The effect of heat-treatment temperature on the superconducting properties of malic acid-doped MgB2/Fe wire

The effects of the heat-treatment temperature on the carbon (C) substitution amount, full width at half-maximum (FWHM) value, critical temperature ( T c ), critical current density ( J c ), irreversibility field ( H irr ) and upper critical field ( H c2 ) have been investigated for 10 wt% malic acid-doped MgB 2 /Fe wires. All the samples were fabricated by the in situ powder-in-tube (PIT) method and heat-treated within a temperature range of 650–1000 °C. As the heat-treatment temperature increased, it seemed that the lattice distortion was increased by a more active C substitution into the boron (B) sites from the malic acid addition. These increased electron scattering defects seemed to enhance the J c – H properties in spite of an improvement in the crystallinity, such as a decrease of the FWHM value and an increase of the T c . Compared to the un-doped wire heat-treated at 650 °C for 30 min, the J c , H c2 and H irr were enhanced by the C doping in a high-field regime. The wire heat-treated at 900 °C resulted in a higher magnetic J c of approximately 10 4 A cm −2 at 5 K and 8 T, and it showed a H irr value of 11 T at 20 K from the resistance ( R ) versus temperature ( T ) curves.