Table of contents

Highly oriented strontium titanate films have been processed on single-crystal lanthanum aluminate and on textured nickel substrates using solution deposition. Precursor solutions of strontium acetate and titanium IV butoxide in acetic acid and methanol were deposited on substrates using spin coating and heat treated in a reducing atmosphere in a single stage to yield textured strontium titanate films. A dense crack-free microstructure with a grain size of the order of 100 nm was achieved along with a texture of the same order as the underlying substrate. These results offer promise for scalable processing of buffer layers on textured nickel.

This paper is the result of the work of a SCENET (The European Network for Superconductivity) material working group's efforts on giving values for present and future expected performance of high-temperature superconducting (HTS) wires and tapes. The purpose of the work is to give input to the design of HTS applications like power cables, motors, current leads, magnets, transformers and generators. The current status performance values are supposed to be used in the design of today's prototypes and the future values for the design of fully commercial HTS applications of the future. We focus on what is expected to be the relevant parameters for HTS application design. The most successful technique by far for making HTS tapes has been on the (Bi, Pb)₂Sr₂Ca₂Cu₃O_x (Bi-2223) material by the powder-in-tube (PIT) technique and this paper therefore focuses on giving the current status and expected future performance for Bi-2223 tapes.

Hysteresis loop measurements of superconducting polycrystalline samples of Tl-2223 were carried out at various temperatures. Using the magnetization remanence (M_R) as measurement of the maximum vortex flux pinning at each temperature, our results reveal that the vortex flux pinning forces show two distinct behaviours of M_R with temperature, a linear behaviour with t at high temperatures and an exponential behaviour with t^β at low temperatures, where t = (1-T/T_c) and β is constant of order 8. The effect of γ-irradiation shows six-fold enhancement in the vortex pinning over all temperatures up to T_c. This increase in the pinning forces is attributed to the induced defects introduced by the γ-irradiation. The same enhancement in the pinning forces at all temperatures implies constant defects induced by γ-irradiation.

Texture development in pure Ag was investigated in order to obtain {110}⟨uvw⟩ textured Ag ribbons that can be used as a substrate for YBa₂Cu₃O₇ superconducting coated tapes without any buffer layer. The starting material was 99.95% pure commercial Ag foils. A 20% deformation reduction was used at each step of the cold rolling process after which an optimal annealing was achieved at 800 °C for 4 h in a primary vacuum. This process leads to large-grain ribbons with the {110}⟨112⟩ orientation. A sharp and pure texture is obtained with a FWHM smaller than 6° in the three x-ray pole figures that were used to calculate the orientation distribution function. Crystallographic orientation maps achieved by electron back-scattering diffraction show a misorientation from the normal direction of less than 4°. A twinning mechanism is used to explain the formation of the {110}⟨112⟩ texture. The stability of the {110}⟨112⟩ texture is confirmed by further annealings up to 900 °C, a practical temperature for coated tapes preparation. Lengths up to 25 cm with a uniform {110}⟨112⟩ texture have been obtained.

Zero-field-cooled (ZFC) and field-cooled (FC) magnetic properties of strip-like Josephson-junction (JJ) arrays with very strong demagnetizing effects are calculated from basic laws. Similar to slab-like JJ arrays without considering demagnetizing effects, a vortex state evolves to a critical state (CS) with increasing maximum JJ currents in the ZFC case, and a vortex state always remains with a negative low-field susceptibility in the FC case. However, the strong demagnetizing effects cause qualitative changes in the CS, where the overall feature of the field and current profiles turns out to be similar to that in type-II superconducting strips, but not like the ordinary Bean CS in slab-like JJ arrays, the CS current profile is never flat and the critical current is no longer a step function of the maximum JJ current as in slab-like JJ arrays. The calculated results of different types of JJ arrays indicate that although the intergranular CS in granular superconductors may have a common origin, the discovered paramagnetic Meissner effect in them is still difficult to explain.

