MgB₂ coated conductors directly grown on flexible metallic Hastelloy tapes by hybrid physical-chemical vapor deposition

MgB₂ coated conductors (CCs), which can avoid the low packing density problem of powder-in-tube (PIT) processed wires, can be a realistic solution for practical engineering applications. Here we report on the superior superconducting properties of MgB₂ CCs grown directly on the flexible metallic Hastelloy tapes without any buffer layer at various deposition temperatures from 520 to 600 C by using hybrid physical-chemical vapor deposition (HPCVD) technique. The superconducting transition temperatures (T_c) are in the range of 38.5-39.4 K, comparable to bulk samples and high quality thin films. Clear (101) and (002) reflection peaks of MgB ₂ are observed in the X-ray diffraction patterns without any indication of chemical reaction between MgB₂ and Hastelloy tapes. From scanning electron microscopy, it was found that connection between MgB ₂ grains and voids strongly depend on the growth temperature. A systematic increase in the flux pinning force density and thereby the critical current density with decreasing growth temperature was observed for the MgB ₂ CCs. The critical current density (J_c) of J _c(5 K, 0 T) ∼10⁷ A/cm² and J_c(5 K, 2.5 T) ∼10⁵ A/cm² has been obtained for the sample fabricated at a low growth temperature of 520 C. The enhanced J_c (H) behavior can be understood on the basis of the variation in the microstructure of MgB₂ CCs with growth temperature.