Critical Current Densities In Excess Research Articles

Transport in Bulk Superconductors: A Practical Approach?

The characterization of the critical current density of bulk high-temperature superconductors is typically performed using magnetometry, which involves numerous assumptions, including, significantly, that $J_{{c}}$ within the sample is uniform. Unfortunately, magnetometry is particularly challenging to apply where a local measurement of $J_{{c}}$ across a feature, such as a grain boundary, is desired. Although transport measurements appear to be an attractive alternative to magnetization, it is extremely challenging to reduce the cross-sectional area of a bulk sample sufficiently to achieve a sufficiently low critical current that can be generated by a practical current source. In the work described here, we present a technique that enables transport measurements to be performed on sections of bulk superconductors. Metallographic techniques and resin reinforcement were used to create an I-shaped sample of bulk superconductor from a section of Gd–Ba–Cu–O containing 15 wt % Ag2O. The resulting superconducting track had a cross-sectional area of 0.44 mm2. The sample was found to support a critical current of 110 A using a field criterion in the narrowed track region of 1 $\mu\text{V cm}^{-1}$ . We conclude, therefore, that it is possible to measure critical current densities in excess of $2.5 \times 10^{8} \text{A m}^{-2}$ in sections of a bulk superconductor.

Strongly enhanced flux pinning in one-step deposition of BaFe2(As0.66P0.33)2 superconductor films with uniformly dispersed BaZrO3 nanoparticles

The high upper critical field and low anisotropy of the iron-based superconductor BaFe2As2 make it promising for its use in the construction of superconducting magnets. However, its critical current density in high magnetic fields needs to be improved. Here we demonstrate a simple, one-step and industrially scalable means of achieving just this. We show that introducing controlled amounts of uniformly dispersed BaZrO3 nanoparticles into carrier-doped BaFe2As2 significantly improves its superconducting performance without degrading its structural or superconducting properties. Our BaFe2(As0.66P0.33)2 films also exhibit an increase in both the irreversibility line and critical current density at all magnetic-field orientations. These films exhibit nearly isotropic critical current densities in excess of 1.5 MA cm−2 at 15 K and 1 T—seven times higher than previously reported for BaFe2As2 films. The vortex-pinning force in these films reaches ~59 GN m−3 at 5 K and 3–9 T, substantially higher than that of the conventional Nb3Sn wire.

Development of Powder-in-Tube Processed Iron Pnictide Wires and Tapes

The development of the PIT fabrication process of iron pnictide superconducting wires and tapes has been carried out in order to enhance their transport properties. Silver was found to be the best sheath material, since no reaction layer was observed between the silver sheath and the superconducting core. The grain connectivity of iron pnictide wires and tapes has been markedly improved by employing Ag or Pb as dopants. At present, critical current densities in excess of 3750 A/cm^2 (Ic = 37.5 A) at 4.2 K have been achieved on Ag-sheathed SrKFeAs wires prepared with the above techniques, which is the highest in iron-based wires and tapes so far. Moreover, Ag-sheathed Sm-1111 superconducting tapes were successfully prepared by PIT method at temperatures as low as 900C, instead of commonly used temperatures of 1200C. These results demonstrate the feasibility of producing superconducting pnictide composite wires, even grain boundary properties require much more attention.

Processing of high performance (LRE)-Ba–Cu–O large, single-grain bulk superconductors in air

We report the fabrication of large (LRE)BCO single-grains with improved superconducting properties for LRE=Nd, Sm and Gd using a practical process via both conventional top seeded melt growth (TSMG) and seeded infiltration-growth (SIG). This process uses a new generic seed crystal that promotes heterogeneous grain nucleation in the required orientation and suppresses the formation of solid solution in a controlled manner within individual grains by the addition of excess BaO2 to the precursor powder. The spatial distribution of the superconducting properties of LRE bulk superconductors as a function of BaO2 addition for large (LRE)BCO grains fabricated in air by TSMG and SIG for LRE=Gd, Sm and Nd are compared. The optimum BaO2 content required to fabricate single-grain (LRE)BCO with high and homogeneous Tc is determined from these experiments for each LRE system. The irreversibility fields of (LRE)BCO bulk superconductors processed in air are as high as those processed in reduced PO2. Critical current densities in excess of 105A/cm2 at 77K and higher trapped fields have been achieved in optimized (LRE)BCO superconductors fabricated in air for the first time.

