JPS6152227B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement of the invention previously proposed by the inventors in Japanese Patent Application No. 55-10537.

More specifically, the present invention focuses on adding a preliminary heat treatment in the process of manufacturing the Nb ₃ Sn-based superconducting material, thereby further improving the critical current value _JC .

In recent years, superconducting magnets have been applied to nuclear fusion devices and large accelerators for nuclear research, and as they are required to be used in high magnetic fields, they generally have higher critical current values than alloy systems. Compound-based superconducting materials that exhibit

One of the obstacles to the practical use of compound-based superconducting materials is firstly that they are difficult to manufacture; surface diffusion methods, vapor phase reduction methods, plasma spray methods,
Although various manufacturing methods such as composite processing methods have been tried, all of them have been complicated and troublesome.
A second bottleneck is that the compound has poor flexibility, which is a characteristic characteristic of the compound.

The method for manufacturing Nb ₃ Sn-based superconducting materials that the inventors previously proposed in Japanese Patent Application No. 55-10537 was developed in view of the above-mentioned circumstances, and was designed to simplify manufacturing and greatly improve flexibility. This is an innovative manufacturing method that solves both the improvement of Out
A cast material is prepared by adjusting the average separation distance of Nb to be 6 μ or less, and the cast material is processed to a degree of 99.5% or more to stretch the Nb phase and the matrix together, thereby forming a filament in the drawn matrix. By forming an Nb phase and then plating the drawn material with Sn in an amount that can generate Nb ₃ Sn corresponding to the amount of Nb present, and then heat-treating at 500 to 700°C. By diffusing Sn into the inside, the Nb filament becomes Nb ₃ Sn
The present invention provides a method for producing a Nb ₃ Sn-based superconducting material, characterized in that the material has a filament phase.

Afterwards, the inventors conducted various studies on this manufacturing method, and found that 240~
If a preliminary heat treatment is applied at 500℃ for 20 to 100 hours, the critical current value JC will be lower than that without this preliminary treatment.
was found to be further improved.

The present invention will be described in detail below based on examples.

First, a Cu-Nb alloy containing 15 to 70% by weight of Nb is melted. If Nb is less than 15%, it is too dilute and it becomes difficult to maintain the average separation distance of Nb dendrites in the cast material to 6μ or less, which is necessary to improve the JC characteristics, so it is excluded. 70 again
% or more, the melting point rises rapidly, making it extremely difficult to obtain the homogeneous and fine casting structure necessary to improve JC properties, and the volume of the copper that becomes the matrix becomes too small, so the so-called stabilizing effect is not guaranteed. Therefore, it is excluded.

Cu-Nb alloys having the above composition range are generally easily melted in arc melting furnaces, beam melting furnaces, etc., but very severe restrictions are imposed on the casting materials.

That is, it is necessary to perform casting so that the average separation distance S of Nb dendrites in the cast material is 6 μ or less.

In this case, it is convenient to measure the average separation distance S of the dendrites using a cross-cut method in which a measurement line crosses the arrangement of dendrites, but other methods may be used.

Fine crystallization of dendrites such that the average separation distance S of crystallized Nb is 6 μ or less can be achieved by adjusting the cooling rate during casting, but it can also be achieved by adding a small amount of certain elements. These details will be proposed separately by the inventors.

The material melted and cast as described above is processed to achieve a processing degree of 99.5% or more in the entire processing. This degree of processing is important, and the degree of processing is
When it becomes 99.5% or more, a rapid rise of the critical current JC occurs when the average separation distance is 6 μ or less. However, even with such strong machining, when the average separation distance becomes coarse, 6 μ or more, the rise is hardly observed. As described above, there is a close correlation between the critical current value, the average separation distance, and the degree of processing. The inventors are currently investigating the academic basis for this.

After the Nb dendrite is strongly processed in this way and made into a filament shape, for these materials,
Sn plating is applied. In this case, the amount of Sn plating is such that it can react with the internal Nb filament to generate Nb ₃ Sn. In reality, the critical current JC increases until the amount of Sn plating reaches a predetermined amount, and when the amount exceeds the predetermined amount, JC becomes saturated.

It is considered that the necessary amount of Nb ₃ Sn is generated near this saturated portion.

After the Sn plating is applied, a heat treatment is performed to diffuse Sn into the interior and react with the Nb filament inside to produce the filament as an Nb ₃ Sn filament.

Therefore, in the present invention, a preliminary heat treatment is applied at 240 to 500° C. for 20 to 100 hours before the final diffusion treatment.

