Table of contents

The temperature dependence of the lower critical field H_c1(T) for the superconducting magnesium diboride, MgB₂, is studied in the vicinity of T_c using a two-band Ginzburg–Landau (G–L) theory. The theoretically calculated H_c1(T) near T_c exhibits a negative curvature. The results are shown to be in good agreement with the experimental data. In addition, two-band G–L theory calculations give a temperature dependence of the Ginzburg–Landau parameter κ (T) = λ (T)/ξ (T), which varies little with the temperature in a manner similar to the microscopic single-band BSC theory.

In the framework of the preparation for the realization of the international thermonuclear experimental reactor (ITER), the construction and test of relevant models of seven different parts of the reactor was decided. Two of them are related to the superconducting coils: the toroidal field model coil (TFMC) and the central solenoid model coil (CSMC). For these superconducting coils, due to the expected high values of the current (≥60 kA) and voltage (≥5 kV with respect to the ground) the adopted technology was that of cable in conduit conductor (CICC). Until recently, little experience of this technology existed. Therefore, an extensive research and development programme has been carried out, in the last 10 years, by the ITER partners and particularly in Europe, to design, industrialize and test these large conductors and their joints. The EURATOM associations CEA and ENEA played a leading part in this phase. The CICC concept is described and the results of the developments are presented. About 7 km of conductors were manufactured in the industry and for that more than 10 tonnes of Nb₃Sn strands were produced in Europe. In this large programme, Europe is particularly in charge of the TFMC, which will be tested this summer at Forschung Zentrum Karlsruhe (Germany). In the framework of this programme, three full size conductors and joint samples were tested at the European Sultan test facility (Centre de Recherches de Physique des Plasmas, Villigen, Switzerland), to validate the technological choices and check that the ITER specifications were met. The results of these tests are presented in detail. Starting from the strand critical properties, the conductors made of about 1000 strands did reach their expected performance. The joints of these large conductors are very special and delicate components. Their behaviour was quite successful and the joint resistance of these samples (of the order of 1 nΩ) was well within the specifications.

The effect of substitution of trivalent Nd ions for divalent Ca ions has been studied in the 110 K phase of Bi_1.84Pb_0.34Sr_1.91(Ca_1–xNd_x)_2.03Cu_3.06O_10+y (x = 0–0.1). We found the solubility limit of Nd in the 110 K phase to be around x < 0.015. Within the solubility limit, the c-axis seemed to decrease with increasing x. In the region of the 110 K single phase, the critical temperature T_c gradually decreased with an increase in x, corresponding to a small change of the carrier concentration.

A non-destructive evaluation (NDE) system was set up in our laboratory. A high-T_c dc superconducting quantum interference device (dc SQUID) made from YBa₂Cu₃O_7−δ thin film on a 24° bicrystal SrTiO₃ substrate was adopted as the magnetic flux sensor. Novel excitation coils and a balance circuitry were proposed and employed for evaluation of multilayer aluminium structure with artificial slits in an unshielded noisy environment. Our experimental data demonstrate that the NDE system with proper location of two thin excitation coils could make full use of the dynamic range of the SQUID and enhance the spatial resolution of detection.

We have investigated the Sr doping effect on the electronic transport behaviour in PrBa_2−xSr_xCu₃O_y and PrBa_2−xSr_xCu_2.7Mo_0.3O_y. With the increase of the doping level, the low-temperature resistivity exhibits a rapid drop at x = 1.0 and, accordingly, the localization radius of carriers (holes) increases significantly. However, a further increase in the doping level does not result in the occurrence of a metallic or superconducting state in the Pr system, although the resistivity still decreases slowly with the increasing doping level. Our results show that Sr doping at the Ba site in PrBa₂Cu₃O_y plays a role in releasing the holes, which are localized in the Fehrenbacher–Rice states, switching the system from a highly localized state to a modest localized state. Our results also suggest that the superconductivity cannot be restored in PrBa₂Cu₃O_y by merely using Sr doping at the Ba site.