Preparation of<tex>$rm Y_1rm Ba_2rm Cu_3rm O_7-delta$</tex>Superconducting Tapes via a Continuous Electrochemical Process

Our recent advancements about a continuous process for the production of electrochemical deposited Y1Ba2Cu 3O7-delta (Y-123) films are presented. The process consists of alternate elemental deposition steps and thermal treatments. By unreeling through sequential modular sections, several tens meter long tapes are processed in a continuous way yielding the final coated superconductor (Y-123/Ag), suitable for industrial applications. The Y-123 phase constituting elements (Cu, Y, and Ba) are sequentially electrodeposited on both sides of an untextured 50 mum-thick silver tape. Following copper and Yttrium hydroxide deposition steps, an intermediate annealing transforms the precursors into a mixture of CuO and Y2Cu2O5. After Barium hydroxide electroplating, a final thermal treatment in pure flowing oxygen allows the formation of the Y-123 superconducting phase. A strict control on deposit stoichiometry and thermal parameters seems to be crucial in order to achieve a good microstructure and high performances. To date the process allows to deposit on both sides of the substrate 0.5 mum-thick Y-123 films that exhibit critical current densities in excess of 20 kA/cm2 (77 K, self-field), the highest reported value for this material obtained via an electrochemical process on untextured metal tape

Fabrication of high performance light rare earth based single-grain superconductors in air

Large, single-grains of (LRE)-Ba–Cu–O (LRE=light rare earth: Nd, Sm, and Gd) bulk high-temperature superconductors with significantly improved properties have been fabricated in an air atmosphere using a practical processing technique based on seeded infiltration growth. This process involves the use of a generic seed crystal, developed recently at the IRC in Superconductivity, to promote epitaxial grain nucleation. The formation of a solid-solution phase within the nucleated single grain is then suppressed effectively by providing excess Ba to the growth front under an air processing atmosphere. Critical current densities in excess of 105A∕cm2 at 77K have been observed in the resulting large grain samples, with an associated and significant improvement in trapped magnetic field. This air-based seeded-infiltration-growth process offers a significant degree of freedom both in terms of the processing parameters and the reproducibility in growth of oriented single grains.

Continuous tape coating by thermal evaporation

Among the various coating techniques, thermal evaporation currently allows the highest YBCO volume deposition rates because of its large area capability. This "simultaneous large area deposition" capability translates into long tape lengths when the deposition area is densely filled up with metal tape. On short rolling-assisted biaxially textured substrate samples thermal evaporation has already established critical current densities in excess of 1 MA/cm/sup 2/ at 77 K, with very promising performance in magnetic fields. Recently, the deposition technique has been scaled up using a multiturn tape winder giving a simultaneously coatable length of about 4 m of 1-cm-wide tape. The reel-to-reel tape deposition system is designed for long term continuous operation based on in-situ refillable evaporation sources and controlled using feedback from atomic absorption spectroscopy measurements in the vapor phase. Here, the authors present results on the first coated conductor tape continuously fabricated by this technique.

