Table of contents

This issue of Journal of Physics: Conference Series contains those contributed papers that were submitted to the Conference Proceedings of the 7th European Conference on Applied Superconductivity (EUCAS '05) on 11–15 September 2005. The plenary and invited papers were published in the journal Superconductor Science and Technology19 2006 (March issue).

The scientific aims of EUCAS '05 followed the tradition established at the preceding conferences in Göttingen, Edinburgh, Eindhoven, Sitges (Barcelona), Lyngby (Copenhagen) and finally Sorrento (Napoli). The focus was placed on the interplay between the most recent developments in superconductor research and the positioning of applications of superconductivity in the marketplace. Although initially founded as an exchange forum mainly for European scientists, it has gradually developed into a truly international meeting with significant attendance from the Far East and the United States.

The Vienna conference attracted 813 participants in the scientific programme and 90 guests: of the particpants 59% were from Europe, 31% from the Far East, 6% from the United States and Canada and 4% from other nations worldwide. There were 32 plenary and invited lectures highlighting the state-of-the-art in the areas of materials, large-scale and small-scale applications, and 625 papers were contributed (556 of these were posters) demonstrating the broad range of exciting activities in all research areas of our field. A total of 27 companies presented their most recent developments in the field. This volume contains 349 papers, among them 173 on materials (49.6%), 90 on large scale applications (25.8%) and 86 on small scale applications (24.6%).

EUCAS '05 generated a feeling of optimism and enthusiasm for this fascinating field of research and for its well established technological potential, especially among the numerous young researchers attending this Conference.

We are grateful to all those who participated in the meeting and contributed to its success.

Harald W Weber (Conference Chairman)

Franz M Sauerzopf (Conference Secretary)

EUCAS'05: The 7th European Conference on Applied Superconductivity was held on 11–15 September 2005 at Vienna University of Technology, Austria.

The bulk of the papers presented at the conference, after peer review, are published in Journal of Physics: Conference Series.

Invited and plenary papers are published in a special issue of Superconductor Science and Technology .

The experimental results of curling effect investigation (sharp increasing of the normal phase propagation speed under influence of the magnetic flux avalanche) in superconducting Nb-Ti and Nb-Zr wires are presented. The external magnetic field was imposed parallel to an axis of a sample of a superconducting wire by thickness from 0.16 mm up to 0.30 mm without a normal stabilizing covering, the transport current was put, and after initiation of a normal phase on a short part of a sample the normal phase propagation and the magnetic flux avalanche along a sample were registered simultaneously. It was found, that the thickness of the magnetic flux avalanche L_mfa and the thickness of the normal phase front L_npf (length of the part of a sample engaged by S-N transition) achieves 3 - 4 mm, that is more than 10 diameters of the sample. The heating of a superconducting wire by the magnetic flux avalanche is accompanied by increase of the speed of normal phase propagation V_npp. In a narrow range of change of a transport current the sharp increasing of the speed of normal phase propagation V_npp up to the speed of a magnetic flux avalanche V_mfa occurs. The further increasing of a transport current is accompanied by reduction of a distance (spacing) between the normal phase and the magnetic flux avalanche. In turn V_mfa increase with increasing of critical current dencity J_c and thickness of a superconducting wire. In particular the speeds V_mfa and V_npp achieve the value 9 - 13 km/s for the wire Nb-25%Zr by thickness of 0.24 mm with the critical current J_c = 2 - 4 MA/cm² in magnetic fields 0.5 - 1 T.

A superconducting magnet which is operated in persistent current mode in SMES, NMR, MRI and MAGLEV has many advantages such as high uniformity of magnetic field and reduced thermal loss. A high temperature superconducting (HTS) persistent current switch (PCS) system was designed and tested in this research. The HTS PCS was optimally designed using two different HTS tapes, second generation coated conductor (CC) HTS tape and Bi-2223 HTS tape by the finite element method (FEM) in thermal quench characteristic view. The CC tape is more prospective applicable wire in these days for its high n value and critical current independency from external magnetic field than Bi-2223 tape. Also a prototype PCS system using Bi-2223 tape was manufactured and tested. The PCS system consists of a PCS part, a heater which induces the PCS to quench, and a superconducting magnet. The test was performed in various conditions of transport current. An initial current decay appeared when the superconducting magnet was energized in a PCS system was analyzed. This paper would be foundation of HTS PCS researches.

The behaviour of a long superconducting strip, placed in a perpendicular external magnetic field, and surrounded with a (ferro)magnetic sheath in different geometrical arrangements is investigated. The numerical method of magnetic energy minimization, based on the critical state model, is used, and the commercial finite element software FEMLAB is utilized in the calculations. Our calculation method enables to insert arbitrary dependence of the permeability of the magnetic sheath material on the local magnetic field, may it be linear or non-linear function. The results of the flux penetration into the superconducting strip in an increasing external filed, together with the curves of initial magnetization are presented, for a few geometrical arrangements and different magnetic properties of the sheath.

A study was made on the effect of small quantities of doped elements Si, Sc and Cu on the microstructure, mechanical and superconducting characteristics of the alloy Nb-48.5 wt.%Ti.

Ta and Ti are the most widely used additions for technical Nb₃Sn multifilamentary superconductors. These elements are known to influence grain growth, grain morphology and chemical composition in the A15 layer, hence the current carrying properties of the wires over a wide magnetic field range. So far only few studies tried to compare systematically Ta and Ti doped and undoped Nb₃Sn wires in the frame of the same work, down to a nanometric scale. We present an investigation on several multifilamentary (Nb, Ta, Ti)₃Sn bronze route wires, fabricated at a laboratory scale, with various amounts of additives. The wires consist of fine filaments embedded in a Cu-Sn or Cu-Sn-Ti Osprey bronze with > 15 wt.% Sn and an external Cu stabilization. Microstructural observations are compared with the results of J_c and n values measured up to 21 T at 4.2 and 2.2 K, and for longitudinal strains up to 0.5%. Non-Cu J_c values up to 300 Amm⁻² and n values up to 50 at 17 T and 4.2 K show clearly that wires with Ti addition to the bronze have a better performance with respect to wires with Ti additions to the filaments.

The magnetic response of different Nb₃Sn samples has been investigated studying the temperature dependence of both the 1st and 3rd harmonics of the AC magnetic susceptibility, in presence of a DC field up to 19 T. In single crystal and in polycrystalline samples, the occurrence of a Peak Effect was observed within DC magnetic fields ranging from 3 T to about 13 T. It corresponds to a transition in the vortex lattice between the Bragg Glass phase and a disordered phase. This transition has been also detected at higher magnetic fields in both the samples, thanks to the 3rd harmonics measurements. In contrast to single crystals and polycrystals, no Peak Effect and no indications about the Bragg/disordered phase transition have been detected in the analysed multifilamentary wires.

To increase the field strength of superconducting High-Field-Magnets, it is necessary to improve the physical properties critical current and upper critical field of the superconducting materials. For (NbX)₃Sn this can be done partly by optimising the heat treatment, which is necessary to form the superconducting phase. In this paper we will present results of two-step heat treatments on a Ta and Ti alloyed bronze route Nb₃Sn conductor. Special attention will be focused on the growth of the Nb₃Sn layer as well as on the influence of the heat treatment temepratures and times on the upper critical field and the maximum pinning force. The differences between one- and two-step heat treatments will be discussed.

Ag-Sn alloy is very attractive for fabricating Nb₃Sn wires through diffusion reaction, because Ag-based alloy including a large amount of Sn is ductile. Therefore we investigated the Nb₃Sn formation through the diffusion reaction between Nb and Ag-Sn alloys. With using Ag-9 at% Sn fcc phase, Ag-12 at% Sn ζ phase, and Ag-24at% Sn phase alloys, we fabricated single-core, 200-core, and 40000-core composite wires with Nb matrix and Ag-Sn alloy cores. With increase of Sn content in Ag-Sn alloys, the obtained superconducting properties of heat treated composite wires were improved. T_c and B_c2(4.2 K) for the Nb/Ag-Sn are similar to those for the Nb/Cu-Sn. However, the I_c values are relatively small, due to the formed very thin Nb₃Sn layers. Ag is apparently not so effective to increase the formation rate of Nb₃Sn layer as Cu. We obtained very interesting results by making the 200-core and 40000-core wires, which show the improved T_c and B_c2(4.2 K) by 0.5-1 K and 2-5 T, respectively. From 10 to 500 times larger I_c(4.2 K) were also obtained for the multifilamentary wires.

The bronze process for the fabrication of niobium-tin superconductors is a mature and successful technology, but the optimisation of these conductors still requires a timeconsuming trial-and-error approach. The choice of heat treatment influences not only the amount of Nb₃Sn formed, but also uniformity across the wire cross-section and the grain size of the A15 phase, and consequently the critical current performance. The heat treatment for a given application must also consider the coil's thermal mass and the capabilities of the furnace. The key to optimisation is knowing the state of A15 phase formation as a function of time and temperature during heat treatment. A combination of reaction-diffusion modelling by finite difference methods and resistivity calculation has been used to develop a system for the optimisation of bronze route superconductors. Given the wire geometry, the radial variation of Nb₃Sn layer thickness and matrix composition can be calculated as a function of time and temperature. A computer program has been written to perform these calculations automatically, and rapidly estimate the optimal heat treatment for the parameters specified by the user. This approach has been applied to European Advanced Superconductors NS6000 conductors and, to test its effectiveness, rapid J_c(B) measurements were performed using a Cryo-BI-Pulse system. Improved critical current performance has been observed in low magnetic fields compared to isothermal samples, but further work is required to optimise high magnetic field performance.

In-situ powder neutron diffraction studies have been performed during the heating treatment of a set of Nb₃Sn wires obtained by the internal tin process. The different phases appearing during the process have been evidenced. A strong reorientation of the deformed copper and niobium metals induces an orientation on the Nb₃Sn formed. Among the phases formed, the δ-CuSn phase is decomposed at 890K to form the γ-CuSn and Nb₃Sn phases. The kinetics of these phases is in agreement with a diffusion-growth mechanism.

The optimization of flux line pinning in superconductors is one of the most efficient ways to improve the transport properties of these materials. The generation of pinning centers in a controlled way and with a projected distribution can contribute to the estimation of the pinning acting on the flux lines and to the improvement of the critical current densities J_c. The present work shows the development and characterization of a Nb₃Sn superconducting wire with nanometric-scale Cu(Sn) artificial pinning centers (APC) introduced in a controlled manner into the superconducting phase. These nanometric APC regions change the properties of the superconducting phase, mainly due to the proximity effect and to the inelastic scattering of the electrons on the interfaces. The final wire, with 1.064.514 Nb filaments, was heat treated to form the Nb₃Sn phase and the Cu(Sn) APC regions at different external diameters, enabling the determination of the flux pinning behavior with the variation of the APC dimensions, in comparison to the superconducting coherence length. The results were analyzed under the microscopic point of view determining the influence of the APC presence on the superconducting properties.

Sumitomo Electric successfully developed drastically innovative Bi-2223 (DIBSCCO), namely, commercially produced Bi2223 long length wires using the controlled over pressure sintering (CT-OP) with unique properties quite different from conventional silver sheathed BSCCO wires. CT-OP prevented pores occurring in BSCCO cores, so it reformed conventional Bi2223 wires to DI-BSCCO with excellent properties of higher critical currents, stronger mechanical strength and better durability against temperature rise in cryogen such as pressurized liquid nitrogen. It enhanced the critical current by 50 percent conventional wires sintered in normal atmospheres. Critical tensile stress was also improved by more than 150 percent. Any ballooning defects and degradation of critical current, one of the critical problems for the conventional BSCCO wires, were not found in full length of several km long DI-BSCCO tapes after 24 hours immersion into 1MPa liquid nitrogen.

Using DTA measurements and SEM/EDX and XRD of isothermally annealed and rapidly cooled/quenched 2212/Ag conductors, we study the effect of precursor composition on the melting temperatures, composition and abundance of phases in the partially molten state.

The procedure of the local magnetic field mapping of Bi-2223 tape was applied to receive self field data for the current gradually increasing from zero level up to over critical current of the tape in zero external magnetic field. The measurement was repeated for several currents, the same as were used in current increase, but during phase of current decrease to zero level. The Tichonov method of regularization was applied in solving Biot-Savart inverse in calculation of current distribution across the tape width. The distribution pattern changes rather irregularly during the increase of the current. On the other hand there is no significant difference in the shape of the current distribution profiles for transport currents 20% over I_c and the I_c. Comparison of up and down profiles show that changes in the current take place predominantly at edges of the tape, remaining central parts in retard. The same trend was observed in Nb₃Sn tape where however most of current flows at edges (non saturated state) contrary to Bi-tapes where central parts are dominant. The method was applied also for frozen currents in Bi-2223/Ag tape. The role of the external magnetic field in the change of the current distribution was studied too.

Considerable fractions of (Bi, Pb)-2223 were recovered after the complete peritectic melting of that phase. The minimization of Pb-losses was a key-feature, so that only about 1.57 wt % of total Pb content was lost over the whole process. Therefore the system remained within the 2223 single-phase region. The new route employed here, including the participation of a stable liquid, should overcome some microstructural problems nowadays present in sintered 2223/Ag tapes, such as voids, cracks and bad grain connectivity. Further optimization of the complete melting-reformation process may reduce the fraction of secondary superconducting phases, which can act as weak-links.

Investigation of AC loss under simultaneous action of transport AC and external AC field is of prime importance for reliable prediction of dissipation in electric power devices like motors/generators, transformers and transmission cables. The experimental rig allowing to perform AC loss measurement in such conditions on short (10 cm) samples of tapes from hightemperature superconductor Bi-2223/Ag has been designed and tested. Both the electrical and thermal method have been incorporated, allowing to combine better sensitivity of former one and a higher reliability of the latter one. Our main aim is to see how the AC loss depends on the phase shift between the current and the field. Such a shift could acquire rather different values in various applications. While in transformer winding, the maximum phase shift at full load will probably not exceed a few degrees, in a three phases transmission cable in tri-axial configuration it is around 120°. Therefore we explored the whole range of phase shifts from 0 to 360°. Surprisingly, the maxima of dissipation do not coincide with zero shift as expected from qualitative considerations.

The use of superconducting ceramics, with higher critical temperatures and higher operation temperatures, leads to economy of cryogenic liquids and systems. One of the largest problems in the practical use of superconducting ceramics is the intrinsic anisotropy of the critical currents. The present work describes a study of the texturing process in Ag/Bi2212 monofilamentary tapes, permitting the optimized choice of the heat treatment profiles to obtain superconducting phases close to the stoichiometric, with high texturing and high critical current densities J_c. The samples of Ag/Bi2212 tapes were obtained using the powder-in-tube (PIT) process with partial melting, treated at different maximum temperatures and with fixed cooling rate and isotherm at low temperature under oxygen flow at atmospheric pressure. The microstructural, electrical and superconducting characterization showed that the Ag/Bi2212 tapes submitted to different heat treatment profiles had significant improvements on the texturing and on the superconducting and microstructural characteristics and properties. The tapes treated with the maximum temperature of 890 °C for only 10 minutes followed by a cooling rate of −5 °C/h and 5 hours at the 830 °C isotherm presented best critical current densities of 7.5 × 10⁴ A/cm², at 4.2 K and 5 T; and 1.4 × 10⁵ A/cm², at 4.2 K and 0 T. This very short step at high temperature is extremely important for magnet development and applications.

