1. Field of the Invention
The invention relates to superconducting conductors such as single strand wires or superconducting cables comprising an aluminum based cryogenic stabilizer.
Unless mentioned otherwise, compositions are expressed as values by weight.
2. Description of Related Art
The use of very high magnetic fields of several teslas for applications such as magnetic levitation of vehicles, nuclear magnetic resonance (NMR) or physics of elementary particles, requires the use of superconducting conductors, especially in the form of cables, capable of carrying high current densities, typically greater than 105 A/cm2 with very low energy losses. The conductors of this type, the most frequently used of which are niobium alloy based conductors such as Nbxe2x80x94Ti, and occasionally Nbxe2x80x94Zr, are only superconducting below a very low critical temperature Tc such that cooling with liquid helium is necessary, and they only remain superconducting if the magnetic field applied to them does not exceed a critical value Hc. Therefore, it is essential to make sure that no localized thermal, mechanical or magnetic disturbance could cause a local loss of superconductivity and propagate throughout the conductor possibly causing irreversible degradation.
For these reasons, superconducting cables are usually composed of a large number of superconducting filaments with a small individual cross-sectional area (typically Ø less than 50 xcexcm) embedded in a metallic matrix, thus forming a xe2x80x9csuperconducting corexe2x80x9d and encased in a metal with high electrical and thermal conductivity such as copper or aluminum, that can quickly transfer heat to the liquid helium bath and mechanically protect the filaments, particularly during shaping operations. These operations comprise successive mechanical working steps (such as extrusion or drawing) and heat treatments (such as annealing) that can give good electrical and thermal contact between the superconducting core and the said encasing metal, which is known under the term xe2x80x9cstabilizing casing (or cladding)xe2x80x9d or xe2x80x9ccryogenic stabilizerxe2x80x9d or simply xe2x80x9cstabilizerxe2x80x9d. In general, superconducting cores occupy 10 to 40% of the cross-section of the superconducting cables and the stabilizing casing occupies 60 to 90%. The superconducting filaments are generally made of a niobium alloy such as NbTi. The said metallic matrix has good electrical and thermal conductivities and keeps the filaments together and provides mechanical protection for them during the cable manufacturing steps. It is usually made of copper or a high purity copper alloy, and less frequently aluminum or a high purity aluminum alloy (at least 99.999% of aluminum).
The technique of using aluminum in cryogenic (or cryostatic) stabilizers for superconducting conductors is known. Aluminum has the advantage that it has very high electrical and thermal conductivities at low temperatures, together with a low density, a low specific heat and good transparency to different types of radiation. It is usually accepted that the choice of a particular aluminum will be made as a function of its resistivity at the temperature of liquid helium (4.2 K), called the xe2x80x9cresidual resistivityxe2x80x9d that is expressed in terms of the ratio (denoted RRR) between the resistivity at ambient temperature and the residual resistivity. Since the thermal conductivity of aluminum and its alloys at 4.2 K is approximately proportional to the RRR, an aluminum with a high RRR can dissipate heat released during a local loss of superconductivity of one or more superconducting filaments more efficiently.
Since the residual electrical resistivity of aluminum depends very closely on the impurities or the alloying elements contained in it, a very pure aluminum is usually used, namely an aluminum with a purity of at least 5N, in other words a pure aluminum containing not less than 99.999% by weight of aluminum, and particularly poor in elements that could degrade the resistivity (such as Ti, V, Zr, Mn or Fe). The use of aluminum bases with this high purity considerably increases the manufacturing cost of stabilizers and superconducting conductors.
In most applications, cryogenic stabilizers must also be capable of resisting high mechanical tension or compression stresses that are largely caused by electromagnetic forces. These stresses, which may be cyclic, cause deformation of the stabilizer and increase the residual resistance over a period of time, or simply at the time of winding or cooling to the temperature of the liquid helium.
In order to overcome these disadvantages, European patent application EP 500 101 (corresponding to American patent U.S. Pat. No. 5,266,416) describes using a cryogenic stabilizer made of an aluminum alloy with a yield stress at 0.2% elongation equal to at least 40 MPa and an RRR equal to at least 250, at very low temperatures (typically 4.2 K). These characteristics may be obtained by using Zn, Si, Ag, Cu or Ce as elements of addition added to an aluminum base with a purity equal to at least 5N. However these mechanical properties are insufficient for applications such as medical imagery by NMR (Nuclear Magnetic Resonance) For this application, a stabilizing casing that is almost universally used at the moment is made of copper with a residual resistivity to liquid helium of less than 5.4 nxcexa9.cm, and a yield stress measured at ambient temperature exceeding 80 MPa. A disadvantage of this solution is the high copper density that very much increases the weight of the windings and increases the direct and indirect cost (for example by the use of larger coil supports).
