Superconducting Characteristics Research Articles

Influence of the heat treatment profiles on the development of highly textured Ag/Bi2212 monofilamentary tapes

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 Jc. 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.

Prebending Strain Effect on<tex>$rm CuNb/Nb_3rm Sn$</tex>Superconducting Wire During Practical React-and-Wind Process

To demonstrate the applicability of the prebending strain effect to the R&W coil winding process, we have developed the prebent react-and-wind (R&W) process, which is the combination of the conventional R&W process and the prebending treatment. The superconducting and mechanical characteristics were investigated for CuNb/Nb3Sn superconducting wires which were picked up from the prebent R&W coil winding process. The results were compared with that of CuNb/Nb3Sn wires which were prebent manually using a prebending former. The prebending treatment using pulleys enhanced Ic more than the manual prebending treatment. This is explained qualitatively by a difference of the prebending strain distribution. The prebending strain distribution is plain symmetric and axis symmetric in cases of the manual prebending and the pulleyed prebending, respectively. The axis symmetrically distributed prebending strain reduces the residual strain more than the plain symmetrical one. The mechanical property was also enhanced. The slope of the stress-strain curve for the pulleyed prebent samples were steeper than that of as-heat-treated sample. The maximum Ic of the Ic -strain curve was enhanced and the strain value corresponding to the maximum Ic shifted to smaller strain by the pulleyed prebending treatment. Though in the large stress region over 200 MPa, the Ic-stress curves of pulleyed prebent samples and as-heat-treated sample were in good agreement, suggesting that the Ic enhancement effect by the prebending treatment was not beneficial, in the small stress region, it is quite beneficial

Influence of small additives of Si, Sc, Cu on properties of Nb-48.5 wt.% Ti superconductor

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.

Trial Manufacture of a km Class Length of Cu Cladding RHQT<tex>$rm Nb_3rm Al$</tex>Flat-Wire

We have produced so far about a 400 m length of Cu stabilized RHQT (rapid-heating, quenching and transformation) Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al flat-wire using a small-sized multi-billet (10 kg). Although our goal is to make a 2.5 km unit length of Cu cladding flat-wire for high-field NMR uses, an attempt was made to fabricate a 1 km class unit-length of conductor as a milestone of commercialization. A long-length of precursor wire over 2.5 km was prepared by using a large-sized multi-billet (50 kg). A 1.3 km length RHQ operation by using a newly installed large-scale RHQ apparatus was performed and then a km-class length Cu cladded flat-wire was fabricated to evaluate the uniformity of resultant superconducting characteristics of wire

Position Control of Active Magnetic Levitation using YBCO Bulk and Multiple Electromagnets

We have been developing an active magnetic levitation system composed of field-cooled disk-shaped YBCO bulk and multiple ring-shaped electromagnets which are vertically piled up. We have demonstrated and reported that the levitation height, as well as stability, can be remarkably improved by adjusting the operating current of each electromagnet individually. In this study, we constructed a noncontact position control system in the vertical direction based on the feedback control theory and the electromagnetic field analyses considering superconducting characteristics of the YBCO bulk. Depending on levitation height, the operating current in the electromagnets is controlled automatically. The system consists of two ring-shaped copper-winding electromagnets without iron core and a ring-shaped YBCO bulk magnetized by the field-cooling process. We carried out experiments to verify the feasibility of noncontact position control system. In the experiment for position control, the levitation height maintained a target position accurately, and responded smoothly to changing target position. In addition, we aimed at the construction of the position control system with higher accuracy using the numerical simulation based on the finite element method analyses. And, the position control system that piled up three electromagnets was designed and constructed. Concretely, accuracies of the position control within 28µm were obtained. Final goal of our study is to apply the levitation system to the inertial nuclear fusion in which the accuracy as high as several µm would be required.