The transport critical current density of multifilament Ag/Bi-Sr-Ca-Cu-O tapes with the addition of magnetic nanopowder (γ-Fe₂O₃) has been measured. The tapes were prepared using the powder in tube method with 30 filaments. Three types of Bi-Sr-Ca-Cu-O starting materials namely from solid state reaction without γ-Fe₂O₃, solid state reaction with γ-Fe₂O₃ addition and ultrafine powders prepared by coprecipitation method with γ-Fe₂O₃ have been used. The starting composition of the γ-Fe₂O₃ added samples is Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀-(γ-Fe₂O₃)_0.01. The first intermediate rolling enhances J_c but subsequent rolling shows no improvement in J_c. Tapes prepared from coprecipitation powders with γ-Fe₂O₃ added showed the highest J_c of 7890 A/cm² at 77 K and in self-field after the first intermediate rolling. These results suggest that intermediate rolling and magnetic nanopowders can be used to improve the critical current density of multifilament Ag/Bi-Sr-Ca-Cu-O superconductor tapes.

We measured the effective thermal conductivity in samples simulating the structure of Bi-2223/Ag impregnated coils at liquid nitrogen temperature. We also studied the effect of the sample orientation with respect to the coolant surface: samples with the cooled surface in the horizontal and vertical positions were measured. The effect of heat drains located between the layers of the tape on the effective thermal conductivity was studied. Its increase is important for coils conductively cooled by refrigerators. Our experimental results are compared with those obtained by other authors using model calculations.

Effect of the over pressure processing on the distribution of the critical current density in mono-core tapes was measured in detail using Campbell's method. It was found that the magnetic flux profile is more straight in the over-pressured specimen, suggesting that the critical current density is more uniform. This seems to be partly caused by the improved uniformity of the thickness due to the disappearance of voids and the reduction in sausaging of superconducting layer by application of the over pressure. The possibility of the further improvement of the critical current density is discussed

We analyzed the current distribution in three Bi-2223/Ag tapes with different filament lay-out, comparing the results of magnetic knife and Hall probe experiments. Detailed knowledge of the current distribution can be useful for the diagnostics of HTS conductors. The lateral current distribution was measured with the non-destructive magnetic knife technique and used to calculate the corresponding magnetic field profile above the tape. These calculated profiles were then compared to those actually measured by a Hall probe. Additionally, the Hall probe data were also compared with ''ideal'' field profiles, assuming uniform current flow across the tapes. This assumption is often used in measurement interpretation. The Hall probe - and magnetic knife data correspond very well, but deviate significantly from the ideal profiles. Further analysis of the current distribution shows that this deviation is mainly due to suppression of the critical current density at the tape edges.

With focus on the application in rotating electric machines we measured the dependence of current capacity of Ag/Bi-2223 wires on temperature and magnetic field. Even for wires stemming from a single manufacturer we observe a significant spread of wire properties. We study different temperature and magnetic field dependence by a parallel path model which allows for a quantitative analysis. The implications of experiments and modelling are discussed with regard to the further wire development and for application within windings.

We studied the influence of the aspect ratio (width/thickness) of a Bi-2223/Ag tape on AC transport current losses in multifilamentary tape. The aspect ratio of the original tape (27.4) was stepwise decreased to 14.5. We used three different methods of sample width reduction (A, B and C). The current - voltage characteristics were measured for all sample widths. We observed that the hysteresis of the current–voltage characteristic was reduced with sample width reduction. At small sample widths the hysteresis practically did not appear. The magnetic field dependence of the critical current, I_C (B), was measured as a function of the sample width. Also the critical current and the engineering critical current density as a function of the tape width were evaluated. The transport current losses of the tape samples with different widths were measured at 57 Hz and compared with the Norris equation for an elliptical model.

The experiment showed that the slope of the transport current loss–current does not depend on the aspect ratio.

The magnetic behavior of iron sheathed magnesium diboride (MgB₂) wires was investigated. Global magnetization measurements have shown unusual critical current density variations compare to a superconductor with no magnetic environment. Local quantitative studies, through MO images, are linked to these global magnetization measurements in order to better understand the observed results. It is shown that the current distribution is affected by the soft magnetic sheath interacting with the superconducting core.

The effect of nano-C doping on the microstructure and superconducting properties of Fe-sheathed MgB₂ tapes prepared through the in-situ powder-in-tube method was studied. Heat treatment was performed at a low temperature of 650°C for 1 h. Scanning electron microscopy investigation revealed that the smaller grain size of MgB₂ in the samples with the C-doping. Further, the a-axis lattice parameter and transition temperature decreased monotonically with increasing doping level, which is due to the C substitution for B. High critical current density J_C values in magnetic fields were achieved in the doped samples because of the very fine-grained microstructure of the superconducting phase obtained with C doping.

The modelling of a single filament, 6 filaments and 19 filaments MgB₂ conductors was performed for two limiting cases: a) isothermal conditions considering J_c(B) dependence, b) considering heating effects but with J_c magnetic field independent. As a starting point of the modelling in case a) we used experimental dependence of J_c(B, 4.2 K) for the best wire. Then Poisson equation for magnetic vector potential was solved by finite element method and self-field critical current densities of the wires with different diameter were calculated. There is no significant dependence of J_c in range of electric fields E = 1–10 µV/cm. The isothermal modelling with J_c independent on magnetic field, case b), gave the results more close to the experimental ones. From the modelling of the heating effects we conclude that the cryogenic stability of the used Cu/SUS316/MgB₂, Fe/MgB₂ and Cu/MgCu₂/MgB₂ wires is adequate. The stabilizing copper layer can take all the current up to the onset of film boiling of LHe where the current starts to decrease with increasing electric field, i.e. at this point the differential resistivity of the Cu/SUS316/MgB₂ wire starts to become negative (indicating the onset of instability). Analysis of mechanically induced defects limiting the J_c in 6 filaments conductor is presented.

MgB₂ wires, tapes and bulk samples produced within the EU-funded HIPERMAG project have been studied by a combination of X-ray diffraction and electron microscopy. The reaction layers forming at the interface between the ceramic core and Fe or Ni sheaths can be studied with both methods. The complementary techniques enable to study both the microstructure and the formation kinetics of the interface layers. Grain sizes can be determined either by direct observation or by analysis of the shape of X-ray diffraction peaks. Electron microscopy can detect B-rich secondary phases and phases present in small fractions that are not accessible by x-ray diffraction. On the other hand, synchrotron diffraction provides a fast and non-destructive method for the study of the main phases and their development during in-situ, high-temperature investigations. The combination of the two techniques is a very valuable tool for the optimisation of MgB₂-based superconducting material.

The relationship between irreversibility field, H_irr, and crystallinity of MgB₂ bulks including SiC or B₄C doped samples was studied. Among the XRD peaks of MgB₂, the FWHM of (110) reflection corresponding to the in-plane disorder was strongly dependent on the samples and H_irr was found to be systematically increased with an increase of the FWHM of MgB₂ (110) peak. Enhanced intra-band scattering and strengthened grain boundary flux pinning are suggested to contribute to the excellent H_irr characteristics. On the other hand, weak correlation between H_irr and FWHM of (002) peak was confirmed. These mean that introduction of disorders into the ab-plane, i.e., distortion in honeycomb boron sheet, is essentially effective to enhance H_irr.

The influence of disorder introduced by neutron irradiation is investigated for sintered bulk material and for in-situ iron sheathed wires. We find strong enhancements of the upper critical field and a decrease of the upper critical field anisotropy. Accordingly, the irreversibility fields, defined as the fields, where the first continuous superconducting current path occurs and the resistivity disappears, are enhanced. The critical current densities increase after irradiation, especially at high magnetic fields. This enhancement is mainly caused by the changes of the reversible properties, but slight changes in flux pinnning are observed as well.

Effects of controlled grain size on the critical current properties of in-situ fabricated polycrystalline MgB₂ bulks were systematically investigated. Grain sizes of MgB₂ bulks were controlled by three different methods: changing the heating conditions, such as temperature and holding time, changing the molar ratio between Mg and B, and starting from B powders with different grain sizes. MgB₂ bulks with smaller grain sizes were obtained when heating at low temperatures or for short time at high temperatures starting from fine B powder. Small MgB₂ grains were also found in the bulks starting from Mg-deficient composition using fine B powder. Through the careful analyses on the grain size of MgB₂ and critical current properties, we found that MgB₂ bulks with smaller grain sizes exhibit higher irreversibility fields and larger maxima of pinning force density. Our present results supported the scheme that grain boundaries are the predominant pinning centres in polycrystalline MgB₂ bulks.

The phase transformations occurring in the ceramic core of Fe-sheathed MgB₂ wires and tapes prepared by in-situ reaction of Mg and B precursor powders, have been studied by means of high-energy x-ray diffraction. In particular, the time evolution of the Fe₂B phase, forming at the interface between the sheath and the ceramic, was studied at different sintering temperatures. The reactivity of the sheath towards Fe₂B formation is strongly dependent on powder pre-treatment. In wires produced with commercial Mg and B powders without additional mechanical activation, the Fe₂B phase starts forming around 650°C. In contrast, in tapes produced from a mixture of Mg and B powders subjected to high-energy ball milling, the interfacial Fe₂B layer forms readily at 600°C. The increase of Fe₂B volume fraction is linear to first approximation, showing that the interfacial layer does not act as a diffusion barrier against further reaction between the sheath and the ceramic core. If the ceramic core is converted to MgB₂ at a temperature, which is low enough to avoid Fe₂B formation, the interface is stable during further annealing at temperatures up to 700°C at least. However, too high annealing temperatures (T > 800°C), would result in formation of Fe₂B, probably following the partial decomposition of MgB₂.

AC transport loss of YBCO coated tape 4 mm wide has been investigated by standard experimental techniques. When used to evaluate the critical current of the tape in an indirect way, the results pointed toward the critical current exceeding 100 A at 77 K and zero superimposed DC field. Taking into consideration the extremely high aspect ratio of the superconducting layer, one would expect to observe the loss behaviour predicted for infinitely thin strip. However, in the case of AC transport, the loss dependence on current follows the formula derived for a strip with elliptical cross-section. The influence of the metal substrate on such behavior was experimentally excluded. To understand this result, additional experiments by field mapping over the sample carrying transport current have been performed. According these results the distribution of the transport current in the cross section of the sample fulfil the elliptical model prediction.

We recently reported on a simple thermal co-evaporation route based on a reel-toreel system, that enables the production of 2 m-long superconducting tapes with I_c values up to 120 A/cm-width (J_c = 2 MA/cm²) and T_c = 88 K. This work describes the development of a new multi-chamber system designed for the continuous production of these tapes suitable for an industrial process. The system consists in three distinct vacuum chambers connected one with another by two specially designed 50 cm long slits. The length and cross-section of these slits have been calibrated in order to achieve a pressure difference up to 5 orders on magnitude across adjacent chamber. The system is originally conceived for the continuous production of CeO₂ buffered RABiTS Ni-based tapes that require a pre-treatment of the bare metallic tapes in forming gas, followed by CeO₂ deposition and a post-treatment in oxygen. To improve the robustness of the whole tape production process, we have designed and built a novel device based on a supersonic oxygen gas expansion for the continuous in situ oxygenation of YBCO during deposition. Here, we present preliminary results demonstrating the effectiveness of this device. Specifically, thanks to the enhancement of the number of collisions of the O₂ molecules with the substrate due to the focused supersonic beam, we find that the oxygen pressure in the vicinity of the substrate can be up to 3 orders of magnitude higher than the background pressure in the chamber. The main advantage of this supersonic device is the insensitivity of the effective pressure to the substrate-nozzle distance, in the 2-5 mm range, which can be easily controlled during a continuous production process.

YBa₂Cu₃O_7-δ thick films have been deposited onto Ag substrates by the Electrophoretic Deposition (EPD) technique. Different microstructures and electrical behaviours were observed depending on the starting powder. Coatings prepared from commercial powder displayed significant porosity and the superconducting transition width was found to be magnetic-field dependent. Films produced from home-made coprecipitated powder are denser but contain some secondary phases. No dependence of the resistive transition as a function of magnetic field (H ⩽ 20 Oe) was observed in that case.

CeO₂ films are technologically important as a buffer layer for the integration of superconducting YBa₂Cu₃O₇ films on biaxially textured Ni substrates. The growth of YBa₂Cu₃O₇ layers on the CeO₂ cap layers by the trifluoroacetate (TFA) route remains a critical issue. To improve the accommodation of YBa₂Cu₃O₇ on CeO₂, surface conditioning or CeO₂ is required. In this work we have applied ex-situ post-processes at different atmospheres to the CeO₂ layers deposited on YSZ single crystals using rf sputtering. XPS analysis showed that post-annealing CeO₂ layer in Ar/H₂/H₂O catalyses in an unexpected way the growth of (001)- terraces. We also report on the growth conditions of YBa₂Cu₃O₇-TFA on CeO₂ buffered YSZ single crystal grown by chemical solution deposition and we compare them with those leading to optimized YBa₂Cu₃O₇-TFA films on LaAlO₃ single crystals. Critical currents up to 1.6 MA/cm² at 77 K have been demonstrated in 300 nm thick YBa₂Cu₃O₇ layers on CeO₂/YSZ system. The optimized processing conditions have then been applied to grow YBa₂Cu₃O₇-TFA films on Ni substrates having vacuum deposited cap layers of CeO₂.

A multi-turning Reel-to-Reel tape conveyance system of the long YBCO coated conductors using TFA-MOD process is desired to be high production rate without large degradation between tape lines. The numerical simulation, which calculated the YBCO growth kinetics, gas element diffusion and gas flow, shown a possibility that the vertical gas flow onto the tape surface realized high production rate of YBCO conductor even on the wider tape. This possibility was also proved by experimental measurements.

We address the use of coated conductors for resistive fault current limiters. Fast quench propagation is required to let the conductor switch on the full length within milliseconds. But the ordinary quench propagation mechanism is too slow because of the small heat diffusivity in typical materials of substrate tapes. Here we present a new mechanism which is not based on heat diffusion. Rather, we have chosen a conductor geometry such that any quench leads to a distortion of the current flow pattern and therefore to current bunching. Thereby the critical current density is locally exceeded and the superconductor turns normal whithout the necessity of heating. In this way the resistive state is quickly spreading until the current flows mainly through the substrate from end to end. Thus, the conductor develops its full normal resistance and is homogenously warming further up. The mechanism was confirmed by a numerical simulation and by experiments on samples of short and medium length. In general, coated conductors can be made self-protecting by the new mechanism, so that the use of thick normal conductiong stabilizers is no longer necessary.

A new processing schema is proposed to fasten the metal-organic decomposition step required to grow high critical current YBa₂Cu₃O₇ thin films based on Trifluoroacetates (TFA). AFM and optical microscope study of quench samples at different temperatures have been performed to determine the temperature window where strong stresses are generated. The thickness evolution with the temperature has been observed and linked to the final microstructure. The influence of the gas flow and temperature annealing ramp for two different types of solution has been investigated. We demonstrate that high critical current YBa₂Cu₃O₇ thin films, i.e. J_c = 3MA/cm², with thickness of 300 nm can be grown with a new fast calcination process lasting only 60 min. This fastened calcination process allows to enhance the coated conductor production rate without the need of any modification of the metal-organic precursor solutions.

Temperature dependence of the ac loss in YBCO superconducting tapes fabricated by IBAD-PLD technique was investigated. The ac loss was measured with a saddle-shaped pickup coil by applying magnetic field perpendicular to the tape. Temperature dependence of I_c-B characteristics was estimated from the observed magnetization curves. The I_c-B curves normalized by the zero-field I_c at the respective temperature coincided with one master curve regardless of temperature. I_c-B characteristics of YBCO superconducting tapes are scaled with temperature. According, when the magnetization curves and the ac loss vs field amplitude curves are normalized by using the zero field I_c at the respective temperature, they coincided with one master curve for any temperature from 4.2 to 77 K. It is verified that the magnetization and the ac loss are scaled with temperature.

