Table of contents

The critical current density, J_c, irreversibility field, B_irr, and magnetic field trapping ability of (LRE)–Ba–Cu–O bulk superconductors, where LRE is a light rare earth element such as Nd, Sm, Eu and Gd, are generally superior to those of the more common melt-processed Y–Ba–Cu–O (YBCO). The lack of availability of a suitable seed crystal to grow large, single grain (LRE)–Ba–Cu–O superconductors with controlled orientation, however, has hindered severely the development of these materials for engineering applications over the past ten years. In this communication we report for the first time the development of a generic seed crystal that can be used to fabricate any rare earth (RE) based (RE)–Ba–Cu–O ((RE)BCO) superconductor in the form of a large single grain with controlled orientation. The new seed crystal will potentially enable large grain (LRE)–Ba–Cu–O bulk superconductors to be fabricated routinely, as is the case for YBCO. This will enable the field trapping and current-carrying characteristics of these materials to be explored in more detail than has been possible to date.

The costs of superconducting magnet strands are compared by calculating a 'production scaling factor' P that relates purchase data to the cost of raw materials. Using a consistent method, we normalize for different conductor geometries and strand diameters to arrive at cost indices in $ kg⁻¹, $ m⁻¹, and $ kA⁻¹ m⁻¹. Analyses of Nb47Ti conductors taken from the past 25 years of high-field magnet projects reveal that the price of raw materials and, to a lesser extent, finished strands, have tracked the price of niobium pentoxide. Performance gains during the 1980s produced $ kA⁻¹ m⁻¹ indices that fell with time ahead of strand cost in $ m⁻¹, a situation that may reflect the present status of Nb₃Sn magnet conductors. Analyses of present materials show that P decreases systematically with billet mass. While production strands in 200–500 kg billets have costs times the cost of raw materials, the 20–50 kg billet size for internal-tin Nb₃Sn composites drives P up to 8–10. Thus, in contrast to LHC-type Nb47Ti strands that cost $150 kg⁻¹, $0.60 m⁻¹, and $1.00 kA⁻¹ m⁻¹ at 5 T, 4.2 K, Nb₃Sn strands required for the next generation of accelerator magnets are $1000 kg⁻¹, $4.00 m⁻¹, and >$5.75 kA⁻¹ m⁻¹ at 12 T, 4.2 K (where J_c is comparable to that for Nb47Ti at 5 T, 4.2 K). This high cost might be reduced by a factor of if a large-scale internal-tin or powder-in-tube Nb₃Sn process can be found. Replacing expensive components with functionally equivalent but cheaper materials can produce 20% changes in $ kg⁻¹ and $ m⁻¹, but this might come at a performance penalty and no net savings in $ kA⁻¹ m⁻¹. Removing stabilizer from the strand cross-section and replacing it elsewhere in the cable can reduce the cost for a given length of cable significantly, but only if the processing cost for the strand remains unchanged after reduction of the stabilizer area. Emerging powder-in-tube composites show promise: Nb₃Sn strands could reach $6 kA⁻¹ m⁻¹ at 12 T, 4.2 K, while Bi-2212 strands could fall below $10 m⁻¹. MgB₂ superconducting strands could have very low raw materials cost at $0.20 m⁻¹, which translates to $1.00 m⁻¹ for finished strands and at 2 T, 4.2 K based on published data.

Non-metallic flexible Y Ba₂Cu₃O_7−δ (YBCO)-coated superconductor tapes were fabricated using zirconia with 3 mol% yttria CERAFLEX_TM (CF) substrates. The CF substrate surface was treated by spin-coating of spin-on-glass (SOG). Yttria-stabilized zirconia (YSZ) buffer layers were deposited on the SOG layer by ion beam assisted magnetron deposition (IBAD). The epitaxial YBCO films were grown on the buffer layer. The crystalline quality and degree of biaxial alignment were measured using the x-ray θ–2θ and ϕ-scans. The YBCO films were c-axis aligned and had an YBCO(103) ϕ-scan full width half maximum (FWHM) of 10.5°. Films with ϕ-scan FWHM of 19° had a zero field critical current density, J_c, of 0.25 MA cm⁻² at 77 K as measured by the induction method.

