Nb3Sn low-temperature superconductors, as the core material for high-field superconducting magnets above 10T, have been widely used in large-scale scientific research facilities such as nuclear fusion reactors, particle accelerators, and nuclear magnetic resonance spectrometers. Their performance directly affects the energy output and research level of these devices. Currently, the main methods for preparing Nb3Sn superconducting wires include the internal tin method and the bronze method. Among them, the internal tin method has become the main choice for preparing 12T~20T strong magnetic field magnets because it can provide a sufficient Sn source, has a short processing cycle, low manufacturing cost, and can carry a larger transport critical current under high magnetic fields.
However, existing internal tin-based fabrication techniques have significant limitations: Traditional methods typically involve inserting a Sn rod into the central through-hole of a CuNb composite rod to form a sub-component, which is then housed in an oxygen-free copper tube containing an external barrier layer. During heat treatment, Sn within the sub-component easily diffuses to adjacent sub-components via the Cu matrix, leading to insufficient reaction between Sn and the Nb core wire, a low superconducting phase ratio, and limiting the wire's current-carrying capacity. Simultaneously, high-field-strength (greater than 14T) magnets place higher demands on the wire's critical current density, thermomagnetic stability, and low AC loss. Traditional wires struggle to meet these requirements in terms of stress concentration mitigation, processing performance, and thermal conductivity. While some techniques have attempted to incorporate a pure metal Ta barrier layer within the sub-component to limit Sn diffusion, an optimized structure that balances distributed tin diffusion and distributed barrier properties has not yet been achieved. Furthermore, improper sub-component size design can easily lead to an imbalance between loss and critical current density, failing to fully meet the needs of high-end scientific facilities.
Among existing Nb3Sn superconducting wire fabrication technologies, the internal tin method, while a primary choice for high-field magnet wires due to its abundant Sn source and lower cost, still has several limitations: First, during heat treatment, Sn elements within the subcomponents easily diffuse to adjacent subcomponents via the Cu matrix, leading to insufficient reaction between Sn and the Nb core wire, a low superconducting phase ratio, and limiting the wire's current-carrying capacity. Second, traditional wires lack an optimized structure that balances uniform Sn diffusion with cross-subcomponent barrier properties, resulting in high AC losses and thermomagnetic stability that fails to meet the requirements of high-field (greater than 14T) magnets. Third, significant stress concentration occurs during processing, and the wire's processing performance, thermal conductivity, and low-temperature operational stability need improvement. Furthermore, although existing technologies use Ta as the internal barrier layer within the subcomponents, they lack a synergistic design of distributed tin and distributed barrier properties, failing to simultaneously address multiple issues such as diffusion uniformity, loss control, and processing performance.
Dual Distributed Structure Collaborative Optimization:A distributed tin structure is employed within each subunit, significantly shortening the Sn diffusion distance and avoiding the uneven diffusion problem inherent in traditional central Sn rods, ensuring uniform Nb3Sn phase formation. Each subunit uses a niobium tube as an independent barrier layer, effectively limiting Sn diffusion across subunits, increasing Nb3Sn generation, and guaranteeing a high critical current density for the wire. Compared to traditional internal tin-based wires, the superconducting phase ratio and current-carrying capacity are significantly improved.
Improved Processing Performance and Stability: The oxygen-free copper core rod in the final billet core effectively reduces stress concentration during processing, improving the wire's yield rate. Simultaneously, it enhances the wire's thermal conductivity, ensuring thermomagnetic stability at low temperatures and resolving the stability issues of existing wires.
By increasing the number of subunits and appropriately reducing their diameter (approximately 30% smaller than traditional wires), magnetization losses can be reduced while maintaining a high critical current density. This avoids the critical current density decay caused by excessively small subunits, achieving an optimized match between losses and current-carrying capacity.
Strong process adaptability: The staged heat treatment system is compatible with the dual-distributed structure, further promoting the uniform formation of Nb3Sn phase; the addition of specific alloying elements to the tin alloy rod, in synergy with the dual-distributed structure, further improves the overall performance of the wire. Compared with the existing technology using Ta barrier layer, it ensures the barrier effect while taking into account the feasibility of the process and the balance of performance.
