Showing papers in "Superconductor Science and Technology in 2005"

Irradiation-free, columnar defects comprised of self-assembled nanodots and nanorods resulting in strongly enhanced flux-pinning in YBa2Cu3O7−δ films

Abstract: The development of biaxially textured, second-generation, high-temperature superconducting (HTS) wires is expected to enable most large-scale applications of HTS materials, in particular electric-power applications. For many potential applications, high critical currents in applied magnetic fields are required. It is well known that columnar defects generated by irradiating high-temperature superconducting materials with heavy ions significantly enhance the in-field critical current density. Hence, for over a decade scientists world-wide have sought means to produce such columnar defects in HTS materials without the expense and complexity of ionizing radiation. Using a simple and practically scalable technique, we have succeeded in producing long, nearly continuous vortex pins along the c-axis in YBa2Cu3O7?? (YBCO), in the form of self-assembled stacks of BaZrO3 (BZO) nanodots and nanorods. The nanodots and nanorods have a diameter of ~2?3?nm and an areal density ('matching field') of 8?10?T for 2?vol.% incorporation of BaZrO3. In addition, four misfit dislocations around each nanodot or nanorod are aligned and act as extended columnar defects. YBCO films with such defects exhibit significantly enhanced pinning with less sensitivity to magnetic fields H. In particular, at intermediate field values, the current density, Jc, varies as Jc~H??, with ?~0.3 rather than the usual values 0.5?0.65. Similar results were also obtained for CaZrO3 (CZO) and YSZ incorporation in the form of nanodots and nanorods within YBCO, indicating the broad applicability of the developed process. The process could also be used to incorporate self-assembled nanodots and nanorods within matrices of other materials for different applications, such as magnetic materials.

Effects of B4C doping on critical current properties of MgB2 superconductor

Abstract: The relationships between microstructures, crystallinity and critical current properties for B4C doped MgB2 bulks, MgB2?5x(B4C)x with x = 0, 0.04, 0.1, 0.2 and 0.4, were systematically studied. Decreases of Tc and the a-axis length and deterioration of the crystallinity of MgB2 due to the carbon substitution occurred upon B4C doping. Substantially improved Jc was observed in the B4C doped MgB2 bulks especially at 5?K under high magnetic fields. Strengthened flux pinning at grain boundaries by carbon substitution was considered to contribute to the improvement of Jc. Furthermore, the reactivity of B4C with magnesium and boron was found to be much higher than that of graphite. These findings suggested that B4C is a promising carbon source dopant for MgB2 materials with excellent Jc properties, particularly under magnetic fields.

Melt-processed Gd–Ba–Cu–O superconductor with trapped field of 3 T at 77 K

Abstract: We fabricated a single-domain Gd?Ba?Cu?O bulk superconductor 65?mm in diameter and studied the microstructure, superconducting and field-trapping properties. Melt-processing was performed under a controlled oxygen partial pressure of 1.0% using a precursor containing Gd123 and Gd211 powders in a molar ratio of 2:1, with 0.5?wt% of Pt and 20?wt% of Ag2O added. The distribution of Ag and Gd211 particles was almost homogeneous. The addition of Ag was very effective in reducing the amount of cracking in the sample. The maximum trapped magnetic field recorded was 3.05?T at 77?K. We also measured the trapped field between two Gd?Ba?Cu?O bulk samples in order to minimize the demagnetizing effect and found that the trapped field reached 4.3?T at 77?K.

Improved critical current properties observed in MgB2 bulks synthesized by low-temperature solid-state reaction

Abstract: MgB2 bulks were synthesized by the solid-state reaction of Mg and B at 600??C and their superconducting properties were compared with samples heated at 850??C. The samples heated at 600??C exhibited improved critical current properties up to high fields at 20?K. Poor crystallinity is found to contribute enhancement of Hc2, Hirr and Jc at high fields. On the other hand, the strongly grain connected network structure and smaller grain size are responsible for high Jc at low fields. Improved Jc up to 3.93 ? 105?A?cm?2 and a high ?0Hirr of ?T, as for undoped MgB2 bulks, guarantees that low-temperature sintering is a promising way to fabricate MgB2 conductors with high critical current performance.

