Table of contents

Thin films of the infinite-layer compound Sr_0.6Ca_0.4CuO_2−δ have been prepared by off-axis rf magnetron sputtering on SrTiO₃ (001). For an optimum level of oxygen vacancy superconductivity was observed (T_{c onset} = 42 K and T_{c(ρ = 0)} = 11 K). Structural and transport data suggesting n-type superconductivity are presented and discussed. Infinite-layer thin films of Sr_0.6Ca_0.4CuO_2−δ, without trivalent cation doping and showing n-type superconductivity, are reported for the first time.

In this letter, we report the results on transport J_c of solenoid coils of up to 100 turns fabricated with Cu-sheathed MgB₂ wires using a wind-reaction in situ technique. Despite the low density of the single core and some reaction between the Mg and Cu sheaths, our results demonstrate that the decrease in transport J_c with increasing length of MgB₂ wires is insignificant. Solenoid coils with diameter as small as 10 mm can be readily fabricated using a wind-reaction in situ technique. The J_c of the coils is essentially the same as that of comparable straight wires. A J_c of 133000 A cm⁻² and 125000 A cm⁻² at 4 K and self-field has been achieved for a small coil wound using Cu-sheathed tape and Cu-sheathed wire, respectively. These results indicate that MgB₂ wires have a great potential for large-scale applications.

This comprehensive survey deals with the melt-growth process and characterization of the novel class of ternary materials, (Nd,Sm,Gd)Ba₂Cu₃O_y 'NSG-123'. Microstructure observations by optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) revealed the presence of sub-micron secondary phase (RE₂BaCuO₅, 211) particles uniformly dispersed in the 123 matrix that enhance flux pinning at low fields and improve the performance of this material. The energy-resolved TEM-EDX analysis clarified that small particles consist mainly of Gd₂BaCuO₅ 'Gd-211', irrespective of the initially added secondary phase (Gd-211 or NSG-211). Dark-field TEM observations showed that the NSG-123 matrix had a high density of RE-rich RE_1+xBa_2−xCu₃O_y (RE-123ss) clusters 3–10 nm in size. These small clusters enhance pinning at intermediate fields. The scaling analyses of the volume pinning force density at 77 K made on NSG-123 samples with varying 211 contents of various secondary phases showed the secondary peak at normalized fields as high as 0.45, which might indicate the activation of ΔT_c pinning mechanism. Trapped field measurements proved that silver was the key element to improve the mechanical strength of NSG-123 materials. Record trapped-field values at high magnetic fields can be obtained at 77 K with appropriately large sized NSG-123 samples.

In this paper we present the results of measurements of the microwave surface impedance of a powder sample and two films of MgB₂. The powder sample has a T_c = 39 K and the films have T_c = 29 K and 38 K. These samples show different temperature dependences of the field penetration depth. Over a period of six months, the film with T_c = 38 K degraded to a T_c of 35 K. We compare the results on all samples with data obtained elsewhere and discuss the implications as far as is possible at this stage.

Single-core MgB₂/Cu and MgB₂/Fe/Cu wires and tapes have been made by the powder-in-tube method with the use of commercial MgB₂ powder (Alfa Aesar) involving hydrostatic extrusion and two-axial rolling processes. High-pressure post-treatments such as cold isostatic pressing, double hydrostatic extrusion and uniaxial pressing were applied. The grain connectivity, I_c(B) curves, pinning in MgB₂ grains, upper critical field, deformation homogeneity, compact density and preferable alignment crystallites for the [001] (c-axis) direction were all improved and enhanced by application of various high-pressure treatments for the MgB₂ compound and composites. Hydrostatically extruded MgB₂ wires have been shown to be better than rolled wires and are very promising for possible application in windings.

