D Abraimov

New Fe-based superconductors: properties relevant for applications

Less than two years after the discovery of high temperature superconductivity in oxypnictide LaFeAs(O, F) several families of superconductors based on Fe layers (1111, 122, 11, 111) are available. They share several characteristics with cuprate superconductors that compromise easy applications, such as the layered structure, the small coherence length and unconventional pairing. On the other hand, the Fe-based superconductors have metallic parent compounds and their electronic anisotropy is generally smaller and does not strongly depend on the level of doping, and the supposed order parameter symmetry is s-wave, thus in principle not so detrimental to current transmission across grain boundaries. From the application point of view, the main efforts are still devoted to investigate the superconducting properties, to distinguish intrinsic from extrinsic behaviors and to compare the different families in order to identify which one is the fittest for the quest for better and more practical superconductors. The 1111 family shows the highest Tc, huge but also the most anisotropic upper critical field and in-field, fan-shaped resistive transitions reminiscent of those of cuprates. On the other hand, the 122 family is much less anisotropic with sharper resistive transitions as in low temperature superconductors, but with about half the Tc of the 1111 compounds. An overview of the main superconducting properties relevant to applications will be presented. Upper critical field, electronic anisotropy parameter, and intragranular and intergranular critical current density will be discussed and compared, where possible, across the Fe-based superconductor families.

The effect of strain on grains and grain boundaries in YBa2Cu3O7−δ coated conductors*

The role of grains and grain boundaries in producing reversible strain effects on the transport current critical current density (Jc) of YBa2Cu3O7−δ (YBCO) coated conductors that are produced with metal–organic deposition (MOD) was investigated. The strain (e) dependence of Jc for full-width coated conductors is compared with that for samples in which the current transport was limited to a few or single grain boundaries by cutting narrow tracks with a laser or focused ion beam, as well as with thin films deposited on bicrystalline SrTiO3 substrates by use of pulsed-laser deposition (PLD). Our results show that the dependences of Jc on e for the grains and for the grain boundaries from the two kinds of YBCO samples can be expressed by the same function, however with a greater effective tensile strain at the grain boundaries than in the grains. The really striking result is that the grain boundary strain is 5–10 times higher for grain boundaries of in situ PLD grown bicrystals as compared to the aperiodic, meandered, nonplanar grain boundaries that develop in ex situ grown MOD-YBCO in the coated conductor of this study.

Anisotropic in-plane reversible strain effect in Y0.5Gd0.5Ba2Cu3O7 − δ coated conductors

Recent experiments have shown that reversible effects of strain on the critical current density and flux pinning strength in the high-temperature superconductor Bi2Sr2Ca2Cu3Ox can be explained entirely by the pressure dependence of its critical temperature. Such a correlation is less simple for RE–Ba2Cu3O7 − δ (RE = rare earth) superconductors, in part because the in-plane pressure dependence of its critical temperature is highly anisotropic. Here, we make a qualitative correlation between the uniaxial pressure dependence of the critical temperature and the reversible strain effect on the critical current of RE–Ba2Cu3O7 − δ coated conductors by taking the crystallography and texture of the superconducting film into account. The strain sensitivity of the critical current is highest when strain is oriented along the [100] and [010] directions of the superconducting film, whereas the critical current becomes almost independent of strain when strain is oriented along the [110] direction. The results confirm the important role of the anisotropic pressure dependence of the critical temperature on the reversible strain behavior of RE–Ba2Cu3O7 − δ. The reversible strain effect in RE–Ba2Cu3O7 − δ is expected to decrease the performance of the conductor in many applications, such as high-field magnets, but the effect may be only minor in coated conductor cables, where strain is generally not aligned with the tape axis.