We measured the velocity of the flux front of an artificially nucleated dendritic instability in Y Ni₂B₂C. The required time resolution in the nanosecond regime was achieved by our magneto-optic pump–probe technique, utilizing a femtosecond laser system. The penetration velocity of the flux front is on the order of 360 km s⁻¹.

We show with the help of simulations that the critical current densities determined from magnetization and transport measurements are modified differently by the presence of macroscopic defects in a superconducting flat conductor like thin film or tape. A particular double-slot geometry, reminiscent of a superconducting tape with transversal cracks, is considered for the simulations, allowing us to analyse systematically how the critical current densities determined in an inductive and transport way change as a function of the size and relative position of the defects within the tape. The differences in the critical current densities obtained from both methods are discussed.

Highly smooth and c-axis oriented superconducting MgB₂ thin films were prepared by pulsed laser deposition (PLD) with off-axis geometry. The films were deposited on Al₂O₃–C substrates perpendicularly aligned to a stoichiometric MgB₂ target in a 120 mTorr high purity Ar background gas. An in situ annealing was carried out at 650 °C for 1 min in a 760 Torr Ar atmosphere. Despite the short annealing time, an x-ray θ–2θ scan shows fairly good crystallization, according to the clear c-axis oriented peaks for the films. Both atomic force microscopy and the x-ray diffraction results indicate that the crystallite size is less than 50 nm. The root mean square roughness of our off-axis film is  nm in a 5 × 5 µm² area. The T_c onset value of the best off-axis film reaches 33.1 K with a narrow transition width of 0.9 K. The films showed no anisotropy in H_{c 2}–T curves when parallel and perpendicular fields were applied. The slope of the H_{c 2}–T curves in the low field regime is 1 T K⁻¹, which is among the highest reported values.

A European consortium has progressed towards AC coil, transformer and cable applications made of low AC loss Bi-2223 conductors. Internally assembled ring-bundle-barrier conductors were used in AC coils and an externally assembled conductor was realized for transformer and reactor windings. For the design and optimization of HTS AC coils, a numerical tool for the prediction of AC losses in coils and coil sets had been developed. The demonstrations with air-core coils and iron-core reactors showed that the numerical tool can already be confidently used in the design and optimization of HTS AC coils at nominal operation conditions. A series of three power cable prototypes was manufactured to estimate the influence of optimized low AC loss tapes on the total losses of a cable. In order to estimate the impact of the twist pitch on the losses we also manufactured cables with short and long twist pitches. The advantages and drawbacks of low AC loss tapes in power applications are discussed on the grounds of the test results.

The thermal runaway phenomenon in Ag/Bi₂Sr₂CaCu₂O₈ composite conductors is studied for the nonlinear temperature and magnetic field dependences of their critical current density J_c(T,B) and matrix resistivity ρ_m(T,B) (nonlinear approximation). The influence of the applied magnetic field, heat transfer coefficient, matrix resistivity and volume fraction of the composite superconductor on the static thermal and electric states were investigated. The used model is based on steady heat and current balance equations. It allows us to investigate the non-isothermal voltage–current characteristic of composite superconductors, that roots the static description of their operating regimes and allows us to determine the limiting quantities of the thermal runaway parameters. It is found that there exist stable jumps of the electric field, current and temperature without the transition of the superconducting composite into the normal state. The presence of these static states is a result of the additional stable branches on the voltage–current characteristics, which appear according to the variation of the differential resistivity of a composite. The latter may have wide range fields due to the coupled thermo-magnetic and current-sharing mechanisms changing the quantities and . This variation takes place when the applied magnetic field or operating parameters of a composite exceed some characteristic values. As a result, the non-monotonic temperature dependence of the Joule heat release will take place. Besides, it is shown that the stable voltage (without thermal runaway) may exist when the temperature of the Ag/Bi₂Sr₂CaCu₂O₈ composite stably increases from the coolant temperature up to the critical temperature of a superconductor during current charging. The thermal runaway parameters as a function of the applied magnetic field are numerically derived accounting for the additional stable branches of the voltage–current characteristic.

