A Nb-61.7at.%Ti-3at.%Hf alloy has been fabricated into multifilamentary wires and studied in terms of high field superconducting properties.
First, effects of precipitation heat treatment at temperatures between 325 and 500°C and cold working up to 99% area reduction ratio after the heat treatment on the superconducting properties at 4.2K were examined. The heat treatment enhances the critical current density,
Jc, in low fields below 8T, but not that inhigh fields above 10T. The cold working after heat treatment is identified as critical to increasing
Jc in high fields.
Jc increases with reduction ratios up to 95%. Thus, to obtain high
Jc in high fields a process emerges where the heat treatment producing a maximum upper critical field,
Bc2, is followed by severe cold working. The concept underlying this may be applicable to alloy superconductors other than NbTiHf.
Second, the superconducting properties at 1.8K were investigated. The critical current density
Jc vs. magnetic field
B curves obtained for 4.2K are found to coincide with those for 1.8K when shifted along the
B axis by 3.5T. The present NbTiHf superconductor has higher
Jc and
Bc2 at 1.8K than those of typical NbTi. For both 4.2 and 1.8K, maximum pinning forces in samples cold-worked after heat treatment appear at the same reduced field of about 0.5, indicating that the temperature scaling law may hold for this NbTiHf superconductor.
View full abstract