Development of the Ultrafine MgB2 Superconducting Wires and Flexible Cables

We successfully fabricated the ultrafine MgB
₂
mono-core superconducting wires through the general in-situ powder-in-tube process by using a starting mixture of commercial Mg and B powders without C doping. In addition, some bundled cables were fabricated by using ultrafine MgB
₂
wires. Those cables showed surprisingly flexible mechanical performance even after the heat treatment. The critical current (
I
_c
) decreased monotonically with decreasing the wire diameter, meanwhile, the critical current density was almost the same value with decreasing the wire diameter down to 0.05 mm. The
I
_c
of the twisted cables was increased almost by the number of bundled wires. This is a very promising result from a practical application point of view. In addition, we recognized the Monel outer sheath was very effective in improving the cold-die-drawability to forward the ultrafine diameter. It was fabricated 4 km of long piece length without the wire breakage for the Monel sheathed mono-core wire 0.05 mm in diameter. Furthermore, we challenged the limits of diameter reduction in the present drawing technique, and eventually, the Monel sheathed MgB
₂
mono-core superconducting wire with 0.015 mm in diameter and 135 m in length was obtained. This is currently the thinnest MgB
₂
superconducting wire in the world.