Processing of oxide superconductors

Details Processing of oxide superconductors Processing of oxide superconductors

2001-05-18

2002-08-20

Kopec, Mark (Department: 1751)

Superconductor technology: apparatus, material, process

Processes of producing or treating high temperature...

Heating, annealing, or sintering

C505S482000, C505S742000

Reexamination Certificate

active

06436876

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to high-performance oxide superconductors and oxide superconductor composites. The present invention further relates to a method for healing defects introduced into the oxide superconductor phase during processing thereby improving superconducting properties. The present invention also relates to the processing of high performance bismuth-strontium-calcium-copper oxide superconductors and oxide superconductor composites and a method for improving the critical transition temperature (T
c
and critical current density (J
c
) of these oxide superconductors.
BACKGROUND OF THE INVENTION
Oxide superconductors of the rare earth-barium-copper-oxide family (YBCO), bismuth(lead)-strontium-calcium-copper-oxide family ((Bi,Pb)SCCO) and thallium-barium-calcium-copper-oxide family (TBCCO) form plate-like and highly anisotropic superconducting oxide grains. Because of their plate-like morphology, the oxide grains can be oriented by mechanical strain. Mechanical deformation has been used to induce grain alignment of the oxide superconductor c-axis perpendicular to the plane or direction of elongation. The degree of alignment of the oxide superconductor grains is a major factor in the high critical current densities (J
c
) obtained in articles prepared from these materials.
Known processing methods for obtaining textured oxide superconductor composite articles include an iterative process of alternating heating and deformation steps. The heat treatment is used to promote reaction-induced texture of the oxide superconductor in which the anisotropic growth of the superconducting grains is enhanced. Each deformation provides an incremental improvement in the orientation of the oxide grains. Additional heat treatment intermediate with or subsequent to deformation is also required to form the correct oxide superconductor phase, promote good grain interconnectivity and achieve proper oxygenation.
Processing long lengths of oxide superconductor is particularly difficult because deformation leads to microcracking and other defects which may not be healed in the subsequent heat treatment. Cracks that occur perpendicular to the direction of current flow limit the performance of the superconductor. In order to optimize the current carrying capability of the oxide superconductor, it is necessary to heal any microcracks that are formed during processing of the oxide superconductor or superconducting composite.
Liquid phases in co-existence with solid oxide phases have been used in processing of oxide superconductors. One type of partial melting, known as peritectic decomposition, takes advantage of liquid phases which form at peritectic points of the phase diagram containing the oxide superconductor. During peritectic decomposition, the oxide superconductor remains a solid until the peritectic temperature is reached, at which point the oxide superconductor decomposes into a liquid phase and a new solid phase. The peritectic decompositions of Bi
2
Sr
2
CaCu
2
O
8+x
, (BSCCO-2212, where 0≦x≦1.5), into an alkaline earth oxide and a liquid phase and of YBa
2
Cu
3
O
7−&dgr;
, (YBCO-123, where 0≦&dgr;≦1.0) into Y
2
BaCuO
5
and a liquid phase are well known.
Peritectic decomposition of an oxide superconductor and the reformation of the oxide superconductor from the liquid + solid phase is the basis for melt textured growth of YBCO-123 and BSCCO-2212. For example, Kase et al. in
IEEE Trans. Mag
. 27(2), 1254 (MarCu 1991) report obtaining highly textured BSCCO-2212 by slowly cooling through the peritectic point. This process necessarily involves total decomposition of the desired 2212 phase into an alkaline earth oxide and a liquid phase.
It is also recognized that an oxide superconductor itself can co-exist with a liquid phase under suitable processing conditions. This may occur by solid solution melting, eutectic melting or by formation of non-eqilibrium liquids.
