2005 年 40 巻 11 号 p. 466-472
The relationship between microstructures and critical current properties of magnesium diboride (MgB2) bulks was studied. MgB2 bulks synthesized by the solid-solid reaction of magnesium and boron below the melting point of magnesium (650°C) exhibited excellent critical current properties up to high magnetic fields. Low-temperature long-time heat treatment was found to be effective for forming a strongly connected MgB2 matrix with small grain size and poor crystallinity, and a very high Jc of 4.0×105 A/cm2 was achieved at 20 K. Furthermore, the strong relationship between Hirr and the crystallinity of MgB2 bulks was discovered. The Hirr was found to increase with an increase of full-width at half-maximum, FWHM, of MgB2 (110) peak, which corresponds to distortion in the honeycomb boron sheet. Excellent Jc characteristics under high magnetic fields were observed in samples with large FWHM of (110) due to the enhanced intraband scattering and strengthened grain boundary flux pinning. The relationship between crystallinity and Hirr explains the large variation of Hirr for MgB2 bulks, tapes, single crystals and films.