Critical current densities and irreversibility fields of MgB2 bulks

We prepared two MgB2 bulks by applying conventional sintering and high-pressure sintering methods, and compared the current carrying properties. J(c) obtained by the resistive method was larger than that obtained by the magnetic method. J(c)-B curves obtained by the resistive method showed no history effect. These results indicate that most of the superconducting currents flowing in the MgB2 bulks were intergrain (transport) currents and intragrain currents were negligibly small. The high-pressure sintered sample with smaller grain size showed smaller field dependence of J(c) and higher B-irr than the conventionally sintered sample with larger grain size. This behavior can be explained by the grain boundary flux pinning. (C) 2001 Elsevier Science B.V. All rights reserved.