During the processing of Powder-in-Tube type Bi-2212 wires the precursor needs to be melted. Overpressure processing enables to vary the oxygen partial pressure pO2 in a wide range. We have studied the influence of pO2 on the phase changes that occur during the Bi-2212 melt processing in situ by high energy synchrotron X-ray diffraction. Heat treatments have been performed in an X-ray transparent high pressure cell in which the air pressure could be varied up to 100 bar during heat treatments up to 900 °C. We report on the phase changes and on the decomposition and recrystallization temperatures as a function of oxygen partial pressure in the process gas.

Understanding the degradation mechanisms and safe operation limits of Ag-sheathed multifilamentary round wires of Bi₂Sr₂CaCu₂O_8+x (Bi2212) is important to solve the quench protection challenge. In this study, a large pool of Bi-2212 wires has been investigated by examining the microstructures and electromagnetic properties in short straight samples that have experienced a gradual critical current degradation induced by a controlled quench experiment. Among these samples with varied conductor architecture and porosity level (porous as-drawn wires compared to dense overpressure processed wires), a consistent quench degradation behavior is observed: wire exhibits small irreversible degradation when the hot spot temperature exceeds 400 K and then large degradation in 500-600 K. The sample set also includes barrel-shape samples and solenoids tested in background fields up to 14 T, which varies the strain state of Bi2212; the safety criterion for quench protection of 350 K is still applicable. Further microstructural study on degraded samples reveals that quench degradation is a thermal-strain driven effect, initiated by cracks in the Bi2212 filament developing perpendicularly to wire axis and then following by ruptures in Ag sheath.

Work is supported by the US Department of Energy, Office of High Energy Physics through the joint University – Fermilab doctoral program in accelerator physics and technology and through a FY12 Early Career Award to TS.

Metallic precursors (MP) starting from Bi (or Bi2O3), Sr, Ca, Cu and Ag were synthesized via mechanical alloying with a controlled stoichiometry and homogeneity. Bi2Sr2CaCu2Ox (Bi2212)/Ag multifilamentary wires are manufactured via the powder-in-tube process using MP. After deformation, the multifilamentary wires are heat treated in flowing oxygen through three stages: oxidation, conversion and partial-melt processing. The MP are oxidized into sub-oxides, converted to Bi2212 phase and melt processed to enhance grain connectivity, texture and thus transport properties. Results show that the oxidation stage plays a crucial role in phase assembly, microstructure evolution and thus transport properties of the wires after complete heat treatment. Here, several factors that impact the oxidation mechanism, formation and growth of Bi2212 grains are discussed. Results of chemical analysis, transport properties, magnetic behavior, microstructure and phase assemblage of multifilamentary wires are also reported.

The authors acknowledge the use of the Analytical Instrumentation Facility (AIF) at North Carolina State University, which is supported by the State of North Carolina and the National Science Foundation. The authors are grateful to William
Nachtrab and Terry Wong (Supercon. Inc.) for assistance with this project.

Bi₂Sr₂CaCu₂O_x (Bi2212)/Ag multifilamentary wires are manufactured via the powder-in-tube process using oxide powders. The properties of the precursor powders, including stoichiometry, purity, grain size and morphology, packing density and phase assembly, have significant impact on wire properties after heat treatment. Most research has focused on the processing of wire after deformation due to limited control of the precursor powders, resulting in several challenging, unsolved problems. In particular, inconsistency in stoichiometry, content of carbon residue and Bi₂Sr₂CuO_y (Bi2201) impurity of the precursor powders limit wire transport. Here, nanosize oxides produced by NanoSpray CombustionTM (nGimat, llc) are used as starting materials to synthesize Bi2212 oxide precursors via solid-state calcination. After calcination, high purity Bi2212 powders with controllable stoichiometries, ultra-low carbon content and absence of Bi2201 are produced. In our study, properties of nanosize oxides, detailed processing parameters of Bi2212 precursor powders and their influence on precursor properties including stoichiometry, phase transformation, carbon content, grain size and morphology are discussed. Furthermore, multifilamentary round wires are made from these powders, melt processed and analyzed. Results of transport property, magnetic property, microstructures and phase assemblage correlated to precursor properties are reported.

Some applications of high temperature superconductors (HTS) will require stacked HTS tapes. Therefore we need to study the performance of HTS tapes in both single and stacked conductors. Previous studies have revealed that the vertical component of the self magnetic field or external magnetic field affect the current transport properties in the tapes. The structure of the self magnetic field strictly related to the current density profiles in HTS tape. In present work we examine the current density profiles in stacked conductor consists of two identical BSCCO tapes placed in parallel one above the other at the distance of 50 micrometer (2 layers of kapton tape). The transport currents in both tapes are running in opposite directions. We measured the vertical component of the magnetic field by Hall probe with active area of 0.05 x 0.05 mm2 above the stacked conductor at the altitude 0.3 mm. The magnetic field profile was measured at different current feeding mode for analysis of its behavior. From the data set for the magnetic field by means of inverse problem solution we calculate the current density profiles in both tapes. To perform these calculations we developed MATLAB code based on method of least squares. The calculation results show the flattening of current density profile in the top tape during various current feeding modes that is expected to be followed by increasing of critical current. Critical current measurements were performed with the standard four-probe method at the liquid nitrogen temperature. The critical current increased in one tape of the stacked conductors and may be attributed to self-field influence of anti-parallel currents in the second HTS tape. Experimental results showed that the critical current is enhanced on the top tape of stacked conductors by antiparallel currents and this may be as a result of strong interaction between them.

