DE2165130B2 - SUPRAL CONDUCTIVE WIRE - Google Patents

Description

The invention relates to a superconducting wire consisting of a plurality of aluminum coated superconductors.

From DT-AS! 7 90 149 a method for producing an electrical conductor is known in which a rod made of a superconductor alloy, preferably at a temperature between 450 and 550'C, heated, and immediately after reaching this temperature in a shell having room temperature Aluminum is used. Immediately after completion of this process, the superconducting rod is through joint extrusion connected to the aluminum shell. When heating the superconductor rod their hardness is reduced, so that there is only a slight difference between the hardness of aluminum and that of the superconductor alloy is present. Therefore, the two metals can work together satisfactorily be extruded.

Furthermore, a superconducting wire of the specified type is known from DT-AS 12 74 703, at a core made of lead is surrounded by an aluminum sheath and the resulting wire is deformed. Since lead has essentially the same hardness as aluminum, this wire can be easily deformed.

In such a superconducting wire, a spatial change in the magnetic flux generates a relatively large amount of heat. Will this amount of heat If the superconducting wire does not immediately lead to the outside, the wire becomes unstable, i.e. it becomes unstable. i.e., he can even go over to the normal line state. To investigate the resulting effects, im The following are the thermal, electrical and magnetic properties of superconducting wires be considered, where copper and aluminum coatings are compared with one another.

First of all, the specific resistances of aluminum and copper in the range of the temperature of liquid helium (4.2 ° K) are to be given with reference to FIG. the specific resistance of aluminum is 3 · 10 "ohm cm and the specific resistance of copper 1 ~ 2 · 10 ⁸ ohm cm. The thermal conductivity of aluminum and copper in the range of 4.2 ° K is 33 (W / cm " K each) ) or 3 (W / cm ° K), as can be seen from FIG. The thermal diffusion coefficient Dth. which is defined by thermal conductivity (Kj / specific heat (C) times specific weight (d) for copper (OFHC, 99.99%) or for aluminum purer than 99.995%) 0.12 ■ 10 ⁴ or 43 ■ 10 ^4th Finally, the magnetic diffusion coefficient is Dm. which is defined by (electrical resistance (R) / μ _α (magnetic permeability) for copper 1 ~ 2 · ΙΟ " ⁸ / ^ or for aluminum 3 · 10 Ύμ ,,.

Since the generated heat generally increases with an increase in the magnetic flux, the magnetic diffusion coefficient has the effect of smoothing the flux change. If the. Relaxation time with respect to the change in flow is denoted by r, the relationship between Dm and r can be represented by: Dm- l / r. The heat dissipation ratio of aluminum to copper therefore assumes the following value per unit area and unit of time:

/) ί / ι (ΑΙ) / τ (Λ!)
Ir (Cu)

2.70 / m / i (All 8.95 iY /) »! (Al)
/) m (Cu)

2.70

J Κ, ^ι ) ί

= .10 - 60.

Since the heat and / or the magnetic flux propagates through the surface of the superconductor, it becomes the value for the ratio for the unit coating thickness \ f3T) ~ \ lffl, that is, the heat dissipation ability of aluminum is 5.5 ~ 7.7 times As large as that of copper By using high-purity aluminum as a coating material, the same stability can be achieved as obtained with a copper coating

can be, but with an aluminum coating of iivva '/ ίο the surface and about -Vn the thickness of the Copper plating is sufficient. The lower-; Area and thickness of an aluminum coating i; i comparison with a copper plating lead to the following advantages when a superconducting wire in many turns a coil is wound:

F i g. 3 shows the current density for the entire cross-sectional area of a superconductor provided with a coating in percent when a current unit / flows through the superconducting core wire and the cross-sectional ratio of core line to coating ratio is 1: 2. If the area ratio of the superconducting wire to the copper coating is 1: 4, it follows from the above explanation that the ratio of superconducting wire to a pure aluminum coating can be in the order of magnitude of 1-0.2 to essentially the order of magnitude of 1-0.2 to achieve the same stability as with a copper coating. When using a pure aluminum coating for a core wire of the same size, this means that the current density for the entire cross-sectional area of the superconductor can be increased from 17% to 75%, i.e. the current density is 4.4 times that which can be achieved with a copper coating. Thus, if aluminum is used to achieve substantially the same stability as a copper plating, the volume or weight of the solenoid is 1/4 and 1/3 of what it would be if a copper plating were used. It is known that the electrical resistance increases with an increase in the magnetic field, as shown in FIG. 4 is shown; For high-purity aluminum, this magnetoresistive effect reaches ^{the saturation range for high magnetic fields at 0.6 · 10 8} Ohm cm, while the corresponding value for copper increases to a multiple of this value when no magnetic field is applied. This means that the above effect improves for an aluminum plating while it deteriorates for a copper plating.

