Microstructures of SiC nanoparticle-doped Mg B2 Fe tapes

We have studied bulk Mg B2 synthesized by reaction of Mg H2 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 Mg B2, including the sample with 10 at. % SiC reacted at 600 °C, which showed Hc2 (0 K) >42 T. Extensive transmission electron microscopy (TEM) and STEM observations show that using Mg H2 instead of pure Mg reduces the concentration of oxide second phases in the tapes, but that adding SiC reintroduces nanoscale grains of MgO, Si O2, and Si Ox Cy, and larger grains of Mg2 Si. SiC causes some C doping of the Mg B2, but electron energy loss spectroscopy and x-ray diffraction measurements show that the C concentration is similar to other bulk C-doped Mg B2. 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 Hc2 of these samples. However, the Hc2 properties are somewhat compromised by very broad transitions that may have their origin in the local variations of nanostructure.