In the past few years magneto-optical flux imaging (MOI) has come to take an increasing role in the investigation and understanding of critical current densities in high-Tc superconductors (HTS). This has been related to the significant progress in quantitative high-resolution magneto-optical imaging of flux distributions together with the model-independent determination of the corresponding current distributions. We review in this article the magneto-optical imaging technique and experiments on thin films, single crystals, polycrystalline bulk ceramics, tapes and melt-textured HTS materials and analyse systematically the properties determining the spatial distribution and the magnitude of the supercurrents. First of all, the current distribution is determined by the sample geometry. Due to the boundary conditions at the sample borders, the current distribution in samples of arbitrary shape splits up into domains of nearly uniform parallel current flow which are separated by current domain boundaries, where the current streamlines are sharply bent. Qualitatively, the current pattern is described by the Bean model; however, changes due to a spatially dependent electric field distribution which is induced by flux creep or flux flow have to be taken into account. For small magnetic fields, the Meissner phase coexists with pinned vortex phases and the geometry-dependent Meissner screening currents contribute to the observed current patterns. The influence of additional factors on the current domain patterns are systematically analysed: local magnetic field dependence of jc(B), current anisotropy, inhomogeneities and local transport properties of grain boundaries. We then continue to an overview of the current distribution and current-limiting factors of materials, relevant to technical applications like melt-textured samples, coated conductors and tapes. Finally, a selection of magneto-optical experiments which give direct insight into vortex pinning and depinning mechanisms are reviewed.

https://iopscience.iop.org/article/10.1088/0034-4885/65/5/202/pdf

Magneto-optical studies of current distributions in high-Tc superconductors

Improvement in the thermal conductivity of aluminum nitride (AlN) can be realized by additives that have a high thermodynamic affinity toward alumina (Al{sub 2}O{sub 3}), as is clearly demonstrated in the aluminum nitride-yttria (AlN-Y{sub 2}O{sub 3}) system. A wide variety of lanthanide dopants are compared at equimolar lanthanide oxide:alumina (Ln{sub 2}O{sub 3}:Al{sub 2}O{sub 3}, where Ln is a lanthanide element) ratios, with samaria (Sm{sub 2}O{sub 3}) and lutetia (Lu{sub 2}O{sub 3}) being the dopants that give the highest- and lowest-thermal-conductivity AlN composites, respectively. The choice of the sintering aid and the dopant level is much more important than the microstructure that evolves during sintering. A contiguous AlN phase provides rapid heat conduction paths, even at short sintering times. AlN contiguity decreases slightly as the annealing times increase in the range of 1--1,000 min at 1,850 C. However, a substantial increase in thermal conductivity results, because of purification of AlN grains by dissolution-reprecipitation and bulk diffusion. Removal of grain-boundary phases, with a concurrent increase in AlN contiguity, occurs at high annealing temperatures or at long times and is a natural consequence of high dihedral angles (poor wetting) in liquid-phase-sintered AlN ceramics.

High‐Thermal‐Conductivity Aluminum Nitride Ceramics: The Effect of Thermodynamic, Kinetic, and Microstructural Factors

High current YBa2Cu3O7−δ (YBCO) thick films on flexible nickel substrates with textured buffer layers were fabricated. Highly textured yttria‐stabilized‐zirconia buffer layers were deposited by using ion beam assisted deposition (IBAD). Pulsed laser deposited YBCO films were not only c‐axis oriented with respect to the film surface but also strongly in‐plane textured. The in‐plane mosaic spread of YBCO films was ∼10°. A critical current density of 8×105 A/cm2 was obtained at 75 K and zero field for thin YBCO films. It was also demonstrated that thick YBCO films with a high critical current and excellent magnetic field dependence at liquid nitrogen temperature can be obtained on flexible nickel substrates by using the textured buffer layers. The result indicates that thick film technology in combination with IBAD buffer layers could be a viable method for fabricating YBCO tapes in long lengths.

High current YBa2Cu3O7−δ thick films on flexible nickel substrates with textured buffer layers

Disclosed is a molten carbonate fuel cell comprising an electrolyte body sandwiched between a pair of electrodes, outflow of an electrolyte from the electrolyte body being diminished to suppress an increase of the internal resistance and occurrence of a gas crossover, which accompany the outflow of the electrolyte, so as to increase the life time of the fuel cell. The molten carbonate fuel cell of the present invention comprises a pair of conductive electrodes, an electrolyte body sandwiched between the pair of electrodes and including a porous body containing a retaining material and a reinforcing material and an electrolyte impregnated in the porous body and containing an alkali carbonate mixture, the retaining material containing fine particles having an average particle diameter of 0.2 to 0.6 μm and the reinforcing material containing short fibers having a diameter of 0.5 to 5 μm and a length of at most 50 μm, fuel gas supplying means for supplying a fuel gas to one of the pair of electrodes, and oxidant gas supplying means for supplying an oxidant gas to the other of the pair of electrodes.

