Table of contents

We present the fabrication and test results of hot-isostatic-pressed (HIPed) powder-in-tube (PIT) MgB₂ coils. The coils' properties were measured by transport and magnetization at different applied fields (H) and temperatures (T). The engineering critical current (J_e) value is the largest reported in PIT MgB₂ wires or tapes. At 25 K our champion six-layer coil was able to generate a field of 1 T at zero external field (I_c>220 A, ). At 4 K this coil generated 1.6 T under an applied field of 1.25 T ( A, ). These magnetic fields are high enough for a superconducting transformer or magnet applications such as MRI. An SiC doped MgB₂ single layer coil shows a promising improvement at high fields and exhibits J_c> 10⁴ A cm⁻² at 7 T.

Anomalous voltage peaks have been observed below the critical current in dc transport measurements of superconducting YBCO coated conductors for specific contact arrangements. Using a simple concentrated constant-equivalent circuit, it is predicted that current redistributions due to non-homogeneities in the sample may explain the observed effect.

An experimental study of the effect of MgO content in the MgB₂ powder used for ex situ made composite wires was carried out. Two single-core MgB₂/Fe/Cu wires were made using commercial MgB₂ powders from Alfa Aesar containing different fraction of MgO. Critical temperature and critical currents of as-deformed and heat-treated wires were measured. The differences between the wires are discussed and correlated with the MgO content. It was found that by increasing the amount of MgO, the inter-grain connectivity worsens, but well distributed and low size MgO particles improve flux pinning.

In this communication, for the first time, we report on a high-T_c 95 K Sm_1+xBa_2−xCu₃O_z (SmBCO) single crystal with a small transition width of less than 0.5 K grown under 1 atm oxygen pressure. We found that the substitution of Ba by Sm could be effectively suppressed by using liquid composition control. J_c–B curves and scaling analysis of samples at 77 K with indicate that a delta T_c style defect is the predominant pinning in SmBCO crystals.

A brief overview of the current state of development of 1201-type, Ba-free, mercury cuprates, (Hg,M)Sr₂CuO_4+δ (M = Cr, Mo or Re), is presented. Our focus here is confined to synthesis methods, chemical stabilization, structural parameters and superconducting properties of this Hg/Sr 1201 system.

Two sets of MgB₂ samples doped with up to 5 at.% of Al were prepared in different laboratories using different procedures. Decreases in the 'a' and 'c' lattice parameters were observed with Al doping, confirming Al substitution onto the Mg site. The critical temperature (T_c) remained largely unchanged with Al doping. For 1–2.5 at.% doping, at 20 K the in-field critical current densities (J_c s) were enhanced, particularly at lower fields. At 5 K, the in-field J_c was markedly improved; for example at 5 T J_c was enhanced by a factor of 20 for a doping level of 1 at.% Al. The improved J_cs correlate with increased sample resistivity, which is indicative of an increase in the upper critical field, H_{c 2}, through alloying.

In this paper we report a Fe_0.5Cu_0.5Ba₂Y Cu₂O_7.35 superconductor with a tetragonal structure and T_c = 80 K. The magnetism of the Fe_0.5Cu_0.5Ba₂Y Cu₂O_7.35 superconductor has been investigated systematically by the measurement of magnetization. Results indicate that superconductivity and ferromagnetism coexist in the Fe_0.5Cu_0.5Ba₂Y Cu₂O_7.35 superconductor by the formation of a SVP (spontaneous vortex phase). Moreover, a scenario is proposed to understand some unique behaviours in the curves of magnetization versus temperature under different external magnetic fields.

An integral formulation based on the stream function of sheet currents is applied to finite length superconductors to model the coupling through a normal matrix. This formulation is an extension of Brandt's 2D formulation for modelling a 3D problem. Thin discs and infinite slabs were studied and the critical was obtained as a function of applied field. While an excellent agreement for fully penetrated infinite slabs was found with existing theories, original results are presented for partially penetrated slabs as well as for thin discs in a wide range of applied fields.

We measured critical current densities (J_cb) at 24°, 30°, 36.8°, and 45° grain boundaries of Y₁Ba₂Cu₃O₇ and Sm₁Ba₂Cu₃O₇ films, and we compared them with the previous measurements on Nd₁Ba₂Cu₃O₇ films. We observed salient hysteretic behaviours in the field-dependent J_cb at various temperatures. The main features of these measurements were consistent with the results of our calculations, where J_cb depends on the vortex density at the grain boundary as expressed by the modified Kim model. The vortex densities were estimated from field distributions around grain boundaries, while the field distributions were obtained from the Brandt formula. By these data analyses, we estimated the four characteristic parameters of grain boundaries and compared them for the three different types of material.

