Internal Mg diffusion(IMD) method is promising to fabricate high Jc and high Je MgB2 wires. We fabricated high performance MgB2 wires by applying IMD method and using 4%C-coated nano-sized B powder(SMI Co.). We found the pre-treatment of SMI B powder with p-methylbenzene(C8H10) eliminated Cl in the powder and increased the critical current properties. Highest Jc of 133kA/cm2 and 76kA/cm2 were obtained at (4.2K, 10T) and (20K, 5T), respectively. These Jc are higher than those of SiC-added wire. SiC addition brings about Mg2Si precipitates which act as barriers of superconducting currents and decrease Jc, while the wire fabricated with C-coated B powder contains no such precipitates. Multi-filamentary wires are easily fabricated with IMD process. We have already fabricated 7-, 19- and 37-filamentary wires. Generally, Jc increased with increasing the number of filaments, i.e. with decreasing the MgB2 layer thickness. Previously, we demonstrated that some kinds of aromatic hydrocarbon additions to the starting powder can introduce C substitution for B sites in MgB2 of both powder-in-tube(PIT)- and IMD-processed wires and increase Jc values. Recently we found that a coronene(C24H12) is more effective in increasing Jc values. C24H12 coating on B powder is easily obtained by heating the B and C24H12 mixture above the melting point of C24H12. By using this C24H12-coated B powder large increase of Jc and Je values were obtained for both PIT- and IMD-processed MgB2 wires. The IMD mono-filamentary MgB2 wires fabricated with 5%C24H12-coated B show Jc and Je of 110kA/cm2 and 12kA/cm2 at 4.2K and 10T, respectively. We have already fabricated 100m-class mono-filamentary IMD MgB2 wires using C24H12 coated B powder. 2m long wires cut from these wires show fairly uniform Ic distribution, suggesting that IMD method together with C24H12 coating on B powder is promising for the fabrication of practical high performance MgB2 wires.

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

One of the main challenges in development of MgB₂ wires is to improve packing factor and grain connectivity in the superconducting material, while keeping the grains small. This is often realized through sample pressing that leads to material densification. Such process can be conducted before (cold pressing) or during material synthesis (hot pressing). For wire samples, it is important to maintain their round shape, thus isostatic pressing is preferred over uniaxial pressing.
We have applied various high pressure procedures to multifilamentary HyperTech Research MgB₂ wires. Superconducting material was either undoped or doped with C, SiC or malic acid. Sintering of samples was performed in one of 3 procedures: ambient pressure annealing, 1 GPa cold isostatic pressing (CIP) followed by ambient pressure annealing or 1 GPa hot isostatic pressing (HIP) where high pressure and high temperature are applied simultaneously.

Samples were analyzed with SEM to determine material porosity, grains size and other effects. Transport resistivity measurements vs. temperature were conducted in fields up to 9 T. Critical current transport measurements were conducted in both 4.2 K and 20 K temperature in fields up to 14 T.

We show that both CIP and HIP improve critical current of MgB₂ wires. However, there are significant differences between these two processes. During HIP material is not only densified, but also the reaction dynamics is changed due to an increase of magnesium melt temperature. As a result, CIPed and HIPed samples have different microstructure and current performance. We present very dense MgB₂ cores with no visible porosity on SEM images.

Finally, we present a new method of isostatic wire pressing, which can be easily applied in industrial production. Key feature of the proposed method is the replacement of a dangerous gas medium with a dry lubricant. Despite the visible 15% flattening of samples, we were able to improve I_c of MgB₂ wires with quasi isostatic, 200 MPa high pressure annealing.

Author of this paper has received funds for PhD thesis preparation from National Science Centre Poland under PhD stipend based on decision number UMO-2013/08/T/ST3/00559.
This project is also partially supported by the URC/AIIM/University of Wollongong, Australia joint partnership grant with IHPP PAS Poland.



