Self insulating substrate tape

Details Self insulating substrate tape Self insulating substrate tape

2000-04-28

2002-05-14

Jones, Deborah (Department: 1775)

Stock material or miscellaneous articles

Composite

Of metal

C505S230000, C505S231000, C505S236000

Reexamination Certificate

active

06387525

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to self insulating substrate tapes, superconducting composite tapes and methods of manufacturing the tapes. In particular, the invention relates to the manufacture and use of self insulating substrate tapes to eliminate the problem of insulation for the construction of high temperature superconductor magnets, and to enable application of “wind and react” processes with insulation away from the high temperature superconductor phase.
BACKGROUND OF THE INVENTION
In the manufacture of high temperature superconductor magnets, it is often desirable to use composite tapes employing high temperature compatible insulating materials. Specifically, pancake coils are wound from high temperature superconductors in which a “wind and react” approach is used due to the brittle nature of the materials. As a result, it becomes necessary to use high temperature compatible electrically insulating materials for turn-to-turn insulation in any magnet built from the high temperature superconductors. In the past, ceramic based papers or tapes have been used for this purpose. However, a disadvantage with this type of approach is that such insulation, of course, takes up valuable winding space, and currently available materials have a thickness of about 0.1 mm or greater. In addition, insulating materials are in direct contact with the superconducting materials when magnets are made from dip coated high temperature superconducting (HTS) tapes. Almost all of the insulating materials react with the HTS materials (especially BSCO) during the final partial melt heat treatment process.
The wind and react method of the prior art 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 in 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. The superconducting coils, like most coils, are formed by winding an electrically 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 performance of the conductor is increased and large magnetic fields can be generated by the coil.
As is well known, certain ceramic materials exhibit superconducting behavior at low temperatures, such as the compound Bi
2
Sr
2
Ca
n
Cu
n+
O
2n+4
where “n” can be either 0, 1, and 2. One material, Bi
2
Sr
2
Ca
2
CU
3
O
10
(BSCCO(2223)), has performed particularly well in device applications because superconductivity and corresponding high current densities are achieved at relatively high temperatures, T
c
≡115° K. Other oxide superconductors include general Cu—O-based ceramic superconductors, such as members of the rare-earth-copper-oxide family (i.e., YBCO), thallium-bariumcalcium-copper-oxide family (i.e., TBCCO), the mercury-barium-calcium-copper-oxide family (i.e., HgBCCO), and BSCCO compounds containing lead (i.e., Bi, Pb)
2
Sr
2
Ca
2
CU
3
O
10
).
Electrically insulating materials surrounding the conductors are used to prevent electrical short circuits within the winding of a coil. From a design point of view, the electrical insulation layer must be able to withstand large electric fields (as high as 4×10
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, in the past, it was desired to make electrical insulation layers as thin as possible (typically 50-150 &mgr;m) so that the amount of superconducting material in the coil can be maximized.
By using existing conducting and electrically 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. 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.
Thus, instead of the “wind and react” process previously discussed, one prior 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 a insulated composite conductor which is then wound into a coil. 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 a 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 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 coil fabrication process, both bend strain and handling, can result in micro-crack formation in the ceramic grains, severely degrading the electrical properties of the composite conductor.
Alternatively, in the “wind and react” process previously discussed, 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 coils fabricated using the “wind and react” approach with composite conductors have limitations related to winding density and current carrying capability. 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. Moreover, currently available insulation takes up a lot of winding space limiting the number of turns achievable, and further limiting the teslas at the highest field achievable in the bore of a magnet.
In accordance with the invention, the problems of the prior art “wind and react” approach are avoided by use and application of insulation which is significantly thinner. As such, the winding space saved can be used to result in more amp turns, allowing for an increase in teslas at the highest field achievable in the bore of a magnet.
SUMMARY OF THE INVENTION
In accordance with one aspect of the invention, there is provided a self-insulating substrate tape which includes a first and a second conducting tape, both having facing sides which face each other. An insulating layer secures the facing side of the first conducting tape to the facing side of the second conducting tape by adhering to the facing sides of the two tapes. The insulating layer is of a material capable of withstanding temperatures sufficiently high to make superconducting materials from refractory materials, or precur

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