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High engineering critical current density ( JE of 1300 A/mm2 at 4.2 K and 15 T) in Bi-2212 round wire has been achieved through a partial melt, overpressure heat treatment process. JE varies strongly with processing conditions, particularly the maximum heat treatment temperature ( T max). Increasing T max results in longer time in the melt (defined as the time between when Bi-2212 melts on heating and when Bi-2212 begins to form on cooling), more bridging between the filaments, lower JE , and higher ac losses. A wide processing window with a large range of T max that has a nearly constant JE is desired for processing large coils with large thermal mass and significant thermal time constants that may make precise control over the desired temperature – time profiles uncertain. Accordingly, we wanted to explore broadening the T max window by controlling the Bi-2212 powder melting or wire architecture design. Here we report on studies of the performance variation with T max for two production wires with a filling factor of about 20% and 85 × 18 filaments where filament size was varied by changing the wire diameter, a process which also shortens the distance between filaments. We found that wires with smaller filament diameter (9 to 11 μm) showed a peak JE at the low end of T max and also a JE that was more sensitive to T max. A JE – T max plot showed a plateau JE (4.2 K, 5 T) of ∼1100 A/mm2 between T max of 886 and 894 °C for 1.0 and 1.2 mm wires, where JE is less sensitive to the wire diameter and T max. This JE plateau range is a preferred processing window for achieving high JE in coils.

Bi-2212 wires have been produced for much of the last decade with varying architectures, powder sources, and great variability of final J C (B) properties. As Bi-2212 transitions from an R&D materials development to a magnet technology, means to predictably forecast J C (B) becomes increasingly valuable. Here we report on characterizations of short samples having J C (15 T, 4.2 K) varying from 1210 to 6560 A/mm 2 , measured in fields up to 31 T, and drawn from 10 billets. Using both 4 K transport and variable temperature vibrating sample magnetometer measurements, capable of probing the 20 K irreversibility field, B irr = μ 0 H k , we found that the vortex pinning properties are very stable across these wires, which all used the same standard powder composition (Bi 2.17 Sr 1.94 Ca 0.89 Cu 2.00 O 8+ x ), even though it has been made by three different manufacturers over nine years. We conclude that a power-law fit J C ∝ B -α , where α = 0.280 (σ = 0.015), works well at 4.2 K over the field range of at least 3-30 T and that μ 0 H k (20 K) remains stable at 8.6 ± 0.4 T. We conclude that the dominant current limiting mechanism predicting J C (B) in Bi-2212 is the effective filament connectivity. J C (B) at high fields may thus be predicted by simple I C measurements at easily accessible fields in the 3-15 T range.

Bi-2212 is the only high-field, high-temperature superconductor (HTS) capable of reaching a critical current density $J_{\text{c}}$(16 T, 4.2 K) of 6500 $\mathrm{A\cdot mm^{-2}}$ in the highly desirable round wire (RW) form. However, state-of-the-art Bi-2212 conductors still have a critical current density ($J_{\text{c}}$) to depairing current density ($J_{\text{d}}$) ratio around 20 to 30 times lower than that of state-of-the-art $\mathrm{Nb-Ti}$ or REBCO. Previously, we have shown that recent improvements in Bi-2212 RW $J_{\text{c}}$ are due to improved connectivity associated with optimization of the heat treatment process, and most recently due to a transition to a finer and more uniform powder manufactured by Engi-Mat. One quantitative measure of connectivity may be the critical current ($I_{\text{c}}$) distribution, since the local $I_{\text{c}}$ in a wire can vary along the length due to variable vortex-microstructure interactions and to factors such as filament shape variations, grain-to-grain connectivity variations and blocking secondary phase distributions. Here we compare $\sim$ 0.1 m length $I_{\text{c}}$ distributions of Bi-2212 RWs with recent state-of-the-art very high-$J_{\text{c}}$ Engi-Mat powder and lower $J_{\text{c}}$ and older Nexans granulate powder. We do find that the $I_{\text{c}}$ spread for Bi-2212 wires is about twice the relative standard of high-$J_{\text{c}}$ $\mathrm{Nb-Ti}$ well below $H_{\text{irr}}$. We do not yet see any obvious contribution of the Bi-2212 anisotropy to the $I_{\text{c}}$ distribution and are rather encouraged that these Bi-2212 round wires show relative $I_{\text{c}}$ distributions not too far from high-$J_{\text{c}}$ $\mathrm{Nb-Ti}$ wires., 6 pages, 2 figures, 2 tables

In order to better understand the chemical changes after wood drying, Eucalyptus wood was dried to an absolutely dry state following the five types of treatment: oven-dried at 60, 90, 120, ...