The complex ac susceptibility was measured as a function of temperature and ac field amplitude on rectangular bar shaped high-temperature superconductors with nominal composition of Bi_1.6Pb_0.4Sr₂(Ca_1−xNi_x)₂Cu₃O_δ (x = 0–0.10) superconducting samples prepared by the solid state reaction method. It is found that as the amount of Ni doping increases, the critical temperature T_c slowly decreases. A small amount of Ni doping (x = 0.03) increases the intergranular critical current j_cm and the flux pinning of the Bi(Pb)-2223 system. The effective volume fraction of the grains and the field dependence of the intergranular critical current density were estimated by comparison of the maximum of the extracted matrix susceptibility and the corresponding calculated data which was obtained from hysteresis losses employing the power law critical state model.

Measurements of the critical current density were conducted on MgB₂ films with the thickness in the range of 0.2–1.1 µm. The films have been prepared by a combined method of chemical vapour deposition and magnesium diffusion. The thinnest film has critical current density over 26 MA cm⁻² at 15 K in zero field. As film thickness is increased, J_c decreases to 4.9 MA cm⁻². Scanning electric microscopy analyses indicate a progressive surface deterioration with the increase of film thickness that leads to the drop in J_c.

We investigated the effect of ZrO₂ addition on superconducting and magnetic properties of oxygen-controlled-melt-growth- (OCMG-) processed (Nd_0.33Eu_0.33Gd_0.33)Ba₂Cu₃O_y (NEG-123) bulk material. The purpose of this study was to ascertain whether the ZrO₂ particles are able to improve flux pinning even without entering into reaction with NEG-123 and Gd₂BaCuO₅ (Gd-211) phases. We observed the expected effect but only at a relatively low concentration of ZrO₂. The best flux pinning was observed in the sample with 0.29 wt% ZrO₂. No trace of Zr was found in the NEG-123 matrix. Our study demonstrates the potential of ZrO₂ particles in enhancing flux pinning in melt-processed REBa₂Cu₃O_y (RE-123).

Bi₂Sr₂CaCu₂O_x (Bi-2212) thick films were prepared on (100) MgO substrates by combining both the tape cast and laser zone melting techniques. The final thickness of the textured samples is of the order of 350–400 µm. The effect of translation rates on the microstructure and, consequently, on the critical current densities (J_c) of the films has been analysed. J_c values measured in zero magnetic field were 1200 A cm⁻² (I_c = 42 A) at 77 K for samples grown at 15 mm h⁻¹, using a total laser power of 12 W and a power density of 2 W mm⁻². These are similar to the best results obtained in Bi-2212 thick films processed with other methods.

High-T_c superconducting YBa₂Cu₃O₇ films with a CeO₂ buffer layer, deposited onto sapphire substrates, were prepared by off-axis rf-magnetron sputtering. It is found that the microstructure of the buffer layer can be controlled by the substrate temperature, and is connected with the formation of the discontinuities in the film, which are the origin of micropores. The microstructural peculiarities of the YBa₂Cu₃O₇/CeO₂/Al₂O₃ films are studied and discussed.

Liquid phase epitaxy (LPE) has a great potential in producing low-cost, high-current, coated conductors due to its fast growth rate in excess of 1 µm min⁻¹ and the capability of growing thick films up to 10 µm without degrading the structural perfection or J_c. The main problem for LPE is the chemical reaction between the films and substrates at elevated growth temperatures. Former efforts have been focused on the reduction of growth temperatures. This has been proved to be unsuccessful due to the limited degree of temperature reduction; reducing the growth temperature also reduces the solubility of YBCO in the liquid, making the growth even more difficult. An alternative solution to this problem is to search for a special buffer which is particularly resistant to the attack of the high-temperature liquid. We have recently developed a new buffer, Nd₂CuO₄, which was very stable in the cuprate solution at temperatures around 950 °C and therefore extremely useful for LPE. Initial growth of YBCO on Nd₂CuO₄ buffered, surface oxidized Ni substrates showed a sharp superconducting transition at 90 K and transport J_c over 10⁵ A cm⁻² (77 K). Although YBCO could be grown on Nd₂CuO₄ by LPE without any other intermediate seed layer, growth of Nd₂CuO₄ on both sides of the NiO/Ni substrate with 100% coverage was not easy and had a low success rate. Complete coverage of Nd₂CuO₄ buffer on the NiO/Ni substrates was the critical step to avoid Ni contamination and achieve a high J_c.

