The wind-and-react method involves winding the precursor to a superconducting material around a mandrel in order to form a coil, and then processing the coil with high temperatures and an oxidizing environment. The processing method results in the conversion of the precursor material to a desired superconducting material, and in the healing of micro-cracks formed in the precursor during the winding process, thus optimizing the electrical properties of the coil.
Superconducting magnetic coils, like most magnetic coils, are formed by wrapping an insulated conducting material around a mandrel defining the shape of the coil. When the temperature of the coil is sufficiently low that the conductor can exist in a superconducting state, the current-carrying performance of the conductor is markedly increased and large magnetic fields can be generated by the coil.
Certain ceramic materials exhibit superconducting behavior at low temperatures, such as the compound Bi.sub.2 Sr.sub.2 Ca.sub.n Cu.sub.n O.sub.2n+4 where n can be either 1, 2, or 3. One material, Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10 (BSCCO (2223)), performs particularly well in device applications because superconductivity and corresponding high current densities are achieved at relatively high temperatures (T.sub.c =115 K.). Other oxide superconductors include general Cu-O-based ceramic superconductors, such as members of the rare-earth-copper-oxide family (ie., YBCO), the thallium-barium-calcium-copper-oxide family (ie., TBCCO), the mercury-barium-calcium-copper-oxide barium-calcium-copper-oxide family (ie., HgBCCO), and BSCCO compounds containing lead (ie.,(Bi,Pb).sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10).
Insulating materials surrounding the conductor are used to prevent electrical short circuits within the winding of a coil. From a design point of view, the insulation layer must be able to withstand large electric fields (as high as 4.times.10.sup.5 V/cm in some cases) without suffering dielectric breakdown, a phenomenon that leads to electrical cross-talk between neighboring conductors. At the same time insulation layers must be as thin as possible (typically less than 50-150 .mu.m) so that the amount of superconducting material in the coil can be maximized.
Using existing conducting and insulating materials, the maximum magnetic field generated by a superconducting coil is ultimately determined by the winding density (defined as the percentage of the volume of the coil occupied by the conductor) and the coil geometry. However, the large tensional forces necessary for high winding densities can leave conductors in highly stressed and/or strained states. The bend strain of a conductor, equal to half the thickness of the conductor divided by the radius of the bend, is often used to quantify the amount of strain imposed on the conductor through coil formation. Many superconducting magnet applications involving high-density conductor windings require conductor bend strains on the order of 0.2%, and in some cases much higher. The critical strain of a conductor is defined as the amount of strain the material can support before experiencing a dramatic decrease in electrical performance. The critical strain value is highly dependent on the formation process used to fabricate the conductor, and is typically between 0.05%-1.0%, depending on the process used. If the bend strain exceeds the critical strain of a conductor, the current-carrying capability of the conductor, and hence the maximum magnetic field generated by a coil, will be decreased significantly.
One approach to manufacturing high-performance conductors having desirable mechanical properties involves starting with a precursor to a high temperature superconducting material, typically a ceramic oxide in a powder form. Despite relatively poor mechanical properties and more complex manufacturing processes which requires high temperatures and an oxidizing environment, high temperature superconducting materials are preferred to low temperature superconducting materials for certain applications because they operate at higher ambient temperatures. Oxide powders are packed into a silver tube (chosen because of malleability, inertness, and high electrical conductivity) which is then deformed and reduced in size using standard metallurgical techniques: extrusion, swaging, and drawing are used for axisymmetric reductions resulting in the formation of rods and wires, while rolling and pressing are used for aspected reductions resulting in the formation of tapes and sheets (Sandhage et al., "Critical Issues in the OPIT Processing of High-Ic BSCCO Superconductors", Journal of Metals 3, 21, 1991).
Following the deformation process, heating and cooling results in the growth and evolution of individual crystalline oxide superconductor grains in the conductor which typically take on a rectangular platelet shape. Further deformation results in a collective alignment of the crystallographic axes of the grains. An iterative heating/deforming schedule unique to the ceramic oxide forms of superconductors is typically carried out until the desired grain size, alignment, and density of the superconducting state are achieved.
Conductors having a single oxide core, classified as mono-filament composite conductors, result from the iterative schedule described above and can have critical strain values as high as 0.1%. Mono-filament composite conductors can be transformed into multi-filament composite conductors using a rebundling fabrication operation involving further reduction in size of the mono-filament composite conductors, and finally concatenation of individual conductors to form a single conductor. Typically, the evolution of cracks in response to bend strains is more likely in mono-filament composite conductors than in multi-filament composite conductors, where critical strain values increase with the number of filaments in the conductor, and can be greater than 1.0%. Other limitations of mono-filament composite conductors include decreases in crack healing ability and oxygen access to the conductor during processing. Furthermore, because mono-filament composite conductors have only a single superconducting region, it is difficult to control the conductor size and shape, and mechanically robust conductors can not be easily fabricated (K. Osamure, et al., Adv. Cryo. Eng., ICMC Supplemental, 38, 875, 1992). Thus, multi-filament composite conductors have desirable mechanical properties, and can be used in coils requiring high winding densities.
One method used to fabricate coils with multi- and mono-filament composite conductors is the react-and-wind process. This method first involves the formation of an insulated composite conductor which is then wound into a coil. In this method, a precursor to a composite conductor is fabricated and placed in a linear geometry, or wrapped loosely around a coil, and placed in a furnace for processing. The precursor can therefore be surrounded by an oxidizing environment during processing, which is necessary for conversion to the desired superconducting state. In the react-and-wind processing method, insulation can be applied after the composite conductor is processed, and materials issues such as the oxygen permeability and thermal decomposition of the insulating layer do not need to be addressed.
In the react-and-wind process, the coil-formation step can, however, result in straining composite conductors in excess of the critical strain value of the conducting filaments. Strain introduced to the conducting portion of the wire during the deformation process can result in micro-crack formation in the ceramic grains, severely degrading the electrical properties of the composite conductor.
Another method used to fabricate magnetic coils with mono-filament composite conductors is the wind-and-react method. In this method, the eventual conducting material is typically considered to be a "precursor" until after the final heat treating and oxidation step. Unlike the react-and-wind process, the wind-and-react method as applied to high temperature superconductors requires that the precursor be insulated before coil formation, and entails winding the coil immediately prior to a final heat treating and oxidation step in the fabrication process. This final step results in the repair of micro-cracks incurred during winding, and is used to optimize the superconducting properties of the conductor. However, these results are significantly more difficult to achieve for a coil geometry than for the individual wires which are heat treated and oxidized in the react-and-wind process.
Due to the mechanical properties of the conducting material, superconducting magnetic coils fabricated using the wind-and-react approach with mono-filamentary composite conductors have limitations related to winding density and current-carrying ability. Although the wind-and-react process may repair strain-induced damage to the superconducting material incurred during winding, the coils produced are not mechanically robust, and thermal strain resulting from cool down cycles can degrade the coil performance over time.
A feature of the invention is a wind-and-react process which is used to manufacture superconducting magnetic coils with multi-filament composite conductors. This processing method can be used to manufacture several variations of coils types, all of which are discussed below.
An advantage of the invention is ability to produce mechanically robust coils requiring high winding densities, without significantly degrading the superconducting properties of the multi-filament composite conductors used to form the coils.