We investigated the melt-growth behavior of YbBa2Cu3O7−δ (Yb123) powder under different atmospheres for potential application in Ag-sheathed Yb123 wires. We chose Yb123 because it has the lowest melting temperature of the RE123 (RE = rare earth) phases. Yb123 needs to be melted under low oxygen partial pressure (pO2) to depress its melt temperature below that of Ag. Here we report that adding Ag powder to Yb123 powder or encasing Yb123 powder in a Ag-sheath further decreases the melting temperature of Yb123. We designed a heat treatment process in which we varied the pO2 during the heat treatment to melt the Yb123 in a Ag-sheathed powder-in-tube (PIT) wire and then regrow Yb123 on cooling. Based on these studies, we have developed a model for Yb123 melting and growth.

Atomic layer controlled growth of epitaxial thin films of unconventional superconductors opens the opportunity to discover novel high temperature superconductors. For instance, the interfacial atomic configurations may play an important role in superconducting behavior of monolayer FeSe on SrTiO3 and other Fe-based superconducting thin films. Here, we demonstrate a selective control of the atomic configurations in Co-doped BaFe2As2 epitaxial thin films and its strong influence on superconducting transition temperatures by manipulating surface termination of (001) SrTiO3 substrates. In a combination of first-principles calculations and high-resolution scanning transmission electron microscopy imaging, we show that Co-doped BaFe2As2 on TiO2-terminated SrTiO3 is a tetragonal structure with an atomically sharp interface and with an initial Ba layer. In contrast, Co-doped BaFe2As2 on SrO-terminated SrTiO3 has a monoclinic distortion and a BaFeO3- x initial layer. Furthermore, the superconducting transition temperature of Co-doped BaFe2As2 ultrathin films on TiO2-terminated SrTiO3 is significantly higher than that on SrO-terminated SrTiO3, which we attribute to shaper interfaces with no lattice distortions. This study allows the design of the interfacial atomic configurations and the effects of the interface on superconductivity in Fe-based superconductors.

A novel melt-growth heat treatment of YbBa 2 Cu 3 O 7-δ (Yb123) powder in Ag-sheathed tape has been established as a necessary first step to form aligned Yb123 grains in a powder-in-tube (PIT) conductor. The heat treatment parameters including the rate at which the atmosphere was changed from Ar to pure O 2, the heat treatment temperature, and the time during which the melt transformed back to Yb123 were investigated to find the optimum parameters for reforming Yb123 in Ag-sheathed tape. The Yb123 purity after being reformed is insensitive to the rate at which the atmosphere was changed. The optimized parameters are 910 °C for 48 h to reform Yb123.

A comprehensive study of upper critical and irreversibility magnetic fields in Ba(Fe0.95Ni0.05)2As2 (large grain and small grain samples), Ba(Fe0.94Ni0.06)2As2, Ba(Fe0.92Co0.08)2As2, and Ba(Fe0.92Co0.09)2As2 polycrystalline bulk pnictide superconductors was made in pulsed fields of up to 65 T. The full magnetic field-temperature (H-T) phase diagrams, starting at 1.5 K, were measured. The higher temperature, upper critical field Hc2 data are well described by the one-band Werthamer, Helfand, and Hohenberg (WHH) model. At low temperatures, the experimental data depart from the fitted WHH curves, suggesting an emergence of a new phase that could be attributed to the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. The large values of the Maki fitting parameter α indicate that the Zeeman pair breaking dominates over the orbital pair breaking and spin-paramagnetic pair-breaking effect is significant in these materials. Possible multi-band structure of these materials is lumped into effective parameters of the single-band model. Table of measured physical parameters allows us to compare these pnictide superconductors for different Co- and Ni- doping levels and granularity.

A comprehensive study of upper critical ( Hc 2) and irreversibility magnetic fields ( H irr) in (Ba0.6K0.4)Fe2As2 (critical temperature $T_{c} = \text{38.3 K}$ ), Ba(Fe 0.95Ni0.05)2As2 ( Tc = 19.2 K), Ba(Fe0.94 Ni0.06)2As2 ( Tc = 18.5 K), Ba(Fe0.92 Co0.08)2As2 ( Tc = 23.2 K), and Ba(Fe0.91 Co0.09)2As2 ( Tc = 25.3 K) polycrystalline bulk pnictide superconductors with different average grain sizes was made in pulsed fields at the Los Alamos National Laboratory. The magnetic field–temperature ( Hc 2– T ) phase diagrams with Hc 2 as high as 65 T at 28 K for the K-doped samples and critical current density $(J_{c})$ measurements as high as 105 A/cm2 for the smallest, submicrometer grain size samples were obtained. The high Hc 2, H irr, and $J_{c}$ data show the suitability of these materials for magnet design as their mechanical strength and random grain alignment show promise in the manufacturing of next-generation magnets.

