EP0048880B1 - Method of fixing the windings of a superconductive magnet coil - Google Patents

Description

The invention relates to a method for fixing the turns of a superconducting magnetic winding, which is initially built up from preliminary conductor products and in which the superconducting properties are produced by reaction annealing of the finished winding.

Superconducting intermetallic compounds with A15 crystal structure, such as the two-component compounds Nb ₃ Sn and V ₃ Ga or the ternary compound Nb ₃ A1 _o , ₈ Ge ₀ , ₂ , have good superconducting properties and are distinguished by high critical values, ie by a high step temperature, a high critical current density and a high critical magnetic field.

They are therefore particularly suitable as conductor materials for superconducting magnetic coils for generating strong magnetic fields.

However, the superconducting intermetallic compounds are generally very brittle, so that their manufacture in a form suitable for solenoids is difficult. The general procedure today is to first of all provide a rod or wire made of the higher-melting element of the connection with a shell made of an alloy, which consists of a carrier metal and the lower-melting element of the connection or, if appropriate, several such elements. For example, a niobium wire is surrounded by a copper-tin-bronze sheath. A large number of these covered wires are then combined into a bundle. However, it is also possible, for example, to store several wires in a block made of bronze with holes. The structure obtained in this way is then subjected to processing which reduces the cross section. A wire or a strip of an alloy matrix is thus obtained, in which a multiplicity of thread-shaped cores from the higher-melting element of the connection is embedded, for example a copper-tin wire with a large number of embedded niobium filaments. To produce the intermetallic superconducting compound, such a preliminary conductor product is then subjected to an annealing treatment in which the element or elements of the compound contained in the matrix diffuse into the cores of the higher-melting element and react with it to form the intermetallic compound (see e.g. DE-A-2044660).

Superconducting magnetic coils from such superconductors have hitherto generally been produced by two different processes (cf. e.g. DE-A-2837199 and 2840526).

In the first process, which is also referred to as the “react first-wind then process”, the preliminary conductor product is wound onto a provisional winding body and then annealed to form the desired superconducting compound. Following the annealing treatment, the superconductor produced in this way is unwound again from the provisional winding body and can then be wound, for example, into a magnetic coil. However, due to the high brittleness of the superconducting intermetallic compounds, there is a risk that cracks will occur in the intermetallic superconducting connections if the bending radius permitted for the completely reacted conductor is undershot and the superconducting properties are impaired accordingly.

To avoid this danger, another method is used, which is also referred to as the “wind and react technique”. The coil body of the magnet to be provided with the winding is wound with the as yet unreacted conductor preliminary product and the entire magnet thus wound is then subjected to diffusion annealing. Since the intermetallic connection is only created in situ in the coil or winding, in this case the diffusion annealing is also called "in situ annealing". This procedure avoids all difficulties in processing a brittle conductor material. However, the materials present in the coil during the diffusion annealing must withstand the required high temperatures for several hours, which, for example in the case of Nb ₃ Sn, are around 700 ° C.

Up to now, therefore, the conductor pre-product wrapped or wound with an insulation made of glass, ceramic or quartz threads has been wound onto a correspondingly high-temperature-resistant coil body and the coil windings are finally fixed only after the diffusion annealing, by covering the finished winding with suitable curable or solidifying organic materials upon cooling , for example epoxy resins or paraffin, preferably impregnated by vacuum impregnation (cf. DE-A-2546198 and 2837199).

However, this procedure is also not without problems. Firstly, there is the difficulty that carbon is deposited between the coil turns during the diffusion annealing and the insulation of the coil is thereby deteriorated. The insulation materials usually used in the coil winding made of glass, ceramic or quartz threads are usually provided with organic sizes or binders to facilitate their handling, which decompose at high temperatures. Special methods are therefore used to remove the sizes or binders from the coil before the reaction annealing (DE-A-2837199). However, such methods require a certain amount of effort. Since the insulation materials mentioned lose strength with increasing temperature, there is also the danger that the turns which are not further fixed during the annealing treatment will change their position or shape. This can also result in a deterioration in the insulation between the turns and, moreover, lead to unforeseeable changes in the field distribution in the winding.

