Showing papers in "Superconductor Science and Technology in 2008"

Ultra-high flux pinning properties of BaMO3-doped YBa2Cu3O7−x thin films (M = Zr, Sn)

Abstract: We studied the flux pinning properties of BaZrO3-doped YBa2Cu3O7−x and BaSnO3-doped YBa2Cu3O7−x films. We found that BaSnO3-doped films showed very high global pinning forces, Fp, of 28.3 GN m−3 (77 K, ) and 103 GN m−3 (65 K, ), twice that of BaZrO3-doped films. Transmission electron microscopy analysis showed that, in both films, nanorods of the dopant phase were incorporated. The BaSnO3 nanorods were nearly straight but the BaZrO3 nanorods became curved with the increasing film thickness.

Crystallographic phase transition and high-Tc superconductivity in LaFeAsO:F

Abstract: Undoped LaFeAsO, the parent compound of the newly found high-Tc superconductor, exhibits a sharp decrease in the temperature-dependent resistivity at ~160?K. The anomaly can be suppressed by F doping with simultaneous appearance of superconductivity appears correspondingly, suggesting a close association of the anomaly with the superconductivity. We examined the crystal structures, magnetic properties and conductivity of undoped (normal conductor) and 14?at.% F-doped LaFeAsO (Tc = 20?K) by synchrotron x-ray diffraction (XRD), DC magnetic measurements, and ab?initio calculations demonstrated that the anomaly is associated with a phase transition from tetragonal (P4/nmm) to orthorhombic (Cmma) phases at ~160?K as well as an antiferromagnetic spin ordering transition at ~140?K. These transitions can be explained by spin configuration-dependent potential energy surfaces derived from the ab?initio calculations. The suppression of the transitions is ascribed to interrelated effects of geometric and electronic structural changes due to doping by F? ions.

Superconductivity at 53.5 K in GdFeAsO1−δ

Abstract: Here we report the fabrication and superconductivity of the iron-based arsenic oxide GdFeAsO1−δ compound with oxygen-deficiency, which has an onset resistivity transition temperature at 53.5 K. This material has the same crystal structure as the newly discovered high-Tc ReFeAsO1−δ family (Re = rare earth metal) and a further reduced crystal lattice, while the Tc starts to decrease compared with the SmFeAsO1−δ system.

Prospects for improving the intrinsic and extrinsic properties of magnesium diboride superconducting strands

Abstract: The magnetic and transport properties of MgB2 films present performance goals yet to be attained for powder-processed bulk samples and conductors. Carbon-doped films have exhibited upper critical fields, ?0Hc2, as high as 60?T and a possible upper limit of more than twice this value has been predicted. Very high critical current densities, Jc, have also been measured for films, e.g.?25?MA?cm?2 in the self-field and 7?kA?cm?2 in 15?T. Such performance limits are still out of reach for even the best MgB2 magnet wire. In discussing the present status and prospects for improving the performance of powder-based wire we focus attention on (1)?the intrinsic (intragrain) superconducting properties of MgB2?Hc2 and flux pinning, and (2)?the factors that control the efficiency with which current is transported from grain to grain in the conductor, an extrinsic (intergrain) property. With regard to item (1), the role of dopants in Hc2 enhancement is discussed and examples presented. On the other hand their roles in increasing Jc, both via Hc2 enhancement as well as via direct fluxoid/pinning center interaction, are discussed and a comprehensive survey of Hc2 dopants and flux pinning additives is presented. Dopant selection, chemistry, methods of introduction (inclusion), and homogeneity of distribution (via the rounding of the superconducting electronic specific heat transition) are considered. Current transport through the powder-processed wire (an extrinsic property) is partially blocked by the inherent granularity of the material itself and the chemical or other properties of the intergrain surfaces. Overall porosity, including reduced density and intergranular blocking, is quantified in terms of the measured temperature dependence of the normal-state resistivity compared to that of a clean single crystal. Several experimental results are presented in terms of per cent effective cross-sectional area for current transport. These and other such results indicate that in many cases less than 15% of the conductor's cross-sectional area is able to carry transport current. It is pointed out that densification in association with the elimination of grain-boundary blocking phases would yield fivefold-to-tenfold increases in Jc in relevant regimes, enabling the performance of MgB2 in selected applications to compete with that of Nb3Sn.

