Niobium-titanium (Nb-Ti) alloy tubes are a key matrix material for manufacturing cryogenic superconducting cables and superconducting coils. Their main value lies in their moderate critical temperature (9.3K-10.3K) and strong resistance to deformation. They play a crucial role in superconducting power transmission and high-precision equipment.
Zero Resistance and Low Loss: In the liquid helium temperature range (approximately 4.2K) or in the superconducting state, niobium-titanium superconductors exhibit zero resistance, enabling lossless transmission of large currents and significantly improving power grid transmission efficiency and capacity. Support and Sheathing Material: In the manufacture of superconducting cables, Nb-Ti alloy rods or wires are often embedded in a high-purity copper matrix and then drawn. Niobium-titanium tubes can serve as the central support tube or outer protective sheath for the cable, used for the circulating cooling of the internal cryogenic fluid (such as liquid helium or supercritical helium), thereby ensuring the stability of the superconducting state. Superconducting Power Cables: Used in urban power grid upgrades to achieve high-capacity, lossless, and highly compact underground power transmission. High-field magnet equipment: Widely used in nuclear magnetic resonance imaging (MRI) equipment, particle accelerators, and controlled nuclear fusion devices to provide strong magnetic fields. High-precision transmission: Also used in cryogenic signal transmission cables within quantum computers. Nb-Ti alloys exhibit excellent conventional superconducting properties; practical materials typically contain 35% to 55% niobium (e.g., alloys containing 47% titanium), and overall superconducting performance can be improved by adding trace amounts of tantalum or zirconium.
Superconducting MRI and NMR devices require closed-loop operation of the superconducting magnet to obtain highly stable magnetic fields. Therefore, the connection resistance between coils within the superconducting magnet, as well as between the coil and the superconducting switch, must be extremely low. It has been reported that for MRI devices, the joint resistance is less than 10⁻¹¹ ohms; for NMR devices, the joint resistance is less than 10⁻¹⁴ ohms. Conventional ultrasonic welding and laser welding processes suffer from complexity, unstable performance, and inconvenience in on-site operation, resulting in low yield rates. A method for fabricating a niobium-titanium (NbTi) superconducting wire joint overcomes the shortcomings of existing technologies. This method offers a simple structure, easy fabrication, stable performance, and low resistance.
The superconducting material must be encased or embedded within a common good conductor to provide a low-resistance bypass in areas where the wire suddenly transitions to normal. Furthermore, the sheathing is important in preventing magnetic field line jumps. In other words, a suitable base material must be used to minimize losses and improve stability. The selection of the base material depends on conflicting requirements. Wilson and colleagues' theory suggests that a highly conductive base allows for the use of finer wires, but multi-strand cables must be tightly helically twisted. The Germans believe that the main reason for choosing highly conductive oxygen-free copper as the base material for the NbTi superconducting alloy is that when a transient disturbance occurs in the superconducting state due to localized temperature increases, the transport current can be transferred to the base, keeping the Joule heating sufficiently low. However, some have pointed out that copper, despite its good stabilizing properties, has not been able to adequately limit the induced current.
