Effect of Two-Stage Heat Treatment on Superconducting Properties for In-Situ ${\rm MgB}_{2+{\rm x}}$ Tapes Using Mg Flakes

MgB_2+x (x = -0.3 - 3.3) tapes were fabricated by the in-situ powder-in-tube method with two-stage heat treatment, starting from amorphous B powder and Mg flakes to avoid excessive oxidation. The MgB₂ tapes without final heat-treatment have a two-stage transition due to inter-granular decoupling. The two-stage heat treatment brought about an enhancement of core density and a strong inter-granular connectivity. Using this method, we studied characteristics of MgB₂ tapes synthesized with non-stoichiometric Mg:B ratios on purpose to introduce effective pinning centers such as non-reacted materials and secondary phases. J_c increased drastically with increasing B composition ratio. The maximum Jc reached 1.7 times 10⁵ A/cm² at 4.2 K, 1 T for MgB_2.8, 3.3 times 10³ A/cm² at 4.2 K, 5 T for MgB_3.3 and 1.9 times 10⁵ A /cm² at 20 K, 0 T for MgB_2.6. These values are 3-30 times higher than that for stoichiometric MgB₂ tape. Analyses based on the grain boundary pinning theory revealed two factors for improvement of J_c. One is an increase of grain boundary density due to suppression of grain growth in Mg poor condition. The other is an enhancement of elementary pinning force at grain boundaries due to the existence of non-superconducting phases. These results are considered to be very useful for further improvement of J_c - B properties in PIT MgB₂ tapes.