Double disordered YBCO coated conductors of industrial scale: high currents in high magnetic field

A significant increase of critical current in high magnetic field, up to 31 T, was recorded in long tapes manufactured by employing a double-disorder route. In a double-disordered high-temperature superconductor (HTS), a superimposing of intrinsic and extrinsic disorder takes place in a way that (i) the intrinsic disorder is caused by local stoichiometry deviations that lead to defects of crystallinity that serve as pining centers in the YBa2Cu3O x−δ matrix and (ii) the extrinsic disorder is introduced via embedded atoms or particles of foreign material (e.g. barium zirconate), which create a set of lattice defects. We analyzed possible technological reasons for this current gain. The properties of these tapes over a wider field-temperature range as well as field anisotropy were also studied. Record values of critical current as high as 309 A at 31 T, 500 A at 18 Tm and 1200 A at 5 T were found in 4 mm wide tape at 4.2 K and B perpendicular to tape surface. HTS layers were processed in medium-scale equipment that allows a maximum batch length of 250 m while 22 m long batches were provided for investigation. Abnormally high ratios (up to 10) of critical current density measured at 4.2 K, 19 T to critical current density measured at 77 K, self-field were observed in tapes with the highest in-field critical current. Anisotropy of the critical current as well as angular dependences of n and α values were investigated. The temperature dependence of critical current is presented for temperatures between 4.2 and 40 K. Prospects for the suppression of the dog-bone effect by Cu plating and upscale of processing chain to >500 m piece length are discussed.

Sample and length-dependent variability of 77 and 4.2 K properties in nominally identical RE123 coated conductors

We present a broad study by multiple techniques of the critical current and critical current density of a small but representative set of nominally identical commercial RE123 (REBa2Cu3O7−δ , RE = rare Earth, here Y and Gd) coated conductors (CC) recently fabricated by SuperPower Inc. to the same nominal high pinning specification with BaZrO3 and RE2O3 nanoprecipitate pinning centers. With high-field low-temperature applications to magnet technology in mind, we address the nature of their tape-to-tape variations and length-wise I c inhomogeneities by measurements on a scale of about 2 cm rather than the 5 m scale normally supplied by the vendor and address the question of whether these variations have their origin in cross-sectional or in vortex pinning variations. Our principal method has been a continuous measurement transport critical current tool (YateStar) that applies about 0.5 T perpendicular and parallel to the tape at 77 K, thus allowing variations of c-axis and ab-plane properties to be clearly distinguished in the temperature and field regime where strong pinning defects are obvious. We also find such in-field measurements at 77 K to be more valuable in predicting 4.2 K, high-field properties than self-field, 77 K properties because the pinning centers controlling 77 K performance play a decisive role in introducing point defects that also add strongly to J c at 4.2 K. We find that the dominant source of I c variation is due to pinning center fluctuations that control J c, rather than to production defects that locally reduce the active cross-section. Given the 5–10 nm scale of these pinning centers, it appears that the route to greater I c homogeneity is through more stringent control of the REBCO growth conditions in these Zr-doped coated conductors.

Significant reduction of AC losses in YBCO patterned coated conductors with transposed filaments

The concentration of internal gettering sites within a semiconductor wafer is controlled by two-step thermal processing. In a concentration reduction phase, the wafer is rapidly heated to an elevated temperature in the range from about 900 DEG to 1350 DEG C., resulting in the partial or total dissolution of precipitable impurities within the wafer. In a concentration enhancement step, the wafers are subjected to a relatively low temperature anneal process where the density of potential internal gettering sites is increased. By properly controlling the processing temperatures and treatment times, the two steps may be performed in either order to obtain wafers having internal gettering site concentrations within a desired range.

Correlation between YBa2Cu3O7 nuclei density and the grain orientation of the CeO2 buffered Ni―W template of the second-generation superconducting wire

The prime goal of the second-generation superconducting wire technology is to grow high-quality epitaxial layers of YBa2Cu3O7 (YBCO) superconductor using high-rate deposition on low-cost, kilometer-long substrates. We analyze the influence of Ni–W RABiTS™ substrate grain misalignment on nucleation of epitaxial YBCO during metal-organic ex situ processing. Electron backscatter diffraction orientation maps are correlated with YBCO nuclei density obtained from scanning-electron microscopy. A critical Ni–W grain tilt misorientation angle of 8.5° was identified above which the YBCO nuclei density was observed to be extremely low, approaching zero. A proposed model explains the reduction in nuclei density as being due to the absence of (001) substrate terraces wide enough to accommodate the critical size for YBCO nuclei. This study emphasizes the strong effect of the out-of-plane tilt of substrate grains on superconducting properties of YBCO layers produced by metal-organic deposition.