If the outermost shell of a superconductor with an aluminum coating is used as the anode in the electrolyte and an electric current is passed through, aluminum oxide (an electrically insulating, thin film of very hard alunite) can form on the surface of the shell. This film has excellent properties in terms of stress resistance, and the thermal conductivity of this film in the radial direction through this insulating film to helium is about 100 times the thermal conductivity of the conventional organic film as compared with the conventional polyvinylfoiirial coating on the copper coating; because the thermal conductivity of the organic film is 0.00154 watt / cm ⁰ C, while that of the alunite is 0.164 watt / cm ⁰ C, the thickness of the alunite being about 1/10 of the thickness of the organic film.

Despite these advantages of using an aluminum overlay as compared to a copper overlay aluminum has only rarely been used up to now for the following reasons:

1) The mechanical strength, ie, the Spannungsfesiigkeii of aluminum is 4 kg / mm ² at 20 ⁰ C and thus significantly lower than that of copper, which at 20 ⁰ C 24 kg / ^mm: is, even though it at a value of aluminum 4.2 ° K is a multiple of the value at 20 ° C.

2) Since the Vickers hardness of the superconducting wire (Nb-Ti) as shown in FIG. 5 greater than 120, there is a very large difference in hardness between the superconducting wire and high-friction

-, nem aluminum; this can cause a great stretch generated by an electromagnetic force applied from the outside.

3) If a coating of high-purity aluminum is applied directly to a superconducting wire, for example

Ki se made of an Nb-Ti alloy, applied and the

resulting superconductor is drawn, so due to the extreme hardness difference between these materials only stretch the aluminum coating. This means that a "flow effect" r> can occur, i.e., the superconducting wire and

the coating will not be deformed evenly.
It is therefore an object of the present invention, a superconducting wire of the type indicated, in which the superconductor and the aluminum 2t) serving be uniformly deformed.

This object is achieved by the invention specified in claim 1.

The advantages achieved with the invention are based in particular on the fact that the above-mentioned »flow : - fect «can be avoided, d. that is, the deformation of the superconductor or the aluminum sheath, respectively is equal to. In addition, no complex heat treatment is required, as is the case with the method according to DT-AS 17 90 149 was required; and finally κ, you can still enjoy the significant advantages explained above an aluminum coating can be used.

The invention is explained in more detail below using exemplary embodiments with reference to the schematic drawings,
j-, it shows

F i g. 1 the change in the electrical resistance of aluminum or copper with temperature,

2 shows the change in the thermal conductivity of aluminum or copper with temperature,
4Ii Fig. 3 shows the percentage current density for a unit of current flowing through a superconducting wire when the superconducting wire consists of a superconductor sheathed by aluminum or copper, which is wound in such a way that a compact solenoid with a cross-sectional area ratio of wire to coating of 1 : 2 arises and the current unit flowing through the wire is averaged over the entire cross-sectional area of the superconductor and the coating,

F i g. 4 the magnetoresistive properties of aluminum and copper,

FIG. 5 shows the relationship between the workability (area reduction factor) and the Vickers hardness, FIG. 6 shows a cross section through a single superconductor before the wire is drawn,
-, i F i g. 7 shows a cross section through a single superconductor after the wire has been drawn,

F i g. 8 the change in voltage with the current (thermal hysteresis) of a superconducting wire which consists of a superconductor with a copper or cn aluminum coating, and

F i g. 9 shows a superconducting wire according to the present invention, which consists of a plurality of mi Aluminum-covered superconductors consists.

In particular in FIG. 6 is the basic one

n5 superconducting wire shown before Pulling the wire; So that the high-purity aluminum coating is not stretched on its own, a strand is made three superconducting wires 1 inserted into a tube 2

which is made from 99.99% pure aluminum; then the tube 2 is in a further tube 3 from a Aluminum alloy with a suitable hardness is used, so that a structure with a triple layer results. When this superconducting wire is drawn, the pipe 2 is made of the pure aluminum at the beginning deformed, as a result of which the gaps or spaces 4 are filled with part of the volume of the tube 2. at the next stage of wire drawing can be "flowing" / wipe the aluminum tube, the aluminum bracing tube and the strand of superconducting wires can be reduced to less than 5% if the tube 3 is made of an aluminum alloy containing 2.5% Mg and 0.25% Cr is produced, the value of the flow after filling the gaps with the pure aluminum is determined by the hardness of the aluminum alloy. After the spaces 4 have been filled, in In the intermediate stage of wire drawing, hydrostatic pressure is applied to the outer surface of the superconducting strand exerted which causes a force acting radially inward on the surface; this gives the finished superconducting wire has a surface reduction factor on the order of 95%, as in Fig. 7 is shown. The mechanical strength of this aluminum alloy is 23 kg / mm-, corresponds to essentially the value for copper.