High critical current density YBa₂Cu₃O_7-x (YBCO) coated conductors were obtained on cube textured Ni-W. The use of a Pd transient layer as a first buffer led to a sharp out-of-plane grains alignment of the CeO₂/YSZ/CeO₂ buffer layer. YBCO films grown on this template exhibit an out-of-plane orientation with a full width at half maximum of about 3°, less than 50% of the respective starting Ni-W value. Despite the complete interdiffusion between Ni-W and Pd after the YBCO film deposition, the coated conductors exhibit good film adherence as well as a crack free and smooth surface of the YBCO film. YBCO thin films show critical temperature values above than 88 K and a critical current density of 2.1 MA/cm² at 77 K and self field.

Fabrication of Hg_1-xRe_xBa₂Ca₂Cu₃O_y [Hg(Re)1223] tapes has been attempted by the PIT(Powder-In-Tube) method using a NiO/Ni sheath. It is confirmed that Hg(Re)1223 tapes with a high Re substitution level, 25% for Hg, showed sharp superconducting transitions at ∼134 K and high irreversibility fields up to ∼9 T at 77 K. In addition, application of magnetic grain alignment was effective for development of c-axis oriented Hg(Re)1223 tapes showing high transport properties.

We achieved a very low resistance at 0.02 ∼ 0.03 µΩ (12 nΩcm²) across the nonsuperconducting joint with an area of 2 × 20 mm² for YBCO coated conductors produced by pulsed laser deposition (PLD). The joint technique was very simple in which YBCO coated conductors were stuck in a face to face manner with Ag surfaces in contact and heat-treated at 500°C in an oxygen atmosphere. We also confirmed that the good superconducting properties of the YBCO coated conductors were maintained after completion of the series of joint procedures. Furthermore, a low contact resistance of 0.5 nΩcm² at the interface between the YBCO film and the Ag stabilizing layer was confirmed.

Recent progress in the research and development of the TFA-MOD processing for the coated conductors is reviewed. For the higher I_c performance, process conditions such as heating rate and water vapor inlet temperature in the calcination step and heating rate and water vapor pressure in the crystallization step have been optimized to obtain YBCO films with good crystallinity. Consequently, a high I_c value of 431A/cm-w was achieved by the combination of the above optimized conditions with the use of highly textured CeO₂ cap layered substrates. Forwards the effort for long tape fabrication with high production rate, the multi-turn system for coating and calcination step was developed. Additionally, the reel-to-reel system for the crystallization step was developed for the longer tape, and a 25m long YBCO tape with a reasonable end-to-end I_c value of 100A, which corresponds to 2500Am as a product of I_cxL, was obtained by the above continuous system. For the higher production rate in the crystallization step, it was confirmed that optimization of the processing parameters such as high water partial pressure, high diffusion constants due to low total pressure, and a high gas flow rate affects the YBCO growth rate.

We have formed SmBa₂Cu₃O_7-δ(Sm-123) films on PLD-CeO₂/IBAD-GZO/metal substrates which are suitable for coated conductors. Characteristics of the SmBa₂Cu₃O_7-δ films were affected by O₂ gas pressure during deposition through film crystallinity. For the film 0.25 m in thickness deposited under 500 mTorr O₂ gas pressure, we obtained a J_c of 2.2MA/cm² and T_c of 92.5 K. We also measured the dependence of J_c on applied magnetic field angle for Sm-123 films and compared to REBa₂Cu₃O_7-δ (RE = Y, Er, Gd) films. The J_c of the Sm-123 films had peaks at the angle where the magnetic field was parallel and perpendicular to c-axis. Under the magnetic field perpendicular to the c-axis of the films, the REBa₂Cu₃O_7-δ (RE = Y, Er, Gd) films which had high J_c of 2 to 3 MA/cm² in self-field showed similar J_c values. On the contrary, under the magnetic field parallel to the c-axis, the J_c exhibited large difference depending on the RE elements.

The realisation of superconducting thin films by MOD is possible using different precursors; among them, the most promising approach is the use of trifluoroacetates (TFAMOD). However, one of the major drawbacks of this approach is the generation of water when trifluoroacetic acid (TFAH) is used. In this case, a lengthy purification process of the solution is necessary. As an alternative, trifluoroacetic anhydride (TFAA) has been used affording anhydrous TFA solutions without any additional purification. Anhydrous TFA solutions have allowed YBa₂Cu₃O_7-x films to be obtained with high critical currents (J_c > 3-4 MA/cm² at 77K, thickness 300nm).

One route to achieve thick YBCO layers by a single deposition on single crystal substrates (LAO) has been to modify the viscosity of the solution by using different polymeric agents in precursor solutions. A screening investigation of several polymers with different molecular weights has been performed to choose the best additive based on the following criteria : to increase viscosity, to avoid any chemical reaction with the precursors and to keep the same decomposition temperature as compared to the pyrolysis of the TFA precursors. Solution viscosity, thermal and thermo-gravimetric analysis measurements have been used to characterize the TFA solutions with the additives. An increase of the YBCO films thickness of 100% (⩾600nm) has been demonstrated keeping a high J_c ⩾ 1.1 MA/cm² (77K).

In general, the sol-gel method is a very attractive route for ceramic synthesis because it permits direct fabrication of multi-component ceramics in different configurations without powder intermediates. This diversity, in which materials can be formed, has made the sol-gel method an important synthesis route in several domains of research. For (RE)Ba₂Cu₃O_7-y superconducting thin film development, chemical solution deposition (CSD) techniques starting from sol-gel precursors can offer a cost-effective and more flexible large-scale alternative synthesis route in comparison to the common used vacuum techniques. This work describes the deposition of thin NdBa₂Cu₃O_7-y layers on SrTiO₃ single crystals based on a new sol-gel dip coating process using aqueous precursor solutions. Two inorganic aqueous sol-gel routes were investigated, a metal nitrate - citric acid based and a metal acetate - triethanol amine based solution.

Dy-Ba-Cu-O filaments have been successfully fabricated by a solution spinning method. The relationship between the partial melting temperature and the transport J_c value at 77 K of the filamentary sample was systematically investigated. Two kinds of precursor filaments with starting composition of Dy:Ba:Cu = 1.18:2.12:3.09 (sample A)and Dy:Ba:Cu = 1.00:2.00:3.00 (sample B) were partially melted in 1%O₂ + Ar atmospheric gas and oxygenated in pure 100% O₂ gas. Samples partially melted at a wide temperature range of 990-1040 °C had J_c values higher than 10⁴ A/cm². Both sample A and sample B partially melted at 1020 °C exhibited the maximum J_c value higher than 4.2 × 10⁴ A/cm² and 2.0 × 10⁴ A/cm², respectively. These samples had a dense microstructure consisting predominantly of Dy123 and finely dispersed Dy211 particles. The Dy211 particles with a diameter of about 1µm were finely dispersed in sample A, while the particles with a diameter of about 5µm were observed in sample B. The J_c values of both samples were maintained about 2.0 × 10³ A/cm² up to 14 T.

Critical current dependence versus strain is obtained for in-situ axial stress experiments on ISD YBCO and DyBCO coated conductors. The drop of critical current due to the apparition of first cracks in the superconducting ceramics is related to the passage in the plastic region of the substrate for a strain of about 0.3% and a stress higher then 500MPa. The superconductivity is preserved between the cracks.

The epitaxial growth of YBCO films both on (001) SrTiO₃ (STO) and Ni-W biaxially textured metallic substrates prepared by metal-organic deposition (MOD) using a trifluoroacetic acid (TFA) solution is reported. The degree of epitaxy of the YBCO films was investigated by X-ray diffraction and scanning electron microscopy (SEM). The as deposited films exhibit good morphological and structural properties. The ω-scan of the YBCO films grown on (001) SrTiO₃ single crystal substrate and on Pd/CeO₂/YSZ/CeO₂ buffered biaxially textured Ni-5at%W (Ni-W) tapes has a full width at half maximum (FWHM) of 0.12° and 3.4°, respectively. The ϕ-scan of (113) peak of YBCO film grown on Ni-W substrate has FWHM of 6.1°. The YBCO/STO film has a zero resistance critical temperature of Tc(R = 0) = 92 K and a critical current density Jc > 2 MA/cm² at 77 K and in zero magnetic field.

Reel-to-reel MOCVD process for continuous growth of electrically conducting buffer layers on biaxially textured Ni5W tapes has been developed. The new buffer layer architechture is presented: 200 nm (La, Ba)₂CuO₄/40 nm (La, Ba)MnO₃/Ni5W. Constituting layers with high structural quality have been grown on moving tapes (in plane FWHM ⩽ 6° and out of plane FWHM ⩽ 3°).

A new MOCVD technique was developed for the deposition of oxides at a controlled low oxygen partial pressure, which enable to coat textured metal tapes with electrical conductive perovskites. In a RTR MOCVD-system metal tapes were coated with electrical conductive (La, Ba)MnO₃, (La, Sr)MnO₃, (La, Sr, Pb)MnO₃ and (La, Ba)₂CuO₄ buffer layers and YBCO. In the same RTR-system YBCO was deposited on different buffered metal tapes. Computer calculations were used to characterise the behaviour for a weak superconducting part of a CC-tape with a conductive buffer layer system.

Long length MOD processing of YBCO conductor requires high performance precursor solution without degradation during the process. In this study, effect of chemical modification of precursor solution on MOD processing was investigated to meet the requirement. Highly stable YBCO precursor solution was synthesized with PVP (polyvinylpyrrolidone) and crack-free thick precursor film was successfully fabricated. However, annealed YBCO film shows porous microstructures originated from large HF gas generation from thick films and residual carbon from additives. Reduction of fluorine content is required to densify microstructures. In order to reduce fluorine content, F-free Cu-precursor solution was synthesized. YBCO film with J_c = 0.8MA/cm² was grown on buffered RABiTS tape. Annealed YBCO film still contains pores and further reduction of pores is required to improve critical current (I_c)

The commercial development of coated conductors is rapidly progressing. As a result we present an economic route to produce second generation HTS tape from the initial substrate preparation to the final metal coating. The most important and technically challenging steps are the deposition of an oriented buffer layer and the superconductor film in a reel-to-reel configuration. New evaporation techniques have been developed to enable reliable, high rate tape coating. Highly oriented MgO - buffer layers are realized by inclined substrate deposition (ISD) and DyBCO is deposited by simple e-gun evaporation yielding critical currents beyond 200 A/cm. Coated conductors have been fabricated up to 40 m length and are currently tested in a variety of applications.

Modelling and design of superconducting fault current limiters (SFCL) requires accurate prediction of current-voltage behaviour and thermal response of HTS tapes carrying electrical current over I_c, since these characteristics affect the electrical behaviour of the SFCL during a fault. An experimental method was developed to measure current, voltage and surface temperature on short HTS tapes for electrical field values above I_c (1µV/cm), up to 10mV/cm. The measurements were performed under different thermal exchange conditions with current pulses having different duration and amplitude. The experimental results allowed to validate a numerical model for HTS tapes in overcurrent conditions.

Magnetic critical temperature of composite superconductors has been studied for providing a new standard method for the critical temperature measurement. Various magnetization-temperature curves of Cu/NbTi, Cu/Nb3Sn and Ag/Bi-2223 wires were measured using a SQUID magnetometer and an ac susceptometer. In the case of SQUID magnetometer measurements, we employed a two-field-direction method to obtain higher resolution near the onset temperature. In ac susceptometer measurements, various amplitudes from 2 Oe to 10 Oe with frequencies from 11.3 Hz to 1033 Hz were applied to three specimens to study the suitable ac magnetic field and frequency range. Analytical comparison of the magnetic curves with the resistive curves was also accomplished to investigate the validity of the new method as compared with the existing one.

Continuous columnar pinning centers result in maximum pinning potential, U_pin. Most scientists have assumed maximum U_pin would maximize J_c. However, several clues pointed to high J_c despite pinning discontinuities. An experiment was performed to directly compare continuous and discontinuous pinning, using high-energy ion damage to create the pinning centers. Results in YBCO show that J_c for discontinuous pinning is much higher than for continuous pinning. At high pinning density (20 Tesla equivalent field), J_c is 60 times higher for discontinuous pinning, and at 5 Tesla equivalent field, it is 4 times higher. Record J_c ≈ 300 kA/cm² at 77 K was achieved in large grain textured HTS for pinning which was 67% discontinuous. This resulted without improving the texturing or purity. Despite reduced U_pin, H_irr for discontinuous pinning is as high as for continuous pinning. The superior T_c, and percolation, achieved by discontinuous pinning, far outweighs the decrease in U_pin.

Single crystals of Bi2212 and Bi(Pb)2212 having various cation compositions were grown by the floating zone method and evaluated their J_c characteristics after control of oxygen content by post-annealing. J_c characteristics of Bi2212 and Bi(Pb)2212 were found to be improved by control of cation compositions towards stoichiometric, Bi:Sr:Ca:Cu = 2:2:1:2. This means that nonstoichiometric cation composition in these systems suppresses inherent superconductivity possibly due to considerably disordered crystal lattice, resulting in poor J_c characteristics. Dramatically enhanced J_c were achieved for cation stoichiometric and small amount of Y or Lu-doped Bi(Pb)2212 single crystals. The highest J_c at 20 K observed were ∼0.85 MAcm⁻² in self-field and ∼0.1 MAcm⁻² under 5 T with H // c. The dilute Y and Ludoping is considered to introduce locally lattice-distorted regions acting as effective pinning sites up to high fields.

Improvement of flux pinning properties of RE123 single crystals and melt-solidified bulks was attempted by dilute doping of impurity elements to various cation sites except the Cu at the CuO₂ plane. All the examined dilute doping for RE123, such as Sr to Ba-site, Lu to Y-site and Co to Cu-site in the CuO-chain, was found to increase critical current density, J_c, particularly under magnetic fields accompanying enhanced second peak effect. These results suggest that the dilute doping is universally effective for improving the critical current properties of the RE123 system independent of the doped cation sites.

Although pinning centers created by irradiation presently produce the highest J_c, it is probable that ultimately these will be emulated by chemical pinning centers. The best pinning centers produced by irradiation nevertheless provide guidelines for desirable morphology of chemical pinning structures. The highest J_c produced earlier in textured HTS was obtained using isotropic high-energy ions produced by fission of ²³⁵U. This so-called U/n process produces pinning centers of diameter ⩽ 4.5 nm, with an effective length of ∼2.7 µm. Maximum J_c occurs for pinning center density of ∼10¹⁰ cm⁻³. We use this as a model for desired chemical pinning centers. Our approach to introducing chemical pinning centers has been to produce precipitates within the HTS containing elements not native to the HTS, and to seek needlelike (columnar) deposits of small diameter. We report here on the formation of needlelike or columnar deposits in textured Y123 containing a dopant foreign to Y123. It serves as a demonstration that self-assembling nanometer diameter columns utilizing a dopant foreign to the HTS system are a feasible goal. These deposits, however, do not fully meet the ultimate requirements of pinning centers because the desired deposits should be smaller. The self-assembling columns formed contain titanium, are ∼500 nm in diameter, and up to 10 µm long. The size and morphology of the deposits vary with the mass of admixed Ti dopant. J_c is decreased for small dopant mass. At larger dopant masses needlelike precipitates form, and J_c increases again. A small range of mass of admixed Ti exists in which J_c is enhanced by pinning. In the range of admixed Ti mass studied in these experiments there is a negligible effect on T_c. Magnetization studies of J_c are also reported.

Basing on the phenomenological model of the microwave response of thin superconducting films with trapped magnetic flux elaborated in Luzhbin et al 2005 Supercond. Sci. Technol.18 1112-7, detailed analysis of the ac linear response of a superconducting strip-line resonator cooled down in FC and ZFC regimes is presented. Special emphasis on a nonuniform distribution of the trapped vortices and on the magnetic field and temperature dependence of the phenomenological model parameters such as critical current density, vortex pinning and viscous motion coefficients is made. For a ZFC case realistic trapped magnetic .eld distribution based on the critical-state model for a vortex penetration is used. Within the framework of the used model the unusual multi-peak dependence of the surface resistance of HTS films observed experimentally is explained as well as clari.ed conditions for its appearance.

