Niobium-titanium tubes (more precisely, Niobium-titanium alloy superconducting wire, usually in the form of multi-core wires with copper or copper-nickel composite matrix) are currently the most widely used and mature practical cryogenic superconducting materials.
The following will systematically explain its superconducting properties:
1. Core superconducting characteristic parameters
For any superconducting material, we mainly focus on the following key characteristic parameters. Niobium-titanium demonstrates a balanced and excellent critical temperature in these aspects: approximately 9.5 K. This is the highest temperature at which it can transition to the superconducting state. This temperature is very low, meaning it must operate within the liquid helium temperature range (4.2 K) or below. This is its greatest limitation, but it also makes its performance very stable. At the critical magnetic field, the critical magnetic field at 4.2 K is approximately 11 teslas. This refers to the maximum magnetic field strength at a specific temperature that can destroy its superconductivity. The irreversible magnetic field is usually slightly lower than the upper critical field, which is the magnetic field boundary at which the magnetic flux pinning force disappears in practical applications. The high critical magnetic field is the fundamental reason for its ability to be used in the manufacture of strong magnetic field magnets.
Critical current density is the most crucial engineering parameter. It refers to the maximum current density that a superconductor can carry without loss under specific temperature and magnetic field conditions. The critical current density of niobium-titanium is particularly outstanding under high magnetic fields. For instance, at 4.2 K and a magnetic field of 5 T, the critical current density of high-performance Niobium-titanium wire can reach > 3000 A/mm². This value decreases as the magnetic field increases.
Coherence length: Approximately 4-5 nanometers. This describes the scale at which the superconducting order parameters vary in space. A shorter coherence length makes niobium-titanium extremely sensitive to crystal defects (dislocations, grain boundaries, etc.), and these defects can precisely serve as effective flux pinning centers, thereby significantly enhancing its current-carrying capacity.
2. The Materials Science Behind the Properties
The superconducting property of nitinol does not originate from a pure single-phase alloy. Instead, it is highly dependent on its microstructure - a two-phase structure: After appropriate processing and heat treatment, the nitinol alloy forms a superconducting matrix rich in niobium, in which dispersed are the normal-state precipitates rich in titanium, the α phase.
The magnetic flux pins these tiny, evenly distributed α phase precipitates, as well as the numerous dislocations and grain boundaries generated during the processing, together form a powerful magnetic flux pinning center. They can "anchor" the magnetic flux lines, preventing them from moving under the current drive, thereby avoiding energy loss (i.e., the generation of resistance).
Excellent machinability: Nitinol alloy has excellent ductility, allowing it to be drawn into extremely fine wires (with diameters reaching several micrometers) and to form multi-core composite wire strands containing thousands or even tens of thousands of superconducting wires, all wrapped in a high electrical conductivity copper or copper-nickel matrix. This significantly enhances the stability and anti-interference capability of the material.