French application FR 2 707 419 (corresponding to American patent U.S. Pat. No. 5,573,861) also proposes using a cryogenic stabilizer made of high purity aluminum (from 99.9 to 99.9999% by weight) with a crystalline structure possessing a specific orientation relative to the longitudinal direction of the conductor. However, this preferred orientation of the grain after extrusion requires the use of extremely pure and only very slightly alloyed aluminum, and therefore with mechanical properties that are far too low for many applications.
For the same reasons, French application FR 2 707 420 (corresponding to American patents U.S. Pat. Nos. 5,753,380 and 5,733,389) also proposes to use a cryogenic stabilizer made of a high purity aluminum (from 99.8 to 99.9999% by weight) containing at least one xe2x80x9cactivexe2x80x9d metallic or semi-metallic element, particularly such as B, Ca, Ce, Ga, Y, Yb and Th, most of which would be in solid solution. Published results also show that the mechanical properties are much lower than for copper.
The article by A. Yamamoto et al., xe2x80x9cDesign and Development of the ATLAS Central Solenoid Magnetxe2x80x9d, published in the IEEE Transactions on Applied Superconductivity, pp. 852-855, Vol. 9, No. 2, June 1999, also describes the use of a 5N based aluminum alloy with 1000 ppm by weight of Ni that can be used to make a stabilizer with an RRR of about 600 and a yield stress at 0.2% elongation of 110 MPa at 4.2 K and 81 MPa at 300 K, after cold drawing corresponding to an elongation of 27% and a 21% reduction in the cross-section (1/1.27=0.79). However, cold elongation of the composite formed by the superconducting core and its stabilizing casing of this magnitude is at the limit of what is possible for this type of composite without local necking or rupture.
International application WO 00/17890 also describes a process for the production of superconducting cables comprising an aluminum alloy stabilizer with hardening by precipitation, with a very pure base containing 100 ppm to 25000 ppm of Ni. According to this process, a precipitation heat treatment is applied to the alloy at a temperature between 250xc2x0 C. and 500xc2x0 C., before covering the superconducting core by hot extrusion. Starting from a very pure aluminum base (typically 5N according to examples 1 to 3), it is possible to add elements other than Ni that do not increase the resistivity of aluminum and that are chosen from among Ag, As, Bi, Ca, Cd, Cu, Ga, Mg, Pb, Sc, Si, Sn and Zn. The sum of the alloying elements other than the latter elements (such as Fe) must not exceed 10 ppm.
The mechanical properties of the composite cable comprising a superconducting core and a stabilizing casing made of an aluminum alloy were measured at the temperature of liquid helium (4.2 K). The values corresponding to the aluminum alloy alone (i.e. without the superconducting core) are not given but they are necessarily very much lower than the values for the composite, since the mechanical properties of the superconducting core that is formed of filaments of the Nbxe2x80x94Ti superconductor encased in work hardened copper that forms a large proportion of the cross-section of the cable, are very high. Furthermore, the mechanical properties of the aluminum alloy measured at 4.2 K are very much higher than the properties measured at ambient temperature (300 K) as will be seen in the following sections.
In a presentation entitled xe2x80x9cProgress in ATLAS Central Solenoid Magnetxe2x80x9d presented at the 16th International Conference on Magnet Technology (Florida, 1999), Yamamoto et al. indicated the following values for Al+0.1% Ni alloy (in other words an alloy containing no alloying element other than Ni) treated by precipitation at 430xc2x0 C. and after final reduction of the cross section by 21% cold working:
yield stress of the alloy at 4.2 K=110 MPa,
RRR of the alloy=570;
yield stress of the composite cable at 4.2 K=146 MPa,
which demonstrates the xe2x80x9creinforcementxe2x80x9d effect due to the superconducting core in the composite cable.
In the same conference, K. Wada et al. made a presentation entitled xe2x80x9cDevelopment of High-Strength and High-RRR Aluminum Stabilized Superconductor for the ATLAS Thin Solenoidxe2x80x9d containing results on alloys with 0.05% and 0.1% of Ni formed by the addition of Ni to 5N base aluminum, without any other element of addition. The values that they obtained on laboratory samples (Y.S. denotes the yield stress) are shown in table A.