As silver is known to improve the mechanical properties of the brittle superconducting oxide YBa₂Cu₃O_7−δ (Y123), studies are performed to texture large single domains of silver doped Y123. Here we report on the attempts realized to texture silver+cerium doped Y123. It is shown that, as silver decreases the peritectic decomposition temperature of cerium doped Y123 to approximately 960 °C, it becomes possible to texture samples with a maximal temperature of the thermal cycle of 1000 °C, which is interesting in terms of energy saving. However, it is very difficult, even impossible, to grow domains larger than 1 cm in diameter. Several thermal treatments have indeed been used, such as slow cooling ramps and different temperature dwells, without any success. The increase of the melting temperature and the use of a melt textured NdBa₂Cu₃O_y seed was proven to be efficient in solving this size limitation problem and a 30 mm diameter single domain was textured.

We have structurally studied the elaborated single crystal of YBa₂Cu₃O_7−δ using high-resolution x-ray diffraction, including φ-scan, rocking curve, reflectivity and small angle scattering. The results show that the single crystal is natively inhomogeneous. The rocking curve of 00l reflection indicates that the c-axis lattice constant of the crystal varies in a narrow range, clearly showing a multilayer characteristic. The reflectivity measurement supports this result; it also shows an obvious multilayer characteristic. The small angle scattering indicates that, although the c-axis lattice constant is little different in the crystal, it shows a long period of about 700 Å. The local work function measurement using a scanning tunnelling microscope demonstrates that the crystal is inhomogeneous, supporting the results of x-ray analyses. These facts tell us that so far we have not completely understood the structure of high-T_c superconductors.

Copper oxy-chloride superconductors La_2−xSr_xCaCu₂O₄Cl₂ and La_1.8−xSr_0.2Ca_1+xCu₂O₄Cl₂ have been prepared at ambient pressure using the traditional solid state reaction method. The structure of these superconductors belongs to that of 212 cuprate superconductors, with lattice parameters a ∼ 3.83 Å and c ∼ 19 Å, and the space group I4/mmm. The highest T_c obtained was 45 K when x = 0.35–0.40 in the La_2−xSr_xCaCu₂O₄Cl₂ system. The carrier concentration in these materials is controlled not only by cation substitution but also by anion substitution.

The physical peculiarities of temperature dependence of heat release in a composite superconductor induced by varying transport current are analysed. The proposed investigation was based on a model describing the simultaneous thermal and electromagnetic processes inside the composite superconductor with nonlinear current–voltage characteristic correctly. Numerical solutions of the set of Fourier and Maxwell's equations were used. The analytical expression of the heat release in the superconducting composite during the flux-creep regime is found for its stable superconducting state. The results are compared with the well-known formulae describing a current-sharing approximation of heat generation in the superconducting composite. It is shown that for a high rate of current charging and with a low heat transfer coefficient, the Joule heat generation in a composite increases both in the stage of the steady superconducting state and during the development of instabilities. In particular, the value of generated heat release may differ by several orders of magnitude from the amount of heat induced by slower rates of a current charge. In these cases a notable increase of the superconductor temperature is observed during the entire process of current penetration. Therefore, for a high rate of current charge and with low heat transfer coefficients, the value of the heat released in a superconducting composite is notably larger, through the entire charging process, than would be calculated from the existing thermal stability theory.

In order to establish the test method of mechanical properties of oxide composite superconductors, an international round-robin test (RRT) has been carried out under the programme of VAMAS activity, for which eight research groups participated. The present RRT made the following guideline clear for assessing the mechanical property of Ag/Bi2223 multifilamentary tapes. Owing to the constitution of the brittle superconductive oxide layer embedded in the ductile metal matrix, the stress–percentage extension (R–A) curve shows a three-stage behaviour, that is, the true elastic region is very narrow and is followed by the quasi-elastic region before reaching the macroscopic plastic region. For assessing the elastic constant, it is recommended to look for the maximum slope carefully by enlarging the initial part of the R–A curve. The following three quantities, elastic constant, yield strength and tensile strength, can be reasonably determined with good accuracy by the procedure reported in this paper. The percentage extension after fracture is, however, excluded from the standard procedure, because it scatters to a great extent owing to the nature of the test sample as well as the experimental limitation.