High-pressure synthesized nanostructural magnesium diboride-based materials for superconductive electromotors, generators and pumps

Additions of Zr can increase critical current density ( j c) of high-pressure synthesized MgB 2 (HPS-MgB 2) in the same manner as additions of Ta or Ti, i.e. due to the absorption of impurity hydrogen (to form ZrH 2). The formation in HPS-MgB 2 of ZrB 2 phase at higher synthesis temperatures (about 950 °C) does not result in the j c increase. Some increase in j c of HPS-MgB 2 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 MgH 2 impurity phase from appearing and may prevent hydrogen from being introduced into the material structure and besides, their presence in HPS-MgB 2 promotes the formation of a higher amount of Mg–B (most likely MgB 2) inclusions in the Mg–B–O material “matrix” that in turn leads to the increase of j c in magnetic fields. The high level of superconductive (SC) and mechanical characteristics attained for HPS-MgB 2 and the possibility to manufacture large samples make its application in the superconductive electromotors, generators, pumps, etc., very promising.

Phase diagram in bilayered-hydrate [formula omitted] revealed by Co nuclear quadrupole resonance

We have performed Co nuclear quadrupole resonance (NQR) measurements in various bilayered-hydrate (BLH) Na x CoO 2 · y H 2 O compounds, showing superconducting (SC) and non-SC characteristics. A weak magnetic order was found in a BLH sample with a longer c -axis lattice parameter. We also found that the NQR frequency ν Q arising from the ± 7 2 ↔ ± 5 2 transition is related to the ground state of this system. A phase diagram is developed, in which the superconducting and magnetic-order temperature T c and T M are plotted with respect to ν Q .

The reactive liquid Mg infiltration process to produce large superconducting bulk MgB 2 manufacts

An alternative and simple manufacturing process is presented to produce high density bulk MgB 2 superconducting objects of large dimensions. The process avoids the use of high pressure apparatus and consists in the reactive infiltration of liquid Mg in B powders preforms. With an appropriately designed stainless steel container for the reactants, several manufacts of different shape have been obtained, including tubes, cylinders, rings or disks, of dimensions of the order of the tens of centimeters. The MgB 2 material, unlike high temperature oxide superconductors, allows an easy percolation of the supercurrents across the grains boundaries, even if it is in the polycrystalline form. Due to this property, the large MgB 2 manufacts obtained by the reactive liquid infiltration present high superconducting characteristics, as demonstrated by transport and magnetic measurements up to 35 K. In particular the magnetic levitation and the magnetic shielding capability appear as the most promising applicative fields in which there is a need for large and homogeneously superconducting bulk pieces. For space applications, the use of MgB 2 superconductors may enable a substantial improvement in the compactness and weight of cryogenic systems, with respect to the actual systems based on the liquid He temperatures. Furthermore, with respect to other high temperature oxide superconductors, MgB 2, besides the disadvantages of the need of a 20–30 K cryogenic system, presents the advantages of a relative lower density (2.4 g/cm 3), higher mechanical strength and easier processability, like that here described.

Analysis of the minority crystalline phases in bulk superconducting MgB 2 obtained by reactive liquid Mg infiltration

The preparation of MgB 2 starting from the elements (in situ process), when performed in conjunction with the Mg liquid infiltration process, gives rise to very high dense manufacts, with outstanding mechanical and superconducting characteristics. The infiltration technique is particularly suitable when realized into preforms of crystalline boron powders of grain size up to about 100 μm; however, the resulting products typically present a microstructure characterized by a composite morphology, indicating a inhomogeneous distribution of different crystalline phases. In this paper the composition and the morphology of typical products has been analyzed with several techniques, including X-ray powder diffraction (XRPD), scanning electron microscopy and X-ray fluorescence microanalysis, aiming to quantify the (spatial) distribution of crystalline phases and the presence of minority phases different from the MgB 2. The minority phases that are commonly present in the MgB 2 infiltration preparation are residual elemental magnesium and a new boron-rich magnesium boride phase (Mg 2B 25), recently discovered and structurally characterized (by unconventional XRPD methods) by our group. We have analyzed the common morphological features of typical MgB 2 samples, and cross-checked the analytical results obtained from different experimental techniques. Our results show that the new Mg 2B 25 phase amounts to less than 10 molar % of the MgB 2 entire product, and that it is mainly concentrated, as submicronic inclusions (inside the larger crystalline MgB 2 grains) where it can reach concentration values up to 40 molar %.

