Critical Current Characteristics Research Articles

Experimental study of yttrium barium copper oxide superconducting tape’s critical current under twisting moment

Critical current (I c) characteristics of 2G YBCO superconducting tape under the influence of twisting moment was experimentally investigated at varying current ramp rates in the self-field. Under a uniform twist, the degradation in the current-carrying capacity of YBCO tape up to 30% was observed at 77 K. The degradation is largely attributed to the shear stress and torsional shear strain resulting from the twisting. The superconductor to resistive transition index, n, is also found to behave in an identical manner with increase in the twisting. Finite element analysis (FEA) of the tape in the experimental configuration with twisting moment being applied on to it has been carried out in COMSOL. The torsional strain calculated analytically as per the experimental configuration matches closely with that of FEA results, which shows that the critical current degradation is a function of strain.

Behaviour of filamentary MgB2 wires subjected to tensile stress at 4.2 K

Different filamentary MgB2 wires have been subjected to tensile stress at 4.2 K. Stress–strain and critical current versus stress and strain characteristics of wires differing by filament architecture, sheath materials, deformation and heat treatment were measured and compared. It was found that the linear increase of critical current due to the pre-compression effect (ranging from 5% up to ≈20%) is affected by thermal expansion and the strength of used metallic sheaths. The values of irreversible strain εirr and stress σirr depend dominantly on the applied outer sheath and its final heat treatment conditions. Consequently, the strain-tolerance of MgB2 wires is influenced by several parameters and it is difficult to see a clear relation between Ic(ε) and σ(ε) characteristics. The lowest εirr was measured for Monel sheathed wires (0.3–0.6%), medium for GlidCop® sheath (0.48–0.6%), and the highest εirr = 0.6–0.9% were obtained for MgB2 wires reinforced by the stainless steel 316L annealed at temperature between 600 and 800 ° C. The highest εirr = 0.9% and σirr = 900 MPa were measured for the work-hardened steel, which is not considerably softened by the heat treatment at 600 ° C/2.5 h.

Ybco Superconductor Characterization under Shear Strain

YBCO based high temperature superconductors in practical applications are subjected to shear strain. `Critical current characteristics’ of such high temperature superconductors are known to get degraded in strain state. In this work, shear stresses resulting from the finite twisting of HTS tape having varying widths have been modeled using FEA analysis for different twisting angles. Supporting the findings of this model, an American Superconductor Corporation (AMSC) 2G YBCO tape of a given width has been twisted and experimentally investigated in self field for a given current ramp-rate. Under uniform twist of the YBCO tape at 77 K, the degradation in the current carrying ability up to 30% was observed. The irreversibility in the current carrying ability of HTS tape was also observed beyond the twisting angle per unit length of 25 degree/cm. The superconductor to resistive transition index, `n’ is found to behave in an identical manner to the critical current as a function of twisting angle. Such degradation is largely attributed to the torsional shear strain resulting from the twisting. High temperature second generation (2G) commercial grade YBCO coated conductors are available now-a-days in long length for extensive uses in electrical power cable (Rutherford cable) in the form of stacked twisted HTS conductors [1-3]. Even the proposal of coated conductor in conduit cable (CCICC) employs the idea of stacked HTS conductor twisted over different transposition lengths [4]. In such applications individual HTS tape subjects to twist induced shear stress/strain. The strain beyond a certain limit can critically affect the transport property as well as can modify the super-current carrying path in the coated conductor [5]. Hence, it has become necessary to ensure the maximum tolerable shear stress/strain applied to HTS which could induce recoverable damage. Previous works on the torsoinal strain dependence of Bi-2223, YBCO coated conductor shows experimental findings on Ic degradation behavior. Finite element analyses along with theoretical co-relation also have been surveyed on torsion experiments of high temperature superconducting tapes [6-8]. In case of pure torsion, the strain induced on the tape cross-section is non-uniform and complex. However, the maximum torsional shear strain could be determined at the midpoint of the width side of the cross-section using the formula (et = tθ/L) [8-9]. In the present study, FEA modeling for shear stress developed on 2G YBCO tape of different widths has been considered. Further, the shear strain dependence of the critical current of coated conductor has been investigated experimentally for a high current ramp-rate. In this paper, the FEA analysis is discussed in section-II. The experimental results and conclusion are discussed in section-III & section-IV respectively.

