JPH0381246B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Background and Objects of the Invention] The present invention relates to a method for manufacturing a Nb-Ti alloy superconducting wire coated with enamel insulation.

Superconducting magnets, especially superconducting magnets for NMR, require high magnetic field uniformity. In order to obtain high magnetic field uniformity, the most effective means is to improve the winding accuracy of the magnet, but another effective means is to improve the dimensional accuracy of the superconducting wire itself.

The superconducting wire used in superconducting magnets is
Usually, the most common insulation method is to apply enamel insulation coating, but with the current enamel insulation coating technology, the limit of dimensional tolerance is about ±5 μm. On the other hand, dimensional accuracy of ±2.5 μm is particularly required for wire rods for superconducting magnets for NMR.

In addition, in order to improve the accuracy of the magnet winding,
It is necessary to wind the wire with high tension, but the heat generated when applying the enamel insulation coating dulls the Cu portion, which is a superconducting stabilizing material, and the mechanical strength of the superconducting wire decreases. However, it is not possible to increase the winding tension.

The object of the present invention is to eliminate the drawbacks of the prior art described above and provide an enamel insulation coating with high dimensional accuracy.
An object of the present invention is to provide a Nb-Ti alloy superconducting wire.

[Summary of the Invention] The gist of the present invention is to coat a Nb-Ti alloy superconducting wire with enamel insulation, such as formal, polyimide, etc., and then reduce the cross section by drawing or the like.
The purpose is to perform heat treatment at a relatively low temperature.

In addition to Nb-Ti in the present invention, Nb-Ti
- Ternary systems such as Zr, Nb-Ti-Ta, Nb-Ti-Hf, etc.
Furthermore, a quaternary system such as Nb-Ti-Zr-Ta can be applied.

The cross-section reduction process in the present invention is intended to improve dimensional accuracy, but if the cross-section reduction rate exceeds 25%, the enamel insulation coating will deteriorate and the breakdown voltage will decrease, making it difficult to obtain a product with good performance. Therefore, the area reduction rate is kept below 25%.

In addition, in the present invention, heat treatment is performed after the cross-section reduction process to reduce the electrical resistance (ρ) of the Cu portion, which is a stabilizing material, at extremely low temperatures (for example, liquid helium temperature of 4.2K), which has increased due to the cross-section reduction process. This is for recovery, but if the temperature is over 350℃
Since the superconducting properties of the Nb-Ti alloy superconducting wire deteriorate, the heating temperature is 350°C or lower.

There are two types of heating below 350℃: heating at a temperature below the recrystallization temperature of Cu and heating at a temperature above the recrystallization temperature of Cu, but the former requires particularly high dimensional accuracy and mechanical strength. The latter is applied when dimensional accuracy and electrical resistance (ρ at4.2K) are required. Even in the former case, it is possible to lower the electrical resistance (ρ at4.2K) of the Cu part. The heating temperature is greatly influenced by the area reduction rate, and if the area reduction rate is large, it will shift to the lower temperature side,
On the other hand, when the area reduction rate is small, there is a tendency to shift to a higher temperature side, so the heating temperature must be selected based on the relationship between the area reduction rate and the required characteristics. According to the results of the examples described later, the heat treatment temperature is 200°C.
That's all.

[Embodiments of the invention] The present invention will be described below with reference to the drawings.

In each case, a filament 1 of superconducting material made of Nb-Ti alloy is embedded in a stabilizing material 2 made of Cu, and an enamel insulation coating 3 is applied by applying formal insulation paint to the outer periphery and baking it. ,(a)
(b) shows a case in which a large number of extremely thin filaments 1 are embedded to obtain a multi-core structure.

Superconducting wires of any structure are coated with enamel insulation 3
After that, the cross-section is reduced by die drawing and finished to the required dimensions, and then heat treated at a temperature of 350°C or less.

Figure 2 shows the outer diameter with formal insulation coating 3.
This figure shows the relationship between the area reduction rate, tensile strength, dimensional accuracy, and dielectric breakdown voltage when processing an ultrafine multifilamentary wire with a thickness of 1.0 mm and an insulation coating thickness of approximately 3.0 μm at various area reduction rates.

From this figure, it can be seen that the tensile strength and dimensional accuracy are significantly improved by applying the cross-section reduction process. However, it can be seen that the degree of deterioration of the dielectric breakdown voltage is small until the area reduction rate is 25%, but the degree of deterioration increases when the reduction rate exceeds 25%.

Figure 3 shows the relationship between heating temperature, tensile strength, and electrical resistance when a wire rod with a cross-section reduction rate of 5% is heat-treated in the example shown in Figure 2. There is almost no decrease in tensile strength up to a temperature of 300℃, but a decrease phenomenon is observed when the temperature exceeds 300℃. It can also be seen that the electrical resistance at liquid helium temperature (ρ at4.2K) begins to decrease when it exceeds 200°C. Therefore, it can be seen that the appropriate range for the heat treatment temperature is 200 to 350°C.

[Effects of the Invention] As is clear from the above description, according to the present invention, the electrical properties of the superconducting wire coated with the enamel insulation can be improved by performing cross-section reduction processing and heat treatment after the enamel insulation coating. It has the advantage of being able to improve dimensional accuracy and mechanical strength without reducing it, and has great industrial utility value.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing an example of a superconducting wire according to an embodiment of the method according to the present invention, and FIG. 2 is a graph showing the relationship between area reduction rate and required characteristics regarding the present invention.
FIG. 3 is a graph showing the relationship between heating temperature and required characteristics regarding the present invention. 1: Filament of superconducting material, 2: Stabilizing material,
3: Enamel insulation coating.

Claims (1)

[Claims] 1. After applying an enamel insulation coating to a Nb-Ti alloy superconducting wire, the wire is subjected to a cross-section reduction process of 25% or less, and then the wire is heat-treated at a temperature of 200 to 350°C. A method for manufacturing Nb-Ti alloy superconducting wire.