We have calculated the alternating current (ac) losses of a 114 MVA high-T_C superconducting (HTS) transmission cable using an electric-circuit (EC) model. The HTS cable is fabricated by Tokyo Electric Power Company and Sumitomo Electric Industries, Ltd. The EC model is comprised of a resistive part and an inductive part. The resistive part is obtained by the approximated Norris equation for a HTS tape. The Norris equation indicates hysteresis losses due to self-fields. The inductive part has two components, i.e. inductances related to axial fields and those related to circumferential fields. The layer currents and applied fields of each layer were calculated by the EC model. By using both values, the ac losses of the one-phase HTS cable were obtained by calculation considering the self-field, the axial field and the circumferential field of the HTS tape. The measured ac loss transporting 1 kA_rms is 0.7 W m⁻¹ ph⁻¹, which is equal to the calculation. The distribution of each layer loss resembles in shape the distribution of the circumferential field in each layer, which indicates that the circumferential fields strongly influence the ac losses of the HTS cable.

AC losses measurements have been performed on a coil wound NbTi CICC magnet in pulsed regimes by the pick-up magnetic method. Owing to the complex set of mutually inducting coils and to the very high field variation rates which are needed to simulate plasma disruption conditions, the simple compensation by two concentric pick-up coils normally adopted for solenoid magnetization measurements cannot be used. In fact, appreciable eddy currents are induced in the metallic parts of the system and a huge 'empty coil effect' is observed. Thus, an extra-compensation procedure is applied by subtracting from the compensated pick-up signal the corresponding one measured in 'warm' conditions, i.e., at a temperature slightly over the superconductor T_c. Experimental evidence is presented for the effectiveness of the adopted procedure for data elaboration.

The software package 3D-MLSI is developed, which allows us to calculate the current distribution and to extract inductances from multi-layered high-T_c and low-T_c superconducting circuits. Both kinetic and magnetic inductances as well as the three-dimensional distribution of the magnetic field are taken into account. We discuss the numerical approach used in 3D-MLSI and some new features such as visualization of sheet currents and analysis of circuits with holes. As an example, we present a simulation of a high-T_c double-layer transformer.

The variation of the phase of the response of YBCO edge-transition bolometers on crystalline MgO substrate is found to be dependent on the normal–superconducting transition. The phase of the response for T < T_c increased at low modulation frequencies where the thermal diffusion length into the sample substrate from the top absorbing YBCO film is longer than the substrate thickness. The substrate/cold-head boundary resistance mainly dominates the thermal conductance of our samples at low frequencies. This transition-dependent change of the phase of the response is interpreted to be due to the effects of the order parameter of the YBCO material on the phonon spectrum and hence the determining Kapitza boundary resistance. The phase of the response decreased at high modulation frequencies where the thermal diffusion length into the substrate is less than the substrate thickness. The results of our analytical model for the low modulation frequencies agree well with the measured values for temperatures close to the T_c-zero.

High quality (Sm_0.5, Eu_0.5)Ba₂Cu₃O_7−δ single domains were grown in air with BaCuO_2−x addition. It was found that a RE–Ba substitution (RE: Sm and Eu) could be effectively suppressed by a small addition of BaCuO_2−x. The 94 K superconductivity with sharp transition was obtained in the composition range of 0.16 ∼ 0.33 BaCuO_2−x and (Sm_0.5, Eu_0.5)Ba₂Cu₃O_7−δ in a molar ratio. At 77 K, a surface field of 0.8 T was trapped in a single-domain pellet of 3.2 cm diameter. A further addition of BaCuO_2−x deteriorated the microstructure and degraded the superconducting performance.

Silver tapes were spray coated using an organometallic solution as the carrier liquid. When this solution was annealed in air at 825 °C, ∼4% of its total weight remained as CaZrO₃. Mixtures of Bi₂O₃, SrCO₃ and CaCO₃ have been prepared with seven different stoichiometries. An organometallic solution was mixed with ceramic oxide powders and spray coated onto silver tapes. Electrical insulation tests were performed applying a dc potential, and insulation survived more than 30 V. Coated tapes were coiled to progressively decreasing diameters down to 1 mm diameter with no sign of cracking, indicating the possibility of metallic bonding and diffusion. This technique appears to be suitable for use in the insulation of Bi-based superconducting tapes and wires, since it does not contain any element that is chemically reactive to this system.

