Introduction
Niobium-titanium (NbTi) is an alloy of niobium and titanium used industrially as a type II superconductor for superconducting magnets. Its
critical temperature is about 9.7K . NbTi alloys are suitable for fabri cating superconducting magnets, with magnetic fields up to about 10T.
In 1962, Berlincourt and Hake discovered that the NbTi alloy showed superconductivity at 4.2K. Two years later, Westhouse produced the first NbTi wire. In 1965, the critical current density of NbTi superconducting wire was significantly improved by low-temperature heat treatment and cold processing by Vetrano. After overcoming the instability of NbTi wire, the critical current density (Jc) was increased to 2000A/mm2 by applying an aging heat treatment process and multifilamentary technology in 1972. To reduce the loss of NbTi superconducting wire, the diameter of NbTi filaments must be small enough , which requires the NbTi alloy to have high ingredient uniformity. In the late 1970s, the high-homogeneity NbTi alloy was successfully produced by vacuum smelting techniques,which greatly improved the critical current density of NbTi superconducting wire. In 1982, Wu from Northwest Institute for Nonferrous Metal Research (NIN) improved the Jc of 61 filaments of NbTi superconducting wire to 3470A/mm2 (5T, 4.2K).
In industrial applications, to obtain high critical current density of NbTi superconducting wire, the α-Ti precipitates must reach nanoscale during deformation process. Meanwhile, to reduce the superconducting hysteresis loss and eddy current loss, NbTi superconducting wires should be multifilamentary . The NbTi alloy with Ti content of 46–50wt% has proven to be the best compromise between precipitate yield for critical current density and alloy composition for the other superconducting properties.
There is one important question: How can NbTi superconducting wires realize engineering and mass production, and be widely used in magnets?
The first reason is that all the raw materials of NbTi superconductors have easy deformation ability (i.e., NbTi alloy and high purity copper). Owing to the good deformation ability of these raw materials, long NbTi superconducting wires can be produced. The second reason is that all the raw materials of NbTi superconductors are readily available. This means that the materials cost is much lower than other superconductors. The third reason is that NbTi superconductors are much more reliable and stable than other superconductors, and its performance degradation is much slower during the fabrication and operation of magnets.
As a result, NbTi superconducting wires have been widely used in magnetic resonance imaging (MRI), nuclear magnetic resonance (NMR), high energy particle accelerators, Tokamak fusion reactors (e.g., International Thermonuclear Experimental Reactor [ITER] project), magnetic separation systems, power systems, superconducting energy storage system (SMES), etc.
NbTi superconducting composition and microstructure
The composition of Nb and Ti not only determines the superconducting properties but also the amount of α-Ti pinning center. The higher the Ti content is, the more the pinning center is, and the lower cost of the alloy is. In addition, the higher Ti content increases the hardness of the NbTi filaments and may result in sausage filament and even filament breakage . As the filament diameter is usually below 10 μm and even below 1 μm, it requires that initial alloy composition must be homogeneous over the entire casting ingot. Thus, any Ti-rich region, hard inclusions, or unmelted Nb is not permitted in the initial Nb-Ti ingot. Therefore, the correct overall alloy composition to optimize Hc2, Tc, and precipitation is typically Nb-(47±1)wt% Ti alloy. A fine and uniform grain size and grain boundaries in the Nb-Ti matrix are to control the distribution of precipitate nucleation sites.
NbTi superconducting wire types
To obtain a high critical current density NbTi wire, the microstructure of the NbTi filament must be a two-phase nanostructure after the final process. Meanwhile, to reduce the hysteresis loss and eddy current loss of the superconducting magnet, NbTi superconducting wires need to be multifilament and fine filament. Because a NbTi superconductor has low processing cost, mature process routes, and stable performance, it has been widely used in various types of magnets. WST has successfully developed different types of NbTi superconducting wires, including monolith NbTi superconducting wire, wire-in-channel (WIC) NbTi superconducting wire, and superconducting cables. Fig. 4 shows the cross-sections of monolith NbTi superconducting wires with different filament numbers, different Cu/NbTi ratios, different shapes, and different low-temperature properties. At present, low-cost superconducting wire has been widely used in MCZ, MRI, NMR, and other applications.
To reduce the cost and increase the market share, WST independently developed WIC superconducting wire (Fig. 5). By soldering rod wire with low Cu/NbTi ratio into the U-shaped copper channel, WIC wire with a high Cu/NbTi ratio is fabricated. This type of wire has excellent stability and has been used in MRI magnets.
In addition, to meet the requirements of some special magnets, WST has also developed round cable-in-channel conductor and Rutherford Cable in Channel conductor with critical current higher than 10,000 A at 4T and 4.2K (see Fig. 6).
(A) (B) (C) (D) (E)
Fig. 4 Cross-section of monolith superconducting wires with different filament arrangement (from WST). (A) Cu/Sc ratio of 4.0 and filament number of 36, (B) Cu/Sc ratio of 1.0 and filament number of 55, (C) Cu/Sc ratio of 2.3 and filament number of 2616, (D) Cu/Sc ratio of 7 and filament number of 24, and (E) Cu/Sc ratio of 1.3 and filament number of 630.
Fig. 5 Cross-section of WIC superconducting wire (from WST). (A) Cu/Sc ratio of 7.0 and filament number of 55, and (B) Cu/Sc ratio of 12.0 and filament number of 55.
(A)
(B)
Fig. 6 Cable in Channel conductor and metallographic section (from WST). (A) Cu/Sc ratio of 4.0 and filament number of 55×7, and (B) Cu/Sc ratio of 1.0 and filament number of 55×10.