Electricity: conductors and insulators – Conduits – cables or conductors – Superconductors
Reexamination Certificate
1999-07-23
2002-09-03
Cuneo, Kamand (Department: 2841)
Electricity: conductors and insulators
Conduits, cables or conductors
Superconductors
C505S885000, C505S237000
Reexamination Certificate
active
06444917
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to composite ceramic superconducting tapes and structures. Tapes including ceramics such as YBa
2
Cu
3
O
7−&dgr;
(YBCO 123), (Pb,Bi)
2
Sr
2
Ca
2
Cu
3
O (BSCCO 2223), and (Pb,Bi)
2
Sr
2
Ca
1
Cu
2
O (BSCCO 2212) can become superconducting at relatively high temperatures, e.g., liquid nitrogen temperatures, and are ideal for carrying electrical current over large distances. The composite superconducting tape usually includes superconducting portions of ceramic material within a conductive metal matrix (e.g., BSCCO filaments within a noble metal matrix) or superconducting portions coated on a conductor (e.g., one or more layers of YBCO or BSCCO supported on a conducting substrate). A support structure such as a metallic tape can be laminated to the composite superconducting tape to provide it with mechanical strength and resilience. During operation the superconducting article (e.g., superconducting tape and support structure) is immersed in fluid cryogen (e.g., liquid nitrogen, liquid helium, or supercritical helium) for an extended period of time. During this time fluid cryogen may infiltrate into the superconducting ceramic material. For example, the infiltration may occur when a portion of the ceramic material, which can be porous, is directly exposed to the cryogen, or when one or more surface defects in the composite material provide a channel between the cryogen and the ceramic material.
Such infiltration can be a serious problem because upon warming the article, the cryogen can quickly vaporize, causing pressure to build up within the article. For example, the density of liquid nitrogen at 77 K is seven hundred times greater than that of nitrogen gas at ambient conditions. The pressure build up within the article can create a large physical defect in the superconducting ceramic and significantly degrade its superconducting properties (e.g., transport properties), thus blocking the desired electrical performance of the article. Because the defect introduces the appearance of a bulge or balloon on the exterior of the superconducting article, this problem is referred to as the “balloon” problem.
SUMMARY OF THE INVENTION
Applicants have discovered that even where composite ceramic superconducting tapes have a metal coating applied to their surface, cryogen may still infiltrate into the ceramic material through porous or microporous defects in the coating and form balloons. Such defects can be difficult to locate prior to balloon formation because they can be exceedingly small and rare along the length of the tape. Thus, a coated tape vulnerable to balloon formation may, to the eye, look perfect prior to cryogenic thermal cycling. Moreover, the likelihood of cryogen infiltration through such defects increases when the fluid cryogen is under pressurized conditions, e.g., up to about 1 to 33 bars, and when the superconducting article is exposed to the fluid cryogen for long periods of time, e.g., several weeks, several years, or many years. Such conditions are typical for superconductive cabling applications.
Applicants have recognized that a surface defect in the composite ceramic superconducting tape can cause an overlapping defect in an applied metal coating. For example, a surface defect may prevent solder from wetting over the defect, thereby causing a microporous defect to form in an applied solder coating. The overlapping defects can provide a channel through which cryogen can infiltrate into the ceramic material. In tapes formed of BSSCO filaments in a noble metal matrix, for example, such surface defects can result from oxides released from the BSSCO powder during the powder-in-tube fabrication of the composite ceramic tape.
More generally, defects in the composite ceramic tape and metal coating can result during handling and applications manufacturing. Microporous defects in the metal coating can also be caused by shrinkage voids during cooling of the metal coating when the corresponding dimensions of the metal coating are too large (e.g., larger than 0.080″). Statistically, some defects in the composite ceramic tape may overlap with defects in the metal coating to form one or more channels through which cryogen can infiltrate into the ceramic material.
Embodiments of the present invention substantially prevent such cryogen infiltration by completely encapsulating the superconducting tape along its length within a sealing structure. The sealing structure hermetically seals the entire surface along the length of the superconducting tape (e.g., the top, bottom, and sides of the tape) from the cryogen bath to prevent cryogen infiltration. For example, in one embodiment, a first stainless steel tape is laminated to the top of the composite ceramic tape and a second stainless steel tape is laminated to the bottom of the composite ceramic tape to sandwich the composite ceramic tape. The stainless steel tapes are selected to be wider than the composite ceramic tape so that they overhang the sides of the composite ceramic tape. Solder fillets can then seal the sides of the ceramic tape because the solder can wet to the overhanging portions of the metallic tapes and form a continuous surface covering the sides of the composite ceramic tape. The combination of the metallic tapes and the solder fillets thus forms the sealing structure.
The sealing structure can generally provide mechanical reinforcement to the composite ceramic tape, e.g., by including one or more metallic laminates. Alternatively, the sealing structure can be separate from such support structure, e.g., it can encapsulate a ceramic tape already having one or more metallic laminates bonded thereto for providing mechanical reinforcement.
In general, in one aspect, the invention features a superconducting ceramic conductor for use in a preselected fluid cryogen including: a composite ceramic superconducting wire having an outer surface along its length; and a sealing structure hermetically surrounding the outer surface to prevent the cryogen from infiltrating into the wire and degrading its superconducting properties.
The superconductor can include any of the following features. The wire and surrounding sealing structure can be greater than 50 meters long. The wire can include a metallic matrix supporting a plurality of superconducting ceramic filaments. Alternatively, the wire can include at least one superconducting ceramic layer and at least one metallic substrate supporting the at least one superconducting ceramic layer. The sealing structure can be metallic. The sealing structure can prevent the cryogen from infiltrating into the wire through the outer surface under pressurized conditions, for example, the pressurized conditions can exceed about 10 atm and the fluid cryogen can be liquid nitrogen.
Furthermore, the wire can be a composite ceramic superconducting tape having a top face, a bottom face, and side faces, and wherein the outer surface is the top, bottom, and side faces. For example, the sealing structure can include: a first metallic tape laminated to the top face of the composite tape; a second metallic tape laminated to the bottom face of the composite tape, the first and second metallic tapes extending beyond the side faces of the composite tape; and non-porous solder fillets adjacent the side faces of the composite tape filling space between the metallic tapes. The metallic tapes can include stainless steel, Cu—Be alloy, aluminum, copper, nickel, or Cu—Ni alloy. The first and second metallic tapes can be at least 5% wider than the composite tape to extend beyond the side faces of the composite tape. The composite tape and the sealing structure can define an aspect ratio for the conductor that is greater than about five. Alternatively, the sealing structure can include: a first metallic tape laminated to the top face of the composite tape and having portions extending beyond the side faces of the composite tape; and a second metallic tape laminated to the bottom face of the composite tape and having portions extending beyond the side fa
Buczek David M.
Caracino Paola
Daly Derek Patrick
Fleshler Steven
Harnois Richard E.
American Superconductor Corporation
Cuneo Kamand
Fish & Richardson P.C.
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