Record current density of 344 A mm−2 at 4.2 K and 17 T in CORC® accelerator magnet cables

One of the biggest challenges in developing conductor on round core (CORC®) magnet cables for use in the next generation of accelerator magnets is raising their engineering current density J E to approach 600 A mm−2 at 20 T, while maintaining their flexibility. One route to increase J E could be to add more RE-Ba2Cu3O7−δ coated conductors to the cable, but this would increase the cable size and reduce its flexibility. The preferred route to higher J E is a reduction in diameter of the CORC® cable, while maintaining the number of tapes wound into the cable. The availability of very thin tapes containing substrates of 30 μm thickness enabled us to wind a 5.1 mm diameter CORC® cable from 50 coated conductors, while maintaining a tape critical current I c of about 97% after cabling. The cable I c was 7030 A at 4.2 K in a background field of 17 T, corresponding to a J E of 344 A mm−2, which is the highest performance of any CORC® cable so far. The magnetic field dependence allowed us to extrapolate the cable performance to 20 T to predict an I c of 5654 A and a J E of 309 A mm−2. The results clearly show that rapid progress is being made on overcoming the J E hurdle for use of CORC® cables in the next generation of accelerator magnets. Further optimization of the cable layout will likely increase J E towards 600 A mm−2 at 20 T in the near future, while further reduction in cable size will also make them even more flexible.

Engineering current density in excess of 200 A mm −2 at 20 T in CORC ® magnet cables containing RE-Ba 2 Cu 3 O 7− δ tapes with 38 μ m thick substrates

Conductor on round core (CORC®) cables wound from RE-Ba2Cu3O7−δ coated conductors are currently being developed for the next generation of accelerator magnets because of their high flexibility and potential for high engineering current densities J E. CORC® cables previously reached J E of 114 A mm−2 at 4.2 K and 20 T in a 7.5 mm diameter cable. Accelerator magnets require a current density of at least 300 A mm−2 and a cable-bending diameter as small as 40 mm, which has so far not been possible with superconducting tapes made on 50 μm thick substrates. CORC® cables made from thinner substrates could have significantly increased J E with greater flexibility as we here demonstrate with a CORC® cable made of tapes with 38 μm thick substrates. A custom cable machine produced higher cable quality and better retention of tape performance compared to previous cables that were wound by hand. The thinner substrate showed an almost twofold increase in engineering current density from 114 A mm−2 to 216.8 A mm−2 at 4.2 K and 20 T, at a reduction in cable diameter from 7.5 mm to 6.0 mm. The results clearly demonstrate that winding CORC® cables from tapes with thinner substrates is a straightforward method for raising their current density and one that shows great promise for use in accelerator magnets.

Broad Temperature Pinning Study of 15 mol.% Zr-Added (Gd, Y)–Ba–Cu–O MOCVD Coated Conductors

BaZrO3 (BZO) nanocolumns have long been shown to be very effective for raising the pinning force Fp of REBa2Cu3Ox (REBCO, where RE = rare earth) films at high temperatures and recently at low temperatures too. We have successfully incorporated a high density of BZO nanorods into metal organic chemical vapor deposited (MOCVD) REBCO coated conductors via Zr addition. We found that, compared to the 7.5% Zr-added coated conductor, dense BZO nanorod arrays in the 15% Zr-added conductor are effective over the whole temperature range from 77 K down to 4.2 K. We attribute the substantially enhanced Jc at 30 K to the weak uncorrelated pinning as well as the strong correlated pinning. Meanwhile, by tripling the REBCO layer thickness to ~2.8 μm, the engineering critical current density Jc at 30 K exceeds Jc of optimized Nb-Ti wires at 4.2 K.