$MgB_2$ coated conductors directly grown on flexible metallic Hastelloy tapes by hybrid physical-chemical vapor deposition

Abstract

$MgB_2$ 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 $MgB_2$ CCs grown directly on the flexible metallic Hastelloy tapes without any buffer layer at various deposition temperatures from 520 to $600^{\circ}C$ 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 $MgB_2$ 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 Jc(5 K, 0 T) ${\sim}10^7A/cm^2$ and $J_c$(5 K, 2.5 T) ${\sim}10^5A/cm^2$ has been obtained for the sample fabricated at a low growth temperature of $520^{\circ}C$. The enhanced $J_c$ (H) behavior can be understood on the basis of the variation in the micro-structure of $MgB_2$ CCs with growth temperature.

Keywords

• $MgB_2$ film;
• Hastelloy tape;
• Coated conductors;
• HPCVD