In order to improve the intrinsic properties of MgB₂ superconductors, the application of mechanical alloying (MA) of elemental Mg and B powders is a very promising fabrication technique. The enhancement of the upper critical field H_c2 and the irreversibility field H_irr as well as of the critical current density J_c shows the potential of this preparation route. Nevertheless, a better understanding of the MA process would allow further optimization of its parameters for MgB₂ preparation. The coaction of the grain refinement of the starting substances Mg and B with the chemical reaction forming MgB₂ by mechanical fracturing, cold-welding and solid-state-reaction of the powder particles leads to a complex behaviour of the whole system. Additionally, the introduction of oxygen from the working atmosphere and the incorporation of W, C and Co impurities stemming from the milling tools has a strong influence. Hence, two opposed processes are taking place which lead—with the milling time as the only parameter—in the beginning to an improvement of the superconducting properties of MgB₂. This can be attributed to the grain refinement resulting in a higher reactivity and, therefore, an optimal grain connectivity and a high density of grain boundaries in hot pressed nanocrystalline MgB₂ bulks, which is due to clean surfaces and a larger surface area of the particles. In contrast, for milling times longer than 50 h this excellent performance degrades rapidly. The saturation of the grain refinement at a final coherent scattering length, which is regarded as a minimal bound for the grain size of about 10 nm associated with an enrichment of the impurities (mainly oxygen) to a maximum content of about 4.5 at% for the longest milling time, causes a porous microstructure with reduced grain connectivity. These results allow us to achieve an optimum MgB₂ microstructure by applying appropriate mechanical alloying conditions, i.e. a medium processing time of 50 h.

There is interest in probing the feasibility and possible advantages of making uniform and cost-effective long-length coated conductors through a low-pressure processing approach. The low-pressure approach, eliminating the boundary layer and gas flow considerations, consumes much less of the processing gases and offers the possibility of improved uniformity and faster growth rate of superconducting films. Here, we have fabricated Y Ba₂Cu₃O_7−δ (YBCO) epitaxial films of thickness 0.1–1.0 µm on SrTiO₃ single crystal substrates using ex situ post-deposition processing of co-evaporated Y, BaF₂ and Cu precursors in a controlled low-pressure gas mixture of oxygen and water vapour. Partial pressures of oxygen (P_O2) and water vapour (P_H2O) as low as 10 and 0.1 mTorr, respectively, were used. X-ray diffraction and scanning electron microscopy inspection were conducted for structure characterization of the films. High critical current densities (J_c) of at 77 K in self-field were obtained, yielding properties comparable to those of in situ films and ex situ films processed under atmospheric pressure condition.

We have observed back-bending structures at high bias current in the current–voltage curves of intrinsic Josephson junctions. These structures may be caused by nonequilibrium quasiparticle injection and/or Joule heating. The energy gap suppression varies considerably with temperature. Different levels of the suppression are observed when the same level of current passes through top electrodes of different sizes. Another effect which is seen and discussed is a super-current 're-entrance' of a single intrinsic Josephson junction with high bias current.

Epitaxial Tl₂Ba₂CaCu₂O_X thin film dc superconducting quantum interference devices (dc SQUIDs) have been fabricated on bicrystal SrTiO₃ (STO) substrates. By using the Tl-2212 single phase film, a flux noise density, S_Φ^1/2, of 2.0 × 10⁻⁵Φ₀ Hz^−1/2 at liquid nitrogen temperature was obtained in the white noise region, which is more than one order of magnitude lower than previous Tl-based SQUIDs made by multiphase thin films. The Tl-2212 thin film bicrystal grain boundary Josephson junctions have demonstrated resistively shunted junction (RSJ) behaviour. The characteristic voltage, V_C, which is the product of critical current I_C and junction resistance R_N, was 45 µV at 95 K. The dependence of the critical current on temperature near T_C (105 K) was measured as . The dc SQUIDs can operate at temperatures up to 99 K.