The magnetic relaxation of a superconducting slab in the flux creep regime has been investigated numerically in the framework of an approximate power-law dependence of the electric field E on the current density J (i.e. Eα\J\^σJ). It has been shown numerically that the magnetization relaxation could be described in time as (t + t₀)^−1/σ for a time in the range of t₀. This description was found to be valid for a short time, compared to t₀, after the start of relaxation. t₀ is a constant, which has also been analysed. Our result is useful in applications when transitory regimes are studied, and it has been extended to the case when the slab is in the flux flow regime.

The ac transport losses at 77 K of a 3 m long high-temperature superconducting (HTS) cable conductor were measured electrically using the conventional voltage signals recorded by two pairs of voltage taps (four probe measurements). Further, the signals from two contactless pick-up loops were employed. The contactless loops were placed on the same path as the wiring of the conventional voltage taps outside the cable, whereas the return path went through the centre of the former conductor, thereby encircling all of the superconducting layers. By this arrangement the magnetic part of the ac transport losses of the HTS cable could be singled out. The measurements confirmed that at currents, I, far below the critical current, I_c, of the cable (I ≪ I_c) the losses are dominated by magnetic losses and above I_c the magnetic losses level off and resistive losses become dominating.

A comparative study of the processing of powders to obtain the high-T_c YBa₂Cu₃O_7−x superconductor has been made. The effects of the starting barium compound and the preparation procedure on compositional homogeneity have been investigated. Our results show that the co-precipitation method using barium nitrate is a simple and convenient sintering method.

Ba₂LaHfO_5.5 (BLHO) has been developed as a new ceramic substrate suitable for both YBa₂Cu₃O_7–δ (YBCO) and (Bi,Pb)₂Sr₂Ca₂Cu₃O_x (Bi-2223) superconductors. BLHO has a complex cubic perovskite (A₂BB'O₆) structure with lattice constant a = 8.395 Å. The DTA studies revealed that there is no phase transition occurring in BLHO material in the temperature range of 30–1200 °C. The dielectric constant and loss factor values of BLHO are in a range suitable for its use as a substrate material for microwave applications. YBCO and Bi-2223 superconductors do not show any detectable chemical reaction with BLHO even under extreme processing conditions. The thick films of YBCO dip-coated on polycrystalline BLHO substrate gave T_c(0) of 92 K and J_c of ∼1.1 × 10⁴ A cm⁻². The Bi-2223 thick films dip-coated on polycrystalline BLHO substrate gave T_c(0) of 110 K and J_c of ∼3 × 10³ A cm⁻². The YBCO and Bi-2223 thick films had excellent adhesion on polycrystalline BLHO substrate.

The formation of CuO nanocrystallite in a precursor has little influence on the critical current density (J_c) of the YBa₂Cu₃O_7−x (YBCO) film by the metalorganic deposition using trifluoroacetate. From the result of transmission electron microscopy, the microstructure of the YBCO film allows the coexistence of isolated CuO grains. Increasing humidity in the firing process is effective to decrease the grain growth of CuO nanocrystallite and attain high J_c of the films. We propose that a quasi-liquid is formed in a Cu-rich Y–Ba–O–F amorphous matrix and CuO/amorphous interface in the firing process with humidity. The quasi-liquid may help to diffuse H₂O and HF in the film during the firing process, therefore, the removal of HF gas from the film surface is only a growth limiting process. Such a growth scheme leads to high J_c with high reproducibility.

Measurement of ac susceptibility (ACS) was performed for an ultra-pure YBa₂Cu₃O_6.993 crystal by cycling temperature (T) warming-up and cooling-down at several dc and ac fields in relative high frequency of 100 kHz. Strong history dependence was observed in these experimental ac susceptibility curves. To explain the experimental result, two distinct weakly pinned states of flux lattice (i.e. one ordered and the other disordered) are proposed on the basis of the phenomenological model. On the basis of this model we can qualitatively explain the experimental results.

