Table of contents

Nanoparticles of BaSnO₃ were incorporated into YBa₂Cu₃O_7−x (YBCO) films on LaAlO₃ substrates for magnetic flux pinning enhancements. More than an order of magnitude improvement in the high field magnetization J_c at 6 T at 77 K was observed as compared to regular YBCO films. The irreversibility field (H_irr) was increased to 8.5 T at 77 K and to 13.4 T at 65 K. The in-field transport current measurements confirmed an order of magnitude improvement in high fields. The angular dependence of the J_c data at 1 T showed that is 1.3 times higher than indicating the presence of c-axis correlated defects. Transmission electron microscopy studies revealed the presence of a large density of uniformly distributed ∼10 nm sized BaSnO₃ precipitates and strain fields around them. A dual sector pulsed laser deposition target is used to produce the films, thus eliminating reactions between BaSnO₃ and YBCO during the target preparation stage, but may allow the BaSnO₃ to react locally and create defects that act as pinning centres.

Epitaxial NdBa₂Cu₃O_7−δ (NdBCO) films with and without the incorporation of self-assembled nanodots of BaZrO₃ (BZO) were grown on rolling-assisted-biaxially-textured substrates (RABiTS) by pulsed laser deposition. Compared to epitaxial YBa₂Cu₃O_7−δ (YBCO) films on RABiTS, NdBCO films are found to have superior performance in applied fields at all field orientations, suggesting the presence of an increased density of pinning centres. The incorporation of columns of self-assembled nanodots of BZO within the NdBCO films results in further enhancement of the in-field J_c at all field orientations. J_c varies as J_c∼H^−α at intermediate fields, with α values for NdBCO and BZO-doped NdBCO films being ∼0.37 and ∼0.43, respectively, which are significantly lower than α∼0.52 for YBCO film.

We review the current status of the growth, characterization and applications of Bi–Sr–Ca–Cu–O (BSCCO) whiskers from the materials science point of view. Our analysis leads to the conclusion that reconsideration of the importance of the whisker studies is necessary; whiskers can play a major role in studies of the growth mechanism and defects, properties control and their understanding, and new devices and applications. Arguments favourable to the idea that BSCCO whiskers can possibly have a significant impact on future synthesis of nano-objects in this system or other high-T_c superconducting systems, as well as in investigation and understanding of the growth processes under elevated magnetic fields, are discussed. Emphasis is made on superconducting whiskers, but non-superconducting ones are also briefly addressed.

We review the scaling relations for the critical current density (J_c) in Nb₃Sn wires and include recent findings on the variation of the upper critical field (H_c2) with temperature (T) and A15 composition. Measurements of H_c2(T) in inevitably inhomogeneous wires, as well as analysis of literature results, have shown that all available H_c2(T) data can be accurately described by a single relation from the microscopic theory. This relation also holds for inhomogeneity averaged, effective, H_c2^*(T) results and can be approximated by , with t = T/T_c. Knowing H_c2^*(T) implies that J_c(T) is also known. We highlight deficiencies in the Summers/Ekin relations, which are not able to account for the correct J_c(T) dependence. Available J_c(H) results indicate that the magnetic field dependence for all wires from  T up to about 80% of the maximum H_c2 can be described with Kramer's flux shear model, if nonlinearities in Kramer plots when approaching the maximum H_c2 are attributed to A15 inhomogeneities. The strain () dependence is introduced through a temperature and strain dependent H_c2^*(T,) and Ginzburg–Landau (GL) parameter κ₁(T,) and a strain dependent critical temperature T_c(). This is more consistent than the usual Ekin unification of strain and temperature dependence, which uses two separate and different dependences on H_c2^*(T) and H_c2^*(). Using a correct temperature dependence and accounting for the A15 inhomogeneities leads to the remarkably simple relation , where C is a constant, s() represents the normalized strain dependence of H_c2^*(0) and h = H/H_c2^*(T,). Finally, a new relation for s() is proposed, which is an asymmetric version of our earlier deviatoric strain model and based on the first, second and third strain invariants. The new scaling relation solves a number of much debated issues with respect to J_c scaling in Nb₃Sn and is therefore of importance to the applied community, who use scaling relations to analyse magnet performance from wire results.

