M Herrmann

Touching the properties of NbTi by carbon doped tapes with mechanically alloyed MgB2

Combining mechanical alloying and powder-in-tube processing is a promising way to obtain tapes with excellent properties. Taking advantage of the properties of nanocrystalline precursor powders, it was possible to obtain Jc values of 104A∕cm2 at 12.1T and 4.2K. Evidential substitution of carbon into the MgB2 changed the electron scattering and therefore raised the Bc2 up to 12T at 10K. Systematic investigation on the influence of the heat treatment showed that, although an interfering Fe2B reaction layer was formed, an excellent Jc of 104A∕cm2 at 14.3T and 4.2K was achieved.

Further increase of the critical current density of MgB2 tapes with nanocarbon-doped mechanically alloyed precursor

The combination of nanocarbon-doped nanosized MgB2 precursor powder with an inert metallic sheath of appropriate hardness gives the possibility to obtain tapes with significantly improved critical current densities at high magnetic fields. In parallel field, Jc-values of 104?A?cm?2 at 16.4?T (4.2?K) and 5.6?T (20?K) could be measured.

MgB2 bulk and tapes prepared by mechanical alloying: influence of the boron precursor powder

The influence of the quality of boron precursor powder on the microstructure and superconducting properties of MgB2 bulk samples and tapes was investigated. The nominal purity specified by the suppliers considers only metallic impurities and is not sufficient for the characterization of the boron precursor powder. Oxygen impurities and the grain size of the B precursor powder were found to affect Tc and the microstructure of the MgB2 tapes. The microstructure was investigated by SEM and TEM. Grains in the boron precursor powders were either nanocrystalline or crystalline, with grain sizes varying between 110 and 500 nm. MgB2 precursor powder was prepared by mechanical alloying, which resulted in a small, 20–60 nm, MgB2 grain size of bulk samples. Bulk samples showed the highest MgB2 phase fraction and a critical current density of 4.7 × 104 A cm−2 (at 20 K, 1 T) if boron precursor powder with small grain size and small fraction of metallic impurities was used. Such powder also yielded compact tapes and required lower annealing temperatures for the MgB2 phase formation. The typical critical current densities of the tapes were 5.0 × 104 A cm−2 (at 20 K, 3 T) and were significantly better than those of samples reported recently. These results underline the importance of mechanical alloying for enhancing the critical current density of MgB2 tapes. Summarizing, the phase content, the density and the superconducting properties of MgB2 bulk and tapes depend on the choice of boron precursor powder.

The effect of reactive nanostructured carbon on the superconducting properties of mechanically alloyed MgB2

Polycrystalline samples of MgB2 doped with reactive nanostructured carbon were synthesized by pressure assisted sintering of mechanically alloyed precursors. Varying the nominal carbon concentration from x = 0 to 0.316, the effects of carbon doping on the lattice parameter, lattice strain, actual amount of incorporated carbon (xactual), grain size, normal state resistivity (?), connectivity, superconducting transition (Tc), critical fields (Birr and Bc2) and critical current density (Jc) as well as the pinning force (Fp) were evaluated. An evident solubility limit of carbon within the MgB2 matrix, forming MgB2?xCx with an xactual?0.125, was observed. In addition to the carbon saturation the superconducting properties, e.g.?Tc, Bc2 and Jc, also reflect saturation effects with respect to the actual carbon concentration. Improved electron scattering in MgB2?xCx seems responsible for the observed enhancement of Bc2 to 11.4?T at 20?K. On the other hand, calculations of the flux-pinning forces show a dramatic decrease of Fp,max with increasing carbon concentration. Therefore we conclude the observed improvement in critical current density at applied fields >6?T to result mainly from the raised upper critical field.

Reactive nanostructured carbon as an effective doping agent for MgB2

A MgB2−xCx superconductor was prepared with reactive nanostructured carbon up to nominal x = 0.316 by high-energy ball milling. These products crystallize at temperatures below 700 °C, forming mainly MgB2−xCx with a particle size of about 20 nm and with lattice-dissolved carbon content up to about x = 0.13 under normal pressure conditions, and minor MgB4. The nominal higher addition of reactive nanostructured carbon does not have an influence on the a-axis of the MgB2−xCx structure. The superconducting behaviour reflects the optimum interplay of the lattice-dissolved carbon, which influences the carrier density, and the homogeneously distributed carbon, which probably acts as a pinning centre. For the sample with a nominal x = 0.221, the critical current density (Jc) increased by approximately one order of magnitude to Jc = 1.7 × 104 A cm−2 at 9 T and 4.2 K compared to the undoped MgB2.

