JPH0644421B2 - Superconducting conductor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a superconducting conductor, and more particularly to a superconducting conductor suitable for application to a tightly wound superconducting coil, a forced refrigeration type superconducting coil, and the like.

[Conventional technology]

Conventional superconducting conductors have a structure in which the surface is covered with an organic insulator for insulation, but organic insulators cannot be applied to wire rods using compound superconducting materials that are heat-treated at high temperatures. There was an important problem. Therefore, as described in JP-A-49-57793,
Ceramic enamel coating was proposed. However, a drawback of this ceramic enamel is that, when baked by high temperature heat treatment, the insulating property is reduced or broken due to shrinkage during decomposition. Also, no attention has been paid to the thermal conductivity of the insulation.

That is, in JP-A-49-57793, it is described that ceramic enamel is coated and baked as an insulating material, and the ceramic is melted and adhered by heating, and the coil is integrated. Is 2000
The temperature must be higher than ℃. However, with this, the melting point of the superconductor is exceeded and it is practically impossible. Also, since enamel is used, it is essentially the same as conventional organic insulation.

[Problems to be solved by the invention]

Therefore, the above conventional techniques have the following problems.

1. In a superconducting coil using a compound-based superconducting conductor that is diffusion heat-treated at a high temperature, when heat treatment is required after winding, it is not possible to use an organic insulation having low heat resistance, depending on the dimensions and manufacturing conditions of the coil. Further, in the ceramic enamel in which the ceramic powder is mixed with the enamel, it is considered that the enamel is burnt out by the heat treatment, so that the ceramic particles are peeled off and the insulating property is not maintained.

2. The thermal conductivity of the organic insulator is significantly lower than that of metal, and the heat generated inside the conductor is transferred to the refrigerant through this insulator. This deteriorates the thermal stability of the coil.

3. In a bundle type forced refrigeration superconducting conductor using a compound superconducting material, a plurality of superconducting element wires are twisted and inserted into a conduit, and heat treatment is performed. For this reason, in the conventional technology, the surface of the superconducting wire cannot be subjected to an insulation treatment, so that in the operation involving an alternating magnetic field or a large magnetic field change, the temperature of the conductor rises due to the coupling loss between the wires. , There is a risk of reaching a quench. Further, the heat generated by the coupling loss becomes a large heat load on the freeze liquefaction machine of the superconducting coil.
In the poloidal coil of a fusion reactor that receives an AC fluctuating magnetic field, reducing the AC loss of the superconducting coil is an important issue.

As a conventional example of inorganic insulation, it is conceivable to form a CuO layer on the surface of a compound superconducting conductor, but there is a problem that the CuO layer disappears during heat treatment. Although alumina may be formed in some cases, the purpose of the present invention is to form an inorganic insulating layer having a performance superior to that of alumina on the surface of the superconducting conductor.

The present invention has been made in view of the above-mentioned problems, and it is an object of the present invention to provide a high-performance superconducting conductor that is thermally stable and has a low AC loss.

[Means for solving problems]

In order to achieve the above object, the present invention is a superconducting material in which a superconducting material is embedded in a stabilizing material, and the surface of the stabilizing material in which the superconducting material is embedded is surrounded by an inorganic insulating layer. Is BeO, SiC, TiC, Si
_{It is made of} a ceramic material of ₃ N ₄ and AlN and has a thermal conductivity of 1 × 10 ⁻² W / cm / K to 2 × 10 at extremely low temperatures.
A superconducting conductor, which is an inorganic insulating layer having high thermal conductivity and heat resistance within a range of ¹ W / cm / K.

[Action]

According to the present invention described above, by coating a superconducting conductor with a highly heat-conductive and heat-resistant inorganic insulating material instead of the conventional organic insulator, diffusion of heat generated in the conductor is stopped,
Thermal stability is increased.

Further, due to heat treatment, it is possible to reduce the AC loss of the bundle type forced refrigeration conductor using the compound-based superconducting material, which cannot be organically insulated.

Fig. 4 shows high thermal conductivity and heat resistant inorganic insulating materials (BeO, Si
The heat conduction measurement result in the low temperature of C) is shown. Referring to FIG. 4, the range of the thermal conductivity of these inorganic insulating materials at low temperature is from 1 × 10 ⁻² / cm / K to 2 × 10 ¹ W / cm.
/ K. For comparison, metal (copper, SUS304), organic insulating material (polyester), and inorganic insulating material (Al ₂ O)
₃ ) shows the thermal conductivity. At extremely low temperatures above 10K,
The thermal conductivity of these highly heat-conductive and heat-resistant inorganic insulating materials is about two orders of magnitude higher than that of organic insulating materials, and equal to or higher than that of SUS304. Furthermore, from FIG. 4, B
The inorganic insulating material of the present invention composed of eO and SiC is Al ₂ O.
It can be seen that it has better thermal conductivity than ₃ .

Here, a simple model (see FIG. 5) is used to show how the thermal conductivity of the insulating material contributes to the stability of the conductor. With the radius r _{i of the} conductor 1 and the thickness r _o −r _{i of} the insulating material 2,
Assuming that the temperature inside the conductor 1 is T _i and the temperature of the refrigerant is T ∞, the thermal conductivity k of the insulating material 2 and the thermal conductivity of the refrigerant to h are conductors per length L of this superconductor. The amount of heat transferred from 1 to the refrigerant through the insulating material 2 is Becomes

Here, when the geometric shapes are the same and the insulating layers are made of organic insulation (polyester) and high thermal conductivity inorganic insulation (SiC ceramic), the thermal conductivity at 4.2K is k ₁ = 7. × 10 ^-4 W / cm · K, k ₂ = 1.5 × 10
^-2 W / cm · K.