The scanning Hall probe microscope (SHPM) is an important imaging tool used for detailed studies of superconductors in basic science as well as in the industrial sector. It can be used for the studies of losses, current distribution, and effects at grain boundaries.

However, only a few SHPMs for magnetic field imaging at temperatures below 77 K have been proposed up to now, most of them designed for small-area () scanning. We present a large-scale low-temperature SHPM developed for imaging the entire magnetic field in close proximity to magnetic and superconducting samples at 4.2–300 K. The microscope combines a large scanned area and high spatial and magnetic field resolution. The instrument is designed as an insert of standard helium flowing cryostats. The Hall sensor scans an area up to 7 × 25 mm² in the whole temperature interval with a spatial resolution better than 5 µm. The presented system is used for the study of ex situ prepared MgB₂ filament. We show that external magnetic field induces local supercurrents in the MgB₂, from which the critical current can be estimated. Moreover, it indicates the microstructure and space homogeneity of the superconductor.

The electronic structure of the layered diboride dicarbide superconductor Y B₂C₂ is calculated using the full potential LAPW method within the framework of ab initio density functional theory. Our results confirm that the crystal structure with P4/mbm symmetry is more stable than the originally claimed structure, which is in accordance with recent interpretations of the diffraction patterns of other related compounds of LaB₂C₂-type. It is found that the metallic conductivity in the stable P4/mbm structure is due to Y d-bands partially hybridized with p_z-states from the B–C planes. Thus the structure of the conduction bands differs from those found in MgB₂. However, a large portion of the Fermi surface of Y B₂C₂ exhibits distinctive two-dimensional features, which can make this compound interesting for experimental studies on superconductivity connected to effects of strong electronic structure anisotropy.

Bi_1.8Pb_0.35Sr_2.0Ca_2.1Cu_3.1Nd_xO_y (x = 0.00, 0.010, 0.020 and 0.030) superconductors were prepared in bulk form. Phase analysis by XRD, microstructural examination by SEM, superconductivity measurements at 77 K, etc were carried out to evaluate the relative performance of the samples. Both the self-field J_C and J_C in the presence of an applied magnetic field of the Nd-added samples are found to be much better than those of the undoped one. Also the peak value of the bulk pinning force density, F_{p max}, of Nd-added samples get shifted towards higher magnetic fields, indicative of enhanced flux pinning strength in these samples. For the sample with Nd = 0.030, small fractions of (Bi, Pb)-2212 were found even after the last stage of sintering. Accordingly the microstructure of this sample shows considerable difference from the other samples, wherein the natural layered growth mechanism of (Bi, Pb)-2223 is disrupted by grain growth and coarsening in random directions. It is likely that Nd ions are substituting at the Ca site, and the point defects thus created act as flux pinners, enhancing the self-field J_C as well as the J_C–B characteristics of the Nd-added samples.

We estimated the distribution of currents and fields in a magnet which consisted of three coaxial solenoids; the solenoids were made by stacking pancakes of coated conductors (CCs). We used a model for the critical current densities which satisfied simple relations with magnitudes and directions of fields. Since various types of pinning centres result in different anisotropies in the field dependence of critical currents, their contributions to the properties of magnet were different. We found that the intrinsic pinning centres in CCs resulted in inefficient performance of the magnet as they were effective only in the pancakes in the central part of the magnet, where the fields were parallel to the ab-planes of the superconducting layers in the CCs. We estimated how much the magnet performances were improved by additions of columnar pinning centres or by addition of random point pinning centres.

A Ag:Y Ba₂Cu₃O_7−δ/SrNb_0.01Ti_0.99O₃ bilayer structure was fabricated by pulsed-laser deposition. Current–voltage (I–V) dependence measurements of the bilayer were carried out in the temperature range 5–250 K with variation of magnetic field from 0 to 50 kOe. Schottky junction behaviour was observed at temperatures higher than the critical temperature of superconductivity (T_c) due to the n-type semiconductor SrNb_0.01Ti_0.99O₃ and the metallic conductivity of Y Ba₂Cu₃O_7−δ. At temperatures lower than T_c, the bilayer becomes a degenerate heterostructure p–n junction. The I–V characteristic shows a sequence of current steps at forward voltage and an oscillation character at backward voltage. Energy diagrams of the Ag:Y Ba₂Cu₃O_7−δ/SrNb_0.01Ti_0.99O₃ bilayer structure have been proposed to explain the peculiar I–V characteristics.

