Superconducting Properties and Microstructure of Nanocrystalline MgB2 Bulk Prepared by Sintering of Ball-Milled Powders

Ultrafine nanostructured MgB2 bulks with an average grain size less than 10 nm have been fabricated by high-energy ball milling and subsequent high pressure sintering. Microstructural evolution in MgB2 subjected to high-energy ball milling has been investigated by means of X-ray diffraction (XRD). The finer grain size of MgB2 powders of about 7 nm has been estimated from Rietveld refinement analysis of XRD data, which is confirmed by a transmission electron microscope (TEM) observation. There is almost no grain growth in the subsequent sintering at low temperature of 600 degrees C under pressure of 3-5 GPa for 10-30 min. The nanocrystalline MgB2 bulks exhibit the lower onset critical transition temperatures (T-c onset) of 32-33 K. The relative wider width of the magnetic hysteresis loops at high external magnetic field and the higher critical current density (J(c)) are obtained in nanocrystalline bulks. J(c) is as high as 10(5) A/cm(2) in 8 T at 10 K and 2.7 x 10(3) A/cm(2) in 4 T at 20 K.