Ba-Cu-O films have been grown by pulsed laser deposition. Investigation of their transport properties and characterization of electronic structure by in-situ photoemission spectroscopy have been carried out. The films grown at 450 ∼ 480 °C from BaCuO₂ target had simple infinite layer structure with lattice parameter of a and c about 0.385 and 0.405 nm, respectively. Conductivity of these films were, however, poor. Usage of BaCu_0.75O_x target and addition of CO₂ into the growth atmosphere resulted in an expansion of c-axis length up to 0.810 ∼ 0.835 nm and a significant rise of conductivity. This structure is represented as ''2c''. For the 2c phase, conductivity above of 10³ S/cm was achieved by adjusting their growth temperature in the range of 500 ∼ 530 °C. Most of the highly conductive 2c phase showed superconductivity. The maximum temperature of onset of the superconducting transition and zero resistance state obtained so far were 60 and 47 K, respectively. Finite spectral weights at Fermi level were successfully observed in in-situ photoemission spectra of the 2c phase. In valence band region, a prominent peak with u 3d character located at a binding energy around 1.6 eV. Though this low binding energy suggested a rather high hole concentration of mobile holes were around middle of 10²¹ cm⁻³ range. These results about electronic structure indicate a coexistence of conventional CuO₂ planes and distorted ones. The latter structure might play charge-reserving blocks, though some portions of holes should localize in them. Correlations between the properties and electronic structure are to be discussed.

High quality YBa₂Cu₃O₇ (YBCO) films and multilayers of ReBa₂Cu₃O₇ superconductors, where Re is rare earth elements (Y and Nd), have been prepared by pulsed laser deposition. Pinning characteristics of the structures obtained have been analysed and attributed to growth conditions and corresponding structural peculiarities. Relatively thick (∼1 µm) multilayers exhibit better performance than mono-layer YBCO films having arbitrary thickness. differences in the films and multilayers are discussed in terms of their structure homogeneity and defects induced by the growth of the layers.

Surface morphology of Bi₂Sr₂CaCu₂O_x (BSCCO) thin films prepared by metalorganic decomposition (MOD) method is successfully improved by using nano-structured SrTiO₃ (STO) buffer layer prepared by MOD method. The nano-structured STO thin films were obtained by 2-step thermal treatment at 870°C for 2 hours followed by high temperature of 1000°C in O₂ atmosphere for 1 hour. In the case of BSCCO thin films on conventional STO substrates, many islands consist of additional phases and large-size steps were observed. In contrast, segregations are extremely suppressed, and terrace structures with a-b plane of BSCCO were clearly observed. The BSCCO thin films on the nano-structured STO showed a critical temperature of 80 K. This result indicates that a control of nucleation is one of the most important factors to obtained high quality thin films in the case of hetero-epitaxy.

We investigated the magnetic field dependence of the longitudinal resistivity ρ_xx, the Hall resistivity ρ_xy, and the Hall angle (θ_H) at various temperatures of fully textured (Hg_0.9Re_0.1)Ba₂CaCu₂O_{6 + δ} (HgRe-1212) thin films prepared by pulsed laser deposition (PLD) and post-annealing. The films exhibit sharp superconducting transitions at T_c = 124 K with transition widths of ΔT_c ≃ 2 K. At higher fields the Hall resistivities of the films have a nearly linear dependence on the field. The sign of the Hall resistivity in the low-.eld region near the transition temperature becomes negative. The total Hall conductivity for HgRe-1212 HTSC thin films can be described well by σ_yx(B) = C₁/B + C₂B + C₃.

We have studied in detail the kinetics parameters of the YBa₂Cu₃O_7-x (YBCO) thin films growth on LaAlO₃ (100) single crystals by the TFA-MOD method, using an i.n-situ fluoride selective electrode. We have considered: temperature, flow carrier gas rates and water partial pressure dependences in the reaction. Kinetics curves show that it exist an intermediate step before the YBCO formation. This step, which starts during the heating ramp (∼400°C) consists in a partial elimination of F from the BaF₂ precursor to forms an oxyfluoride intermediate compound. The total YBCO formation appears at temperatures as higher as 700°C. We have found that at low flow carrier gas rates, the reaction is controlled by diffusion mechanisms and the apparent order of YBCO formation is n = 1 respect to the stirring rate, but at higher flow rates the YBCO formation is controlled chemically, then the apparent order is zero. The apparent reaction order of YBCO formation respect to the water pressure also has been studied.

High quality superconducting thin films of HTS have been grown by MOCVD on substrates with artificial steps of predefined height and width. The surface of the films grown on the steps having width equal to the 'double of the migration length' of the atomic species depositing on the substrate is totally free of precipitates: precipitates are gathered at the step edges where the free energy is lowest. The method has several advantages: it is simple, universal (it is independent of the materials, substrates, deposition technique or application) and allows control of precipitates segregates so that the quality and growth conditions of the films are the same as for the films grown on conventional substrates. The method is expected to result in new opportunities for the device fabrication, design and performance.

Calcium doping is well known to increase the grain boundary critical current, J_c^gb, in polycrystalline Y₁Ba₂Cu₃O_7-δ, (YBCO). High resolution Secondary Ion Mass Spectroscopy (nanoSIMS) analysis has been used to map the distribution of Ca⁺ ions in three dimensions in an YBCO grain boundary grown on a 24° SrTiO₃ bicrystal. High resolution SIMS imaging has produced Ca, Sr, and Ti elemental maps at 20 nm intervals from the YBCO surface. From the variation of Ca concentration with depth a Ca grain boundary diffusion coefficient can be estimated. Moreover, localised 'hot spots' of Ca at the YBCO grain boundary have been observed, perhaps at regions with a high dislocation density. There is also some diffusion of Sr and Ti ions from the substrate along the boundary. This is an important observation because it has been proposed that these ions are electrically active.

The interest of this paper centers on fabrication and characterization and modeling of vortices in high temperature superconducting thin films. As a first step, the magnetic vertices of the superconducting matrix were modeled. As a second, Bi-Sr-Ca-Cu-O thin films were grown using Pulsed Laser Ablation (PLD) on single crystal MgO substrates as magnetic templates for the potential use for Nano and Microelectronic circuits, and were characterized by x-ray diffraction, electron, and atomic force microscopy. The third step (future work) will be observation and pinning of these vortices using Bitter decoration.

Tl-1223 and Tl-2223 superconducting films have been prepared using spray pyrolysis and MOCVD for the deposition of the precursor layer and a solid source of thallium for the subsequent thallination step. For the former phase, critical temperatures and critical current densities up to 113 K and 2.5 MA/cm² (77 K, 0 T) have been obtained after 3 to 6 hours at 860-870°C. Pure Tl-2223 films are by far more difficult to prepare. This can be achieved by operating at 890-900°C or with a fluorine containing source. The resulting films have then poor superconducting properties due to the presence of holes. Working at 875°C yields multiphase films (Tl-2223 + Tl-2212 or Tl-2223 + Tl-1223) with however better properties

The corrosion resistance of high temperature superconducting (HTC) Y-Ba-Cu-O (YBCO) films and doped YBCO films (lanthanum La, calcium Ca, silver Ag) was studied and their lifetimes in saturated steam environment at 75°C were determined. The results indicate an enhancement of stability against environmental degradation for the films with increasing La content. We suppose, that this enhancement is attributed to poor water solubility of lanthanum hydroxide protective films, which were formed in the first step of corrosion.

We report the superconducting properties of (Cu, C)Ba₂CuO_y thin films deposited by RF magnetron sputtering from targets with a nominal composition of Ba₂Cu_1.5O_y. The thin films were grown in mixed Ar (5∼40 mTorr), O₂ (0∼1.2 mTorr) and CO₂ (0∼1.8 mTorr) atmosphere. The epitaxial c-axis oriented thin films with c-axis length in the range between 7.96 Å and 8.82 Å were obtained on SrTiO₃(100) substrates, but superconductivity was observed only for the films which have the c-axis length with 8.01 Å and 8.32 Å. By optimizing deposition conditions, the excellent (Cu, C)Ba₂CuO_y thin film with T_c = 62K and superconducting transition width ΔT_c = 1.5 K was successfully obtained.

Critical magnetic behaviour of MgB₂ thin films grown on SiC-buffered Si substrate was investigated in comparison with MgB₂ films on NbN-buffered Si substrate. MgB₂ films were prepared by sequential evaporation of boron and excess magnesium followed by in situ annealing in an Ar atmosphere. The upper critical field H_c2 estimated from the onset of AC diamagnetic susceptibility was larger in films with SiC buffer than those with NbN buffer. The temperature dependence was almost linear with |dH_c2/dT| up to 8 kOe/K, especially under parallel magnetic field to the film surface. Although the critical current density evaluated from DC magnetization hysteresis approached 1 MA/cm² at the lowest temperatures, the maximum value was smaller than that in MgB₂/NbN/Si. The irreversibility field also inferred effects of weak links, possibly due to some impurity phases.

We report spin injection from nearly 100 % spin-polarized ferromagnet La_0.67Ca_0.33MnO₃ into hot isostatically pressed MgB₂. The injected spin-polarized current suppresses the supercurrent in MgB₂ in the proportion of about 100 units of critical current for each unit of the injected current. Such unusually high current gain could pave the way for using MgB₂ in spintronics applications. We overcome incompatibility of MgB₂ and La_0.67Ca_0.33MnO₃ by preparing a small-area ab-plane mechanical contact.

Superconducting MgB₂ films were obtained following a two steps approach, using the multilayered structures of MgB_x/Mg as precursors deposited at low substrate temperature by sputtering of an MgB₂ stoichiometric target, and thermal evaporation of pure Mg. Precursors were subsequently annealed in-situ in the range of 350-600°C. The films showed a zero resistance critical temperature up to 31 K.

The as-obtained MgB₂ films were investigated by X-Ray Photoelectron Spectroscopy (XPS) and X-Ray Auger Electron Spectroscopy (XAES) using non-monochromatic Al Kα radiation (1486.6 eV) and a double-pass CMA operating in retarding mode. The electronic structure was studied by monitoring the B1s, Mg2p, O1s core-levels and the Mg KL_{2, 3} L_{2, 3} Auger peak structure. For comparison, the electronic structure of an MgB₂ superconducting sputtering target, of a precursor film and of a non-superconducting sample obtained by sputtering from the MgB₂ target have also been investigated. Electron spectroscopy showed that in the superconducting systems the Mg KL_{2, 3} L_{2, 3} Auger peak kinetic energy position is always higher by about 0.7eV with respect to the energy position of the same Auger line measured in the nonsuperconducting samples.

We have synthesized as-grown superconducting MgB₂ thin films on polyimide film substrates, making use of a low substrate temperature growth of MgB₂ thin films, and found that T_c's above 30 K were obtained on polyimide films, like on Al₂O₃-C substrates. For highquality MgB₂ thin film fabrication on substrates completely out of lattice-matching with MgB₂ lattice, we have examined Ti/Ag double-layered buffers on glass substrates, and clarified that a double-layered buffer composed of Ag bottom layer 50 nm thick and Ti upper layer 50 nm thick prepared at an ambient temperature is effective for this purpose.

Superconducting MgB₂ thin films were prepared in situ using a combination of rf magnetron sputtering of B and thermal evaporation of Mg. The films exhibited T_c of up to 36 K. The microwave measurements were performed on 14 × 14 mm² films using both Cu-shielded and Nb-shielded sapphire puck resonators at the frequency of 18.8 GHz. The hf surface resistance (R_S) and the change of the hf surface reactance (ΔX_S) were determined. The films exhibited low R_S matching the literature results for high-quality MgB₂ films. Below 3K R_S reached 3-5 µΩ which was the resolution limit of our measurement. The temperature dependences of both R_S and ΔX_S were in good agreement with BCS theory. From the R_S(T) dependence we obtained an energy gap Δ(0) ≈ 3 meV. The measured variation of the London penetration depth with temperature, Δλ_L(T), was also in good agreement with the BCS model. Using the BCS relation between the energy gap and the penetration depth we fitted our experimental Δλ_L(T) data and obtained λ_L(0) values, which ranged for different films from 85 to 100 nm.

In this paper we present measurements concerning the current noise produced during the resistive transition in a MgB₂ polycrystalline thin film. The power spectrum of the current noise, observed when the temperature is slowly changed across its critical value, presents a large electrical noise of the 1/fⁿ type (n ≅ 3) over a quite wide range of frequencies. This noise may be considered as generated by the abrupt creation of resistive strips across the specimen constituted by grains which have undergone the resistive transition. A computer model that takes into account fluctations of the grain critical currents and of the number of grain per strips, has been developed to simulate the resistive transition and to evaluate the noise power spectrum. When the temperature is incresed and reaches its critical value, resistive strips are formed according to a percolative process, giving rise to resistance steps which are at the origin of the noise. The theoretical results obtained by this model are in good agreement, concerning both the shape and intensity of the noise power spectrum, with the experimental data directly measured on the specimen.

In the last two decades, non conventional behavior of the d.c. transport properties of superconductors, with the appearance of anomalous peaks at the transition, have been investigated and interpreted in different ways. In several cases it was recognized that the behavior can be due to current redistribution effects related to the non-homogeneous nature of the measured superconducting sample.

In this paper we will briefly review and discuss these effects and, referring to simple concentrated constant equivalent circuits, we will show that sample non-homogeneity can produce the observed features. Then, in the same framework, by performing specific simulations on planar resonators, we will show that anomalous peaks in temperature dependence of the resonant frequency and of the extracted surface reactance can occur at the transition temperature of minority, lower Tc, superconducting phases.

The heteroepitaxial growth of YBa₂Cu₃O_7-x (YBCO) films prepared by the trifluoracetate TFA route was characterized by TEM/EELS, µ-Raman spectroscopy and X-Ray diffraction of specimens quenched from various temperatures. We find that after the pyrolysis, the film consists of a homogeneous, partly amorphous, nanocrystalline matrix of Ba_1-xY_xF_{2 + x} and CuO. Upon heating, such a precursor undergoes a strong phase segregation on a length scale of 100 nm. Simultaneously, the Ba_1-xY_xF_{2 + x} solid solution is decomposed into BaF₂ and Y₂O₃, and part of this Y₂O₃ eventually reacts with the CuO to give Y₂Cu₂O₅. Our results make evident that the nucleation of YBCO takes place exclusively at the interface with the substrate, within the fluoride phase. The fluoride phase appears highly textured from the early stages of phase evolution, above 600 °C, and determines the orientation of the YBCO. The microstructural heterogeneity of the precursor film prior to the nucleation of YBCO strongly suggests that more than one reaction path may operate simultaneously.

We have studied the surface morphology of SrTiO₃ (STO) and YBa₂Cu₃O_7-δ (YBCO) thin films fabricated by pulsed laser deposition technique in a multilayer structure. Study was focused on the surface morphology of an STO thin film deposited on an YBCO layer that received chemical and mechanical polishing (CMP) treatment. The CMP process could remove the presence of the outgrowth on the STO thin film, improve its smoothness and facilitate a good epitaxial growth of the next deposited YBCO film. As a result, we could fabricate a high quality YBCO thin film on the YBCO/STO structure with T_c = 91 K and J_c = 4 × 10⁶ A/cm².

Results of deposition of YBa₂Cu₃O_7-x/CeO₂/(La_0.7Ca_0.3MnO₃ or La_0.7Sr_0.3MnO₃) structures on the standard oriented and tilted ( = 8°) NdGaO₃ substrates and results of investigation of electrical parameters of YBa₂Cu₃O_7-x (YBCO) films in such structures are presented. The YBCO component of the structure exhibits lower value of the critical parameters in comparison with those of single YBCO films. The contribution of the magnetic layer to the microwave losses of the YBCO film in the layered structure is evaluated.

