JPS6217809B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a composite superconductor used in superconducting power transmission cables, various superconducting magnets, and the like.

A composite superconductor consists of a superconducting material and a normal conducting material, for example, a superconducting wire with a diameter of 5 to 250 μm,
Embedded in normal conductive material such as copper or aluminum,
It is finished into round wire, flat wire, and hollow wire, and is wound as appropriate to form a coil.

Since strands exhibit superconductivity, they do not generate heat under a constant magnetic field and constant current, but they generate loss due to magnetization when the current changes over time. Furthermore, if the superconductivity is broken for some reason, a large amount of Joule heat will be generated. Therefore, a part of the composite superconductor becomes a cooling surface that is cooled by being in direct contact with liquid helium.

When superconductivity is broken, the conditions for preventing the expansion of the normal conduction state region with Joule heat generation are given by the following equation.

ρI ² /APq<1 ......(1) Here, ρ: Electrical specific resistance of the normal conductor, I: Current current, P: Cooling perimeter, A: Cross-sectional area of the normal conductor, q: Cooling heat flow It's a bunch.

According to equation (1), to increase the current carrying current (I), use a normal conductor with a small electrical resistivity (ρ) and a large cross-sectional area (A), or increase the cooling capacity (P x q). You'll know what to do.

The former has limitations due to the material of the normal conductor or the design of the composite superconductor, and the main focus of efforts to improve performance is on the latter.

As an example of this measure, a rectangular groove with a depth of 1 mm and a width of 1 mm is provided on the surface of the conductor in contact with liquid helium.
A method of increasing the cooling perimeter (P) is known.

As another example, attempts have been made to provide a cellulose film on the surface of the conductor, but in both cases the improvement in cooling capacity cannot be said to be sufficient.

This invention was made in view of these points,
The cooling capacity is improved by covering the surface of the conductor in contact with liquid helium, which is a refrigerant, with a thin film made of a metal or metal compound that has higher thermal resistance than the surface material.

The following will be explained with reference to the drawings. FIG. 1 shows an embodiment of the composite superconductor according to the present invention, in which a cooling surface is formed on one side of the cross section of the conductor finished into a rectangular wire. .

The cooling surface of the composite superconductor, in which a large number of thin superconductor wires 2 are embedded in a normal conductor 1 made of high-purity copper, has parallel grooves 3 that cross the conductor.
A large number of raised portions 4 are formed separated by grooves 3, and each raised portion 4 has a tapered tip with an acute angle, and the tip side is formed by a V-shaped crack 5 shallower than the depth of the groove 3. It is divided into two parts. This rough cooling surface is covered with a thin copper oxide film 6.

Such a cooling surface can be obtained by performing the following processing.

After forming first grooves with a V-shaped cross section that are adjacent to each other in a continuous manner on the smooth surface of the conductor corresponding to the cooling surface, grooves deeper than the first groove are formed by cutting in a plow-like manner in a direction intersecting the first grooves. Form a second groove. A groove 3 is formed by this second groove, and a raised portion 4 that stands between them forms a V-shaped crack 5 that is a remnant of the first groove.
, and exhibits a sharp, pointed taper. For example, if the first groove has a pitch of 0.6 mm and a depth of 0.4 mm, and the second groove has a width of 0.2 mm, a depth of 0.8 mm, and a pitch of 0.4 mm, the surface area will be approximately 3.5 times that before processing. If the first groove, which is the split 5, is not formed, the surface area will be approximately three times that before processing.

After this surface processing, the processed side is, for example,
It is treated by anodization using a sodium hydroxide aqueous solution bath heated to about 90°C. By this treatment, a thin film 6 of copper oxide is formed on the processed surface which will be the cooling surface.

After surface processing, the film 6 is formed in an atmospheric furnace at 400°C for 5 minutes and at 150°C for 10 minutes.
There is no problem with the method of heating for a certain amount of time. The effect of the cooling surface thus obtained will be explained with reference to FIG. 2. FIG. 2 shows the boiling performance in liquid helium. The vertical axis shows the heat flux, and the horizontal axis shows the temperature difference between the liquid helium and the cooling surface.

Curve A is the characteristic of a normal smooth surface of high-purity copper,
Curve B is the characteristic of the same smooth surface with rectangular grooves of 1 mm depth and 1 mm width, curve C is the characteristic of the surface of the above example, and C' is the characteristic of the surface without film 6.

As the heat flux is increased, heat transfer is first performed by nucleate boiling, and at a certain heat flux q max it transitions to film boiling. If the heat flux is lowered here, it returns to nucleate boiling again at a certain heat flux q min.

In the unlikely event that the temperature of the superconductor increases for some reason,
In order to quickly recover the superconducting properties when the cause of the transition to the film boiling region disappears, q
In addition to the height of max, it is also desirable that qmin be as high as possible.

Both q max and q min of curve B show high values when compared with curve A, and although the effect of roughening the surface is recognized, q max and q
The difference with min is not getting smaller.

On the other hand, q min of curve C shows a value close to q max. Although not shown in the drawings, such a tendency occurs when the processed surface in the above example is treated with an aqueous calcium sulfide solution to form a copper sulfide film, and of course when the surface is coated with nickel by the plating method. This was also observed when a similar film was provided on an ordinary smooth surface.

This applies not only to the special substrate configuration shown in the examples, but also to the film 6 constituting the surface zone.
This can be said to be due to the high thermal resistance of That is, in a film boiling state, the vapor film covering the surface is constantly fluctuating, and it is thought that locally the film is very thin or completely disappears. Nucleate boiling occurs temporarily in this area, but if the surface is made of copper, which has good heat conductivity, for example, the temperature in this area does not drop that much and returns to film boiling instantly, further increasing the heat load. If the temperature is not lowered, nucleate boiling will not occur. Moreover, if the surface is covered with a film with high thermal resistance, the temperature will drop locally and nucleate boiling will be maintained for a certain period of time even with the heat load that returns to film boiling. A coexistence state of boiling occurs, and q min becomes higher than in the case without a high heat resistance film.

In addition, if this film with high thermal resistance is formed with minute irregularities or porousness to the extent that it becomes a boiling nucleus of liquid helium, q max will be obtained.

Therefore, with such a cooling surface, even if the superconductor becomes normal conductive for some reason, a rapid temperature rise will not occur and the superconductor can be quickly returned to the superconducting state.

As described above, according to the present invention, the critical heat flux of film boiling can be increased, so the cooling capacity used in the stabilization design of composite superconductors can be increased, and therefore normal conductors This has the effect of reducing the cross-sectional area of the magnet, increasing the effective current density, reducing the size of the superconducting magnet, reducing the amount of superconducting material used, and reducing the refrigeration capacity, which greatly contributes to reducing the cost of the entire system.

[Brief explanation of the drawing]

FIG. 1 is a diagram illustrating an embodiment of the composite superconductor according to the present invention, and FIG. 2 is a graph showing the characteristics of its cooling surface. 1: Normal conductor, 2: Superconductor, 3: Groove, 4: Protuberance, 5: Cracks, 6: Thin film.

Claims (1)

[Claims] 1. On part or all of the surface of a composite superconductor made of a superconducting material and a normal conducting material, a large number of closely spaced grooves are formed, and a large number of grooves are separated by the grooves and have tapered ends with acute angles. A composite superconductor comprising a raised part, and a part or all of these grooves and raised parts are covered with a thin film of a metal or metal compound having higher thermal resistance than the surface part of the conductor. Each body.