Bi-2223 superconducting wires were produced by the in situ powder-in-tube (PIT) method using Ag and Ag-Cu alloy tube with four different ratio (% 1 Cu -% 99 Ag, % 2 Cu -% 98 Ag, %3 Cu -% 97 Ag and %4 Cu -% 96 Ag). Structural, electrical and magnetic characterizations of the samples were performed by the scanning electron microscopy (SEM), R-T, I-V and AC susceptibility measurements. Surface morphology, critical transition temperature (Tc), and critical current (Ic) of samples were measured to investigated experimentally the effects of heating rates with thermal heat treatments. We found that slow heating rates (1 οC /min) are to give better superconducting performance of the samples. All samples are kept for 120 h at 845 οC and then cooled by 1 οC/min rates from annealing temperature to 200 οC in air atmosphere. All the samples in the study shows the superconducting properties with Tc ranges between 106 K and 110 K, as determined by ac susceptibility measurements. Cu addition as the matrix with Ag, is shown to weaken superconducting properties. Detailed analysis of the results, will be presented.

This work has been supported by Turkish Research and Scientific Council (TUBITAK) under grant contract No: 113F150. We also acknowledge The Republic of Turkey, Ministry of Development under the Project Number 2010K120520.

Bi2223 tapes have been developed as high-T_c superconducting material for various practical applications, such as power cables, high field generating magnets and motors. In general, Bi2223 tapes have been fabricated through long-time sintering at 830 ~ 845ºC in P_O2 ~ 0.08 atm. Our previous studies revealed that sintering in low oxygen partial pressure (P_O2 ~ 0.03 atm) promotes formation of Bi2223 phase in a short sintering time. In the present study, effect of low oxygen partial pressure sintering and high oxygen partial pressure annealing on Bi2223 tape were systematically studied. As-rolled tapes which contain Bi2212 and other phases at the oxide core were sintered in low oxygen partial pressure, 0.03 ~ 0.05 atm, at various temperatures for 12 ~ 48 h. After the first sintering, tapes were rolled again, and re-sintered under various conditions, temperature, P_O2 and time. All re-sintered tapes were annealed under P_O2 ~ 0.01 atm for controlling cation composition and were finally annealed at low temperatures below 400ºC to control oxygen content under P_O2 = 1 atm or 10 atm. A tape fabricated by the first sintering at 820ºC in P_O2 = 0.03 atm for 24 h, and second sintering at 815ºC in P_O2 = 0.05 atm for 24 h, showed relatively high I_c ~ 140 A at 77 K. In addition, the tape sintered in P_O2 = 0.03 atm at 820ºC for 12 h and re-sintered at 800ºC for 6 h exhibited I_c = 120 A at 77 K. This suggests damages due to rolling process after first sintering were recovered by second sintering for a short time. After oxygen annealing treatment, sharpness of superconducting transitions and critical current density at high magnetic field were found to be improved.

There is demand for 2212 superconductor wire in NMR, accelerator and high field magnet applications, if it is available in long, higher current, defect-free and robust lengths. However, fine filament 2212 wire that is suitable for advanced prototype and commercial applications development cannot be produced until the 2212 powder and silver billet into which it is packed are improved. This paper describes the development of a single step aerosol spray pyrolysis (ASP) process for making high purity 2212 powder that does not contain large hard particles, in combination with the immediate automated packing and sealing of this powder into silver billets before it can be contaminated or agglomerated. The impurity content of ASP powder is at less than half the levels specified in standard powder, while carbon is at 40 ppm compared to 100 ppm standard levels and the powder is also free from foreign or agglomerated large hard particles. In order to assist powder and powder-packed billet development, we have established a capability for low cost, rapid production of small scale 2212 multifilament wire that we are applying to develop superior powder types and upstream wire attributes with regards to wire Jc, Je and quality. With standard powder, 2212 Jc in small scale wire is comparable to much higher fill factor standard wire. Small scale wires have been produced containing powders with stoichiometry and phase content variations. Ic testing and metallography of samples with different diameters and filament sizes indicate that Jc, Je and Ic, as well as filament uniformity and quality of full scale 2212 wires will be improved by using the best powder types identified in this study.


This work was supported by Phase 2 SBIR DE-SC0011334. The authors thank Oxford Superconductor Technologies and SupraMagnetics for providing fabrication services and the Applied Superconductivity Center and North Carolina State University for testing.

There is demand for HTS conductors that must operate with high winding current and current density in magnetic fields above 20 T, with mechanical properties that support large Lorentz force induced stresses and with acceptably low losses in ramped fields. As yet there is no HTS conductor available that meets all these requirements. This paper presents test results and development work that have established the feasibility of a novel approach for producing high current density, strengthened rectangular 2212-based wire by a low cost long length method. The square to slightly rectangular cross-sectioned wire is formed by hydrostatic pressure rolling of as –drawn standard round 2212 wire. This approach is shown to greatly increase the density of 2212 powder in the filaments, without impairing filament uniformity. A 50% improvement in Jc (4.2K, 5T) is attained compared to as drawn round parent wire when melt texture heat treated in a 1 atm oxygen, for example 550 A/mm2 as compared to 360 A/mm2. A further improvement to is realized by application of a modest level of mechanical compression on the wire’s flat surfaces during the above type of melt texturing. The feasibility of attaching very high modulus, high yield strength reinforcement strips to these wires has been demonstrated, with tensile stress tolerance increased above 300 MPa and bend tolerance improved by 2.5 fold as compared to round wire with the same cross-sectional area. In combination, these developments greatly advance the opportunity for utilizing 2212-wire and cable by the react and wind approach after it is fully melt textured, because winding 2212 wire before full processing limits the sizes and complexities of coils that can be produced.