Essentially single-phase samples of the CuBa₂Ca₃Cu₄O_10+δ (Cu-1234:P) phase were synthesized by a high-pressure technique utilizing an internal-oxidizing-agent method. The oxygen content of the samples was determined by coulometric titration. A novel high-oxygen-pressure heat treatment technique to introduce additional oxygen to the as-synthesized Cu-1234:P sample was developed. The critical current density (J_c) measured at 77 K under magnetic fields of 1.0-4.4 T decreased with the increase of the oxygen content and the peak effect in J_c, which was observed for the as-synthesized sample, disappeared after the introduction of additional oxygen. Thus, it is likely that the oxygen-deficient regions in the as-synthesized Cu-1234:P sample would work for some kind of pinning of the vortices to cause the peak effect.

The effect of lattice defects at the unit cell level created by the stress field when two rare earths with different ionic radii are mixed at the rare earth site, on the flux pinning and critical current density in Y_1-xDy_xBa₂Cu₃O_7-δ thin films prepared by the pulsed laser deposition technique, was investigated using SQUID magnetometry at different temperatures for x = 0 to 1 in steps of 0.2. From the isothermal magnetic hysteresis recorded up to 5.5 T at 5, 35 and 77 K, the critical current density, pinning force density and irreversibility fields are estimated and their variation as a function of x analysed. An inverse relation between current density and irreversibility field is observed at all the temperatures except for the film having a 20% concentration of Dy in Y which also exhibits the highest critical current density among the films investigated, suggesting this to be the optimum composition for effective flux pinning from stress field induced lattice defects.

The intrinsic limits of J_c in composite Bi(Pb)-2223 multifilamentary tapes due to the reduction of the effective cross section and the enlargement of the twisted filaments have been analysed. A simple model of the twisted filaments as elliptic helices grouped in concentric shells have been used to derived analytical results for the intrinsic variation of J_c with the twist pitch, t_p. In tapes with high thickness to width aspect ratios and large t_p values, this model predicts a linear dependence of J_c with 1/t_p, when the shape and superconducting filling factor of the tapes are constant. The J_c(t_p) values depend on a set of parameters that define the helices, which may be derived from the analysis of the morphology of the filaments inside the tapes. To assess the model we have compared the predictions with the results here reported on Ag and Ag_0.94Au_0.06 alloy sheathed tapes with 19 and 37 twisted filaments, respectively. In tapes with high J_c, we have found a good agreement between the variation of J_c with t_p experimentally measured and the intrinsic limits imposed by the filament geometry.

Stability and quench experiments were performed on a cryocooler refrigerated Bi-2223/Ag magnet with an artificial hot spot. The hot spot was realized by making a poor, high-resistivity joint between the current terminal and the bottom pancake coil of the magnet. According to the experiments the magnet could tolerate considerable power dissipation at the hot spot for long times still avoiding a thermal runaway. Processes preceding the quench occurred much slower if compared to quenches in typical LTS windings. For a deeper understanding of the measured results computer simulations were done by using a model based on the heat conduction equation coupled with Maxwell's equations and solved using the finite-element method. Simulated temperature distributions inside the magnet showed that considerable temperature differences exist between different parts of the magnet, despite the slowness of the processes. Before a quench there were no large temperature gradients around the hot spot. During the quench the hot spot temperature increased rapidly but the temperature of other parts of the magnet increased only with a delay.

We have melt quenched a series of Ga substituted Bi_2-xGa_xSr₂Ca₂Cu₃O_10+y samples with the compositions with x = 0.2, 0.4, 0.6, 0.8 and 1.0. Microstructure, thermal properties including crystallization activation energy, magnetic and superconducting properties were investigated. We have found that Ga³⁺ ions have a solid solubility limit in the BSCCO system and this limit was passed at x = 0.6. Below this limit HT_c BSCCO n = 3 phase was obtained, but after the solid solubility limit samples yield multiphase superconducting properties. Extended heat treatment at 840°C displayed no suppression of the solid solubility limit. The crystallization kinetic studies were investigated using non-isothermal kinetic models of Kissinger and Augis-Bennet. Higher crystallization activation energies, E_a, were obtained compared to unsubstituted BSCCO materials, glassification improved and glass handling time increased. The substitution had a slight effect on J_c, the flux pinning in the material was improved and better J_c values were obtained below the solid solubility limit.

