Juliane Scheiter

High trapped fields in bulk MgB2 prepared by hot-pressing of ball-milled precursor powder

Bulk superconducting MgB2 samples, 20?mm in diameter, were prepared by hot-pressing of ball-milled Mg and B powders using fine-grained boron powders. High maximum trapped fields of B0?=?5.4?T were obtained at 12?K in one of the investigated trapped field magnets (height 8?mm) at the centre of the bulk surface. Investigating the temperature dependence of the trapped field for short MgB2 samples (height ?1.6?mm), trapped fields of up to B0?=?3.2?T at 15?K were achieved. These high trapped fields are related to extremely high critical current densities of up to 106?A?cm?2 at 15?K, indicating strong pinning due to nanocrystalline MgB2 grains. Expected trapped field data for long trapped field magnets prepared from the available MgB2 material are estimated.

AC Magnetization Loss and Transverse Resistivity of Striated YBCO Coated Conductors

It is well known that the separation of thin (RE)BCO superconducting films into electrically isolated stripes (striation process) leads to significant reduction of the magnetization losses. However, in practice, achieving the theoretically predicted loss reduction is quite complicated, due to imperfect separation of the stripes: techniques used for striation leave resistive bridges between the stripes, and coupling currents are free to flow. Very little is known about the precise paths of the coupling currents, other than the fact that the transverse resistivity may play a major role. In this paper, we investigate the magnetization ac loss and the transverse resistivity profile on samples with different numbers of filaments and with different thicknesses of the stabilization layer. The reduction of stabilization layer thickness leads to better control of the laser grooves and substantially suppresses coupling loss. The total loss in those tapes was reduced significantly and is very close to the theoretical expectation.

Magnetization ac loss reduction in HTS CORC® cables made of striated coated conductors

High temperature superconductors (HTSs), like for instance REBCO (RE?=?rare earth) coated conductors, are of high potential for building large superconducting magnets. Some magnets, such as accelerator magnets, require the use of superconducting cables to allow fast ramping, and low magnetization loss to mitigate field quality issues. One of the methods to lower ac loss is to divide the superconducting layer in the tape into filaments. In this paper, conductors with copper stabilization for practical applications are laser scribed into narrow filaments. Striated tapes are then wound into conductor on round core (CORC?) cables. The critical current and magnetization ac loss of single tapes were measured. We found that the stabilizing copper layer causes difficulties for laser scribing. The degradation of the critical current is more pronounced than in the case of non-stabilized tapes. The selection of the number of filaments is therefore a compromise between critical current degradation and reduction of ac loss. Based on the results obtained from single tape experiments, the optimum number of filaments in 4 mm wide tapes was chosen, and CORC? cables with 2, 3 and 4 layers of tapes with and without filaments were manufactured. Magnetization ac loss measurements at 77 K showed a reduction of ac loss in the cables with filaments. This reduction corresponds almost to the number of filaments. Measurement at different frequencies also showed that the coupling loss in CORC? cables with a short twist-pitch is relatively small in comparison to hysteretic loss.

Properties of

MgB2 single-filament tapes and wires with Monel/Nb sheath were produced by the Powder in Tube (PIT)-technique using MA in-ex situ powder. This powder prepared by mechanical alloying (MA) of Mg and B was finally reacted to MgB2 at 700°C. The amount of MgB2 after annealing was about 91 wt%. Additionally 9 wt% MgO secondary phase was found. The primary crystallite size was about 25 nm. Critical current density measurements have shown relatively high values of Jc = 104 A/cm2 at 6.7 T for unsintered tapes in parallel magnetic field. Heat treatment at temperatures of up to 900°C could not improve these values, probably due to MgO hindering the sintering process.

{\rm MgB}_{2}

We present a new high-deposition-rate coating technique that allows us to produce at room temperature long thick films of MgB₂ on flexible steel substrates. Such a technique might give rise to new tapes with higher filling factors compared to the standard processed tapes. With the so-called aerosol deposition technique, MgB₂ films were prepared on Hastelloy substrates with commercially available <italic>ex situ</italic>-prepared MgB ₂ powder (<italic>T</italic>_C,onset of 38 K, <italic>J</italic>_C of 3.8·10³ A/cm² at 4 K and 1 T). Microscopic analyses yield nanocrystalline dense films with high film stresses. The as-deposited films have so far a superconducting transition temperature <italic>T</italic>_C0 of 18.1 K and a critical current density <italic>J</italic>_C up to 5·10³ A/cm² at 4 K and self-field obtained.

Tapes Prepared by Using MA In-Ex Situ Powder

MgB2 bulk magnets are one of the attractive bulk superconductors because they have homogeneous trapped field distribution, low material costs, lightweight, and so on. Pulse-field magnetization (PFM) also has some effective advantages in the point of view of compact setup, shorter magnetizing time, and an inexpensive way to magnetize the bulk samples. However, the trapped field by PFM is inferior to those by field cooling magnetization due to the heat generation caused by sudden changes of the magnetic flux motion. Especially, the temperature rise is remarkable for MgB2 bulk magnets because of its low specific heat. In this study, we have carried out the PFM technique for five-stacked or three-stacked MgB2 bulk samples to analyze the flux-invasion behavior. These samples were fabricated by ball-milling and hot-pressing. As a result, maximum trapped field of BT = 0.64 T was obtained at 14.6 K at the center of the topmost sample when the external magnetic field Bex of 0.90 T was applied. As Bex increases further, BT were dramatically decreased because of heat generation. Furthermore, flux jumps frequently took place in the applied field higher than 2.0 T.

Superconducting Properties of Thick Films on Hastelloy Prepared by the Aerosol Deposition Method With Ex Situ MgB2 Powder

Abstract Long superconducting MgB2 wires and tapes were produced by the “powder in a tube” method (PIT) using a mechanically alloyed nano-powder consisting of Mg, B and MgB2 as the precursor material. Both single-core (SC) and multi-core (MC) conductors were made by swaging and wire drawing the powder contained within a new type of sheath material (ODS-Cu) which, amongst others, also has the advantage of thermally stabilising the conductor. Metallographic investigations on specimens of the conductors in the as-drawn condition and after heat treatment were carried out both to monitor the manufacturing process as well as to gain information regarding important parameters such as the changes in hardness and ductility of the sheath and filament occurring during the forming process and the effect which they have on the final properties of the conductors. The metallographic and scanning electron microscopic (SEM) investigations of the microstructures were carried out on longitudinal (LS) and cross (XS) sections of the conductors which made it possible to understand the correlation between the microstructures and properties of the conductors and to draw important conclusions in order to be able to achieve the optimum design and best electrical properties for the finished conductors. By heat treating the cold formed conductor in the temperature range 500–650°C, very good superconducting properties were able to be produced with critical temperatures (Tc) of 36 K and critical current densities (jc) of 104 A/cm2 in fields of 11 T (parallel to the tape surface), measured at 4.2 K. Using this new sheath material, a conductor of over 100 m in length was able to be manufactured.