Multilayered cuprates of Ba₂Ca_n-1Cu_nO_2n(O, F)₂ (F-02(n-1)n) with n = 5 - 9 have been synthesized by using high pressure synthesis method in order to investigate the variation of T_c. The temperature dependence of susceptibility showed that the T_c (about 80 K) does not depend on n for n = 5 - 9. This result can be explained using the carrier imbalance model in multilayered cuprates. Charge reservoir layers supply most of the carriers to adjacent CuO₂ planes (OP) and the OP keeps the T_c constant even for large n.

AFM and STM scans on SmBa₂Cu₃O_x (SmBCO) melt processed samples revealed nanoscale stripe-like structures, sometimes parallel over several micrometers, sometimes wavy. These structures consist of chemical compositional fluctuations and act as effective δT_c pinning centres due to their wavelength of typically 10-60 nm which is comparable to the ideal pinning center size 2ξ (∼10 nm for YBa₂Cu₃O_x in the ab-plane). Compared to similar structures in ternary (Sm, Eu, Gd)Ba₂Cu₃O_x (SEG) and (Nd, Eu, Gd)Ba₂Cu₃O_x (NEG) systems, where the stripes appear either as plateau-like stripes or as chains of aligned clusters, the stripes in SmBCO always appear as plateau-like stripes with a height of 1Å–8Å. These pinning structures throughout the whole sample volume may be a key to improve critical current densities especially at high external magnetic fields.

Copper rich Cu-Ni alloys have been textured with the RABiTS method in order to get non-magnetic and cost efficient substrates for coated conductor wires. The study is focused on two industrial compositions: Cu₅₅Ni₄₅ (Constantan) and Cu₇₀Ni₃₀. Studies on surface roughness have been done. The effects of annealing conditions (temperature and atmosphere) on the grain boundaries deepness were analyzed. Electrolytic polishing was also applied to improve the surface quality. RX pole figures and micro hardness measurements have been achieved on samples annealed up to 950°C. Attention has been paid to the rolling texture and to the annealing processes. The rolling texture has been found to be mainly copper-type (C, S and B contributions). Beginning of recrystallization occurred between 400°C and 600°C and stabilized between 900°C to 950°C, depending on the nickel content of the alloy. Finally both samples, rolled and annealed under the appropriate conditions, have been characterized. Pole figure measurements gave the global in plane and out of plane disorientations of our samples which are in-plane 7.4° and out-plane (RD) 4.3° for both samples. EBSD maps have shown the details of the distribution and have allowed us to quantify the ratio between cubic {100}<001> and twined {122}<21-2> orientations.

We present results of crack free layers of CeO₂ and La₂Zr₂O₇ deposited by means of CSD on cube textured Ni-4 at.% W substrates. EBSD-data show histograms with very good inplane- and out-of-plane textures and were used to simulate the critical current density in the YBCO layer. The surface roughness, a sensitive feature for good deposition results, was analyzed with a profilometer. In the CSD process we applied, the 2, 4-pentanedionates of the metal cations in glacial acetic acid and methanol served as starting substances.

YBa₂Cu₃O₇ (YBCO) film was prepared by fluorine-free metalorganic deposition (MOD) on a 2-inch-diameter CeO₂-buffered yttria-stabilized zirconia (CbYSZ) substrate. The MOD-YBCO films on a flat CbYSZ prepared using a metal acetylacetonate-based coating solution demonstrated high critical current density (J_c), the average being higher than 4 MA/cm² at 77.3 K and in a self field using an inductive method. The J_c distribution was uniform in the whole area of the MOD-YBCO films on the large size substrates. Cross sectional transmission electron microscopic images of the MOD-YBCO film exhibited a very smooth CeO₂/ YSZ and CeO₂ buffer layer/ YBCO film interfaces. Both the buffer and the YBCO layers were dense; however, a small amount of (100)-oriented YBCO occurred inside the matrix of (001)-oriented YBCO layers. Also, some distinct dislocations were observed at the CeO₂/ YBCO interface. From around the stepped surface of the CeO₂ buffer layer there seemed to grow a slightly distorted (001)-oriented YBCO domain, however, the domain restored its (001)-oriented periodicity in the region at some distance above the steps. Such growth mode is considered to correlate the high J_c of the MOD-YBCO film with very small average roughness of CeO₂ buffer layer.

By growing CeO₂ buffer layers on deliberately miscut Al₂O₃ (102) surfaces, we have successfully fabricated stoichiometric YBa₂Cu₃O_7-δ (YBCO) thick films by pulsed laser deposition. Atomic force microscopy and scanning electron microscopy observations demonstrated that YBCO films with thickness up to t ∼ 1.0 µm were microcrack-free. Characterization of the films revealed a porous morphology, consisting of interconnected islands and deep holes (pores). This feature is considered as one of the sources contributing to the strain-relieving mechanism responsible for the increase in film thickness without microcracking. Microcrack-free thick YBCO films revealed T_c = 90.5 ± 0.5 K, J_c(77.3 K, 0 T ) = 2.0-3.0 × 10⁶ A/cm², and a substantial enhancement of J_c × t (77.3 K, 0 T ) up to 246 A/cm.

Cube textured Cu/Cu-based tapes are shown to be an alternative to Ni/Ni alloy substrates widely used in high current capability YBCO-coated conductors. Copper, other than nickel, is non-magnetic and has a larger thermal and electrical conductivity, keeping up thermal stabilization of the superconductor at cryogenic temperatures. Jointly with the cube texture of exceptional strength that develops after rolling and recrystallization, Cu is, therefore, a candidate material for coated conductor architecture.

In this work, we report on the texturing of pure copper and different copper alloys, such as Cu- Mn and a dispersion hardened Cu-B₄C tape. For Cu and Cu-B₄C, the maximum found in the cube texture histograms are 3.8° and 4.4°, respectively.

CeO₂, BaZrO₃ as well as LaAlO₃ buffer layers were deposited on {100}<001> Ni + 5 weight-% W substrates by a wet chemical technique. The solutions were prepared by dissolving the metal nitrates or acetates and zirconiumacetylacetonate, respectively, in mixtures of acetic acid, methanol and water. The solutions were applied by dip- or spincoating, dried at 135 °C and annealed at temperatures between 900 and 1 400 °C depending on the buffer layer for 15 min. under Ar-5% H₂ gas flow. Pole-figure measurements proved the exact texture of each buffer layer. Electron microscopy showed dense and smooth buffer layers.

1 cm ×12 cm rectangular YBCO films for fault current limiter devices were prepared by MOD. Usually, solution coating in MOD is carried out by spin coating. However, there are some problems associated with spin coating of large-size rectangular substrates. Namely, the maximum substrate dimension is limited by cup size and motor torque of the apparatus. It was found that wire-bar coating is a promising alternative; this method is more easily scaled up to a large size at lower cost. By both methods, YBCO films with high J_c's around 2 MA/cm² were obtained on CeO₂-buffered sapphire. So far, the uniformity of J_c in the YBCO films prepared by spin coating was better than those by wire-bar coating.

Epitaxial YBa₂Cu₃O₇ (Y123) films were obtained on CeO₂-buffered sapphire substrates by a fluorine-free MOD process. A 170-nm-thick homogeneous film revealed a high critical current density (J_c > 4 MA/cm²) at 77 K, whereas a 320-nm-thick film had a low J_c of 0.7 MA/cm². Both of the films have neither a-axis orientation nor BaCeO₃ phase by x-ray diffraction. In this article, the microstructures of these Y123 films were observed by crosssectional transmission electron microscopy. The epitaxial c-axis layers were observed successively from the interface with CeO₂ buffer to the surface. Selected area diffraction (SAD) exhibited that the orientation relationship among the Y123, CeO₂-buffer and sapphire substrate was: Y123(001)[110] ∥ CeO₂(100)[100] ∥ Al₂O₃(-1012)[1-210]. The impurity phase on the surface was revealed to be a CuYO₂ compound by energy dispersive X-ray spectroscopy and SAD. The Y123 grains on the surface of the 320-nm-thick film were somewhat inclined to the epitaxial Y123 layer; this is one of the reasons for degraded J_c in the thicker films.

We studied microstructure and superconducting properties of melt-processed RE123 (RE = Y and Dy) bulk superconductors with a large amount of BaCeO₃ addition. The melttextured bulk samples with compositions of Y123 + xBaCeO₃ (x = 0.05-0.4) and 0.5 wt% of Pt were fabricated in air. The BaCeO₃ particles with the diameter of 1-2 µm were trapped in the melt-textured samples. The critical current density (J_c) in a low field region increased with the amount of BaCeO₃ inclusions. This suggests the BaCeO₃ particles act as effective flux pinning centers, as well as Y211 particles in conventional melt-textured Y-Ba-Cu-O bulk materials. We found that the size of BaCeO₃ particles could be reduced by substituting Zr on Ce site. The Y123 bulk sample with 30 mol% of Ba(Ce_0.75Zr_0.25)O₃ and 0.5 wt% Pt showed a high J_c value of 110 kA/cm² at 77 K. The Dy123 bulk samples dispersed BaCeO₃ and Ba(Ce_0.75Zr_0.25)O₃ particles also exhibited excellent J_c-B properties.

We have succeeded recently in synthesizing a chemically stable, inert family of materials of composition Y₂Ba₄CuMO_y (Y-2411 where M = Nb, Ta, Mo, W, Zr, Hf) within the superconducting YBa₂Cu₃O_7-δ (Y-123) phase matrix that forms effective flux pinning centers of nano-scale dimensions. In this paper we report the synthesis of the Y₂Ba₄CuBiO_y phase with nano-scale dimensions that is similarly compatible with the Y-123 matrix and which does not impair the properties of the bulk superconductor. YBa₂Cu₃O_7-δ/Y₂BaCuO₅ (Y-123/Y-211) precursor powders enriched with various amounts of Bi₂O₃ and Y₂Ba₄CuBiO_y have been fabricated successfully in the form of large, single grains by the top seeded melt growth (TSMG) process. Microstructural studies of these composites reveal the presence of nanometer-sized Y₂Ba₄CuBiO_y and much larger Y-211 phase particles (∼1 µm) embedded in the Y-123 phase matrix. The critical current density of the nano-composites is observed to increase significantly compared to undoped YBCO.

The multiseeded seamless bulk technique (MUSLE) developed by Sawamura et al for Dy/Gd/Ho-BaCuO compositions have been adapted for Y-based superconductors. This method allows overcoming the major problem encountered with the multiseeding technique, which is the trapping of non superconducting phases between the different domains. The MUSLE process requires the choice of two compositions for the bi-layer system. The criterion is a sufficient difference in the peritectic decomposition temperatures. In this condition, the higher peritectic temperature layer serves as nucleating site for the final YRE123 domain. Differential thermal analyses have been performed to select suitable compositions of as the upper layer and YYb123 as the lower layer. The influence of (Y, RE)₂BaCuO₅ (YRE211) composition on the crystal growth rate and on the superconducting properties was also investigated. Finally, compositions and thermal cycle have been defined to texture 2 cm multiseeded bi-layers samples. Electrical properties, coupled with SEM observations reveal the sample quality.

We study a pulsed-field magnetization of HTS bulks and application to rotating synchronous machine of axial-gap type. To increase the output performance of the rotating machine, the multiple rotor structure is designed with the alternation of the rotor with Gd-bulks and the fixed armature. Thus, we obtain a redundancy for rotor magnets operation without additional current leads nor slip rings. To assess the present redundancy, we show the result of the pulsed-field magnetization for Gd-bulks in the above mentioned multiple rotor geometry. Two Gd-bulks with 60 mm in diameter and 19 mm thick were inserted into three vortex-type Cu coils, alternatively. They are immersed into liquid nitrogen. The pulsed current was applied to three vortex-type coils as serial. The maximum applied magnetic filed was 5.7 T with the rise time of 6.6 ms. The trapped field for two Gd-bulk were 0.851 T and 0.835 T, respectively. We evidenced it is possible to perform the pulsed field magnetization for two Gd-bulks sandwiched among three armature vortex-type Cu coils, i.e., double layered rotor assembly.

We present a post-fabrication treatment that improves thermal conductivity of bulk Gd-Ba-Cu-O magnets. A small hole about 1 mm in diameter was artificially drilled into the centre of bulk Gd-Ba-Cu-O 25 mm in diameter and 18 mm in thickness. An aluminium wire was inserted into the hole, and then the sample was subjected to the impregnation by using Bi- Su-Cd alloy. A pulsed-field magnetization was performed for Gd-Ba-Cu-O bulk with and without the alloy impregnation treatment. The temperature of samples was monitored with thermocouples and local magnetic field density was measured with a Hall sensor at several positions. The maximum temperature rise was depressed by 4 K and the trapped field was increased by 25 % at 44 K. The result shows that the alloy impregnation is very effective in enhancing the thermal conductivity and thereby improving the field trapping ability.

An isothermal kinetics study of the transformation of Tl-2212 into Tl-2223 has been carried out at four different temperatures. The Tl-2223 and Tl-2212 weight fraction have been deduced from Rietveld analysis of XRD patterns and the transformed fraction α has been calculated for each sample. The results indicate that the transformation is limited by diffusion and particularly by the thickening of large plates after lateral impingement.

Melt Textured YBa₂Cu₃O₇/Y₂BaCuO₅ (Y123/Y211) and Y123/Y211/CeO₂ composites, have been grown by Top Seeding Melt Texturing growth (TSMTG). The initial powder mixture has been densified, through a pre-melting process carried out at 1000-1050°C under O₂ atmosphere. This process leads to a strongly decreased porosity of the pellet. Afterwards, TSMTG is carried out in air atmosphere. The influence of the pre-melting process on the final microstructure has been analyzed by optical and scanning electron microscopy (SEM). Microstructural analysis reveals a high microcracking density. Three well defined microcrack orientations have been detected, one parallel the (001) planes and a new class of microcrack parallel the {100} planes. The latter is being produced by the pre-melting densification process, while the usual microcrack parallel to (00l) planes also prevail. The influence of the oxygenation process and the Ce additives on the development of this new class of microcrack pattern will be discussed. Finally, the influence of the new microcrack network on the critical current density will be presented.

The microstructural and magnetic properties of YBa₂Cu₃O₇ (YBCO) welds with different crystallographic [001]-tilt misorientation, prepared by the Ag surface melting induced welding technique, have been studied. The inter- and intra-grain critical current densities have been simultaneously obtained by solving the Inverse Problem from the remanent local magnetization magnetic field maps measured by Hall Probe imaging. The obtained dependence of the inter-grain current density with the angle, J_c^GB(θ), is compared to previous results for thin-film bicrystals and bulk boundaries.

We report a new pulse field magnetizing (PFM) method using the modified multi pulse technique combined with a stepwise cooling (MMPSC) to obtain the trapped field B_T^P over 4 T on a bulk superconductor. The B_T^P value as high as 4.47 T has been attained on the surface of the GdBaCuO bulk, which is the present highest using PFM. The key points to obtain the higher B_T^P are the introduction of a small amount magnetic flux (B_T^P∼1 T) into the bulk with the ''M-shaped'' trapped field distribution at a higher starting temperature T_s∼45 K, and the following application of the optimum stronger pulse fields of B_ex∼6.7 T at a lower T_s∼30 K. The reduction in the temperature rise ΔT due to the already existing trapped flux is a very important issue.

