A new heat treatment was used to prepare dense polycrystalline or monocrystalline high-quality samples in a single process. Resistivity measurements for the polycrystalline part of the sample gave a residual resistivity ratio of 16.6 and a very low normal state resistivity ρ0(40K) of 0.28µcm. Both scanning electron microscopy and SQUID studies of polycrystals revealed the high quality, dense nature and good coupling of the grain boundaries. On the other hand, the high-quality single crystals had a unique shape that resembled the hexagonal crystal structure. The SQUID measurements revealed a very weak flux pinning character, implying that the single crystals were largely free of flux-trapping centers. It was concluded that heat treatment played a major role in defining the characteristics of both polycrystalline and monocrystalline MgB2. Samples were thoroughly characterized by using X-ray, resistivity, direct-current SQUID and scanning electron microscopy methods.

A New Heat Treatment to Prepare High-Quality Polycrystalline and Single Crystal MgB2 in a Single Process. M.H.Badr, K.W.Ng: Superconductor Science and Technology, 2003, 16, 668-71