In order to understand low-temperature melting during the 'BaF₂ process', equilibria in the quaternary Ba, Y, Cu//O, F reciprocal system have been investigated using a compositional model which can be represented as a trigonal prism. This prism is comprised of three tetrahedra: BaF₂–Y F₃–CuF₂–CuO_x, BaF₂–Y F₃––CuO_x, and BaF₂–BaO––CuO_x. Systematic differential thermal analysis (DTA) studies of compositions spaced along compositional vectors extending from the fluoride end of the prism to the oxide end gave evidence of low-melting liquids (<600 °C) near the fluorine-rich region in the BaF₂–Y F₃–CuF₂–CuO_x tetrahedron. In the intermediate BaF₂–Y F₃––CuO_x tetrahedron, a low-temperature DTA peak (550–570 °C) was also identified; this has been shown to be due to a reversible phase transformation in crystalline YOF. In the oxide-rich BaF₂–BaO––CuO_x tetrahedron, where, in the presence of water vapour, the principal defluorination process is generally thought to occur, the lowest melting temperature observed was 815 °C at p_O2 = 20 Pa and p_H2O = 2.1 kPa. However, published observations on films undergoing the BaF₂ process have suggested the presence of an amorphous phase thought to be a liquid at . Based on our results, the low-melting liquids reported in the literature do not appear to exist as a stable liquid phase in the BaF₂–BaO––CuO_x–H₂O system under the conditions of our experiments. Rather, the formation of low-melting liquid in Ba–Y–Cu–O–F films at may require (a) relatively fluorine-rich compositions, (b) metastable melting, (c) formation of hydroxide- or hydroxyfluoride-based liquids, or (d) some combination thereof.

In this paper we analyse the structural and physical properties of rutheno-cuprate samples of the phase Ru–Gd(1212) subjected to different thermal treatments. We have observed the variations induced by these treatments by structural measurements (x-ray powder diffraction and high-resolution transmission electron microscopy) and physical measurements (dc magnetic measurements and magnetoresistive measurements).

Such variations involve both the intrinsic properties of the Ru–Gd 1212 phase (for example the temperatures of the magnetic and superconducting ordering), and the extrinsic properties, i.e. those related to the microstructure of the samples, such as the domain dimension, and to their granular nature. In particular, while a prolonged annealing in flowing oxygen induces an extended microstructural homogeneity, annealing in vacuum, besides producing loss of oxygen, leads to the formation of intragranular disordered microdomains and extended planar defects. We verify how the physical properties are dependent on the level of microstructural order that determines the passage from metallic to semiconducting behaviour and, possibly, from bulk to spin glass superconducting behaviour.

By saying 'bulk' superconducting behaviour we mean the behaviour commonly observed in high-T_c superconductors where, as is well known, granularity is present and intergranular and intragranular properties of the sample may be observed: in appropriate experimental conditions it is possible to separate the two contributions to study the intrinsic properties of the material or to observe the character of the Josephson coupling between grains (or clusters). In contrast, the 'spin glass' is modelled as superconducting grains weakly linked into closed loops, where different supercurrent carrying states of nearly equal energy may install, producing, among other effects, metastable configurations. We suggest that the microstructure is an important cause of the different behaviour of samples otherwise very similar: strong microstructural disorder decreases the domain size inside the grains until they are comparable with the coherence length ξ (and much smaller than the penetration depth λ). In such conditions the superconducting volume of the sample becomes smaller and smaller; granularity or weak-link behaviour becomes ubiquitously present and practically determines the physical properties in the whole phase diagram.

