Showing papers in "Superconductor Science and Technology in 2003"

The widely variable resistivity of MgB2 samples

Abstract: In this brief review, I consider a rather mundane property of MgB2, namely its resistivity as a function of temperature. It turns out that a comparison of resistivity data for a wide variety of samples, from single crystals to films, wires and polycrystalline bulk, is surprisingly informative. The majority of samples of MgB2 exhibit resistivities that are much higher than the low values that are seen in a relatively small number of single crystals, bulk samples and films. In many cases, the resistivity is increased over the single crystal values by orders of magnitude. Even at these high values, there is often still a metallic temperature dependence of the resistivity, and surprisingly, Tc is often at or near the bulk value of 39 K. The resistivity increase has been ascribed to a reduction in the effective current-carrying cross-sectional area of the sample. If this loss of cross-sectional area is the dominant factor contributing to the increase in resistivity of MgB2 samples, then the critical current density must be decreased by the same loss in the effective area. Comparisons of the MgB2 resistivity with the properties of disordered 'cluster compound' superconductors, of HTS materials and of granular Al (heavily contaminated with oxygen), show similarities between the MgB2 behaviour and the results of the HTS and Al studies. I discuss various effects that might contribute to the reduction in effective sample area in MgB2 samples, to the increase in resistivity and to a reduction in Jc. I speculate that a Josephson junction model of the grain boundaries in MgB2 might apply to samples with extremely high resistivities. Alternatively, it has been suggested that the two-band nature of MgB2 can result in an unusual behaviour of its resistivity and Tc as the material changes from the 'clean' to 'dirty' limits. I conclude that measurements of both the resistivity and the transport critical current density in a wide variety of bulk and film samples would be very useful, as would a study of the resistivity and Jc changes as a function of irradiation damage in MgB2 bulk and films of initially low resistivity.

Review of a chemical approach to YBa2Cu3O7−x-coated superconductors—metalorganic deposition using trifluoroacetates

Abstract: Large-area, uniform, high critical current density (Jc) YBa2Cu3O7−x (YBCO) superconductor films are now routinely obtained by metalorganic deposition using trifluoroacetates (TFA-MOD) This method does not require any expensive vacuum apparatus at any time during the whole process Thus, TFA-MOD is regarded as one of the most suitable candidates for fabricating a YBCO tape for many high-power applications This method originated from an electron beam process using BaF2 developed by Mankiewich et al Afterwards, Gupta et al reported using TFA-MOD to prepare a similar precursor film These two ex situ processes used fluorides instead of BaCO3 to avoid the fatal deterioration in Jc, which is caused in the resulting films through metal carboxylic groups Fluorides not only avoid such deterioration but also lead to perfectly c-axis-oriented epitaxial crystal growth In conventional metalorganic deposition, nucleation in the precursor film causes random orientation in the resulting film However, in TFA-MOD, nanocrystallites in the precursor film never cause such disorder Furthermore, during the firing process of TFA-MOD, water and HF gas diffuse quickly between the film surface and growth front of the YBCO layer This diffusion never limits the growth rate of YBCO What distinguishes TFA-MOD from conventional metalorganic deposition? What happens during heat treatment? In this paper, we discuss all the TFA-MOD processes and the peculiar growth scheme of the YBCO layer in TFA-MOD using the model of a quasi-liquid network In addition, we review the history of TFA-MOD and recent results and discuss the prospects of future applications

Effect of SiO2 and SiC doping on the powder-in-tube processed MgB2 tapes

Abstract: Effect of SiO2 and SiC nano-powder doping was investigated for the powder-in-tube processed MgB2/Fe tapes. Mg or MgH2 powder was used as the Mg source of starting materials, and heat treatment was carried out at 600 °C for 1 h. These heat treatment conditions of lower temperature and shorter heating time are advantageous from the aspect of practical production processes. MgH2 powder improved the connection of MgB2 grains and prevented oxidation of MgB2. SiC and SiO2 doping greatly enhanced the critical current density (JC) values of the tapes prepared with Mg + B powder. However, only the SiC doping was effective in enhancing JC values for MgH2 + B powder. SiC doping decreased magnetic field sensitivity of JC, while SiO2 doping did not change the field dependence of JC. The SiC doped tape showed transport JC value of about 6 500 A cm−2 at 4.2 K and in the magnetic field of 12 T. The irreversibility field increased from 17 T to 23 T by the SiC doping.