Chemical solution growth of epitaxial YBa₂Cu₃O₇ thin films on single crystalline substrates has been investigated using trifluoroacetate precursors. The concentration of the starting solution was selected to achieve a final film thickness of 250 nm. First, it is shown that high quality films can only be achieved if an homogeneous nanocrystalline film is obtained after the spin coating deposition and pyrolysis steps. Secondary phases such as BaCuO₂, Y₂BaCuO₅ and CuO remain after the high temperature growth process, when macrosegregation is detected after these initial processing steps. A local degradation of texture associated with macrosegregation is detected by μ-Raman spectroscopy. The influence of growth temperature on the film quality has been analysed and the reaction times have been optimized at each temperature according to the reaction kinetics. In-situ fluoride analysis and μ-Raman spectroscopy have been used to determine the advancement of the formation reaction. The most apparent microstructural modification of these thin film samples with optimized annealing times has been found to be an enhanced porosity for low processing temperatures. The influence of porosity on the normal state resistivity and the critical currents has been evidenced. Optimized processing parameters lead to samples with very high critical currents (J^ab_c = 3.2 × 10⁶ A cm⁻² at 77 K and 2.7 × 10⁷ A cm⁻² at 5 K) which demonstrates the capability of the trifluoroacetate metalorganic deposition method for thin film and coated conductor preparation.

In view of the development of superconducting fault current limiters, the properties of switching and recovering of YBa₂Cu₃O_7−δ/Au (YBCO/Au) thin films are studied at 77 K and 50 Hz for overcurrents. The bilayers present an abrupt transition to a high dissipative state leading to a current limitation at a maximum value of about 2.5 times the critical current I_c, and allow the development of electric fields of 3 kV m⁻¹ without any damage. After the overcurrent regime, the recovery of the superconducting state under the rated current I_n is studied as a function of overcurrent parameters. These results show clearly the strong potential of YBCO/Au thin films which can recover their superconducting state under nominal mode. This last point is crucial for transformer connection, as experimentally shown in this paper.

Bulk Y_3/4Lu_1/4Ba₂Cu₃O₇ + BaPb_1−xFe_xO₃ composites representing a model of a network of superconductor–normal metal–superconductor (SNS) Josephson junctions with paramagnetic impurities (Fe) in the N layers have been prepared. The transport properties of the composites have been studied. An exponential decay of the critical current with increasing Fe content in the non-superconducting ingredient of the composite has been observed. Such a behaviour is accounted for in terms of the de Gennes proximity effect theory.

The critical currents of 1 cm wide YBCO coated tapes, fabricated within the EC-funded project READY, have been measured. Critical current data as a function of temperature from 4.2 K to 77 K in magnetic fields up to 6 T, together with data at 4.2 K in fields up to 20 T, are presented. The future use of such conductors in high-field magnet-insert coils is considered through comparison of the data with those for Bi-2212, Bi-2223, NbTi and Nb₃Sn conductors at liquid helium temperatures.

We report on the analysis method to extract quantitative local electrodynamics in superconductors by means of the magneto-optical technique. First of all, we discuss the calibration procedure to convert the local light intensity values into magnetic induction field distribution and start focusing on the role played by the generally disregarded magnetic induction components parallel to the indicator film plane (in-plane field effect). To account for the reliability of the whole technique, the method used to reconstruct the electrical current density distribution is reported, together with a numerical test example. The methodology is applied to measure local magnetic field and current distributions on a typical YBa₂Cu₃O_7−x good quality film. We show how the in-plane field influences the MO measurements, after which we present an algorithm to account for the in-plane field components. The meaningful impact of the correction on the experimental results is shown. Afterwards, we discuss some aspects about the electrodynamics of the superconducting sample.

The behaviour of the resistivity of underdoped Y₁Ba₂Cu₃O_x samples (x = 6.4–6.7) at temperatures above T_c, i.e., in the case when the samples are in a normal, non-superconducting state, is analysed. It is shown that the model of negative-U centres describes the transition correctly at T ≈ T* from the state with metallic conduction to that with semiconducting conduction, which occurs as temperature is lowered. In this model, the state with the so-called pseudo-gap is treated as a state in which the Fermi level, pinned half-way between the levels associated with negative-U centres, lies at a distance on the order of T* above the valence band top.