RbOs₂O₆, the third superconducting pyrochlore oxide (known so far), has been synthesized by encapsulation and by high pressure techniques. Suitable post chemical treatment of the as-prepared sample allowed us to eliminate the impurity phases. Bulk superconductivity with T_c = 6.4 K was observed in magnetization and specific heat measurements. The transition temperature of RbOs₂O₆ was found to be the same for both preparation methods. Structural investigations showed that Rb atoms occupy the 8b site in the pyrochlore lattice with a lattice parameter of 10.1137(1) Å.

On five Bi-2223/Ag tapes with different aspect ratios from 5 to 26, AC losses have been measured at 77 K while a parallel AC magnetic field or a perpendicular AC magnetic field or a longitudinal AC transport current is applied. It has been found that at any frequency the perpendicular magnetic losses per cycle increase, but the parallel magnetic losses per cycle and the transport losses per cycle decrease as the aspect ratio increases. These experimental results are in accord with theoretical results. Meanwhile, we investigated the geometry dependence of the decay time constant of coupling current and that of full penetration field.

The critical current density (J_c) was measured at 4.2 K for MgB₂ strands with and without SiC additions. In some cases measurements were performed on long (1 m) samples wound on barrels, the transport results being compared to the results of magnetic measurements. Most measurements were performed on short samples in fields of up to 18 T. It was found that in situ processed strands with 10% SiC additions heat treated at 700–800 °C showed improved irreversibility fields (H_r) and bulk pinning strengths (F_p) as compared to control samples; an increase in H_r of 1.5 T was noted. Heat treatment to 900 °C gave even larger improvements, with H_r reaching 18 T and F_p values maximizing at 20 GN m⁻³.

Results on an established batch process preparing melt-textured YBCO of high quality and in large quantities are reported. We used a standard composition Y_1.5Ba₂Cu₃O_7−x+1 wt % CeO₂ without further doping to fabricate single domain YBCO monoliths in different sizes and shapes (cylindrical, quadratic) as well as rectangular multi-seeded YBCO monoliths. Up to 2–3 kg of melt-textured YBCO blocks were grown, reproducible in one box furnace run. Top seeding by a self-made SmBCO was improved and rationalized. Optimization of an oxygen annealing treatment led to macro-crack free YBCO monoliths. Each YBCO monolith was characterized by integral levitation force and field mapping. In a single domain, a quadratic monolith with a edge length of 38 mm, a maximum induction of 1.44 T at 77 K and a distance of 0.5 mm was frozen. The reproducibility of the batch process is guaranteed. Mean maximum induction from 1.1 to 1.2 T at 77 K per batch was reached. A trapped magnetic field of 2.5 T was achieved between two single domain monoliths in a gap of 1.5 mm at 77 K.

Depending on the application, function elements with different sizes, designs and more or less complex geometry are constructed in several working steps by cutting, machining, bonding and passivation. Selected function elements were checked with field mapping at 77 K. The results of our function elements in HTSC reluctance motors with an output power of up to 200 kW using single domain material are shown. We report on a fly wheel system DYNASTORE and a system to levitate people.

In this investigation we have taken the viewpoint of the Landau theory for the normal state and have suggested a covariant equation for current density. This suggestion is based on the exact analysis of electrodynamic relations in the intermediate state of superconductors. Our analysis is restricted to the simplest electrodynamic relations. At first we have employed Maxwell's equations with the general Ohm–London equation and have found the general covariant current which takes a covariant form after Lorentz transformation. Finally we have checked it by considering two inertial frames in a simple state.

Axial and radial high temperature superconducting (HTS) magnetic bearings are evaluated by their parameters. Journal bearings possess advantages over thrust bearings. High magnetic gradients in a multi-pole permanent magnet (PM) configuration, the surrounding melt textured YBCO stator and adequate designs are the key features for increasing the overall bearing stiffness. The gap distance between rotor and stator determines the specific forces and has a strong impact on the PM rotor design. We report on the designing, building and measuring of a 200 mm prototype 100 kg HTS bearing with an encapsulated and thermally insulated melt textured YBCO ring stator. The encapsulation requires a magnetically large-gap (4–5 mm) operation but reduces the cryogenic effort substantially. The bearing requires 3 l of LN₂ for cooling down, and about 0.2 l LN₂ h⁻¹ under operation. This is a dramatic improvement of the efficiency and in the practical usage of HTS magnetic bearings.