Magnetic measurements in a wide temperature range of several ternary LREBa₂Cu₃O_y samples differently doped by secondary phase nanoparticles of different sizes are analysed in terms of a thermally activated depinning process. Evolution of the involved parameters with temperature and particle size is discussed. Zr contamination of the secondary phase particles facilitates the particles size reduction during meltgrowth process up to nanometre scale. Critical current at low fields, facilitated by multiple vortex pinning, is inversely proportional to (a small power of) the mean particle size. With such defects both critical currents and irreversibility field are high enough for industrial applications up to 90.2K (boiling point of liquid oxygen).

Electrical characteristics of the Pr-doped cuprate Nd-123 (NdPr-123) have been investigated in reference to an inhomogeneous structure consisting of Nd-123 and Pr-123 phases. The superconductor-insulator (S-I) transition takes place there when Pr-concentration x exceeds a threshold point x_c ≈ 0.32. At x just below the threshold point x_c, the resistance has been found to become highly non-ohmic, appearing as an increasing function of the current I. In contrast, at x much higher than x_c, the resistance turns into a decreasing function of I. The non-ohmic resistance having such dual aspects was observed also for another series of samples of NdPr-123 that was prepared in a different way and had a much lower threshold x_c ≈ 0.14. For this degraded samples, the resistance becomes non-ohmic more easily, indicating that the structural inhomogeneity is closely related to the non-ohmic behaviour. The non-ohmic resistance can be explained in terms of the current distribution governed by a percolation process pertain to the inhomogeneous structure consisting of unit cells of Nd -123 and Pr-123.

The ternary high-T_c superconductor (Nd_0.33, Eu_0.39, Gd_0.27)Ba₂Cu₃O_y ''NEG-123'' doped by 5 mol% of the nonsuperconducting secondary phase (Nd_0.33, Eu_0.33, Gd_0.33)₂BaCuO₅ ''NEG-211'' was prepared by oxygen controlled melt growth (OCMG) process using MgO seeds. Various contents of ZnO up to 0.07 mol% were added. At the ZnO concentration around 0.035 mol%, a substantial reduction of liquid phase loss was observed, accompanied with a perfect facet line up to the bottom of the sample, with c-axis reflection plane. Magnetization measurements showed that at this optimum ZnO addition the superconducting transition temperature was slightly depressed. On the other hand, flux pinning performance around the secondary peak, observed at liquid nitrogen temperature, was continually improved with increasing ZnO content up to 0.035 mol%. It was found that the optimum content of ZnO is crucial for flux pining enhancement and orientation control of MgO seeded melt-textured NEG-123 system.

We have recently reported a practical processing method for the fabrication in air of large, single grain (LRE)-Ba-Cu-O [where LRE = Nd, Sm, Eu and Gd] bulk superconductors that exhibit high T_c and high J_c. The process is based initially on the development of a new type of generic seed crystal that can promote effectively the epitaxial nucleation of any (RE)-Ba-Cu-O system and, secondly, by suppressing the formation of (LRE)/Ba solid solution in a controlled manner within large LRE-Ba-Cu-O grains processed in air. In this paper we investigate the degree of homogeneity of large grain Sm-Ba-Cu-O superconductors fabricated by this novel process. The technique offers a significant degree of freedom in terms of processing parameters and reproducibility in the growth of oriented single grains in air and yields bulk samples with significantly improved superconducting and field-trapping properties compared to those processed by conventional top seeded melt growth (TSMG).

Liquid phase sintering of YBCO superconductor has been investigated using 900°C- and 950°C-calcined precursors. Magnetic hysteresis, i.e. critical current density (Jc), is enhanced at elevated magnetic fields for the samples with the 900°C-calcined precursor of fraction more than 80%. Jc increases steeply as fraction of 900°C-calcined precursor increases from 80% to 100%, which is coincident with anomalous grain growth in the samples. The samples showing Jc enhancement compose of rather rectangular grains with length more than 100 µm, whereas grains of the samples showing no enhancement are rather round and a few tens of µm in diameter. Structural developments with fraction of the precursor and oxygenation behavior suggest that low angle grain boundary pinning is observed in the liquid phase sintered samples.

An important phenomenon in bulk superconductors fabricated by top-seeded-melt growth (TSMG) is the formation of cracks due to the inherent brittleness of the YBa₂Cu₃O_7-δ (Y-123) phase matrix. These form during the fabrication of the superconducting monolith and play an important role in the limitation of current flow. However, cracks may also form during cooling cycles of the sample to liquid nitrogen temperatures. In this investigation, macrocracks along the c-direction, in particular were analyzed microscopically before and after cooling. In addition we attempt to resolve the c-axis macrocrack formation pattern using the magnetoscan technique.

The joining of bulk high temperature superconductors (HTSs) with good transport properties has been investigated. The join is achieved by using a 'solder' that is predominately HTS with a slightly lower peritectic temperature than the parent material, in this study Y₁Ba₂Cu₃O_7-. This technique results in the formation of a superconducting seam with a critical current density and varying degrees of micro-structural defects. Solder reagents used were YbBa₂Cu₃O_7-, Yb₂Ba₁Cu₁O₅, and Y₁Ba₂Cu₃O_7-/Ag, which resulted in a total of eight different superconducting 'solders'. Micro-structural analyses using a scanning electron microscope was conducted on each sample as well as transport measurements. It was found that silver doping lowered the peritectic temperature of Yb₁Ba₂Cu₃O_7- resulting in a solder that gave best joins.

Melt-textured YBCO samples processed with added Y₂O₃ and depleted uranium oxide (DU) contain nano-particles, which have been identified previously as Y₂Ba₄CuUO_x (U-411). This phase has a cubic unit cell, which is clearly distinct from the orthorhombic Y-123 and Y-211 phases within the YBCO system. In samples with a high amount of DU addition (0.8 wt-% DU), U-2411 particles have sizes between 200 nm and several µm, so identification of the Kikuchi patterns of this phase becomes possible. Together with a parallel EDX analysis, the particles embedded in the Y-123 matrix can be identified unambiguously. In this way, a three-phase EBSD scan becomes possible, allowing also the identification of nanometre-sized particles in the sample microstructure.

Since melt-textured RE-system bulk can trap strong magnetic flux, it acts as a quasipermanent magnet stronger than conventional permanent magnets. Large single domain Dy-123 system superconductors about 48 mm in diameter and 15 mm in thickness were prepared with a seeding and temperature gradient method in air. DyBa₂Cu₃O_x (Dy-123) and Dy₂BaCuO_y (Dy-211) powders were mixed in molar ratios of 1:0.3. Ag₂O of 10 wt% and Pt of 0.5 wt% were added to the mixture. The pellets were partially melted at 1070 °C for 4 hour, and rapidly cooled to 980 °C for one hour. When the top temperature of the sample reached 980 °C, the ab-plane of Nd-123 seed crystal was placed in contact with the top surface of the bulk. After the seeding, the samples were cooled to 930 °C with a cooling rate of 0.07-0.08 °C/hr, and then the bulks were cooled to room temperature with furnace cooling. Finally, the samples were annealed in pure oxygen atmosphere from 500 °C to 250 °C for 500 hours. Two samples of the melt-textured single domain were fabricated. Homogeneous morphology was obtained in the samples. When magnetic field of 1.5 T was applied for the samples, about 0.7 T was trapped at 77.3 K.

We have recently developed a new generic seed crystal that has been used successfully to fabricate any oriented, single grain (RE)-Ba-Cu-O bulk superconductor by a cold seeding technique. In this paper we report the chemical, structural and microstructural properties of these seed crystals, including the variation of melting point, crystallographic parameters and volume fraction of Mg-rich inclusions in the Nd_{1 + x}Ba_2-x(Cu_1-yMg_y)₃O_z matrix as a function of externally added MgO content. The influence of Mg-doping on the superconducting transition temperatures of YBCO grains fabricated using these seeds is investigated. Finally, an optimum MgO content of the generic seed that effectively controls the orientation of the seeded grain without compromising its superconducting properties is suggested from the many seed crystals fabricated with a wide range of Mg-rich addition.

Bulk superconducting samples of type Tl₂Ba₂Ca₂Cu_3-xFe_xO_10-δ; with 0 ⩽ x ⩽ 0.4, have been prepared using a single step of solid-state reaction. The prepared samples have been characterized using X-ray powder diffraction (XRD), scanning electron microscope (SEM) and microprobe analysis (MPA). The tetragonal structure of Tl-2223 did not change with the partial replacement of Cu²⁺ by Fe³⁺ ions, whereas the lattice parameters were found to vary as function of Fe-content. The superconducting transition temperature T_c determined from electrical resistivity and ac magnetic susceptibility measurements shows suppression in its value as Fecontent increases. The suppression in T_c was attributed to the magnetic disorder and Cooperpairs breaking. The critical current density J_c and field irreversibility B_ir were calculated as function of Fe-content.

We fabricated bulk GdBa₂Cu₃O_7-δ (Gd123) superconductors with additions of Gd₂BaCuO₅ (Gd211) and Gd₂Ba₄CuMoO_x (Gd2411). We analyzed the relation between the addition amounts of Gd211/Gd2411 particles and critical current J_c. We found Gd2411/Gd211 particles formed a band structure which resulted in the inhomogeneous distribution of J_c in the Gd123 bulks. Gd2411 particles can stably exist in the vicinity of the seed site compared to Gd211 that is easy pushed out to edge area of single domain. This enhanced the critical current J_c in the vicinity area of bulks.

Hot plastic deformation of bulk Bi2212 strongly improves pinning by introducing strong pinning centres accompanied by the formation of a high degree of texture. This is proved by an increase in critical current density and mean effective activation energy obtained from relaxation measurements, as well as from a large shift of the irreversibility line. The analysis shows that a simple interpretation of the U(J)-relation in terms of collective creep theory is not possible. Admixture of MgO particles gives similar, but less good results.

Thallium-free precursors for bulk superconductors with the overall composition (Tl_0.5Pb_0.5)(Sr_1-xBa_x)₂(Ca_1-yGd_y)Cu₂O_z (x = 0.0, 0.05, 0.1, 0.15, 0.2, 0.25, y = 0.0, 0.05, 0.1, 0.2, 0.3, 0.4) were prepared via wet chemical gel techniques. The respective amounts of Tl₂O₃, PbO and Gd₂O₃ were co-milled to the precursor and the mixture was subjected to a heat treatment first at 850 °C followed by sintering at 925 °C in order to optimize the superconducting properties of the bulk material. X-ray diffraction showed that samples with x and y equal to or smaller than 0.2 were nearly phase-pure. While changes in the x value had little effect on the properties of the materials, a Gd stoichiometry of 0.2 yielded the best results in this series. Critical temperatures around 100 K were obtained for samples with the composition (Tl_0.5Pb_0.5)(Sr_1-xBa_x)₂Ca_0.8Gd_0.2)Cu₂O_z.

The crystal structure and thermal stability of Ba₄CaCu₃O_{8 + δ} and Ba₆CaCu₃O_{10 + δ} been investigated by neutron powder diffraction, differential thermal analysis and thermogravimetry. Both phases are metastable at low temperature in p(O₂) = l bar although they can be obtained with normal cooling conditions after sintering. Ba₄CaCu₃O_{8 + δ} has a cubic structure (Im3m, a = 8.1452(1) for δ = 0.7) for high oxygen contents as reported in the literature but undergoes a transformation into a tetragonal structure (P4/mmm, a = 8.1976(3), c = 8.0709(3) for δ = - 0.8) when the oxygen content is lowered. The structure of Ba₆CaCu₃O_{10 + δ} is confirmed (I4/mmm, a = 4.0416(1), c = 21.7884(7) for δ = −0.4) with however some differences with previously reported results

The effect of Zr and Zn doping on transport critical current density and microstructure of filamentary Dy-Ba-Cu-O superconductor was investigated. Precursor filaments prepared by a solution spinning method were partially melted in atmosphere gas and oxygenated. Samples doped with Zr and Zn showed dense microstructure, and finely dispersed Dy₂BaCuO₅ particles in the Dy123 matrix were observed. It was found that the superconducting properties of Dy123 filaments doped with Zr and Zn were strongly dependent on the heating condition and the atmosphere. A high J_c value more than 10⁴ A/cm² with good reproducibility was obtained when these samples partially melted in flowing 20%O₂ + Ar gas. By controlling the heat treatment condition, the maximum J_c value of 2.7 × 10⁴ A/cm² at 77 K and self-field were attained for the sample doped with 0.1 at % Zr. In the case of the sample doped with 0.1 at % Zn, the J_c value around 2.5 × 10⁴ A/cm² was obtained, which was comparable with that of sample without doping. The relation between the field dependence of J_c and microstructure of the filamentary sample was also examined.

We have investigated the effect of Na substitution at Ca sites on the lattice parameters, electrical resistivity, magnetic susceptibility and thermopower coefficient for TlBa₂Ca_2-xNa_xCu₃O_9-δ; with 0 ⩽ x ⩽ 0.4. The lattice parameters elongate with increasing the Na-content. A little enhancement in the superconducting transition temperature T_c has been observed as x increases from 0 to 0.05 and then it depresses gradually for x > 0.05. This observation was attributed to the fact that Na has converted the Tl- 1223 phase from over-doped regime to under-doped regime which is in conformity with the thermopower results, that found to change from negative (over-doped regime) to positive (under-doped regime). The inter-grain critical current density as function of Na has been also reported from ac magnetic susceptibility measurements.

The microwave response of three high-density bulk MgB₂ samples has been investigated in the linear and nonlinear regimes. The three samples, characterized by different mean size of grains, have been obtained by reactive infiltration of liquid Mg in powdered B preforms. The linear response has been studied by measuring the microwave surface impedance; the nonlinear response by detecting the power radiated by the sample at the second-harmonic frequency of the driving field. Our results suggest that bulk MgB₂ prepared by the liquid Mg infiltration technique is particularly promising for manufacturing resonant cavities operating at microwave frequencies.

Al-alloyed MgB₂ was prepared by solid state reaction of Mg_1-xAl_x alloy and crystalline B powder particles. The nominal composition of the samples was Mg_1-xAl_xB₂, with x = 0, 0.1, 0.2, 0.3. Residual resistivity (ρ₀) of the samples measured by four-probe technique is comparable to the single crystals, indicating the excellent quality of these samples. The lattice parameters c and a decrease at a rate of 1.15 pm and 0.17 pm per at.% Al, respectively. Superconducting transition temperature (T_c) determined by measuring the resistive superconducting transition, decreases with increasing Al alloying at a rate of 1.56 K/at.% Al. Microstructure and chemical composition have been investigated by scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and transmission electron microscopy (TEM). Al is incorporated into MgB₂ grains of ∼1 µm size by substitution of Mg lattice sites with Al. Al was found to be distributed inhomogenously which partially explains the broadening of the superconducting transition width (ΔT_c) with increasing Al mole fraction. Al alloyed samples contain large (∼15 µm) and small (∼ 2 µm ) secondary phases embedded in the (Mg, Al)B₂ matrix. The composition of large secondary phases is found to be (Mg, Al)B_7+δ, with the mole fraction of excess boron (δ) increasing from 0.99 at.% in pure sample to 4.14 at.% in highest alloyed sample. The Al to Mg mole fraction ratio in these large secondary phases is about half of that in the matrix. The size and density of the large secondary phases increased with increase in Al mole fraction. The secondary phases constitute less than 4% of the total sample volume and are thus not likely to affect the bulk superconductivity.

J_c enhancements in M/MgB₂ (M = Ag, Cu and Nb) system at 20 K and 1 T were first observed in the sintered bulk samples with small amount of M. They were prepared from the mixture of Mg, B and M_mO_n in the 10%H₂/Ar atmosphere at 900 °C for 30 minutes. The largest J_c of M = Ag, Cu and Nb system were 1.33-, 1.48- and 1.70-times larger than that of pure MgB₂, respectively.