Here, assuming that the same energy q flows out from the conductor to the refrigerant, when the temperatures of the conductors are T _i1 and T _i2 , Becomes

If ΔT ₁ = T _i1 −T ∞ and ΔT ₂ = T _i2 −T ∞, then in the general design, Therefore, It can be seen that the temperature rise of the conductor is inversely proportional to the thermal conductivity of the insulator.

Therefore, from the above examination, when comparing the case of using the organic insulation as the insulator and the case of using the high thermal conductivity inorganic insulation, the rate of temperature rise of the conductor is k ₂ / k ₁ = 21.
You can see that it is different. That is, the superconductor provided with the high thermal conductivity inorganic insulation is thermally stable with a temperature rise of 1/21 when removing the same amount of heat generation.

〔Example〕

The present invention will be described in detail below with reference to the embodiments shown in the drawings.

FIG. 1 shows a typical superconducting conductor 5 composed of a superconducting material (filament + barrier) 3 and a stabilizing material 4 on the surface of which has a high thermal conductivity and a heat resistant inorganic insulator (eg, BeO, Si).
C, TiC, Si ₃ N ₄ , AlN ceramic material) 6 is coated. This coating layer is 1
The thickness is set to μm to 50 μm. That is, when the thickness is 1 μm or less, the insulating property is deteriorated, and when the thickness is 50 μm or more, cracks may occur due to strain of thermal stress.

FIG. 2 shows a part of the winding bobbin 7 of the tightly wound superconducting coil made by using the superconducting conductor according to the present invention shown in FIG. Unlike conventional organic insulation, it has high heat conduction,
Moreover, since the heat-resistant inorganic insulator 6 is coated on the surface of the superconducting conductor 5, the heat generated in the superconducting conductor 5 propagates not only in the conductor longitudinal direction but also to the superconducting conductors 5 adjacent to each other vertically and horizontally. As a result, the temperature of the heat-generating conductor portion rapidly drops below the shunt start temperature, and the propagation of the normal-conducting region is blocked, so that the superconducting coil has higher thermal stability than the conventional superconducting coil using organic insulation. A coil can be obtained. In addition, unlike the conventional superconducting coil, even if the coil is quenched, the heat stays in one place, and that part runs thermal runaway, melts or the temperature rises, and the resistance of the stabilizing material 4 increases. Therefore, it is possible to prevent an accident of dielectric breakdown between the superconducting conductors 5 due to the generation of high voltage.

FIG. 3 shows an example of a bundle-type forced refrigeration superconducting conductor housed in a conduit 8 using a plurality of superconducting conductors 5 according to the present invention shown in FIG. 1 as a stranded wire. Note that reference numeral 9 indicates a refrigerant flow path. A conductor using a compound-based superconducting material such as Nb ₃ Sn is designed to be inserted into the conduit 8 after being twisted and heat-treated at a high temperature. For this reason, in the conventional organic insulation, it is impossible to insulate the superconducting stranded wires and suppress the coupling loss between the stranded wires. However, when a plurality of superconducting conductors 5 according to the present invention are used to form the superconducting stranded wires. For that,
It is possible to manufacture a thermally stable high performance forced refrigeration type superconducting conductor with low AC loss.

As a method for coating the surface of the superconducting conductor with a highly heat-conductive and heat-resistant inorganic insulating material (BeO, SiC, TiC, Si ₃ N ₄ , AlN ceramic material), sputtering, CVD (gas phase reaction method), Pressure sintering, dynamic range method, etc. can be considered.

〔The invention's effect〕

As described above, according to the superconducting conductor of the present invention, the heat generated in the superconducting conductor is promptly and adjacently formed by forming the heat-resistant inorganic insulating layer on the superconducting conductor surface with high heat conduction. Since it can be diffused into the conductor and the coolant, it has an effect of preventing the generation and propagation of the normal conducting region. Further, according to the present invention, by using a high thermal conductive and heat resistant inorganic insulation for the insulation of the superconducting stranded wire of the bundle type forced refrigeration conductor (using a compound superconducting material such as Nb ₃ Sn), the insulation during heat treatment can be improved. Damage can be prevented, and a great effect can be obtained in reducing AC loss.

[Brief description of drawings]

FIG. 1 is a partially cutaway plan view showing an embodiment of a superconducting conductor according to the present invention, FIG. 2 is a sectional view of a main part of a superconducting coil using the superconducting conductor of the present invention, and FIG. 3 is a superconducting conductor of the present invention. Partially cutaway plan view of a bundle-type forced refrigeration superconducting conductor using a stranded wire, FIG. 4 is a heat conduction characteristic at low temperature of an inorganic insulator (SiC, BeO), Cu, SUS304L and an organic insulator (polyester) of the present invention. 5 is a graph showing the principle of the present invention. 3 ... Superconducting material, 4 ... Stabilizing material, 5 ... Superconducting conductor,
6 ... Inorganic insulator.

Claims (1)

[Claims] 1. A structure in which a superconducting material (3) is embedded in a stabilizing material (4), and the surface of the stabilizing material (4) in which the superconducting material (3) is embedded is surrounded by an inorganic insulating layer (6). In the superconducting conductor (5), the insulating layer (6) is made of a ceramic material such as BeO, SiC, TiC, Si ₃ N ₄ and AlN and has a thermal conductivity of 1 × at extremely low temperature.
A superconducting conductor, which is an inorganic insulating layer having a high thermal conductivity and a heat resistance in the range of 10 ^-2 W / cm / K to 2 × 10 ¹ W / cm / K.