 

Thermally stabilized filamentary MgB2 wires with different barrier materials (Nb, Ti, NbTi and C) and not magnetic outer sheath have been made by in-situ PIT and IMD process. Critical current densities and AC losses of MgB2 wires and cables with different twisting and barriers have been measured and compared. AC loss measurements were done at variable external AC magnetic fields and temperatures between 18 K and 40 K by the system utilizing a calibration free method. External AC field up to 70 mT in RMS and frequency range from 2.3 Hz up to 1152 Hz were applied. The effect of barrier material (different resistivity) on the coupling AC loose is shown and compared with theory. A strong coupling effect was observed in non-twisted 30-filament wire with Ti barriers and also for cable made from 30-filament strands. The reduction of coupling losses by twisting was studied and related to the critical currents of not twisted wires. The minimal AC losses and no critical current reduction were measured for cables made from single-core MgB2 strands. High current densities and small transport current degradation by twisting typical for wires made by IMD process. Application of NbTi barrier has shown an effective way for the reduction of coupling losses in MgB2 wires at temperatures around 20 K. Consequently, filamentary MgB2 wires with NbTi barriers and made by IMD process are very promising for future low AC loss conductors.

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

We report a study of thermal transport in MgB₂ tapes produced by Columbus Superconductors differing in architecture, stabilization and composite materials. For the first time, the temperature and field dependence of thermal conductivity has been investigated both along the conductor and in the direction perpendicular to the tape. These data provide fundamental input parameters for describing the 3D heat diffusion process in a winding. Thermal transport properties – even in field – are typically deduced using semi-empirical formulas from the RRR of the stabilizer measured at B = 0. We have evaluated the accuracy of these procedures, comparing the calculated k(T,B) values with the measured ones. Based on the experimental thermal conduction properties k(T,B) and critical current surface J_C(T,B) we have determined the dependence of minimum quench energy and normal zone propagation velocity on the operating parameters of the conductor. The correlation between thermal properties and tape layout has allowed us to propose solutions for optimizing the thermal stability of MgB₂ conductors. Thermal conduction and stability properties of MgB₂ have also been compared with those of REBCO coated conductors [1,2].

[1] M. Bonura, C. Senatore, Supercond. Sci. Technol. 28 (2015) 025001
[2] M. Bonura, C. Senatore, IEEE Trans. Appl. Supercond., Vol. 25, No. 3, (2015) 6601304

Round wires made of multifilamentary Bi2Sr2CaCu2Ox (Bi-2212) are an appealing alternative to Coated Conductors for high magnetic field applications (> 25 T) such as accelerator dipoles and NMR magnets. The main drawback in obtaining high critical currents densities (Jc) in long-length wires is the internal gas pressure generated during the heat treatment, which expands the wire diameter and dedensifies the superconducting filaments. Different routes have been proposed to increase the density of the Bi-2212 filaments and as a consequence increase Jc along the conductor by acting on the final as-drawn wire through cold isostatic pressure (CIPping) and swaging after drawing or through a very high (up to 100 bar) over pressure (OP) applied during heat treatment.
At CNR-SPIN we tried to overcome this issue pursuing a different approach, i.e. changing the fabrication process in order to avoid as much as possible the porosity inside the as prepared wire. In particular, we applied the groove-rolling at different stages of the deformation process, obtained a larger powders compaction with respect to the drawing, and fabricated samples with round, square or rectangular shape depending on the application requirements.
Through a comparison between the Jc values measured in open- and closed-ends samples, we demonstrate the capability of this alternative deformation process of increasing the density in Bi-2212 wires. We will compare the results obtained in wires fabricated with different powders – e.g. produced by Nexans and nGimat - different architectures and shapes through the characterization of transport and electromechanical properties measured at different temperatures.