Bi2Sr2CaCu2Ox (Bi-2212) conductor is the only high-temperature superconductor manufactured as a round wire and is a very promising conductor for very high field applications. One of the key design parameters of Bi-2212 wire is its filament size, which has been previously reported to affect the critical current density ( Jc ) and ac losses. Work with 1 bar heat treatment showed that the optimal filament diameter was about 15 μ m, but it was not well understood at that time, that gas bubbles were the main current limiting mechanism. Here, we investigated a recent Bi-2212 wire with a 121 × 18 filament architecture with varying wire diameter (1.0–1.5 mm) using 50 bar overpressure processing. This wire is part of a 1.2 km piece length of 1.0 mm diameter made by Oxford Superconducting Technology. We found that Jc is independent of the filament size in the range from 9 to 14 μ m, although the n value increased with increasing filament size. A new record Jc ( 4.2 K, 15 T ) of 4200 A/mm2 and JE ( 4.2 K, 15 T ) of 830 A/mm2 were achieved.

The route to 30 T NMR endorsed by a recent National Academy report clearly still has many challenges to achieve high stability and homogeneous high temperature superconducting (HTS) magnet. As the only HTS conductor with round wire (RW) geometry, Bi2Sr2CaCu2O8-x (Bi-2212) RW conductor is very attractive for NMR magnet applications. At present, an NMR quality demonstration magnet with Bi-2212 RW wound insert coils is under development at the National High Magnetic Field Laboratory (NHMFL). The target of this demonstration magnet is to generate a total field of 23+ T with ppm level homogeneity. Since Bi-2212 coils require Wind-and-React (W&R) technology, our initial major concern was that large Bi-2212 coils might deform during the typical partial melt Bi-2212 heat treatment (HT) due to their large self-weight. To experimentally mimic the HT of large Bi-2212 coil, several small test coils were heat treated under deadweight loads. After 1 bar Bi-2212 full reaction, these coils were characterized in terms of coil geometry, transport critical current properties, oxygenation status and insulation performance. Coil geometry and individual wire shape was in fact not distorted, nor was transport properties degradation was induced by mechanical loading. Uniform oxygen equilibration was achieved in these coils even though they were coated with dense oxide insulation. However, although the TiO2-based insulation coating was well preserved on the wire surface, several coils developed electrical shorts.

Although nanoscale deformation, such as nanostrain in iron-chalcogenide (FeSexTe1−x, FST) thin films, has attracted attention owing to its enhancement of general superconducting properties, including critical current density (Jc) and critical transition temperature, the development of this technique has proven to be an extremely challenging and complex process thus far. Herein, we successfully fabricated an epitaxial FST thin film with uniformly distributed nanostrain by injection of a trace amount of CeO2 inside an FST matrix using sequential pulsed laser deposition. By means of transmission electron microscopy and geometric phase analysis, we verified that the injection of a trace amount of CeO2 forms nanoscale defects, with a nanostrained region of tensile strain (ezz ≅ 0.02) along the c-axis of the FST matrix. This nanostrained FST thin film achieves a remarkable Jc of 3.5 MA/cm2 under a self-field at 6 K and a highly enhanced Jc under the entire magnetic field with respect to those of a pristine FST thin film. The maximum amount of current that can be carried by iron-based superconductors can be improved by introducing deliberate defects during fabrication. Iron–selenium–tellurium thin films have recently been identified as high-temperature superconductors that may pass large quantities of current. Sanghan Lee from the Gwangju Institute of Science and Technology in South Korea and co-workers have now used lasers to deposit small amounts of cerium oxide atoms between layers of an iron–selenium–tellurium thin film. The cerium oxide particles create defects, such as selenium-deficient sites, that slightly strain the crystal lattice and let more current move through the film. Mapping these nanostrain effects with electron microscopy enabled the team to find conditions needed to enhance superconductivity while minimizing cerium oxide by-products. To enhance supercurrent of iron-chalcogenide (FST) superconductor thin films, we induced nanostrain in FST thin films. The nanostrain was generated around nanoscale defects which were formed by the inserted a trace amount of oxide artificially inside FST matrix during the growth of FST thin film using sequential pulsed laser deposition. In particular, the critical current density (Jc) of the nanostrained FST thin films was significantly improved without dominant degradation of critical transition temperature.

Multifilamentary Bi 2 Sr 2 CaCu 2 O x (Bi-2212) wire made by the powder-in-tube technique is the only high-temperature superconductor made in the round shape preferred by magnet builders. The critical current density (J C ) of Bi-2212 round wire was improved significantly by the development of overpressure heat treatment in the past few years. Bi-2212 wire is commercially available in multiple architectures and kilometer-long pieces and is a very promising conductor for very high-field NMR and accelerator magnets. We studied the effects of precursor powder and heat treatment conditions on the superconducting properties and microstructure of recent Bi-2212 wires. Short samples of recent wire with optimized overpressure processing showed J C (4.2 K, 15 T) = 6640 A/mm 2 and J C (4.2 K, 30 T) = 4670 A/mm 2 , which correspond to engineering critical current densities J E (4.2 K, 15 T) = 1320 A/mm 2 and J E (4.2 K, 30 T) = 930 A/mm 2 .