Hole doping and superconductivity characteristics of the s=1, 2 and 3 members of the (Cu,Mo)-12 s2 homologous series of layered copper oxides

Superconductivity characteristics have been systematically evaluated for a two-CuO 2-plane copper oxide system, (Cu,Mo)-12 s2, upon increasing the number of fluorite-structured layers, s, between the two CuO 2 planes. Essentially single-phase samples of (Cu 0.75Mo 0.25)Sr 2YCu 2O 7+δ ( s = 1 ), (Cu 0.75Mo 0.25)Sr 2(Ce 0.45Y 0.55) 2Cu 2O 9+δ ( s = 2 ) and (Cu 0.75Mo 0.25)Sr 2(Ce 0.67Y 0.33) 3Cu 2O 11+δ ( s = 3 ) were synthesized through a conventional solid-state route in air. To make the samples superconductive an additional high-pressure oxygenation (HPO) treatment was required. Such treatment (carried out at 5 GPa and 500 °C in the presence of 75 mol% Ag 2O 2 as an oxygen source to maximize the T c) compressed the crystal lattice for the three members of the (Cu 0.75Mo 0.25)-12 s2 series equally, i.e., by 0.01 Å for the a parameter and by 0.07 Å for the c parameter per formula unit. From both Cu L-edge and O K-edge XANES spectra the s = 1 sample was found to possess the highest overall hole-doping level among the HPO samples. Accordingly it exhibited the best superconductivity characteristics. With increasing s, both the T c ( s = 1 : 88 K, s = 2 : 61 K, s = 3 : 53 K) and H irr values got depressed, being well explained by the trend of decreasing CuO 2-plane hole concentration with increasing s as revealed from O K-edge XANES spectra for the same samples. Hence, the present results do not suggest any significant (negative) impact on the superconductivity characteristics from the gradually thickened fluorite-structured block itself.

Growth of EuBa2Cu3O7−δ films under an ozone atmosphere

We have investigated the dependence of the crystallinity and the superconducting characteristics in the EuBa2Cu3O7−δ (EuBCO) thin films on oxygen content. The EuBCO films were grown using a pulsed laser deposition (PLD) under an oxygen and an ozone atmosphere on (100) SrTiO3 substrates. The c-axis length of as-grown EuBCO films under the oxygen atmosphere was longer than that of the bulk. The c-axis length of in situ annealed EuBCO films was shortened as compared with the as-grown EuBCO films. Additionally, the surface resistance (Rs) of the in situ 3 and 6h annealed EuBCO films was lower than that of as-grown EuBCO films. However, the surface resistance of in situ annealed more than 24h EuBCO films was not lower than that of in situ 6h EuBCO films. The c-axis length of the as-grown EuBCO films grown under an ozone atmosphere showed 1.170nm. It must be noted that the lowest surface resistance of EuBCO films was obtained by EuBCO films grown under the ozone atmosphere without any in situ annealing. We found that the growth of EuBCO films under the ozone atmosphere was good to dope oxygen in the films.

Fabrication of reliable via conductors for niobium SFQ devices

The via-wiring process for niobium large-scale single-flux-quantum (SFQ) circuits was studied from two viewpoints: niobium deposition and inter-level-dielectric (ILD) SiO2 deposition. It was found that superconductive critical current density (Jc) strongly depends on the target-to-wafer distance (TS) during niobium deposition. Transmission-electron-microscope observation revealed that the niobium film sputtered under a long-TS condition has enough step coverage, but less crystallinity and lower density. Moreover, deposition at a lower incident angle at the via walls was found to cause the degradation. Grain growth at an inclined crystal orientation during the deposition is thought to produce high porosity in the film, resulting in poor superconductive characteristics. Vias formed in the SiO2 deposited without substrate biasing were also found to degrade niobium film crystallinity. The most probable cause of this niobium degradation is the influence from water (moisture) absorbed in the SiO2 vias. These results show that reliable vias can be fabricated by niobium sputtering with optimized TS and by SiO2 formation with substrate biasing.