Porosity in Bi₂Sr₂CaCu₂O_x (2212) round wire is the main current-limiting factor when standard 1 atm processing is employed because gas-filled pores agglomerate into large filament-size bubbles that block current transport when the 2212 melts. As we showed in 2014, these bubbles can be eliminated by using overpressure (OP) heat treatment at 50-100 atm to densify the wires which significantly increases the engineering critical current density (J_E) up to >600 A/mm² at 4.2 K and 20 T, which makes 2212 a serious candidate for high-field magnet applications. OP treatments also decrease the wire diameter by about 4%, which introduces some complexities when designing and heat treating coils. Ideally, it would be better to wind after densifying the wire, then move straight into OP processing. Several methods to densify the wire before winding have been tried, including swaging, cold isostatic pressing, and rolling. Here we report on wire densification using OP processing at 820°C, which is below the melting point of 2212, then winding the partially-densified wire followed by a full OP heat treatment at 100 atm. Densifying at 820°C decreases the wire diameter by ~3.5%, greatly reducing shrinkage of the winding pack during the full OP heat treatment to ~ 0.5%. We are also re-examining phase formation during cooling using cooling rates from 1.25°C/h to 20°C/h. Our goal is to simplify the standard heat treatment, adding margin to what has been thought to be a very constrained and complex protocol and at the same time develop a more controllable microstructure with less filament coupling. The critical current density and coil design implications of these new processing methodologies will be reported.

The work at the NHMFL was supported by a grant from the US Department of Energy Office of High Energy Physics under DE-SC0010421 and by the National High Magnetic Field Laboratory, which is supported by the National Science Foundation under NSF/ DMR-1157490, and by the State of Florida.
 

Despite 25 years of progress in the performance of Bi₂Sr₂CaCu₂O_x (Bi2212)/Ag multifilamentary round wires (RWs), understanding the impact of microstructural defects on multiple length scales on the transport behavior remains a significant challenge. On the micron-scale, voids and Bi₂Sr₂CuO_x (Bi2201) grains, and on the nanoscale grain bounderies (GBs), dislocations and Bi2201 intergrowths, are the primary defects. Here, we present results on multiscale microstructural features of several Bi2212 condcutors: cold isostatic pressed (CIP) and over-pressure (OP) partial melt processed (PMP) Bi2212/Ag multifilamantry RWs which carry very high critical current density, and tapes and wires with different precursor powder compositions and therefore carrying very different critical current density. The mesoscopic microstructures are analyzed by cross-sectional scanning electron microscopy (SEM). Bi2212 grains are extracted from filaments via a lift-out technique using a focussed ion beam (FIB), and the nanoscale defects within Bi2212 colonies are studied via an aberration corrected scanning transmission electron microscope. Results show that CIP and OP-PMP increase transport through increased filament density, however greater improvement in OP-PMP compared to CIPed wires is due to the improved grain connectivity on the nanoscale. Multiscale studies on tapes with different precursor compositions show varying Sr/Ca leads to a significantly different phase assemblage in the melt, and therefore different impurity distribution in the PMPed conductors. We correlate the uniform dispersion of micronscale impurities, in particular Bi2201 grains, and high Bi2201 intergrowth density, to high Sr/Ca ratio and high critical current density. This study confirms that after porosity on multiple length scale, Bi2201 grains have the greatest negative impact on Bi2212 wires and significant enhancements in Bi2212 wire performance requires either avoiding the formation of Bi2201 grains, or ensuring complete conversion of Bi2201 to Bi2212 grains. Bi2201 intergrowths are not detrimental to wire transport and even could be beneficial through enhancing magnetic flux pinning.

A project is underway at the NHMFL to develop and build a 32 T superconducting
magnet. The 32 T magnet will consist of a 15 T LTS outer magnet and two HTS coated–conductor inner coils generating 17 T, constructed from double-pancake modules. Prototype HTS coils, very similar to the final coils except for a reduced number of modules, have been rigorously tested in the 15 T outer magnet. The prototype coils contain 2 km of 4-mm wide coated conductor and generate 9 T at 200 A.
The test protocol included a) a sensitivity study of the HTS coil quench behavior versus background field, quench heater current and coil current (100-200 A);
b) load cycling of the HTS coils above design stress and c) observing the response of the HTS coils to deliberate quenches of the LTS magnet and vice versa from self-field up to the full-field case at 24 T central field.
Both the HTS and LTS coils proved robust through load cycling and numerous deliberate quenches and the quench protection schemes proved effective. No spontaneous quenches occurred. After the above tests, the HTS coil was operated at 264 A in a 15 T background field for 15 minutes before ramping down. A central field of 27.0 T was generated, which is the highest field so far for an all superconducting magnet.