On nanocrystalline materials, which were grown by an appropriate annealing of quasi-amorphous Bi₂Sr₂CaCu₂O_8+δ, the static magnetic susceptibility χ and electrical conductivity σ have been measured between 1.8 and 300 K. Up to grain sizes of d_c = 50 nm, the crystalline phase displays the Bi₂Sr₂CuO_6+δ (Bi2201) structure. Upon increasing size of the Bi2201 nanocrystals, an onset of a variable-range-hopping type σ(T) is observed along with a rapid reduction of χ which comprises contributions from both the Curie term and the temperature-independent background. We propose to associate these novel features with the depletion of the intergranular phase, containing rather large amounts of paramagnetic centres and of antiferromagnetically coupled pairs, at the expense of the formation of metallic 2201-nanocrystals.

We study the collective modes of flux line liquids such as occur in the type-II superconductors of recent interest composed of two-dimensional layered structures. Starting from the vortex-vortex interaction and employing the dielectric formalism within the random-phase approximation, we find propagating sound modes in the long-wavelength limit.

A new method is proposed for decreasing transport ac losses in multifilamentary superconductors by the decoupling of the filaments using a magnetic material in the form of thin layers surrounding the individual filaments. For a superconductor with an elliptical cross section, the magnetic material surrounding the filaments affects the local magnetic field distribution that both reduces the critical current of the filaments and induces the transport ac losses in the magnetic material. Even by taking into account any detrimental influences of the presence of the magnetic material around the filaments, the analysis of the experimental data supported by computer modelling confirmed that for a Bi2223 tape with 100 filaments individually covered by magnetic material, such as iron powder, the transport ac losses should be 65 times lower than for the same multifilamentary conductor without the magnetic coating on the filaments. With an increasing number of filaments, the ac loss decrease would be even larger.

A modified phenomenological model of the microwave surface impedance is proposed. The model is based on the experimental data of measurements of the microwave characteristics of high-temperature superconducting (HTS) films. Numerous experimental results are analysed. The model description of the HTS single crystals and the HTS epitaxial films are compared. It is shown that these two objects are of different physical nature and should be described by different models. The phenomenological model of the HTS film surface impedance based on the modified two-fluid model takes into account specific features of the HTS films. The model of the real and imaginary parts of the surface impedance uses the model parameter γ, which is responsible for the superconducting and normal charge carrier density. This parameter is used for a description of the temperature dependence of the London penetration depth λ_L(T) including λ_L(0). The relaxation time model also uses the γ parameter in combination with the residual resistance parameter α. One can conclude that the parameter γ is the main characteristic of the model and can be considered as a figure of merit of the HTS film microwave properties.

We describe a new device in which the asymmetry parameter in a SQUID can be varied by a temperature difference between the electron temperature in the normal metal barriers of the superconductor/normal/superconductor junctions that form the SQUID. We demonstrate that such a device would have desirable properties for a range of applications.

YBa₂Cu₃O_7-δ (YBCO) films were grown on CeO₂-buffered r-cut sapphire (CbS) substrates using an off-axis dc magnetron sputtering method. Both as-grown and post-annealed CbS substrates were used for this purpose, with the post-annealed CbS prepared by annealing the as-grown CbS at 1000 °C for 1 h. The YBCO films on post-annealed CbS showed significantly improved surface morphology and structures than those on the as-grown CbS, with a peak-to-valley roughness of 3.2 nm and a full-width at half-maximum of (005) YBCO rocking curve of 0.47° for a 300 nm thick YBCO. The surface resistance of the 300 nm thick YBCO film on post-annealed CbS appeared as small as about 110 µΩ at 60 K and about 230 µΩ at 77 K at about 8.6 GHz.

