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[en] In this study, we used a systematic route to optimize the fluorine-free MOD process to achieve a high critical current density (Jc) in BaZrO3 (BZO)-doped YBCO films on RABiTS substrates. The BZO content is given by 1 YBCO+x BZO films, where x is moles of BZO per 1 mole of YBCO. We found x = 0.10 to be the optimal BZO content and ∼795–805 °C to be the optimal growth temperature window with 60–90 min processing time. TEM studies show the BZO nanoparticles are ∼20 nm in size and spaced ∼50–100 nm apart. The in-field Jc and the peak pinning force (Fp) of the film grown at the optimal conditions were greatly increased at 77 K relative to pure YBCO films, achieving ∼6.7 GN m−3 at 77 K, H ‖ c in a ∼800 nm thick x = 0.10 film. The angular dependence of in-field Jc measurements also shows greatly reduced angular anisotropy at 1 and 4 T at 77 K due to isotropic pinning by BZO nanoparticles. (paper)
[en] In this study, BaZrO3 (BZO)-doped YBCO films were fabricated on SrTiO3(100) single-crystal substrates by a fluorine-free metal–organic deposition (MOD) process. We added extra Ba and Zr organic salts, which formed well-dispersed ∼10–25 nm sized BaZrO3 nanoparticles in the YBCO films. The in-field critical current density (Jc) and the peak pinning force (Fp) were greatly enhanced in the BZO-doped sample at 77 K relative to pure YBCO films. The optimal BZO content that gave the highest peak pinning force of ∼10 GN m−3 in a ∼180 nm thick film was found to be x = 0.10 for YBCO + xBZO films, where x is moles of BZO per 1 mol of YBCO. The angular dependence of in-field Jc measurements shows the BZO nanoparticles increased Jc over the entire angular range and also reduced the angular anisotropy measured at 4 T at 77 K. (paper)
[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] We have recently shown that the gas present in the only ∼ 70% dense filaments of as-drawn Bi-2212 wire agglomerates into large bubbles that fill the entire filament diameter during the melt phase of the heat treatment. Once formed, these bubbles never disappear, although they can be bridged by 2212 grains formed on cooling. In order to test the effect of these bubbles on the critical current Ic, we increased the density of the filaments after drawing using 2 GPa of cold isostatic pressure, finding that the bubble density and size were greatly reduced and that Ic could be at least doubled. We conclude that enhancement of the filament packing density is of great importance for making major Ic improvements in this very useful, round superconducting wire. (rapid communication)
[en] Increasing the critical current density (Jc) of the multifilamentary round wire Ag/Bi2Sr2CaCu2Ox(2212) requires understanding its complicated microstructure, in which extensive bridges between filaments are prominent. In this first through-process quench study of 2212 round wire, we determined how its microstructure develops during a standard partial-melt process and how filament bridging occurs. We found that filaments can bond together in the melt state. As 2212 starts to grow on subsequent cooling, we observed that two types of 2212 bridges form. One type, which we call Type-A bridges, forms within filaments that bonded in the melt; Type-A bridges are single grains that span multiple bonded filaments. The other type, called Type-B bridges, form between discrete filaments through 2212 outgrowths that penetrate into the Ag matrix and intersect with other 2212 outgrowths from adjacent filaments. We believe the ability of these two types of bridges to carry inter-filament current is intrinsically different: Type-A bridges are high- Jc inter-filament paths whereas Type-B bridges contain high-angle grain boundaries and are typically weak linked. Slow cooling leads to more filament bonding, more Type-A bridges and a doubling of Jc without changing the flux pinning. We suggest that Type-A bridges create a 3D current flow that is vital to developing high Jc in multifilamentary 2212 round wire.
[en] It is well known that the critical current density Jc of multifilamentary Bi-2212 wires tends to decline as the wire length increases, but the reasons for and the magnitude of this decline remain obscure and quantitatively unpredictable. Here we report on the Jc and mass density variation with length on ∼ 1 m long samples taken from two recent and representative wires, in which we find a strong decrease of Jc with distance from the end and a strong correlation between Jc and the local mass density. The mass density variations occur on length scales of centimeters, many times the nominal 15 μm filament diameter. The cause of the mass density variation appears to be internal gas pressure that generates bubbles which almost fill the filament diameter when the Bi-2212 melts. Control of this internal pressure seems to be vital to moderating or avoiding the length dependence of Jc.