Progress in high temperature superconductor coated conductors and their applications

Abstract: Second generation (2G) high temperature superconductor (HTS) wires are based on a coated conductor technology. They follow on from a first generation (1G) HTS wire consisting of a composite multifilamentary wire architecture. During the last couple of years, rapid progress has been made in the development of 2G HTS wire, which is now displacing 1G HTS wire for most if not all applications. The engineering critical current density of these wires matches or exceeds that of 1G wire, and the mechanical properties are also superior. Scale-up of manufacturing is proceeding rapidly, with several companies already supplying the order of 10 km annually for test and demonstration. Coils of increasing sophistication are being demonstrated. One especially attractive application, that relies on the specific properties of 2G HTS wire, is fault current limitation. By incorporating a high resistivity stabilizer in the coated conductor, one can achieve high resistance in a quenched state during a fault event and at the same time provide significant heat capacity to limit the temperature rise. A test of a 2.25 MVA single phase system at 7.5 kV employing such wire by the Siemens/AMSC team has demonstrated all the key features required for a cost-effective commercial system. A novel approach to providing fault current limiting functionality in HTS cables has also been introduced.

Evidence for two distinct scales of current flow in polycrystalline Sm and Nd iron oxypnictides

Abstract: Early studies have found quasi-reversible magnetization curves in polycrystalline bulk rare-earth iron oxypnictides that suggest either widely spread obstacles to intergranular current or very weak vortex pinning. In the present study of polycrystalline samarium and neodymium iron oxypnictide samples made by high pressure synthesis, the hysteretic magnetization is significantly enhanced. Magneto-optical imaging and study of the field dependence of the remanent magnetization as a function of particle size both show that global currents over the whole sample do exist but that the intergranular and intragranular current densities have distinctively different temperature dependences and differ in magnitude by about 1000. If the highest current density loops lie only within grains, their magnitude is ~5 × 106 A cm−2 at 5 K and self-field. Whole sample current densities, though two orders of magnitude lower at 1000–10 000 A cm−2, are some two orders of magnitude higher than in random polycrystalline cuprates. We cannot yet be certain whether this large difference in global and intragrain current density is intrinsic to the oxypnictides or due to extrinsic barriers to current flow, because the samples contain a significant second phase, some of which wets the grain boundaries and may cause a superconducting–normal–superconducting proximity effect in the whole sample critical current.

Magnetic field generated by shielding current in high T c superconducting coils for NMR magnets

Abstract: Numerical electromagnetic field analyses of high Tc superconducting tape in coils were carried out to calculate the magnetic field generated by the shielding (magnetization) current in superconducting tape. The numerical model employs the power law electric field–current density characteristic and the thin strip approximation, in which the current component normal to the wide face of the tape is neglected. The shielding (magnetization) currents lead to non-uniform current distributions in the superconducting tape in the coils. The magnetic field generated by the shielding (magnetization) current can deteriorate the field quality and could be a concern in insert coils for NMR magnets using high Tc superconducting tape.

Vortex pinning in chemical solution nanostructured YBCO films

Abstract: A methodology of general validity to study vortex pinning in YBa2Cu3O7−δ (YBCO) is described. It is based on measuring the angular-dependent critical current density in a large range of temperatures and magnetic fields and it permits one to identify, separate and quantify the three basic vortex pinning contributions associated to anisotropic–strong, isotropic–strong and isotropic–weak pinning centers. The corresponding vortex pinning phase diagrams for these three contributions are built up. This methodology is applied to the recently discovered solution-derived YBCO–BaZrO3 nanocomposites which display a maximum vortex pinning force of 78 GN m−3 at 65 K, being five times larger than that of NbTi at 4.2 K. These nanocomposites show an effective anisotropy tending to unity and the best superconducting properties observed so far in YBCO. We present the richness and complexity of their microstructure and the correlation with the vortex pinning properties. The application of the vortex pinning methodology and comparison with a standard solution-derived YBCO film enable us to identify the isotropic–strong defect contribution being at the origin of their unique properties.