A European consortium has progressed towards elementary and assembled conductors viable for AC applications in the project ACropolis. The main goal was to achieve low AC losses and high critical current densities in tapes developed for AC coils, transformers and cables. In order to verify the performance of tapes an extensive characterization programme was carried out. The dependence of critical current and AC losses on the parameters of the tape manufacturing process were scrutinized. Furthermore, SEM-EDX and DTA/TG analyses were performed to study the material microstructure, and the diffusion coefficient of oxygen through different matrix metals was measured. The homogeneity of the tapes was verified with magnetic knife and Hall sensor experiments. Transport current measurements were performed at different mechanical stress conditions and also the distribution of critical current along long tapes was determined with a quasi-continuous four-point measurement. The results confirmed that the low AC loss conductors can be produced on an industrial scale and they are competitive in power engineering applications.

We have chemically created a superconducting–paramagnetic dual magnetic system by introducing molecular magnetic species into layered high-T_c cuprate, where the weak π–π interactions between planar π-conjugated radicals appear to form a Pauli-type paramagnetic subsystem in the interlayer space of cuprate blocks. The intercalation-induced free radicals lead to a heterostructured spin-system, providing not only a probe for high-T_c superconductivity but also a new way of creating materials with spin- or magnetic field-dependent functionality unattainable from conventional solid-state materials.

For making trilayer superconducting devices based on Y Ba₂Cu₃O_7−δ (YBCO) thin film processing, we developed a new technique by employing Ge ion implantation. A YBCO thin film of 150 nm thickness having high c-axis orientation and a transition temperature, T_c, of 90 K was implanted with 80 keV, 1 × 10¹⁶ Ge ions cm⁻² at room temperature. By the result of TRIM calculation, Ge ions were found to penetrate into the YBCO thin film approximately 60 nm below the surface of the film, thus leaving the lower part of the film as a superconductor. Upon implantation with Ge ions, the implanted upper part of the sample lost its electrical conductivity and diamagnetism while its original crystalline structure was preserved. The implanted ions we found did not alter the overall crystal structure of the YBCO thin film; this allowed us to grow an epitaxial superconducting upper layer of YBCO on top of the implanted area, leaving no need to use any buffer layer. The superconducting properties of the upper layer were similar to those of the pure YBCO base layer with an increased room temperature resistivity and a lowered T_c (88 K). This process provides an effective method for fabrication of a trilayer HTS device structure.

Recently, the '(p+h)-wave' form of pairing symmetry has been proposed for the superconductivity in the PrOs₄Sb₁₂ compound (Parker et al 2004 Preprint cond-mat/0407254). In the present paper, a stationary Josephson junction as a weak-link between PrOs₄Sb₁₂ triplet superconductors is theoretically investigated. The quasiclassical Eilenberger equations are analytically solved. The spin and charge current-phase diagrams are plotted, and the effect of misorientation between crystals on the spin current and spontaneous and Josephson currents is studied. It is found that such experimental investigations of the current-phase diagrams can be used to test the pairing symmetry in the above-mentioned superconductors. It is shown that this apparatus can be applied as a polarizer for the spin current.

MgB₂ thin films were fabricated on sapphire substrate by using electron beam deposition and without any post-treatment. The films have columnar grains aligned perpendicular to the film surface. Superconducting properties were investigated in magnetic fields, B, in the range 0–28 T and/or at temperatures, T, ranging from 4.2 K to the critical temperature, T_c, by using the DC four-probe transport method. T_c-values of the films are 33.12–33.90 K in self-field. The films show very high J_c at 4.2 K, e.g., 4.7–7.1 × 10⁶ A cm⁻² in self-field, and 0.8–1.1 and 0.3–0.4 × 10⁶ A cm⁻² in 4 T of the perpendicular and the parallel fields, respectively. At 4.2 K, the films carry 0.4–1.0 and 0.8 × 10⁵ A cm⁻² even in a perpendicular field of 8 T and the parallel fields, respectively. J_c exceeds 10⁵ A cm⁻² at 27 K in 1 T and 15–17 K even in 5 T of the perpendicular fields. The upper critical fields, B_c2 and irreversibility fields, B_irr, at 4.2 K are 27.6–27.9 T and 16.7–19.2 T in the parallel fields, and 17.5–20.4 T and 10.7–13.4 T in the perpendicular fields, respectively. The strong pinning at grain boundaries between columnar grains is confirmed at high temperatures and below 7–8 T at 4.2 K.