In this work, superconducting YBa₂Cu₃O₇ (YBCO) thin films were grown on different substrates under the same conditions and their behaviours were investigated. At a higher substrate temperature, because of the occurrence of a chemical reaction between the substrate and film, the film quality was impaired, especially in case of the Al₂O₃ (sapphire) substrate. Films were also deposited onto other substrates, SrTiO₃, MgO and ZrO₂, and the effects of these substrates on the film quality were compared by x-ray diffraction.

We have developed a transformer technique to supply currents over 500 A into a tape from a high-temperature superconductor (HTS). The superconducting tape under test could form either the whole or a part of the secondary circuit winding. The ferromagnetic core, as well as the copper primary, is immersed in an LN₂ bath. This results in a simple and compact design of the whole system. Our approach to the loss determination is based on the fact that the voltage on the entire secondary circuit can be easily registered with the help of a loop embracing the transformer core. In practice, it will also contain the signal from a resistive part of the secondary circuit (joint). We show how this voltage can be subtracted to obtain the pure superconductor loss signal. In our method, the problem of correctly placing the voltage signal wires in the vicinity of the superconducting object, carrying transport current, is avoided. This could be of substantial advantage in the case when the loss of a HTS object that is more complicated than the single tape (e.g. superconducting cable or coil) needs to be measured.

A series of high temperature superconductor samples of type Tl_1−xYb_xBa₂Ca₂Cu₃O_9−δ with x ranging from 0 to 0.5 have been synthesized at 860 °C for 4 h. The x-ray powder diffraction and the electrical resistivity measurements for the prepared samples have been reported. X-ray data indicate that the solubility limit x_s between Tl³⁺ and Yb³⁺ ions in Tl_1−xYb_xBa₂Ca₂Cu₃O_9−δ phase is about 0.3. The transition temperature T_c, determined from the electrical resistivity data, shows a depression in its value with increasing Yb-content. The critical concentration x_c, at which the superconductivity disappeared, is estimated to be 0.58. The effect of external magnetic field on the electrical resistivity behaviour has been studied. The activation energy, calculated from these data, shows a decrease in its value as the applied magnetic field and Yb-content increase.

Bulk superconducting samples of the type Hg_0.3Tl_0.7Ba₂Ca₃(Cu_1−xR_x)₄O_10+δ have been synthesized at normal pressure. The x-ray powder diffraction (XRD) for the prepared samples is studied and the lattice parameters 'a' and 'c' are calculated. The transition temperature T_c determined from both electrical resistance and ac magnetic susceptibility showed a decrease in its value by increasing both Ni and Ag contents. The electrical resistance data are measured at different applied magnetic fields which show that this field does not affect the first stage of the transition, while the transition width is enlarged by increasing the magnetic field. Critical current density, calculated from the imaginary part of ac magnetic susceptibility, is well fitted according to the formula J_c(T) = J_c(0){1 − T/T_c}^γ.

We discuss and characterize the fabrication process of superconductor–insulator–superconductor (SIS) junctions based on a Nb/Al–AlN_x/Nb tri-layer. Utilization of the AlN_x tunnel barrier, produced by Al nitridation in a nitrogen glow discharge, enables us to produce high-quality SIS junctions with low R_nA values (a product of junction resistance and area). We characterize the tunnel barrier formation and investigate the correlation of plasma characteristics and junction properties. The experiment shows that an increase in nitridation time and applied power results in an increase in junction resistance, while variation in nitrogen pressure has almost no influence on the junction characteristics. Analysing the correlation of junction resistance and plasma properties, it is concluded that the mechanism of tunnel barrier formation is based on nitrogen implantation into the Al layer with subsequent diffusion of nitrogen, stimulated by plasma heating.

A set of standard rapid-single-flux-quantum logic cells has been established in order to build blocks for complex high-speed superconducting digital applications. Special emphasis has been put on the implementation with Nb and NbN technologies as well as with high-T_c superconductors. The influence of the different technological parameters on the projected performance of some basic cells is presented along with a comparative study of their margins of operation. In particular, the performance of a D-flip-flop has been studied as a function of frequency of operation for the different technologies.