High-temperature superconductors (HTS) could enable high-field magnets stronger than is possible with Nb-Ti and Nb3Sn, but two challenges have so far been the low engineering critical current density JE, especially in high-current cables, and the danger of quenches. Most HTS magnets made so far have been made out of REBCO coated conductor. Here we demonstrate stable, reliable and training-quench-free performance of Bi-2212 racetrack coils wound with a Rutherford cable fabricated from wires made with a new precursor powder. These round multifilamentary wires exhibited a record JE up to 950 A/mm2 at 30 T at 4.2 K. These coils carried up to 8.6 kA while generating 3.5 T at 4.2 K at a JE of 1020 A/mm2. Different from the unpredictable training performance of Nb-Ti and Nb3Sn magnets, these Bi-2212 magnets showed no training quenches and entered the flux flow state in a stable manner before thermal runaway and quench occurred. Also different from Nb-Ti, Nb3Sn, and REBCO magnets for which localized thermal runaways occur at unpredictable locations, the quenches of Bi-2212 magnets consistently occurred in the high field regions over a long conductor length. These characteristics make quench detection simple, enabling safe protection, and suggest a new paradigm of constructing quench-predictable superconducting magnets from Bi-2212.

High-temperature superconductors (HTS) could enable high-field magnets stronger than is possible with Nb-Ti and Nb3Sn, but two challenges have so far been the low engineering critical current density JE, especially in high-current cables, and the danger of quenches. Most HTS magnets made so far have been made out of REBCO coated conductor. Here we demonstrate stable, reliable and training-quench-free performance of Bi-2212 racetrack coils wound with a Rutherford cable fabricated from wires made with a new precursor powder. These round multifilamentary wires exhibited a record JE up to 950 A/mm2 at 30 T at 4.2 K. These coils carried up to 8.6 kA while generating 3.5 T at 4.2 K at a JE of 1020 A/mm2. Different from the unpredictable training performance of Nb-Ti and Nb3Sn magnets, these Bi-2212 magnets showed no training quenches and entered the flux flow state in a stable manner before thermal runaway and quench occurred. Also different from Nb-Ti, Nb3Sn, and REBCO magnets for which localized thermal runaways occur at unpredictable locations, the quenches of Bi-2212 magnets consistently occurred in the high field regions over a long conductor length. These characteristics make quench detection simple, enabling safe protection, and suggest a new paradigm of constructing quench-predictable superconducting magnets from Bi-2212.

The distinctive quasi-biaxial texture of Bi2Sr2CaCu2O x (Bi-2212) plays an important role in enabling high critical current density (J c) in Bi-2212 round wires (RWs). Here we studied three over pressure heat treated wires with J c varying by a factor of ∼10, all being fully dense. Using electron backscatter diffraction, we observed the differences in biaxial texture in these three wires. Transmission electron microscopy also revealed differences in grain boundary (GB) cleanliness and connectivity. These analyses showed that high J c is unambiguously correlated to the best biaxial texture, which is in turn correlated to slow cooling from the liquid melt into solid Bi-2212. However, at 4.2 K, there is a negligible difference in intragrain pinning in the three wires, suggesting that the J c variation by a factor of ∼10 is primarily due to variable filament and intergrain connectivity. The principal determinants of intergrain connectivity is the quasi-biaxial texture and GB cleanliness. Overall, J c optimization of the Bi-2212 RW is a complex multi-variable process, but this study shows that maximizing the biaxial texture quality is an important first step in such an optimization process.

A comprehensive study of upper critical and irreversibility magnetic fields in Ba(Fe0.92Co0.08)2As2 and Ba(Fe0.91Co0.09)2As2 polycrystalline bulk pnictide superconductors was made in pulsed fields of up to 65 T. The full-magnetic field-temperature (H−T) phase diagrams, starting at 1.5 K, were obtained. Possible multi-band structure of these materials is lumped into effective parameters of the single-band model. The higher temperature, upper critical field H c2 data is well described by the one-band Werthamer–Helfand–Hohenberg (WHH) model. The large values of the Maki parameter (6.5 and 4.4) indicate that the Zeeman pair breaking dominates over the orbital pair breaking and spin-paramagnetic pair-breaking effect is significant in these materials. The fitted WHH curve departs from the experimental data at low temperatures, particularly for the Ba(Fe0.91Co0.09)2As2 sample, suggesting an emergence of a new phase that can be attributed to the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state. The H c2 temperature behavior can be described within a single-band model used in WHH with added finite-Q dependence to introduce FFLO instability. The anisotropy in the behavior of the upper critical field could also explain the data.