Such pre-treatment is necessary for the Sn plating layer and the Cu-Nb layer.
By providing familiarity with the alloy for diffusion in advance, this prevents the Sn plating layer from sagging and becoming eccentric during subsequent diffusion heating at a higher temperature, causing non-uniform diffusion. This heat treatment is intended to prevent
This is an attempt to further improve the JC characteristics. Perhaps this relatively low temperature heat treatment promotes diffusion into the processed material, resulting in a homogeneous
It is thought that something beneficial to the formation of Nb ₃ Sn, such as the movement of dislocations, may occur, but the detailed investigation is currently underway.

This kind of heat treatment is too low below 240℃.
No effect on JC characteristics was observed, and temperatures above 500°C would become too high, causing sagging of the Sn plating layer, which would actually impede homogenization, so it was excluded. In addition, this heat treatment is excluded if it is too long, such as 100 hours or more, as the effect will be saturated and there is no point in spending any more time, and if it is less than 20 hours, it is still too short, and the effect of the heat treatment will not be fully manifested.

After such preliminary heat treatment, the
Perform diffusion treatment at a temperature of ℃ to ensure sufficient internal
Generates Nb ₃ Sn. In this case, temperatures below 500°C are too low and the formation of Nb ₃ Sn is inhomogeneous and insufficient, making it impossible to obtain a material with the desired JC value, while temperatures above 700°C are too high. The plating layer becomes unstable and inhomogeneous, the shape of the filament becomes unstable, and crystal growth of Nb ₃ Sn occurs, which inhibits the so-called pinning effect, so the Jc value also decreases. It is excluded because it does.

Example 1 Cu-24.5%Nb alloy was arc melted to a total amount of 150g, cooled in a water-cooled crucible, and made into a 15mm square
A cast material with a length of 80 mm was made, and this was cold drawn to a diameter of 0.24 mm. 200, 250, 300, 350, which is plated with Sn in an amount of 7.1% and without preheat treatment.
The samples were preheated for 24 hours and 96 hours at 400, 450, and 500 degrees Celsius, and then finally diffused at 600 degrees Celsius for 96 hours, and their critical currents Jc were measured in a magnetic field of 9 Tesla.

FIG. 1 is a diagram plotting the results.
At 200℃, there is almost no difference from the untreated case, but
It is clearly seen that the Jc value improves rapidly when the temperature rises above 250°C. The critical value for this rise is 240°C, which is thought to be related to the melting point of Sn.

Example 2 A Cu-28.1%Nb alloy was melted and cast as described above.
After drawing the wire to 0.24mmφ, plate it with 10.1% Sn, and then
We prepared samples preheated at 400°C for 24 hours and samples without pretreatment.
After 96 hours of diffusion treatment at each temperature of 700℃,
Jc was measured with 9Tesla.

FIG. 2 is a diagram plotting the results. As is clear from Figure 2, the Jc properties of the specimens preheated at 400°C for 24 hours showed an excellent improvement in Jc characteristics compared to those without treatment, and this tendency becomes more pronounced as the temperature increases. .

The inventors conducted many more experiments, and were able to confirm that in each case, the effect of the preheat treatment according to the present invention was remarkable, as in the previous example.

Since the present invention is a Cu-Nb binary alloy, it is extremely easy to process at the cold working stage, and even when a Nb ₃ Sn filament is produced afterwards, it has excellent flexibility due to thinning. We have been able to provide a Nb ₃ Sn-based superconducting material that maintains the same properties and has significantly improved Jc properties through preliminary heat treatment, and together with its ease of manufacture and economic efficiency, it will become a high-performing material that will replace alloy-based superconducting materials in the future. It must be said that the effect of opening the way to the application of flexible materials to higher magnetic fields is enormous.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between pretreatment temperature and critical current value, and FIG. 2 is a diagram showing the effect of presence or absence of preheat treatment after diffusion treatment.

Claims (1)

[Claims] 1 An alloy consisting of 15 to 70% by weight of Nb, balance Cu, and unavoidable impurities is melted, and during casting, the average distance between the crystallized Nb is adjusted to be 6 μ or less to obtain a cast material, and the cast material is 99.5% By performing the above processing and stretching the Nb phase and the matrix together, a filamentary Nb phase is formed in the stretched matrix, and then Nb ₃ is added to the stretched material in accordance with the amount of Nb present. Sn
By plating Sn in an amount that can generate the Nb A method for producing a Nb ₃ Sn-based superconducting material, characterized in that the filament is a filament phase consisting of an Nb ₃ Sn phase.