser deposition to form micron-sized HTS strip lines with different widths and lengths. A typical line has a normal resistance ranging from 10 Ω to 100 KΩ. A CCSS was constructed using such a HTS line as a resistor in parallel with a load resistor. The operation of CCSS is similar to that of a current divider. Applications of CCSS includes HTS logic gates and circuit breakers.

A switch has been developed which uses a thin film of Y-Ba-Cu-O as a switching element. The two important process parameters in the fabrication of the switch - contact resistance between a metal and a superconductor and the passivation of the superconducting films by SiO2 have been studied. The thin film of superconductor is patterned into a narrow line (gate) crossed by a metal control line with a deposited SiO2 as an insulation layer between them. When the control line carries current large enough to switch the gate into the normal state the gate is in the ‘off’ state. When no current is present, the gate is superconducting and it is in the ‘on’ state. The operation of the device was verified by testing at 77 K. Applications of the device include amplifiers, digital logic circuits, and microwave switches.

Intensity oscillations of the specular reflection high energy electron diffraction (RHEED) beam contain useful information on the mode of growth and the evolving structure of thin films. We present RHEED studies of the growth of DyBa2Cu3O7−x films and DyBa2Cu3O7−x/DY2O3/DyBa2Cu3O7−x structures on SrTiO3; substrates deposited by ozone-assisted molecular beam epitaxy. The effect of substrate temperature, ozone flux and surface step densities on the epitaxial relationship and evolving microstructure will be discussed. The strong damping of the oscillations and identical time periods under different nuoleation and growth conditions suggest that the intensity oscillations are a consequence of to diffuse scattering from step edges rather than a layer-by-layer growth mode. It was also found that Dy2O3 can be used to fabricatee tri-layer type structures but not superlattice structures.

Metal-insulator-semiconductor capacitors are fabricated from a YBa2Cu3O7-δ (YBCOVYttria-Stabilized Zirconia(YSZ)/Si structure. Current-voltage(I-V) measurements reveal that thicker YBCO films(≤150Å) tend to result in more stable capacitors. Results of capacitance-voltage(C-V) measurements during bias-temperature cycling suggest the presence of a thermally activated process in the YSZ and/or YSZ/Si interface. This process is probably related to trapping/detrapping mechanisms in the SiOx, layer formed between YSZ and Si. It is shown that the distribution of mobile ions in YSZ can be “set” with biases at room temperature and then “frozen” by lowering the temperature, giving rise to adjustable threshold voltages at 80K.

Switching from superconducting (S) to normal (N) state was investigated using short rise time (∼ 10−10 s) electric pulses in MBE grown Bi-Sr-Ca-Cu-O and rf magnetron sputtered Y-Ba-Cu-O submicron thickness microstrips. It was found that two reversible processes: fast electronic (τ∼10∼−12 s) and slow thermal (τ∼10∼−8s) one take place during switching. At high current densities (∼107 A/cm2) irreversible changes in strip material induced by thermal runaway occurred. The use of a superconducting element as fast fault-current limiter or pulse sharpening device is pointed out.

We have modelled the grain boundaries in high-Tc superconducting oxides and determined the critical current density. The tunneling of superconducting pairs across the coalesced regions is used to determine the boundary effects. The length of the coalesced regions, with continuity of the Cu-O planes maintained by relaxation of the atom positions, is determined by minimization of the energy of the configuration. The depression of the order parameter is evaluated using the continuity conditions at the boundary in the proximity effect formulation. The excess charge distribution at the core of the boundary, determined from the solution to the Poisson's equation, is used to determine the scattering of the superconducting pairs. The width of the boundary, evaluated from modelling, determines the transmission coefficient for tunnelingof superconducting pairs. The critical current density is expressed in terms of these four important factors associated with the grain boundary. All the experimental results are explained by the present modelling of the grain boundary effects.

The transport critical current density (Jc) for high-Tc thin films, bicrystals, and bulk ceramics is shown to be determined by magnetic field penetration into the grain boundaries. The gross grain orientations may not in all cases be an important factor in determining this penetration. The parameter (λG/λj)2can characterize the strength of the grain boundary coupling, which depends mainly on the crystal coherence and connectivity at the boundary area.

The results of a UK-based industrial consortium, aimed at the development of high-Tcsuperconducting wires, tapes and bulk conductors, are reviewed. The powder-in-tube method has been used to fabricate 10's of metre lengths of bismuth-based, Ag-clad, single and multi-filamentary wires. Performance highlights include current densities of 23,000Acm-2 [7A] at 77K; (zero field) for 2223 bismuth-based wire and greater than 105Acm-2 [25A] and 104cm-2 at 4.2K, zero field and 15T, respectively, for 2212-based wire. Ag-backed, 2212, tape-cast conductors have carried currents of over 60A [80,000Acnrr-2] at 4.2K; (zero field). Bulk ceramic processing techniques, sol-gel chemistry and plasma-spraying have also been used to fabricate a range of YBCO and bismuth-based components. Currents in excess of 100A have been carried in bulk 2223 bismuth-based samples at 77K.