Vortex dynamics and nonlinear ac response are studied in a Ba(Fe0.94 Ni0.06)2As2(T c = 18.5 K) bulk superconductor in magnetic fields up to 12 T via ac susceptibility measurements of the first ten harmonics. A comprehensive study of the ac magnetic susceptibility and its first ten harmonics finds shifts to higher temperatures with increasing ac measurement frequencies (10 to 10,000 Hz) for a wide range of ac (1, 5, and 10 Oe) and dc fields (0 to 12 T). The characteristic measurement time constant t 1 is extracted from the exponential fit of the data and linked to vortex relaxation. The Anderson-Kim Arrhenius law is applied to determine flux activation energy E a /k as a function dc magnetic field. The de-pinning, or irreversibility lines, were determined by a variety of methods and extensively mapped. The ac response shows surprisingly weak higher harmonic components, suggesting weak nonlinear behavior. Our data does not support the Fisher model; we do not see an abrupt vortex glass to vortex liquid transition and the resistivity does not drop to zero, although it appears to approach zero exponentially.

We have studied the influence of the oxygen partial pressure $\text{pO}_{2}$ up to 5.5 bar on the phase changes that occur during the melt processing of a state-of-the-art Bi-2212 multifilamentary wire. Phase changes have been monitored in situ by high energy synchrotron X-ray diffraction. We found that the stability of the Bi-2212 phase is reduced with increasing $\text{pO}_{2}$ . For $\text{pO}_{2} > 1\ \text{bar}$ , a significant amount of the Bi-2212 phase decomposes upon heating in the range of 400 °C–650 °C. The extent of decomposition strongly increases with increasing pO2, and at $\text{pO}_{2}=5.5\ \text{bar}$ Bi-2212 completely decomposes in the solid state. Textured Bi-2212 can be formed during solidification when pO2 is reduced to 0.45 bar when the precursor is molten. Since the formation of current-limiting second phases is very sensitive to pO2 when it exceeds 1 bar, we recommend to reduce the oxygen partial pressure below the commonly used $\text{pO}_{2}=1\ \text{bar}$ , in order to increase the pO2 margins and to make the overpressure process more robust.

The present study aimed to examine the apoptotic effects of adenovirus (ADV)-mediated herpes simplex virus thymidine kinase (ADV-TK) combined with ganciclovir (GCV) in tissues obtained from patients with hepatocellular carcinoma in order to provide a theoretical basis for the development of this gene therapy program. Apoptosis detection was conducted using the terminal deoxynucleotidyl-transferase-mediated dUTP nick end labelling assay and the apoptosis index was compared between the experimental; and control groups. Furthermore, the protein expression levels of caspase-3, B-cell lymphoma-2 (Bcl-2), Bcl-2-assoicated protein X (Bax) and nuclear factor (NF)-κB were examined in pathological specimens using immunohistochemical staining. The Bax/Bcl-2 ratio and the release of cytochrome c were examined using western blot analysis. Results indicated that combined ADV-TK and GCV treatment significantly increased the number of apoptotic cells compared with the control group (P

Author(s): Zhang, K; Higley, H; Ye, L; Gourlay, S; Prestemon, S; Shen, T; Bosque, E; English, C; Jiang, J; Kim, Y; Lu, J; Trociewitz, U; Hellstrom, E; Larbalestier, D | Abstract: We fabricated three racetrack coils (RC1, RC2, and RC3) from Bi-2212 Rutherford cables (17-strand, thickness × width = 1.44 mm × 7.8 mm, strand diameter = 0.8 mm) and applied overpressure processing heat treatment (OPHT). The quench currents of RC1 and RC2 reached 5268 A and 5781 A, respectively, despite them still, surprisingly, exhibiting some Bi-2212 leakage to the surface. After removing most of the leakages using a simple-to-implement insulation scheme, the quench current of RC3 improved to 6485 A, which is about three times the average quench current of a dozen racetrack coils that had been fabricated and reacted using the conventional 1 bar heat treatment. The results confirm the effectiveness of the OPHT technology and the new leakage control scheme for coils made from Bi-2212 Rutherford cables. Coils exhibited an increased quench current with increasing the current ramp rate from 5 to 200 A s-1; they were quite stable against point and transient disturbances, and were capable of adsorbing persistent Joule heating at ∼80 mW for g15 s before quenching. These behaviors are different from Nb-Ti and Nb3Sn accelerator magnets. Overall, our results provide a critical evaluation and verification of Bi-2212 wire and magnet technologies (wire, insulation, heat treatment, coil fabrication, and coil operation), reveal crucial new stability features of Bi-2212 magnets, and demonstrate technological options for it to become a practical high-field magnet technology.

