C Rodig

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.

Preparation of MgB2 tapes using a nanocrystalline partially reacted precursor

Fe-cladded MgB2 conductors have been prepared by the powder-in-tube method using mechanically alloyed nanocrystalline Mg+2B powder mixtures consisting of the constituents Mg, B, and MgB2 as precursor. Despite low Tc values of about 31 K, maximum critical current densities (Jc) of 22 and 7 kA/cm2 in external magnetic fields of 7.5 and 10 T, respectively, are achieved at 4.2 K. These excellent values can mainly be attributed to the very fine-grained microstructure of the superconducting phase. Higher values have only been reported for tapes doped with SiC nanoparticles. The irreversibility fields Hirr of these tapes are 9.5 and 4.2 T at 10 and 20 K, respectively.

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.

Highly alloyed Ni–W substrates for low AC loss applications

Cube texture formation has been studied in Ni?W alloys with a W content of 9?at.% and above. These alloys show a low magnetization at 77?K and below, and are therefore excellent candidates for use as substrates of coated conductors in AC applications. The application of a modified deformation and annealing sequence leads to a highly textured surface of Ni9W and Ni9.5W tapes with cube texture fractions above 96%. YBCO (YBa2Cu3O7??) layers obtained on these substrates using a standard buffer architecture showed a critical current density exceeding 1.5?MA?cm?2 at 77?K, similar to those for films on commercial Ni5W tapes. In contrast, only a weak cube texture was achieved in Ni10W tapes. The rolling texture of this alloy showed a significantly increased Goss component, which could not be reduced by applying intermediate annealing treatments. The influence of this texture on the cube texture formation will be discussed in detail.

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.

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)

Microstructure and electrical properties of BSCCO tapes with ceramic barriers

The AC-losses in multifilamentary tapes can be reduced mainly by insulating barriers and/or by twisting. We prepared multifilamentary Bi-2223/Ag tapes with the PIT process using monofilaments dip-coated with a ceramic barrier (BaZrO/sub 3/, YSZ, CeO/sub 2/). All tapes with ceramic barriers showed, besides lower losses, a degradation of the critical current density due to interactions between the barrier material and the BSCCO-filaments during heat treatment. Furthermore, we studied the influence of the twist pitch on the AC-losses and the critical current density of twisted tapes without barriers. Microstructural investigations showed a partial damaging of the inner filaments during flat rolling.