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Although high-temperature superconducting materials have been discovered for nearly 20 years, due to the current shortcomings such as difficulties in forming materials, low performance under magnetic fields, and high manufacturing costs, low-temperature superconducting materials such as niobium-titanium alloys will not be used for a long time in the future. It will still occupy a dominant position in the field of strong current applications. Superconducting materials are currently the most widely used low-temperature superconducting materials due to their good superconducting properties, excellent mechanical properties and low manufacturing costs. Their usage accounts for more than 90% of the entire superconducting market. In order to improve the critical current density of NbTi superconductors and expand their application range, multiple aging heat treatment processes and “artificial pinning” technologies are currently used, and significant progress has been made. With the expansion of application scope, further simplifying its production process and reducing manufacturing costs have always been the goals that people are constantly pursuing. In the late 1990s, a variety of new processes using niobium and titanium to form conformers and then undergo diffusion reactions to form NbTi superconductors emerged and made certain progress, becoming a new hot spot in current NbTi superconductor research. NbTi superconductors were prepared through the interdiffusion reaction between Nb sheets and Ti sheets, and ideal superconducting properties were obtained. Compared with the traditional process, this niobium and titanium interdiffusion process has a simple preparation process, low manufacturing cost and good performance, and has good application prospects.
Sample preparation and experimental method: First process the niobium sheet and titanium sheet into sheet materials with thicknesses of 0.4mm and 0.8mm respectively, and then alternately assemble multiple layers of Nb and Ti flakes into NbTi single core rods. The assembled single Xin The rods are evacuated and then welded and sealed. From single-core rods to multi-core rods and then to the finished composite wire, they need to go through processes such as hot isostatic pressing, two hot extrusions, multi-pass drawing and heat treatment. When the diameter of the NbTi wire is stretched to 5.0mm, diffusion heat treatment is carried out. The system is: 800℃, 5 hours. When the wire diameter is stretched to 1mm, aging heat treatment is performed. The system is: 400℃, 40 hours; then 20% The processing rate of each pass is reduced to the final 0.1mm wire.
The microstructure observation of the sample was carried out on JSM6460 and JSM-6700F scanning electron microscopes, and the components of each point on the sample were obtained by electron energy spectroscopy (EDS). The critical current test of the sample was performed at the high field of the French National Center for Scientific Research (CNRS). Conducted in the laboratory and the University of Science and Technology of China. Using the four-lead method, take a wire about 1 meter long, wind it into a solenoid coil, and immerse it in liquid helium to pass direct current. The potential lead spacing is 300~800mm, and the criterion is 0.1μv/cm ; Use the weighing method to measure the cross-sectional area S of the NbTi core wire and calculate Jc-Ic /S.
microstructure
In order to determine the reasonable diffusion heat treatment system for superconducting composite wires, we first prepared a diffusion couple and studied the diffusion behavior of elements near the Nb/Ti bonding surface and the composition change rules of the diffusion layer under different heat treatment conditions. The 600°C values were given respectively. and the cross-section morphology near the Nb/Ti diffusion layer after treatment at 800°C for 5 hours. From 600°C, it can be seen that under a magnification of 1000 times, the Nb and Ti interface of the sample diffused at 600°C for 5 hours is clear and cannot be observed. There is an obvious diffusion layer, indicating that Nb and Ti basically do not diffuse under such conditions. However, the Nb/Ti diffusion layer can be clearly observed in the sample treated at 800°C, and the single-phase diffusion layer is 5~6um. The results show that: Nb/ The thickness of the Ti diffusion layer increases with the increase of diffusion temperature and diffusion time; under the existing process conditions, diffusion at 800°C for 5 hours can obtain a single β-phase diffusion layer with the largest thickness and the highest Ti content, which is more ideal. Heat treatment system. Too high diffusion temperature or too long diffusion time will only increase the thickness of α+β3 dual-phase zone, and the performance of the material will be reduced. Therefore, when the NbTi wire is pulled to 5.0mm for diffusion heat treatment, The heat treatment system we adopt is: 800℃, 5 hours.
The SEM photo of the sample when the NbTi/Cu wire was stretched to a diameter of 1.0mm after 5 hours of diffusion heat treatment at 800°C. It can be seen that due to the uneven processing deformation, there are still incompletely reacted Ti and Ti in local areas of the sample. Nb; the darker area is the Ti-rich area, the lighter area is the Nb-rich area, and in the middle is the NbTi alloy area formed after diffusion, and its composition changes in a gradient. After diffusion heat treatment and multiple major processing After cold stretching at a high rate, the lamellar arrangement of Nb/Ti sheets is further bent and presents a disordered strip shape. The large number of dislocation defects generated in this way can serve as pinning for the diffusion remaining phase between Nb and Ti sheets. center, which is very beneficial to increasing the critical current density.
The aging heat treatment temperature is an important factor affecting the critical current density. Generally, the aging treatment temperature of NbTi alloy is about 350~420℃. If the temperature is too high, the dislocations will be annihilated and the precipitation phase will grow, which will reduce the Jc value of the material. If the temperature is too high, the Low, the plasticity of the alloy is not good. Therefore, when the NbTi wire is stretched to a diameter of 1.0mm for diffusion heat treatment, the aging heat treatment system we adopt is: 400°C, 40 hours.
The diameter of the superconducting wire is 1.0mm. The cross-sectional SEM photos and EDX line scans of Ti/Nb inside the core wire show that after diffusion heat treatment and aging heat treatment, the concentration of Ti/Nb alloy elements inside the core wire is There are fluctuations. In the darker Ti-enriched area, the Ti content increases significantly, while the Nb content decreases significantly. In the lighter-colored Nb-enriched area, the Nb content increases significantly, while the Ti content decreases significantly. This shows that during the heat treatment process, incomplete diffusion of Nb/Ti inside the core wire will cause composition differences in local areas within the superconducting wire.
Superconducting performance comparison
Through the observation and analysis of the microstructure of non-alloyed superconducting wire samples, it can be seen that appropriate heat treatment can cause thermal diffusion of Nb sheets and Ti sheets to form an NbTi solid solution alloy, and can produce the same two-phase structure to replace the traditional process. Alloy. Since Nb and Ti can appear in each Nb/Ti layer, and their quantity and uniform distribution are guaranteed, the diffusion remaining phase between Nb and Ti sheets can become the pinning center, and at the same time, the α-Ti Defects such as precipitated phases and a large number of dislocations generated during deformation can also serve as pinning centers, which is very beneficial to increasing the critical current density.
Performance comparison curve of NbTi superconducting wire Jc prepared by traditional process and non-alloying process. As can be seen from the figure, the superconducting performance of the sample prepared by non-alloying process is corresponding to the critical current at 3T and 4.2K. The density value is 4200A/mm2; the corresponding critical current density value at 5T and 4.2K is 2800A/mm2, and it has more advantages under low magnetic fields. Under high magnetic fields, the performance is slightly better than that of superconductors prepared by traditional processes. Poor, generally speaking, the performance is equivalent to that of NbTi superconductors prepared by traditional processes. After process optimization, it is expected to obtain higher Jc performance.
After Nb and Ti sheets are diffused at 800°C for 5 hours, a single superconducting phase diffusion layer with the largest thickness and high Ti content can be obtained. The non-alloying preparation process shortens the preparation cycle of NbTi superconductors, and the manufacturing cost is low and the performance is good. .The superconducting properties of samples prepared through non-alloying processes are 4200A/mm2 at 3T and 4.2K, and 2800A/mm2 at 5T and 4.2K. The performance is equivalent to that of superconductors prepared by traditional processes.