Atomic layer controlled growth of epitaxial thin films of unconventional superconductors opens the opportunity to discover novel high temperature superconductors. For instance, the interfacial atomic configurations may play an important role in superconducting behavior of monolayer FeSe on SrTiO

The residual-resistivity ratio (RRR) of the normal-metal matrix is a key parameter for the electrical and thermal stability of technical superconductors. In Bi2Sr2CaCu2 ${{ m{O}}}_{8+x}$ (Bi-2212) round wires, the precursor powders are embedded in a Ag matrix without any diffusion barrier, and elemental diffusion from the superconducting filaments into the Ag might be expected to contaminate the matrix during the melt processing required for high critical current density development. This work shows that the overpressure processing, which is adopted to enhance the critical current performance, improves the thermal and electrical conductivities of the conductor, too. In the case of wires reacted with a standard processing performed in 1 bar O2, the RRR of the Ag matrix is about 90, in spite of the simple conductor design that does not include diffusion barriers. Increasing the total reaction pressure to 100 bar improves the RRR to about 200. The differences in RRR reflect on the thermal conductivity of the whole conductor, which has been investigated in magnetic fields up to 19 T.

Thermally assisted flux flow (TAFF) and flux pinning energiesare studied in a Ba(Fe 0.91 Co 0.09 ) 2 As 2 ( T c = 25.3 K) sample via resistivity and AC susceptibility measurements in magnetic fields up to 18T. The flux pinning energy U ( T,H ) is determined from the Arrhenius law. The pinning maxima well determined by resistivity measurements ranged from 1724 K at 0 T to 585 K at 18 T with a sharp drop off so that U(T=T c ) varied with the applied field H as . The pinning activation energies determined from the AC susceptibility data but were by a factor of three higher, which is explained here. Both inter- and intra-granular pinning energies are determined in low fields. The onset of TAFF temperature and the crossover temperature T x from TAFF to flux flow are determined, showing the limitations of the Anderson-Kim model.

Overpressure (OP) processing of wind-and-react Bi2Sr2CaCu2Ox (2212) round wire compresses the wire to almost full density, decreasing its diameter by about 4% without change in wire length and substantially raising its JC. However, such shrinkage can degrade coil winding pack density and magnetic field homogeneity. To address this issue, we here present an overpressure predensification (OP-PD) heat treatment process performed before melting the 2212, which greatly reduces wire diameter shrinkage during the full OP heat treatment (OP-HT). We found that about 80% of the total wire diameter shrinkage occurs during the 50 atm OP-PD before melting. We successfully wound such pre-densified 1.2 mm diameter wires onto coil mandrels as small as 10 mm diameter for Ag–Mg-sheathed wire and 5 mm for Ag-sheathed wire, even though such small diameters impose plastic strains up to 12% on the conductor. A further ~20% shrinkage occurred during a standard OP-HT. No 2212 leakage was observed for coil diameters as small as 20 mm for Ag–Mg-sheathed wire and 10 mm for Ag-sheathed wire, and no JC degradation was observed on straight samples and 30 mm diameter coils.

Flux pinning dynamics are studied in a Ba(Fe\(_{0.91}\)Co\(_{0.09})_{2}\)As\(_{2}\) (\(T_\mathrm{{c}}=25.3\) K) bulk samples via ac susceptibility measurements. Ac susceptibility curves shift to higher temperatures as the frequency of small ac fields is increased from 75 to 1997 Hz in all magnetic fields ranging from 0 to 18 T. The temperature profile of the ac susceptibility curves shows narrower ac loss distribution in temperature for higher frequencies and gradually narrowing frequency shift as the temperature sweeps the full range from 2 K to the upper critical field temperature. The frequency (\(f)\) shift of the susceptibility curves is modeled by the Anderson–Kim Arrhenius law \(f = f_{0} \mathrm {exp}(- {E}_\mathrm{{a}} /kT)\) to determine flux activation energy \(E_\mathrm{{a}}/k\) as a function of magnetic field. Extensive mapping of the irreversibility lines shows broad dependence on the magnitude and the frequency of the ac field, in addition to the dc magnetic field. The irreversibility lines were just below the upper critical field \(H_\mathrm{{c2}}\) lines at 0 T in the \(H-T\) plane, but they moved significantly below the \(H_\mathrm{{c2}}\) line at higher magnetic fields, placing constraints on the use of these materials at higher magnetic fields such as 10 T and above.