Deposition and microwave performance of YBCO films on technical ceramics

YBCO films for microwave applications are usually deposited on expensive, single crystal substrates. Here, technical ceramics offer a variety of advantages with respect to rf-filters. They are available in arbitrary large size, the dielectric constant is isotropic, and can be selected from a range between 10 and 110 for ceramics with appropriate composition. The main shortcoming is the polycrystalline nature of such ceramic substrates. However, the ion beam assisted deposition (IBAD) technique allows YBCO growth with excellent crystalline orientation and yields superconducting properties comparable to epitaxial films on single crystal substrates. As an example we tested substrates made of SM 210, a standard low loss microwave ceramic from Kyocera, with a dielectric constant of 21 close to that of LaAlO 3. After proper polishing the 2 ′′ substrates a 2 μm thick YSZ-IBAD buffer was applied by PLD. YBCO films of various thicknesses were deposited by thermal evaporation. These films exhibited critical current densities in excess of 1.5 MA/cm 2 at 77 K. We report evaluation of the surface resistance of the YBCO films and the substrate losses by a standard dielectric resonator technique in the frequency regime around 10 GHz and discuss the technological impact of these results on future filter development.

Critical current across grain boundaries in melt-texturedYBa2Cu3O7−δrings

We determine the critical current ${J}_{\mathrm{cB}}$ of grain boundaries fabricated by ``welding'' of melt-textured YBCO with various degrees of $\mathbf{ab}$-plane misalignment. ${J}_{\mathrm{cB}}$ is determined by monitoring the magnetic moment due to persistent shielding currents, which were induced in rings containing two sections of a single grain boundary. The voltage drop across the junctions is estimated to be below ${10}^{\ensuremath{-}12}\mathrm{V},$ much smaller than in typical transport measurements. As the temperature or magnetic field is increased, an abrupt decrease (kink) is observed in the magnetic moment of the ring when the induced shielding current in the ring exceeds the critical current across the boundaries. The kink signals that flux has begun to penetrate through the grain boundaries into the bore of the ring. This behavior is confirmed by magneto-optical imaging. We observe that grain boundaries with [001] tilt mismatch angles up to 5\ifmmode^\circ\else\textdegree\fi{} have critical current densities in excess of ${10}^{4}{\mathrm{A}/\mathrm{c}\mathrm{m}}^{2},$ comparable to the bulk current density. At larger mismatch angles, the critical current rapidly decreases with increasing angle. The qualitative behavior of ${J}_{\mathrm{cB}}$ as function of temperature, magnetic field, and misorientation angle is similar to that observed YBCO grain boundaries manufactured by other methods.

Continuous YBCO – Tape Coating by Thermal Evaporation

ABSTRACTCurrently, due to simultaneous large area coating thermal evaporation (TE) allows the highest YBa2Cu3O7 (YBCO) volume deposition rates among various coating techniques. The simultaneous large area deposition translates into long length when the deposition area is closely filled up with metal tape. On short RABiTS samples thermal evaporation has already established critical current densities in excess of 1 MA/cm2 at 77K. Recently, the deposition technique has been scaled up using a multi-turn tape winder for a simultaneously coatable length of 4 meters of 1 cm wide tape. The reel to reel tape deposition system is designed for long term operation based on a vapor composition control by atomic absorption spectroscopy and in situ refillable evaporation sources. In the initial setup phase the system has been able to reproduce the above results on small samples. The first results on continuously moving tape substrate coating up to a length of several meters will be reported.

R&D of coated conductors in Japan

The recent achievements of critical current densities in excess of 1MA/cm 2 at 77K in YBCO films deposited over appropriate bi-axially textured buffer layers/metal substrates have stimulated interest in the potential future applications of coated conductors at liquid nitrogen temperature and high magnetic fields, which are called as the next/second generation tape conductors. Several different processes for obtaining the biaxially textured buffer layers/metal substrates as well as thicker film deposition processes and non-vacuum processes are reviewed. This paper reviews the research goals of the project including the recent development of the related research by individual research group.

R&D of Coated Conductors in Japan

ABSTRACTThe recent achievements of critical current densities in excess of 1MA/cm2 at 77K in YBCO films deposited over appropriate bi-axially textured buffer layers/metal substrates have stimulated interest in the potential future applications of coated conductors at liquid nitrogen temperature and high magnetic fields, which are called as the next/second generation tape conductors. Several different processes for obtaining the bi-axially textured buffer layers/metal substrates as well as thicker film deposition processes and non-vacuum processes are reviewed. This paper reviews the research goals of the project including the recent development of the related research by individual research group in Japan.

