Advances in high-field superconducting composites by addition of artificial pinning centres to niobium-titanium

Abstract

Artificial pinning-centre (APC) niobium-titanium composites attain critical current density J c values higher than 4000 A mm −2 at 5 T, 4.2 K, surpassing the barrier reached by the conventional Nb-Ti composite process. At 2 T APC composites achieve more than double the J c of conventional composites, making them particularly well suited for low-field applications. On the other hand, APC composites are inferior to conventional composites at 8 T, due to weak high-field pinning and reduced upper critical field. This review discusses fabrication techniques, microstructural development and superconducting and flux-pinning properties of APC composites. Key elements and underlying issues for achieving higher J c are identified and discussed in terms of the current state of the art.

A Nb0.36Ti0.64 superconductor with ferromagnetic artificial pinning centers for low magnetic field applications

“…While the results demonstrated the J c performance in Nb 0.36 Ti 0.64 utilizing nickel artificial pinning centers, additional J c performance improvements is expected by further reduction in the pin spacing via the APC approach as well as increase in the APC pin fraction. Past developments of APC demonstrated critical current densities exceeding 13,000 A/mm 2 with pin volume percent as high as 12% but were performed on laboratory scale samples with only 10% to 15% superconductor fraction [3]. Increasing the pin volume percent to 10% to 12% within a 32 volume percent superconductor design is expected to result in higher J c performance if the processing is successful.…”

“…A benefit of the ferromagnetic pins is that they produce stronger flux pinning characteristics as compared to normal state pins and, therefore, require relatively smaller pin volumes to achieve similar J c performance characteristics at low applied magnetic fields. For example, approximately 6 volume % Cu-clad Ni pinning centers in Nb 0.36 Ti 0.64 wire produces about 13, 000 A/mm 2 at 2 T compared to about 11,500 A/mm 2 at 2T for 15 to 24 volume % Ti and Nb pinning centers [3]. The basis for such stronger pinning is the extremely short coherence length which is a consequence of the much larger magnetic exchange energy than the pairing interaction energy.…”

“…Moreover, the APC approach does not require subsequent intermediate anneals during the cold work processing normally required to develop the precipitates for flux pinning in commercially produced Nb 0.36 Ti 0.64 wires. The artificial pin materials used have been either Nb (low field superconductors), Cu or Ti (normal state metals) [3]. In addition, ferromagnetic artificial pinning centers also have been studied [4,5].…”

“…In addition, previous laboratory studies have only produced APC Nb 0.36 Ti 0.64 superconductors with fill factors below 20%. Moreover, APC designs have been produced by cold processing and only in laboratory scale quantities [3].…”

Superconducting materials for large scale applications

“…Critical current density of conventional and artificial pinning center composites, showing that it is possible to further enhance the current density of Nb-Ti by tuning the pinning center properties. Data is sourced from [41], [43], and [45].…”