Ni tapes were textured by taking advantage of their secondary recrystallization. The 18 cm long tapes obtained had textures of single crystalline quality with their [001] axes tilted with respect to the surface and their [010] axes parallel to the rolling direction. YBCO/CeO₂/YSZ/CeO₂ films grown on the Ni tape had the same crystalline orientations. Magnetic field dependent I-V relations were measured on a 5 cm section of the tape. J_c defined by 1 mV criterion was 1.5×10⁵ A cm^-2 at 77 K under zero field and was reduced by ~50% under the applied magnetic field of 5 T.

Using a magneto-optical technique, magnetic field distributions have been measured in melt-textured YBa₂Cu₃O_7-x, joined to form an artificial grain boundary, in an external magnetic field perpendicular to the sample surface. An inversion scheme using the inverse-matrix method has been applied to obtain the current distribution from the magnetic field distribution. Both magnetic field and current distributions around the grain boundary show different behaviour between the field increasing up to 150 mT and decreasing down to 0 T after zero-field cooling. The magnetic field at a weakly coupled section of the grain boundary in the increasing field is higher than that in the decreasing field, even in the same external field. This study supports a model in which such differences in the magnetic field at the weak-link grain boundaries give rise to the hysteresis behaviour in the field dependence of transport critical current density in polycrystalline samples.

The resistivity, ρ, and the critical current density at zero applied magnetic field, J_c, of ceramic Y₁Ba₂Cu₃O_7-δ samples with different average grain size, a (from 5 to 14 µm), are reported. These results show that at a given temperature J_c depends only on the specific contact resistance of the average grain boundary junction, ρ_□, so that the product J_cρ_□ is sample independent. The relevance of ρ_□ and not of a on J_c has been further tested on a subset of samples having very different average grain size but similar ρ_□ for which negligible variations of J_c are observed.

The phase content and grain alignment of individual filaments in Bi2223 multifilamentary tapes have been successfully analysed by means of a micro-x-ray diffraction system. According to the 2223 phase formation rate and texture, filaments can be divided into three groups. The filaments in the centre part of the tape have higher 2223 phase content and better texture due to their thinner, flatter shape. The filaments near the tape edges are poorer because of their low density and irregular shape. The strong variation in filament shape and density could be two of the most important factors that affect the homogeneity of 2223 phase conversion and texture quality. Using a more reactive powder, a harder sheath and making heat treatment under 7% oxygen, as well as using rectangular wire to start the rolling process, can all increase the filament uniformity and enhance the tape's J_c value.

The effects of laser irradiation on high-temperature superconducting YBa₂Cu₃O_7-x thin films have been investigated by micro-Raman spectroscopy. Laser heating affects YBa₂Cu₃O_7-x thin films to varying degrees at different laser power levels. For a laser power density below 1 mW µm^-2, laser irradiation causes only thermal expansion within the focused area. When the laser power density is increased to about 1.7 mW µm^-2, oxygen out-diffusion occurs. The orthorhombic to tetragonal structural phase transition was observed at 3.1 mW µm^-2. Above 4.3 mW µm^-2, chemical decomposition is inferred from the appearance of Raman bands due to the impurity phase BaCuO₂. Comparison between the laser-heated results and the furnace-heated results was made, and the role played by temperature gradient and stress in the laser-heated sample is discussed in detail.