Fabrication of high-performance MgB2 wires by an internal Mg diffusion process

Abstract: We succeeded in the fabrication of high-Jc (critical current density) MgB2 /Fe wires applying the internal Mg diffusion (IMD) process with a pure Mg core and SiC addition. A pure Mg rod with a diameter of 2 mm was placed at the center of a Fe tube with an outer diameter of 6 mm and an inner diameter of 4 mm, and the space between the Mg and the Fe tube was filled with B powder or a powder mixture of B–(5 mol%) SiC. The composite was cold worked into a wire with a diameter of 1.2 mm and finally heat treated at temperatures above the melting point of Mg (~650 °C). During the heat treatment, liquid Mg infiltrated into the B layer and reacted with B to form MgB2. X-ray diffraction analysis indicated that the major phase in the reacted layer is MgB2. Scanning electron microscopy (SEM) analysis shows that the density of the MgB2 layer is higher than that of the usual powder-in-tube (PIT) processed wires. The wires with additional 5 mol% SiC heat treated at 670 °C showed Jc values higher than 105 A cm−2 in 8 T and 41 000 A cm−2 in 10 T at 4.2 K. These values are much higher than those of the usual PIT processed wires, even compared to those with additional SiC. The higher density of the MgB2 layer obtained by the diffusion reaction is the major cause of these excellent Jc values.

Superconducting properties of granular SmFeAsO1−xFx wires with Tc = 52 K prepared by the powder-in-tube method

Abstract: We report the fabrication of Ta-sheathed superconducting SmFeAsO1−xFx wires by the powder-in-tube method for the first time. The transition temperature of the SmFeAsO0.65F0.35 wires was confirmed to be as high as 52 K. High critical fields Hc2(0)≥120 T as well as current density Jc of 3900 A cm−2 at 5 K were also demonstrated. It should be noted that the Jc exhibits a very weak field dependence behavior, indicating a very encouraging prospect for application of the new superconductors.

Modeling of coated conductor pancake coils with a large number of turns

Abstract: The current distribution, the magnetic field and the AC loss are numerically studied for pancake coils with up to 200 turns, taking into account the real thickness of the conductor and the magnetic interaction between the turns. All calculations assume the critical state model with a constant critical current density. The results show a slab-like current distribution, with the absence of magnetization currents due to the radial magnetic field. Consequently, longitudinal striation, with or without twisting, cannot reduce the self-field AC loss of this system. Finally, an analytical formula has been deduced for the AC loss at current amplitude: Im = Ic, where Ic is the critical current, for large mean coil radius and any number of turns.

Upper critical field, Hall effect and magnetoresistance in the iron-based layered superconductor LaFeAsO0.9F0.1−δ

Abstract: By using a two-step method, we successfully synthesized the new iron-based superconductor LaFeAsO0.9F0.1−δ. The resistive transition curves under different magnetic fields were measured, leading to the determination of the upper critical field Hc2(T) of this new superconductor. The value of Hc2 at zero temperature is estimated to be about 50 T roughly. In addition, the Hall effect and magnetoresistance were measured in a wide temperature region. A negative Hall coefficient RH has been found, implying a dominant conduction mainly by electron-like charge carriers in this material. The charge carrier density determined at 100 K is about 9.8 × 1020 cm−3, which is close to the cuprate superconductors. It is further found that the magnetoresistance does not follow Kohler's law. Meanwhile, the different temperature-dependent behaviors of resistivity, Hall coefficient and magnetoresistance have anomalous properties at about 230 K, which may be induced by some exotic scattering mechanism.

Growth and anisotropy of La(O, F)FeAs thin films deposited by pulsed laser deposition

Abstract: LaFeAsO1−xFx thin films were deposited successfully on (001)-oriented LaAlO3 and MgO substrates from stoichiometric LaFeAsO1−xFx polycrystalline targets with fluorine concentrations up to x = 0.25 by pulsed laser deposition (PLD). Room temperature deposition and post annealing of the films yield films with a pronounced c-axis texture and a strong biaxial in-plane orientation. Transport measurements show metallic resistance and the onset of superconductivity at 11 K. μ0Hc 2(T) was determined by resistive measurements and yield μ0Hc 2 values of 3 T at 3.6 K for and 6 T at 6.4 K for .

Systematic study of the BaSnO3 insertion effect on the properties of YBa2Cu3O7−x films prepared by pulsed laser ablation

Abstract: YBa2Cu3O7−x+BaSnO3 (YBCO + BSO) mixed films with increasing BSO content were prepared on SrTiO3 substrates by pulsed laser deposition from YBCO+x wt% BSO mixed targets (x = 2, 3, 4, 5, 6, 8 and 9 wt%). Transmission electron microscopy (TEM) images reveal that BSO is incorporated into the YBCO matrix in form of nanorods, whose density and size increase with increasing BSO content. The pinning properties for the mixed YBCO + BSO film were systematically studied. The 4 wt% sample presents the maximum global pinning forces FP at all considered temperatures: 28.3 GN m−3 near 2 T at 77 K, 103 GN m−3 near 5 T at 65 K, and>241 GN m−3 near 9 T at 40 K.