High performance new MgB2 superconducting hollow wires

Abstract: MgB2 hollow wires have been produced with a new technique which uses a conventional wire manufacturing process but is applied to composite billets containing the elemental B and Mg precursors in an appropriate shape. The technique has been applied to the manufacture of both monofilamentary and multifilamentary wires of several tens of metres in length. The superconducting transport properties of the MgB2 hollow wires have been measured in a magnetic field and in the temperature range from 4.2 to 30 K. Promising results are obtained, which indicate the possibility of the application of these wires as superconductors in the temperature range of 15–30 K and at medium-high values of magnetic field.

MOD approach for the growth of epitaxial CeO2 buffer layers on biaxially textured Ni–W substrates for YBCO coated conductors

Abstract: We have grown epitaxial CeO2 buffer layers on biaxially textured Ni–W substrates for YBCO coated conductors using a newly developed metal organic decomposition (MOD) approach. Precursor solution of 0.25 M concentration was spin coated on short samples of Ni–3 at%W (Ni–W) substrates and heat-treated at 1100 °C in a gas mixture of Ar–4%H2 for 15 min. Detailed x-ray studies indicate that CeO2 films have good out-of-plane and in-plane textures with full-width-half-maximum values of 5.8° and 7.5°, respectively. High temperature in situ XRD studies show that the nucleation of CeO2 films starts at 600 °C and the growth completes within 5 min when heated at 1100 °C. SEM and AFM investigations of CeO2 films reveal a fairly dense microstructure without cracks and porosity. Highly textured YSZ barrier layers and CeO2 cap layers were deposited on MOD CeO2-buffered Ni–W substrates using rf-magnetron sputtering. Pulsed laser deposition (PLD) was used to grow YBCO films on these substrates. A critical current, Jc, of about 1.5 MA cm−2 at 77 K and self-field was obtained on YBCO (PLD)/CeO2 (sputtered)/YSZ (sputtered)/CeO2 (spin-coated)/Ni–W.

High-Tc and low-Tc dc SQUID electronics

Abstract: Superconducting quantum interference devices (SQUIDs) are commonly operated in a flux-locked loop (FLL). The SQUID electronics amplifies the small SQUID signal to an acceptable level without adding noise, and it linearizes the transfer function of the SQUID in order to provide sufficient dynamic range. In this paper, the fundamentals of SQUID readout are reviewed including a discussion of preamplifier noise. The basic FLL concepts, direct readout and flux modulation readout, are discussed both with dc bias and bias reversal. Alternative readout concepts such as additional positive feedback (APF), two-stage SQUIDs, SQUID series arrays, relaxation oscillation SQUIDs and digital SQUIDs are briefly described. The FLL dynamics are discussed on the basis of a simple model with finite loop delay. It is shown that with optimized SQUID electronics a system bandwidth of ≈18 MHz and a corresponding slew rate of ≈8 Φ0 µs−1 are possible. A novel FLL scheme involving a Smith predictor is presented which allows one to increase the FLL bandwidth to about 100 MHz. The theoretical predictions are experimentally checked using a high-speed SQUID electronics prototype with a small-signal bandwidth of 300 MHz. Methods for increasing the dynamic range of SQUID systems are described: flux-quanta counting and dynamic field compensation (DFC). With DFC, the residual magnetic field at the SQUID can be kept close to zero even if the device is moved in the Earth's field. Therefore, the noise level of a high-Tc magnetometer measured inside a magnetically shielded room (60 fT Hz−1/2 with a 1/f corner at 2 Hz) remained unchanged after moving the device in the magnetic field outside the room (60 µT dc plus 0.8 µT peak-to-peak power line interference).

The influence of Nb3Sn strand geometry on filament breakage under bend strain as revealed by metallography

Abstract: The non-Cu critical current density of Nb3Sn strands has been pushed towards 3000 A mm−2 (12 T, 4.2 K) by increasing the Sn content and reducing the inter-filamentary Cu. We compare the susceptibility to A15 filament breakage (under 0.5% bend strain) of the new high-Jc internal Sn conductor geometries with both high-Jc powder-in-tube (PIT) and low hysteresis loss distributed filament ITER designs. In all but the PIT designs, there was significant filament breakage on the tensile side of the strand cross-section with little if any evidence for cracking on the compressive side. Where there is significant inter-filamentary Cu remaining after reaction the highest frequency of A15 filament breakage is observed at the edges of the filament packs. This suggests that the breakage is most likely to occur where filaments receive less mechanical support from the filament-Cu matrix. In very high Jc strands, where individual Nb filaments coalesce into large A15 tubes during reaction, breakage can occur across the entire sub-element. In the PIT design composite, filament breakage did not occur at 0.5% bend strain. At 0.6% bend strain the PIT filaments cracked in both the tensile and compressive regions.