Two kinds of Y₂O₃ buffer layers have been directly deposited on cube-textured Ni substrates by electron beam evaporation under different gas pressures. One is deposited at 10^-5 Torr air (a single-layer buffer). In order to reveal the improvement of the microstructure, the other is formed at first under 5×10^-4 Torr and then 10^-6 Torr of N₂ (a double-layer buffer). As a result, the double-buffer layer obtained is more than 97% {100} orientation and is a relatively denser structure. YBa₂Cu₃O_7-x overlayers are deposited by pulsed-laser deposition on these buffer layers. Large YBa₂Cu₃O_7-x pop-off areas are seen for the single-buffer samples. These areas are eliminated on the double-layer buffer. However, the critical current does not increase appreciably for the YBa₂Cu₃O_7-x film on the double-layer buffer, even though the microstructure of the buffer is improved. Other defects probably yielded percolative current flow and a low critical current density in the YBa₂Cu₃O_7-x layers.

The magnetic properties of a series of superconductors (Eu_1-xY_x)Ba₂Cu₃O_7-y (x = 0, 0.2, 0.3, 0.5, 0.7 and 0.8) have been investigated by magnetic measurements from 20 to 90 K. Magnetization hysteresis loops of the Y-doped sample exhibit a strongly developed second peak in the higher temperature range. We discuss the experimental results in the framework of a stress-field pinning model. The stress field induced by the lattice mismatch is also an effective pinning centre, in addition to pinning by 211 phase precipitates and oxygen vacancies. We further suggest that in the (Eu_1-xY_x)Ba₂Cu₃O_7-y samples the oxygen deficient regions induced by the stress field in Cu-O chains also act as pinning centres, thus causing the second peak in the J_c-B curves of (Eu_1-xY_x)Ba₂Cu₃O_7-y.

A powder-in-tube (PIT) process employing groove and flat rolling has been successfully used for the fabrication of Bi₂Sr₂CaCu₂O_x (Bi-2212)/Ag tapes. The tapes were melt processed at 890 °C, which was followed by controlled cooling to 855 °C in order to complete the phase formation. The effect of processing atmosphere (air versus oxygen) on the current transport properties, and microstructure were studied. The average J_c at 4.2 K, self-field with 1 µV cm^-1 criteria, was higher for tapes processed in oxygen (5×10⁴ A cm^-2) compared to tapes processed in air (6.25×10³ A cm^-2). Microstructural analysis on the heat treated tapes revealed that the oxygen processed tapes had a higher Bi-2212 content, better grain alignment and smaller secondary phases compared to the tapes processed in air. Bubble defects, which severely deteriorate I_c of the tapes, were more pronounced for tapes processed in oxygen. TEM studies on the Bi-2212-Ag interface illustrate the oxygen heat treated tapes to have sharp and straight interface compared to air processed tapes which exhibit a tendency to facet in the direction of [0 0 1]/[2 0 6].

The influence of tellurium (Te) addition to the Bi-Pb-Sr-Ca-Cu-O (B(Pb)SCCO) system on its microstructure and phase formation is investigated. Electron probe microanalysis and scanning electron microscopy analyses show Te diffusion from the superconducting core into the silver sheath of the B(Pb)SCCO tapes. It is established that the initial Te presence in the core does not lead to any essential changes with respect to the superconducting transition temperature (T_c) and critical current density (J_c). However, Te addition appears to be a supporting factor for 2223 phase formation and crystal growth. Final Te incorporation in the silver sheath has an additional strengthening effect on the sheath that makes it more stable to external mechanical damages and gives the opportunity for better protection of the superconducting core.

Step-edge Josephson junctions in the c-axis oriented YBa₂Cu₃O₇ films were fabricated on the CeO₂-buffered sapphire substrates. The step angles were controlled in the wide range of 20-75° by an Ar ion milling technique. The I-V curves of junctions fabricated on the 35° step with the film thickness to the step height ratio of about 0.8 exhibited a resistively shunted junction-like behaviour with an I_CR_N product of about 250 µV and a critical current density of about 2×10⁴ A cm^-2 at 77 K. The critical currents of the step-edge junctions increased approximately linearly with decreasing temperature but the normal resistance remained almost constant. All the samples of the step-edge Josephson junctions satisfied a scaling behaviour of I_CR_N∝(J_C)^0.5, and a dc SQUID made out of those junctions exhibited a typical voltage modulation in an applied magnetic field.