Present status and future prospect of coated conductor development and its application in Japan

Abstract: The current national project on coated conductors using Y-system superconductors has been carried out over the project period (FY2003–FY2007). In this paper, the current status and the future prospect of this project are reviewed. The high performance tape development group, consisting of Fujikura and SRL-NCCC, has worked on the tape by PLD-REBCO superconducting layers on PLD-CeO2/IBAD-GZO buffered substrates. A high product of Ic and L, higher than 112 166 A m, was achieved in a 368 m–304.8 A GdBCO tape whose Ic value is mostly above 350 A/cm in width. The performance under magnetic field was also improved up to 42 A at 3 T in a GdBCO short film with doping of ZrO2. 61 m long GdBCO tape with ZrO2 doping showed a high Ic value of 220 A at self field and 30 A at 3 T. On the other hand, another group focusing on low production cost has worked on TFA-MOD and MOCVD processes. The extremely high Ic value of 735 A/cm-w was obtained in TFA-MOD films on PLD-CeO2/IBAD-GZO/Hastelloy substrate due to the effect of Ba-poor nominal composition. In efforts towards long tape production by the SWCC group, a 200 m long tape with a high Ic value of 200 A/cm-w was obtained using a batch-type furnace. The Ic × L value of this tape was 40 000 A m, which is the highest value in the world obtained by the TFA-MOD process. Based on the above achievements in coated conductor process development, two new additional goals were set in the project. One is the development of extremely low cost tape and the other is the development of the basic technologies for making electric power devices of cables, transformers, motors, current-limiters and cryocoolers. Some of the new investigations have already revealed marvellous results, such as a 15 kW motor, low AC loss coils, low AC loss cables, etc.

Essential factors for the critical current density in superconducting MgB2: connectivity and flux pinning by grain boundaries

Abstract: Electrical connectivity and flux pinning strength of grain boundaries are essential factors in determining the critical current density in superconducting polycrystalline MgB2. The effect of these factors is quantitatively investigated for a series of MgB2 bulk samples prepared by various methods. The electrical connectivity is evaluated using the percolation model and the obtained electrical connectivity is used for the estimation of the residual resistivity of superconducting MgB2 grains. The elementary pinning force of grain boundaries is evaluated using the theoretical result of Yetter et al based on the electron scattering mechanism with the residual resistivity. It is found that the critical current density is approximately proportional to the product of the electrical connectivity, the elementary pinning force of grain boundaries and the reciprocal grain size. This confirms that the critical current density is dominantly determined by the electrical connectivity and the flux pinning strength of grain boundaries. The flux pinning property in MgB2 under the condition of full electrical connectivity is compared with that in Nb3Sn. The obtained result shows that the flux pinning ability of pure MgB2 is comparable to or even higher than that of Nb3Sn, indicating a much higher potential in carbon-doped MgB2. This proves that the MgB2 is a promising superconductor for practical applications.

The effect of substitution of Eu on the critical current density and flux pinning properties of (Bi, Pb)-2212 superconductor

Abstract: The critical current density and flux pinning properties of Bi1.6Pb0.5Sr2−xEuxCa1.1Cu2.1Oy (where x = 0.0, 0.1, 0.2, 0.3, 0.4 and 0.5) prepared by solid state synthesis in bulk polycrystalline form were studied. The samples were characterized by powder x-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with energy-dispersive x-ray analysis (EDS) and superconductivity measurements. The critical temperature (TC), critical current density (JC) and field dependence of JC of the Eu-substituted samples were found to be highly enhanced for optimum doping levels. The peak position of the normalized pinning force density (Fp/Fpmax) is found to shift to higher fields (0.88 T) for optimally doped samples in contrast to 0.2 T for the undoped sample. The enhancement of the JC–B characteristic and pinning force density FP (FP = JC × B) due to Eu substitution is of great technological significance.

