Application of Niobium and Titanium Superconducting Alloys

Niobium-titanium alloy superconducting materials have been applied in large-scale devices such as superconducting high-energy accelerators, superconducting nuclear magnetic resonance imaging diagnostic instruments, superconducting maglev high-speed trains, and superconducting strong magnetic concentrators;

It is also applied in the development of energy sources such as controlled nuclear fusion, magnetic fluid power generation, generators, power transmission, and energy storage.

In addition, it is used in strong magnetic propulsion systems (ships, ships, high-speed launch devices, etc.) and military defense.

In conclusion, the plastic niobium-titanium alloy superconducting material plays an important role in large-scale superconducting application devices, and it is the superconducting material with the largest amount (>95%) in the world.

Properties of Niobium and Titanium Superconducting Alloys

After the composition of the niobium-titanium superconducting material is determined, its Tc and Hc2 values generally do not change much, and its Jc has a great relationship with the microstructure produced by cold working-aging treatment.

If the morphology, size, spacing, and quantity of the cold-worked dislocation cell walls and the heat-treated precipitates can match the quantum magnetic flux, the maximum magnetic flux pinning force will be generated (see the magnetic flux pinning theory), making the niobium The Jc of the titanium superconducting material reaches the maximum value.

In the production of niobium-titanium superconducting materials, the Jc is significantly improved after one-time aging treatment after strong cold deformation. When the cold deformation aging treatment is used for several times, after the last aging, the process flow chart of the manufacturing process of niobium-titanium superconducting materials for pure niobium and pure titanium products is the process technology of further cold deformation of niobium n (see the multiple aging heat treatment technology of ferroniobium superconducting materials). ), can produce a more ideal microstructure (see the microstructure of niobium-titanium superconducting material), so that Jc obtains a larger value.

Niobium-titanium alloy superconductor

Niobium-titanium alloy superconductors are "leading materials" in the superconducting industry. It is the material of choice for practical superconducting magnet applications along with Nb3Sn superconductors, it has a high upper critical magnetic field (~11T at 4.2K, ~14T at 2K) co-draws well with copper, has good processing Plasticity, high strength and good superconductivity.

The raw materials and manufacturing costs of niobium-titanium alloy superconductors are much lower than other superconducting materials: this superconducting material can be subjected to a heat treatment process to improve superconductor performance before the assembly process of twisting, winding and other applications: its yield strength Similar to steel, these excellent properties will keep NbTi superconducting alloys widely used for a long time to come.

Fabrication of practical NbTi alloy superconductors

①Alloy smelting. The best alloy ratio contains T at 46%~50% (mass percentage).

②NbTi alloy rod processing, high uniformity Nbi rod in addition to uniform composition, but also the uniformity of mechanical properties.

③Cover the stabilization material, so that hundreds of NbTi filaments are buried in the stabilized high-conductivity oxygen-free copper or high-purity aluminum to prevent thermal quench or provide current bypass. The residual resistance ratio (RRR) is often used to describe the quality of the stabilized material. RRR refers to the ratio of the resistivity at room temperature of 293K divided by the resistivity at low temperature (4.2K). Generally, the RRR ratio of the stabilized material is required for superconducting magnets. at 30 and above.

④ Wrap the blocking material. Nb is a widely used barrier layer material. Its purpose is to avoid the formation of intermetallic compounds such as TiCu4 between the NbTi alloy and the Cu matrix, and to prevent the wire from breaking and significantly reduce the critical current density (Jc).

⑤ Combined design of multi-core composites. The combined design is carried out before hot extrusion. The design is based on the parameter requirements of the finished superconducting wire, such as wire diameter, core diameter, copper ratio, etc. to calculate the size and Number of squeezes. In the multi-core composite wire, the NbTi core wires are closely arranged, and they are arranged in hexagonal layers from the core to the outside. , 18, 24, 30, 36, ….

⑥ Extrusion and stretching, using extrusion to form a metallurgical bond with high bonding strength between the NT rod, the Nb barrier layer and the Cu layer matrix.

Technology of NbTi Alloy Composite Superconductor Development

The superconducting material must be encapsulated, or embedded in ordinary good conductors, to provide a low-resistance bypass during the sudden transition of certain line segments to normal. In addition, encapsulation is also important in preventing magnetic field lines from jumping forward. In other words, in order to reduce losses and improve stability, suitable base materials must be used. The choice of matrix material depends on conflicting requirements. Wilson and colleagues' theory suggests that using a highly conductive matrix allows the use of thinner wires, but its stranded cables must be twisted in a tighter helical shape. The Germans believe that the main reason for choosing high-conductivity oxygen-free copper as the matrix material of the superconducting material NbTi alloy is that when the superconducting state is instantaneously disturbed due to the temperature increase, the transport current can move into the matrix. , so that the Joule heating that has occurred can be kept sufficiently small. However, it has also been pointed out that copper, which has good stabilization properties, has never been able to adequately limit the induced current.

The characteristics of aluminum substrate compared with copper substrate in this paragraph

1. Under any magnetic field strength, the resistivity of high-purity aluminum is lower than that of oxygen-free copper. It must be pointed out that at higher field strengths it makes more sense. Because of the lower resistivity of aluminum, when aluminum is used as the substrate, Joule heating is reduced due to the shunting effect, and the disturbance to the magnetic field is controlled.

2. The thermal conductivity of high-purity aluminum is at least an order of magnitude better than that of oxygen-free copper, which reduces the local temperature rise of the coil;

3. The heat capacity per unit volume of aluminum is smaller than that of copper, so the cooling energy of aluminum can be 64% smaller than that of copper; if the conditions are stable, the heat requirement of aluminum is smaller than that of copper, thereby reducing the cooling cost.

4. The density of aluminum is only one-third of that of copper. Because the amount of aluminum is reduced, the weight of the magnet is reduced by a quarter. In this way, the application fields of superconducting magnets can be made wider, such as being used as aviation magnets and suspension magnets.

5. High-purity aluminum has the characteristics of self-annealing at room temperature, which can eliminate the resistance caused by stress when winding the wire once.

6. The surface of aluminum can be anodized for insulation. Its advantages are that the insulating layer has good contact with the conductor and has high mechanical strength; its thickness is thinner than that of organic insulation, and it is not easy to break, and its thermal conductivity is better than that of organic insulation. In terms of thickness and thermal conductivity, the thermal conductivity of the coil can be increased by two or three times.