Critical Current Characteristics and Flux Pinning in Fe-Based Pnictide Superconductor

The superconducting Ba1-xKxFe2As2(x = 0.4) single crystals were prepared by the so-called FeAs self-flux method. The critical temperature by ac susceptibility measurement was estimated to be about 36 K. The magnetic field and temperature dependences of critical current densities were investigated by an ac inductive measurement (Campbell’s method). Unlike the phenomenon of co-existence of the global and local critical current densities observed in many polycrystalline Fe-based superconducting pnictides, it was found that only a uniform critical current density (Jc) flows through the whole sample. The value of Jcat 20 K and 1 T was about 5×108A/m2, which is much smaller than the local critical current density observed in polycrystalline samples. This result implies that a dissimilarity of flux pinning mechanism exists between these two kinds of materials. The force-displacement characteristic of fluxoids in sample was investigated. The Labusch parameter was found to increase monotonously with increasing magnetic field, while the interaction distance was proportional to the fluxoid spacing. These results are consistent with the prediction based on a simple flux pinning mechanism.

The low-temperature, high-magnetic-field critical current characteristics of Zr-added (Gd,Y)Ba2Cu3Ox superconducting tapes

Critical current performances of state-of-the-art Zr-added (Gd, Y)BaCuO tapes have been investigated over a temperature range of 20–77 K, in magnetic fields up to 9 T and over a wide angular range of magnetic field orientations. The peak in critical current that is commonly observed in the field orientation perpendicular to the tape in BaZrO3 (BZO) containing superconducting tapes is found to vanish at 30 K in magnetic fields at 1–9 T. While the critical current of 15% Zr-added tapes was about 40% lower than that of 7.5% Zr-added tapes at 77 K, the pinning force values of the former were found to be 18–23% higher than those of the latter in the temperature range of 20–40 K and in magnetic fields of 3–5 T. The results from this study emphasize the importance of optimization of coated conductor fabrication processes for optimum performance not just in low magnetic fields at 77 K but also at the operating conditions of low temperatures and high magnetic fields that are of interest, especially for rotating superconducting machinery applications.

Alternating current loss of second-generation high-temperature superconducting coils with magnetic and non-magnetic substrate

It is widely believed that the second-generation high-temperature superconducting (2G HTS) tapes with magnetic substrates suffer higher transport loss compared to those with non-magnetic substrates. To test this, we prepared two identical coils with magnetic and non-magnetic substrates, respectively. The experimental result was rather surprising that they generated roughly the same amount of transport loss. We used finite element method to understand this result. It is found that, unlike in the single tape where the magnetic field-dependent critical current characteristic can be neglected and the effect of magnetic substrate dominates, the magnetic field-dependent critical current characteristic of 2G tape plays as an equally important role as magnetic substrate in terms of HTS coils.

Influence of Wire Parameters on Critical Current Versus Strain Characteristics of Bronze Processed ${\hbox {Nb}}_{3}{\hbox {Sn}}$ Superconducting Wires

In order to develop bronze processed Nb3Sn wire for the ITER CS coil operating under higher compressive strain, the influence of various parameters of wires such as filament diameter, barrier materials, barrier thickness, heat treatment pattern and Ti addition on critical current (Ic) versus intrinsic strain eν(-1.0% <; eν <; +0.1%) characteristics was investigated. The change of these parameters brought significant changes to superconducting properties such as Ic and n-value. In spite of different wire parameters, the strain dependency of normalized Ic was almost the same, except that a Ti addition affects the upper critical field Bc2. This result suggests that assuming the same Ti-addition level, Nb3Sn wire with higher performance at a certain eν would exhibit higher performance at any eν in the compressive regime. Based on the result, bronze processed Nb3Sn wires with non-Cu critical current density more than 1100 A/mm2 at 12 T, 4.2 K, zero applied strain have been successfully developed for the CS coil.

