Role of SiC Nanoparticles in the Electromagnetic Properties of SiC-doped MgB$_{2}$ Tapes*

MgB$_{2}$ tapes with H$_{c2}$(2.3K) = 42 T and J$_{c}$(4.2K) $\sim $ 4$\times $10$^{5}$ A/cm$^{2}$ have recently been demonstrated by Matsumoto et al. These tapes are synthesized by powder-in-tube-processing from MgH$_{2}$, B, and nanoparticle SiC precursors. TEM and scanning TEM analysis of samples as a function of SiC content and reaction temperature shows that the role of SiC is more complicated than simply providing a source of C to dope the MgB$_{2}$. The sample with the highest H$_{c2}$ and J$_{c}$ contains unreacted SiC nanoparticles, which may act as pinning centers directly or may distort the adjoining MgB$_{2}$ lattice, increasing intraband scattering. Compared to pure MgB$_{2}$ samples, samples with SiC contain a significantly higher concentration of oxygen, leading to a higher density of second phases. SiC also tends to reduce the already small grain size of the pure MgB$_{2}$ from $\sim $50 nm to $\sim $20 nm, making grain boundaries another import source of pinning.