[en] We have made extensive low temperature and high field evaluations of a recent 2.1 μm thick coated conductor (CC) grown by metal-organic chemical vapor deposition (MOCVD) with a view to its use for high field magnet applications, for which its very strong Hastelloy substrate makes it very suitable. This conductor contains dense three-dimensional (Y,Sm)2O3 nanoprecipitates, which are self-aligned in planes tilted ∼7 deg. from the tape plane. Very strong vortex pinning is evidenced by high critical current density Jc values of ∼3.1 MA cm-2 at 77 K and ∼43 MA cm-2 at 4.2 K, and by a strongly enhanced irreversibility field Hirr, which reaches that of Nb3Sn (∼28 T at 1.5 K) at 60 K, even in the inferior direction of H parallel c axis. At 4.2 K, Jc values are ∼15% of the depairing current density Jd, much the highest of any superconductor suitable for magnet construction.
[en] The recent discovery that gas bubbles formed in the melt state are a major current-limiting mechanism in Bi_2Sr_2CaCu_2O_x (Bi2212) round wires has prompted explicit examination of the bubble density in split melt processed samples which, under optimized 1 bar processing conditions, can exhibit significant (30–50%) enhancement of critical current density, J_c. By examining quenched and furnace-cooled samples from different points in the split melt processing (SMP), we found that the bubble size correlates well to the J_c. Compared with standard processed samples, the bubble size is smaller in SMP samples which are cooled directly to room temperature by an intermediate cooling from the first melt before being reheated to the second melt. Bubble size and density observations suggest that J_c can only be increased when bubble growth in the second melt is prevented by very tight control of the reheat temperature. Smaller bubble size is favorable for J_c because filament connectivity is determined by the effectiveness of bubble bridging by Bi2212 grain growth on cooling from the second melt. Because SMP appears to allow higher J_c by shrinking bubble size rather than by diminishing the bubble volume fraction, we conclude that SMP is unlikely to offer benefits to newer processes like over-pressure processing which raise J_c much more significantly by full Bi2212 densification and bubble elimination. (paper)
[en] Magnetic susceptibility, electrical resistivity, and heat capacity results are reported for the chemical substitution series URu2Si2−xPx for . This study expands in detail on work recently reported in Gallagher et al (2016 Nat. Commun. 10712), which focused on the small x region of this substitution series. Measurements presented here reveal persistent hybridization between the f- and conduction electrons and strong variation of the low temperature behavior with increasing x. Hidden order and superconductivity are rapidly destroyed for and are replaced for by a region with Kondo coherence but no ordered state. Antiferromagnetism abruptly appears for . This phase diagram differs significantly from those produced by most other tuning strategies in URu2Si2, including applied pressure, high magnetic fields, and isoelectronic chemical substitution (i.e. Ru → Fe and Os), where hidden order and magnetism share a common phase boundary. Besides revealing an intriguing evolution of the low temperature states, this series provides a setting in which to investigate the influence of electronic tuning, where probes that are sensitive to the Fermi surface and the symmetry of the ordered states will be useful to unravel the anomalous behavior of URu2Si2. (paper)
[en] Recent study of the current-limiting mechanisms in Bi-2212 round wires has suggested that agglomeration of the residual Bi-2212 powder porosity into bubbles of filament-diameter size occurs on melting the Bi-2212 filaments. These pores introduce a major obstacle to current flow, which greatly reduces the critical current density (Jc). Here we present an in situ non-destructive tomographic and diffraction study of the changes occurring during the heat treatment of wires and starting powder, as well as a room temperature study of ex situ processed wires. The in situ through-process study shows that the agglomeration of residual porosity is more complex than previously seen. Filament changes start with coalescence of the quasi-uniform and finely divided powder porosity into lens-shaped defects at about 850 0C when the Bi-2201 impurity phase decomposes before the Bi-2212 starts to melt. These lens-shaped voids grow to bubbles of a filament diameter on melting of the Bi-2212 and continue to lengthen and then to agglomerate across multiple filaments while the filaments are in the liquid state. The experiment makes clear why melt processing is vital to developing high Jc and also shows how rearrangement of the residual filament porosity on melting imposes a strong longitudinal inhomogeneity in each filament. Reducing the bubble density is clearly an important path to reaching much higher Jc values in Bi-2212 round wires. Synchrotron micro-tomography is an exceptionally powerful technique for studying the spatial extent of the porosity on a scale of about 2 μm and larger.