Uniform performance of continuously processed MOD-YBCO-coated conductors using a textured Ni–W substrate

Abstract: Second-generation coated conductor composite HTS wires have been fabricated using a continuous reel-to-reel process with deformation-textured Ni–W substrates and a metal-organic deposition process for YBa2Cu3O7−x. Earlier results on 1 m long and 1 cm wide wires with 77 K critical current performance greater than 100 A cm−1 width have now been extended to 7.5 m in length and even higher performance, with one wire at 132 and another at 127 A cm−1 width. Performance as a function of wire length is remarkably uniform, with only 2–4% standard deviation when measured on a 50 cm length scale. The length-scale dependence of the deviation is compared with a statistical calculation.

Doping effect of nano-diamond on superconductivity and flux pinning in MgB2

Abstract: The doping effect of diamond nanoparticles on the superconducting properties of MgB2 bulk material has been studied. It is found that the superconducting transition temperature Tc of MgB2 is suppressed by the diamond doping, however, the irreversibility field Hirr and the critical current density Jc are systematically enhanced. Microstructural analysis shows that the diamond-doped MgB2 superconductor consists of tightly-packed MgB2 nano-grains (~50–100 nm) with highly dispersed and uniformly distributed diamond nanoparticles (~10–20 nm) inside the grains. The high density of dislocations and diamond nanoparticles may be responsible for the enhanced flux pinning in the diamond-doped MgB2.

An improved method for quantitative magneto-optical analysis of superconductors

Abstract: We report on the analysis method to extract quantitative local electrodynamics in superconductors by means of the magneto-optical technique. First of all, we discuss the calibration procedure to convert the local light intensity values into magnetic induction field distribution and start focusing on the role played by the generally disregarded magnetic induction components parallel to the indicator film plane (in-plane field effect). To account for the reliability of the whole technique, the method used to reconstruct the electrical current density distribution is reported, together with a numerical test example. The methodology is applied to measure local magnetic field and current distributions on a typical YBa2Cu3O7−x good quality film. We show how the in-plane field influences the MO measurements, after which we present an algorithm to account for the in-plane field components. The meaningful impact of the correction on the experimental results is shown. Afterwards, we discuss some aspects about the electrodynamics of the superconducting sample.

Current distribution simulation for superconducting multi-layered structures

Abstract: The software package 3D-MLSI is developed, which allows us to calculate the current distribution and to extract inductances from multi-layered high-Tc and low-Tc superconducting circuits. Both kinetic and magnetic inductances as well as the three-dimensional distribution of the magnetic field are taken into account. We discuss the numerical approach used in 3D-MLSI and some new features such as visualization of sheet currents and analysis of circuits with holes. As an example, we present a simulation of a high-Tc double-layer transformer.

SQUIDs: some limits to measurement*

Abstract: SQUIDs are used extensively across the world of precision measurement in physics and many other sciences, including earth science studies and medicine, providing some of the most precise measurements possible. The paper will discuss some of the limits to SQUID measurements which emerge in these various fields. One of the newer areas of interest is the use of SQUIDs to measure the properties of nanomagnetic particles or even single spins. This is driven by the needs of future ultra high density magnetic storage and quantum information processing. The paper will focus on this issue and will propose some hard limits as well as some means by which the present state-of-the-art may be extended towards those limits.

Critical current degradation behaviour in Bi-2223 superconducting tapes under bending and torsion strains

Abstract: In this paper, we have investigated the Ic degradation behaviours in Ag alloy sheathed Bi-2223 superconducting tapes under bending and torsion strains. In particular, we have examined the homogeneity of Ic degradation along the longitudinal direction of tapes by adopting multiple voltage terminals. The bending modes influenced the Ic degradation behaviour in Bi-2223 tapes. A gradual and consistent decrease of Ic could be observed at all sections, which was a different behaviour from that under tensile loading. Up to eirr, Ic showed a nearly similar degradation at each section, due to a simultaneous initiation of cracks from defects indicating that uniform deformation occurred in tapes with bending. Over eirr, some variations in the Ic degradation occurred due to the difference of crack growth rates at each section. In particular, the behaviour appeared significantly with hard bending. The degradation of Ic in Bi-2223 tapes by torsion occurred gradually when the torsion angle exceeded 150°. However, the Ic degradation by torsion strain was small compared with other types of loading. Also, the Ic degradation behaviour at each section along the longitudinal direction of tapes was gradual and consistent, representing a uniform torsional deformation in the tapes.