The phase assemblage and particle sizes of precursor powders have been optimized in a sequence for fabricating Ag/BSCCO-2223 composite tapes. Firstly, an optimal calcination temperature was determined based on the experimental results. Then, the precursors calcined at the optimal temperature were ball-milled for different dwell times to obtain varied particle sizes. The effects of both the phase assemblages and particle sizes of the precursor powder on the phase formation, microstructure and transport J_c of Bi-2223/Ag tapes have been investigated. The results show that the precursor phase assemblage has a large impact on the reaction routes, microstructure, and J_c property. Meanwhile, a fine powder is beneficial for the grain growth, alignment, and J_c enhancement in fully reacted tapes. The best J_c was achieved in the tape made from the powder after optimizing the phase assemblage and particle size.

We have shown that oxygenation of the melt-grown YBa₂Cu₃O₇/Y₂BaCuO₅ single grain superconductors is associated with the formation of macro-cracks parallel to the a/b-plane. The origin of these cracks is associated with tensile stress induced into the oxygenated surface layer due the c-lattice parameter shortening. The critical strain calculated according to Thoules's model for the cracking of brittle films on an elastic substrate suggests that the macro-cracking starts much earlier than the formation of the homogeneous layer of equilibrium oxygen content at the surface.

Thin films of the infinite-layer (IL) compound, Sr_1−xCa_xCuO_2−δ (x = 0, 0.3 and 0.4), have been prepared by off-axis radio-frequency (rf) magnetron sputtering on SrTiO₃ (100) and KTaO₃ (100). The c-axis lattice parameter of the IL phase shrinks with the increase of the substrate temperature and with the decrease of oxygen partial pressure, whereas the a-axis lattice parameter expands accordingly. This behaviour suggests the availability of the incorporation of oxygen vacancies in the CuO₂ planes and the mechanism of electron doping. For the lower doping level, for example, when the doping level is less than 0.08, the temperature dependence of resistivity shows insulating or semiconducting behaviour. On the other hand, the large amount of in-plane oxygen deficiencies will also lead to a high degree of disorder in the CuO₂ planes, especially when the in-plane oxygen vacancies are larger than 10%. This will result in a variable range hopping transport mechanism. Moreover, we have demonstrated that electrons can be doped in the IL phase by reducing the oxygen content.

The effect of Ag₂O addition on the Bi–(Pb)–Sr–Ca–Cu–O system has been investigated in terms of ac susceptibility, phase evolution, critical current density and critical temperature. It was found that as the amount of Ag₂O addition increases, the intergranular critical current density decreases in our samples (Bi_1.84Pb_0.34Sr_1.91Ca_2.03Cu_3.06O₁₀) fabricated by ammonium nitrate technique. The analysis for comparison is based on the suppression degree of the diamagnetic behaviour with respect to fields, rapid or slow shift of the summit in χ'(T) to lower temperature with increasing field amplitude and the sharpness of the transition of χ'(T) for intergranular component for the same field amplitude. We also qualitatively discuss experimental results in the framework of the critical state model. The room temperature XRD diagram indicates the presence of large amount of high-T_c (2223) phase. The percentage of Bi-2223 phase in the phase mixture was estimated from the intensities of high-T_c (2223) and low-T_c (2212) phase peaks as 78% for the pure BSCCO sample. Among the Ag₂O-added BSCCO samples studied, the one in which 5 wt%Ag₂O was added shows the highest rate of Bi-2223 formation as 92%. The SEM analysis reveals some morphological changes induced by silver addition.