Approximately 50 µm thick epitaxial c-axis orientated (Hg_0.8Re_0.2)Ba₂Ca₂Cu₃O_x superconducting films have been prepared on MgO(100) substrates using a spraying process and post-Hg-vapour annealing. The effects of the heating temperature–time combinations and the filling factor of Hg (ff_Hg) on the physical, electrical and magnetic properties of the thick films have been investigated. The interesting observation was the formation of a thick interfacial layer between the film and the substrates. This interfacial layer was found to be protective against excessive diffusion and dissolution of the Mg²⁺ ions from the substrate or counter-diffusion of Ba, Ca and Cu ions from the films. The XRD investigations showed that the a–b plane of the HgRe-1223 phase aligns parallel to the substrate surface. The best T_c and T_zero were found to be 131.9 and 129.3 K, respectively. Magnetic properties up to 5 T have been investigated. The calculated value of critical current density, J_c, was found to be 4.82 × 10⁵ A cm⁻² at 4.5 K. The results obtained suggested that fabrication of HgRe-1223 thick film superconductors using a spraying process with 130 K transition is possible and they are very promising particularly for electronic applications.

The background magnetic field dependence of the critical current (I_c) is studied on magnetic field processed (MFP) Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) multifilamentary tapes in increasing and decreasing magnetic fields. The critical currents are measured in magnetic fields up to 5 T at 4.2 K, with several orientations of the applied magnetic field relative to the tape plane. Two field dependences of I_c are found: a stronger one when the magnetic field is applied perpendicular to the tape plane () and a weaker one when it is applied parallel to the tape plane . The angular dependence of I_c as a function of Bsinθ, which is the component of the magnetic field perpendicular to the tape plane, exhibits that I_c(Bsinθ) coincides with in the high-field region, indicating that the tape behaves as a two-dimensional (2D) superconductor. Because of the existence of misalignment in Bi-2212 grains, an average misalignment angle φ_avg is introduced and is found within the range of 4.5°–6° for the MFP Bi-2212 tape. φ_avg is independent of the magnetic field, indicating that it is an intrinsic property of the tape. To determine the critical current behaviour for all orientations of the magnetic field, a scaling function f(θ) is introduced. The field-angle dependence of the critical current can be scaled to one curve where the field applied perpendicular to the tape plane at 4.2 K within the field range between 0 and 5 T, both in increasing and decreasing fields, which is of primary importance.

Silver paint has been tested as a soldering agent for DyBaCuO single-domain welding. Junctions have been manufactured on Dy–Ba–Cu–O single domains cut either along planes parallel to the c-axis or along the ab-planes. Microstructural and superconducting characterizations of the samples have been performed. For both types of junctions, the microstructure in the joined area is very clean: no secondary phase or Ag particle segregation has been observed. Electrical and magnetic measurements for all configurations of interest are reported (ρ(T) curves, and Hall probe mapping). The narrow resistive superconducting transition reported for all configurations shows that the artificial junction does not affect significantly the measured superconducting properties of the material.

2212 intergrowths in the (Bi, Pb)2223 phase have been investigated using in situ high temperature synchrotron XRD technique. With a high energy synchrotron x-ray and high resolution diffractometer, we could obtain high resolution powder XRD patterns of (Bi, Pb)2223 and (Bi, Pb)2212 from the whole bulk inside the Ag-sheath. This gave us more detailed information on 2212 intergrowths in the (Bi, Pb)2223 phase than ever before. During in situ observation, the Ag-tubed precursor was kept at 1095 K with flowing Ar–7.8% O₂ mixed gas. The profiles of the diffraction peaks were analysed by Rietveld analysis to evaluate the isotropic and anisotropic lattice strain of (Bi, Pb)2223 and (Bi, Pb)2212. Considering the evolution of anisotropic lattice strain during the heat treatment, it was concluded that 2212 intergrowths in the (Bi, Pb)2223 phase are not in the untransformed region of (Bi, Pb)2212 after incommensurate intercalation, but a stacking fault-like defect contained in the (Bi, Pb)2223 phase during its nucleation and growth. A new model of 2212 intergrowth formation in the (Bi, Pb)2223 phase was suggested and discussed.