Magnetization curves and the scaled Jc characteristics in various superconductors

From the shape of magnetization curves, the important superconducting characteristics can be evaluated. The critical current densities, the irreversibility fields and the losses of the superconductors can be estimated from hysteretic curves. Magnetic field and temperature dependence of the pinning strength F p in type-II superconductors is known to be well described by the scaling law of the pinning force as F p = AH c 2 m ( B / B c 2 ) γ · ( 1 - B / B c 2 ) δ , where H c2 is the upper critical field, γ, δ and m are the pinning parameters. In high temperature superconductors, the scaling law fails to describe the pinning properties since the irreversibility field B irr is much smaller than the upper critical field B c2. Conversion of field dependence term in the scaling law is accomplished by replacing B c2 by B irr. The magnetization curves differentially computed using the scaled J c with the pinning parameters of γ, δ and m show various shapes and can be fitted well to the observed curves of MgB 2 and Ag–Hg1223 superconductors in a wide range of temperature and magnetic field.

Role of rare-earth Ba2+ doping in governing the superconducting and mechanical characteristics of Bi–Sr–Ca–Cu–O

The effect of barium doping on the superconducting and mechanical properties in theBi2Sr2Ca2BaxCu5Oy (BSCCO) system has been investigated by thermoelectric power (TEP),Vickers microhardness (VHN), x-ray diffraction (XRD), and scanningelectron microscope (SEM) analysis. Barium was incorporated withx ranging from 0.0 to 0.82. The temperature variation of the thermopower (Seebeck coefficient)S shows that the critical transition temperatureTc0 is enhanced (to93 K) up to x = 0.45 andthen disappears for x = 0.82. The results have also suggested an increased interaction of electronsand holes with phonons at low temperatures. The enhancement ofTc0 isascribed to the improvement in electric connection between superconducting grains. XRD data show that thehigh-Tc (2:2:2:3) phaseincreases and the low-Tc (2:2:1:2) phase decreases as the barium concentration changes fromx = 0.0 to 0.45. SEManalysis of a sample (x = 0.45) shows that plate-like grains (2223 phase) are formed. Also, an appropriate level ofBa dopant improves the microhardness and fracture toughness remarkably. Theoptimum dopant level of Ba from the point of view of mechanical properties isx = 0.45. The improvement of mechanical characteristics is due to the fact thatBa2+ can fill the intergrain spaces, and thereby reinforce the coupling between granules. From the aboveresults, it is inferred that the added Ba has the positive effect of decomposing the structure of thelow-Tc phase, leadingto the production of BaBiO3 and BaCuO2 andenhancing the high-Tc phase creation.

Preparation of superconductive layer on the press formed bulk from Bi system superconductor powder by laser irradiation

A study on the preparation of superconductive layer on the press formed bulk from Bi system superconductor powder by laser melting method was conducted. Pulverized and filtered with a net of 325 mesh superconductor powder of the system was pressed to form a bulk of 2 mm thick and 10 mm diameter disk. The bulk specimen showed no superconductive characteristics as formed. Then, a CO 2 laser beam of output power 6–9 W was scanned at 2 or 10 mm/min on the surface of the specimen in the air. This laser treatment yields gradual change in temperature from the melting point to the non-heat-affected temperature range on the surface and inside the bulk. The influence of the specimen forming pressure on the critical temperature was also discussed. It was found that (1) surface layer laser melted shows the characteristics of semiconductor, (2) superconductor layer is formed underneath the melted layer, i.e., semiconductor layer, (3) this superconductor layer prepared mainly consists of mainly 2212 phase, (4) the thickness of superconductor layer can be controlled in the range between 60 and 130 μm by changing output power, and (5) the critical temperature of Tc(end) is influenced by laser treatment conditions as well as the pressure to form the bulk specimen.