The author thanks the Applied Superconductivity Center at the National High Magnetic Field Laboratory for testing samples and providing access to use of deformation equipment.

As industrial quality standards for 2G HTS wire become more and more strict, the focus in the wire development is shifting towards meeting the maximum number of special customer requirements. Each HTS application is implemented through a unique device; therefore, it ideally requires 2G wire with a unique set of properties. The product development at SuperOx is directed towards: (1) customisation of 2G wire itself and (2) fabrication of basic integrated modules made with HTS wires.
There are common parameters of the wire, which are critical for almost all applications, such as high Ic value both in magnetic field and in self-field, high mechanical strength, etc. At the same time, in order to offer wire that fits best specific applications, we provide a set of additional wire parameters upon customer demand. SuperOx offers customised wire solutions to reflect different key property profiles sought by the developers of various HTS devices. At present, customisation options include on-demand thickness of silver and copper coatings, surround polyimide insulation, low resistance soldered joints, solder plating, and lamination. We can combine and/or alternate different finishes along the length of a single piece of wire, for example, make a polyimide-insulated wire with ends cleared of insulation coating and pre-soldered to facilitate electric contact. Under development are filamentisation of wires for reduced AC losses, fabrication of stacks of wires, and Roebel cables.
In addition to customised wire, we make HTS device modules such as coils (pancake, racetrack, and bifilar) and current leads. Another development are solid 2D blocks of any size and shape consisting of multiple layers of 2G HTS tapes that represent an advanced and easily engineerable alternative to bulk HTS materials for applications that require trapping and/or shielding of magnetic field, including magnetic levitation.

Recent success in development and up-scaling of the manufacturing process of HTS coated tapes with extra-high critical currents in 4mm wide conductors triggered our motivation to up-scale further the processing to 12 mm conductor wide that meets the specifications of EUCARD2 CERN project. Particularly, the HTS tape was developed for use in high-field Roebel cable with improved engineering critical current density at 4.2 K .
A multi-beam pulsed laser deposition (MB-PLD) was used to deposit the YBCO based HTS layer onto a stainless steel substrate preliminary coated with bi-axially textured yttria-stabilized-zirconia buffer template. A double precipitation method based on combination of intrinsic and extrinsic precipitations was employed to gain critical current in high magnetic field. A drum concept of our MB-PLD enabled easy conversion of the 4mm-width technique to 12 mm wide tapes. Deposition of further metallic layers of silver and copper were performed by modified physical vapour deposition and plating techniques respectively.
As a result, engineering current density, J_e, of 820 A/mm² was achieved in 12 mm wide and 36 m long HTS coated tapes at 4.2 K, 18 T, B//c (measured in particular fragments of entire tape). This is the highest current density achieved in 12 mm wide coated tapes so far. Earlier, we demonstrated J_e = 950 A/mm² as a record engineering current density in 4 mm wide tape, 22 m long, measured at same temperature and field.
Nowadays, we report an engineering current density, J_e, which corresponds to 87 % of earlier record Je-value. Remarkable is that this new Je degree is achieved in up-scaled by a factor of 5 tape areas (length x width).
Further step of reduction of substrate thickness in order to gain J_e by further 40% is discussed.
Results from application of the tapes in Roebel cable are presented.

This work was supported in part via European Project EuCARD-2

Excimer lasers have a long history in PLD research on superconducting YBCO thin films where they are routinely operated at wavelengths of 248 and 308 nm. From the laser perspective upscaling the deposition rate commands both, the increase of the pulse energy and of the laser pulse frequency Scaling up the deposition rate via pulse energy increase generally implies a multi-plume design of the PLD system. Increasing the laser pulse frequency increases the deposition rate linearly. Today coated conductor volume manufacturing via PLD relies on solid-state switched based excimer laser platforms delivering a stabilized output power of up to 600 W.
State-of-the-art high-power excimer lasers routinely achieve maintenance-free operating times of 10,000 hours in thin film deposition and annealing applications. Flow-loop systems optimize the laser gas flow in the discharge region. With careful design using wind tunnel simulation, gas speed and uniformity has been optimized enabling homogeneous beam cross sections over the entire tube life.
Multi-plume, multi-lane approaches have emerged as the standard parallel approach to growing coated conductors at high speed. The substrate tape is wound on sets of large-diameter rollers and moves through the deposition zone many times until the desired layer thickness has been achieved. A substrate heater thereby ensures a constant tape temperature. Plumes are created by synchronous rastering with the laser beam. This distributes the energy on the target and leads to large and homogeneous deposition area.
PLD production systems have matured into turn-key solutions transforming high laser power into high average deposition rates. Both metal oxide buffer and superconducting layer can be deposited on up to 1000 m long coated conductor tapes.

