Superconductor technology: apparatus – material – process – High temperature – per se
Reexamination Certificate
1999-11-29
2001-07-03
Gupta, Yogendra (Department: 1751)
Superconductor technology: apparatus, material, process
High temperature , per se
C505S450000, C505S451000, C505S490000, C252S521100, C252S519100, C252S519150, C252S518100
Reexamination Certificate
active
06255255
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an approximately 90K-class (critical temperature Tc=approx. 90 degrees Kelvin) oxide superconducting material and a method of producing the same.
2. Description of the Related Art
The conventional method of raising the critical current density (Jc) of oxide superconducting materials is to establish pinning sites in REBa
2
Cu
3
O
7−x
(superconducting phase; abbreviated: 123 phase) by dispersion of about 1-micron particles of RE
2
BaCuO
5
(211) or RE
4
Ba
2
Cu
2
O
10
(422). (RE in the foregoing notations designates one or a combination of two or more rare earth elements including Y.) It is known that 211 can be refined to around 1 micron by addition of Pt or Rh and that 422 can be refined to about the same level by addition of Ce. It is also known that a portion of the added Ce forms fine CeBaO
3
of a particle size of 1 micron or less that disperses into the superconducting phase.
JP-A-(unexamined published Japanese patent application)4-16511 teaches a structure having BaMO
3
(M representing Zr, Sn, Ce or Ti) finely dispersed together with 211 in stacked plate-like 123 phase. Materials having this structure are produced in a temperature gradient. JPA-5-279033, JP-A-5-286719, JP-A-6-1609 teach methods of producing superconductors in which 211 is finely dispersed by adding cerium oxide. They also teach structures having cerium oxide finely dispersed in 123 phase together with 211 and a noble metal such as silver. JP-A-5-58626 describes a superconductor having 211 finely dispersed in 123 phase containing Ce and method of producing the superconductor and teaches that addition of Ce enables fine dispersion of 211 by suppressing its agglomeration and enlargement.
From the viewpoint of achieving high Jc, it is preferable to introduce a large amount of fine non-superconducting particles and other pinning centers in the 123 phase. While 211 and 422 are currently the main non-superconducting phases used to produce pinning sites, the CeBaO
3
, SnBaO etc. taught in the foregoing literature also contribute to the pinning site formation, although at a lower rate than 211. The development of still other new substances capable of forming pinning sites is a challenge that demands attention.
SUMMARY OF THE INVENTION
That CeBaO
3
, a Ce—Ba—O system compound, is capable of forming pinning sites was thus known in the prior art. Now, the present inventors have discovered that particles composed of Ce, Ba, Cu and O (hereinafter called Ce—Ba—Cu—O system particles) constitute a new class of pinning site forming materials. They also discovered that when the particles are given a composition of Ce
x
Ba
y
Cu
z
O
w
, their effect is enhanced in the composition range of 2.4≦x≦3.6, 2.4≦y≦3.6, 0.8≦z≦1.2, 8≦w≦12. They further discovered that when the particle composition includes Ce
3
Ba
3
CuO
10
, the particles particularly enhance Jc by dispersing into the superconducting phase as submicron-diameter particles to become still more effective pinning sites. They additionally found that the Ce—Ba—Cu—O system particles coexist with 211 and 422 in the 123 phase.
When the Ce
x
Ba
y
Cu
z
O
w
content is too large, critical temperature and Jc decrease rather than increase because a portion of the Ce disperses in the 123 phase. The content is therefore preferably 15 mass % or less, more preferably 1-10 mass %.
The inventors also discovered that the Ce—Ba—Cu—O system particles can coexist with 211, 422 and 123 phases containing Pt, Rh or Ag. When the Pt content is less than 0.05 mass %, 211 is not refined, and when it is greater than 2.0 mass %, the addition efficacy decreases because the excess Pt forms many extraneous phases. The Pt content is therefore preferably 0.05-2.0 mass %, more preferably 0.3-0.6 mass %. When the Rh content is less than 0.01 mass %, 211 is not refined, and when it is greater than 1.0 mass %, the addition efficacy decreases because the excess Rh forms many extraneous phases. The Rh content is therefore preferably 0.01-1.0 mass %, more preferably 0.1-0.3 mass %. When Ag is added, about 0.1 mm-diameter Ag particles precipitate into the 123 phase to enhance the mechanical strength. When the amount of Ag added is less than 5 mass %, Ag particles do not precipitate, and when it is greater than 20 mass %, a tendency to inhibit crystal growth of the 123 phase arises. The amount of Ag addition is therefore limited to 5-20 mass %, more preferably 10-15 mass %.
The inventors discovered a method of producing a material containing effectively dispersed Ce
x
Ba
y
Cu
z
O
w
by mixing Ce
x
Ba
y
Cu
z
O
w
with a starting material powder containing RE (RE designating one or a combination of two or more rare earth elements including Y), Ba, Cu and O. When a powder compact formed by compacting the mixed powder is placed in an oxidizing atmosphere in the temperature range of 850-1,250° C., it assumes a semi-molten state in which 211 or 442 coexists with a liquid phase. When the compact is heat-treated in the atmosphere, it assumes a semi-molten state at 850-1,250° C. The 123 phase grows while incorporating Ce
x
Ba
y
Cu
z
O
w
to provide a 123 phase containing pinning sites of Ce
x
Ba
y
Cu
z
O
w
and 211 or 422 by carrying out slow cooling from the above described semi-molten state to a temperature range at which a 123 phase grows as a crystal, or slow cooling or maintaining an isothermal temperature in a temperature range for growing a 123 phase to crystallize.
Preferably, slow cooling or maintaining an isothermal temperature is carried out from a starting temperature for growing a 123 phase crystallization (Tp: peritectic temperature) to Tp-30° C. This method can provide a material containing effectively dispersed Ce
x
Ba
y
Cu
O
w
because the Ce
x
Ba
y
Cu
z
O
w
remains in the material without decomposing during processing.
A material containing dispersed Ce
3
Ba
3
CuO
10
can be obtained by adding Ce
3
Ba
3
CuO
10
to a starting material powder containing RE, BA, Cu and O.
REFERENCES:
patent: 5308799 (1994-05-01), Morita et al.
patent: 5508253 (1996-04-01), Morita et al.
patent: 5786304 (1998-06-01), Kimura et al .
patent: 5849668 (1998-12-01), Yamaguchi et al.
patent: 5968878 (1999-10-01), Kojo et al.
patent: 4-16511 (1992-01-01), None
patent: 5-58626 (1993-03-01), None
patent: 5-279033 (1993-10-01), None
patent: 5-286719 (1993-11-01), None
patent: 6-1609 (1994-01-01), None
Morita Mitsuru
Nose Tetsuro
Gupta Yogendra
Hamlin Derrick G.
Kenyon & Kenyon
Nippon Steel Corporation
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