The behaviour of the electric field in the magnetization process for a superconducting slab of infinite length is studied by numerical simulation. On the basis of the results of our numerical calculations, a new treatment is suggested, the results of which are compared with those from the straight-line approximation. Both sets of results are further compared with the results of numerical calculations.

The evolution of the rate of flux creep along the major hysteresis envelopes and along bridges between the envelopes in a commercial Bi:2223/Ag tape at 77 K has been investigated. Magnetic relaxation rates depend strongly on the prior magnetic history. It was found that the Bi:2223/Ag tape sample has large flux creep along the hysteresis envelope. Flux creep activation energy was calculated to be 0.12 eV from the rate of decay for the remanent magnetization of 6 mT.

Measurements of ac loss on superconducting tapes are important, but non-trivial. It is well known that in the potential tap method the ac voltage contains a large reactive component, and that its magnitude depends upon the current distribution and the position of the voltage contacts; these are particularly uncertain in multi-filamentary conductors. Bolometric measurements of ac loss are intrinsically more reliable, but previous approaches have required long lengths (~1 m) of conductor, which introduces other complications. We describe a novel bolometric technique that measures ac losses of HTS tape samples only a few centimetres in length. It has a sensitivity approaching 10^-4 W m^-1, and can be used over a wide frequency range; a single measurement can be taken in 1 min or less. It may be adapted for measurements of short-sample losses in the presence also of an ac magnetic field.

Melt textured single domains of Mg-doped YBa₂Cu₃O₇ superconductors with homogeneous cation distribution and narrow superconducting transitions have been prepared by top seeding growth. The strong reduction in the transition temperature, T_c, found in Mg substituted YBa₂(Cu_1-xMg_x)₃O₇ samples, suggests that Mg ions occupy the CuO₂ planes. For low doping concentrations, the decrease in T_c is estimated as dT_c/dx≈-12 K/at% Mg. Instead, for high doping values, a saturation of Mg substitution occurs within the matrix, leading to MgO segregation. The maximum amount of Mg atomic substitution has been estimated as x≈4 at%, shifting T_c down to T_c≈45 K. Studies of anisotropy of the superconducting state from magnetoresistance measurements reveal that, concomitant to the decrease in T_c, the mass anisotropy decreases with Mg doping. These results are interpreted in the framework of non-magnetic impurity scattering effects in d-wave superconductors, where the main distinguishing feature has been the development of new quasi-particle states within the superconducting gap.

We experimentally studied the effects of a single artificial defect and a linear array of artificial defects on I-V curves, critical currents and transport current ac losses of 55 filament untwisted Bi-2223/Ag tapes. The artificial defect was a small hole drilled into the tape. The reduction in the critical current measured on a 1 cm long section due to one hole of diameter 0.9 mm was 33% and that due to a linear array of seven similar holes was 62%. The slopes of the I-V curves, n, measured in this section were 33, 16 and 5.8 in the original sample, in the sample with one defect and the sample with seven defects, respectively. Both I_c and the slope reduction were smaller if the distance between the potential taps was increased. The transport current ac losses at 50 Hz and I_rms =  10 A in the sample with one defect measured in a 1 cm long section were practically the same as those in the original sample (4.1×10^-4 W m^-1), but they increased by 83% in the sample with a linear array of seven defects. The measured increase in losses per unit length was the smaller, the larger the distance between the potential taps. A comparison between the measured and calculated losses revealed that a formal application of the Norris equations for loss calculations in samples with local defects leads to an overestimation of the ac losses. A procedure for the calculation of transport current losses in samples with local defects based on the Norris model is proposed and verified.