The new THELMA code, including a thermal-hydraulic (TH) and an electro-magnetic (EM) model of a cable-in-conduit conductor (CICC), has been developed. The TH model is at this stage relatively conventional, with two fluid components (He flowing in the annular cable region and He flowing in the central channel) being particular to the CICC of the International Thermonuclear Experimental Reactor (ITER), and two solid components (superconducting strands and jacket/conduit). In contrast, the EM model is novel and will be presented here in full detail. The results obtained from this first version of the code are compared with experimental results from pulsed tests of the ENEA stability experiment (ESE), showing good agreement between computed and measured deposited energy and subsequent temperature increase.

Thin filament MgB₂/Fe tapes have been made by the powder-in-tube technique using a precursor powder prepared by mechanical alloying and deformed by two-axial rolling. Increasing critical current density (J_c) in a parallel external magnetic field has been measured for a reduced filament thickness up to 18.5 µm; this could be effective for future practical fine-filamentary MgB₂ conductors. This increase in J_c is attributed to an apparent improvement in texture introduced by rolling. Further increase in J_c with reduction of filament size is limited due to chemical reaction at the filament/Fe interface during the final heat treatment at 600 °C for 3 h. On the other hand, improved texture leads to a strong critical current anisotropy, which increases with the reduced filament thickness and temperature. High I_c anisotropy can lead to an apparent reduction of the total magnet current due to the effect of the radial field component, as in the case of high temperature superconducting coils.

We report the observation of a phase transformation which can occur in a microscopic double phase BSCCO whisker at room temperature. We performed electrical resistivity measurements by thermally cycling the whisker in the range 78–300 K in a helium atmosphere and observed a decrease of the Bi-2223 phase amount in favour of the Bi-2212 phase, accompanied by an increase in the T_c of the Bi-2212 phase. A longer ageing of the whisker increased its resistance by about a factor of four at room temperature and caused a semiconducting behaviour at low temperature. We demonstrate that a simple electrical model can account for the experimental data and disentangle the contributions of the two phases.

The AC losses of a rolling-assisted-biaxially-textured-substrate (RABiTS™) processed YBa₂Cu₃O_7−δ (YBCO) coated conductor (Goyal et al 1996 Appl. Supercond. 4 403–27) with Ni-alloy substrate and Cu stabilizer were measured at 77 K by both calorimetric (CM) and electromagnetic (EM) methods. In the CM method, improvements in the measurement were observed when a Cernox temperature sensor was used to measure the temperature rise on the sample instead of a differential thermocouple. The effect of heat transferred from current leads was taken into account to improve the accuracy of the CM results. In the EM method, the magnetization losses of the sample in a perpendicular applied field were measured by an in-plane pick-up coil. The calculation of the calibration factor C of the pick-up coil is discussed. Good agreement between AC loss results obtained from the CM and EM methods confirmed the validity of our measurements and calculations.

An independently oxidized AgMn alloy is used to gauge the overall tensile strength of technical Ag/Bi2223 composite flat wire through the contributions of individual constituent strengthenings. Theoretical analysis based on the rule of mixtures shows similarity between measured and calculated curves of the composite at room temperature (RT) and 77 K. The elastic modulus of the composite wire, deduced from initial and loading/unloading curves, is found to be around 95–98 GPa, while for Bi2223 filaments, the modulus of elasticity was estimated to be 129 GPa. The effect of thermal cycling between RT and 77 K on the electromechanical properties of the wire is also examined. The critical current is significantly reduced by tensile strain above 0.4% but not affected by the short thermocycle when measured across different specimens. Finally, the relation between the fracture strain and the strain span of the Bi2223 filaments is found to accommodate an intrinsic filament fracture strain of about 0.04–0.11% in the wire.

The changes in phase evolution, structural, superconducting and flux pinning properties of (Bi, Pb)-2212 superconductor due to the addition of the rare earth Dy have been studied. Bulk polycrystalline samples were used for the study in which Dy content was varied from x = 0.0 to 0.5 on a general stoichiometry of Bi_1.7Pb_0.4Sr₂Ca_1.1Cu_2.1Dy_xO_y. It was found that the Dy atoms enter into the structure, replacing Sr and Ca with significant changes in the lattice parameters, microstructure, hole concentration, normal state resistivity and flux pinning strength of the system. Consequently, the critical temperature (T_C), critical current density (J_C) and the field dependence of J_C (J_C–B characteristics) of the Dy added samples were found to be enhanced considerably for optimum doping levels. A maximum T_C−onset of 93.4 K (for x = 0.4) and a maximum J_C of 692 A cm⁻² at 64 K (for x = 0.2) were observed for doped samples, as against 75.8 K and 107 A cm⁻², respectively, for the pure sample. The enhancement in the superconducting properties, particularly in the J_C–B characteristics, due to Dy addition seems to have great technological significance.

