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.
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.
In this contribution we present systematic studies on the purification and subsequent
functionalization of magnetically pure single-walled carbon nanotubes. We show through a
combination of burning treatments and microwave digester treatments in aqua regia that
single-walled carbon nanotubes can be purified without incurring any damage,
with 90 wt% of catalyst material being removed. It is also shown that multiple
microwave digester treatments lead to incremental functionalization of the nanotubes.
The obtained functional groups are easily removed by annealing the sample in
vacuum.
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