Isothermal melt processed Bi-2212/Ag tapes containing MgO and Al 2 O 3 additions

The development of the superconducting phase in melt-processed Bi-2212 tapes depends on controlling the development of solid phases in the partial melt and the conversion of the partial melt into a highly-aligned superconducting phase. MgO and Al2O3 additions have been used with isothermal melt processing (IMP) for the grain refinement of phases in the partial melt and the prevention of secondary phase growth during solidification. Controlled solidification with reduced oxidation rates also significantly increased the transport properties of the tapes with or without the oxide additions. The optimal oxidation rate was found to depend upon the processing temperature. Critical current densities in excess of 100 kA cm−2 were obtained in tapes melt-processed below 800 °C.

High critical current density YBa2Cu3Ox tapes using the RABITs approach

Progress in the fabrication of epitaxial, high-Jc, biaxially aligned YBCO thick films on Rolling-assisted biaxially textured substrates (RABiTs) is reported. RABiT substrates comprise a biaxially textured metal substrate with epitaxial oxide buffer layers suitable for growth of superconductors. Oxide buffer layers have been deposited using three techniques: laser ablation, electron-beam evaporation, and sputtering. Epitaxial YBCO films grown using laser ablation on such substrates have critical current densities approaching 3 × 106 A/cm2 at 77 K in zero field and have field dependences similar to epitaxial films on single crystal ceramic substrates. Critical current densities in excess of 0.2 MA/cm2 have been obtained on stronger, nonmagnetic substrates. In addition, samples with Je of 12.5 kA/cm2 at 77 K have been fabricated. The highest strain tolerence obtained so far is 0.7% in compression and 0.25% in tension. Deposited conductors made using this technique offer a potential route for the fabrication of long lengths of high-Jc wire capable of carrying high currents in high magnetic fields and at elevated temperatures.

Development of Ag-Sheathed Bi(2223) Tapes with Improved Microstructure and Homogeneity

The development of the fabrication process of Ag-sheathed Bi(2223) tapes has been carried out in order to improve their transport and mechanical properties, as required by the power applications which are so far under study. Critical current density values of 28 kA/cm2 at 77 K have been achieved on long multifilamentary Bi(2223) tapes, with a fabrication process that has been successfully employed in the fabrication of samples longer than 50 m. The microstructure and homogeneity of Ag-sheathed multifilamentary Bi(2223) tapes has been markedly improved by employing an alternative deformation technique. In a substantial part of the fabrication process, swaging, drawing, and rolling have been replaced by deformation with an active turks-head machine, which allows the deformation of rectangular shaped wires. At present, critical current densities in excess of 25 kA/cm2 at 77 K have been achieved on long samples prepared with this technique. Moreover, innovative filament configurations have been employed for the fabrication of square-shaped Bi(2223) wires with reduced anisotropy and with critical current densities exceeding 20 kA/cm2 at 77 K.

Comparison of the powder-in-tube processing properties of two (Bi2−xPbx)Sr2Ca2Cu 3O10+δpowders

The processing and formation kinetics in air of two commercial (Bi 2-x Pb x )Sr 2 Ca 2 Cu 3 O 10+ δ powders (Bi 1.87 Pb 0.34 Sr 1.89 Ca 1.94 Cu 3.00 O 10+ δ and Bi 1.84 Pb 0.33 Sr 1.84 Ca 2.07 Cu 3.06 O 10+ δ ) in a silver-sheathed tape using the powder-in-tube manufacturing technique have been investigated. The highest concentration of the (Bi 2-x Pb x )Sr 2 Ca 2 Cu 3 O 10+ δ phase was found to occur at the ceramic/silver interface. The results over a range of times and processing temperatures were fitted to an Avrami equation. In the process range that yielded the optimum volume fraction of (Bi 2-x Pb x )Sr 2 Ca 2 Cu 3 O 10+ δ the Avrami exponent was found to bem=1.2, indicative of platelike growth. In this same region the critical current densities were found to be the maximum and showed that if the entire ceramic cross-section can be processed to have a high volume fraction of the (Bi 2-x Pb x )Sr 2 Ca 2 Cu 3 O 10+ δ phase, then critical current densities in excess of 5×10 8 A m -2 should be routinely achieved. The difference in physical properties of the two powders was attributed to the different starting morphologies of the powders, which determined the properties of the final product.