Influence of the milling energy transferred to the precursor powder on the microstructure and the superconducting properties of MgB2 wires

A systematic study of the influence of the milling energy of the precursor powder on the microstructure and the superconducting properties of MgB2 bulk samples and wires, and, in addition, the deformation behavior of the wires is presented. An explicit approximate formula for the energy transferred to the powder sample during milling and its dependence on the parameters of the milling process is developed and used for the data analysis. For higher milling energies the amount of the reacted MgB2-phase shows a strong increase. The transport critical current density of wires can be enhanced by using precursor powder milled with higher energy. Because the deformation properties are degraded to some extent, one has to find a compromise of the preparation parameters between current density and deformation behavior.

MgB2 tapes made of mechanically alloyed precursor powder in different metallic sheaths

MgB2 tapes have been made by the powder-in-tube technique using precursor powder prepared by mechanical alloying and deformed in Fe, Nb and Ti sheaths by two-axial and flat rolling. The best core uniformity and the highest core density were obtained for Ti sheathed tape. Different Jc values and current anisotropies were measured for applied sheath materials after the same deformation and heat treatment at 600 °C for 3 h or at 650 °C for 0.5 h, which is discussed and related to the interface reaction and to MgB2 core density and texture. The MgB2/Ti tape having the strongest sheath and the smoothest core/sheath interface has the best texture of MgB2 and consequently also the largest anisotropy ratio.

Anisotropy of the critical current in MgB 2 tapes made of high energy milled precursor powder

For applications of MgB2 wires or tapes, high critical currents in high magnetic fields are essential. By using tapes in superconducting coils the anisotropic behaviour of the critical current, i.e. the dependence on the direction of the external field in relation to the tape surface, has to be taken into account. The anisotropy of MgB2 tapes with mechanically alloyed (MA) precursor powder and different sheath materials which can be much higher than the intrinsic anisotropy is discussed. Furthermore tapes with pure and C doped MA-MgB2 precursor are compared. Tapes with a hard Fe-sheath and undoped precursor show a high extrinsic anisotropy of the critical current density which can be reduced considerably by carbon doping. A texture of the MgB 2 phase in the tape filaments introduced by flat rolling was observed by synchrotron x-ray diffraction. Using this texture information the observed macroscopic Ic-anisotropy of the tapes can be explained by calculations based on the percolation model. (Some figures in this article are in colour only in the electronic version)

Ex?situ?MgB2 barrier behavior of monofilament in?situ?MgB2 wires with Glidcop? sheath material

Cost-effective MgB2 wires are highly sought after to replace the widely used NbTi conductors in superconducting magnets, e.g.?for magnetic resonance imaging (MRI). One method of choice for lowering conductor costs is to use a less expensive barrier and sheath material. From this point of view copper is a good candidate for the sheath, and additionally has other advantageous properties, e.g.?the best electrical and thermal conductivity at operating temperature. One major disadvantage of copper is its high chemical reactivity. This material reacts fast with magnesium and forms Mg?Cu alloys, removing the starting element necessary for MgB2 formation. To prevent this reaction, a special coaxial architecture was applied using ex?situ? MgB2 powder as a chemical barrier between the copper sheath and the in?situ? MgB2 powder core. The Glidcop? (dispersed strengthened copper) sheathed MgB2 wires with an ex?situ barrier have been fabricated by the conventional powder in tube (PIT) method. Besides avoiding the reaction of Mg and Cu, the barrier also contributes to the superconducting core fraction and increases the filling factor up to 50%. The Glidcop? sheathed wires with an ex situ?MgB2 commercial powder used as a barrier have been successfully drawn to a diameter of 1.2?mm and then a pressure-assisted heat treatment was applied. For our case of specified in?situ and ex?situ powders the pressure-assisted preparation of at least 0.15?GPa was found to be important for the densification of the ex?situ barrier.

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.