Porous magnesium diboride samples have been prepared by heat treatment of a pressed mixture of Mg and MgB₂ powders. It was found that linked superconducting structure is formed down to the minimum normalized density γ_c = d/d_o ≃ 0.16 (percolation threshold), where d is the density of MgB₂ averaged over the sample, d_o = 2.62 g/cm³ is the X-ray density. The critical temperature of the porous samples decreases with increasing porosity (decreasing γ) and T_c2 ≃ 32K is minimal at γ_c ≃ 0.16.

The developed method of high-pressure synthesis (HPS) allows producing nanostructural highly dense material based on MGB₂, which possesses the highest superconducting and mechanical characteristics among the known world analogues, in the form of blocks that are suitable for application in SC electromotors and pumps. Additions of Zr can increase critical current density (j_c) of synthesized at 2 GPa and 750-800 °C MGB₂ in the same manner as additions of Ta or Ti, i.e. due to the absorption of impurity hydrogen forming the ZrH₂. The formation of ZrB₂ phase at higher synthesis temperatures (about 950 °C) in HPS MGB₂ does not result in the j_c increase. Some increase in j_c of HPS MGB₂ at 10 K in the fields higher than 8 T was observed when nano-SiC was added. The additions of Zr, Ta or Ti can prevent the harmful MgH₂ impurity phase from appearing and hydrogen from being introduced into the material structure. Besides, the presence of additions in HPS MGB₂ promotes the formation of a larger amount of Mg-B (most likely MGB₂) inclusions in the Mg-B-O material ''matrix'' that in turn leads to the increase of j_c of the material in magnetic fields.

We report on a thorough characterisation of MgB₂ bulk superconductors doped with 10 at% Ti (T_c = 36.54 K) or 10 at% Zr (T_c = 37.10 K). The upper critical field was determined resistively. It extrapolates to 28 T at 0 K in both cases and is, therefore, considerably larger than for pure MgB₂ (∼20 T at 0 K). The irreversibility lines were also determined resistively (offset of the resistive transition). The irreversibility fields are much higher than in pure MgB₂ and reach, e.g., 15 T at 7.5 K in MgB₂ (10 at% Ti). The critical current densities were determined from ac measurements employing the flux profile technique. They are enhanced by a factor of 2 and more at 20 K compared to pure MgB₂. Finally, we report on results obtained after neutron irradiation of the samples, which show further significant improvements of their transport properties, especially at low temperatures and high magnetic fields.

When bulk RE-BCO superconductors are used as permanent magnets in engineering applications, they are likely to experience transient variations of the applied magnetic field. The resulting vortex motion may cause a significant temperature increase. As a consequence the initial trapped flux is reduced. In the present work, we first focus on the cause of a temperature increase. The temperature distribution within a superconducting finite cylinder subjected to an alternating magnetic field is theoretically predicted. Results are compared to experimental data obtained by two temperature sensors attached to a bulk YBCO pellet. Second, we consider curative methods for reducing the effect of heat flux on the temperature increase. Hall-probe mappings on YBCO samples maintained out of the thermal equilibrium are performed for two different morphologies : a plain single domain and a single domain with a regularly spaced hole array. The drilled single-domain displays a trapped induction which is weakly affected by the local heating while displaying a high trapped field.

This communication reports experimental results on the superconducting properties of YBaCuO thick films prepared by electrophoretic deposition on silver substrates. The magnetic shielding properties of the coatings were characterised by various methods. First, the electrical resistance and the transport critical current density, J_c, were determined. Our coatings exhibit a superconducting transition at a temperature of 90 K. Next, shielding characterisations were carried out at 77 K for samples having either a slab or a cylindrical geometry. In both cases, the frequency of the applied magnetic field was 103 Hz; the field behind the shielding wall was measured by a pick-up coil connected to a lock-in amplifier. In the case of cylindrical samples and for an applied induction lower than 1 G, the field inside the shielding enclosure is reduced by a factor greater than 10⁶ (i.e. 120 dB) with respect to the applied field.

Mechanical stress is introduced into the bulk of large YBCO single domains during oxygenation, mainly because of the tetragonal to orthorhombic phase transition. Due to the slow oxygen diffusion rate, full oxygenation is nearly impossible in reasonably large samples without the occurence of macro-cracks. An alternative morphology of the bulk, i.e. a periodic array of holes drilled parallel to the c-axis prior to oxygenation, could lead to a solution of this problem, because the perforation limits the wall thickness (diffusion distance) to the distance between the holes and provides additional paths for oxygen flow during processing. Conventional flux density mapping of the fully magnetised bulks showed, that the artificial holes were beneficial for the material properties, resulting in an enhancement of the trapped flux density compared to reference samples without these holes. However, variations of the flux density profile with the periodicity of the perforation are observable at higher resolution. This indicates that the array of holes influences the local current distribution. The bulks were also investigated more locally, by the magnetoscan technique. These measurements clearly display the array of holes and provide additional information about the cracks.

We present the latest progress of our computation method for critical currents in bulk YBCO samples from magnetic field measurements with Hall probe, based on discretization of the magnetization M and resolution of the linearized inverse Biot-Savart problem by QR inversion. While computationally harder than other linear/FFT methods, it is more robust in front of measurement errors, and allows the computation of detailed current maps without assuming any symmetry in the OXY plane. In particular, we are able to detect irregularities such as multiple domains, or the perturbation induced by a welding. If the current in the bulk sample is not homogenous along the OZ axis, we obtain the average along the OZ axis of the current density. Moreover, the inhomogeneity of the current along the vertical axis can be detected by the user by measuring the magnetic field on the side of the bulk sample and comparing it with that induced by the computed OZ-averaged current. Finally, we discuss what must be done in the measurement of the magnetic field and mathematical processing of the data to avoid numerical pitfalls, and discuss the limits to the computation of 3-dimensional current maps.

Recently, particles with the chemical composition Y₂Ba₄CuMO_x where M = U, Nb, Zr, etc., and sizes in the range of 50 - 200 nm have been generated within the YBCO matrix of bulk, melt-processed superconductors in order to serve as effective flux pinning sites. By means of AFM and electron backscatter diffraction (EBSD) measurements, we analyse the spatial distribution and the size distribution of these nanoparticles within the superconducting YBCO matrix.

Local grain orientations were studied in melt-textured YBCO samples processed with various amounts of depleted uranuim oxide (DU) and Y₂O₃ by means of electron backscatter diffraction (EBSD) analysis. The addition of DU leads to the formation of Ucontaining nanoparticles (Y₂Ba₄CuUO_x) with sizes of around 200 nm, embedded in the superconducting Y-123 matrix. The orientation of the Y₂BaCuO₅ (Y-211) particles, which are also present in the YBCO bulk microstructure, is generally random as is the case in other melttextured Y-123 samples. The presence of Y-211 particles, however, also affects the orientation of the Y-123 matrix in these samples.

In the present work specific heat measurements have been carried out for the Mn-substituted compounds Y_0.95Pr_0.05Ba₂(Cu_1-xMn_x)₃O_y (0 ⩽ x ⩽ 2%) using the semi adiabatic heat pulse method. Resistivity measurements indicate that resistivity increases with Mn concentration. In the present setup, we could get absolute accuracy in specific heat of about 0.5% for samples of about few hundred mg. The present results show that specific heat jump of 3.8 J/mol K, was observed only for pure Y-123 sample. However no jump was observed in other samples.

We report nucleation and growth of dense MgB₂ phase in two advanced methods for compacting MgB₂ powder: hot isostatic pressing (HIP) and resistive sintering (RS). Both methods produce a compact with nearly theoretical mass density and high critical current density: up to 8·10⁵ A/cm² at 20 K. A liquid phase is responsible for the propagation of dense MgB₂. The additions of Mg and Ni are beneficial for rapid formation of dense compact. The process of compacting is further improved by introducing single crystal-dense MgB₂ seeds.

Progress in pulse magnetization technique for high-temperature superconductor bulks of melt-textured RE-Ba-Cu-O with large diameter is important for the realization of power applications. We studied the pulsed power source and pulsed field waveforms to enhance to improve the magnetization properties for Gd-Ba-Cu-O bulk. The risetime and duration of pulse waveform effectively varied distribution of magnetic flux.

Comprehensive mechanical characterisation of stainless steel reinforced Bi2223 superconducting tapes was conducted using purpose built rigs. This included the effect of axial (tensile and compressive) and bending strain on the critical current (I_c) of reinforced Bi2223 tape. In addition, the axial tensile and transverse compressive stress properties of the tape were measured and evaluated. It was found that the I_c-bending strain characteristic of the tape was improved as a result of sheath reinforcement, compared to an identical tape without reinforcement. A mechanism for this improvement based on the homogenising effect of the reinforcement on strain is proposed. To avoid I_c degradation, the maximum axial stress on the reinforced tape should not exceed 350 MPa, or the tensile strain should be less than 0.65%. Furthermore, no I_c degradation was observed for transverse compressive stresses up to 100 MPa, while an axial compressive strain of about 0.3% causes about 10% drop in the critical current.

YBCO coated conductor has high prospects for coils used in high magnetic fields, but not only its higher transport characteristics but also its adequate workability are required for coil applications. In these studies, mechanical properties of YBCO tape fabricated by multistage chemical vapor deposition were investigated by flat-wise and edgewise bend strain tests. After estimation of influences of its bend strain and its self magnetic field, a small coil of the long YBCO tape could be manufactured. Basic characteristics of the coil were investigated and maximum magnetic field of 0.4 T class was achieved in decompressed liquid nitrogen by the small YBCO coil.

We have studied the magnetic dependence of the surface resistance (Rs) of YBa₂Cu₃Oy (YBCO) thin films by changing the direction of an applied magnetic field by mean of a micriostrip line resonator method (MLRM). We measured Rs(0), Rs(90) and Rs(45) to which the direction of the applied magnetic field was respectively normal, parallel and at 45° to the film surface. In the low temperature region, (below 40 K), the Rs(0) had low magnetic dependence; however, the Rs(90) and Rs(45) had high magnetic dependence, even below 10 K. We examined the magnetic field dependence of Rs (90) and Rs(0) using the London equation, and found that Rs(90) in the higher temperature region could be mostly explained by the theory.

Recently, evidence has been presented by this group that the measured intermodulation distortion (IMD) in YBa₂Cu₃O_7-x (YBCO) films of high quality is consistent with the intrinsic limit, given by the d-wave order parameter. This intrinsic nonlinearity is also known as the nonlinear Meissner effect. The measurements and theory of the IMD agree in magnitude, temperature dependence, and power dependence. We present our latest results of IMD measurements that strongly support the agreement with the d-wave theory. Thus, a detailed understanding of the nonlinearity in YBCO is emerging from the measurements and calculations. We present results of modeling the IMD in YBCO resonators with very good agreement with measurement. We also present simulations of the IMD in multipole filters assuming intrinsic nonlinearity. While the IMD results depend on geometry and filter design parameters, our simulation results can be used to draw some conclusions regarding IMD in filters and the effect on the dynamic ranges of systems with YBCO filters at the front end.

Influence of ferromagnetism of the substrates on transport current losses in HoBaCuO-123 tape-shaped coated conductors with ferromagnetic Ni-alloy substrates was investigated by measuring the losses as a function of an external DC magnetic field. The losses were measured by electric and calorimetric methods simultaneously to prove the validity of the measured data. The experimental results showed significant influence of ferromagnetism of the substrates, when the substrates were not magnetically saturated by an external DC magnetic field. On the other hand, when the substrates were saturated, the loss characteristics were less influenced.

DC current-voltage characteristics and transport ac losses of striated and non-striated Y₁Ba₂Cu₃O_7-δ (YBCO) coated conductors have been measured. Transport ac loss measurements have been performed in frequencies from 40 Hz up to 2 kHz at 77.4 K. Degradation in the critical current of about 40% due to the laser striation process has been found. The striated sample was much more thermally stable than the non-striated one. Transport ac losses were weakly frequency dependent up to 2 kHz, and mainly determined by the losses in the substrate and Ni layer up to currents of about 50% of the critical current. Striation had no effect on normalized transport ac losses, which are basically hysteretic. Comparison of transport ac losses with in-field losses showed that the dominant loss contribution is the in-field loss.

We derive exact analytic representations of the Meissner state and the critical state of a current-carrying superconductor strip located inside a cylindrical magnetic cavity. Our results show that, when the distance between the superconductor and the magnet is small, the penetration of magnetic flux fronts is strongly reduced as compared to the respective situation in an isolated strip. Even for total currents almost matching the critical current of the strip, the major part of the superconductor remains flux free, so that AC losses in it are greatly depressed; a fact which facilitates the use of such a configuration as a coaxial-type transmission line. From our generic representation it is possible to derive current profiles in cavities of various other closed geometries too by means of conformal mapping of the basic configuration addressed.

A study of the time-dependent electric field, E(t), and E-I curves of BSCCO/Ag tapes at 77 K is presented. Measurements were performed on tapes carrying DC transport current and exposed to external AC (0-500 G, 4-400 Hz) magnetic fields perpendicular to the broad side of the tape. E-I curves measured at frequencies exceeding 50 Hz and at amplitudes of the AC field below 400 G are well described by power law in the range between 0.1 and 10 µV/cm of the electric field. The results within this domain of parameters are discussed, assuming a narrowing down of the DC current path under the application of a perpendicular AC field. Nearly linear dependency of electric field on transport current was observed at the frequency-amplitude domain below 10 Hz and above 500 G. Using magneto-optical techniques we are able to determine that in this domain the AC magnetic field fully penetrates the tape. Behaviour of E-I curves under these conditions can be elucidated using the ''dynamic resistance'' model in conjunction with the assumption that the magnetic field fully penetrates the superconductor.

We study the magnetization of a cylindrical type-II superconductor filament covered by a coaxial soft-magnet sheath and exposed to an applied transverse magnetic field. Examining penetration of magnetic flux into the superconductor core of the filament on the basis of the Bean model of the critical state, we find that the presence of a non-hysteretic magnetic sheath can strongly enhance the field of full penetration of magnetic flux. The average magnetization of the superconductor/magnet heterostructure under consideration and hysteresis AC losses in the core of the filament are calculated as well.

The AC loss performance of two BSCCO and two YBCO conductors of different geometry, characterized by the same self-field critical current of 150 A, is analysed and compared quantitatively. The comparison is made using the finite-element method with a nonlinear B-dependent E-J relation. A new shell-region model is utilised for the simulations of thin YBCO strips. Different AC working conditions are simulated: self-field, applied external field, and combined transport current and external perpendicular field application. Magnetic field and current density profiles are investigated in order to illustrate the reasons for the loss difference in the conductors. Depending on the application, the advantages of using BSCCO or YBCO conductors with specific geometry are outlined.

We have been considering about standard methods to estimate total AC loss in oxide-superconducting tapes under practical electromagnetic conditions in future power devices. We adopt 4 candidates of the standard methods to measure the AC loss in superconducting wires exposed to AC transverse magnetic field and alternating transport current. We also introduce two pickup coil methods for comparative references of external magnetic field loss. The specimen is a Bi-2223 Ag-alloy-sheathed multifilamentary tape without twisting. 60-100 mm short pieces of specimen were cut from a terminal of a 50 m long specimen for 4 in 6 methods. A double-layer non-inductive coiled specimen was also prepared from the long specimen for the other 2 methods. The scattering in AC loss property among the short specimens prepared is less than 1% in the perpendicular field with the amplitude from 0.01 to 0.1 T at 10 Hz. The scattering in repeated measurements of the loss during 3 heat cycles is within 2-3%, which almost corresponds to that of the critical current in the long specimen. We processed the results measured by each method for the external field loss and total AC loss, summarized dispersion among the observed results of all methods and discussed about the candidates of the standard methods.