Flux pinning and thermally assisted flux flow are studied in a (Ba $$_{0.6}$$ K $$_{0.4})$$ Fe $$_{2}$$ As $$_{2}(T_\mathrm{c}$$ =38.3 K) bulk samples in magnetic fields up to 18 T via ac susceptibility measurements. Ac susceptibility curves shift to higher temperatures as the frequency is increased from 75 to 1,997 Hz in all fields. The frequency ( $$f)$$ shift of the susceptibility curves is modeled by the Anderson-Kim Arrhenius law $$f = f_{0}\mathrm{exp}(-E_\mathrm{a}{ /kT})$$ to determine flux activation energy $$E_\mathrm{a}/k$$ as a function of ac field $$H_\mathrm{ac}$$ and dc magnetic flux density $$\mu $$ $$_\mathrm{0} H_\mathrm{dc}$$ . $$E_\mathrm{a}/k$$ ranges from 8,822 K (761 meV) at $$\mu $$ $$_{0} H_{dc}$$ = 0 T to 1,100 K (95 meV) at 18 T for $$H_\mathrm{ac}=$$ 80 A/m (1 Oe). The energies drop very quickly in a non-linear manner as $$\mu $$ $$_{0} H_\mathrm{dc}$$ increases from 0 to 1 T, and more gradually, in a linear-like manner, as $$\mu $$ $$_{0} H_\mathrm{dc}$$ increases further to 18 T, suggesting some kind of vortex transition. For ac fields of 400 A/m (5 Oe) and higher, the Arrhenius model starts breaking down, at around $$\mu $$ $$_{0} H_{ \mathrm dc}$$ = 2 T. As the dc magnetic flux density increases further, this breakdown becomes significant for $$\mu _{0} H_\mathrm{dc}$$ = 15 and 18 T at ac fields of 400 A/m and higher. Extensive mapping of the de-pinning, or irreversibility, lines shows broad dependence on the magnitude of the ac field, frequency, in addition to the dc magnetic flux density.

Thermally assisted flux flow (TAFF) based on magneto-resistivity and ac susceptibility measurements is studied in a Ba(Fe0.91Co0.09)2As2(T c=25.3 K) sample in magnetic fields up to 18 T. In addition to the upper critical field μ 0 H c2 and the coherence length ξ(0), the flux flow activation energy U(T,H) has also been determined. The resistive transition width is proportional to μ 0 H, in contrast to Tinkham’s theoretical prediction. By applying Fisher’s model, the glass melting transition temperature T g, which occurs in the upper TAFF state and not in the zero resistivity vortex solid regime, is calculated. The onset of TAFF temperature and the crossover temperature T x from TAFF to flux flow are determined. By contrasting the ac susceptibility data with the resistivity data, considerable flux penetration appears even in the zero resistivity state, in addition to ac losses. The H-T phase diagram is drawn and shows weak pinning regime as the field approaches μ 0 H c2, and the strength of the weak pinning decreases to 0 with increasing magnetic field from 0 to 18 T.

Thermally assisted flux flow (TAFF) is studied in bulk Ba(Fe0.95 Ni0.05)2As2 (T c = 20.4 K) and Ba(Fe0.94 Ni0.06)2As2 (T c = 18.5 K) superconductors by transport measurements in magnetic fields up to 18 T. In addition, the upper critical field μ 0 H c2(0) and the coherence length ξ(0) are determined. The data is analyzed in the context of the widely accepted Anderson-Kim model and Fischer model. The onset TAFF temperature and the crossover temperature T x from TAFF to flux flow are determined. The flux pinning activation energy U is modeled as U(T,H) = U 0(H) f(t) where f(t) is some temperature function and the modified Anderson-Kim model is used to extract U 0, which is graphed as a function of magnetic field μ 0 H near T c. The resistive regime is observed, which is caused by fluctuations. Fisher’s model is applied to determine the glass melting transition temperature; it occurs in the upper TAFF state and not in the expected zero-resistivity vortex solid regime. Furthermore, the resistive transition width is proportional to μ 0 H, in contrast to Tinkham’s prediction. The H-T phase diagram is drawn.

Magnets are the principal market for superconductors, but making attractive conductors out of the high-temperature cuprate superconductors (HTSs) has proved difficult because of the presence of high-angle grain boundaries that are generally believed to lower the critical current density, J(c). To minimize such grain boundary obstacles, HTS conductors such as REBa2Cu3O(7-x) and (Bi, Pb)2Sr2Ca2Cu3O(10-x) are both made as tapes with a high aspect ratio and a large superconducting anisotropy. Here we report that Bi2Sr2CaCu2O(8-x) (Bi-2212) can be made in the much more desirable isotropic, round-wire, multifilament form that can be wound or cabled into arbitrary geometries and will be especially valuable for high-field NMR magnets beyond the present 1 GHz proton resonance limit of Nb3Sn technology. An appealing attribute of this Bi-2212 conductor is that, being without macroscopic texture, it contains many high-angle grain boundaries but nevertheless attains a very high J(c) of 2,500 A mm(-2) at 20 T and 4.2 K. The large potential of the conductor has been demonstrated by building a small coil that generated almost 2.6 T in a 31 T background field. This demonstration that grain boundary limits to high Jc can be practically overcome underlines the value of a renewed focus on grain boundary properties in non-ideal geometries.