The object of the invention is to manufacture of superconducting magnetic windings, the superconducting properties of which are produced by reaction annealing (in-situ annealing) of the finished winding.

This is achieved according to the invention in that when the conductor intermediate products are wound, a putty made of insulating inorganic material, which is solidified in the solidified state at the low temperatures required to bring about the superconducting state and at the high temperatures required for reaction annealing, is introduced into the winding and after completion the winding is solidified before performing the reaction annealing.

The method according to the invention leads to a fixed fixation of the individual turns of the coil winding even before the reaction annealing and thus avoids the shape and position changes occurring in unfixed turns due to thermal expansion or contraction and their disadvantageous consequences. it has also been shown that the formation of carbon bridges is also avoided by the putty introduced into the winding. It is therefore no longer necessary to remove the sizes or the binders of the insulating materials from the winding before the annealing treatment.

Suitable putties are inorganic materials which, if necessary after prior mixing with water or other suitable inorganic liquids, are curable or otherwise harden or solidify and in the solidified state both at the high temperatures required for reaction annealing of, for example, about 700 ° C. at Nb ₃ Sn as well as at the low temperatures required to bring about the superconducting state, for example the temperature of the liquid helium of 4.2 K, are stable. The putties should also have good resistance to temperature changes so that no damage occurs in the winding even when the coil is repeatedly cooled to low temperatures. For similar reasons, the putties should not differ too much from the superconductor material with regard to their thermal expansion. Furthermore, good thermal conductivity of the cement is also advantageous for effective cooling of the coil winding. Unstable connections that release metals during reaction annealing should not be included in the kit, since the insulation of the winding would be impaired by the released metal.

A putty made from water glass (sodium silicate solution) and talcum (soapstone powder) has proven to be particularly suitable. This putty solidifies into a hard mass made of double silicates, which very well fulfills the aforementioned requirements. A mixture of about 60% by weight of water glass and 40% by weight of talc is preferably used, which can still be processed well even after a long standing time.

Especially when using water glass talcum putty, a two-stage heat treatment of the winding before the reaction annealing has proven itself

The winding is first used to dry the cement of a heat treatment at a. first temperature of preferably 35 to 40 ° C and then to harden the putty a heat treatment at a second higher temperature, preferably about 100 to 120 ° C, subjected. This two-stage heat treatment can be combined with the reaction annealing to a three-stage heat treatment.

In addition to the preferred mixture of water glass and talcum, other mineral-based cements are also suitable as putties, provided that they meet the putty requirements already explained.

A particularly stable winding structure can be obtained by first applying an insulating mat made of high-temperature resistant material, for example a glass fiber mat, to a high-temperature-resistant coil former, for example made of stainless steel, and then impregnating it with the putty, then applying the successive layers of the conductor winding and in each case with the putty are coated and finally the finished winding is surrounded by another high-temperature-resistant insulating mat, which is also to be soaked with putty, and finally the putty is cured at elevated temperature. In addition, further high-temperature-resistant insulating mats can also be inserted between the individual layers of the winding.

The invention will be explained in more detail using an exemplary embodiment.

For the production of a magnetic coil with Nb ₃ Sn as the superconductor material, a strip-shaped, 1 mm wide and 0.3 mm thick conductor pre-product made of a copper-tin matrix with 1159 embedded niobium filaments was assumed. For the insulation, the preliminary conductor product was provided with a glass fiber covering on which an organic size was applied.

A water glass-talcum mixture with about 60% by weight of sparkling water glass and about 40% by weight of talc was used as the putty. If the water glass remains open for a long time before mixing with the talcum powder, it becomes thinner. It is then advisable to increase the proportion of talcum to increase the viscosity of the putty.

The bobbin used to wind the bobbin was made of non-magnetic stainless steel and was equipped with two disk-shaped side flanges and a central bore with a diameter of 25 mm. The inner winding diameter was 30 mm, the outer winding diameter 50 mm, the winding length 40 mm.