Spatially heterogeneous character of superconductivity in MgB 2 as revealed by local probe and bulk measurements

Controversial tunnel and point-contact experimental data on superconducting gap values in MgB 2 are analyzed and interpreted in terms of the spatial inhomogeneity and the proximity effect appropriate to this layered compound. Existing specific heat measurements are explained in the same way. Break-junction tunnel studies of polycrystalline MgB 2 samples are carried out and a clear-cut multiplicity of gaps is observed, although the shapes of the I– V characteristics vary substantially.

Inductive Type Fault Current Limiter Using Bi-2223 Thick Film on MgO Cylinder With Bi-2212 Buffer Layer

We have been developing an inductive type superconducting fault current limiter with a superconducting cylinder. For practical use, a high critical current density (Jc) is needed in a large diameter. To achieve these items, we have developed a Bi-2223 thick film on a MgO substrate, with a Bi-2212 buffer layer. This cylinder was 450 mm in diameter and 120 mm in length. We describe the superconductive characteristics of this cylinder, and the limiting properties of the fault current limiter using these cylinders.

Trial Manufacture of a 2.5 km Class Long-Length of Nb-Matrix Jelly-Roll Nb/Al Precursor Wire for RHQT Process

Since the RHQT-Nb/sub 3/Al conductor shows excellent superconducting characteristics in high magnetic fields, its application to a 1 GHz NMR insert coil is in particular expected. For such an application, the development of a long-length of RHQT-Nb/sub 3/Al wire is indispensable. However, the unit length of wire achieved was no more than 400 m so far, due to a restricted size of the multi-billet, while about a 2.5 km-length of wire is necessary for the NMR insert coil application. Thus, an attempt was made to produce a 2.5 km-length of precursor wire by using a large-sized billet, and we have succeeded in producing such a long-length of precursor wire (2.6 km) without any breaking of wire during hydrostatic extrusion and subsequent drawing processes.

Entanglement and off-diagonal long-range order of an η-pairing state

Off-diagonal long-range order (ODLRO) is a quantum phenomenon not describable in classical mechanical terms. It is believed to be one characteristic of superconductivity. The quantum state constructed by eta-pairing which demonstrates ODLRO is an eigenstate of the three-dimensional Hubbard model. Entanglement is a key concept of the quantum information processing and has no classical counterpart. We study the entanglement property of eta-pairing quantum state. The concurrence is a well-known measure of quantum entanglement. We show that the concurrence of entanglement between one-site and the rest sites is exactly the correlation function of the ODLRO for the eta-pairing state in the thermodynamic limit. So, when the eta-pairing state is entangled, it demonstrates ODLRO and is thus in superconducting phase, if it is a separable state, there is no ODLRO. In the thermodynamic limit, the entanglement between M-site and other sites of the eta-pairing state does not vanish. Other types of ODLRO of eta-pairing state are presented. We show that the behavior of the ODLRO correlation functions is equivalent to that of the entanglement of the eta-pairing state. The scaling of the entropy of the entanglement for the eta-pairing state is studied.

Superconducting Characteristics of Polycrystalline Magnesium Diboride Ceramics Fabricated by a Spark Plasma Sintering Technique

Highly densified MgB2 superconductors were successfully fabricated using a spark plasma sintering (SPS) technique, and their superconductivity with respect to microstructural evolution was evaluated. Full densification with final density close to the theoretical density was achieved at a temperature of 1000°C within a total SPS processing time of 40 min. Both an MgB2 specimen sintered at 1000°C for 30 min and one sintered at 1050°C for 10 min exhibited a high critical transition temperature (Tc) similar to that of an MgB2 single crystal (39 K), and a very sharp superconducting transition width (ΔT) less than 0.5 K. In addition, high critical current densities (Jc) of 7.7 × 105 A/cm2 in a field of 0.6 T at 5 K and of 8.3 × 104 A/cm2 in a field of 0.09 T at 35 K were obtained. These excellent superconducting characteristics of the SPS‐processed MgB2 are attributed to uniformly distributed secondary MgO phase nanoparticles and well‐developed dislocations in the microstructure that may act effectively as extrinsic flux pinning sites, resulting in the strong pinning force showing a high Jc of 8.7 × 104 A/cm2 even in the condition of a field of 4 T at 5 K.