Nanoscale grains, high irreversibility field and large critical current density as a function of high-energy ball milling time in C-doped magnesium diboride

Abstract: Magnesium diboride (MgB2) powder was mechanically alloyed by high-energy ball milling with C to a composition of Mg(B0.95C0.05)2 and then sintered at 1000 °C in a hot isostatic press. Milling times varied from 1 to 3000 min. Full C incorporation required only 30–60 min of milling. The grain size of sintered samples decreased with increased milling time to 80 000 A cm−2 with 1200 min of milling compared with only ~26 000 A cm−2 for 60 min of milling. This non-compositional performance increase is attributed to grain refinement of the unsintered powder by milling, and to the probable suppression of grain growth by milling-induced MgO nanodispersions.

Preparation of LaFeAsO0.9F0.1 wires by the powder-in-tube method

Abstract: We report that Fe sheathed LaFeAsO0.9F0.1 wires with Ti as a buffer were fabricated by the powder-in-tube (PIT) method for the first time. Compared to the common two-step vacuum quartz tube synthesis method, the PIT method is more convenient and safe for synthesizing the novel iron-based layered superconductors. Structural analysis by means of x-ray diffraction shows that LaFeAsO0.9F0.1 is the main phase in wires produced by this synthesis method. The transition temperature of the LaFeAsO0.9F0.1 wire is around 25 K. The micrograph shows a homogeneous microstructure with a grain size of 1–3 µm. The results suggest that the PIT process is promising for preparing iron-based layered superconductors.

High-efficiency and low-cost permanent magnet guideway consideration for high-Tc superconducting Maglev vehicle practical application

Abstract: In order to improve the cost performance of the present high-Tc superconducting (HTS) Maglev vehicle system for practical application, the multi-pole permanent magnet guideway (PMG) concept was introduced. A well-known double-pole Halbach PMG was chosen as a representative of multi-pole PMGs to compare with traditional monopole PMGs from the point of view of levitation efficiency and cost. Experimental results show that YBCO bulks above the double-pole Halbach PMG can exhibit better load capability and guidance performance as well as dynamics stability at the applied working height between the bulk HTSC and the PMG due to a more reasonable magnetic field distribution at the working range of bulk HTSC. Furthermore, the double-pole PMG configuration can play a more important role in improving guidance performance due to the potential-well field configuration. By comparing with former 'century' PMGs, the double-pole Halbach PMG shows another remarkable advantage in reducing the cost of levitation. As another necessary issue, magnetic field homogeneity and the corresponding magnetic drag force of a double-pole Halbach PMG has been considered by experiment in spite of the above highlights. Synthetically, the multi-pole Halbach PMG design is concluded to be one important choice for future HTS Maglev vehicle applications because of its high efficiency and low cost.

Dependence of the critical current of YBa2Cu3O7−δ coated conductors on in-plane bending

Abstract: A new method to measure the effect of in-plane bending on the critical current of YBa2Cu3O7−δ coated conductors is presented. Such a bending mode can be important in transmission cables, saddleback magnets, and double-pancake windings. A linear strain distribution over the width of the conductor develops in this bending mode, where one half of the conductor is under axial compressive strain and the other half is under axial tensile strain. A reversible reduction in critical current of up to 5% is measured in 4 mm wide conductors at a critical bending radius of 0.25–0.28 m. The critical current degrades irreversibly for bending radii less than this because the strain at the edge of the conductor that is under tension irreversibly damages the conductor. The results are described by use of a model that calculates the critical current as a function of in-plane bending radius by taking into account the strain gradient over the width of the sample and the measured dependence of the critical current on axial strain. A similar approach can be used to calculate the degradation of the critical current n of other deformation modes, such as torsion, or other more complex geometries.

Further increase of the critical current density of MgB2 tapes with nanocarbon-doped mechanically alloyed precursor

Abstract: The combination of nanocarbon-doped nanosized MgB2 precursor powder with an inert metallic sheath of appropriate hardness gives the possibility to obtain tapes with significantly improved critical current densities at high magnetic fields. In parallel field, Jc-values of 104?A?cm?2 at 16.4?T (4.2?K) and 5.6?T (20?K) could be measured.