We studied the magnetization losses of four different types of filamentary YBCO coated conductors. A 10 mm wide YBCO coated conductor was subdivided into 20 filaments by laser ablation. We measured the frequency dependence of the total losses in the frequency range 0.1 Hz < f < 608 Hz. The coupling loss was obtained from the total by subtracting the hysteresis loss. The latter was measured at low frequencies since only hysteresis loss is nonnegligible at frequencies below 1 Hz. The transverse resistivity, ρ_tr, was determined from the coupling losses; it was assumed that the sample length is equal to half of the twist pitch. The values of ρ_tr deduced from the loss data were compared with those obtained by the four-point measurements with current flowing perpendicular to the filaments. Preliminary results indicate the existence of electrical contacts between the superconducting filaments and the substrate in some areas of the samples. This was also confirmed by the Hall probe mapping of the magnetic field in the vicinity of the tape. The measured transverse resistivity was close to the resistivity of the substrate (Hastelloy).

HTS bulk can trap higher magnetic field than a permanent magnet and electric motor and actuators are promising applications. The HTS bulks in those electric machines are exposed to AC magnetic field perturbations, which cause AC losses in the bulks. Therefore, it is important to understand the AC loss characteristics. In the previous works, we measured the AC losses by an electric method using the double pick-up coils method. This time, we measured the AC losses by a calorimetric method using a cryo-cooler and heater at different temperature. In this paper, the AC losses measured by the electric and calorimetric methods are compared with each other to investigate the validity of the measurement methods.

Magnetization relaxation in superconductors is under the study from the very beginning of superconductivity. Now the knowledge about the nature of flux creep is suggested and confirmed by many experiments, but mainly by macromeasurement of average magnetization. Studying the relaxation in finite size superconductors, melt textured and single crystals YBCO, we have found the relaxation is determined not only by material properties, pinning center sizes, dimensionality and distribution, but sample size and shape as well as magnetic flux distribution. Magneto-optic technique, used to visualize the spatial magnetic flux distribution and redistribution in superconductors during relaxation, allowed us to understand the origin of these effects.

Theoretical analysis of the irreversibility fields B_irr was obtained from the flux creep theory based on a depinning mechanism caused by thermally activated flux creep. Near the critical temperature T_c, the calculated B_irr depends on power of ( 1-(T/T_c)²). The measured B_irr, however, increases more rapidly than the power law at low temperature. This enhancement from the power law in low temperatures and high fields has been ascribed to a different pinning mechanism. The experimental temperature dependence of B_irr is obtained from the AC susceptibility measurements in Hg-based composites of Ag_x(HgBa_1.9Bi_0.1Ca₂Cu₃O_8+δ)_1-x(x = 0.1 and 0.3) superconductors. AC and DC magnetizations were measured using a SQUID magnetometer and a PPMS susceptometer at temperature range 5-150 K under magnetic fields up to 14 T. The irreversibility fields B_irr are estimated from the peaks of imaginary parts of AC susceptibilities and shown to agree well with the numerical estimation of the original equation. The magnetization characteristics are also successfully obtained from the pinning parameters.

High T_c superconductors (HTSCs) for the application of magnets and cables are usually used in a magnetic field parallel to the superconducting layers, using the intrinsic pinning of vortices in the layered structures. However, vortex states in the parallel field have not been well understood in strongly anisotropic superconductors. We have studied the vortex states by measuring the flow resistivity of Josephson vortices (JVs). From the measurements, we can determine the 3D-ordered phase of JVs, which means that JVs are well ordered and are considered not to influence the dissipation of the superconducting state. However, by the R-T measurements, we have found the excitation of pancake vortex/anti-vortex pairs to reduce the flow resistance. This limits the application of HTSC materials in the parallel magnetic fields.

We report on detailed investigations of the mixed state in the mercury based high-T_c superconductor HgBa₂CuO₄ (Hg-1201) by means of ac and dc magnetization measurements. Over large parts of the superconducting phase diagram the vortex mobility turned out to be remarkably high. From knowledge of the phase boundaries and the irreversibility line also determined, it became clear that in Hg-1201 thermal activation strongly supports the flux diffusion in both liquid and solid vortex phases. Thermally assisted flux diffusion widely stops here only for temperatures well below the melting line. In particular, the strong decrease of vortex mobility within the solid phase suggests a transition between different regimes originating from a different nature of vortex pinning, i.e. collective or single, and indicates a weak to strong pinning crossover of vortex matter in pure Hg-1201. A phenomenological description of the flux dynamics based on the classic diffusion formalism has been performed.

Interaction of vortex matter with defects arranged in a mesh minimally ordered have been conducted in a number of different superconducting systems possesing the unusual feature of exhibiting a fairly narrow distribution of weak-link critical currents. This contribution reports on the common characteristic, detected for all studied samples, of a field-dependent threshold temperature T*(H) at which vortex dimensionality changes. This feature was revealed by measurements of AC-susceptibility at different excitation fields, as well as by magneto-transport experiments in a 6-terminal flux-transformer arrangement. The reported behavior is a consequence of a very narrow distribution of weak-link critical currents. Although extrinsic in nature, H*(T) is a characteristic border on the magnetic phase diagram of such class of samples, being thus of practical relevance to study this boundary, above which the effectiveness of flux pinning is considerably reduced. Results on selected superconducting systems are numerous, three of which are summarily presented in this article.

We propose a description of the transition of YBa₂Cu₃O_7-δ films from the superconducting to the normal state. The discontinuity observed in the current-voltage curves at the transition is explained by the sudden increase in the dissipated power rate due to vortex depinning. However, near T_c, this phenomenon does not occur because the vortex activation energy in this range of temperature is near zero. We also show how the current at the transition to the normal state is related to the critical current of the films. The predictions of this description are compared to the data published by González et al. [Phys.Rev. B 68, 054514 (2003)].

Flux cutting, where two contiguous flux lines can interact and exchange line segments, has been studied in low T_c superconductors since the 1970s. Recently there has been renewed interest in this phenomenon [Olsen-Reichardt C and Hastings M. B. PRL 92 (15): 157002 (2004)]. We present studies of flux channelling in both extrinsic and intrinsic geometries which demonstrate that flux cutting occurs in the cuprate superconductor YBa₂Cu₃O_7-δ. The angular dependence of the flux cutting effect can be straightforwardly modelled by considering a length independent cutting force, f_cut per vortex segment. We describe the behaviour of the weakly pinned vortex segments prior to cutting and relate the experimentally derived cutting force to the properties of the vortex line.

Systematic slow transport relaxation (V-t curves) and magnetovoltage measurements (V-H curves) have been carried out in a carved out superconducting polycrystalline Bi-Sr-Ca-Cu-O sample as a function of current (I), temperature (T), and external field (H). The V-t curves reveal the details of the time evolution of the penetrated state within the granular structure of the sample and also give a direct evidence of the relaxation of the flux trapped inside the drilled hole on the time scale of the experiment. On the other hand, V-H curves exhibit several unusual interesting properties upon cycling of the external magnetic field in forward and reverse directions, and, in addition to irreversibilities, strong reversible effects are observed, which is associated with the trapping of the macroscopic flux bundles in the drilled hole. It is also observed that the field sweep rate influences dramatically the reversible and irreversible behavior of V-H curves. The experimental results were mainly interpreted in terms of current and field induced organization of the vortices.

Magnetovoltage measurements (V-H curves) were carried out in superconducting polycrystalline bulk Y₁Ba₂Cu₃O_7-x (YBCO) material as a function of current (I), temperature (T), field sweep rate (dH/dt) and field orientation with respect to the transport current. A relative decrease in the dissipation measured in V-H curves was observed as dH/dt is increased, which implies that the time spent to plot the whole cycle has an importance on the evolution of the V-H curves. Thus, it could be possible to observe the relaxation effects in magnetovoltage measurements. In addition, the several significant steps and plateaus in V-H curves evolve depending on the magnitude of the transport current and also dH/dt. These observations were attributed to locking of the flux lines to decrease or increase in size of the easy motion flow channels. The strong hysteresis effects in V-H curves were discussed mainly by means of the flux trapping within the granularity of sample and the different degree of the inhomogeneous flux motion with respect to the sweeping of the external magnetic field up and down.

The problem of vortex depinning from extended linear defect in 3D -anisotropic superconductor under the Lorentz force influence is considered for the case T =0. We treat it as a problem of classical mechanics behavior of an elastic vortex string which is settled in a potential well of linear defect and exerted to the Lorentz force action on its end within the screening layer of width λ (λ - is the London penetration depth) near the specimen surface. We search the stability problem and obtain conditions for the instability threshold, as well as the critical current density j_c, at which the instability leading to vortex depinning first emerges near the surface and then propagates inside the superconductor. This scenario of vortex depinning mechanism at low temperatures is strongly supported by some recent experiments.

The influence of magnetic field and microwave irradiation on dynamical phase separation in submicron Bi₂Sr₂CaCu₂O_8+x bridges has been studied. Strong effect on the shape and metastable character of the step-like I-V characteristics are found. Under a weak field H < 2 Oe and low level microwave irradiation the step-like structure of the I-V characteristics smears out and disappears completely. The average frequency of switching between metastable states grows by 5 orders under increase of magnetic field by only 1 Oe. This behavior is explained in terms of the model of dynamical vortex lines.

Experimentally, multivortex states and giant vortex states in mesoscopic superconductors can be distinguished directly by using the multiple-small-tunnel-junctions, and indirectly by studying the temperature dependence of the expulsion fields. These experimental results are compared with the theoretical prediction from the nonlinear Ginzburg- Landau theory.

We report on the effects of neutron irradiation on the irreversible magnetic properties of MgB₂ single crystals. The size of the newly created defects is comparable to the superconducting coherence length of MgB₂, which makes these defects particularly suitable for pinning flux lines. Indeed, we observe significant quantitative and qualitative modifications of the critical current density. In particular, a second peak (fishtail effects) emerges in the field dependence and is accompanied by history effects. The fishtail effect is thoroughly studied with respect to different neutron fluences and good agreement with a recent theoretical explanation by an order-disorder transition is obtained.

Measurements of higher harmonics of the ac voltage response in YBCO thin film strips under low amplitude and low frequency harmonic excitation, as a function of temperature, show a non linear response of the conductivity in the superconducting transition interval. The third and fifth harmonics of the local voltage as a function of T exhibit a peak near T_c and their amplitudes seem to be closely related to the T-derivative of the first harmonic. The peaks are linearly dependent on the current amplitude and do not depend on frequency. The observed data are partially interpreted in terms of ac current induced thermal modulation of the sample temperature added to strong thermally activated fluctuations in the transition region. The fit of the model to the data gives information of some sample properties such as zero temperature critical current, zero onset resistance and thermal boundary conductance.

In the paper is investigated the interaction of the nanoscale pinning centers with vortices in layered HTc superconductors, which interaction determines the critical current. Capturing of the vortices in parallel magnetic field in the buffer layer of nanometer thickness is analyzed. In perpendicular magnetic field the pinning of the pancake vortices on the nanodimensional pinning centers is considered. The potential barrier height, current-voltage characteristics, critical current and pinning forces of the HTc superconductors are determined. Important result of the considerations is the statement that for too small dimensions of the pinning centers the critical current vanishes, what has essential meaning from the point of view of the superconducting tapes technological process. In the total energy balance the elastic energy of the vortices lattice has been taken into account, while for calculations of the pinning forces the statistical occupation of the pinning centers by vortices considered. In the paper the author's works devoted to the construction of the model of the fault current limiter will be reviewed too.

The hysteretic magnetic flux dynamics of ceramic type-II superconductors under superimposed transport current cycles has been investigated.

Field cooled and zero field cooled experiments were carried-out on polycrystalline YBa₂Cu₃O_7-x cylinders by using both temperature and applied field control, to drive the sample into the semi-reversible granular regime. The transport current magnitude was near and above the dissipative threshold. The interplay between shielding and transport due to vortex-pinning interactions, equilibrium magnetization effects, and diamagnetic/paramagnetic relaxation phenomena are shown to play an important role in the range of practical applications.

We report current voltage curves (CVC) on c-axis oriented YBa₂Cu₃O_7-δ superconducting thin films patterned in several microbridges of different width and the same width/length ratio (1/10). The microbridge widths range from 5 µm to 100 µm. Our present measurements extend previous experiments performed in our research group to microbridges with different geometries. Measurements of CVC spanned the whole range: from the critical current J_c (when the dissipation starts) to the supercritical current J* (when an abrupt jump to a highly dissipative state occurs). We have found that J_c values do not change for all the microbridges of the same film whereas J* decreases as the microbridge width increases. These findings indicate that near J_c the current is almost uniformly distributed over all the microbridge volume. Moreover, although our results do not exclude an intrinsic origin for the transition to the normal state induced by high current densities, they suggest that J* could be deeply affected by the thermal interchange between the films and their substrates.

Resistivity measurements on ultrathin (36 and 42 nm thick) YBCO films were performed with a novel pulsed-current technique that allows for application of current densities up to several MA/cm² with minimal self-heating even in the resistive state. Using current pulses as short as 50 ns we found that the onset of resistance in the I-V curves takes place at substantially higher current densities as compared to results achieved with the commonly used d.c. technique. Both the temperature dependence and the absolute values for such critical current measurements indicate that indeed the depairing current has been probed. The results are in quantitative agreement with the predictions from Ginzburg-Landau theory. Thus, our measurements can determine the ultimate thermodynamic limit for dissipation-less current transport in a high-temperature superconductor that is more than an order of magnitude larger than previous d.c. technique results.

New phenomena - peak-effect and angular hysteresis - in field/angle J_c(H, θ) dependencies are detected for YBCO epitaxial films at moderate dc magnetic fields H parallel to the film. Films (300-350 nm thick) are deposited by off-axis dc magnetron sputtering onto rcut sapphire substrate buffered with CeO₂. Surface roughness (peak-to-valley) determined by AFM does not exceed 2 nm. J_c(H, θ)-curves are measured by low-frequency ac magnetic susceptibility and four-probe transport technique. J_c(H) at H∥ab-plane for the most smooth films reveal dome-shape enhancement of J_c(up to 10 p.c.) above J_c(0) value, starting from the field H* ascribed to the first critical field H_c1 of thin film. J_c(H)-plots at H∥c-axis with a plateau at low fields followed by monotonic fall-down are consistent to our model of vortex lattice depinning from the out-of-plane linear defect network (growth-induced edge dislocations). Field dependencies of J_c at arbitrary inclination angles may be recalculated from J_c(H, θ = 0) and J_c(H, θ = π/2), assuming independent effects of normal Hcosθ and parallel Hsinθ field components on J_c. Angle J_c(θ)-dependencies evolution with H is shown to be consistent with dominant mechanism of pinning on edge dislocations. The most surprizing feature of this evolution is emergence of the peak in J_c(θ)-dependence for H∥c-direction, becoming observable only above threshold magnetic field H_p dependent on film thickness and surface roughness. Angular hysteresis in J_c(H, θ) dependence is detected for magnetic field directions close to H∥ab-plane. This hysteresis is sensitive to magnetic/angular pre-history and together with observed peak-effect at H∥ab-plane can be understood by account for surface (and/or geometrical) barrier as additional pinning source for Abrikosov vortices.

The measurement of energy dissipation in superconducting high T_c tapes in an external ac-field is of major concern for their technical application at 50 Hz. The widely used technique is based on the measurement of the magnetization loop which is proportional to the dissipated energy. Since the proportionality constant depends on the geometry of the sample and of the pick-up coils used, calibration of the system is necessary. A convenient way is the calibration with a calorimetric measurement which however leads, at 77 K, to the problem of long thermal time constants. It requires furthermore a special preparation of the sample which must be thermally insulated from the cooling bath. This paper gives a simple relationship between the sample geometry and the calorimetric calibration factor, empirically deduced from measurements on tapes and stacks of tapes with rectangular cross sections. The accuracy of the method is within about ±10 %, which is sufficient for most technical applications.