The inclusions of Mg–B (MgB12?) as potential pinning centres in high-pressure–high-temperature-synthesized or sinteredmagnesium diboride

Abstract

A systematic study of the structure and superconductive characteristics of high-pressure–high-temperature (2 GPa,700–1000 °C)-synthesizedand sintered MgB2 without additions from different initial powders was performed. Amongvarious secondary phases Mg–B inclusions with a stoichiometry close toMgB12 were identified. With an increasing amount of these inclusions the criticalcurrent density increased. So these inclusions can be feasible pinning centres inMgB2. The highestjc values in zerofield were 1300 kA cm−2 at10 K, 780 kA cm−2 at 20 K and62 kA cm−2 at 35 K andin 1 T field were 1200 kA cm−2 at 10 K, 515 kA cm−2 at20 K and 0.1 kA cm−2 at 35 K for high-pressure-synthesized magnesium diboride and the field of irreversibility at 20 Kreached 8 T. The average grain sizes calculated from x-ray examinations in materials having highjc were 15–37 nm.