Improved transport critical current and irreversibility fields in mono- and multifilamentary Fe/MgB2 tapes and wires using fine powders

Abstract: Mono- and multifilamentary Fe/MgB2 tapes and wires with high transport critical current densities have been prepared by the powder-in-tube technique using fine powders. The influence of the initial MgB2 grain size on critical current density, upper critical and irreversibility fields has been studied. After reducing the MgB2 grains to micrometer size by ball milling, the critical current density, Jc, was enhanced, while the upper critical field, μ0Hc2, remained unchanged. The anisotropy ratio between the upper critical fields parallel and perpendicular to the tape surface was determined to be 1.3, reflecting a deformation induced texture. A good agreement has been found between resistive and inductive Jc values, measured at various temperatures between 4.2 and 25 K. On monofilamentary tapes, Jc values close to 105 A cm−2 were measured at 25 K/1 T, while Jc values ≈106 A cm−2 were extrapolated for 4.2 K/0 T. Fe/MgB2 tapes exhibit high exponential n factors for the resistive transition: n values of 60 and 30 were found at 4 T and 6 T, respectively. Multifilamentary wires (with seven filaments) show slightly lower Jc values, 1.1 × 105 A cm−2 at 4.2 K/2 T. The improvement of thermal and mechanical stability of MgB2/Fe tapes and wires appears clearly as a challenge for future developments.

Properties of superconducting MgB 2 wires: in situ versus ex situ reaction technique

Abstract: We have fabricated a series of iron-sheathed superconducting wires prepared by the powder-in-tube technique from (MgB2)1−x:(Mg+2B)x initial powder mixtures taken with different proportions, so that x varies from 0 to 1. It turned out that ex situ prepared wire (x = 0) has considerable disadvantages compared to all the other wires in which in situ assisted (0 0. Pinning of vortices in MgB2 wires is shown to be due to grain boundaries. Jc(Ba) behaviour is governed by an interplay between the transparency of grain boundaries and the amount of ‘pinning’ grain boundaries. Differences between thermo-magnetic flux-jump instabilities in the samples and a possible threat to practical applications are also discussed. (Some figures in this article are in colour only in the electronic version)

MgB2 single crystals: high pressure growth and physical properties

Abstract: Single crystals of MgB2 with a size up to 1.5 × 0.9 × 0.2 mm3 have been grown with a high pressure cubic anvil technique. The crystal growth process is very peculiar and involves an intermediate nitride, namely MgNB9. Single crystals of BN and MgB2 grow simultaneously by a peritectic decomposition of MgNB9. Magnetic measurements with SQUID magnetometry in fields of 1–5 Oe show sharp transitions to the superconducting state at 37–38.6 K with a width of ~0.5 K. The high quality of the crystals allowed the accurate determination of magnetic, transport (electric and heat) and optical properties as well as scanning tunnelling spectroscopy (STS) and decoration studies. Investigations of crystals with torque magnetometry show that H//cc2 for high quality crystals is very low (24 kOe at 15 K) and saturates with decreasing temperature, while H//abc2 increases up to 140 kOe at 15 K. The upper critical field anisotropy γ = H//abc2/H//cc2 was found to be temperature dependent (decreasing from γ 6 at 15 K to 2.8 at 35 K). The effective anisotropy γeff, as calculated from reversible torque data near Tc, is field dependent (increasing roughly linearly from γeff 2 in zero field to 3.7 in 10 kOe). The temperature and field dependence of the anisotropy can be related to the double gap structure of MgB2 with a large two-dimensional gap and small three-dimensional gap, the latter of which is rapidly suppressed in a magnetic field. Torque magnetometry investigations also show a pronounced peak effect, which indicates an order–disorder phase transition of vortex matter. Decoration experiments and STS visualize a hexagonal vortex lattice. STS spectra in zero field evidence two gaps 3 meV and 6 meV with a weight depending on the tunnelling direction. Magneto-optic investigations in the far-infrared region with H//c show a clear signature of the smaller of the two superconducting gaps, completely disappearing only in fields higher than H//cc2.

