Table of contents

A series of high-quality La_2−xSr_xCuO₄ (LSCO) superconductor crystals in the under-doping region with x = 0.063, 0.07, 0.09, 0.10, 0.111 and 0.125 have been successfully prepared by the travelling-solvent floating-zone (TSFZ) technique. The crystals are large and free of subgrains and foreign phases. The high crystal quality has been revealed by double-crystal x-ray rocking curves and Rutherford backscattering spectrometry combined with ion-beam channelling effect. We find that the evolutions of the superconducting transition width and volume fraction as a function of carrier concentration exhibit interesting anomalies in the vicinity of some 'magic number' doping levels such as x = 1/4² ( = 0.0625) and 1/3² ( = 0.111). We argue that these behaviours are of intrinsic electronic origin.

Recently, an electromagnetic generation was detected from superconducting films caused by low-frequency oscillating magnetic fields. We propose a possible explanation of this interesting effect based on the nonlinear dependence of the magnetization curve of the superconducting film caused by the vortices motion in a film made from type II superconductors. Our calculations can explain the experimentally observed voltage spectral density and the generation of odd harmonics in it.

In this paper, the annealing effects on the magnetic properties of a multifilamentary Cu–15vol%Nb composite were investigated. During vacuum annealing, noticeable changes take place in the microstructure, mostly the partial spheroidization and further coarsening of the niobium filaments. Results show that spheroidization becomes noticeable at about 700 °C and, even after annealing at severe conditions, e.g. 1050 °C for 32 h, the continuity of the niobium-conducting path is partially preserved. The influence of these microstructural changes on the magnetic properties of the multifilamentary Cu–Nb composite conductor is discussed.

In some electrical apparatuses, superconducting tapes are exposed to the longitudinal magnetic field. In this work, AC losses were measured in twisted and untwisted Bi-2223 tapes carrying AC transport current in the AC longitudinal magnetic field. In twisted tapes, the transport, magnetization and total losses depend on the relative direction of the longitudinal magnetic field to the direction of the transport current, while the field direction does not influence the AC loss characteristics in untwisted tapes. In the Z-twisted tapes, the total AC loss is larger in the longitudinal magnetic field that is anti-parallel to the transport current than in the longitudinal magnetic field of another direction. Numerical analysis shows that this field direction dependence of the total AC loss results from the change in the current distribution. In the longitudinal magnetic field that is anti-parallel to the transport current, the total AC loss in the Z-twisted tape is more than that in the untwisted tape. This dependence on the field direction is reversed in S-twisted tapes. It is to be noted that the twist increases the total AC loss in a longitudinal magnetic field of a certain direction, while it reduces the AC loss in the transverse magnetic field.

Under the sponsorship of the US Department of Energy (USDOE), the University of Tennessee Space Institute (UTSI) carried out an engineering evaluation of various options to manufacture long lengths of high-temperature superconductor (HTS) wire/tape. As a result, the leading candidates to apply buffer and/or YBCO (HTS) materials were identified as based on metal organic decomposition (MOD), sol–gel, metal organics chemical vapour deposition (MOCVD) and pulsed laser deposition (PLD). Earlier, Chapman had developed economics for manufacturing schemes based on MOCVD, e-beam and PLD. Using his approach, preliminary economics for a sol–gel based manufacturing scheme has been estimated. In this paper, major findings from our economic evaluation of the sol–gel based film coating scheme are reported.

We present strong evidence observed in a YBCO superconducting tube that helical flux lines can concurrently enter and leave across the inner surface of the wall of a hollow cylinder of sintered high-T_c superconductors at 77 K, and hence cut and traverse each other. The evidence for flux line cutting and cross-flow is examined in the perspective of basic concepts. The traffic of the flux lines is also described mathematically. The penetration fields across the wall and into the grains of the sintered ceramic tube are seen to correlate with salient features (peaks and valleys) of the flux line cutting and cross-flow phenomena. This enables us to claim that we are witnessing flux line cutting and cross-flow in the weak links as well as in the interior of the grains in the YBCO tube. The Clem/Perez-Gonzalez phenomenological theory is exploited in a simplified framework to describe the crucial features of the data semi-quantitatively. This analysis confirms the above conclusions and provides estimates of j_c||m and j_c||g, the critical current densities for intergranular and intragranular flux line cutting and their dependence on the magnetic flux density.

High-temperature superconducting (HTS) transformers promise decreased weight and volume and higher efficiency. A 1 MVA HTS railway transformer was built and tested at Siemens AG. This paper deals with the prediction of ac loss in the BSCCO/Ag-tape windings. In a railway transformer the tape carries ac current in alternating field, the temperature differs from 77 K, tapes are stacked or cabled and overcurrents and higher harmonics occur. In ac-loss literature these issues are treated separately, if at all. We have developed a model that predicts the ac loss in sets of BSCCO/Ag-tape coils, and deals with the above-mentioned issues. The effect of higher harmonics on the loss in HTS tapes is considered for the first time. The paper gives a complete overview of the model equations and required input parameters. The model is validated over a wide range of the input parameters, using the measured critical current and ac loss of single tapes, single coils and sets of coils in the 1 MVA transformer. An accuracy of around 25% is achieved in all relevant cases. Presently the model is developed further, in order to describe other HTS materials and other types of applications.