Isothermal melt processing of Bi 2Sr 2CaCu 2O y round wire

Isothermal melt processing (IMP) has been used to process oxide-powder-in-tube (OPIT) Bi 2Sr 2CaCu 2O y (Bi-2212) round wire with high critical current densities. Isothermal melt processing is a process in which the melting and solidification steps are carried out at the same temperature by melting in an inert atmosphere and solidifying in a high oxygen partial pressure. For each particular oxidizing atmosphere used during IMP, the highest critical currents were found in wires processed in the temperature range between 790°C to 820°C. The highest critical current densities obtained in 1.0, 0.75, and 0.5 mm diameter wires at 4 K and self-field were 4.3 × 10 4, 6.0 × 10 4, and 6.0 × 10 4 A/cm 2, respectively. Transport measurements on a 0.75 mm wire revealed that critical current densities in excess of 3 × 10 4 A/cm 2 could be achieved at 4 K in applied magnetic fields of 9 T. Anisotropy measurements and SEM analysis were used to examine the radial alignment of the Bi-2212 grain colonies within the wires.

Advances in the fabrication and characterization of HTSC composite conductors

Recent advances in the performance of high-temperature superconductor (HTSC) multifilamentary composite conductors have been dramatic. Using powder-in-tube techniques that are based on scalable processes, conductor (J e) and filament (J c) critical current densities in excess of 6,700 and 26,500 A/cm 2 (77 K, self field) have been achieved, respectively. Pilot production scale conductors have achieved J e and J c values of 2,100 and 8,900 A/cm 2 (77 K, self field) across 1.16 km using a 1×10 -11 Ω cm criterion. Magneto-optic results have been correlated with microstructural features within these high-performance samples.

Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7//MgO/Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7/ edge Josephson junctions

Josephson edge junctions have been fabricated using thin sputtered films of MgO (8-32 AA) to produce a weak connection between high-T/sub c/ electrodes. The current-voltage characteristics of these junctions can be well modeled by the resistively shunted junction (RSJ) equation, and they possess excellent Josephson properties, exhibiting strong magnetic field modulation and microwave response at 10 and 100 GHz up to 85 K. The best high temperature (T>70 K) operation has been obtained with devices which have critical current densities in excess of 10/sup 5/ A/cm/sup 2/ at 4.2 K, and junction critical current resistance products I/sub c/R/sub n/ equivalent to 0.10 mV at 83 K. Experiments indicate that improvements in device quality are expected as the base electrode junction surface is improved. Preliminary results indicate that the cyclability of these junctions is good. >

Y1Ba2Cu3O7/MgO/Y1Ba2Cu3O7 edge Josephson junctions

Josephson edge junctions have been fabricated using thin sputtered films of MgO (8–32 Å) to produce a weak connection between high Tc electrodes. The current voltage characteristics of these junctions can be well modeled by the resistively shunted junction (RSJ) equation and they possess excellent Josephson properties, exhibiting strong magnetic field modulation and microwave response at 10 and 100 GHz up to 82 K. The best high-temperature (T≳70 K) operation has been obtained with devices which have critical current densities in excess of 105 A/cm2 at 4.2 K, and junction critical current resistance products IcRn≂0.15 mV at 60 K. Experiments indicate that improvements in device quality are expected as the base electrode junction surface is improved.