Mechanical bending of HTS tapes can result in the deterioration and destruction of their transport properties. Depending on the magnitude and type of strain (tensile or compressive), the films can be permanently damaged by fracture and recoverable only by post heat treatment, or simply elastically strained with recoverable transport properties upon release of the strain. Here we present results of strain effects on BSCCO (2212) /Ag tape transport properties and post heat treatment recovery of those properties.

The 2212 phase exhibits an extended single phase region having a variable Sr/Ca ratio and Bi-content of 2.1 < Bi < 2.3. At T = 820°C the single phase region extends between 2.05/0.95 > Sr/Ca > 1.2/1.8, whereas, at T = 870° C the single phase region extends between 2.2/0.8 > Sr/Ca > 1.6/1.4. The 2223 phase exhibits a significant higher Bi-content of Bi ∼ 2.5 compared to the formal composition Bi2Sr2Ca2Cu3O10. The Sr/Ca ratio varies between 2/2 > Sr/Ca > 1.9/2.1 reffering to Cu-content of Cu = 3. Samples with the nominal composition Bi2.5Sr2Ca2Cu3Ox and Bi2.5Sr1.9Ca2.1Cu3Ox have been sintered at 880 – 885°C for 90 h in air and consist of 90 – 95 volume percent 2223 phase. Tc depends on the chemical composition of the phases. With decreasing Ca- and Bi-content of the phases Tc increases. Both phases are metastable at roomtemperature. 2212 and 2223 rapidly decompose at 600 °C and 800 °C in air, respectively. At 300°c two new Bi-Sr-Cuprates have been found.

The fundamental problems of applying high-Tc superconductors to power transmission lines were studied, and prototypes of multilayer-wound conductors were fabricated and evaluated.

Multifilamentary silver sheathed wires have good anti-strain. properties. A 61-filament bismuth(2223) wire proved to maintain 90% of initial transport current property after 150 cycles of bending (bent-straightened-bent reversely-straightened) with 0.18% strain. A 61-filament wire of 114m unit-length was produced and proved to have a Jc of 9,700 A/cm2 with an Ic of 9.34A for the whole length at 77.3K.

Using these multifilamentary wires, prototypes of multilayer-wound conductors were produced by the react-and-wind technique. A 1.4m long three layer-wound conductor could carry 590A (Jc=7.020A/cm2) in liquid nitrogen.

These results indicate that the basic problems of high-Tc superconductor application to power transmission lines will be overcome using this technology.

Because of the natural preference for crystal growth in the a-b direction, melt-processed YBCO as well as BSCCO tends to show a local texture with large parallel plates aligned in the direction of CuO2 planes. In YBCO, macroscopically layered and bi-axially textured material can be achieved over extended sample lengths through the use of temperature gradient during melt-texture processing. In BSCCO, a layered structure is relatively easily obtained by subjecting thin ribbon samples in contact with silver to a partial melt processing. While the nature and the mechanism of layer formation may not be the same, the layer configuration in both YBCO and BSCCO is essential for overcoming the grain boundary weak link problem and achieving high transport Jc. In this paper, the process and the mechanism of layer formation will be described, and the implications on superconductor properties will be discussed.

Many processing parameters affect the critical current of silver-clad tapes of Bi2−xPbxSr2Ca2Cu3O10+y (2223) made by the powder in tube process. In this paper the effect of oxygen pressure during cooling from the final sintering step will be considered. It is shown that high oxygen pressures will destabilize the 2223 phase with formation of calcium plumbate and the lower Tc Bi2Sr2Ca1Cu2O8+y (2212) phase. This decomposition can drastically lower the critical current. It is shown that cooling in atmospheres containing about 5 % oxygen will avoid the calcium plumbate formation region.

The critical current density Jc of bulk ceramic superconductors is often several orders of magnitude lower than that of a single crystal or oriented fílm of the same material due to the weak links residing at the boundaries between single crystals within the ceramic sample. Assuming that the rate of electron transport within each single crystal is much faster than the average rate of electron transport between adjacent single crystals, a working hypothesis of Jc enhancement by granular reorientation is formulated which includes the“brickwall”model and melt-texturing as special cases with long-range granular alignment which usually leads to rigid and brittle bulk samples. Simple geometric considerations show that only short-range local granular alignment is necessary for high Jc. For mis reason, it should be possible to make flexible superconducting wires and tapes for commercial applications at or above 77K.

Superconducting films of“1223”Tlx Ca2Ba2Cu3Oy (0.65<×<l.00) have been prepared on polycrystalline yttria stabilized zirconia substrates via the reaction of thallium oxide vapor with spray-deposited Ca-Ba-Cu-oxide films containing silver. The silver addition permits the formation at 860C of oriented, interconnected polycrystalline superconducting films with Tc(0) values of 104–107K. Samples have been prepared routinely with 77K zero-field dc transport critical current densities >10,000A/cm2 with values as high as 105,000A/cm2 having been measured. The range in Jc values measured is believed to be related, at least in part, to inhomogeneities in the films. The excellent in-field characteristics of the“1223”thick films are reflected in the behavior of the“105,000A/cm” sample for which J >10,000A/cm was measured at 60K in a 2 Tesla field applied parallel to the crystallographic c-axis