Transport ac loss studies of YBCO coated conductors with nickel alloy substrates

Abstract: Transport alternating current (ac) loss measurements were performed on a series of rolling-assisted biaxially textured substrate (RABiTS) processed YBa2Cu3Ox (YBCO) coated conductors at 77 K. While each sample possessed a 1 µm layer of YBCO and a 3 µm silver cap layer, two different nickel alloy substrates were used and their impact on the ac loss was examined. Both substrates possessed a 75 µm Ni–5 at%W base, but one substrate also had a 2 µm nickel overlayer as part of the buffer layer architecture. The ac losses, which were determined by thermal and electrical measurements, contained two dominant contributions: superconductive hysteresis in the YBCO and ferromagnetic hysteresis in the substrates. The superconductive component followed the Norris elliptic model for the substrate with the nickel overlayer and the Norris thin strip model for the substrate without the nickel overlayer. The substrates' ferromagnetic loss was determined separately through magnetization measurements, which showed that this loss contribution was independent of the presence of the nickel overlayer for effective ac currents less than 50 A. While the overall loss was lower for the thin-strip-like conductor with no nickel overlayer, further research is necessary to strengthen this connection.

Effect of Ag2O addition on the intergranular properties of the superconducting Bi–(Pb)–Sr–Ca–Cu–O system

Abstract: The effect of Ag2O addition on the Bi–(Pb)–Sr–Ca–Cu–O system has been investigated in terms of ac susceptibility, phase evolution, critical current density and critical temperature. It was found that as the amount of Ag2O addition increases, the intergranular critical current density decreases in our samples (Bi1.84Pb0.34Sr1.91Ca2.03Cu3.06O10) fabricated by ammonium nitrate technique. The analysis for comparison is based on the suppression degree of the diamagnetic behaviour with respect to fields, rapid or slow shift of the summit in χ'(T) to lower temperature with increasing field amplitude and the sharpness of the transition of χ'(T) for intergranular component for the same field amplitude. We also qualitatively discuss experimental results in the framework of the critical state model. The room temperature XRD diagram indicates the presence of large amount of high-Tc (2223) phase. The percentage of Bi-2223 phase in the phase mixture was estimated from the intensities of high-Tc (2223) and low-Tc (2212) phase peaks as 78% for the pure BSCCO sample. Among the Ag2O-added BSCCO samples studied, the one in which 5 wt%Ag2O was added shows the highest rate of Bi-2223 formation as 92%. The SEM analysis reveals some morphological changes induced by silver addition.

Fabrication and properties of monofilamentary MgB2 superconducting tapes

Abstract: Nickel-sheathed MgB2 tapes were fabricated by means of the powder-in-tube method with a monocore configuration of the superconductor. The metallic tubes were filled with commercial reacted powders and cold-worked, respectively, by groove rolling, drawing, and rolling to flat tapes of about 4 mm in width and 0.35 mm in thickness. Portions of the manufactured conductors were heat-treated in an argon atmosphere at temperatures up to 940 °C. Conductors were characterized by transport measurements to determine the magnetic field dependence of the critical current density, the irreversibility line, as well as to establish an initial relationship between the thermo-mechanical treatment and the current carrying capacity of the manufactured MgB2 samples. Our optimized samples for the low field operation carry a critical current density of at least 4 × 105 A cm−2 at 4.2 K, 1 T. The main outcome of this study, however, is that the optimal heat treatment condition changes if the MgB2 conductor has to be employed respectively for a low- or high-field operation. Finally, the first successful demonstration of the current carrying capability of longer MgB2 tapes is also reported in this paper.

Improvement of critical current density in Fe-sheathed MgB2tapes by ZrSi2, ZrB2and WSi2doping

Abstract: Fe-sheathed MgB2 tapes were prepared by the in situ powder-in-tube technique by 5 at% ZrSi2, ZrB2 and WSi2 doping, respectively. The doping effect of these compounds on the microstructure and superconducting properties of MgB2 tapes has been investigated by using x-ray diffraction, scanning electron microscope, transport measurements and dc susceptibility measurements. Compared to the undoped samples, Jc for all the doped samples were much improved; the best result in terms of Jc was achieved for ZrSi2 doping, by up to a factor of 3.4 at 4.2 K in magnetic fields up to 12 T. Moreover, these dopants did not significantly decrease the transition temperature. The Jc–B curves of WSi2-doped tapes show better performance in higher magnetic fields in comparison to undoped tapes, suggesting that pinning centres effective in a high-field region were possibly introduced.