High-quality YBa₂Cu₃O_7−x (YBCO) films were fabricated on yttria-stabilized zirconia (YSZ)-buffered Hastelloy C276 substrates by pulsed laser deposition. YSZ was grown by ion-beam-assisted deposition. A thin (≈10 nm) CeO₂ layer was deposited before the deposition of YBCO. The crystalline structure and biaxial texture of the YBCO film and the buffer layer were examined by x-ray diffraction 2θ-scan, ϕ-scan and pole-figure analysis. Epitaxial growth of the YBCO film on the buffer layer was observed. Full width at half maximum (FWHM) value of 7.4° was measured from the ϕ-scan of YBCO(103). Raman spectroscopy showed compositional uniformity and phase integrity in the YBCO films. Surface morphologies of the YBCO films were examined by scanning electron microscopy. Comparative studies indicated that the CeO₂ buffer layer significantly improves the structural alignment and superconducting properties of YBCO films. T_c = 90 K, with sharp transition, and transport J_c = 2.2 × 10⁶ A cm⁻² at 77 K in zero-external field were obtained on the 0.5 μm thick YBCO films. The dependence of J_c on the FWHM of the YBCO(103) ϕ-scan indicated that high J_c is associated with low FWHM.

YBa₂Cu₃O_X thin films were grown on MgO by ion beam sputtering at various substrate temperatures T_S and oxygen partial pressures P_O with a supply of either oxygen molecules or plasma (PL) to study fundamental crystal growth of perovskite oxide. Growth of the a-phase dominates at low T_S while that of the c-phase dominates at high T_S. This indicates that a thermal surface migration is an important mechanism for the a/c orientations. The c-phase dominates at lower P_O, and it decreases while the a-phase increases with increasing P_O at T_S = 600 °C. This is interpreted by a particle energy assisted surface migration controlled by collision between sputtered particles and the supplied oxygen species. The two results indicate that the surface migration is a crucial factor for the a/c orientations. The a-phase growth is enhanced by PL due to two mechanisms, the collision retarded surface migration, and plasma energy assisted atomic rearrangement and surface energy reduction. Intragrain and intergrain crystallinities show a regular correlation of P_O dependences at T_S = 600 °C, while a P_O dependence of surface roughness shows an inverse correlation with the intragrain and intergrain crystallinities. The mixed film can release a strain energy from interior grain to ambient media, then it has good crystallinity but a rough surface. In contrast, the pure single a-phase film has poor crystallinities but the smoothest surface because the strain energy is stored in the interior. In-plane orientation is improved by increasing the a-phase ratio, and in-plane crystallinity can be improved by PL.

A narrow dip is observed around zero magnetic field in magnetization curves M(B) of superconducting YBCO films with about 10 nm thickness. This anomaly occurs in the same field range with an anomaly of ac surface impedance Z(B) found recently in thin YBCO films. Because the thickness of our films is considerably less than the London penetration depth, two-dimensional limit of the critical state model is applied. In the framework of this model the magnetic field dependence of the critical current density j_c(B) is found. The obtained j_c(B) function agrees well with that found in the ac surface impedance investigation.

Evolutions of diamagnetic magnetization (M_dia), critical current density (J_c) and irreversibility field (H_irr) for MgB₂ have been investigated by changing the water-exposure time. The evolutions are found to follow a universal exponential law of A_N(t_R) = A_N(0) exp(−t_R/t_N), where A_N represents M_dia, J_c and H_irr for N = 1, 2 and 3, respectively, and t_N is the corresponding time constant. When M_dia shows the most rapid decay with exposure time, H_irr exhibits the slowest degradation, with a difference between the decay-time constants of more than two orders of magnitude. For the case of Ti-doped MgB₂, the exponential decay law can be still applied, but with a much longer decay-time constant. Microstructure analyses reveal that the grain boundaries of MgB₂ are corroded by water when the exposing time is long enough. However, Ti doping significantly enhances the resistance to the corrosion of water.

Diffusion method has been developed to preferentially dope the grain boundaries of YBa₂Cu₃O_y bicrystals and polycrystals. Ca and Fe have been used as dopants in this study. The distribution of the dopants is highly localized in the grain boundaries. The critical current density of a YBa₂Cu₃O_y textured polycrystalline sample is significantly enhanced by doping Ca in grain boundaries; however, it is significantly decreased by doping Fe in grain boundaries. The Ca-doping effect has been explained by charge-carrier compensation in the grain boundaries, which reduces the grain boundary resistance and thus increases the grain boundary critical current. The Fe-doping effect has been explained by worsening the hole loss in grain boundary region, which increases the grain resistance and thus decreases the grain boundary critical current.