On the winding core of the bobbin and the side flanges, glass fiber mats with a thickness of 90 _fl m were placed on top of each other for insulation against the winding to be applied and coated with putty. Then the first winding layer was wound onto the ribbon-shaped conductor pre-product on the glass fiber mat surrounding the winding core and putty was applied to this layer. A 30 _f lm thick glass fiber mat was then placed on the putty applied for layer insulation, the second winding layer wound on it and putty applied again. This continued until after 22 layers the outer winding diameter was reached. After the application of a kit layer, the last winding layer was again surrounded with a bandage consisting of several layers of glass fiber mats. Putty was applied to each layer again. The coil produced in this way had a total of 637 turns. The length of the pre-processed conductor was 81 m.

The coil was then first heated to about 37 ° C. for a few hours to dry the putty. This is sufficient if the water vapor can escape from the winding, for example through holes provided in the disk-shaped side flanges or in other ways. This drying step prevents cracks from appearing in the putty during subsequent curing. A second heat treatment at about 117 ° C for several hours then gave the putty the required strength. After appropriate determination of the conductor ends for later assembly of the contacts, the coil was finally wrapped in a zirconium foil serving as getter and subjected to reaction annealing at about 700 ° C. under argon for about two days. After cooling, the coil was cast with casting resin in a protective trough. Instead of the casting resin, an inorganic putty can also be used.

The finished coil was then inserted into the bore of a larger superconducting magnet and tested in its magnetic field. With a magnetic induction of 7 Tesla generated by the outer coil, a magnetic induction of 9 Tesla was achieved in the bore of the inner coil.

A training behavior of the coil produced according to the method according to the application could not be determined. The critical current density of the Nb ₃ Sn conductor of the inner coil was approximately 90,000 A / cm ² in an external field of 2 Tesla, and approximately 45,000 A / cm ² in an external field of 7 Tesla ^.

The method according to the application has proven itself in the same way for larger coils of up to 177 mm winding length and 119 mm outer winding diameter. The packing density of the winding could be increased even further by omitting the insulating mats between the individual winding layers. In this case, putty is applied to each winding layer and the next winding layer is wound directly over it. However, omitting the layer insulation is only recommended if the actual conductor insulation consists of a fiber wound. If the pre-products for insulation are only wrapped with glass fiber, for example, then the insulating mats should be retained to insulate the winding layers from each other. If, especially in the case of thicker insulation mats, the putty applied to the previous winding layer is not sufficient to impregnate the insulation mat, putty is expediently applied to it again before the next winding layer is applied. Even when using glass fiber wrapping or wrapping impregnated with organic binders, for example with polyvinyl butyral, there were no difficulties in the process according to the application. Apparently, the carbon released during the thermal decomposition of the polyvinyl butyral during the heating of the winding for reaction annealing is prevented by the putty from forming conductive bridges between the turns. Carbon-releasing substances therefore do not need to be removed or at least not completely removed before the reaction annealing when winding the coil.

Claims (6)

1. A method of fixing the turns of a superconductive magnetic winding which is initially composed of conducting initial products and in which the superconductive properties are produced by a reactive annealing of the finished winding, characterised in that when winding the conducting initial products a cement, made of an insulating inorganic material and which, in the hardened state, is stable at the low temperatures required for bringing about the superconductive state and at the high temperatures required for the reactive annealing, is introduced into the winding and, on completion of the winding, is hardened before the reactive annealing is carried out.

2. A method according to Claim 1, characterised in that a mixture of water glass and talc is used as the cement.

3. A method according to Claim 2, characterised in that a mixture of approximately 60% by weight of water glass and approximately 40% by weight of talc is used.

4. A method according to one of Claims 1 to 3, characterised in that, prior to the reactive annealing, the winding is first subjected to a heat treatment at a first temperature in order to dry the cement and is supsequently subjected to a further heat treatment at a second higher temperature in order to harden the cement.

5. A method according to one of Claims 1 to 4, characterised in that an insulating mat, which is made of a high temperature-resistant material, is first applied to a high temperature-resistant coil body and is then saturated with the cement, the successive layers of the conductor winding are subsequently applied and respectively coated with the cement and the completed winding is finally surrounded with a further high temperature-rersistant insulating mat to be saturated with cement, and the cement is finally hardened at a higher temperature.

6. A method according to Claim 5, characterised in that further high temperature-resistant insulating mats are additionally inserted between the individual layers of the winding.