Superconducting (Tl_0.5Pb_0.5)(Sr_0.9Ba_0.1)₂Ca₂Cu₃O_y films were fabricated on lanthanum aluminate substrates by screen printing of the precursor material followed by ex situ thallination. The thickness of the films varied between 1.3 and 2.6 μm. X-ray diffraction and optical microscopy showed that the films were highly textured. Samples with critical temperatures T_c(0) between 116 and 120 K and with transition widths ΔT_(90−10%) of 1 K have been fabricated. Critical current densities between 0.6 and 0.9 MA cm⁻² were measured.

Thin films of Tl(Ba, Sr)₂Ca₂Cu₃O_y superconductor have been grown on CeO₂ buffered r-cut sapphire substrate with the use of a template TlSr₂CaCu₂O_y by the amorphous phase epitaxy method. By using this template containing Sr, we have overcome the problem of chemical reactions between Ba and Ce present in the superconducting and buffer layers, respectively, at the normal processing temperatures of 840–890 °C. Films grown by this method have an excellent c-axis orientation as well as in-plane alignment with the TlSr₂CaCu₂O_y template, which in turn is aligned with the CeO₂ layers. These films have superconducting critical temperature values up to 104 K and critical current density values as high as 1 MA cm⁻² at 77.3 K and 0 T.

Temperature and time dependence of the phase formation in the (Cu_0.5,Tl_0.5)-1234 composition have been studied. The liquid–vapour deposition mechanism for the formation of 12(n − 1)n superconducting phases is presented. XRD, SEM, EDS and weight loss data are discussed. Samples were superconducting with maximum T_c and J_c (at 60 K and 0.5 T), determined from M(H) loops of 118 K and 4 × 10⁵ A cm⁻², respectively.

Superconducting samples with a starting composition Cu_0.5Tl_xBa₂Ca₃Cu₄O₁₂ (x = 1–0.35) were prepared in Au tubes from oxides by using different heating rates. The optimum heating rate was 0.11 °C min⁻¹ applied between 860 and 880 °C. These samples show the highest content of 1234 phase and the lowest of Ba–Cu–O phases as well as the highest critical temperatures. The highest values were T_c = 119.2 K and T_c0 = 115.1 K. All samples show J_c values around 6 × 10⁵ A cm⁻², at 60 K and 0.5 T. In the rest of the samples, heated by using lower or higher heating rates, a high concentration of 1223 and/or 1245 phases is detected. The content of the 1234 phase and of Ba(Ca,Tl)–Cu–O residual liquid phase, in the samples synthesized for the optimum heating rate, depends on the Tl content in the starting mixture. The influence of the oxygen content in the starting mixture on phase composition is not as strong as of Tl, but it has a major role in establishing the superconducting behaviour of the 1234 grains. Some arguments that suggest the dependence of the transport properties of the non-superconducting matrix on oxygen content will be discussed. In our technological arrangement the determined optimum heating rate can be applied to the synthesis of other single-layered superconducting phases, e.g. 1212 phase from a Cu_0.25Tl_0.75Ba₂CaCu₂O_8.115 starting composition.

We have realized four-pole, quarter wave, parallel coupled, microstrip band pass filters at 4035 ± 40 MHz, using high-temperature superconducting thin films of YBCO on both sides of 0.5 mm thick LaAlO₃ substrates. The achieved results are close to the predicted results except for the return loss. Detailed results are presented in the paper.

Fully c-axis-oriented YBCO films were directly deposited on polycrystalline silver substrates by inclined substrate pulsed laser ablation. The orientation and microstructure of the YBCO films were characterized by x-ray diffraction 2θ-scans, Ω-scans and pole figure analysis. Surface morphology was examined by scanning electron microscopy. Irregular-mosaic-shaped supergrains were observed in the films. Raman spectroscopy was used to evaluate the quality of the YBCO films. The superconducting transition temperature (T_c) and the critical current density (J_c) of the films were determined by inductive and transport measurements, respectively. T_c = 91 K with sharp transition and J_c = 2.7 × 10⁵ A cm⁻² at 77 K in zero field were obtained on a film that was 0.14 μm thick, 5 mm wide and 10 mm long. This work demonstrated a promising approach to obtain high-J_c YBCO films on nontextured polycrystalline silver substrate.