An improvement of the transport critical current density, J_c, of MgB₂ wires was obtained after the addition of 10 wt% B₄C powders, after reaction at 800 °C: J_c values of 1 × 10⁴ A cm⁻² at 4.2 K and 9 T were obtained for wires of 1.11 mm diameter in a Fe matrix. The starting mixture of Mg and B was doped with submicrometric B₄C, the ratio being Mg:B:B₄C = 1:2:0.08, corresponding to 10 wt% B₄C. For T>800 °C, a decrease of J_c was found, due to the reaction with the Fe sheath. In order to investigate the origin of the improvement of the transport properties for heat treatments up to 800 °C, x-ray diffraction measurements were performed. A decrease of the lattice constant a from 3.0854 to 3.0797 Å was found, thus suggesting an effect of the substitution of carbon on the properties of the wires. A comparison with the literature data shows that the addition of B₄C powders leads to the second highest improvement of J_c reported so far after SiC, thus constituting an alternative for future applications.

The specific heat of a melt-textured-growth Y_0.8Ca_0.2Ba₂Cu₃O_y crystal has been measured from 100 to 70 K in an applied magnetic field up to 9 T and from 30 to 2 K in a magnetic field up to 4 T, respectively. The abnormal electronic specific heat near T_c can be simulated correctly by the London model, indicating that there is a coexistence of the quasiparticles and the Cooper pairs in the mixed state. The field-induced specific heat C_DOS at low temperature (T<5 K) behaves as αT² in zero field and AT(μ₀H)^1/2 in nonzero fields, which is well-known evidence for d-wave paring. The values of the Sommerfeld constant γ_n and the upper critical field μ₀H_c2(0) deduced from the specific heat near T_c are consistent with that at low temperature (T<5 K).

The current distributions of untwisted infinitely long superconductors have been studied during the current sweep and under an external field, using the inductance matrix among superconducting finite elements which are generated from a superconductor. The self- and mutual inductances of general polygonal conductors with a uniform current density over each cross section are precisely calculated from the analytical expressions for the geometrical mean distances. The current distributions among each superconducting element are obtained by solving the circuit equation with the Bean model and a nonlinear E–J relation based on the power law. In addition, the magnetic field and vector potential distributions of an untwisted superconducting composite are also obtained, using the analytical expressions for the magnetic field and vector potential due to polygonal conductors.

The effect of 200 MeV Au ion irradiation on the temperature and field dependence of the critical current density J_c of high quality MgB₂ thin films on Al₂O₃(0001) substrate is reported. The films were irradiated by 200 MeV Au ions with fluences 5 × 10¹⁰ and 1 × 10¹² ions cm⁻². Substantial enhancement of the critical current density J_c with increasing fluence was observed. J_c was also increased when the measurements were made at 10–5 K. Columnar defects are expected because of high electronic energy loss (23 keV nm⁻¹) for the present ion energy combination; these act as pinning centres to enhance J_c. Decrease in the transition temperature T_c resulted after irradiation with a dose of 5 × 10¹⁰ ions cm⁻²; however, at the higher dose of 1 × 10¹² ions cm⁻², T_c increased marginally.

The gain bandwidth of a superconducting NbN hot-electron mixer was measured at local oscillator (LO) frequencies 2.5, 0.6 and 0.3 THz and compared to values from presently known bolometric mixer models. At 2.5 THz variations of the bandwidth with the LO power agree rather with the hot-spot mixer model than with any of the homogeneous bolometric models, whereas an increase of the bandwidth at low LO frequencies plausibly evidences non-bolometric direct interaction of magnetic vortices with the radiation field.