The scaling law for the strain dependence of the critical current density in Nb3Sn superconducting wires

Abstract: Comprehensive measurements are reported of the critical current density (JC) of internal-tin and bronze-route Nb3Sn superconducting wires as a function of magnetic field (B?23?T), temperature (4.2?K ?T?12?K) and axial strain (?1.6%??I?0.40%). Electric field?temperature characteristics are shown to be equivalent to the standard electric field?current density characteristics to within an experimental uncertainty of ~20?mK, implying that JC can be described using thermodynamic variables. We report a new universal relation between normalized effective upper critical field (BC2*(0)) and strain that is valid over a large strain range for Nb3Sn wires characterized by high upper critical fields. A power-law relation between BC2*(0,?I) and TC*(?I) (the effective critical temperature) is observed with an exponent of ~2.2 for high-upper-critical-field Nb3Sn compared to the value ?3 for binary Nb3Sn. These data are consistent with microscopic theoretical predictions and suggest that uniaxial strain predominantly affects the phononic rather than the electronic properties of the material. The standard Summers scaling law predicts a weaker strain dependence than is observed. We propose a scaling law for JC(B,T,?I) based on microscopic theory and phenomenological scaling that is sufficiently general to describe materials with different impurity scattering rates and electron?phonon coupling strengths. It parametrizes complete datasets with a typical accuracy of ~4%, and provides reasonable predictions for the JC(B,T,?I) surface from partial datasets.

Assembling the puzzle of superconducting elements: a review

Abstract: Superconductivity in the simple elements is of both technological relevance and fundamental scientific interest in the investigation of superconductivity phenomena. Recent advances in the instrumentation of physics under pressure have enabled the observation of superconductivity in many elements not previously known to superconduct, and at steadily increasing temperatures. This article offers a review of the state of the art in the superconductivity of elements, highlighting underlying correlations and general trends.

From RE-211 to RE-123. How to control the final microstructure of superconducting single-domains

Abstract: This paper reviews the usual techniques for producing YBCO-type single-domains and the microstructure of the as-obtained samples. The problems of seed dissolution and parasite nucleations are discussed in detail. The formation of microstructural defects, such as pores and cracks, is examined. An important part of this review is devoted to the study of the influence of RE-211 particles (RE2BaCuO5 where RE denotes Y, Yb, Nd, Sm, Dy, Gd, Eu or a mixture of them. Generally Nd4Ba2Cu2O10 is preferred to Nd2BaCuO5) for the microstructure and properties of RE?Ba?Cu?O single-domains. Pushing/trapping theory is described in order to explain the spatial distribution of RE-211 particles in the RE-123 ((RE)Ba2Cu3O7??) monoliths. The formation of RE-211-free regions is discussed. Different ways to limit the RE-211 coarsening are reviewed. Microstructural defects in the RE-123 matrix caused by the RE-211 particles are presented. It is also shown that RE-211 particles play a significant role in the mechanical properties of single-domain samples. We finish this review by discussing the infiltration and growth process as a good technique to control the microstructure.

Calibration free method for measurement of the AC magnetization loss

Abstract: A calibration free measurement method for determination of the magnetization loss of superconducting samples exposed to the external AC magnetic field is presented. The idea is based on the measurement of the part of the power which is supplied by the AC source to the AC magnet generating the magnetic field, in which the sample is located. It uses a coil wound in parallel to the AC field magnet as the measurement coil. To achieve the necessary sensitivity, two identical systems are used, each consisting of an AC magnet and a measurement coil, one of them containing the sample and the other left empty. No measurement and/or calculation of the calibration constant is required. To confirm the suitability of this method, the loss of a Cu sample with known dissipation was measured. The applicability to the AC magnetization loss measurements of superconducting tapes is presented.

High transport critical current density and large Hc2 and Hirr in nanoscale SiC doped MgB2 wires sintered at low temperature

Abstract: We report a systematic study on the effect of sintering temperature on the phase formation, critical current density, upper critical field and irreversibility field of nanoscale SiC doped MgB2. Bulk and Fe sheathed wires doped with different nano-SiC particle sizes have been made and heat treated at temperatures ranging from 580 to 1000 °C. A systematic correlation between the sintering temperature, normal state resistivity, RRR, Jc, Hc2, and Hirr has been found in all samples of each batch. Samples sintered at a lower temperature have a very fine and well consolidated grain structure while samples sintered at a high temperature contain large grains with easily distinguishable grain boundaries. Low temperature sintering resulted in a higher concentration of impurity precipitates, larger resistivity, higher Jc up to 15 T and lower Tc values. These samples show higher Hc2 and Hirr at T near Tc but lower Hc2 near T = 0 than samples sintered at high temperature. It is proposed that huge local strains produced by nano-precipitates and grain boundary structure are the dominant mechanism responsible for higher Hc2 at T near Tc. However, higher impurity scattering due to C substitution is responsible for higher Hc2 in the low temperature regime for samples sintered at a higher temperature. In addition to high Hc2, it is also proposed that the large number of nano-impurities serve as pinning centres and improve the flux pinning, resulting in higher Jc values at high magnetic fields up to 15 T.

