The application of NbTi alloy tubes in nuclear magnetic resonance (MRI) and superconducting cables in the United States

The niobium-titanium alloy tube (Nb-47 wt% Ti) has become the "low-temperature king" in the fields of MRI and superconducting cables in the United States due to its critical temperature of 9.2 K, high critical current density of 3000 A/mm² (4.2 K, 5 T), and mechanical strength of 300 MPa. In the 4.2 K liquid helium temperature range, it has zero resistance, can withstand strong magnetic fields or large currents, and has a cost much lower than high-temperature superconductors such as Nb₃Sn. Therefore, it still dominates in medical imaging scenarios of 1.5 - 7 T and urban high-power power transmission scenarios.

In MRI equipment, niobium-titanium is widely used in the form of "multi-core composite wires": thousands of 10-50 µm Nb-Ti alloy wires are embedded in a copper matrix and wound to form the main coil of the magnet, achieving a magnetic field uniformity of ±1 ppm; at the same time, the low-temperature helium circulation pipelines in the equipment will also use seamless niobium-titanium tubes to withstand repeated thermal shocks and electromagnetic stresses. Although systems above 7 T have begun to trial use materials such as MgB₂, niobium-titanium remains the preferred choice for medium and low-field MRI due to its mature process and cost-effectiveness.

In the field of superconducting cables, Nb-Ti multi-core wire bundles are encapsulated within copper or stainless steel stabilizing layers to form flexible kilometer-scale cables. The proposals for power grid renovations all indicate that their current density can reach hundreds of times that of copper cables, and the loss can be halved. Accelerator beam lines also utilize Nb-Ti tubes as the structural support for low-temperature transmission lines. However, the operation and maintenance costs and infrastructure requirements brought about by liquid helium cooling have kept these projects at the demonstration stage for now.

The comparison shows that MRI uses Nb-Ti to emphasize high field uniformity and long-term stability, with the form mainly consisting of fine wire windings; while superconducting cables aim for high current and low loss, and adopt a composite structure of wire bundles and low-temperature protective tubes. Both share the liquid helium cooling system.

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