Patent Application
20080163474
This invention relates generally to the manufacture of superconductors, and more specifically, relates to a method enabling the addition of a hard insulation film to a multifilamentary superconductor that has been soldered to a large amount of copper, thus minimizing production cost and giving increased control of copper content. A hard insulation film gives the minimum insulation layer thickness, thus maximizing conductor volume for use in a electromagnetic coil.
Superconducting wire composites used in electromagnets, such as those used for magnetic resonance imaging (MRIs), consist of a multifilamentary superconductor, copper stabilizing matrix, and an outer electrical insulation layer. The superconducting alloy in the multifilamentary strand is usually niobium titanium. The electrical insulation layer on the superconductor wire is critical to its proper performance in most applications. This is particularly true in the construction of solenoid magnets, where multiple layers of wire are wound together under high winding tensions. Turn-to-turn or layer-to-layer electrical shorts seriously degrade performance, and must be prevented.
For large superconducting magnets, a high volume fraction of copper stabilizer is often needed for optimal magnet performance. In the superconducting wire manufacturing process, the most cost effective way to produce a conductor having a high copper content is to make a composite of multifilamentary superconductor containing a small volume fraction of copper matrix, and then join it with a large amount of additional copper at the final stage of manufacturing. The most common method to integrate the components is soldering. The majority of superconductor composite used for MRI is made by soldering a low copper content superconducting strand into the round groove of a relatively large volume generally rectangular shaped copper channel. This process has been referred to as “wire-in-channel”. These conductors are usually insulated with glass or polymer braid, or tape. After winding the magnet, the entire wire assembly is infiltrated with an epoxy to bond the winding together and to prevent any wire movement during operation. It should be noted that while the composite has a generally rectangular cross-section (for maximum compaction in a magnet coil), the corners are rounded to allow evacuation of gasses and the potting of the coil. Other types of soldered conductors are also known in the prior art.
Another type of insulation, a much thinner (and therefore preferred) organic film insulation (or “varnish”), has been employed in situations where the insulation can be cured at an elevated temperature onto the wire. The modified polyvinyl acetal resin Formvar® is the most common type of organic film insulation used for this purpose. This curing temperature typically exceeds the melting point of traditional Sn—Pb solder alloy, which are standard alloys used because of their excellent soldering characteristics. Due to this curing temperature/melting temperature incompatibility, the employment of organic film insulation has so far excluded wire-in-channel composites, and required so-called “monolith” superconducting composites. Monoliths are single-piece multifilamentary superconducting wires which are manufactured with all of the needed copper from start of manufacturing, and thus never need to be bonded with solder into a copper channel.
The drawbacks of these approaches are as follows:
1. Although the glass or polymer braid is a very effective insulation scheme, it also is relatively thick and therefore consumes a considerable fraction of the cross-section of an insulated superconducting composite and thus coil space. This results in a much larger, heavier, and less efficient system.
2. Although Formvar® and similar thin film insulations save considerable space in the cross-section of an insulated superconducting composite they required (in the prior art) the use of monolith superconducting composites which are not cost effective because a considerable volume of copper is co-processed with the superconductor starting at an early processing stage, which is an inefficient manufacturing process.
Formvar® had not been successfully applied to a soldered composite super-conductor used in MRI magnets, because of the low 181° C. melting point of the eutectic Pb—Sn solder alloy. A number of attempts had been made to modify the Formvar® formulations and application processes themselves, without success. During the application of Formvar®, the conductor is coated with liquid Formvar®, and then the coated composite passes through a heat treatment tower furnace at a temperature significantly higher than the solder melting temperature. This causes the Sn—Pb solder to melt, disrupting the composite bonding, and creates bubbles in the insulating film, preventing proper application and adherence of the insulation.
The present invention overcomes the problems listed above by use of a high melting point solder to bond the superconductor strand to the copper channel. The solder must be able to withstand, without re-melting, the exposure to the elevated temperature and environment in the insulation processing tower. The temperature at which the solder melts is critical to the proper application of the insulating film. Through the use of a high temperature solder alloy, soldered conductor can be successfully coated with film insulation. While the temperature in the processing tower may be twice the melting point of even the high temperature solder, one can limit the transit time of the composite. This process results in the actual temperature of the composite being brought into the range of ˜200° C., i.e. in the range that would melt a eutectic Sn—Pb solder but not a high temperature solder, yet is still suitable for curing the Formvar® layer.
The superconducting composite of choice for soldering into the channel is typically Nb—Ti in a Cu matrix, although a Cu—Ni matrix may be chosen. Typically the strand is round in cross-section but a rectangular cross-section conductor may also be used. Typically the high copper fraction composite is made by bonding a shaped channel to the superconducting core, but a copper strip instead could be wrapped around the superconducting core.