VII. IDEAL COMPONENTS AND CURRENT CHARACTERISTICS OF ALTERNATIVE CARE OPTIONS FOR CHILDREN OUTSIDE OF PARENTAL CARE IN LOW‐RESOURCE COUNTRIES

Utilizing the United Nations Convention on the Rights of the Child and the Guidelines for the Alternative Care of Children, this paper examines critical components and current characteristics of alternative care for children in low‐resource countries. It begins by exploring the role of values within policy and practice related to child welfare. Then a brief examination comparing alternative care in high‐ versus low‐resource countries is presented. Alternative care includes a continuum approach beginning with family support and reunification, kinship care, foster care, domestic adoption, and ending with intercountry adoption. Specific examples are provided from Eastern Europe, Latin America, and Africa. The paper concludes with the need for more research related to alternative care outcomes that could inform policy and practice.

Full angular critical current characteristics of coated conductors studied using a two-axishigh current goniometer

Coated conductors are considered to be the most suitable candidates for applicationsin various devices. Recently, the critical current anisotropy of (RE)BCO (rareearth barium copper oxide) has become the focus of particular research interest,due to the changes in the angular dependence of the critical current density,Jc(φ), resulting from different tape architectures and pinning landscapes. In this work, wepresent advanced anisotropy studies of coated conductors in low magnetic fields using atwo-axis high current goniometer. Two different commercially available coated conductorswere characterized and will be discussed: the first is based on an IBAD (ion beamassisted deposition) template with the YBCO (yttrium barium copper oxide)ab planes slightly tilted, and the second is highly symmetric with a template grown by theRABiTS (rolling assisted biaxially textured substrate) method. In addition, therequirements to be met by coated conductors for applications will be discussed.

Full Vector Magnetic Field Critical Current Characterization of Coated Conductors Deposited on Solution Deposition Planarized IBAD Templates

Second generation high temperature super-conductor (HTS) wire consists of metal substrate tapes textured with intermediate buffer layers that are coated with a superconducting layer. The applications of these coated conductors require a high current in magnetic fields that are not restricted to a narrow range of angles. Ion beam assisted deposition (IBAD) template tilt, in combination with REBCO vapor deposition, results in asymmetric sample growth. This requires Vector Magnetic Field (VMF) measurement over 180 degrees to be fully described. We have developed a piece wise characterization and analysis method of the VMF using a number of Ic scaling models. Solution Deposition Planarization (SDP) permits REBCO film deposition on 100 nm rms roughness substrates. To test buffer layer effects on film growth, with and without SDP, superconducting films were deposited at Superconductor Technologies, Inc. by co-evaporation of REBCO on IBAD buffer layer templates from Los Alamos. Critical current measurements in VMF up to a magnitude of ~ 0.9 Tesla show behaviors traceable to the effects of buffer layer template on pinning. The ability to discriminate between buffer layers based on in-field Ic behavior is a useful technique for the optimization of buffer layers for future REBCO film deposition.

AC loss and critical current characterization of a noninductive coil of two-in-hand RABiTSYBCO tape for fault current limiter applications

The combination of noninductive winding and the two-in-hand tape configuration hasbeen found to be very effective in reducing the AC loss of coils of RABiTS™ YBCOcoated conductor for fault current limiting (FCL) applications. In this study, anoninductive coil composed of 11 turns of two-in-hand RABiTS™ YBCO tape wasdesigned and constructed for characterizing and investigating the AC loss, criticalcurrent and inductance. The AC losses in this coil were also calculated by finiteelement modeling (FEM) using the COMSOL Multiphysics package. With themagnetic field dependent permeability and ferromagnetic loss of the substratematerial taken into account, the simulation results are in good agreement withthe experimental data. The simulations therefore were employed to study theeffect on the AC loss behavior of the coil of the width of YBCO tape and of thespatial separation between the coil turns to suggest a more effective coil design.

Numerical Simulation of Explosively Compacted Powder-in-Tube MgB&lt;sub&gt;2&lt;/sub&gt; Superconductor

High Tc MgB2 superconductors were fabricated using the ex-situ and in-situ powder in tube (PIT) technique. During treatment, the precursor materials were shock consolidated under high strain-rates using PETN as the explosive medium. After compaction, the superconducting properties of the steel-sheathed MgB2 samples were examined by means of magnetization measurements using a SQUID magnetometer. Bean’s critical state model was applied to investigate the critical current density characteristics of the samples at fields up to 5T. The superconducting transition temperature of the specimens was determined by examining the material temperature dependence of magnetisation in zero field cooled (ZFC) and field cooled (FC) states. An assessment of the explosive compaction technique was carried out by simulating the procedure using the LS-DYNA explicit finite element code. The sample is modelled as a porous soil-like material with a customised yield surface. The numerically estimated final product dimensions, porosity and hardness are compared to the experimental results; furthermore, the numerically obtained pressure, temperature and strain rate profiles are used to assess the efficiency of the compaction process for different explosive quantities and powder compositions.