(Tl, Pb)(Ba, Sr)-1223 superconducting films with a thickness of about 1 µm were prepared on highly polished, untextured silver as substrate. Excellent c-axis orientation of the superconducting material was obtained. The transition temperature (T_{c (0)}) was 116 K with a very narrow transition range (T_90−10%) of 2 K. Critical current densities of 80 kA cm⁻² at 77 K in self-field were obtained.

To study the phase relations in the Bi-2212 and Y b₂O₃ system, Bi₂Sr₂Ca_1−xY b_xCu₂O_y thick films are prepared by partial melt processing via an intermediate reaction between Bi-2212 and Y b₂O₃. When Bi-2212 and Y b₂O₃ are partially melted and then slowly cooled, solid solutions of Bi₂Sr₂Ca_1−xY b_xCu₂O_y form by reactions between liquid and solid phases which contain Yb. Following these reactions, Ca is partially replaced in Bi-2212 matrix and participates in the formation of secondary phases, such as Bi-free, (Ca,Sr)O_x and CaO. Variation of the Bi-2212–Y b₂O₃ ratios and processing parameters changes the balance between the phases and leads to different Yb:Ca ratios in the Bi-2212 matrix of processed thick films. When the partial melting process is optimized for each sample to minimize the growth of secondary phases, x = 0.42–0.46 for the samples prepared at pO₂ = 0.01 atm, x = 0.24–0.29 for the samples prepared at pO₂ = 0.21 atm, x = 0.18–0.23 for the samples prepared at pO₂ = 0.99 atm are obtained regardless to the starting compositions.

It is found that superconducting properties of Bi₂Sr₂Ca_1−xY b_xCu₂O_y thick films strongly depend on the processing conditions, because the conditions result in different Yb content in the Bi-2212 matrix and the volume fraction of the secondary phases. The highest T_c(0) of 77, 90 and 91 K were obtained for the samples processed at 0.01, 0.21 and 0.99 atm of O₂, respectively.

The growth of 0.9–1.0 µm thick Y Ba₂Cu₃O_7−δ (YBCO) films on biaxially textured Ni–3 at.% W (NiW) substrates using the BaF₂ ex situ process was investigated at reduced pressures. By varying the water vapour pressure (P_H2O), Y–BaF₂–Cu–O (YBFCO) precursor films deposited by e-beam co-evaporation were converted at a reduced total pressure (P_total) of 50–55 Torr and conversion temperature (T_S) of 740 °C for a wet conversion time (t_W) of 60 min. Critical current density (J_C) values greater than 1 MA cm⁻² for the thick YBCO films were obtained under the condition of varying P_H2O from low pressure to 10 Torr. The transition temperatures (T_C) of the samples were over 90 K with ΔT_C = 1.8–2.5 K. Pre-heat treatment of the precursor films on CeO₂/Y SZ/Y₂O₃/Ni/NiW substrates under an O₂ atmosphere condition before the conversion resulted in smooth surfaces without large secondary phase particles embedded in the films.

A number of liquid phase epitaxy (LPE) related growth methods have been investigated. These hybrid-LPE processes enable high rate 'liquid assisted' growth of epitaxial YBa₂Cu₃O₇ films without the many disadvantages of classical LPE. Growth occurs by diffusive transport of Y through a thin liquid flux layer. This layer may be pre-deposited onto the substrate by various means including vacuum and non-vacuum techniques, or deposited at the growth temperature. The composition of the liquid layer is maintained during film growth by feeding YBa₂Cu₃O₇, or the separate components, either from the vapour or by a powder route. Growth rates up to 10 nm s⁻¹ have been demonstrated. Deposition of c-axis oriented epitaxial YBa₂Cu₃O₇ is reported on both seeded and non-seeded substrates; the process is tolerant of a high substrate mismatch. Films 1–2 µm thick with  K and a critical current density J_c> 2 MA cm⁻² have been grown on a range of single crystal substrates as well as on buffered textured metallic tapes. The mechanism of nucleation and growth from a thin liquid layer is described within the general theoretical framework of crystal growth. Particular features of the growth are the short time constant for equilibration of transients in the deposition conditions, the wide range of relative supersaturation spanned by the process, and dominance of interface kinetic effects compared to volume diffusion in the liquid flux.