The effect of Nb_xB₂ addition on the critical current density in applied magnetic fields (J_c–B characteristics) of iron sheathed MgB₂ tapes prepared via an in situ powder-in-tube (PIT) technique was investigated. The results show that the J_c–B characteristics of MgB₂ tapes are enhanced by the addition of an appropriate amount of Nb_xB₂. This effect is considered to be due to the introduction of a pinning centre and the densification of the superconducting core. Furthermore, the effect of Nb_xB₂ addition is not dependent upon the stoichiometry of Nb. The Nb_xB₂ has the same crystal structure as MgB₂ (AlB₂ type), and the lattice parameter is close to that of MgB₂, hence utilization of Nb_xB₂ is expected to have some advantages in the tape fabrication of MgB₂.

Recent differential thermal analysis studies on green Bi-2223 tapes showed two phase transitions, with onset temperatures 800 and 820 °C. From microstructural examination of the long-term phase formation in the tapes the higher temperature endotherm was associated with a partial melt. In this study it is demonstrated how control of the heating rate can promote the partial melt (characterized by DTA), and thereby improve the transport current, I_c. XRD, SEM and transport measurements in field show microstructure features typical of an increased volume of liquid phase: a reduction in secondary phase volume and pores with a corresponding increase in c-plane texture.

We study the effect of the substrate on the effective surface impedance of a Y Ba₂Cu₃O_7−δ thin film grown over a silicon substrate. The microwave measurements, performed with a resonant cavity technique at two distinct frequencies (24 and 48 GHz), show anomalies both in the temperature and in the frequency dependences. By properly taking into account a relevant complex substrate contribution, with an important role played by the finite thickness effect, the whole set of data finds a satisfactory explanation in terms of standard impedance transformations. Using independent measurements at a single temperature, it is possible to extract the temperature dependence of the Y Ba₂Cu₃O_7−δ bulk surface impedance, which shows the expected frequency and temperature behaviours.

The nucleation of superconductivity in a hybrid film (HF) that consists of randomly distributed CoPt ferromagnetic nanoparticles (FNs) embedded in a 200 nm Nb layer was studied by combined magnetic and magnetoresistance measurements. The FNs exhibit both soft and hard magnetic behaviour. It is found that both magnetic phases contribute to the modulation of bulk and surface-like superconductivity by broadening their intermediate regime. More specifically, while the soft FNs (SFNs) promote bulk superconductivity, the hard FNs (HFNs) suppress the bulk superconducting state. In contrast, both SFNs and HFNs promote surface-like superconductivity since on the H–T phase diagram the respective boundary line presents abrupt upturns at their corresponding saturation fields. The dependence of the observed effects on the magnetic history gives clear evidence that they are mainly induced by the cooperative action of the FNs. Our experimental results are compared with current theoretical studies on relevant hybrid systems. Finally, the possible applications that such HFs could find in the near future are discussed.

MgB₂/Fe(Cu) wires fabricated by the PIT method with involvement of the hydrostatic extrusion (HE) process show a noticeable core homogeneity and refined microstructure which result in high transport J_c values. The highest J_c for sintered wire were measured for HE SiC doped wire 10⁴ A cm⁻² (Fe clad) and 0.5 × 10⁴ A cm⁻² (Cu/Fe clad), both at 4.2 K and 5.5 T. Combination of HE with two-axial rolling (TAR) has led to J_c = 0.4–0.5 × 10⁴ A cm⁻² at 4.2 K and 5.5 T. High n-exponent values (between 21–110 at 4.2 K and 5.5 T) prove significant grain refinement (individual grains below 200 nm), homogeneity and density within the hydrostatically extruded cores. After deformation by hydrostatic extrusion the enhanced J_c at higher fields for pure MgB₂ and MgB₂ SiC nano-doped wires in comparison to only TAR wires were measured. Wires sheathed with Fe gave better results than those sheathed with a bimetallic Cu/Fe sleeve. The beneficial effect of hydrostatic extrusion on J_c(B) characteristics can be attributed to very high core density in comparison to other deformation routes.