Compact pilot line (CPL) is aimed for manufacturing of HTS coated conductors via pulsed laser deposition (PLD) and alternating beam assisted deposition (ABAD) on different metallic substrates including Cr-Ni stainless steel tape. The line developed by Bruker HTS is finally put in operation at Kurchatov Institute (Moscow). This is a result of 3 year project between Russian Superconductors, Kurchatov Institute and Bruker HTS.
The line represents a “tool” for learning the fundamental physical and technological principles employed in processing chain, starting from the polishing of metallic substrate and finishing with the Cu-plating and tape characterization.
Results of research program directed towards further improvement of coated conductors regarding their mechanical and electrical properties are presented.
Characterization results for 100m-long-class coated conductors with critical current above 300A/cm-width (77K, SF) are reported.
Conditions required for suppression of critical current non-homogeneities are defined for each particular technological step of tape processing. Investigations of process stability and reproducibility are discussed and summarized.

The authors are thankful to ROSATOM Corporation who provided a support of the project.

One of the most promising commercially produced high temperature superconductors is (RE)Ba2Cu3O7-x in the film of a coated conductor on a Ni-alloy substrate. Its high critical current and mechanical strength make it promising for many applications. Yet the inability to create low-cost superconducting joints has long been one of the barriers in the use of (RE)Ba2Cu3O7-x tapes for applications which require long lengths of superconductor, such as wound magnets. Current joining techniques utilize thermally driven diffusion, which is difficult to accomplish in the c-direction of YBCO, or a non-superconducting solder. These approaches often require impractical processing conditions such as long-term annealing, and both yield low-resistance joints at best. However, several groups have shown that the application of an electric field during annealing can reduce the processing temperature required to produce diffusion in ceramics; this technique is known as electric field assisted sintering. Here, we investigate the use of an electric field to reduce the temperature and time required to create direct contact joints between the superconducting (RE)Ba2Cu3O7-x layers of coated conductors. We present a study which produces joints using a large range of sintering temperatures, times, and electric field/current. The critical current, magnetic behavior, and microstructure of the joints were then characterized using low temperature four-point probe measurements, magneto optical imaging, and electron microscopy.

We would like to thank the US DOE Office of High Energy Physics for their funding on this work.

In these days, although it is available to produce a 1 km long CC tape, it is still not sufficient in length to fabricate cables or coil windings. Therefore a joint for long coated conductor tapes having a low joint resistance is essential in fabricating coils, magnets, and power cables. Various joint techniques such as solder joint and diffusion joint have been tried and reported. A practical and good joint for CC tapes should be cost-effective and meet both purposes of electrical and mechanical connections with low contact resistance without critical current degradation and strong adhesion. In this study, CC tapes were joined by a hybrid welding technique which employs both ultrasonic welding (UW) and mechanically controlled soldering processes. Cu-surround GdBCO CC tapes adopting stainless steel substrate were supplied as samples. The CC tapes were joined by splice joint with face-to-face configuration. In order to improve the specific resistance at the jointed part, dominant parameters like initial gap, tip patterns of mating horn and anvil, and vibration amplitude were varied and their effects on joint characteristics were investigated. The critical current Ic and joint resistance Rj of the jointed CC tapes were measured at 77 K. Electro-mechanical properties of joint parts were also investigated at 77 K and self-field.

This work was supported by a grant from National Research Foundation of Korea (NRF-2014-002640) funded by the Ministry of Science, ICT and Future Planning (MSTP), Republic of Korea.

This talk describes a programme of work aiming to understand the fundamental properties of jointed superconductors for large-scale applications such as demountable fusion reactors and particle accelerators. Jointed high-temperature superconductors (HTS) were fabricated from In₅₂Sn₄₈ and Pb₃₈Sn₆₂ soldered joints consisting of a thin layer of solder sandwiched between two Rare-Earth-Ba₂Cu₃O₇ (REBCO) 2G high temperature superconducting coated conductors. While the joints for low-temperature superconductors (LTS) consist of Pb₃₈Sn₆₂ solder and two NbTi strands. The NbTi strands include the wire-in-channel (WIC) strands and those in the form of Rutherford cables. The electrical and thermal properties of the HTS joints are presented together with numerical modelling data, critical current and resistivity measurements on the joints from 300 K to 4.2 K in applied magnetic fields up to 12 T, as well as scanning electron microscopy (SEM) studies. Our results show that the copper/silver layers significantly reduce the effects of heating in the joints to less than a few hundred mK. When the REBCO alone is superconducting, the joint resistivity (R_J) predominantly has two sources, the solder layer (~20 - 30 nW×cm²) and an interfacial resistivity (R_i) at the REBCO/silver interface which is ~25 nW×cm2 in the as-supplied coated conductors. Furthermore, the HTS joints have zero or very weak magnetoresistance in fields up to 12 T. Electrical resistivity measurements on the LTS joints were performed at 4.2 K and in an applied magnetic field up to 10 T. The resistivity of the LTS joints can be made as low as ~2 nW×cm² at 4.2 K. The copper stabilizer in the WIC strands was¹ very effective in removing heat from the joints to the surrounding liquid helium. The WIC strands remained at 4.2 K when ~0.7 W power was generating at the joints. However, the magnetoresistance of the LTS joints is much higher than that of the HTS joints. We attribute the high magnetoresistance of the LTS joints largely to the copper stabilizer of the NbTi strands. We will also comment on how RJ depends on the joint configurations and suggest other choices of low-temperature superconducting alloys for fabricating HTS or LTS joints.