The intrinsic strain effect on critical current under a magnetic field parallel to the c axis for a MOCVD-YBCO-coated conductor

Abstract: A new experimental set-up was developed to evaluate the strain dependence of critical current (Ic(ea)) for YBa2Cu3O7−δ (YBCO)-coated conductors under a magnetic field in a variable temperature environment. In this paper, we report the first results on the effect of a magnetic field parallel to the c axis () on Ic(ea) up to 10 T at temperatures of 60–77 K. We found that the magnetic field affects Ic(ea) in a different manner, depending on the field region. When the magnetic field increases from B = 0 T, normalized Ic(ea) is first improved and the optimal situation is realized under the characteristic magnetic field of Bp = 0.2 T at 77 K and Bp = 0.4 T at 70 K, respectively. For higher magnetic field, Ic(ea) degrades further with increasing strain. From the results of a fitting analysis, we confirmed that the strain at the peak of Ic(ea) shifts to higher strain and reaches the maximum value at Bp. The peak shift as a function of magnetic field found in the YBCO-coated conductors is in contrast with the Ic(ea) behavior for conventional low temperature superconducting composites, in which all of the critical parameters are optimized when the intrinsic strain of the superconductor is zero. From the present result, we can conclude that the peak strain of the Ic(ea) curve under a magnetic field is not determined only by the thermal residual strain of the YBCO film in the coated conductor. For the high field region, the curvature of the Ic(ea) curve increases with increasing magnetic field and temperature. It results in a steep decrease in Ic(ea) at high magnetic fields and temperatures.

Growth and characterization of A1−xKxFe2As2 (A = Ba, Sr) single crystals with x = 0–0.4

Abstract: Single crystals of A1−xKxFe2As2 (A = Ba, Sr) with high quality have been grown successfully by an FeAs self-flux method. The samples have sizes up to 4 mm with flat and shiny surfaces. The x-ray diffraction patterns suggest that they have high crystalline quality and c-axis orientation. The non-superconducting crystals show a spin-density-wave (SDW) instability at about 173 and 135 K for the Sr-based and Ba-based compound, respectively. After doping K as the hole dopant into the BaFe2As2 system, the SDW transition is smeared, and superconducting samples of the compound Ba1−xKxFe2As2 (0

The effect of internal pressure on the tetragonal to monoclinic structural phase transition in ReOFeAs: the case of NdOFeAs

Abstract: We report the temperature dependent x-ray powder diffraction of the quaternary compound NdOFeAs (also called NdFeAsO) in the range between 300 and 95 K. We have detected the structural phase transition from the tetragonal phase, with P4/nmm space group, to the orthorhombic or monoclinic phase, with Cmma or P112/n (or P2/c) space group, over a broad temperature range from 150 to 120 K, centered at T0 ∼ 137 K. Therefore the temperature of this structural phase transition is strongly reduced, by about ∼30 K, by increasing the internal chemical pressure going from LaOFeAs to NdOFeAs. In contrast, the superconducting critical temperature increases from 27 to 51 K going from LaOFeAs to NdOFeAs doped samples. This result shows that the normal striped orthorhombic Cmma phase competes with the superconducting tetragonal phase. Therefore by controlling the internal chemical pressure in new materials it should be possible to push toward zero the critical temperature T0 of the structural phase transition, giving the striped phase, in order to get superconductors with higher Tc. (Some figures in this article are in colour only in the electronic version)

Integral equations for the current density in thin conductors and their solution by the finite-element method

Abstract: The current density and magnetic field distributions in thin conductors are important for several applications, and they can be computed by solving integral equations This paper describes the implementation of a one-dimensional (1D) integral equation in a finite-element model This numerical method does not require the use of ad hoc assumptions to avoid logarithmic divergences of the current density at the conductor's edges and, by using a coupling with 2D electromagnetic models, it can be used to solve cases of increasing complexity With respect to commonly used 2D models, it overcomes the typical problems linked to the mesh of conductors with high aspect ratio, such as the use of large memory and long computing times

Radiation effects on MgB2 : a review and a comparison with A15 superconductors

Abstract: The study of the effects of irradiation damage in superconducting materials is crucial for the twofold aim of helping to understand the nature of superconductivity and of improving the superconducting properties for applications. In this paper we review the main experiments performed on irradiated MgB2, with the main purpose being to investigate the suppression, or in some cases enhancement, of the superconducting properties. However, we do not treat the important but substantially different topic of irreversible properties such as magnetization and critical current densities. All the results obtained recently on damaged MgB2, primarily using neutrons and α particles, are compared with similar experiments performed in the past on A15 superconductors. We discuss the suppression of the critical temperature (Tc) with disorder, showing the existence of a universal dependence of Tc on the residual resistivity. We review the experiments investigating the defect structure and the effect of annealing after irradiation. The behaviours of the upper critical fields, specific heat, resistivity and magnetoresistivity upon irradiation are also presented. The experiments of specific heat, tunnelling and point contact spectroscopies, which provide an evaluation of the energy gaps as a function of disorder, are summarized. Finally, phenomenological models for the changes in the superconducting properties are discussed in connection with the experimental evidence outlined before. The differences, as well as the important similarities, arising from the comparison between the behaviour of irradiated MgB2 and A15 samples provide significant indications of an overall understanding of radiation damage in superconducting materials.