Three series of (Hg,Tl)-22(n-1)n (n = 2,3) samples were synthesized by a high-pressure technique. One series of samples was prepared by substituting Y for the Ca site in (Hg_0.7,Tl_0.3)₂Ba₂(Y_x,Ca_1-x)Cu₂O_8-2δ (x = 0.4, 0.6, 0.8 and 0.9), while two other series were prepared by annealing (Hg_0.7,Tl_0.3)₂Ba₂(Y_0.8,Ca_0.2)Cu₂O_8-2δ and (Hg_0.6,Tl_0.4)₂Ba₂Ca₂Cu₃O_10-2δ samples in Ar. The crystal structures of the samples were determined by x-ray Rietveld refinement. The variation tendencies of their lattice parameters as well as several particular inter-planar distances with the change of the hole doping level in these three series were investigated and compared with one another. Based on the comparison, a qualitative explanation for the structural evolution tendency under hole doping for these series was proposed.

Despite numerous attempts to optimize the deposition conditions, we have found that there is always a substantial resistive tail below the bulk T_c of EuBa₂Cu₃O_7-δ and SmBa₂Cu₃O_7-δ films deposited on nearly perfect lattice matching SrTiO₃ (STO) substrates. This is in contrast to the same films deposited on YSZ, LaAlO₃ (LAO) and NdGaO₃ (NGO) substrates under similar conditions. The films on YSZ, LAO and NGO substrates have a sharp resistive transition without any tails, although the lattice match is poorer. We have developed an interpretation of this adverse effect and a simple bi-layer process to get rid of the resistive tails.

Single-feed circularly polarized microstrip patch and patch array antennas for `direct-to-home' receiving system at around 12 GHz are studied by theoretical analysis and experiments. A full-wave microwave circuit simulator (Em), based on the method of moments and capable of handling the microwave surface impedance, has been used in the theoretical analysis of the antennas. Antennas have been fabricated from both normal conductor (gold) and YBa₂Cu₃O_7-x (YBCO) superconductor for comparison. Measured results on resonant frequency (f_r), return loss, gain, bandwidth, and axial ratio are presented. The antennas are found to show a very low axial ratio and a moderate bandwidth. In the comparison of gain, the superconducting antennas showed a remarkable improvement over their gold counterparts. The receiving power of a four-element array fabricated from a single-side YBCO thin film on (100) MgO single crystal substrate is found to be 1.8 dB higher than that of a gold array with identical configuration and both measured at 77 K.

When a multi-filamentary superconductor is exposed to an alternating external magnetic field, coupling-current loss is a significant part of the magnetization loss. The coupling-current loss at low magnetic-field frequencies is determined by the effective transverse resistivity of the inner filamentary region. The effective resistivity is calculated for various shapes and structures of the inner filaments. Bi-2223/Ag tapes typically have filaments with a high aspect ratio, causing an anisotropic transverse resistivity. When there is no contact resistance between the filaments and the matrix material, the presence of the filaments decreases the effective resistivity. The decrease is greatest for overlapping filaments whose wide sides are oriented parallel to the external magnetic field. Good insulating barriers result in a high contact resistance from the filaments to the matrix. Then the effective resistivity is greatly increased, especially for overlapping filaments that are oriented perpendicularly to the magnetic field. The results are relevant for the calculation of the coupling-current loss in Bi-2223 tapes with twisted filaments.

A series of high-temperature superconductors of type Tl_1-xEr_xBa₂Ca₂Cu₃O_9-δ with x ranging from 0 to 0.3 are prepared at 860 °C for 3.5 h. The x-ray diffraction, scanning electron microscopy and electrical resistivity measurements for the prepared samples are reported. The x-ray data indicate that the replacement of the Tl³⁺ ions by the magnetic Er³⁺ ions does not change the structure until x = 0.3 and the solubility limit is found to be around x_s = 0.2. The transition temperature T_c, determined from electrical resistivity data, show a depression in its value by increasing the Er content. The depression in T_c is explained to be, according to the Abrikosov and Gor'kov model, due to the breaking of electron pairs or to the localization effect. The critical content of Er³⁺ ions, the content at which the superconductivity disappears, is estimated to be at x_c = 0.28.