Stress/Strain Induced Flux Pinning in Highly Dense ${\rm MgB}_{2}$ Bulks

We have systematically studied the flux pinning behavior of MgB₂ bulks synthesized by direct diffusion of Mg into pressed pellets of high purity crystalline B powder, with and without mixing with C and SiC nanoparticles, at a reaction temperature of 850^degC for 10 hrs. All of the samples showed very high purity and high density, but their microstructure and flux pinning behavior showed significant differences. It was found that the pure MgB₂ agrees with the deltaT_c pinning model, nano-C doped MgB₂ agrees with the deltal pinning model, while the SiC+MgB₂ composite agrees with the deltaepsiv pinning model (stress/strain field pinning), since the dominant micro-defects that influence the flux pinning in these three samples are different.