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

Abstract

Ultrafine nanostructured MgB 2 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 MgB 2 subjected to highenergy ball milling has been investigated by means of X-ray diffraction (XRD). The finer grain size of MgB 2 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°C under pressure of 3-5 GPa for 10-30 min. The nanocrystalline MgB 2 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 × 10 3 A/cm 2 in 4 T at 20 K.Keywords MgB 2 · Nanocrystalline bulk · High pressure sintering · Magnetic hysteresis loops · Critical current density Y. Sun ( ) · L. Zhu · X. Song