The strain effect on critical current in YBCO coated conductors with different stabilizing layers

Abstract: The tensile strain dependences of the critical current (Ic) in YBa2Cu3O7−δ (YBCO) coated conductors fabricated by using the rolling-assisted biaxially textured Ni–W substrates (RABiTS)–pulsed laser deposition (PLD) method were examined at 77 K and in self magnetic field. Cu and stainless steel layers were used as stabilizers to the YBCO coated conductor, and the effects of stabilizing layers on the strain tolerance of Ic were investigated, compared with the case without a stabilizing layer. The lamination of stabilizer produced an increase in the yield strength and strain tolerance of Ic in coated conductors. All YBCO coated conductors tested showed a reversible strain effect and a peak in the relation between Ic and applied strain. The peak strain of Ic and the irreversible strains for Ic degradation were enhanced when the YBCO coated conductor was laminated with a stabilizing layer. For the case laminated with a stainless steel layer, Ic recovered reversibly until the applied strain reached to about 0.5% and showed its peak at a strain of 0.42%, comparing to the case without a stabilizing layer, which were 0.21% and 0.18%, respectively. It can be predicted that the lamination of a stabilizing layer produced a significant residual compressive strain to the YBCO film during cooling to 77 K, which influenced the axial strain tolerance of YBCO coated conductors. Therefore, the Ic–tensile strain relation in YBCO coated conductors could be explained by a two-stage deformation; stage I is the region where YBCO film behaves elastically and Ic recovers when the stress is released. Stage II is the region where Ic decreases irreversibly attributable to the cracking induced in the YBCO film due to the significant plastic deformation of the substrate or the stabilizing layer.

Strong influence of boron precursor powder on the critical current density of MgB2

Abstract: The influence of the nature of the boron precursor on the superconducting properties of polycrystalline MgB2 was studied. Critical current densities (Jc) for MgB2 made from high purity amorphous boron are at least a factor of three higher than typical values measured for standard MgB2 samples made from amorphous precursors. Two possible mechanisms are proposed to account for this difference. Samples made from crystalline boron powders have around an order of magnitude lower Jc compared to those made from amorphous precursors. X-ray, Tc and resistivity studies indicate that this is as a result of reduced current cross-section due to the formation of (Mg)B–O phases. The samples made from amorphous B contain far fewer Mg(B)–O phases than crystalline B despite the fact that the amorphous B contains more B2O3. The different reactivity rates of the precursor powders accounts for this anomaly.

Seeded infiltration and growth of large, single domain Y–Ba–Cu–O bulk superconductors with very high critical current densities

Abstract: Single domain Y?Ba?Cu?O (YBCO) composed of a YBa2Cu3Oy (Y-123) superconducting bulk matrix with discrete, non-superconducting Y2BaCuO5 (Y-211) phase inclusions has been fabricated by a seeded infiltration and growth (IG) technique in the form of cylindrical pellets up to 32?mm in diameter. Sample shrinkage in the radial direction for single domains prepared by this technique is relatively low at 5% and independent of sample size, in contrast to the shrinkage observed in samples grown by conventional melt processing, which increases significantly with increasing sample diameter. Furthermore, samples grown by the IG technique exhibit low porosity of typically 0.9% of the bulk volume fraction, compared with a corresponding value of around 4.9% observed in samples fabricated by conventional melt processing. Fine Y-211 particles were observed to be embedded within the Y-123 superconducting matrix for the IG processed samples, leading to a high critical current density, Jc, of over 100?000?A?cm?2 at 77.3?K in self-field. The distribution of Y-211 particles in the IG sample microstructure, however, was inhomogeneous (unlike in previous reports), which leads to a variation in the spatial distribution of Jc. The volume fraction of Y-211 in the vicinity of the seed crystal (i.e.?corresponding to the initial c-sector growth stage), in particular, is typically around 5%, compared with a value of up to 30% in the a growth sectors more distant from the seed crystal (which corresponds well to the theoretical value for the sample composition studied here). The volume fraction of Y-211 inclusions in the c growth sector more distant from the seed was around 22%. Finally, a trend of the variation in the distribution of Y-211 particles in the Y-123 matrix grown by the IG technique was similar to that in sample grown by conventional melt processing.

