Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1996-06-13
2001-06-05
Arbes, Carl J. (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C174S125100, C505S432000, C505S433000
Reexamination Certificate
active
06240619
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a method of manufacturing superconductor wire or tapes by the powder in tube process and to the material produced thereby. It has long been known that at sufficiently low temperatures certain materials become superconductors of electric currents by virtue of extremely low or the absence of electrical resistance. Substantial research is being directed to finding materials which become superconducting at temperatures above the atmospheric boiling point of liquid nitrogen. To date, these materials have been found to be ceramic and more particularly, oxides of certain combinations, one of which is the bismuth, strontium, calcium, copper oxide system known as the 2223 superconductor or the Bi
2
Sr
2
Ca
2
Cu
3
O
x
or BSCCO.
In the powder in tube method, an oxide precursor of the ultimate superconductor is packed into a silver or silver alloy tube which is used for both mechanical and thermal stabilization. Much of the research is directed toward commercializing methods of manufacture and to improving the current density or J
c
values of the superconducting material.
BRIEF DESCRIPTION OF THE INVENTION
The invention relates to a new method of preparing ceramic powders for introduction into silver or a silver alloy tube which is then swaged and then mechanically and thermally treated to produce the final tape or wire. It has been found that the process of preparing the powder is important as is the process for mechanically and thermally treating the filled tube to obtain high J
c
values.
A variety of methods are taught in the prior art, for instance those taught in the Boeke U.S. Pat. No. 4,980,964 issued Jan. 1, 1991, the disclosure of which is herein incorporated by reference as well as a paper entitled “Microstructural Study of Bi(2223)/Ag Tapes With J
c
(77K, 0T) Values of Up to 3.3×10
4
A cm
−2
” by Yamada et al., see
Superconducting Science Technology
, 4 (1991) 165-171. However, the methods of the prior art do not produce material with sufficiently high J
c
values and have not recognized certain important aspects of processing which are the subject matter of this invention.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide a method of producing a powder for an improved Bi (2223) superconductor which can be formulated into a tape or wire having superior J
c
's;
Another object of the invention is to provide a Bi-2223 tape or wire having improved J
c
's and a method for commercially making same.
The invention consists of certain novel features and a combination of parts hereinafter fully described, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
A number of techniques are used to prepare Pb-doped Bi-2223 precursor powders, as it has been found that Pb present in an amount of 0.4 atom percent improves the characteristics of the Bi 2223 superconductor. They are solid state mixing of ingredients, such as carbonates and/or oxides; sol-gel methods; spray-pyrolysis of cation solution; calcination of the powders in ambient pressure or reduced pressure conditions. In some cases, two powder mixtures are calcined separately, then physically mixed to prepare the desired precursor powders which are then loaded into metallic tubes to prepare flexible conductors by the powder-in-tube technique, see for instance the Boeke U.S. Pat. No. 4,980,964 patent. All of the above techniques have resulted in silver clad Bi-2223 tapes with a range of transport J
c
's Often, very long milling/mixing is done at early and intermediate stages of powder processing. Surprisingly, we have found that this aggressive milling actually degrades the superconducting properties of the conductor, and we have found that by milling dry without any grinding media for short periods of time not to exceed a total of about 30 minutes, improved powders are fabricated.
Appropriate amounts of Bi
2
O
3
, PbO, SrCO
3
, CaCO
3
, and CuO were dry mixed in a SPEX mill for about ten minutes. The pre-mixed oxides and carbonates were heat treated in air at 830° C. to about 842° C. for about 24 to about 48 hours in divided treatments of from about 12 to about 24 hours. An intermittent milling in the SPEX mill was done for about 5 minutes. The calcined powders/pellets were then milled for one last time. It is critical in this invention to use a total milling time of less than about 30 minutes and preferably no more than about 20 minutes in order to prevent severe damage of the structure, size and morphology of the formed phases. The milled powders were characterized by XRD, DTA, SEM, and particle size analyzers and found to be between about 4 and about 5 microns in average diameter. Powders were thereafter packed in silver or silver alloy tubes and processed into flexible conductors. Transport J
c
's of about 30,000 A/cm
2
were obtained in short, rolled tapes when heat treated for less than 100 hours in air at 830-850° C. J
c
's as high as 50,000 A/cm
2
were obtained in rolled and pressed tapes. Long lengths of tapes with good properties were fabricated using the powders prepared as described above with alternate thermal and mechanical treating of the filled tubes as will hereinafter be described.
Previous to the present invention, ceramic powder has been packed in a silver tube and mechanically swaged and drawn into a fine, approximately 2 millimeter diameter wire. Thereafter the wire was rolled, that is passed between two rotating rollers, to produce flexible flat conductor tapes which were then sintered to make them superconducting. Prolonged, that is in the neighborhood of 300-350 hours sintering at temperatures exceeding 800° C. has been necessary to form the 2223 phase (T
c
about 110 K).
REFERENCES:
patent: 4980964 (1991-01-01), Boeke
patent: 5081072 (1992-01-01), Hosokawa et al.
patent: 5204316 (1993-04-01), Argeot et al.
patent: 5288699 (1994-02-01), Sato et al.
patent: 5807808 (1998-09-01), Hikata et al.
patent: 5877125 (1999-03-01), Sato et al.
Microstructural Study of Bi (2223) /Ag Tapes with Jc(77 K, OT) Values up to 3.3 ×104A cm-2Supercond.Sci Tech. 4 (1991) 165-171.
Reaction Mechanism of High-TcPhase (Tc=110K) Formation In the Bi-Sr-Ca-Cu-) Superconductive System, Jap. Journal of Applied Physics, No. 11, Nov. 1989, pp. L1918-1921.
Balachandran Uthamalingam
Haldar Pradeep
Motowidlo Leszek
Poeppel Roger
Arbes Carl J.
Emrich & -Dithmar
The University of Chicago
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