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 combination of nanocarbon-doped nanosized MgB 2 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, J c -values of 10 4 A cm −2 at 16.4 T (4.2 K) and 5.6 T (20 K) could be measured.

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The mass density of superconducting (bismuth, lead)-strontium-calcium-copper-oxide (PBSCCO) cores of silver-sheathed tapes, subject to different states of cold working and thermal treatment, has been determined taking advantage of a modified hydrostatic weighing. It turned out that the densification was more effective the smaller the initial density of the parent material. A core density of about 5 g cm −3 was attained, but could not be exceeded, independent of the deformation method and core density prior to deformation. When starting with high initial densities, drawing even reduced the core density. The density drop increased with reduced diameter reduction per drawing step, whereas rolling improved the core density. Heat treatment (reacting from the 2212 to the 2223 phase) resulted in a small loss of relative density, which raised with increasing core thickness. Nevertheless there is a positive correlation between densities before and after heat treatment. It is therefore important to obtain optimum core densities during the final steps of cold working.

Abstraceuperconducting Bi-(2223) multifilamentary tapes with AglWxY, alloy sheaths, with Y=Cu, Mn and Pd, were made by the oxide-powder-in-tube technique. All resistivities of the Ag alloy sheaths exceed that of the pure Ag sheath by at least a factor of 2. The maximam resistivity was achieved for AgwPdl with ~7~1 . 3 pQcm. Tapes sheathed by Ag90Cu10 exceed the critical current density, Jc, of pure Ag sheathed ones. Jc up to 26 W c m 2 were achieved by pressing AgWCulo Sheathed tapes during the thermo-mechanical-treatment, TMT. For thinner AgWCu,, sheathed tapes up to 31 kA/cm2 were obtained employing flat rolling during TMT. Measured lines of thin and thicker tapes with comparable Jc bicker tapes have a larger pinning strength and a better grain texture but thinner ones a better grain-to-grain connectivity.