Solid solution melting may arise in a phase system, in which the oxide superconductor is a solid solution. As the temperature (or some other controlling parameter) of the system increases (or decreases), the oxide superconductor phase changes from a solid oxide phase to a liquid plus oxide superconductor partial melt (this happens at the solidus). A further increase in temperature (or some other controlling parameter) affords the complete melting of the oxide superconductor (this happens at the liquidus).
A phase diagram containing a eutectic point may provide an oxide superconductor partial melt, known as eutectic melting, when the overall composition is chosen so as to be slightly off stoichiometry. As the temperature (or some other controlling parameter) of the system increases (or decreases), the mixed phase of oxide superconductor-plus-non-superconducting oxide (solid
1
+solid
2
) changes to a liquid-plus-oxide superconductor partial melt (solid
1
+liquid).
Non-equilibrium liquids may also promote partial melting in oxide superconductor systems. A non-equilibrium liquid is established through the relatively rapid heating of a mixture of oxides to a temperature above the eutectic melting point of local stoichiometries present in the heterogeneous mixture of phases. As the oxides form the desired oxide superconductor, the solid and liquid phases can co-exist, if only temporarily.
Partial melting of (Bi,Pb)
2
Sr
2
Ca
2
Cu
3
O
10+x
((Bi,Pb)SCCO-2223, where 0≦x≦1.5) and (Bi)
2
Sr
2
Ca
1
, Cu
2
O
10+x
((Bi)SCCO-2223, where
0≦x≦1.5
), 25 BSCCO-2223, at temperatures above 870° C. in air has been reported; see, for example, Kobayashi et al.
Jap. J. Appl. Phys
. 28, L722-L744 (1989), Hatano et al. Ibid. 27(11), L2055 (Nov. 1988), Luo et al.
Appl. Super
. 1, 101-107, (1993), Aota et al.
Jap. J. Appl. Phys
. 28, L2196-L2199 (1989) and Luo er al.
J. Appl. Phys
. 72, 2385-2389 (1992). The exact mechanism of partial melting of BSCCO-2223 has not been definitively established.
Guo et al. in
Appl. Supercond
. 1(1/2), 25 (January 1993) have described a phase formation-decomposition-reformation (PFDR) process, in which a pressed sample of (Bi,Pb)SCCO-2223 is heated above its liquidus to decompose the 2223 phase, followed by a heat treatment at a temperature below the solidus. The sample was subsequently pressed again and reannealed. The final anneal of the PFDR process includes a standard single step heat treatment in which there is no melting.
The “high T
c
” oxide superconductor Bi
2−y
Pb
y
Sr
2
Ca
2
Cu
−10+x
, where 0≦x≦1.5 and 0≦y≦0.6 (BSCCO-2223 and (Bi, Pb)SCCO-2223, hereinafter referred to as “BSCCO-2223” to indicate both lead-doped and undoped compositions), is particularly desirable because of its high critical transition temperature (T
c
~110 K) and high critical current (I
c
, J
c
). The superconducting art constantly seeks to improve electrical properties, such as, critical current density and critical transition temperature.
Partial melting in the processing of oxide superconductors has been used either to increase the yield of the BSCCO-2223 phase or to improve the contiguity and texturing of the oxide superconductor grains. The issue of healing defects, such as microcracks, which develop during processing of the oxide superconductor, has not been addressed. Further, the prior art has not addressed the possibility of using a two-step process where the oxide superconductor is stable in both steps for the healing of cracks and defects.
Wang et al. (“Advances in Superconductivity”, Springer-Verlag, New York, Editors: Y. Bando and H. Yamauchi, pp.
291
-
294
(1993)) report an increase in T
c
by carrying out a post-anneal step at 790° C. at reduced total pressures. Wang et al. observed T
c
by DC magnetization of 115 K and T
c,zero
of 111 K by resistivity measurement. The technique used by Wang et al. (vacuum encapsulation at 104 Torr of oxide superconductor pellets, followed by annealing at 790° C.) does not permit determination of the oxygen pressure of the system. The encapsulated pellets reach an equil

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