Experimental data for a superconducting wire with a copper plating compared with a Superconducting wire with an aluminum coating are shown in FIGS. 8 (A) and 8 (B). From this data it is found that the copper-clad superconducting wire has thermal hysteresis. during the superconducting wire with aluminum coating has no thermal hysteresis. The in the F i g. 8 (A) and 8 (B) were determined by normalizing the cross-sectional area ratios.

The aluminum-coated superconducting wire as manufactured by the method described above is being further processed so that it can be used in a large electromagnet. It will a large number of such wire-shaped superconductors, each provided with an aluminum coating, bundled and twisted or intertwined. To avoid any electromagnetic coupling between the neighboring superconductors due to a possible change in the magnetic flux density of the To avoid magnets completely, the instability effects in the superconductor, such as flux jumps, diamagnetic currents, uneven currents, eddy currents etc .. will affect the an alunite coating is applied to the outer surface of each superconductor before they are interwoven, which completely insulates the superconducting wire electrically and as a short circuit from a thermal point of view acts; because of the use of organic insulation, this coating was previously not possible are provided. The superconducting wire is inserted into an aluminum alloy coating, which consists, for example, of 2.5 Mg-0.25 Cr-Al or duralumin; if necessary, he can also

-, first provided with a coating of such an aluminum alloy and then pressed or to be pulled.

Since the superconducting wire is made entirely of inorganic materials, it can undergo heat treatment subjected to that at appropriate values for the duration and temperature of the treatment is carried out after the mechanical structure of the superconducting wire is finished and / or the Production of the Alunit coating has been completed

ι-, is. Because the center of gravity of the Alunit coating is very high no combustion occurs. and the mechanical strength of the coating is very great. If necessary, can this superconducting wire can still be subjected to further processing. For example, can another electrical insulating layer with suitable mechanical strength by electrical means on the outer surface of the outer aluminum coating. And finally one can Lightly wrap superconducting wire into a solenoid.

Fig.9 shows a cross section through a superconducting Wire according to the invention. This wire is made up of a multitude of superconductors, each three Have layers. The superconductors are in a tube or in a sheath made of an aluminum alloy

_{i (} i are inserted (Fig. 9 (A)) or are surrounded by a coating of an aluminum alloy (Fig. 9 (B)). In Fig. 9, the areas shown in white indicate pure aluminum, while those provided with oblique lines Areas an aluminum alloy and / or Alunit-

j-, mark coatings.

The superconducting wire produced with this method can be used in many applications for such Use electromagnets, for example, to generate magnetic fields with high field strength and the very have to be exposed to high Lorentz forces; they can also be used with electromagnets graze, which are supposed to keep a linear motor in suspension, which must therefore be very compact and can absorb a high current density (in practice

4-, can be the weight and volume of a magnet made of superconducting wires according to the present Invention is wound on at least Vb or '/ j of one conventional magnets): such electromagnets are especially used for MHD. fin

-, ο the particle acceleration, for spark chambers unc Bubble chambers, for magnets that hold the plasma; Nuclear fusion reactor, for magnets, which al: electrical lenses of an electron microscope are used for electromagnets for electrical energy transfer

\ i used via cable, etc.

HU ^- IViI 5 Hlati

Claims (6)

Patent claims: 1. Superconducting wire, consisting of a large number of superconductors coated with aluminum, characterized in that each superconductor of three or more superconducting wires, the sheathing made of high-purity aluminum and an outer layer made of an aluminum alloy with a Vickers hardness of more than 50, consists and that the covered superconductors are bundled and twisted or intertwined and with are provided with a common coating made of an aluminum alloy. 2. Superconducting wire according to claim 1, characterized in that on the outer surface of each superconductor prior to bundling and twisting or interweaving by means of an Alunit treatment an electrically insulating layer is provided. 3. Superconducting wire according to claim 1, characterized in that on its outer surface by an alunit treatment of the common coating made of the aluminum alloy an electrical one insulating layer is provided. 4. A method for producing a superconducting wire with an aluminum coating according to a of claims 1 to 3, characterized in that a tube made of high-purity aluminum in another Tube made of an aluminum alloy is used, the Vickers hardness of which is greater than 50. the existence A strand of more than three superconductors is inserted into the pure aluminum tube that the strand and the tube are mechanically deformed to form a superconductor that a plurality this superconductor is bundled and twisted or intertwined, and that the outer Surface of the interwoven superconductors with a common coating of one Aluminum alloy is provided. 5. A method for producing a superconducting wire according to claim 4, characterized in that that on the superconductor by an alunite treatment of its outer surface before the interweaving an electrically insulating layer is applied. 6. A method for producing a superconducting wire according to one of claims 4 or 5, characterized in that on the outer surface of the common coating of the Aluminum alloy through an Alunit treatment of the outer surface of the aluminum alloy an electrically insulating layer is applied.