The quench current of a superconducting magnet, I_q, is the current at which a thermal runaway occurs. In trained LTS magnets I_q can be estimated from a short sample critical current, I_c, due to a steep electric field (E)–current density (J) characteristic. Anisotropy and the slanted E(J)-characteristic make the situation more complicated in HTS magnets. Furthermore, the I_q of a conduction-cooled magnet depends strongly on the geometry of the thermal interface. Several criteria, such as the average electric field of 0.1 μV cm⁻¹ and the maximum electric field of 1 μV cm⁻¹ have been suggested for I_q of an HTS magnet. However, in order to determine I_q accurately a detailed stability analysis is required. In this paper different I_q criteria for conduction-cooled HTS magnets are computationally compared at the operation temperatures of 4.2, 20 and 77 K. Computations are based on the I_c data measured with a Bi-2223/Ag tape. 150 different solenoidal magnets having the wire length of 2, 5 and 10 km have been studied. The effect of the thermal interface geometry on I_q has also been investigated. Rules of thumb for the quick estimation of I_q at the given operation temperature are suggested.

We studied YBa₂Cu₃O_7−x ceramic samples during external magnetic field increase from 200 to 4000 Oe at 77 K using acoustic emission and magnetization methods. We observed a small magnetic anomaly near 2500 Oe. This observed magnetic anomaly was accompanied by acoustic emission impulses of activity equal to ∼20 s⁻¹. These impulses are thought to be Bean's mixed-state field H_s. Based on Bean's theory, the full penetration field H* has been calculated to be ∼1200 Oe. The possible mechanism of acoustic emission signals due to interaction between flux lines during the formation of a mixed state inside a sample is discussed.

We have investigated the electrical transport properties for two MgB₂ wires containing four MgB₂ filaments in an Fe matrix, made by an ex situ process. The two wires differed in their final treatment. While only the mechanical deformation steps were applied to one of them (wire A), the other (wire B) was annealed in an Ar atmosphere to perform sintering of the powder grains. Critical currents were measured by the transport method, at temperatures ranging from 4.2 K to 30 K, and in magnetic fields up to 8 T. Due to the sintering step, critical currents and pinning energies of the wire B strongly increased with respect to the wire A. The analysis of the R(T) curves and current–voltage characteristics revealed that the irreversibility line was affected by the sintering process. An explanation of the observed behaviours is given in terms of the different activation volumes for pinning energies.

Several workers have observed that, after fast field cooling of weak-linked granular high T_c type II superconductors, the diamagnetic magnetization will increase and trace a valley, hence a peak of enhanced Meissner effect, as the temperature approaches T_c during slow warming to the normal state with the applied field H_a kept fixed throughout the temperature excursion. We present a simple extension to interconnected granular systems of the established model for isotropic single crystals which qualitatively and quantitatively reproduces our observations of this phenomenon over a large range of H_a, the extensive measurements of Jung et al on YBCO samples and the results of Hyun on compacted Nb₃Sn polycrystals.

Dependence of the microwave surface resistance (R_S) of YBa₂Cu₃O_7−δ (YBCO) films on their crystal structures was studied for YBCO films grown on as-prepared CeO₂-buffered sapphire (CbS) and CbS post-annealed at 950–1100 °C. At temperatures below 70 K, the R_S appeared to be strongly dependent on the degree of the in-plane alignment of the YBCO films as determined by x-ray ϕ-scan analysis. Optimum in-plane alignment could be observed for YBCO films on CbS post-annealed at 1000–1050 °C, with the R_S appearing significantly smaller than for YBCO films prepared on as-grown CbS or CbS post-annealed at the other temperatures. Conversely, we observed a small correlation between the R_S and the degree of the c-axis alignment as determined using x-ray ω-scan analysis. Similarly, surface roughness of the YBCO films did not strongly influence R_S at temperatures below 70 K.

Highly-texturized polycrystalline fibres of the Bi–Sr–Ca–Cu–O system have been grown by the laser floating zone technique at seven different pulling rates: (1.1, 2.2, 4.17, 8.3, 16.7, 33.3, 60.5) × 10⁻⁶ m s⁻¹. The assessment of the cation segregation at the solid/liquid interface allowed us to calculate their equilibrium and effective distribution coefficients. The equilibrium distribution coefficients (k_0,Bi = 0.55, k_0,Sr = 0.97, k_0,Ca = 1.67, k_0,Cu = 1.10) were estimated using the Burton, Primm and Slichter (BPS) theory by taking into account the determined effective values. The effective distribution coefficients tend to unity as long as the pulling rate increases. The composition profiles along the initial transient region of the solidified fibres show a fast approach to the nominal composition as the pulling rate increases. The outstanding effect of the growth speed on superconducting phase type development is explained based on the solute trapping phenomena. The sequence of crystallization for superconducting phases ('2212' → '4413' → '2201') with pulling rate is a spontaneous effect of the system thermodynamics in order to balance the Bi trapping. This phase sequence corresponds to the smallest change of Bi chemical potential from the liquid phase to the solid phase. A diagram of free energy curves of the interdendritic superconducting phases illustrates the partitionless solidification phenomena at the highest growth speed.