We would like to thank: Simone Smith1, Prapaiwan Sunwong¹, Mark J. Raine¹, Robert Edge¹ and Tom Lee^1,2. This work was funded by the RCUK Energy Programme under grant EP/I501045.
¹Department of Physics, University of Durham, Durham, DH1 3LE, UK.
²EURATOM/CCFE Fusion Association, Culham Science Centre, Abingdon, Oxon, OX14 3DB, UK

Due to the further development of metal coated, thin film high temperature superconductors (HTS) which operate at the temperature of boiling liquid nitrogen (LN2, -195,8 °C) a wide field of application in superconducting fault current limiters, generators or motors arises. In order to apply HTS in these devices, long term stable electrical joints between normal- and superconducting materials as well as joints between HTS are needed. In order to develop and evaluate these electrical joints it is necessary to measure and investigate the electrical (e. g. contact resistance, power loss), thermal as well as the mechanical behaviour of the conductors themselves and their joints at the temperature of LN2. This paper presents and evaluates three different joining techniques to establish an electrical joint for low temperature applications. Therefore, a low temperature test rig was built up which provides the possibility to simulate network conditions from normal operation to fault conditions. The given paper describes the design, mounting and implementation of the test rig, including the arrangement and adaptation of the measurement system. At first the different conductors are measured. After the presentation of the selected joining techniques, the electrical joints with normal- as well as with superconductors are established. The different joining techniques will be evaluated according their usability at low temperatures. All results are compared with reference measurements at room temperature.

The gluing and RMS soldering of the conductors were performed in cooperation with the Institute of Manufacturing Technology of the Technische Universität Dresden. The authors would like to thank Maurice Langer (LOT), Evelyn Hofmann and Theresa Kühne (both FM).

Aiming to realise magnets for MRI applications, a necessary step consists in introducing superconducting joint in complex windings to connect variuos elements (magnet to magnet, magnet to switch) to enable the persistent mode and thus improving field stability and removing current feeding. To test and progressively improve the joint technology, in a step by step plan, a series of 30 single turn windings with single joint has been successfully realised and put in persistent mode with resistance of the whole circuit les than 10-14 Ohm, thus proving extreme reliability of the process. Consequently, a 30 meter long winding was realised and, finally, a 200 meter long solenoid was put in persistent mode at various temperatures. The conductor used is a commercial 19 filaments, nickel clad with dimensions of 3x0,5 millimiters and colaminated with a 3x0,2 millimiters OFHC copper strip as stabilizer. Construction, testing results and discussions will be presented.

We are developing MgB₂ wires and magnets aiming at high temperature operation without liquid helium. Since highly-stable magnetic field is required, persistent current mode (PC mode) operation of closed-loop coils is desirable. During PC mode operation, persistent current flowing in a superconducting coil decays owing to resistance caused by flux-creep in wires and joints. In this paper, as a first step for PC operation of MgB₂ coils, dependence of temperature and magnetic field on current decay of MgB₂ wires was investigated using a SQUID magnetometer. Next, PC mode operation of a small closed-loop coil was demonstrated under conduction-cooling condition in self-field. The coil was pre-reacted and jointed with MgB₂ bulk. By measuring trapped field decay, the resistance of the closed-loop, which represents the resistance of MgB₂ joint, was estimated. It was found that the resistance was sufficiently low.

MRI and NMR magnets are critically dependent upon the production of true superconducting joints that can enable persistent mode operation. Although several well established jointing techniques exist for NbTi conductors, the key relationships between processing conditions, joint microstructure and superconducting performance remain poorly understood. Joint failures are common and costly occurrences on the magnet production line. The end of current EU exemptions on the use of Pb in superconducting magnets may soon render the industry standard Pb-Bi alloy soldering technique un-usable. There is therefore a significant driving force for developing Pb-free joints for NbTi conductors.

Presented here are the conclusions of a study into the fundamental performance characteristics of soldered, cold pressed and spot welded joints between NbTi wires. Using a new SQUID magnetometry technique, we have measured the superconducting properties of numerous joints. By combining these results with detailed microstructural analysis, we have shown that we can understand the joint performance by correlating the local chemistry in the key regions of current transfer and the resultant superconducting properties of the joints. True persistent mode joints can be made by crimping, spot welding and with Pb-free solders, although reproducibility is hardest to achieve with spot welding. These results are a useful step towards the development of simple, reliable and Pb-free superconducting joints for next generation magnets.

EPSRC for doctoral funding via DTA. P Noonan and H Rugg at Agilent Technologies for funding and materials. D Prabhakaran and A Boothroyd for use of the MPMS in the Clarendon Laboratory. A. Dobrynin for use of the MPMS at Diamond Light Source.

Superconducting joints are one of the key components needed to make Ag-alloy clad Bi2Sr2CaCu2O8+x (Bi-2212/Ag) superconducting round wire (RW) successful for high-field, high-homogeneity magnet applications like nuclear magnetic resonance (NMR) magnets in which persistent current mode (PCM) operation is highly desired. In this study, a procedure for fabricating superconducting joints between Bi-2212/Ag RWs was developed and tested. Melting temperatures of various Bi-2212 + Ag precursor mixtures to be used in the joint region were investigated by differential thermal analysis (DTA). Test joints were fabricated and heat treated, and their voltage-current (V-I) properties were measured using the conventional four-point method at 4.2 K and in magnetic fields up to 14 T. Microstructures of the joint areas were examined by scanning electron microscopy (SEM). A Bi-2212/Ag RW close loop solenoid with a superconducting joint was fabricated, and the resistance of the solenoid was characterized using the field decay method. The details of the superconducting joint fabrication procedure and measurement results will be presented and discussed.

