Metal treatment – Process of modifying or maintaining internal physical... – Superconductive metal or alloy
Reexamination Certificate
2000-11-06
2003-01-21
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Superconductive metal or alloy
C148S437000, C420S550000, C428S614000, C428S651000, C428S652000, C428S660000, C428S930000
Reexamination Certificate
active
06508888
ABSTRACT:
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 10
5
A/cm
2
with very low energy losses. The conductors of this type, the most frequently used of which are niobium alloy based conductors such as Nb—Ti, and occasionally Nb—Zr, 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<50 &mgr;m) embedded in a metallic matrix, thus forming a “superconducting core” 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 “stabilizing casing (or cladding)” or “cryogenic stabilizer” or simply “stabilizer”. 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 “residual resistivity” 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 n&OHgr;.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 “active” 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., “Design and Development of the ATLAS Central Solenoid Magnet”, 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 250° C. and 500° 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
Aluminium Pechiney
Dennison, Schultz & Dougherty
Sheehan John
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