Enhancement of flux pinning in YBa2Cu3O7−δ thin films embedded with epitaxially grown Y2O3 nanostructures using a multi-layering process

Abstract: Nanodot arrays of Y2O3 were dispersed in thin films of YBa2Cu3O7−δ (YBCO) by growing alternating layers of these two species using a pulsed laser deposition method. As a result, critical current density Jc both in applied magnetic field and self-field is enhanced by as much as an order of magnitude, along with a significant increase in the irreversibility field Hirr. High-resolution scanning transmission electron microscopy (STEM) and Z -contrast STEM show that the nanoparticles are crystalline and coherent with the YBCO matrix. Whereas in most other studies pinning has been attributed to the strain fields around the nanoparticles, in this case pinning may actually be due to the nanoparticles themselves ,s incethe delineation between the two species is very sharp and STEM reveals no discernible strain fields in the superconducting material around the nanoparticles. (Some figures in this article are in colour only in the electronic version)

The influence of growth conditions on the microstructure and critical currents of TFA-MOD YBa2Cu3O7 films

Abstract: The influence of three processing parameters, temperature, gas flow rate and water pressure, on the YBa2Cu3O7 film growth on LaAlO3 single-crystal substrates from trifluoroacetate precursors has been investigated and the optimal film processing conditions to achieve high critical currents have been determined. We have found that the growth conditions maximizing the critical current density are those where the nucleation of a-axis oriented grains is minimized, as determined by μ-Raman spectroscopy. Under these conditions the normal state resistivity is very near to that of single crystals because a vanishingly small film porosity is achieved. Transmission electron microscopy analysis of films quenched from the growth temperature gives some hints for understanding the mechanism linking the film porosity with the concentration of a-axis grains. A cross-linked influence of different processing parameters, such as temperature and water pressure, or water pressure and gas flow, has been demonstrated. The optimal growth temperatures are 790–830 °C, but at these growth temperatures, the critical current density is still dependent on the gas flow rate and water pressure. The optimal processing ranges are a compromise between two different competing phenomena influencing the quality of the films: inhomogeneous film formation due to HF gas stagnancy at small nominal growth rates (low gas flow rate or water pressure) and perturbed crystallinity at high gas flow rates or water pressures.

Development of a generic seed crystal for the fabrication of large grain (RE)-Ba-Cu-O bulk superconductors

Abstract: The critical current density, Jc, irreversibility field, Birr, and magnetic field trapping ability of (LRE)–Ba–Cu–O bulk superconductors, where LRE is a light rare earth element such as Nd, Sm, Eu and Gd, are generally superior to those of the more common melt-processed Y–Ba–Cu–O (YBCO). The lack of availability of a suitable seed crystal to grow large, single grain (LRE)–Ba–Cu–O superconductors with controlled orientation, however, has hindered severely the development of these materials for engineering applications over the past ten years. In this communication we report for the first time the development of a generic seed crystal that can be used to fabricate any rare earth (RE) based (RE)–Ba–Cu–O ((RE)BCO) superconductor in the form of a large single grain with controlled orientation. The new seed crystal will potentially enable large grain (LRE)–Ba–Cu–O bulk superconductors to be fabricated routinely, as is the case for YBCO. This will enable the field trapping and current-carrying characteristics of these materials to be explored in more detail than has been possible to date.

Ac loss in striped (filamentary) ybco coated conductors leading to designs for high frequencies and field-sweep amplitudes

Abstract: AC losses of YBCO coated conductors are investigated by calculation and experiment for the higher frequency regime. Previous research using YBCO film deposited onto single-crystal substrates demonstrated the effectiveness of 'striping' or filamentary subdivision as a technique for AC loss reduction. As a result of these studies the idea of subdividing YBCO 'coated conductors' (YBCO, overlayer, and even underlayer) into such stripes suggested itself. The suggestion was implemented by burning grooves into samples of coated conductor using laser micromachining. Various machining parameters were investigated, and the striping and slicing characteristics are presented. Loss measurements were performed on unstriped as well as striped samples by the pick-up coil technique at frequencies from 50 to 200 Hz at field sweep amplitudes of up to 150 mT. The effect of soft ferromagnetic Fe shielding was also investigated. The results of the experiments form a starting point for a more general study of reduced-loss coated conductor design (including hysteretic, coupling, normal eddy current, and transport losses) projected into higher ranges of frequency and field-sweep amplitude with transformer and all-cryogenic-motor/generator applications in mind.