We observed crack formation in transformation-processed Nb₃Al wires at room temperature, the wire being bent with a small clamp fixture with a curvature. The polished cross-section parallel to the longitudinal axis was observed, using a high power optical microscope or a field-emission scanning electron microscope. The bend strain limit for microcrack formation is found, changing the radius of the curvature of the clamp. The bend strain limit was found to be around 0.3% for standard Nb₃Al wires. This corresponds to the irreversible tensile strain limit of the I_c characteristics determined with a 0.1 µV cm⁻¹ criterion. Reduction of the barrier thickness should be avoided to keep to the bend strain limit. A new configuration of the Nb₃Al wire is demonstrated to improve the bend strain limit. The filament is divided into segments in the transverse cross-section. The wire is fabricated by a double-stacking method. The bend strain limit is enhanced to about 0.85% for the wire surface; the equivalent strain of the outermost filament location is about 0.66%. A simple react and wind test for this wire was performed, where the wire experienced 0.86% bend strain. The degradation of J_c was found to be very small.

The present work presents the steps for the manufacture of an Nb–Ti artificial pinning centre (APC) multifilament wire for AC applications. The wire production starts from a Ti bar inserted in a Nb tube. The superconductor Nb–Ti alloy was obtained using heat treatments during the wire production steps through the growth of the diffusion layer between the pure Nb and Ti regions. The final wire is composed of many filaments dispersed in a Cu–Ni matrix. The transport critical current density was measured in samples for different conditions of heat treatment, followed by wire diameter reduction. Magnetization hysteresis loops were measured to extract the hysteretic losses in wires with filament diameters of 76 and 92 µm and compared to the results in commercial wires with comparable filament diameters. We obtained an Nb–Ti APC wire with hysteretic losses of the same order of magnitude as the commercial wires. However, critical current densities J_c are higher for fields lower than 2 T, where AC applications are most important. The J_c results confirmed a previous result that the decrease of normal region sizes is a less important pinning source in this material than the pinning from variations in the composition of the diffusion layer.

We report a newly developed method, namely the in situ postannealing texture (IPAT) technique, based on a two-step principle to fabricate CeO₂ single buffer layer on biaxially textured Ni–5 at.%W substrate by a reactive direct-current sputtering method. The annealing temperature and time dependence of the grain alignment in CeO₂ buffer layers was studied. High-quality CeO₂ layers were achieved at annealing temperatures from 750 to 850 °C, with annealing times from 10 to 20 min. Atomic force microscopy revealed, via the novel IPAT method, a continuous, dense, and crack-free surface morphology for CeO₂ thin films with a thickness of up to 300 nm, which act as a sufficiently good single buffer for YBa₂Cu₃O_7−δ (YBCO) coated conductors (CCs). The preparation of subsequent YBCO CCs grown on the IPAT-prepared CeO₂ single buffer layers was also investigated.

We have successfully synthesized MgB₂ thick films on 4H-SiC substrate by hybrid physical–chemical deposition (HPCVD). The films have transition temperature T_c above 40 K. X-ray diffraction (XRD) shows the c-axis oriented structure of MgB₂, with Mg and small MgO impurities. The critical current density J_c, estimated using the measured magnetic hysteresis loop and the Bean model, is 6 MA cm⁻² in self-field at 10 K.

Four-filament ex and in situ MgB₂ wires were prepared with the rectangular wire-in-tube (RWIT) technique. Based on experience with single-core wires, 10 wt% of W was added to the ex situ and 10 wt% of SiC to the in situ powders, which were packed into Fe and Nb/AgMg tubes, respectively, and two-axially rolled into composite conductors. The ex and in situ conductors are compared in terms of field-dependent transport critical current density, effects of filament size reduction and twisting, mechanical behaviour and thermal stability.

The effect of site selective doping of Sm on the superconducting and flux pinning properties of (Bi, Pb)-2212 superconductor has been investigated. Pure, Sm substituted at different cationic sites such as Bi, Sr and Ca, and Sm added (Bi, Pb)-2212 superconducting samples are prepared by solid state synthesis. Remarkable improvements in the T_C, J_C and flux pinning properties have been observed both in the Sm substituted and Sm added samples and the properties are found to be highly site dependent. The bulk flux pinning force (F_P) calculated from the field dependent J_C values shows that the irreversibility lines of Sm doped samples are shifted towards higher fields to different extents depending on the site in which the substitution or addition is made. These changes are due to a decrease in hole concentration and the creation of point-like defects because of the replacement of cations by rare earth ions and also may be due to nanoscale secondary phases present in the samples.