W Häßler

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.

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.

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.

Stoichiometry dependence of superconductivity and microstructure in mechanically alloyed MgB2

The correlations among stoichiometry variation, preparation route, and microstructure as well as superconducting properties were investigated in MgB2 bulk samples. We demonstrate that the different initial weight ratios of Mg and B between Mg0.9B2 and Mg1.1B2 are only slightly shifted towards Mg depletion during the mechanical alloying preparation at room temperature followed by a short annealing step. The samples with Mg surplus show the highest critical temperature Tc∼36K, highest critical current density Jc∼1.5×106A∕cm2 at 7.5 K in self-field, and a homogeneous microstructure. In contrast, the Mg-depleted samples exhibit a rather porous and multiphase microstructure with a broad grain size distribution. A direct correlation of stoichiometry variation and superconducting properties was not found, but rather a strong influence of the Mg content on lattice strain and grain connectivity.

Critical current density enhancement in strongly reactive ex?situ? MgB2 bulk and tapes prepared by high energy milling

The ex?situ technique is promising for its ease of preparation, low material shrinkage during annealing and the fact that it is already superconducting without annealing. However the critical current density (Jc) needs to be enhanced. To improve the Jc values of commercial powder high energy milling was used?a method commonly use in industry and easy to apply. It was found that high energy milling is successful in improving Jc in bulk and tapes. The reason for the improvement is reduced crystallite size and the large fraction of fresh grain surfaces. In this paper we also experimentally investigate the problem of decomposition of MgB2?ex?situ during heat treatment.

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.

Low temperature preparation of MgB2 tapes using mechanically alloyed powder

Instead of commercially available MgB2 powder, we have used partially reacted powder prepared by mechanical alloying. This precursor powder consists of grains with a size of only a few nanometres and contains reacted MgB2, and also the starting material Mg and B, and is, therefore, more reactive than fully reacted commercial powders. Using copper as a sheath material, tapes were prepared by the usual powder-in-tube process. After annealing at relatively low temperatures (770–870 K) in inert atmosphere, the tapes have good superconducting properties. Magnetically we have measured a critical current density of 400 kA cm−2 at 4.2 K.

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.

Hot isostatic pressing of multifilamentary MgB2 wires in solid state media for large scale application

Multifilamentary MgB2 wires were annealed under the high pressure of low-friction, solid state media such as BN and graphite powders. The idea was to replace the hot isostatic pressing in argon medium, which is beneficial to wires transport properties but is difficult for application on an industrial scale. The experimental results show that annealing in BN or graphite under 200 MPa leads to enhancement of the critical current, similar to that observed earlier for high pressure argon annealing. An unwanted effect of the process is the flattening of the round wires, which after the treatment yields an elliptical cross section with b to a ratio of around 85 ± 5%. Comparison of critical currents and microstructure of samples synthesized with different parameters are presented.