Reversible axial-strain effect in Y Ba Cu O coated conductors

Abstract: The recently discovered reversible strain effect in Y?Ba?Cu?O (YBCO) coated conductors contrasts with the general understanding that the effect of strain on the critical-current density Jc in practical high-temperature superconductors is determined only by crack formation in the ceramic component. Instead of having a constant Jc as a function of strain before an irreversible drop when cracks form in the superconductor, Jc in YBCO coated conductors can decrease or increase reversibly with strain over a significant strain range up to an irreversible strain limit. This reversible effect is present in samples fabricated either with rolling-assisted biaxially textured Ni?W substrates or with ion-beam-assisted deposition on Hastalloy substrates. The reversibility of Jc with strain is observed for thin as well as thick YBCO films, and at two very different temperatures (76 and 4?K). The reversible effect is dependent on temperature and magnetic field, thus indicating its intrinsic nature. We also report an enhancement of the irreversible strain limit ?irr where the reversible strain effect ends and YBCO cracking starts. The value of ?irr increases from about 0.4% to more than 0.5% when YBCO coated conductors are fabricated with an additional Cu protection layer.

Flux pinning by NiO-induced nano-pinning centres in melt-textured YBCO superconductor

Abstract: Nano-pinning centres ( nm) have been successfully introduced into melt-textured YBCO by doping NiO. The doping of NiO was realized by adding NiO nanoparticles to presintered YBCO followed by melt-texture processing. This method can reduce the substitution level of Ni in the YBCO matrix and can create Ni-rich second-phase precipitates with a size around 10 nm. The critical current density of these Ni-doped samples is significantly increased and the Jc–H behaviour is remarkably improved. Our results suggest that NiO-induced nano-pinning centres may have high pinning efficiency at a temperature around 70 K and be one of the main factors for improving the performance of YBCO. Our study reveals that nano-pinning centres are also a feasible solution to solve the pinning degradation at high temperature in YBCO.

Operating strain effects in Nb3Sn cable-in-conduit conductors

Abstract: Nb3Sn superconductors show a dependence of the critical current and temperature on the strain state of the superconducting material. The basic causes of Nb3Sn strain effects, primarily differential thermal contraction between elements of the strand, have been known for 30 years, but have received more attention lately as part of a drive to achieve much higher operating current densities and make use of them in practical multistrand cables. The use of the cable-in-conduit (CICC) type of conductors to achieve high current capacity has proved popular, as the conductors offer good local cooling of the strands and distributed electrical contact between strands that is essential to provide stability against the inevitable current non-uniformity that arises with parallel connection of the strands. However, the essential openness of the cable means that the strands have to support local magnetic loads as well as being exposed to the overall magnet strain displacements. Simple structural models are developed based on mechanical measurements on cable-in-conduit conductors which are able to successfully simulate the measured superconducting performance. These suggest that degradation observed in large cables is due to a combination of the repeated bending strain experienced by the strands and filament fracture, which is starting to occur to a significant extent in some large cables. Superconducting performance improvements in strands can only be properly utilized with improved support of the strands in the cable, implying a more ordered structure than in a multistage.

AC losses in a type II superconductor strip with inhomogeneous critical current distribution

Abstract: Analytical formulae derived by Brandt and Indenbom (1993 Phys. Rev. B 48 12893–906) and Norris (1970 J. Phys. D: Appl. Phys. 3 489–507) are often used to calculate the magnetization and AC transport current losses in HTS strip conductors, respectively. In these formulae, homogeneous distribution of critical sheet current density σc in the strip is assumed. However, it is considered that σc distributions are inhomogeneous in actual HTS strips and that the inhomogeneous σc distributions cause deviations of the measured AC loss data of actual HTS strips from those formulae. A semi-analytical method to calculate AC transport current and magnetization losses is derived for a type II superconductor strip with inhomogeneous distribution of σc in the direction of the strip width. The method is derived modifying the analysis of Brandt et al. The validity of the semi-analytical method is shown by comparing the results calculated by this method with those calculated by the Norris and Brandt formulae and by a different method of our previous work and also with experimental data. Moreover, it is shown that the deviation of the measured data from the Norris and Brandt models can be estimated by assuming proper σc distributions.