This work is funded by the National Science Foundation (DMR-1157490) and by the State of Florida.

Quench process of high temperature superconductor in case of overcurrent has been an hot and important research topic for many years. A better understanding concerning the quench process can help engineers and researchers to design and manufacture superconducting device in a more reasonable and optimal way. In this work, the quench initiation and quench development of coated conductors in case of fault current are investigated. In the experiment, high temperature superconducting tapes are tested under different fault currents, where the voltage distribution along the tapes and current in the tapes are measured. Then the quench initiation and quench development are analyzed based on the voltage distribution as well as heat generated along the samples. Finally, the influence of critical current and n value to the quench initiation and development is analyzed and summarized.

REBaCuO coated conductors (CCs) are currently the most promising HTS tapes thanks to their high current-carrying capability and good in-field behavior. Their use in several applications is however hindered by their large AC magnetization losses. Striating the tapes into narrow filaments is a well-known way of reducing such losses. Nonetheless, in practice, achieving the theoretically predicted loss reduction is complicated due to the difficulty of controlling the quality of the grooves, which make it possible for coupling currents to flow. This is particularly true for samples with thick copper stabilization. The technological feasibility of the striation process has therefore to be thoroughly tested.
In this work, the AC magnetization losses for various frequencies and the transverse resistances were measured on various types of prepared tapes. The samples differ in terms of manufacturers (SuperPower and SuperOx), geometries (various tape widths and thicknesses of Cu-stabilization), number of filaments (from 3 to 120) and processes for copper deposition (electroplating before or after creating the grooves).
The AC loss measurements are performed in a wide range of frequencies of the external magnetic field, which allows determining when coupling currents are present and the extent of the importance of the coupling losses. Complementary DC measurements of the transverse resistivity between filaments help understand the path of the coupling currents: for example if those currents cross the tape’s width by flowing in and out of each superconducting filament or by flowing only in the metallic substrate. The results of the AC and DC characterization of the tapes are compared against numerical models specifically developed for evaluating the AC losses and determining the path of the coupling currents.
Thanks to the variety of tested samples and operating conditions, this work is expected to provide useful information on the applicability of the striation process in commercially available HTS coated conductors for realistic power applications.

This work has been partly supported by the Helmholtz Association (Young Investigator Group grant VH-NG-617).

Second generation coated conductor tapes are able to carry high currents which are needed for power applications. However, their use in practical applications is restrained due to their high AC magnetization losses, which is an ultimate consequence of their very large aspect ratio. Numerous works from different research groups conducted over the past few years have already shown that striating the tapes into filaments significantly reduces these magnetization losses. However, much effort remains to be made to make the striation process a technologically viable process, especially for tapes with substantial copper stabilization. For this reason only Ag-cap tapes were laser-structured and afterwards electroplated with copper.

In this work, (RE)BCO coated conductors were structured with a Pico-second laser burning grooves of 18 µm to 21 µm in width and 4-5 µm in depth (Ag-cap). Up to 120 parallel filaments in 12 mm wide conductors were made. After structuring the tapes were heat treated in flowing oxygen to avoid the Cu deposition inside the grooves. The Cu electroplating was made by SuperOx Company with thicknesses of about 5 µm, 7 µm and 10 µm, respectively.

The critical current and the total AC-magnetization loss were measured as a function of the filament count and the electroplated Cu thickness. Electron microscope images from top view and cross-section of the laser cuts show that the Cu rapidly overgrows the grooves causes coupling of the filaments. With increasing Cu thickness the remaining groove width decreases. Even for 5 µm of Cu the grooves are partially overgrown. To overcome this problem one could increase the groove width. However, this decreases the critical current. In comparison with our previous work we conclude that for low to medium thick Cu layers (up to 10 µm) the direct laser-structuring of Cu-cap tapes seems to be a more reliable way for making striated conductors.

The Current-Voltage characteristics of superconductors are commonly determined by means of a standard 4-probe DC measurement technique. In coated conductors, however, we often encounter problems due to weak or no stabilization and in such case, when the risk of burning is high, this approach can fail. In other cases such as in HTS cable structures, besides vulnerability and high risk of damage, it may also be difficult to achieve a high critical current level with a power supply that is normally used to characterize single strands. We investigated therefore the AC transport loss as a function of frequency in overcritical current range from which we deduce the prediction of the DC I-V curve. In the AC regime, both, hysteretic loss component and joule dissipation are present for applied transport currents exceeding the critical current Ic. In this work, AC loss measurements at different frequencies are related to different electric fields in DC and a curve similar to the DC I-V is obtained. From this curve, the N and Ic values are estimated. AC regime operation can be more beneficial first, because very high currents can be achieved at ease by using a transformer. And second reason is substantially lower effective joule heat dissipation at overcritical currents as opposed to the dissipation in DC at same amplitudes. In the experiment, we used 1 mm wide YBCO model tape without Cu-stabilization, used to simulate weak narrow current path of CC tape. Such sample burns by thermally triggered quench when measured directly in DC. For comparison with AC transport loss, the sample was connected in parallel with a thin copper plate, so the sample remains stable even in overload condition. To support our study, numerical simulations, reproducing both transient and stationary operation conditions, were performed. A similar relation between the AC current transport loss and the I-V curves was also observed in the simulations

This work has been partly supported by the Helmholtz Association (Young Investigator Group grant VH-NG-617)
 

We report on the mechanical properties of different MgB2 wires and the influence of wire layout, elemental composition, microstructure and texture on the wire degradation under uniaxial tensile loading. Stress-strain curves have been acquired at 77 K and the lattice distortions of the different wire phases have been measured simultaneously by high energy synchrotron X-ray diffraction. The microstructure and the texture in the different phases were characterised using FIB nano-tomography and X-ray diffraction, respectively.