A solution for transverse load degradation in ITER Nb3Sn CICCs: verification of cabling effect on Lorentz force response

Abstract: We present the latest results of the novel model for transverse electromagnetic load optimization (TEMLOP) especially developed for the ITER type of cable-in-conduit conductors (CICCs). The Nb3Sn CICCs for the International Thermonuclear Experimental Reactor (ITER) showed a substantial degradation in their performance correlated with increasing electromagnetic load. Not only do the differences in the thermal contraction of the composite materials affect the critical current (Ic) and temperature margin, but electromagnetic forces cause a significant transverse strand contact and bending strain in the Nb3Sn layers, resulting in localized filament cracking and permanent degradation. The most essential feature of the a priori TEMLOP predictions presented in May 2006 is that the severe degradation in CICCs can be improved greatly and straightforwardly by increasing the pitch length in subsequent cabling stages and by reducing the void fraction. These corrective measures give more support to the strands, sufficiently reduce the strain, and therefore avoid filament damage at the strand crossover points in the cables. It was the first time that an increase of the cable twist pitches has been proposed and no experimental evidence was available at that time. A full-size European prototype TF conductor sample (TFPRO-2), manufactured in autumn 2006, was adapted according to this new insight and tested in April 2007 in SULTAN for experimental validation of the predictions. The results were outstanding: for the first time an Nb3Sn CICC conductor achieved the performance that can be expected based on the single-strand properties, with high n value and no sign of degradation. As input, besides the cable properties, the model directly uses the measured data from single strands under uni-axial stress and strain, periodic bending and contact loads. The recent test results of the ITER OST strands used for the manufacture of the TFPRO-2 obtained with the TARSIS set-up are presented. With these most recent strand results, the model substantiates that not only strand bending is causing degradation but, depending on the strand and cable layout, the strand contact stress can also play a critical role. TEMLOP demonstrates that the twist pitch scheme and void fraction, of the proposed ITER reference TF conductor layout with a first-stage triplet twist pitch of 45 mm, turns out to be practically a worst-case scenario. It is also shown that shorter pitches can lead to an improvement but this requires more Nb3Sn material per metre composite conductor. However, it has been experimentally proven now that the proposed changes recover the ITER TF conductor operational margin up to the expected strand performance. The ITER TF conductor specification is being adapted now and it becomes possible to gain significant savings on the strand design, as degradation no longer needs to be compensated for.

La2Zr2O7 single buffer layer for YBCO RABiTS coated conductors

Abstract: Chemical deposition methods like MOD and MOCVD are promising approaches for coated conductors (CCs) due to their reduced cost and easy scaling. High quality La2Zr2O7 (LZO) buffer layers were prepared by MOD on Ni–5 at.% W (NiW) RABiTS and subsequent YBCO layers (450–800 nm thick) were deposited by pulsed injection MOCVD, leading to a simple low cost architecture NiWRABiTS/LZOMOD/YBCOMOCVD. In this novel combination of MOD and MOCVD approaches, a single LZOMOD buffer layer is sufficient to ensure structural compatibility between YBCO and NiW, and protect the substrate from oxidation during YBCO MOCVD. YBCO films were epitaxially grown on LZO and exhibited critical current densities Jc close to 1 MA cm−2 at 77 K with a critical temperature Tc = 91 K and ΔTc<1 K.

Superconductivity in Co-doped SmFeAsO

Abstract: Here we report the synthesis and characterizations of SmFe1−xCoxAsO (x = 0.10, 0.15) for the first time. The parent compound SmFeAsO itself is not superconducting but shows an antiferromagnetic order near 150 K, which must be suppressed by doping before superconductivity emerges. With Co doping in the FeAs planes, antiferromagnetic order is destroyed and superconductivity occurs at 15.2 K. Similar to LaFe1−xCoxAsO, the SmFe1−xCoxAsO system appears to tolerate considerable disorder in the FeAs planes. This result is important, suggesting a different mechanism for cuprate superconductors compared to the iron-based arsenide ones.