For industrial cables, the measured values on the stabilizer made of a 0.1% Ni alloy after cold working to reduce its final cross-section by 21% were:
RRR 591 to 593,
0.2% Y.S. at ambient temperature 80 to 83 MPa,
0.2% Y.S. at 4.2 K=110 MPa.
This example shows the large difference between the mechanical properties of the aluminum alloy at ambient temperature and at 4.2 K.
Furthermore, the authors consider that this significant improvement in the performances of the industrial product compared with laboratory samples is due to slightly different and better transformation conditions.
The applicant looked for means of simultaneously obtaining a residual resistivity at 4.2 K less than 5.4 nxcexa9.cm (giving an RRR at 300 K greater than about 500) and reinforced mechanical properties, in other words a yield stress at 0.2% elongation (Y.S. at 0.2% elongation) measured at ambient temperature greater than 75 MPa, and preferably greater than 85 MPa, starting from an aluminum base less expensive than the 5N purity base normally used for this type of product, all at acceptable costs for industrial use (such as series production of superconducting coils designed for use in NMR imagery devices).
The subject of the invention is a superconducting conductor such as a superconducting wire or cable comprising at least a superconducting core and a cryogenic stabilizer composed entirely or partly of a high purity aluminum alloy with the following composition:
200 ppmxe2x89xa6Fe+Nixe2x89xa61500 ppm;
0.20xe2x89xa6Fe/(Fe+Ni)xe2x89xa60.65;
optionally, B less than 100 ppm;
remainder aluminum with purity greater than 99.99% by weight.
Another subject of the invention is a cryogenic stabilizer preform composed of the said high purity aluminum alloy.
The applicant observed that quite surprisingly, the simultaneous presence of iron (Fe) and nickel (Ni) as alloying elements in the claimed proportions could give a significantly better compromise between the RRR and mechanical properties than are possible with binary alloys, using a base aluminum with 4N purity, with lower quantities of alloying elements and more moderate final cold drawing ratios, avoiding risks of failure (or xe2x80x9crupturexe2x80x9d) of the conductor during this drawing operation. For example, it is possible simultaneously to obtain an RRR greater than 600 and a yield stress at ambient temperature higher than 85 MPa with an alloy containing less than 700 ppm of Fe+Ni. In general, iron is not recommended as an element of addition to a pure aluminum base, despite its very low solubility limit in solid aluminum at moderate temperatures (200 to 400xc2x0 C.) since its coefficient of diffusion in aluminum is very low and the holding times (i.e. thermal treatment durations) necessary to approach this solubility limit are incompatible with industrial use (several weeks are necessary).
Another subject of the invention is a process for obtaining a cryogenic stabilizer preform for the manufacture of a superconducting conductor according to the invention, that includes the formation of an initial preform in the unprocessed state and a hot mechanical working operation on the said preform, with a reduction of the cross-section of at least 90% at a temperature preferably between 200xc2x0 C. and 400xc2x0 C. Preferably, the said process for obtaining this preform also comprises a precipitation annealing at a temperature of between 300xc2x0 C. and 400xc2x0 C. for at least 8 hours, and preferably between 320 and 380xc2x0 C., after the said mechanical working operation.
The applicant observed that the use of a preform precipitation annealing after the mechanical working operation rather than before it, can significantly improve the combination of the RRR and the mechanical properties for the final stabilizer. The combination of mechanical working and precipitation annealing can significantly limit the time necessary to reach optimum precipitation of the iron and nickel.
Another subject of the invention is a production process for a superconducting conductor according to the invention that comprises at least one operation to insert at least one superconducting core in a stabilizer starting from a preform according to the invention. Preferably, this process also comprises an operation to reduce the cross-section of the conductor in order to obtain a cross section preferably between 1.10 and 1.33 times the final cross-section of the superconducting conductor followed by a restoration heat treatment and a final cold working operation to reduce the conductor cross-section to the required final cross-section.
The applicant observed that the use of an xe2x80x9cintermediatexe2x80x9d restoration heat treatment, in other words on a superconducting conductor blank with a cross-section slightly greater than the cross-section of the final conductor, can significantly increase the mechanical properties of the final stabilizer without excessively degrading the RRR, thus giving an optimized and adjustable compromise between the RRR and the mechanical properties.
Another subject of the invention is the use of at least one superconducting conductor according to the invention in a magnetic device such as a superconducting magnetic coil.