Superconductor technology: apparatus – material – process – High temperature devices – systems – apparatus – com- ponents,... – Superconducting wire – tape – cable – or fiber – per se
Reexamination Certificate
1999-02-01
2001-02-13
Beck, Shrive (Department: 1762)
Superconductor technology: apparatus, material, process
High temperature devices, systems, apparatus, com- ponents,...
Superconducting wire, tape, cable, or fiber, per se
C505S704000, C427S062000
Reexamination Certificate
active
06188921
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to oxide superconductor composites having high sheath resistivity.
BACKGROUND OF THE INVENTION
Oxide superconducting wires and cables typically consist of many filaments of superconducting material within a metal matrix which separates the filaments from each other and from the local environment. The matrix is typically a non-superconducting metal. Silver and its alloys represent the matrix metals of choice because the silver is ductile, chemically benign with respect to the oxide superconductor material and relatively transparent to oxygen.
Recent advances in the development of oxide superconductors have demonstrated their utility in applications such as power transmission cables, fault current limiters, utility inductors, motors and generators. For optimal performance, however, many of these applications require matrix resistivities which are much higher than that of pure silver at use (i.e., cryogenic) temperatures. Pure silver has a resistivity at 80 K of about 0.2-0.5 &mgr;&OHgr;-cm, and this value decreases by an order of magnitude as the temperature drops to 4 K. As the term is used herein, resistivity is defined as the bulk resistivity as determined by measuring the current flow in a wire and applying the formula
ρ
=
V
A
I
x
(
1
)
where &rgr; represents resistivity, V represents voltage measured over wire length x, A represents the cross-sectional area of the wire, and I represents current.
There are many technical difficulties associated with the manufacture of an oxide superconductor having a high resistivity sheath. For example, processing steps associated with the formation of the high resistivity sheath may not be compatible with the processing of the oxide superconductor. In particular, under high temperature conditions used to form oxide superconductor phases, many of the sheath components likely to impart high resistivity to the sheath react with and poison the oxide superconductor. In addition, metals which are chemically compatible with the oxide superconductor and the sheath metal typically are highly electrically conductive.
One approach to increasing matrix resistivity consists of the introduction of fine oxide particles into the metal matrix to form a dispersed oxide/matrix metal alloy (“oxide-dispersion strengthened” or ODS silver); however, this requires relatively large volume fractions of the oxide phase in order to sufficiently raise the bulk resistivity of the matrix. Such an approach is limited because an increase in the oxide content of the matrix metal increases its brittleness. Thus, only modest increases in resistivity, e.g., 1-2 &mgr;&OHgr;-cm, are possible while maintaining a matrix with acceptable mechanical properties. In order not to crack in ordinary coiling and winding operations, the matrix should have a tensile fracture strain of at least 0.5%. Fracture strains of higher than 1% are preferred for practical handling of the superconducting composite. In addition, the oxide precipitates used in ODS silver often interact detrimentally with the oxide superconductor and tend to degrade the superconducting properties of the composite.
In another approach, a metal may be alloyed with the sheath metal prior to composite fabrication to raise the resistivity of the sheath. While many metals may be readily alloyed and incorporated into the metal sheath, this process requires that the solute metal be present during high temperature processing of the oxide superconductor. Unfortunately, known low-cost solutes which significantly increase resistivity typically poison the superconductor or themselves are subject to oxidation under these processing conditions.
Shiga et al. in U.S. Pat. No. 5,296,456 disclose alloying a variety of metals with the metal sheath covering the oxide superconductor to obtain high conductivity (low resistivity) and low conductivity (high resistivity) regions in the sheath. As is discussed in greater detail below, most of the metals disclosed by Shiga et al. are not very effective for increasing electrical resistivity. Further, many metals which are highly effective in raising the net resistivity of the matrix are not good candidates for alloying with the metal sheath because they tend to readily form second phases, e.g., intermetallic compounds, within the matrix metal. Intermetallics tend to embrittle the matrix, and do not raise net resistivity sufficiently.
Due to the limitations of prior art processes, a need remains for sheathed oxide superconducting composites which combine suitably high resistivity with good superconducting properties.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an oxide superconductor composite with high sheath resistivity and, in particular, high bulk sheath resistivity.
It is a further object of the invention to provide a method of obtaining a high resistivity sheath without deleterious effect on the electrical properties of the article.
It is yet a further object of the invention to provide a process for making an oxide superconductor composite having a high sheath resistivity, where the process is adaptable to continuous or bulk processing of composites.
It is another object of the invention to identify materials of suitable resistivity and compatibility with the oxide superconductor for use in the high resistivity sheath of the invention.
In one aspect, the invention comprises a composite oxide superconductor article comprising one or more oxide superconducting members in a silver-containing matrix. The matrix has a resistivity of at least 3 &mgr;&OHgr;-cm, preferably of greater than 5 &mgr;&OHgr;-cm, and most preferably in the range of 5-25 &mgr;&OHgr;-cm. The matrix does not comprise any metallic element having a boiling point below 380° C. at one atmosphere pressure. The matrix further has a tensile fracture strain of at least 0.5%, and preferably of at least 1%. The matrix may also have a bend fracture strain of at least 0.5%, or preferably of at least 1%. The matrix may comprise a silver-rich solid solution with one or more other elements, such as gallium, tin, cadmium, zinc, indium, or antimony. The solute element may be selected so as to be able to diffuse into a silver matrix in less than twenty hours at a temperature of less than 550° C., and/or to have a diffusivity in silver at less than 550° C. of at least 10
−12
cm
2
/s. The solute element may represent at least 2 atm % of the matrix composition, preferably at least 4 atm %, more preferably 4-50 atm %, and most preferably 4-18 atm %. The matrix material may be fine grained, with a grain size of less than 50 &mgr;m, or preferably in the range of 0.1-15 &mgr;m. The oxide superconducting member preferably may comprise BSCCO 2223 phase, BSCCO 2212 phase, or a member of the YBCO family, and may have a critical temperature of at least 70 K. The engineering current density of the superconductor article may be at least 3,000 A/cm
2
at a temperature less than or equal to 90 K (as measured by a 1 &mgr;V/cm criterion).
In another aspect, the invention comprises a method of preparing an oxide superconductor. After formation of an article comprising at least one oxide superconductor member in a silver-containing matrix, the method comprises coating the article with a solute capable of forming a silver-rich solid solution, heating the coated article for a time sufficient to allow the solute to diffuse into the matrix to form the solid solution, and cooling the article to a temperature at which substantially no further diffusion occurs. The heating step is carried out at a temperature below the boiling temperature of the solute at one atmosphere pressure, and preferably at a temperature at which the vapor pressure of the solute is less than or equal to 0.1 atmospheres. The solute element may be chosen from a material system which possesses a thermodynamically stable second phase with silver, and cooled sufficiently rapidly to prevent formation of this phase. The article may be coated by any of a number of processes
Christopherson Craig J.
Mason Ralph P.
Otto Alexander
Roberts Peter R.
American Superconductor Corporation
Beck Shrive
Chen Bret
Choate Hall & Stewart
Nugent Elizabeth E.
LandOfFree
Superconducting composite with high sheath resistivity does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Superconducting composite with high sheath resistivity, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Superconducting composite with high sheath resistivity will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2602247