The solder selection critically depends on the amount of intermetallic formation during the superconducting strand-to-copper channel solder bonding operation, and any subsequent thermal treatment the conductor may undergo. An interface layer thickness of 1-3 microns between the solder and copper composite superconductor is ideal for most solder operations to obtain the maximum solder bond strength. An intermetallic layer much thicker could result in a brittle interface.
Solder selection is also critical in that full and even filling of the groove in the channel is necessary to prevent eventual bubble formation in the insulation layer. This is controlled by the wetting and surface tension characteristics of the solder alloy, and the degree of superheat used in the soldering process.
Good thermo-mechanical fatigue properties are required to ensure that neither the varnish nor solder bond rupture during cycling of the device from room temperature to liquid helium temperature, or that the various films themselves do not undergo fatigue causing failure of the conductor or insulation. The coefficient of thermal expansion is important so that the composite does not undergo excessive strain on cooling down to liquid helium temperature (4.2K). It is not necessary to determine the actual coefficient of temperature expansion but only to cycle the coated conductor repeatedly from room temperature to liquid helium temperature and evaluate the adherence of the insulating film.
The success of this process lies in the higher melting point of the solder alloy such that the integrity of the solder bond is not compromised during the insulation film application process, that the solder fills the channel groove, and that the solder strongly bonds to the copper itself. Solder alloys containing Sn, Ag, Sb, and Cu with higher melting temperatures are all suitable for this application. This is even true for alloys with a relatively low liquidus but high solidus temperature. The alloy will not flow in the intermediate liquidus/solidus range due to the high viscosity. Generally solder having a high Pb content melts at higher temperature, but the use of Pb is not environmentally friendly, and its use is being restricted by legislation in many regions. An excellent wettability of solder to the copper base metal is very important since any trapped gas in voids can outgas during the Formvar® curing process and prohibit bonding of insulation to conductor. Tin rich solders tend to have excellent wettability. A few examples of suitable alloys are given in Table 1. It should be noted that any solder employed cannot have a melting point far above 300° C. as this would degrade the superconducting properties of the niobium-titanium alloy in the wire. Also, any solder alloy under consideration should be evaluated for “tin pest”, the transformation of the crystal structure of tin to the brittle diamond cubic structure (gray tin α, a semiconductor). This must be avoided, and has been known to occur in lead-free solders.
Surface roughness of the solder-coated superconducting composite is important only when the adherence of the film is marginal. Providing a rough surface allows the insulating varnish to mechanically grip the surface and provide a better bond. The surface modification can be done by any of the conventional techniques such as abrasion with sandpaper, grit blasting, or embossing. When grit blasting is applied, the composition of the grit and the angle that it is applied is important in providing the best bonding surface, and this depends on the particular varnish and solder combination. The embossing can be done prior to, during or after the soldering operation. Surface roughness can also be obtained by the selection of the dies used for shaping after the soldering and via the quenching operations on the soldering line. It is also possible to remove by electroplating off the surface solder (but not the solder bonding the core to the channel) to have a cleaner surface for insulation application.
The polymer hard insulation coating can be an epoxy, enamel or varnish type insulation with a curing temperature of over 300° C. The polymer can be applied by running the soldered strand through a tank of liquid insulation or by extruding the insulation with the strand. Examples of extrudable polymers include nylon, epoxy, PVC, polyethylene and floro-chloro-carbons.
In the drawings appended hereto:
a) and 4(b) are two cross sectional photomicrographs, showing in 4(a) a product produced by the method of the invention, and in 4(b) a product produced by prior art methodology.
A multifilamentary superconducting composite is created as shown by the example illustrated in
A solder alloy Sn-5 wt % Sb having a melting temperature range of 232-240° C. was used to solder a copper-matrix (copper alloy UNS 10100) superconductor wire consisting of 31 filaments of Nb-47 wt % Ti into a copper channel at 260° C. This soldered composite was cleaned and then coated with Formvar® and passed through a tower furnace operating at 400° C. The wire temperature measured at the exit of the tower was 205° C. The coating and curing steps were repeated several times to build an acceptable layer of Formvar® on the composite surface. The resulting product had excellent properties. As is seen from an example cross-section in
While the present invention has been disclosed in terms of specific embodiments thereof, numerous variations on the invention will now be enabled to those skilled in the art, without however departing from the invention taught herein. Accordingly, the invention is to be broadly construed, and limited only by the scope and spirit of the claims now appended hereto.