Nowadays long lengths of MgB2 conductors on the km scale fabricated by the powder-in-tube process are commercially available. Due to its relatively low cost, high critical current density (Jc), low anisotropy and a critical temperature of about 40 K, MgB2 may be attractive for several applications. Examples of these are as MgB2 coils for superconductor transformers, as induction heaters or as MRI magnets. MgB2 conductors also promise to be a usable option for superconducting coils in wind generators or as a Superconducting Magnetic Energy Storage (SMES) for the recently proposed LIQHYSMES approach. LIQHYSMES as a new hybrid energy concept combines the use of LIQuid HYdrogen as the primary, high-density energy carrier with SMES for a fast and efficient buffering. In this paper we present the physical properties of MgB2 wires tested in several experimental configurations of a projected MgB2-based solenoid of a small LIQHYSMES test set-up.

MgB2 is regarded as one of the most promising superconductor materials due to its high transition temperature, low raw material cost and good upper critical field and critical current density values. The MgB2 wire was fabricated with milled 5 h precursor powders by the in-situ powder in tube (PIT) method using Nb as a barrier and copper as a stabilizer. The high-energy ball milling is effective to obtain high density and fine grain size MgB2 wire, however, the drawing process of MgB2 wire with milled precursor powders becomes difficult compared with that of wires with unmilled precursor powders. The microstructure of wire has a great influence on its fracture behavior. It shows that there are complicated correlations between fracture behavior and the main influencing parameters. The possible causes of fracture behavior have been discussed.

This work was financially supported by the National Natural Science Foundation (51302226 and 51372207), the National Key Basic Research Program (No. 2011CBA00104) of China and National Natural Science Foundation (2014JQ6198) of Shaanxi province.

We present the results of magnetization AC losses of MgB₂ superconductors with one and four filaments made by internal magnesium diffusion (IMD) into boron process and compared to samples made by powder in tube (PIT) technology. For AC loss measurement a system based on a calibration-free method was used. Short samples of MgB₂ wires were exposed to an external magnetic field with amplitudes up to 0.07 T, frequencies up to 1200 Hz, and a temperature range between 15 K and 40 K. A strong effect of eddy current losses was found in single-core wire containing pure copper sheath, which was proved by the same wire measurement after Cu etching. The impact of coupling current losses in non-twisted four filament wire and the decoupling effect after twisting was observed. Coupling current losses in a low-frequency region were effectively reduced in agreement with theoretical assumption. The degradation of transport currents due to torsion stress by twisting was taken into account and the normalized AC losses of MgB₂ wires made by IMD and powder-in-tube processes were compared. It appears that the IMD process is more perspective for AC applications due to much higher current densities and smaller degradation of current-carrying capability by twisting.

This work was supported by the Slovak Scientific Agency under the Project APVV-0495-10 and Grant agency VEGA 2/0121/12.

Superconducting cables are widely used in high field magnets and recently extended for electricity transport. The run for higher performances (magnetic fields or transport capabilities) pushes them closer to their mechanical limit. Cables are composed with a complex assembly of superconducting strands. Furthermore, the degradation of current transport capability of cables due to electrical-strain dependence in individual strands calls for a multi-scale analysis. In order to predict the electrical behavior of a strand, its accurate mechanical behavior is needed. However, strands are subjected to various loadings during their life. Building an accurate model means reproducing the loadings of the manufacturing process, of the cabling process and in-service loadings. These various loadings applied to cables lead to tension-compression and bending in individual strands with a large range of loadings.
The main objective of this project is to characterize and model various types of superconducting strands (MgB2 and Nb3Sn) with different manufacturing process and applications. Monotonous tensile tests with unloadings and cyclic tensile tests at various mean stresses were used to identify an elasto-plastic constitutive model in a range representative for in-service loadings. Observations and analyses will also be performed to understand the origins of the cyclic degradation of the electrical performances.
The paper presents the testing procedure developed and the results obtained for the mechanical behaviour under monotonous and cyclic loadings of a commercially MgB2 strand with Monel matrix fabricated by the Powder-In-Tube (PIT) method. Nano-indentation tests were performed to study the local properties of each material composing the strand. These steps are required to build an accurate model adapted to the complex structure of the superconducting composite. A strain localization was notably observed under mechanical loading, characterized by a plateau on the stress-strain curve and was specifically investigated as it may be linked to the drop in electrical performances under monotonous and cyclic loadings.
This work is partly funded by the French “Agence Nationale de la Recherche” within the COCASCOPE project.

As MgB2 is a new alternative superconducting strand for CIC conductor, the technology of cabling is important. As we know, the damage to the strand would happen during the cabling process, and the main kind of damage is the indentation on the cross section, caused by the adjacent strands. In this paper, the artificially indentations on the performance of MgB2 round wire will be studied. In the experiment, the MgB2 samples will be marked with various depths artificially before heat treated. Then, the performances such as critical current, n-value, residual resistance ratio and AC losses will be measured. The influence of indentation will be observed and discussed according to the experiment results.