‘Magnetoscan’: a modified Hall probe scanning technique for the detection of inhomogeneities in bulk high temperature superconductors

Abstract: We present a novel technique for the investigation of local variations of the critical current density in large bulk superconductors. In contrast to the usual Hall probe scanning technique, the sample is not magnetized as a whole before the scan, but locally by a small permanent magnet, which is fixed near the Hall probe, during the scanning process. The resulting signal can be interpreted as a qualitative measure of the local shielding currents flowing at the surface.

New chemically stable, nano-size artificial flux pinning centres in (RE)-Ba-Cu-O superconductors

Abstract: Chemically stable, nanoscale (<50 nm) Y2Ba4CuMOy (M = Nb, Ta, W, Mo, Zr, Hf, etc,) phase inclusions, which potentially form a new type of artificial pinning centre, have been introduced successfully for the first time into large, single grain (RE)–Ba–Cu–O bulk superconductors.

Modelling and measurement of ac loss in BSCCO/Ag-tape windings

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

High quality YBa2Cu3O7 thin films grown by trifluoroacetates metalorganic deposition

Abstract: Chemical solution growth of epitaxial YBa2Cu3O7 thin films on single crystalline substrates has been investigated using trifluoroacetate precursors. The concentration of the starting solution was selected to achieve a final film thickness of 250 nm. First, it is shown that high quality films can only be achieved if an homogeneous nanocrystalline film is obtained after the spin coating deposition and pyrolysis steps. Secondary phases such as BaCuO2, Y2BaCuO5 and CuO remain after the high temperature growth process, when macrosegregation is detected after these initial processing steps. A local degradation of texture associated with macrosegregation is detected by μ-Raman spectroscopy. The influence of growth temperature on the film quality has been analysed and the reaction times have been optimized at each temperature according to the reaction kinetics. In-situ fluoride analysis and μ-Raman spectroscopy have been used to determine the advancement of the formation reaction. The most apparent microstructural modification of these thin film samples with optimized annealing times has been found to be an enhanced porosity for low processing temperatures. The influence of porosity on the normal state resistivity and the critical currents has been evidenced. Optimized processing parameters lead to samples with very high critical currents (Jabc = 3.2 × 106 A cm−2 at 77 K and 2.7 × 107 A cm−2 at 5 K) which demonstrates the capability of the trifluoroacetate metalorganic deposition method for thin film and coated conductor preparation.

Macro-cracking in melt-grown YBaCuO superconductor induced by surface oxygenation

Abstract: We have shown that oxygenation of the melt-grown YBa2Cu3O7/Y2BaCuO5 single grain superconductors is associated with the formation of macro-cracks parallel to the a/b-plane The origin of these cracks is associated with tensile stress induced into the oxygenated surface layer due the c-lattice parameter shortening The critical strain calculated according to Thoules's model for the cracking of brittle films on an elastic substrate suggests that the macro-cracking starts much earlier than the formation of the homogeneous layer of equilibrium oxygen content at the surface

Critical current versus strain research at the University of Twente

Abstract: At the University of Twente a U-shaped spring has been used to investigate the mechanical properties of a large variety of superconducting tapes and wires. Several mechanisms are responsible for the degradation of critical current as a function of applied strain. A change in its intrinsic parameters causes a reversible critical current dependence in Nb3Sn. The critical current reaches a maximum at a wire-dependent tensile strain level, and decreases when this tensile strain is either released or further increased. In Bi-based tapes the critical current is virtually insensitive to tensile strain up to a sample-dependent irreversible strain limit. When this limit is exceeded, the critical current decreases steeply and irreversibly. This behaviour is attributed to microstructural damage to the filaments. This cracking of the filaments is verified by a magneto-optical strain experiment. Recent experiments suggest that in MgB2 the degradation of critical current is caused by a change in intrinsic properties and damage to the microstructure. Magneto-optical imaging can be used to investigate the influence of applied strain on the microstructure of MgB2, as is done successfully with Bi-based tapes. In all these conductors the thermal precompression of the filaments plays an important role. In Nb3Sn it determines the position of the maximum and in Bi-based and MgB2 conductors it is closely related to the irreversible strain limit.