The superconducting transition temperature T_c has earlier been correlated with coherence length and effective mass for a series of heavy fermion (HF) materials at atmospheric pressure. Here, a further physical property, the dc electrical conductivity σ(T_c), is one focal point, another being the pressure dependence of both T_c and σ(T_c) for several HF materials. The relaxation time τ(T_c) is also studied in relation to an Uncertainty Principle limit, involving only the thermal energy k_BT_c and Planck's constant.

A study regarding the epitaxial growth of Y₂O₃ film on textured metal substrate and its use as a single buffer layer of a YBCO coated conductor is reported. Different deposition conditions were employed and compared in order to obtain good epitaxial Y₂O₃ film followed by deposition of YBCO superconducting films. The YBCO film deposited by PLD on top of the Y₂O₃ films has good microstructure and superconducting properties. The effect of the thickness of Y₂O₃ film on the texture of Y₂O₃ film and the critical current density (J_c) of YBCO film deposited on top of that were analysed. The J_c of YBCO films on Y₂O₃/Ni and Y₂O₃/NiW were 1.0 × 10⁶ A cm⁻² and 1.1 × 10⁶ A cm⁻² at 77 K and self-field respectively, which indicated that Y₂O₃ is a suitable candidate as a single buffer layer for the fabrication of a YBCO coated conductor.

Automated orientation measurements using electron back-scatter pattern analysis have been taken on samples of (BiPb)₂Sr₂Ca₂Cu₃O_x (Bi-2223) tapes produced using the powder-in-tube method. A strong c-axis fibre texture was found, with almost 90% of the Bi-2223 material having a crystal direction normal to the tape surface within 10° of [001]. The intensity along the fibre texture was found to be almost random, in contrast to some previously reported investigations. An analysis of the misorientations between adjacent plates revealed strong correlations. The measured misorientation angle and axis distributions were compared against those for random pairs of orientations corresponding to the same overall texture, revealing a preference for low angle misorientation boundaries, and a preference for misorientation axes to lie along [010]–[110] for low (<20°) misorientation angles. Although many plates are tilted with respect to the tape surface, the majority of the boundary area per unit volume is more twist than tilt in character.

Al-alloyed MgB₂ was prepared by solid state reaction of Mg_1−xAl_x alloy and crystalline B powder particles. The nominal composition of the samples was Mg_1−xAl_xB₂, with x = 0, 0.1, 0.2, and 0.3. Microstructure and chemical composition have been investigated by scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and preliminary transmission electron microscopy (TEM). The Al concentration of the matrix is close to, but less than, the nominal Al concentration and the difference increases with increasing nominal Al concentration. Al is incorporated into MgB₂ grains of size by substitution of Mg lattice sites with Al. Al was found to be distributed inhomogeneously, which partially explains the broadening of the superconducting transition width (ΔT_c) with increasing Al mole fraction. Al-alloyed samples contain large and small secondary phases embedded in the (Mg,Al)B₂ matrix. The composition of large secondary phases is found to be (Mg,Al)B_7+δ, with the mole fraction of excess boron (δ) increasing from 0.99 at.% in the pure sample to 4.14 at.% in the highest alloyed sample. The Al to Mg mole fraction ratio in these large secondary phases is about half of that in the matrix. The size and density of the large secondary phases increased with increase in Al mole fraction. The secondary phases constitute less than 4% of the total sample volume and are thus not likely to affect the bulk superconductivity. Magnesium diffusion and evaporation governed the formation of secondary phases and can be explained by considering the effects of annealing temperature, annealing time, and boron powder particle size.

The residual resistivity (ρ₀) of pure MgB₂ (x = 0) and Al-alloyed MgB₂ (x = 0.1,0.2,0.3), measured by the four-probe technique is comparable to the single crystals, indicating the excellent quality of these samples. The lattice parameters c and a decrease at a rate of 1.15 pm and 0.17 pm/at.% Al, respectively. The superconducting transition temperature (T_c) determined by measuring the resistive superconducting transition decreases with increasing Al alloying at a rate of 1.56 K/at.% Al.