Microstructures of SiC nanoparticle-doped MgB_2/Fe tapes

摘要

We have studied bulk MgB_2 synthesized by reaction of MgH_2 and B with and without SiC nanoparticles and at a range of reaction temperatures. All of the samples showed enhanced upper critical fields compared to most bulk MgB_2, including the sample with 10 at. % SiC reacted at 600℃, which showed H_(c2)(0 K) > 42 T. Extensive transmission electron microscopy (TEM) and STEM observations show that using MgH_2 instead of pure Mg reduces the concentration of oxide second phases in the tapes, but that adding SiC reintroduces nanoscale grains of MgO, SiO_2, and SiO_xC_y, and larger grains of Mg_2Si. SiC causes some C doping of the MgB_2, but electron energy loss spectroscopy and x-ray diffraction measurements show that the C concentration is similar to other bulk C-doped MgB_2. In all the samples with and without SiC, the grain size is very small, 10-60 nm. Electron scattering from the high density of grains and second-phase boundaries is responsible for the enhanced H_(c2) of these samples. However, the H_(c2) properties are somewhat compromised by very broad transitions that may have their origin in the local variations of nanostructure.