Metal treatment – Process of modifying or maintaining internal physical... – Superconductive metal or alloy
Reexamination Certificate
2000-06-02
2002-04-16
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Superconductive metal or alloy
C505S918000, C505S919000
Reexamination Certificate
active
06372054
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an ultrafine multifilamentary Nb
3
Al superconducting wire improved in characteristics by adding Ge or Si, and provides a process for producing an ultrafine multifilamentary superconducting Nb/Al compound wire into which Ge or Si is added.
2. Description of the Related Art
It has long been known that Nb
3
(Al,Ge) or Nb
3
(Al,Si) produced by arc melting yield a superconducting critical temperature T
C
and a superconducting critical magnetic field H
C2
far higher than those of Nb
3
Al.
In utilizing a production method as shown schematically in
FIG. 13
, recently proposed is a production process comprising subjecting a composite wire material of Nb and Al to a rapid hating and quenching treatment to thereby form a solid solution phase supersaturated with a bcc Nb-25 at. % Al alloy (the term “at. %” as referred herein signifies “% by atomic”) in the composite wire material, and then subjecting the resulting product to heat treatment in the temperature range of from 650 to 900° C. In this manner, it is possible to deposition superfine crystals of Nb
3
Al precipitates near to stoichiometric composition. Since the resulting product enables an extremely high critical current density J
C
, this method is attracting attention in producing practically useful wire material.
Since the highest magnetic field generated by using a metallic superconducting wire material on record is 21.7 T, the upper limit attainable in generated magnetic field using a practical Nb
3
Al wire material is presumably about 23.5 T.
An oxide superconducting wire material enables the generation of a higher magnetic field, however, it disadvantageously requires a production cost about 100 times as large as necessary for producing a metallic superconducting wire material.
Concerning the method for producing a metallic superconducting wire material, on the other hand, there is known a production method comprising preparing an ultrafine multifilamentary composite wire comprising an Al—Ge alloy core embedded in a Nb matrix, and subjecting the wire to a rapid heating and quenching treatment comprising rapidly heating the wire to a temperature of ca. 2,000° C. by resistive heating and continuously introducing it to a molten metal. In this manner, provided that the addition of Ge into Al is as small as 2% or less, a supersaturated solid solution is generated in the resulting composite wire, and by applying a post precipitation heat treatment thereto, the resulting product enables a high J
C
, but there is no considerable increase in T
C
and H
C2
.
On the other hand, if the amount of added Ge increases, the supersaturated solid solution becomes unstable, and the production of a supersaturated solid solution becomes available only under super quenching conditions.
However, the production process above is practically unfeasible because continuous super quenching is extremely difficult in an industrial production.
SUMMARY OF THE INVENTION
In the light of the aforementioned circumstances, in accordance with an aspect the present invention, there is provided a process for producing an ultrafine multifilamentary superconducting Nb
3
(Al,Ge) wire comprising: preparing a composite core material comprising an Al—(2-30)at. % Ge alloy (where at. % represents % by atomic) 1 &mgr;m or less in thickness uniformly incorporated into a Nb matrix at a volume ratio in a range of 1:2.5 to 1:3.5 and forming a composite therewith; fabricating a composite wire having an ultrafine multifilamentary structure by embedding a plurality of the resulting composite core materials in a cylindrical matrix material containing Nb; forming a A15-phase filament having a lower order in crystallinity inside the composite wire having the ultrafine multifilamentary structure by a rapid heating and quenching treatment comprising rapidly heating the composite wire having the ultrafine multifilamentary structure to a temperature of 1,700° C. or higher in 2 seconds, followed by continuously introducing it into a molten metal; coating the composite wire having the ultrafine multifilamentary structure in the state above with copper (Cu) which functions as a superconductivity stabilizing material; and applying a post heat treatment in the temperature range of from 650 to 900° C. to the resulting product to recover the degree of crystallinity of the Nb
3
(Al,Ge) in the A15 phase compound (claim
1
).
According to another aspect of the present invention, there is provided a process for producing an ultrafine multifilamentary superconducting Nb
3
(Al,Si) wire, comprising the same process steps as claimed in claim
1
, except for using an Al—(2-20)at. % Si alloy (where at. % represents % by atomic) as the starting material in the place of the Al—(2-30)at. % Ge alloy (claim
2
).
In accordance with still other aspects of the present invention, there are provided processes for producing an ultrafine multifilamentary superconducting Nb
3
(Al,Ge) wire or Nb
3
(Al,Si) wire as claimed in claim
1
or claim
2
, wherein instead of coating the composite wire with copper (Cu) prior to the additional heat treatment, the step of Cu coating for stabilizing superconductivity is performed after the additional heat treatment (claim
3
); a process for producing an ultrafine multifilamentary superconducting Nb
3
(Al,Ge) wire or Nb
3
(Al,Si) wire as claimed in claim
1
or claim
2
, wherein copper (Cu) is surrounded beforehand with a diffusion barrier material and then embedded into the matrix material, followed by wire drawing to fabricate the composite wire having the ultrafine multifilamentary structure, and subjecting the resulting composite wire to the rapid heating and quenching treatment (claim
4
); or a process for producing an ultrafine multifilamentary superconducting Nb
3
(Al,Ge) wire or Nb
3
(Al,Si) wire as claimed in one of claims
1
,
2
,
3
, or
4
, wherein an alloy expressed by Al—(2-30) at. % Ge—(0-7)at. % X or Al—(2 -20) at. % Si—(0-7)at. % X (where at. % represents % by atomic), where X represents at least one element selected from the group consisting of Mg, Zn, Li, Ag, Cu, and B, is used as the starting material in the place of the Al—(2-30)at. % Ge alloy or the Al—(2-20)at. % Si alloy (claim
5
).
The present invention has been accomplished based on the following findings of the present invention.
On carrying out the rapid heating and quenching treatment on a conventional Al—Ge core material embedded in Nb matrix as described above, filaments of A15 phase compounds having a low degree of ordering in crystallinity are formed, but by subjecting them to a heat treatment in the temperature range of from 650 to 900° C., the long range ordering is recovered to yield a T
C
of 19.4 K and a H
C2
(4.2 K) of 39.5 T. However, this product yields a J
C
(4.2 K) of 130 A/mm
2
at 15 T, a value somewhat inferior as compared with that of a practical wire material. Still, a decrease in J
C
with increasing magnetic field for the material above of the product is small, and the J
C
(4.2 K) under a magnetic field of 25 T is about 100 A/mm
2
, i.e., the highest among the metallic superconducting wire materials. However, a practical wire was still unfeasible because a J
C
(4.2 K) of at least 150 A/mm
2
was necessary for use as a practical superconducting magnet at the targeted magnetic field.
The present inventors successfully fabricated a composite ultrafine multifilamentary wire comprising an Al—Ge alloy core reduced in diameter from the conventional 1.5 &mgr;m to 0.3 &mgr;m, and, on applying a rapid heating and quenching treatment to the composite wire, a J
C
(4.2 K) of over 250 A/mm
2
was finally attained under 21 T, and a value of 150 A/mm
2
was achieved at 25 T.
From the fact above, it was understood that, presumably, by optimally designing a superconducting magnet using the Nb
3
(Al,Ge) ultrafine multifilamentary wire material above, it is possible to generate a magnetic field of 25 T under the operation at 4.2 K and 27 T under the operation at 1.8 K (that is, cooling from 4.2 K to 1.8 K
Iijima Yasuo
Inoue Kiyoshi
Kikuchi Akihiro
Japan as represented by Director General of National Research In
Wyszomierski George
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