Enhanced critical current density in Gd-added (Bi, Pb)-2212 bulk superconductor

Abstract: The effects of Gd addition on the phase evolution and superconducting properties of (Bi, Pb)-2212 prepared in the bulk polycrystalline form were studied. The Gd content in the samples was varied from 0.0 to 0.5 on a general stoichiometry of Bi1.7Pb0.4Sr2.0Ca1.1Cu2.1GdxOy. Phase analysis by means of x-ray diffraction, microstructural examination by scanning electron microscopy and superconducting property studies were carried out to evaluate the relative performances of the samples. It is observed that Gd addition enhances the transition temperature (TC) and critical current density (JC) of the system. Moreover no secondary phase containing Gd ions or any other cations was observed after the final stage of heat treatment. Microstructural examination shows clear and distinct morphologies for the Gd-added samples, wherein the grain growth is suppressed by the addition of Gd and the edges of the grains become more and more rounded. As a result, there is an increase in the porosity of the Gd-added samples, leading to a reduction in the sintered density for these samples.

An all chemical solution deposition approach for the growth of highly textured CeO2 cap layers on La2Zr2O7-buffered long lengths of biaxially textured Ni–W substrates for YBCO-coated conductors

Abstract: A reel-to-reel, dip coating process has been developed to continuously deposit epitaxial La2Zr2O7 (LZO) and CeO2 on 5 m long cube-textured {100} (001)Ni tapes. Recent results for La2Zr2O7 and CeO2 buffer layers deposited on long lengths of Ni substrate for the realization of YBa2Cu3O7−x (YBCO)-coated conductors are presented. The major achievement is the development of a new all chemical solution deposition (CSD) process leading to the formation of highly textured buffer layers at moderate annealing temperatures. Reproducible highly textured, dense and crack-free LZO buffer layers and CeO2 cap layers were obtained for annealing temperatures as low as 900 °C in a reducing atmosphere (Ar–5 at.%-H2). The thickness of the LZO buffer layers was determined to be (200 ± 10) nm per single coating; prepared cerium oxide layers showed a thickness of 60 nm ± 10 nm. Pulsed laser deposition (PLD) was used to grow YBCO films on these substrates. A Tc 0 of T = 90.5 K and ΔTc = 1.4 K was obtained on PLD-YBCO/CSD-CeO2 /CSD-LZO/Ni–5 at.% W, which shows the outstanding features of this new buffer layer architecture processed by CSD. The large layer thickness combined with low annealing temperatures is the main advantage of this new process for low-cost buffer layer deposition on Ni-RABiTS (rolling-assisted biaxially textured substrates).

Tensile fracture behaviour of RE-123 coated conductors induced by discontinuous yielding in Hastelloy C-276 substrate

Abstract: We investigated the intrinsic strain effect on critical current, Ic, and tensile fracture behaviour in the RE-123 coated conductors (RE = Y, Dy or Sm) with Hastelloy C-276 substrates. It is found that the SmBCO coated conductors exhibit an intrinsic strain effect on Ic which is similar to that in the DyBCO ones. Ic decreases monotonically with increasing applied tensile strain but it can recover reversibly up to the strain of 0.32% when the strain is relieved. Quenching occurred for the coated conductors with Hastelloy C-276 substrate in the narrow strain region of 0.27–0.35% regardless of the buffer layers and superconducting materials. In order to reveal the reason for such quenching, we measured mechanical properties of the Hastelloy C-276 tapes alone and coated conductors at 77 K. The as-received and as-polished Hastelloy tapes show continuous yielding, which results from homogeneous plastic deformation. On the other hand, the Hastelloy tape annealed at 963 K corresponding to the deposition condition for the superconducting layer exhibits discontinuous yielding. In the annealed Hastelloy tapes, occurrence of Luders bands is observed, indicating that plastic deformation progresses inhomogeneously. Discontinuous yielding is also confirmed in the coated conductor. The yield strains of the coated conductors are almost identical to the strains at quenching. Characteristic fracture behaviour is observed, that transverse crack arrays are initiated from the tape edge and they propagate into the central part. The crack arrays orient to a certain angle to the loading axis, which is similar to that of the Luders bands. The similarities in the initiation site and the angle to the loading axis between the Luders bands and the crack arrays and close values of quenching strains to the yielding one strongly suggest that quenching is attributable to the discontinuous yielding in the Hastelloy substrate.