High-J_c YBa₂Cu₃O_7−x (YBCO) films on SrTiO₃ substrates are successfully obtained by post-annealing of precursor films which are deposited by a co-evaporation technique with Y, BaF₂ and Cu evaporation sources at room temperature. In the case of using fluoride as the starting material for YBCO fabrications, water vapour is commonly introduced to decompose the fluoride in an annealing process. However, in the present process, we intentionally did not introduce water vapour. The obtained YBCO films have a high J_c of over 1 MA cm⁻² at 77 K and a self-field. This work has demonstrated that the additional water vapour is not required to obtain a high-J_c YBCO film regardless of whether fluoride is used as a starting material or not.

The fabrication of single domain YBa₂Cu₃O_y (123) thick films (10–100 μm) on metallic substrates is reported. The process involves the formation of the 123 phase by a peritectic reaction between an air-brushed dense Y₂BaCuO₅ (211) layer on a Ag12Pd substrate and infiltrated liquid phases containing barium cuprates and copper oxides. Single domain growth is achieved by seeding the green films with a c-axis oriented NdBa₂Cu₃O_y crystal prior to processing. The maximum processing temperatures are lowered to 970 °C by modifying the characteristics of the liquid phases meant for infiltration by addition of Ag powder. The fabrication technique, processing conditions for single domain growth and the resulting microstructures are discussed.

In high-temperature superconducting (HTS) microwave filters, the requirements for miniaturization and high power handling are difficult to match simultaneously. In this paper, we report on the design and realization of HTS filters realized by combining dual-mode cross-slotted and stepped resonators in order to obtain a compact size and a relatively high power handling. Prototype HTS four-pole filters operating in C-band and L-band, with quasi-elliptical responses, on both MgO and LaALO₃ substrates, have been designed and tested at T = 77 K. The filters present a high degree of miniaturization as they save more than 50% of the area compared with those realized with dual-mode and/or hairpin resonators. The measured responses agree well with the simulation even if the in-band ripple should be optimized. As far as the power handling is concerned, the C-band filter grown on LaAlO₃ presents a power handling of P_in = 20 dBm limited by the power handling of the dual-mode cross-slotted resonator, while we have observed no power degradation in the L-band filter grown on MgO up to P_in = 33 dBm limited by power supply.

We have measured the surface resistance of YBCO superconducting thin films deposited on sapphire and lanthanum aluminate substrates at the film side and at the interface between the substrate and the film. The measured difference in the R_s values of the interface and the film sides was between 30% and 60% for the YBCO films on LAO in the temperature range 15–80 K. For the Ag-doped films on sapphire the difference in R_s monotonically varied from 10% to 65% when temperature increased from 30 K to 80 K.

In order to study chemical resistance to the Y–Ba–Cu–O melts, dense BaCeO₃ ceramics have been obtained from coprecipitated oxalate precursors using CuO and BaCuO₂ sintering aids. BaZrO₃ ceramics obtained in a similar way were used for comparison. SEM and EDX investigation of ceramic BaCeO₃ after 12 h contact with molten cuprates at 1020 °C demonstrated no traces of macroscopic Ce dissolution or grain boundary corrosion. Nanoprobe EDX analysis of both ceramics detected no Cu within the BaCeO₃ and BaZrO₃ grains. Trace amounts of Ce and Zr were observed in several intergrain necks of frozen cuprate melt. Barrier properties of BaCeO₃ (BaZrO₃) materials can be further enhanced by surface plasma processing of presintered porous ceramics resulting in the formation of a molten crack-free layer, stable both to chemical corrosion and to physical penetration of the melt.

We report the in situ epitaxial growth of NdBa₂Cu₃O_7−δ thin films on (100) MgO substrates with Ba₂NdTaO₆, a new dielectric buffer layer, by pulsed laser deposition for the first time. It is found that by using Ba₂NdTaO₆ dielectric material as a buffer layer for the growth of NdBa₂Cu₃O_7−δ films over (100) MgO substrate, the in-plane misalignment of the NdBa₂Cu₃O_7−δ grains can be eliminated. The NdBa₂Cu₃O_7−δ films grown on Ba₂NdTaO₆ buffered MgO gave a zero resistance superconducting transition of 93.5 K, which was remarkably higher than the NdBa₂Cu₃O_7−δ films grown on MgO. Also the NdBa₂Cu₃O_7−δ films grown on Ba₂NdTaO₆ buffered MgO substrates show a narrower superconducting transition width (∼0.5 K) and better normal state metallicity (R₃₀₀/R₁₀₀) than the NdBa₂Cu₃O_7−δ films grown on bare MgO substrate. Our results indicate that Ba₂NdTaO₆ is a promising buffer layer material for the growth of epitaxial NdBa₂Cu₃O_7−δ films on MgO substrate.