MgB2 wires made by internal magnesium diffusion proved to be a good alternative to those prepared usually by powder-in-tube method. Compared to in-situ or ex-situ PIT process, IMD wires show considerably higher critical current density and improved in-field dependence. Unlike traditional IMD process, when Mg rod is placed at the centre of metal tube and the space between the Mg and outer tube is filled by boron powder, wires in this study were made differently. Mg rod placed in Nb tube was drilled and filled with boron powder and deformed by rolling into wire. After final sintering the MgB2 core has been extracted from Nb sheath and used for further measurements. The set of MgB2 cores was investigated by magnetic measurement up to 9 T. In this study, we present the properties of MgB2 cores doped with SiO2 nanoparticles and C nanoparticles in comparison to reference sample. Hysteresis loops were recorded and critical current density was determined using Beans critical state model. At 1 T and 3 T field sweep was interrupted and magnetic relaxations were measured. Andersons flux creep theory was applied to calculate mean effective activation energy of flux lines. The effect of various doping on critical current density and pinning is presented and discussed.

For the further development of MgB₂ wires, investigation of microstructures of MgB₂ filaments and sheath materials is important. However, typical cross-section observation such as optical microscopy and scanning electron microscopy are essentially two dimensional, therefore, not enough to understand the three dimensional filament structures. In this study, we have adopted X-ray computed tomography (CT) and carried out three dimensional analysis for MgB₂ wires. X-ray micro-CT (SkyScan 1172, Bruker) allows us to visualize internal filament structure of a cm-long-MgB₂ wire with high spatial resolution, typically 5 micrometers, with non-destructive manner. We investigated the boundary between sheath material and MgB₂ filament of internal Mg diffusion (IMD) processed MgB₂ wire. From the result, we have successfully estimated the longitudinal fluctuation of the position of MgB₂ filament. Furthermore, by improving the signal-to-noise ratio for the X-ray micro-CT, we have also identified the boundary between MgB₂ filament and air (pore) in spite of low X-ray absorption coefficient for boron. From the three dimensional microstructure view, we can revealed the shape and the thickness of MgB₂ filament quantitatively, and also their statistical distribution. These results show that the X-ray micro-CT is one of the most useful diagnostics for microstructural analysis for MgB₂ wires.

This work was supported by Japan Science and Technology Agency (JST) as Advanced Low Carbon Technology Research and Development Program (ALCA).

The high pressure and especially isostatic one has a wide potential to modify microstructure of the superconducting material and can change the thermodynamic properties during materials synthesis. Thus by use of high pressure the microstructure can be modified by promoting the nano-grains stabilization and effectively increasing the connectivity between the grains. It is possible to change the stability region of desired phases and avoid formation of the liquid phase in certain processes. It is very important to understand and use the possibilities provided by high pressure for high connectivity in ex situ and in situ materials and for controlling the kinetic of reaction in which soft in situ material is transformed to the very hard ex situ one. By using sequences of pressure and temperature gradients it is possible to program the grains distribution in the wires. Properly selected combination of the reactive elements and barrier material, followed by a modification of plastic properties of hard phases under high pressure enables elimination of the porous structure and helps in deformation of the wire core according to the desired geometry.
Such processes have been realized at Institute of High Pressure Physics in Warsaw in collaboration with several institutes in the world. The high pressure synthesizes and annealing have been used mainly to the MgB2 family wires and Fe122 and FeSe materials – often doped by carbon, SiC, or the rare earth oxides.
We show the comparison between the ambient pressure annealed samples and the same or similar, made by use of several high pressure procedures (CP, CIP, HP, HIP) in gas, plastic solid media or eutectic liquids at pressure up to 1.4 GPa. For the majority of the high pressure treated samples the Jc is remarkably higher than for normal pressure ones. For MgB2 it can be even 8 times higher at 14 T. Additionally, pristine MgB2 samples rich almost the same Jc at 14 T/4.2 K as doped ones – only by increasing the connectivity and limiting the nano-grains growth. The same effect is presented for Fe122 materials. Several practical methods for long wire production are presented. The Jc level of 100 A/mm2 at 14 T /4.2 K is achieved. The other high pressure methods adapted for wire technology and especially for poisonous materials are presented. The microstructures and transport properties results are presented.

This project was partially supported by the URC/AIIM/University of Wollongong, Australia joint partnership grant with IHPP PAS Poland, and the own sources of IHPP Unipress PAS

Magnesium diboride conductors are considered for use on fully superconducting wind turbine generators and, as a results, being able to quantify the AC losses in MgB2 wound stators is paramount. AC losses were measured on several MgB2 conductors based on NiCu matrix material. The measurements were done calorimetrically at about 15 K and at applied magnetic fields up to 100 mT and frequencies up to 400 Hz. The AC losses measured are consistent with magnetization losses in magnetic materials and not with classic AC losses (magnetization, coupling and eddy). As previously reported in literature, the matrix material composed of NiCu alloys, while non-magnetic at room temperature, might present magnetic properties at cryogenic temperature. This paper deals with the characterization of the matrix material of several MgB2 conductors from different manufacturers through the measurement of the susceptibility with respect to temperature and the measurement of the major hysteresis loop showing the saturation field and the maximum energy dissipated per cycle. The results are then used to correlate the AC losses measurements.