Axial strain characterization of the Nb 3Sn strand used for China's TF conductor

Oxford Superconducting Technology (OST) has developed a type of Nb 3Sn strand which can be used in the ITER TF coils. The strand (billet number is OST 10424FE) is made by an internal tin process. The critical current ( I c) has been measured subjected to uniaxial strain in the Twente University. The strand is destined for the cable-in-conduit conductors (CICC) of the China first short conductor sample for ITER toroidal field coil. For the uniaxial strain characterization, the voltage–current characteristics were measured with an applied axial strain from −0.8% to +0.5%. The strand appears to be fully reversible in the compressive regime during the axial strain testing, while in the tensile regime, the behavior is already irreversibly degraded when reaching the maximum in the critical current versus strain characteristic. The parameters for the improved deviatoric strain description are derived from the I c data, giving the accuracy of the scaling with a standard deviation of 1.5 A, which is by far within the expected deviation for the large scale strand production of such a high I c strand. Analysis of the relation between I c, n index and the axial strain is reported.

Critical current characteristics and history dependence in superconducting SmFeAsOF bulk

The superconducting SmFeAsO1−xFx (x=0.2) polycrystalline bulks were prepared by the powder-in-tube (PIT) method. The magnetic field and temperature dependences of critical current densities in the samples were investigated by resistive and ac inductive (Campbell's) methods. It was found that a fairly large shielding current density over 109 A/m2, which is considered to correspond to the local critical current density, flows locally with the perimeter size similar to the average grain size of the bulk samples, while an extremely low transport current density of about 105 A/m2 corresponding to the global critical current density flows through the whole sample. Furthermore, a unique history dependence of global critical current density was observed, i.e., it shows a smaller value in the increasing-field process than that in the decreasing-field process. The history dependence of global critical current characteristic in our case can be ascribed to the existence of the weak-link property between the grains in SmFeAsO1−xFx bulk.

Cooling cycle test of DC superconducting power transmission cable

We constructed a test stand of a 20 m DC superconducting power transmission cable in Chubu University in 2006. The cable uses thirty-nine Bi-2223 tapes. Four cycles of a cooling and current-feeding test have been carried out after the construction of the test stand. The cable suffered rapid temperature decrease of 20 K/hour at the daytime during the cooling process by cold nitrogen gas and liquid to the liquid nitrogen temperature. Nineteen HTS tapes in the cable are electrically isolated from each other, and the superconducting characteristics of these HTS tapes can be measured separately. At every cooling cycle, critical current of these isolated HTS tapes were measured. Consequently, no reduction of the critical current characteristics was observed through the 4 cooling cycles. We considered that this endurance of the cable against the cooling cycle is due to unfixed cable end structure of the cable, and that the cable was free from the mechanical stress in spite of the 6 cm shrinkage of the cable length under the low temperature.

Analysis of Temperature Dependent Quench Characteristics of the YBCO Coated Conductor

Studies on (YBCO) coated conductor (CC) are actively underway for the practical application of high temperature superconducting (HTS) devices such as magnets, cables and fault current limiting devices because the CC has higher critical current characteristics under high temperature and magnetic field. However, it is hard to make a long-length of YBCO CC tape with uniform critical current density using the present processing techniques. The hot spot will emerge in a local region if the critical current is non-uniform along the HTS tape, and eventually, the hot spots may cause serious damage to the HTS tape. Therefore, it is important to examine the thermal stability characteristic of the YBCO CC tape from the quench protection point of view. In this paper, the segmental critical current of a YBCO CC sample tape is measured in liquid nitrogen, and a heater is installed at the center of the sample tape in order to examine the normal zone propagation characteristics in the longitudinal direction under the adiabatic condition with a Gifford-McMahon cryocooler. Measurements are performed as a function of heat input and transport current for the operating temperatures of 77 K, 65 K and 40 K, respectively, and the results are discussed in detail.