In this study, we extensively used electron backscatter diffraction orientation imaging microscopy to visualize the grain structure in the flat-rolled (Bi,Pb)2Sr2Ca2Cu3O x (Bi-2223) tapes. The thermomechanical process made the grains’ c-axes oriented normal to the tape surface. The 24% difference in critical current density J c was caused by the ∼5° difference in the degree of out-of-plane texture. Although the in-plane orientations are not controlled, the Bi-2223 grains can form the domains, each of which consists of the grains with similar in-plane orientation. Controlling the domain formation could be the next protocol to raise the J c of Bi-2223 tapes.

In order to develop a high current density in coils, Bi-2212 wires must be electrically discrete in tight winding packs. It is vital to use an insulating layer that is thin, fulfils the dielectric requirements, and can survive the heat treatment whose maximum temperature reaches 890 °C. A thin (20-30 µm) ceramic coating could be better as the insulating layer compared to alumino-silicate braided fiber insulation, which is about 100 μm thick and reacts with the Ag sheath during heat treatment, degrading the critical current density (Jc). At present, TiO2 seems to be the most viable ceramic material for such a thin insulation because it is chemically compatible with Ag and Bi-2212 and its sintering temperature is lower than the maximum temperature used for the Bi-2212 heat treatment. However, recent tests of a large Bi-2212 coil insulated only with TiO2 showed severe electrical shorting between the wires after over pressure heat treatment (OPHT). The origin of the shorting was frequent silver extrusions that penetrated the porous TiO2 layer and electrically connected adjacent Bi-2212 wires. To understand the mechanism of this unexpected behaviour, we investigated the effect of sheath material and hydrostatic pressure on the formation of Ag extrusions. We found that Ag extrusions occur only when TiO2-insulated Ag-0.2%Mg sheathed wire (Ag(Mg) wire) undergoes OPHT at 50 bar. No Ag extrusions were observed when the TiO2-insulated Ag(Mg) wire was processed at 1 bar. The TiO2-insulated wires sheathed with pure Ag that underwent 50 bar OPHT were also free from Ag extrusions. A key finding is that the Ag extrusions emanating from the Ag(Mg) sheath actually contain no MgO, suggesting that local depletion of MgO facilitates local, heterogeneous deformation of the sheath under hydrostatic overpressure. Our study also suggests that predensifying the Ag(Mg) wire before insulating it with TiO2 and doing the final OPHT can potentially prevent Ag extrusion.

We study here the effect of axial strain on the degradation of the critical current Ic for bare and reinforced, overpressure processed Bi-2212 conductors. We show that reinforcement markedly improves the conductor's stress limit, doubling it from ~150 MPa in the bare conductor to ~300 MPa when reinforced. We find also that certain processes used to reinforce the conductor slightly reduce the Ic degradation strain limit from ~0.6% to ~0.4%. Stress vs strain data taken from the samples studied here has been used to create a finite element model to explore the feasibility of using a reinforced Bi-2212 strand (produced by Solid Material Solutions) in a small test coil. The model predicts an IC limited coil with a maximum hoop strain of 0.31%, well below the experimentally verified strain limit, and is designed to lead to Bi-2212 coils that are not strain limited, but Ic limited.

Werthamer, Helfand, and Hohenberg (WHH) model is applied to the measured upper critical magnetic fields of Ni-and Co-doped pnictide bulk superconductors. This approach incorporates both orbital and paramagnetic effects in the upper critical field’s temperature dependence. The relative importance of both the orbital and paramagnetic effects in the suppression of the superconductivity is described by the Maki parameter. In most superconductors, the Maki parameter is usually much less than unity suggesting that the influence of the paramagnetic effect is small. However, in materials with heavy electron effective mass, in which the Fermi energy is small, or in layered materials in a magnetic field parallel to the layers, the Maki parameter can be larger than unity. This is the case for our bulk pnictides. By fitting the measured temperature dependence of the upper critical field, we derive the Maki parameter and the spin orbit scattering constant and examine the correlation between the Maki parameter, the spin orbit scattering constant, and the reduced upper critical field for our curve fits vs. temperature. The obtained Maki parameters indicate that the Zeeman pair breaking dominates over the orbital pair breaking and spin-paramagnetic pair-breaking effect is significant. At the same time, our spin orbit scattering constants indicate that spin-orbit scattering needs to be also included in description of the Hc2(T) data in these materials.