Enhanced in-field critical currents of YBCO second-generation (2G) wire by Dy additions

Abstract: The addition of dysprosium oxide nanoparticles is shown to improve the critical current in perpendicular magnetic fields for second-generation (2G) wire formed by metal–organic deposition (MOD). Typical enhancements in Jc are from 0.17 MA cm−2 to over 0.33 MA cm−2 at 77 K and Bperp = 1.5 T. TEM analysis shows that we are introducing (Y,Dy)2O3 nanoparticles with dimensions of 10–50 nm. A simple theoretical analysis shows that the maximum pinning effect for additions is expected at excess concentrations of approximately 70% DyO1.5, i.e. for YBa2Cu3O7−δ+0.7DyO1.5 if the added nanoparticles are randomly dispersed and a strong pinning model is valid. An interesting feature is that the critical current in parallel field is reduced in these samples. We present evidence that shows this may be due to reduced planar defects in the YBCO.

Upper critical field, irreversibility field, and critical current density of powder-in-tube-processed MgB2/Fe tapes

Abstract: We fabricated pure and SiC-added MgB2/Fe composite tapes using a MgH2 starting powder and applying heat treatments at 600–900 °C and systematically investigated their superconducting properties. For both the pure and SiC-added tapes, the critical temperature (Tc) increased with increasing heat-treatment temperature due to the improved crystallinity of MgB2.The SiC addition decreased the Tc but increased the slope of the Bc2–T and Birr–T curves, d Bc2/d T and d Birr/d T, for all heat-treatment temperatures. The d Bc2/d T and d Birr/d T of the pure tape decreased with increasing heat-treatment temperature from 600 to 700 °C because of the longer coherence length associated with the improved crystallinity. However, the SiC addition significantly decreased the heat-treatment temperature dependences of d Bc2/d T and d Birr/d T. At a temperature of ~20 K, which is easily obtained using a cryocooler, the Birr is governed by both the Tc and d Birr/d T. The Birr of a pure tape at 20 K decreased with increasing heat-treatment temperature from 600 to 700 °C, but the Birr of the 10 mol% SiC-added tape increased with the temperature. These behaviours can be explained by the heat-treatment temperature dependence of the Tc and d Birr/d T. At 20 K the highest Birr of 10 T was obtained under the conditions of a 10 mol% SiC addition and heat-treatment temperature of 900 °C. This Birr at 20 K is comparable to that of commercial Nb–Ti at 4.2 K. The 10 mol% SiC-added tape heat treated at 900 °C and the 5 at.% SiC-added tape heat treated at 800 °C showed Jc (MgB2 core) values higher than 104 A cm−2 at 20 K in 5 T.

Impact of spatial periodic bending and load cycling on the critical current of a Nb3Sn strand

Abstract: Differences in the thermal contraction of the composite materials in a cable in conduit conductor (CICC) for the International Thermonuclear Experimental Reactor (ITER) in combination with electromagnetic charging cause significant axial, transverse and bending strains in the Nb3Sn layer. These high strain loads degrade the superconducting properties of a CICC. Here we report on the influence of periodic bending load, using different bending wavelengths from 5 to 10 mm on a Nb3Sn powder-in-tube processed strand. The strand axial tensile stress–strain curve, the critical current versus applied axial strain results, the influence of cyclic loading on the RRR and assessment of the current transfer length from AC loss measurements, required for the analysis, are presented as well. For the strand under investigation, we find an influence of bending strain on the Ic that corresponds well to the predictions obtained from the applied classical relations, distinguishing ultimate boundaries of high and low interfilament electrical resistance. The reduction versus applied bending strain is similar for all wavelengths and equivalent to the low transverse resistance model, which is consistent with the estimated current transfer length. The cyclic behaviour in terms of critical current and n-value involves a component representing a permanent reduction as well as a factor expressing reversible (elastic) behaviour as a function of the applied load. The results from the set-up enable a discrimination in performance reduction per specific load type and per strand type. In this paper, we discuss the results of the pure bending tests.

Axial tensile stress-strain characterization of ITER model coil type Nb 3 Sn strands in TARSIS