A review of weak/strong links and junctions in high-Tc superconductors as a transition to a Mott insulator

Abstract: In high-Tc superconductors (HTS), which are layered, doped Mott insulators, weak links occur easily in preparation and growth, being weakened further by irradiation, by impurities and by disorder. CuO-plane weak links are the major obstacle for HTS currents, both dc and radio frequency. They are tunnel junctions that show reduced critical Josephson currents jcJ (A cm?2) and enhanced normal Rbn (? cm2) and leakage resistances Rbl (T 0.2 nm and of height ? ? 2 eV, housing nL? ? 1021 cm?3 localized states causing R?bl 1/nL?, are quantified for the first time by the resonant tunnel model in agreement with all experimental data, especially R?bl-, j?cJR?bn- and j?cJR?2bn degradations. In interface engineered junctions, a modified, seemingly crystalline (YBCO)* layer of low barrier height c 20 meV and width dc ? 1?5 nm, covered by the standard Mott insulating YBCO, acts as a controllable and integrable tunnel barrier. Perpendicular weak links with the quasi-insulating blocking layer as the tunnel barrier are dominated by d? ? 0.8 nm and by nBL < 1021 cm?3, as the density of intermediate states depending strongly on doping yields j?cJR?bn and j?cJR?2bn values which are larger than for in-plane junctions. Comparing HTS junctions with Nb/Nb2O5?y and Nb/Al/AlOx(OH)y junctions shows the way out of this interface chemistry deadlock.

Magnetostrictive behaviour of thin superconducting disks

Abstract: Flux-pinning-induced stress and strain distributions in a thin disk superconductor in a perpendicular magnetic field are analysed. We calculate the body forces, solve the magneto-elastic problem and derive formulae for all stress and strain components, including the magnetostriction ΔR/R. The flux and current density profiles in the disk are assumed to follow the Bean model. During a cycle of the applied field the maximum tensile stress is found to occur approximately midway between the maximum field and the remanent state. An effective relationship between this overall maximum stress and the peak field is found.

Composite Cu/Fe/MgB2 superconducting wires and MgB2/YSZ/Hastelloy coated conductors for ac and dc applications

Abstract: We discuss the results of a study of MgB2 multifilamentary conductors and coated conductors from the point of view of their future dc and ac applications. The correlation between the slope of the irreversibility line induced by neutron irradiation defects and in situ structural imperfections and the critical temperature and critical current density is discussed with respect to the conductor performance and applicability. We debate the possible origin of the observed anomalous decrease of ac susceptibility at 50 K in copper clad in situ powder-in-tube MgB2 wires. Different conductor preparation methods and conductor architectures, and attainable critical current densities are presented. Some numerical results on critical currents, thermal stability and ac losses of future MgB2 multifilamentary and coated conductors with magnetic cladding of their filaments are also discussed.

Superconducting properties, microstructure and chemical composition of MgB2 sheathed materials

Abstract: The superconducting properties, the microstructure and the chemical composition of sheathed MgB2 tapes and of one wire, all synthesized by the powder-in-tube method, were investigated At 42 K critical current densities up to 105 A cm−2 (0 T) and 15 × 104 A cm−2 (25 T) were obtained by transport measurements in the wire and the tapes, respectively In the MgB2 matrix of all samples, oxygen was identified and mole fractions of 0–10 at% were determined by electron probe microanalysis It was found by scanning electron microscopy that only the tapes showed boron-rich secondary phases about 10 µm in size Comparing different tapes, the critical currents increase with the aspect ratio and decrease with the oxygen mole fraction in the MgB2 material In the tapes, aspect ratios were inhomogeneous and critical current densities at low fields were limited by insufficient thermal stabilization For understanding the internal oxidation in the MgB2 tapes, one tape was investigated by analytical transmission electron microscopy (TEM) The combination of energy-dispersive x-ray spectroscopy and electron spectroscopic imaging in the TEM yielded phase maps of this sample It showed a heterogeneous microstructure, MgB2 grain sizes ranged between 20 nm and 1 µm Oxygen was primarily bound in 20 nm–1 µm MgO precipitates and secondary phases, and no boron oxides could be evidenced Randomly distributed 50 nm–1 µm boron-rich secondary phases (MgB4+δ, MgB7+γ) embedded in the MgB2 matrix were identified The possible reasons for the oxidation of the superconducting matrix are discussed