Design and evaluation of 66 kV class RE-123 superconducting cable

In the new national project, as wire and cable development, Sumitomo Electric Industries, Ltd. (SEI) is trying to develop a 66kV/5kArms high temperature superconducting (HTS) cable with its own REBa2Ca3Ox (RE-123, RE=rare earth element) coated conductors. In an actual power grid, an inflow of current higher than the normal rated power (fault-current) can occur. The maximum fault-current condition is set at 31.5kArms for 2s for 66kV-class transmission system in this project. Therefore, one of the technical targets in this project is to design the wires and cables to survive at this fault-current condition. As the first step, the prototype HTS cable was designed against the fault-current condition of 31.5kArms for 2s. The wires have copper plating as a stabilizer layer not to be damaged by over-currents. The designed HTS cable has a copper former and copper shield layers in parallel with the HTS conducting layers and HTS shield layers, respectively, to control the temperature rises in the cable core at fault-current conditions. As the second step, a sample HTS cable core was fabricated and fault-current tests were conducted to check its soundness. The temperature rises in the cable core were measured under various fault-current conditions. The critical current of the cable core was measured before and after the fault-current tests, and no deterioration in the critical current characteristics was observed. However, there was slight degradation of the n value. The investigation will be conducted, and the results will be fed back to designs of the RE-123 wires and cables.

Characterization of ${\rm Nb}_{3}{\rm Al}$ Strands Subjected to an Axial-Strain for a Fusion DEMO Reactor

A Nb3Al strand, produced by the ldquorapid-heating, quenching and transformation annealing (RHQT)rdquo process, has been developed in collaboration between JAEA and NIMS aiming at application to the conductor in a fusion DEMO reactor. A detailed description of critical current characteristics of the Nb3Al RHQT strand may assist in predicting accurate conductor performance. Calculations of the critical current density are adjusted to 5 scaling laws. Advantages of the newly developed strand compared to the Nb3Sn ITER strand and the Nb3Al Insert strand include improved strain tolerance and a higher current density at around 16 T. Performance parameters of the Nb3Al RHQT strand include 450 A/mm2 at 15.9 T, 5.7 K and -0.4%. These performance parameters fulfill the requirements of the TF conductor of the fusion DEMO reactor. Therefore, the Nb3Al RHQT strand represents an improved performance with prospective application in the fusion DEMO reactor. In addition, the Nb3Al RHQT strand has the potential to enhance further the critical current density.

AC Magnetic Flux Profile and Critical Current Density Obtained by Campbell's Method in Polycrystalline ${\rm MgB}_{2}$

In this study, we focused on the estimation of the global critical current density, super-current path, grain connectivity and their relationships with the faults volume fraction in the MgB2 bulks prepared by a modified PIT (powder in tube) method. Campbell's method was applied for the purpose of acquiring the penetrating AC magnetic flux profile and the characteristic of AC magnetic field vs. penetration depth from the specimen's surface. A computer simulation on the penetrating AC flux profile in MgB2 bulks with certain amount of randomly distributed voids, oxidized grains and other faults was also carried out. The global critical current density obtained by Campbell's method turned out to be smaller than that obtained from the SQUID measurement, implying that the global super-current was reduced by the existence of various kinds of faults and the lack of the electrical connectivity between grains. It was verified that the relationship between the global critical current characteristics and the faults contained in MgB2 specimens can be quantitatively clarified by comparing the simulated critical current densities and other factors with the experimental results.

Critical Current Characteristics of Bi2212 Round Wires Prepared by Isothermal Partial Melting Method

We prepared Bi2212 round wires using the isothermal partial melting process, in which partial melting and solidification are carried out at the same temperature but in changing oxygen partial pressure. We found that the homogeneity of Bi2212 phase inside the wire was important for the J c performance of the wire and the homogeneity was improved by extending the heat treatment time. The J c value of 6.6times104 A/cm2 at 4.2 K and 10 T were obtained. We can expect further enhancement of J c property by optimizing the heat treatment conditions and improving the homogeneity of Bi2212 filaments of the wires.