[en] Foreseeing practical applications of recently discovered new series of superconductor -Fe-pnictides, one of the most important parameters is current transport across grain boundaries. We combined SQUID measurements, magneto-optical (MO) imaging, scanning and transmission electron microscope (SEM and TEM) and low temperature laser scanning microscope (LTLSM) in order to understand the relationship between the microstructure and intergrain current transport, so called global current, in a random polycrystalline SmFeAsO_0.85 (Sm1111) bulk. Our Sm1111 bulk showed significant global critical current density (J_c) which is more than one magnitude higher compared to random polycrystalline pure YBCO bulks at self field and the same temperature. However there was different temperature dependence of intergrain and intragrain J_c, exhibiting granularity at low temperature which was caused by large difference of J_c on two distinct scales. Strikingly most of intergrain current transport at self field relied on SNS Josephson weak links where supercurrent passed across the conductive impurity phase of FeAs, strongly suggesting the need of eliminating such a wetting phase in order to explore internal blocking effects at grain boundaries which are not fully understood yet. (author)

No abstract available

[en] 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 (REBa_2Cu_3O_7_−_δ, RE = rare Earth, here Y and Gd) coated conductors (CC) recently fabricated by SuperPower Inc. to the same nominal high pinning specification with BaZrO_3 and RE_2O_3 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. (paper)

[en] 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-Ba_2Cu_3O_7_−_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. (paper)

[en] The work describes the capabilities of laser scanning microscopy (LSM) as a stability-resolved method of testing high-T_c materials and devices. The earlier results obtained by the authors are briefly reviewed. Some novel applications of the LSM are illustrated, including imaging the HTS responses in rf mode, probing the superconducting properties of HTS single crystals, development of two-beam laser scanning microscopy. The existence of the phase slip lines mechanism of resistivity in HTS materials is proven by LSM imaging

No abstract available

[en] We performed a feasibility study on a high-strength Bi${}_{2-<!--\; ???\; -->x}$Pb_xSr₂Ca₂Cu₃O${}_{10-<!--\; ???\; -->x}$(Bi-2223) tape conductor for high-field solenoid applications. The investigated conductor, DI-BSCCO Type HT-XX, is a pre-production version of Type HT-NX, which has recently become available from Sumitomo Electric Industries. It is based on their DI-BSCCO Type H tape, but laminated with a high-strength Ni-alloy. We used stress–strain characterizations, single- and double-bend tests, easy- and hard-way bent coil-turns at various radii, straight and helical samples in up to 31.2 T background field, and small 20-turn coils in up to 17 T background field to systematically determine the electro-mechanical limits in magnet-relevant conditions. In longitudinal tensile tests at 77 K, we found critical stress- and strain-levels of 516 MPa and 0.57%, respectively. In three decidedly different experiments we detected an amplification of the allowable strain with a combination of pure bending and Lorentz loading to $⩾<!--\; ???\; -->0.92\mathrm{\%<!--\; \%\; -->}$ (calculated elastically at the outer tape edge). This significant strain level, and the fact that it is multi-filamentary conductor and available in the reacted and insulated state, makes DI-BSCCO HT-NX highly suitable for very high-field solenoids, for which high current densities and therefore high loads are required to retain manageable magnet dimensions. (paper)

[en] REBCO (RE = rare earth) based high temperature superconducting (HTS) wires are now being utilized for the development of electric and electromagnetic devices for various industrial, scientific and medical applications. In the last several years, the increasing efforts in using the so-called second generation (2G) HTS wires for some of the applications require a further increase in their engineering current density ( J _e). The applications are those typically related to high magnetic fields where the higher J _e of a REBCO wire, in addition to its higher irreversibility fields and higher mechanical strength, is already a major advantage over other superconducting wires. An effective way to increase the J _e is to decrease the total thickness of a wire, for which using a thinner substrate becomes an obvious and attractive approach. By using our IBAD-MOCVD (ion beam assisted deposition-metal organic chemical vapor deposition) technology we have successfully made 2G HTS wires using a Hastelloy^® C276 substrate that is only 30 μ m in thickness. By using this thinner substrate instead of the typical 50 μ m thick substrate and with a same critical current ( I _c), the J _e of a wire can be increased by 30% to 45% depending on the copper stabilizer thickness. In this paper, we report the fabrication and characterization of the 2G HTS wires made on the 30 μ m thick Hastelloy^® C276 substrate. It was shown that with the optimization in the processing protocol, the surface of the thinner Hastelloy^® C276 substrate can be readily electropolished to the quality needed for the deposition of the buffer stack. Same in the architecture as that on the standard 50 μ m thick substrate, the buffer stack made on the 30 μ m thick substrate showed an in-plane texture with a Δ ϕ of around 6.7° in the LaMnO₃ cap layer. Low-temperature in-field transport measurement results suggest that the wires on the thinner substrate had achieved equivalent superconducting performance, most importantly the I _c, as those on the 50 μ m thick substrate. It is expected the 2G HTS wires made on the 30 μ m thick Hastelloy^® C276 substrate, the thinnest and with the highest J _e to date, will greatly benefit such applications as high field magnets and high current cables. (paper)

[en] The role of grains and grain boundaries in producing reversible strain effects on the transport current critical current density (J_c) of YBa₂Cu₃O_7-δ (YBCO) coated conductors that are produced with metal-organic deposition (MOD) was investigated. The strain (ε) dependence of J_c 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 J_c on ε 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.

[en] Recent experiments have shown that reversible effects of strain on the critical current density and flux pinning strength in the high-temperature superconductor Bi₂Sr₂Ca₂Cu₃O_x can be explained entirely by the pressure dependence of its critical temperature. Such a correlation is less simple for RE–Ba₂Cu₃O_7−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–Ba₂Cu₃O_7−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–Ba₂Cu₃O_7−δ. The reversible strain effect in RE–Ba₂Cu₃O_7−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.