Molten carbonate fuel cell

A polar phase of NaNbO(3) has been successfully synthesized using sol-gel techniques. Detailed characterization of this phase has been undertaken using high-resolution powder diffraction (X-ray and neutron) and (23)Na multiple-quantum (MQ) MAS NMR, supported by second harmonic generation measurements and density functional theory calculations. Samples of NaNbO(3) were also synthesized using conventional solid-state methods and were observed to routinely comprise of a mixture of two different polymorphs of NaNbO(3), namely, the well-known orthorhombic phase (space group Pbcm) and the current polar phase, the relative quantities of which vary considerably depending upon precise reaction conditions. Our studies show that each of these two polymorphs of NaNbO(3) contains two crystallographically distinct Na sites. This is consistent with assignment of the polar phase to the orthorhombic space group P2(1)ma, although peak broadenings in the diffraction data suggest a subtle monoclinic distortion. Using carefully monitored molten salt techniques, it was possible to eradicate the polar polymorph and synthesize the pure Pbcm phase.

The polar phase of NaNbO(3): a combined study by powder diffraction, solid-state NMR, and first-principles calculations.

Large critical currents with a relatively weak magnetic field dependence are obtained in thick films of TlBa2Ca2Cu3Oz. Transport measurements indicate Jc ≳105 A/cm2 at 77 K, zero field, and Jc≳104 at 60 K in a 2 T field applied along the c‐axis. The observed behavior is attributed to a large degree of uniaxial alignment of platelike grains, and to superior intragranular flux pinning. These results are consistent with recent theories concerning the nature of vortices in highly anisotropic (layered) superconductors and ‘‘brick wall’’models of intergranular current transport.

Transport critical currents in spray pyrolyzed films of TlBa2Ca2Cu3Oz on polycrystalline zirconia substrates

Transport and magnetization measurements are compared for ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{z}}$/Ag tapes with critical current densities approaching ${\mathit{J}}_{\mathit{c}}$=${10}^{4}$ A/${\mathrm{cm}}^{2}$ at 77 K, and at zero field. Upon subdivision of the sample, the magnetic hysteresis is found to scale approximately with the sample dimension, indicating that persistent currents flow throughout the superconducting core. The ${\mathit{J}}_{\mathit{c}}$ determined using the critical-state analysis is a factor of \ensuremath{\sim}2--3 higher than that obtained in transport. A weak magnetic-field dependence of ${\mathit{J}}_{\mathit{c}}$ is demonstrated at low temperatures, and yet, a comparison with data for powder samples indicates that the transport ${\mathit{J}}_{\mathit{c}}$ is limited by weak intergranular coupling. At low temperature, the transport ${\mathit{J}}_{\mathit{c}}$ is 2 orders of magnitude below the intragranular ${\mathit{J}}_{\mathit{c}}$. At high temperatures, weak intragranular flux pinning appears to be the dominant limiting mechanism. The degree to which transport properties might be enhanced through the introduction of pinning centers and better intergranular coupling is explored.

Critical-state scaling and weak links in Ag-sheathed Bi 2 Sr 2 Ca 2 Cu 3 O z

The differential thermal expansion of Bi 2 CaSr 2 Cu 2 O x , Bi-(2122), has been measured using high-temperature neutron powder diffraction in air, 0.980% oxygen in nitrogen, and pure argon. Thermal expansion in the c -axis direction is greater by a factor of about 1.4 than that in either the a - or b -axis direction for all sample environments. The neutron results for Bi-(2122) in air are compared with results from similar X-ray powder diffraction measurements.

Thermal expansion measurements using neutron diffraction of Bi2CaSr2Cu2Ox

A particulate mixture of an alkali chloride solvent salt, titanium oxide and an alkaline earth reactant selected from the group consisting of barium oxide, strontium oxide, and mixtures thereof, is heated to melt the chloride salt solvent in which the titanium oxide and alkaline earth reactant dissolve and react precipitating a titanate selected from the group consisting of barium titanate, strontium titanate and mixtures thereof.

Molten salt synthesis of barium and/or strontium titanate powder

A processing protocol for the preparation of Ag sheathed tapes of the bismuth superconductor composition Bi 1.7 Pb 0.3 Ca 2.05 Sr 2 Cu 3.05 O z , Bi-(2223), with critical currents densities, J c , exceeding 10,000 A cm −2 at 77 K and 0 T, is presented. The protocol is based on the combined in-situ reaction and sintering of a mixture of Bi 2 CaSr 2 Cu 2 O z , Bi-(2122), with the additional constituents needed to form the Bi-(2223), within the pre-fabricated tape. The conversion to a dense Bi-(2223) ceramic core was accomplished with a series of anneals at ∼830°C, for periods of up to 100 h, separated by uniaxial cold-pressings of the tape between the anneal. The effects on J c of variations in the anneal temperature, annealing time, and the number of pressings are presented.

Ronald H. Arendt

Papers

Transport critical currents in spray pyrolyzed films of TlBa2Ca2Cu3Oz on polycrystalline zirconia substrates

Critical-state scaling and weak links in Ag-sheathed Bi 2 Sr 2 Ca 2 Cu 3 O z

Thermal expansion measurements using neutron diffraction of Bi2CaSr2Cu2Ox

Molten salt synthesis of barium and/or strontium titanate powder

The fabrication of high critical current capability bismuth superconductor tape