Abstract: For a few years there has been an increasing effort to study the impact of (bending) strain on the transport properties of superconducting wires. As the stress distribution, originated by differences in the thermal expansion and electromagnetic load, is the driving factor for the final strains, the axial and transverse stiffness of the strand play a crucial role in the final performance. Since the strain state of the Nb3Sn filaments in strands determines the transport properties, basic experimental stress?strain data are required at the strand level for accurate modelling and analysis and eventually for optimizing cable and magnet design. We performed axial tensile stress?strain measurements on several types of Nb3Sn strands used for the manufacture of the International Experimental Thermonuclear Reactor (ITER) central solenoid and toroidal field model coils and a powder-in-tube processed wire. In total 48 wire samples were tested at boiling helium, boiling nitrogen and at room temperature. We present the computation of the stress?strain characteristic with a straightforward 1D model using an independent materials database, obtaining a good agreement with the experimental results. The details from the take-off origin of the measured stress?strain curves are discussed and the data are evaluated with respect to some commonly used functions for fitting stress?strain curves. The measurements are performed in the new setup TARSIS (test arrangement for strain influence on strands). A double extensometer connected to the sample enables us to determine the strain level whereas a load cell is used to monitor the stress level. For higher levels of applied stress (100?MPa), we found typically a higher strain for bronze route wires compared to a powder-in-tube and internal tin type of strand. Stress?strain results are essential to assess more accurately the impact of thermal and electromagnetic induced stress on the strain state of the Nb3Sn filaments for wires from various manufacturing processes.

A review on the synthesis of Y?Ba?Cu-oxide powder

Abstract: The preparation of Y?Ba?Cu-oxide (YBCO) powder by various techniques is critically reviewed and the synthesis of ultrafine superconducting YBa2Cu3Ox(123) powders is specially emphasized. The fine powder has assumed a large importance, requiring the minimization of processing parameters (time and temperature) during calcination, homogenization and sintering to avoid the problems of evaporation of the constituent oxides. From the voluminous research activities on YBCO it is known that by adopting any preparation technique the superconducting transition temperature (Tc) value of ?92?K could be achieved in the bulk samples, thus in this review the Tc values are not emphasized. Also, the other transport properties such as critical current densities (Jc) and magnetic properties are not discussed, since these properties are mainly controlled by many other parameters related to the weak link across the grain boundaries of the specimens, and a very little role is played by the synthetic procedure. The weak links across the grain boundaries are known to be the cause for the poor Jc values in bulk specimens and those weak links are observed due to the presence of impurities at the grain boundaries, misorientation of grains and oxygenation of the samples. By selecting the proper synthetic procedure, only the impurity problem can be solved, and to remove the other problems, a proper fabrication method needs to be adopted. This review will guide the new researcher to find the appropriate technique to synthesize good quality YBCO powder.

Reluctance motors with bulk HTS material

Abstract: In recent years we have successfully designed, built and tested several reluctance motors with YBCO bulk material incorporated into the rotor, working at 77 K. Our last motor type SRE150 was tested up to 200 kW. The aim of our investigations is the construction of motors with extremely high power density and dynamics. In comparison to conventional motor types the advantage of HTS reluctance motors with respect to size and dynamics could be demonstrated. Some fields of possible future applications will be described. These motors show a significant improvement in performance using high quality HTS bulk elements in the rotor. Until now the motor parameters have been limited by the current density which could be obtained in the bulk material at 77 K and by the geometric dimensions of the segments available. Therefore we expect further improvements in the case of these materials. Since the total motor including stator and rotor is working at low temperature we have to optimize the windings and the magnetic circuit to these operation conditions. A new design of a 200 kW motor in order to achieve increased power density and the theoretical results of our calculations will be shown.

Magnetic field dependence of Jc for Gd-123 coated conductor on PLD-CeO2 capped IBAD-GZO substrate tapes

Abstract: We prepared GdBa2Cu3O7?? (Gd-123) coated conductors by the pulsed laser deposition (PLD) method on PLD-CeO2/ion-beam assisted deposition (IBAD)-Gd2Zr2O7 (GZO)/hastelloy metal substrate tapes. The Gd-123 film showed a higher critical current density (Jc) in magnetic fields (B) and higher critical temperature (Tc) than those of YBa2Cu3O7?? (Y-123) film. The Gd-123 film exhibited a high Jc value of 0.19?MA?cm?2 at 3?T () in liquid nitrogen (77?K), and the Tc value was 93.8?K. The Jc value at 3?T was twice as high as that of Y-123 film. Moreover, the angular dependence in magnetic fields of Jc for Gd-123 was also superior to that of Y-123. The reduction of Jc by the magnetic field angle for Gd-123 was less sensitive than that in Y-123. Furthermore, Gd-123 clearly exhibited a peak at 0? () as well as at 90? () in the relationship of Jc on the magnetic field angle curve (Jc??), while Y-123 had no peak at 0?. These superior characteristics are considered to be due to the existing defects, such as stacking faults, as observed by transmission electron microscopy. PLD-Gd-123 coated conductors, with pinning-effective defects, high Jc and low anisotropy in Jc??, show promise for future applications.